Optimising The Lamination Properties Of Textile Composites

A thesis submitted to

The University of Manchester

For the degree of Doctor of Philosophy

in the Faculty of

Engineering and Physical Sciences

By

Ali Hasan Mahmood

Textiles Science & Technology

School of Materials The University of Manchester

2011

2

Table of Contents Table of Contents .............................................................................................................. 2

Table of Figures ................................................................................................................ 5

List of Tables ..................................................................................................................... 8

List of Equations ............................................................................................................... 8

Abstract ............................................................................................................................. 9

Declaration .......................................................................................................................10

Copyright Statement ........................................................................................................11

Acknowledgements ...........................................................................................................13

CHAPTER 1 INTRODUCTION ....................................................................................14

1.1. RESEARCH BACKGROUND ..................................................................................14 1.2. PROJECT AIM AND OBJECTIVES ..........................................................................15 1.3. BRIEF CONTENT OF REMAINING CHAPTERS .........................................................16

CHAPTER 2 LITERATURE REVIEW .........................................................................17

2.1. INTRODUCTION ..................................................................................................17 2.2. COMPOSITES ......................................................................................................17

2.2.1. Matrix ...........................................................................................................17 2.2.1.1. Thermoplastic resins .............................................................................18 2.2.1.2. Thermoset resins ...................................................................................18

2.2.2. Reinforcement fibres .....................................................................................19 2.2.2.1. Glass fibre ............................................................................................19

2.3. FIBRE REINFORCED COMPOSITES ........................................................................22 2.4. MANUFACTURING OF COMPOSITES ....................................................................22 2.5. COMPOSITE FAILURE .........................................................................................24

2.5.1. Delamination .................................................................................................25 2.5.2. Importance of filling yarn ..............................................................................26 2.5.3. Effect of thickness and number of laminated layers .......................................28 2.5.4. Effect of thermal conditioning on glass composite failure ..............................29 2.5.5. Effect of hygro-thermal exposure on glass composites ...................................29 2.5.6. Effect of water absorption .............................................................................30

2.6. THROUGH-THE-THICKNESS REINFORCEMENT .....................................................30 2.6.1. Through-the-thickness stitching .....................................................................31 2.6.2. Z-Fibre Pinning .............................................................................................33

2.7. YARN TEXTURING FOR INCREASING THE BONDING STRENGTH ............................35 2.7.1. Air-jet texturing ............................................................................................36

2.7.1.1. Types of operations in air-jet texturing process......................................37 2.7.1.2. Texturing nozzles..................................................................................38

2.7.2. Key considerations for the air-jet texturing process ........................................46 2.7.2.1. Wetting of the yarn before entering the jet .............................................46 2.7.2.2. Primary flow length ..............................................................................47 2.7.2.3. Filament fineness ..................................................................................48 2.7.2.4. Reduction in strength of textured yarn ...................................................48 2.7.2.5. Overfeeding ..........................................................................................48 2.7.2.6. Filament cross-section...........................................................................49

2.8. COMMINGLING PROCESS ....................................................................................49 2.8.1. Jet design for the commingling process .........................................................50 2.8.2. Commingled yarns for composites .................................................................51 2.8.3. Glass filament commingling process .............................................................52

3

2.9. SELECTION CRITERIA FOR THE AIR-JET TEXTURING PROCESS ..............................53 2.10. SUMMARY .........................................................................................................54

CHAPTER 3 GLASS YARN TEXTURING, WEAVING AND COMPOSITE MANUFACTURING PROCESS .....................................................................................56

3.1. INTRODUCTION ..................................................................................................56 3.2. AIR-JET TEXTURING MACHINE ...........................................................................56

3.2.1. Texturing machine components .....................................................................56 3.2.2. Feeder yarn creel ...........................................................................................56 3.2.3. Rollers arrangement ......................................................................................57 3.2.4. The jet box ....................................................................................................58 3.2.5. Oil application device ....................................................................................59 3.2.6. Winding unit .................................................................................................59 3.2.7. Suction gun ...................................................................................................60 3.2.8. Gearing arrangement .....................................................................................61 3.2.9. Texturing machine set up for glass yarn .........................................................62 3.2.10. Alteration in the drawing zone...................................................................62 3.2.11. Alteration in the winding zone...................................................................63 3.2.12. Type of jet used ........................................................................................63 3.2.13. Selection of the overfeed value ..................................................................64 3.2.14. Selection of the air pressure value .............................................................65

3.3. WARPING PROCESS ............................................................................................67 3.4. GLASS FABRIC PRODUCTION ..............................................................................68

3.4.1. Problems during weaving process ..................................................................71 3.5. COMPOSITE MANUFACTURING ...........................................................................73

3.5.1. Vacuum bagging technique ...........................................................................73

CHAPTER 4 CHARACTERISATION, EQUIPMENT AND PROCEDURES ............77

4.1. INTRODUCTION ..................................................................................................77 4.2. BREAKING STRENGTH (TENACITY) TESTING OF GLASS YARNS ............................77 4.3. DENSITY, FIBRE VOLUME FRACTION AND VOID CONTENT .................................78 4.4. TENSILE TESTING ...............................................................................................80 4.5. FLEXURE TESTING (THREE POINT BENDING) .......................................................82 4.6. INTER-LAMINAR SHEAR STRENGTH (ILSS) .........................................................85 4.7. INTER-LAMINAR FRACTURE TOUGHNESS ............................................................86

4.7.1. Mode I Inter-laminar fracture toughness ........................................................87 4.8. SCANNING ELECTRON MICROSCOPE (SEM) ........................................................90

CHAPTER 5 EFFECT OF THE TEXTURING PROCESS ON GLASS YARN TENACITY ......................................................................................................................92

5.1. INTRODUCTION ..................................................................................................92 5.2. TENACITY OF THE FEED YARNS ..........................................................................92 5.3. TENACITY OF THE 300 TEX CATEGORY ...............................................................93 5.4. TENACITY OF THE 600 TEX CATEGORY ...............................................................97 5.5. TENACITY OF COMBINED CORE-AND-EFFECT FEED YARNS ................................ 100 5.6. BROKEN FILAMENTS AND LOSS IN LINEAR DENSITY ......................................... 101 5.7. SUMMARY ....................................................................................................... 103

CHAPTER 6 COMPOSITES MADE WITH TEXTURED YARNS: MECHANICAL TESTING, RESULTS AND DISCUSSION .................................................................. 104

6.1. INTRODUCTION ................................................................................................ 104 6.2. COMPOSITES NOMENCLATURE ......................................................................... 104 6.3. FIBRE VOLUME CONTENT ................................................................................. 105 6.4. TENSILE TESTING OF COMPOSITES .................................................................... 106

6.4.1. Tensile properties of 300 tex plain weave composites .................................. 107

4

6.4.2. Tensile properties of 300 tex twill weave composites ................................... 108 6.4.3. Tensile properties of 600 tex plain and twill composites .............................. 109

6.5. FLEXURE TESTING OF COMPOSITES .................................................................. 113 6.5.1. Flexure properties of 300 tex plain weave composites .................................. 114 6.5.2. Flexure properties of 300 tex twill weave composites .................................. 115 6.5.3. Flexure properties of 600 tex composites ..................................................... 116

6.6. INTER-LAMINAR SHEAR STRENGTH (ILSS) TESTING ......................................... 118 6.6.1. ILSS of 300 tex plain and twill weave composites ....................................... 118 6.6.2. ILSS of 600 tex plain and twill composites .................................................. 120 6.6.3. Microscope and SEM Analysis .................................................................... 121

6.7. FRACTURE TOUGHNESS (MODE I) TESTING ...................................................... 125 6.8. SUMMARY ....................................................................................................... 131

CHAPTER 7 COMPOSITES WITH TEXTURED AND NON-TEXTURED CORE YARNS ........................................................................................................................... 133

7.1. INTRODUCTION ................................................................................................ 133 7.2. CORE TEXTURED YARN COMPOSITES ................................................................ 133

7.2.1. Fibre volume content of CT composites ....................................................... 133 7.3. MECHANICAL PROPERTIES OF CT COMPOSITES ................................................ 134

7.3.1. Tensile properties of 600 tex CT composites ............................................... 134 7.3.2. Flexure properties of 600 tex CT composites ............................................... 136 7.3.3. ILSS of 600 tex CT plain and twill composites ............................................ 137

7.4. MIXED YARN COMPOSITES ............................................................................... 138 7.4.1. Fibre volume content of WfW composites ................................................... 138

7.5. MECHANICAL PROPERTIES OF WFW COMPOSITES ............................................ 138 7.5.1. Tensile properties of 600 tex WfW composites ............................................ 138 7.5.2. Flexure properties of 600 tex WfW composites ........................................... 140 7.5.3. ILSS of 600 tex WfW composites................................................................ 141

7.6. COMPARISON OF MECHANICAL PROPERTIES ..................................................... 142 7.7. PRODUCTION OF MIXED YARN FABRIC ON A POWER LOOM................................ 145 7.8. SUMMARY ....................................................................................................... 147

CHAPTER 8 CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK ................................................................................................................................ 148

8.1. CONCLUSIONS ................................................................................................. 148 8.1.1. Tenacity of yarn after texturing ................................................................... 148 8.1.2. Tensile properties of composites .................................................................. 149 8.1.3. Flexure properties of composites ................................................................. 149 8.1.4. Inter-laminar shear strength and fracture toughness of composites ............... 149 8.1.5. Weave structure .......................................................................................... 150 8.1.6. Composites with combination of textured and non-textured yarns ................ 150

8.2. RECOMMENDATIONS FOR FUTURE WORK ......................................................... 150

REFERENCES............................................................................................................... 152

APPENDIX A: CALCULATIONS FOR DRAW RATIO AND OVERFEED ............. 162

APPENDIX B: MECHANICAL PROPERTIES .......................................................... 166

Word count: 38232 words

5

Table of Figures Figure 2.1 Schematic diagram of the filament winding process [Mazumdar 2002] ..23 Figure 2.2 Schematic diagram of the sequence of delamination crack propagation between the layer in a woven-fabric laminate as viewed from the top [Kim and Sham 2000] ......................................................................................................................26 Figure 2.3 Resin rich areas in woven fabric composite ............................................27 Figure 2.4 Schematic diagram of the stitched preform [Nie et al 2008] ....................31 Figure 2.5 Schematic diagram of Z-pinning process Mouritz [2007] .......................34 Figure 2.6 Mechanism of air-jet texturing [Acar et al 2006] ....................................37 Figure 2.7 First Air-Jet Process “Taslan” by Du Pont ..............................................39 Figure 2.8 Taslan jets (a) Type 7 (b) Type 8 (c) Type 9 (d) Type 10 (e) Type 11 (f) Type 14...................................................................................................................41 Figure 2.9 Taslan Type 20.......................................................................................42 Figure 2.10 Standard-core Hemajet [Heberlein guide 1991] ....................................43 Figure 2.11 (Hemajet LB-02 Universal Housing with T-Series Jet Core) [Heberlein guide 1991] .............................................................................................................43 Figure 2.12 Heberlein Hemajet EO-52 [Oerlikon 2010] ..........................................44 Figure 2.13 Hemajet jet cores (a) A and T series, (b) A-2, S-2 and T-2 series [Oerlikon 2004a, 2007b] .........................................................................................45 Figure 2.14 Heberlein Jet Housing (a) Hemajet LB-04, (b) Hemajet LB-24 [Oerlikon 2007a, 2009b] .........................................................................................................46 Figure 2.15 Commingling process [Alagirusamy et al 2005] ...................................50 Figure 2.16 Air Inlet Configurations for Commingling Process [R. Alagirusamy et al 2005] ......................................................................................................................51 Figure 3.1 Creel Section ..........................................................................................57 Figure 3.2 Rollers Section .......................................................................................58 Figure 3.3 Jet box and components .........................................................................59 Figure 3.4 Oil application roller ..............................................................................59 Figure 3.5 Winding unit ..........................................................................................60 Figure 3.6 Suction gun ............................................................................................60 Figure 3.7 Gearing arrangement ..............................................................................61 Figure 3.8 Modified thread line diagram of Stähle RMT-D air-jet texturing machine for glass yarn ..........................................................................................................62 Figure 3.9 Jet housing (Heberlein hemajet LB-13) ..................................................63 Figure 3.10 Jet core (T-370) ....................................................................................63 Figure 3.11 Core-and-effect textured glass yarns .....................................................66 Figure 3.12 Single end warping machine (made by the Shirley Institute) .................67 Figure 3.13 Glass yarn warping in process ..............................................................68 Figure 3.14 Hand loom ...........................................................................................69 Figure 3.15 Dead weight for warp yarn tensioning ..................................................70 Figure 3.16 (1/1) Plain weave fabrics ......................................................................71 Figure 3.17 (1/3) Twill weave fabrics......................................................................71 Figure 3.18 Entanglements during the shedding process ..........................................72

6

Figure 3.19 Entanglements in 300 + 34 tex 3 bars pressure textured warp yarns ......73 Figure 3.20 Configuration diagram of the vacuum bagging process .........................74 Figure 3.21 Vacuum bag .........................................................................................74 Figure 4.1 Glass yarn specimen undergoing breaking strength testing .....................78 Figure 4.2 Composite specimen undergoing tensile testing ......................................81 Figure 4.3 Flexure testing assembly (a) three point bending (b) four point testing [Hodgkinson 2000] .................................................................................................83 Figure 4.4 Potential failure modes for flexure testing [BSI 14125 1998]..................83 Figure 4.5 Composite specimen undergoing Inter-laminar shear strength (ILSS) testing .....................................................................................................................86 Figure 4.6 Schematic diagrams of the basic modes of fracture, mode I (opening), mode II (shear), mode III (tearing) [Robinson and Hodgkinson 2000] .....................87 Figure 4.7 Double cantilever beam (DCB) specimen geometry, (a) end-blocks, (b) piano hinges [Robinson and Hodgkinson 2000] ......................................................88 Figure 4.8 DCB test specimen undergoing fracture toughness testing ......................89 Figure 4.9 Section of DCB with piano hinges indicating “t” ....................................90 Figure 4.10 Prepared samples for scanning electron microscopy (SEM) ..................91 Figure 5.1 Tenacity of the feed yarns ......................................................................93 Figure 5.2 Tenacity of textured and non-textured glass yarns of 300 tex category ...94 Figure 5.3 photomicrographs of 300 + 34 tex 5 bars textured yarn structure ............95 Figure 5.4 Photomicrographs of 300 + 68 tex 5 bars textured yarn structure ............95 Figure 5.5 Tenacity of textured and non-textured glass yarns of 600 tex category ...97 Figure 5.6 Comparison of tenacity of 300 and 600 tex textured yarns ......................98 Figure 5.7 Photomicrographs images of 600 + 34 tex 5 bars textured yarn structure 99 Figure 5.8 Photomicrographs of 600 + 68 tex 5 bars textured yarn structure ............99 Figure 5.9 Comparison of tenacity of non-textured feed yarns ............................... 100 Figure 5.10 Linear density (tex) of textured glass yarns (a) 300 tex (b) 600 tex category ................................................................................................................ 102 Figure 6.1 Tensile strength of 300 tex plain weave composites.............................. 107 Figure 6.2 Tensile modulus of 300 tex plain weave composites ............................. 107 Figure 6.3 Tensile strength of 300 tex twill weave composites .............................. 108 Figure 6.4 Tensile modulus of 300 tex twill weave composites ............................. 109 Figure 6.5 Tensile strength of 600 tex plain & twill weave composites.................. 110 Figure 6.6 Tensile modulus of 600 tex plain & twill weave composites ................. 110 Figure 6.7 Tensile tested samples of 600 tex non-textured plain weave composites ............................................................................................................................. 112 Figure 6.8 Tensile tested samples of 600 + 68 tex 5 bars textured plain weave composites ............................................................................................................ 113 Figure 6.9 Flexure strength of 300 tex plain weave composites ............................. 114 Figure 6.10 Flexure modulus of 300 tex plain weave composites .......................... 114 Figure 6.11 Flexure strength of 300 tex twill weave composites ............................ 115 Figure 6.12 Flexure modulus of 300 tex twill weave composites ........................... 116 Figure 6.13 Flexure strength of 600 tex plain & twill weave composites ............... 117 Figure 6.14 Flexure modulus of 600 tex plain & twill weave composites .............. 117

7

Figure 6.15 ILSS of 300 tex plain weave composites ............................................ 118 Figure 6.16 ILSS of 300 tex twill weave composites ............................................. 119 Figure 6.17 ILSS of 600 tex plain & twill weave composites ................................ 120 Figure 6.18 600 tex non-textured twill weave composite ....................................... 122 Figure 6.19 600 + 34 tex 5 bars twill weave composite ......................................... 123 Figure 6.20 SEM images 600 + 34 tex 5 bars twill weave composite ..................... 124 Figure 6.21 SEM image 600 + 34 tex 5 bars plain weave composites .................... 125 Figure 6.22 SEM image 600 without textured plain weave composite ................... 125 Figure 6.23 Typical load versus crosshead displacement curves for mode I specimens of the 600 non-textured twill weave and the 600 + 68 tex 5 bars twill weave composites ............................................................................................................ 127 Figure 6.24 Initiation and propagation values for mode I testing of 600 + 68 tex 5 bars textured and 600 non-textured twill weave composites .................................. 128 Figure 6.25 Comparison of the mean values of G1c (visual, 5 % offset and propagation) for mode I DCB testing of 600 + 68 tex 5 bars textured and 600 non-textured twill weave composites............................................................................ 129 Figure 6.26 SEM micrographs of fracture surfaces of 600 tex twill weave non-textured composite ................................................................................................ 130 Figure 6.27 SEM micrographs of fracture surfaces of 600 + 68 tex 5 bars twill weave textured composite ................................................................................................ 131 Figure 7.1 Tensile strength of 600 tex CT plain & twill weave composites ............ 135 Figure 7.2 Tensile modulus of 600 tex CT plain & twill weave composites ........... 135 Figure 7.3 Flexure strength of 600 tex CT plain & twill weave composites ........... 136 Figure 7.4 Flexure modulus of 600 tex CT plain & twill weave composites .......... 136 Figure 7.5 ILSS of 600 tex CT plain & twill weave composites ............................ 137 Figure 7.6 Tensile strength of 600 tex plain & twill weave WfW composites ........ 139 Figure 7.7 Tensile modulus of 600 tex plain & twill weave WfW composites ....... 139 Figure 7.8 Flexure strength of 600 tex plain & twill weave WfW composites ........ 140 Figure 7.9 Flexure modulus of 600 tex plain & twill weave WfW composites ....... 140 Figure 7.10 ILSS of 600 tex plain & twill weave WfW composites ....................... 141 Figure 7.11 Tensile strength of 600 tex plain & twill weave composites ................ 142 Figure 7.12 Tensile modulus of 600 tex plain & twill weave composites ............... 143 Figure 7.13 Flexure strength of 600 tex plain & twill weave composites ............... 143 Figure 7.14 Flexure modulus of 600 tex plain & twill weave composites .............. 144 Figure 7.15 Inter-laminar shear strength of 600 tex plain & twill weave composites ............................................................................................................................. 145 Figure 7.16 Production of mixed yarn fabric on a power loom .............................. 146

8

List of Tables Table 2.1 Available glass types and their properties [Vaughan 1998] ......................20 Table 2.2 Fibre glass filament designations [Vaughan 1998] ...................................21 Table 3.1 Fabric specifications ................................................................................71 Table 3.2 Consumable materials required for the vacuum bagging [Cripps 2000] ....75 Table 5.1 Number of filaments in glass yarns ..........................................................96 Table 6.1 Fibre volume content of glass composites .............................................. 105 Table 7.1 Fibre volume content of CT composites................................................. 134 Table 7.2 Fibre volume content of WfW composites ............................................. 138

List of Equations

Density of specimen = ρS (g/cm3) = LSAS

WAS

mmm

,,

,

(4.1) [BS ISO 1183-1, 2004]

79

f

Cff WV

(4.2) [Khan 2010]

.......................................................................79

10012

13

MMMMW f

(4.3) [BS ISO 1172, 1999] ........................................80

R

Cf

f

Cfo WWV

100100

(4.4) [Khan 2010].......................80

bhF

(4.5) [BS 2782-10: Method 1003 1977] ..............................................81

2

2

2 36123

Lsh

LS

bhFL

f (4.6) [BSI 14125 1998] ......................84

sF

bhLE f 3

3

4 (4.7) [BSI 14125 1998] .....................................................84

bhF

ILSSmax

43

(4.8) [BS ISO 14130 1998] ....................................86

FPG c

a 2b3

1

(4.9) [ASTM D 5528-01 2007]

............................................89

22

31031

2

at

aF

(4.10) [ASTM D 5528-01 2007]

.........................90

9

Abstract Woven glass composites have been used for many years in commercial applications

due to their light weight, competitive price and good engineering properties.

Absorption of energy by laminated composite material results in damage in various

forms, the most common of which is delamination. Inter-laminar fracture causes the

layers of composite to separate, resulting in a reduction in stiffness and strength of

the composite structure, matrix cracking and in some cases fibre breakage takes

place. The aim of this project was to improve the inter-laminar bond strength

between woven glass fabric and resin. Air jet texturing was selected to provide a

small amount of bulk to the glass yarn. The purpose was to provide more surface

contact between the fibres and resin and also to increase the adhesion between the

neighbouring layers. These were expected to enhance the resistance to delamination

in the woven glass composites.

Glass yarns were textured by a Stähle air jet texturing machine. Core-and-effect yarn

was produced instead of a simple air textured yarn. Hand loom and vacuum bagging

techniques were used for making the fabric and composite panels from both textured

and non-textured yarns. Density and fibre volume content were established for

physical characterisation. Breaking strength (tenacity) of the yarns and tensile,

flexure, inter-laminar shear strength (ILSS) and fracture toughness (mode 1)

properties of the composites were determined. Projection microscopy and SEM

imaging techniques were used to assess the fractured surfaces of the composite

specimens. The yarn tenacity and the tensile properties of the composites were

significantly reduced after the texturing process, whereas flexure properties were

unchanged. However, significant improvement was observed in the ILSS and

fracture toughness of the composites after the texturing process. It was also observed

that the composites made from the fabrics with textured yarns in only the weft

direction are the most advantageous as they maintained the tensile and flexure

properties but have significantly higher inter-laminar shear strength.

10

Declaration

No portion of the work referred to in the thesis has been submitted in support of an

application for another degree or qualification of this or any other university or other

institute of learning.

Ali Hasan Mahmood

11

Copyright Statement

I. The author of this thesis (including any appendices and/or schedules to this

thesis) owns certain copyright or related rights in it (the “Copyright”) and he

has given The University of Manchester certain rights to use such Copyright,

including for administrative purposes.

II. Copies of this thesis, either in full or in extracts and whether in hard or

electronic copy, may be made only in accordance with the Copyright,

Designs and Patents Act 1988 (as amended) and regulations issued under it

or, where appropriate, in accordance with licensing agreements which the

University has from time to time. This page must form part of any such

copies made.

III. The ownership of certain Copyright, patents, designs, trade marks and other

intellectual property (the “Intellectual Property”) and any reproductions of

copyright works in the thesis, for example graphs and tables

(“Reproductions”), which may be described in this thesis, may not be owned

by the author and may be owned by third parties. Such Intellectual Property

and Reproductions cannot and must not be made available for use without the

prior written permission of the owner(s) of the relevant Intellectual Property

and/or Reproductions.

IV. Further information on the conditions under which disclosure, publication

and commercialisation of this thesis, the Copyright and any Intellectual

Property and/or Reproductions described in it may take place is available in

the University IP Policy (see

http://www.campus.manchester.ac.uk/medialibrary/policies/intellectual-

property.pdf), in any relevant Thesis restriction declarations deposited in the

University Library, The University Library’s regulations (see

http://www.manchester.ac.uk/library/aboutus/regulations) and in The

University’s policy on presentation of Theses.

12

This thesis is dedicated to my (late) father (Mr. Jafar Mahmood), mother (Mrs.

Shahina Mahmood), my wife (Mrs. Sana Ali), my son (Master Saami Ali), my

brothers (Mr. Faiq Ali, Mr. Ammar Hasan, Mr. Hani Hasan), my sister (Mrs.

Aisha Faiq) and my nephew and niece (Master Hadi and Miss Manal).

13

Acknowledgements First and foremost, praises and thanks to Allah S.W.T who bestowed upon us all the

blessings and the faculties of thinking, learning and searching.

This study would not have been possible without the financial support of my

employer and sponsor, NED University of Engineering & Technology funded

through the Higher Education Commission (HEC) of Pakistan.

I would like to express my deepest gratitude for my supervisors, Prof. Porat and Dr.

Gong, whose encouragement, guidance and most importantly support from the initial

to the final level enabled me to think independently and to develop an understanding

of the subject.

I would also like to thank my parents, my brothers and sister, and my wife for

keeping up with me and my demands and their moral encouragement. They boosted

my ego, when it was needed and supported me in various ways but, all through their

unconditional love.

I would also like to sincerely thank Prof. Peter Foster, Dr. Sheraz Hussain Yousfani,

Dr. Laraib Alam Khan, Dr. Syed Naveed Rizvi, Dr. Alan Nesbitt, Dr. Chris Wilkins,

Dr. Chi Zhang, Mr. Steve Butt, Mr. Adrian Handley and Mr. Tom Kerr for their

valuable help, advice and technical assistance.

Many thanks go to PPG Industries for providing the glass filaments and Mr. Keith

Wilson for providing the best advices and support for texturing glass yarn.

Last but not least, I am indebted to any of my colleagues and staff members, and in

fact anyone else who has supported and assisted me in conducting this work.

14

Chapter 1 Introduction

1.1. Research background

Composite materials have gained substantial popularity for a wide range of

applications in structural components because of their high strength-to-weight and

stiffness-to-weight ratios. However, failure due to delamination (the separation of

laminate layers) is of great concern. Delamination, as indicated by various

researchers, is the most common cause of damage in glass composites. This happens

under the impact of load and results in fibre-matrix de-bonding. The purpose of this

research is to improve the bond strength between the glass and the matrix by using

textured yarns developed through the air jet texturing process. The concept was to

produce bulk in the yarn through texturing in order to provide more surface contact

between the fibre and resin, and between the neighbouring layers. The technique of

air jet texturing was utilised by Ma et al [2003] to improve the coated ratio and the

bond strength of glass/PVC fabrics. Koc et al [2008] found improvement in adhesion

of PET yarns to rubber by incorporating a very small amount of texturing. Langston

[2003] also found improvement in inter-laminar shear strength of composites by

texturing Aramid yarns and the reason was the anchoring and entanglement between

the layers due to the bulkier yarn structure.

One potential disadvantage of using textured yarns is the reduction in in-plane

mechanical strength due to the disorientation of filaments introduced in the texturing

process. Therefore, this study was based on the production of core-and-effect

textured reinforcement (glass) yarns. The intention was to keep the disorientation of

filaments as small as possible to minimise strength reduction while producing

sufficient texture to enhance the inter-laminar bonding strength. With the core-and-

effect yarn, the core yarn was processed with a minimum overfeed ratio to maintain

the strength of the final yarn. The effect yarn, however, was subjected to moderate

overfeed for the development of loops and bulk.

15

The yarns produced were then woven and a number of weave structures were

investigated and optimised, these fabrics were then used to produce composites

which were subjected to various tests.

1.2. Project aim and objectives

The aim of this project was to minimise the problem of delamination in composites

by increasing the bond strength between the reinforcement glass yarn fabric and the

resin and between the neighbouring layers.

In order to achieve the aim, the following tasks were planned:

review the literature in the fields of textile composites, delamination

behaviour of composites and the causes of delamination, other means for

improving the lamination strength, air jet texturing and commingling

processes;

manufacture the core-and-effect textured glass yarn through air jet texturing

and investigates the optimum texturing parameters;

investigate the effect of texturing parameters on the tenacity of glass yarns;

manufacture woven glass fabrics on a hand loom from both the textured and

non-textured glass yarns;

producing multi-layered thermoset composites by using a suitable technique

of composite manufacturing;

investigate the effect of texturing on the physical and mechanical properties

of these composites.

16

1.3. Brief content of remaining chapters

Chapter 2 covers the literature review including a short introduction to composites, a

literature survey of delamination and the preventive measures that are commonly

used and the air jet texturing and commingling processes.

Chapter 3 describes the equipment and techniques employed for the production of

samples used in this study together with their merits and constraints. This includes

the study of air jet texturing machine, texturing of glass yarns, fabric development

and finally the fabrication of composite panels.

The physical and mechanical test methods and equipment used to characterise the

textured and non-textured glass composites and the scientific principles involved in

the techniques are described in detail in Chapter 4.

Chapters 5, 6 and 7 cover the experimental work, results and discussion parts of this

study. The comparison of the tenacities of textured and non-textured glass yarns and

the effect of texturing on their tenacity are investigated in Chapter 5.

Chapter 6 includes the results and discussion regarding the effect of texturing on the

mechanical properties of the fabric composites made from core-and-effect textured

glass yarns.

Chapter 7 concerns the effect of texturing on the mechanical properties of

composites made from textured and non-textured core yarns. These composites were

developed by changing the composition of fabrics on the basis of their constituent

yarns.

Chapter 8 presents the conclusions of this work and suggests future work.

17

2. Chapter 2 Literature review

2.1. Introduction

This project is concerned with improving the lamination strength of glass reinforced

composites by modifying the fabric surface using air-jet textured yarn. The work is

based on a combination of textiles and composites technologies and relevant topics

to this work are reviewed below. This chapter includes a short introduction to

composites followed by a literature survey of delamination and the preventive

measures that are commonly used. It includes studies regarding the air-jet texturing

process, the commingling process and their importance for composites.

2.2. Composites

Composite materials are engineered, heterogeneous materials comprising two or

more constituent materials with a discrete and recognisable interface separating

them. These are macroscopic combinations and the most common naturally

occurring composite is wood. The two constituent materials are the matrix and the

reinforcement. Reinforcement fibres are usually of high strength/stiffness and are

generally orthotropic (having different properties in different directions depending

upon the direction of the applied load). The matrix material is ordinarily of a high

performance type. Moreover, both fibres and matrix may be organic or inorganic in

nature [Reinhart 1998, Peters 1998].

2.2.1. Matrix

The matrix acts as a binder for the fibres because it has adhesion and cohesion

characteristics. It helps in transferring of load to the fibres and between the fibres

and also guards them from environmental impacts. Orientation and location of the

fibres in the composite structure are maintained by the matrix. By distributing the

load evenly among the fibres, it resists damage and crack propagation. The matrix

contributes to the electrical and chemical properties of the composite [Reinhart 1998,

Peters 1998].

Most commercially produced composites use a polymer matrix material often called

a resin which is classified into two types, namely thermoplastic and thermoset resins.

18

2.2.1.1. Thermoplastic resins Thermoplastic resins are usually cheaper for fabrication. They can be stored safely

for long periods of time before moulding. They have the ability to be re-moulded by

application of temperature and pressure as the molecules are generally not cross-

linked. They are characterised by toughness and high impact strength. However, they

suffer thermal degradation with repetitive temperature cycling [Reinhart 1998].

The examples include Polyether ether ketone (PEEK), Polyphenylene sulfide (PPS),

Polyether ketone ketone (PEKK), Polyamide (PA or Nylon), Polybutylene

terephthalate (PBT), Polyethylene terephthalate (PET), Polyethylene (PE),

Polypropylene (PP), Polyvinyl chloride (PVC).

2.2.1.2. Thermoset resins Thermoset resins are generally available in liquid form and after mixing with other

ingredients they solidify. They form cross-linkages between the molecules during the

curing process and thus once cured, they cannot be remoulded. Thermosets are

relatively easy to process and usually do not require pressure or high temperature to

form. They normally possess a short workable shelf life [Peters 1998, Varma and

Gupta 2000].

Examples of thermosets resins include Epoxy, Polyester, Vinylester, Polyurethane,

Polyimide, Cyanate ester, Phenolic triazine.

Epoxy resins are relatively lower molecular weight polymers and are used as a

matrix for fibre composites in structural applications. They have a number of

advantages over the other types of polymers. They are inherently polar in nature

which provides excellent adhesion to a wide range of fibres. They have relatively

lower curing shrinkage and no volatile by-products which prevent undesirable void

formation. After curing, the epoxy resins possess high chemical and corrosion

resistance and good mechanical, thermal and electrical properties. However, they

have higher viscosity, are higher in cost and their major limitations are a longer

19

curing time and poor performance in hot-wet environments [Penn and Wang 1998,

Varma and Gupta 2000].

2.2.2. Reinforcement fibres

The purpose of fibre as reinforcement is to provide integrity and strength to the

structure by carrying the majority of the applied structural loads. Fibres are stronger

because while having smaller diameter, they have fewer defects and have the

possibility to align the crystal or molecular structure. Flaws or defect propagation

usually cause failure of the material. However, due to the presence of many fibres in

the composite structure, sudden damage does not usually occur. Most of the fibres

have to rupture before the complete failure of the composite and hence usually

warning signs are there before the collapse.

Fibre reinforcement, which is the discontinuous phase, is responsible for the primary

engineering properties of composites. The mechanical properties of composites

increase by increasing the fibre volume content up to a level where enough matrix

material is available to support the fibres and transfer the load within the composite

[Reinhart 1998].

Some examples of the reinforcement fibres are: glass, carbon, Kevlar (Aramid),

boron, polyethylene, silicon carbide, silicon nitrite, silica, etc.

Glass yarn was chosen for this project because it has a very wide appeal for

structured composites due to its low cost, easier handling and it is relatively easier to

process in the university research environment. Glass yarns possess a wide range of

properties and tailored performance for specific purposes which suited them for

many applications from small electrical products such as printed circuit boards to

boats and larger ships [Sims and Broughton 2000]. The next section describes the

types and properties of glass fibre.

2.2.2.1. Glass fibre Glass fibre is most widely used as a reinforcement for structural composites. Glass is

described as an amorphous material. It is made up of elements such as silicon, boron

and phosphorus which are transformed into glass by mixing with oxygen, sulphur,

20

tellurium and selenium. There are several glass compositions available (Table 2.1)

depending upon the desired properties for end use [Vaughan 1998]: Table 2.1 Available glass types and their properties [Vaughan 1998] Glass type Key features

A-glass High alkali or soda glass for good chemical resistance

E-glass Low alkali glass (aluminium borosilicate) for excellent electrical

insulation properties

C-glass Composed of soda borosilicate for excellent chemical resistance

S-2 glass Composed of magnesium, aluminium silicate and offers higher

physical strength (40% higher tensile strength than E-glass)

D-glass Superior dielectric constant than E-glass

R-glass Resistant to alkali and is used in reinforcing concrete

Low K An experimental fibre similar in properties to D-glass

Hollow

fibre

Tube-like or hollow fibre glass specific applications in light weight

reinforced aircraft parts

The properties of glass fibre depend on the composition of the original glass melt.

Some of the properties which glass fibre usually exhibits are:

High tensile strength In some applications the strength to weight ratio

exceeds steel wire.

Heat and fire resistance Due to its inorganic nature, glass fibre does not

support combustion.

Chemical resistance Not susceptible to fungal, bacterial or insect attack.

Moisture resistance Due to non-absorbency of water, glass fibre never

swells, rots, stretches or disintegrates in a moist atmosphere.

Thermal properties With having a low coefficient of thermal linear

expansion and a high coefficient of thermal conductivity, it performs well in

thermal functions.

Electrical properties As it has a non-conductive nature, it is efficiently used

for electrical insulation.

21

Glass yarns are created in many varieties so a particular system for yarn

classification is essential. Therefore, glass yarn nomenclature has been developed

based on both alphabetical and numerical designations.

For example ECG 150 4/2 s:

Where;

E Identifies the glass composition (E-glass).

C Recognizes filament type (C = continuous).

G Filament designation indicates filament diameter (from Table 2.2, G = 9

micron).

150 Stands for 1/100th of the single strand yield i.e. (15000 yards/pound).

4 Indicates the number of single strands twisted together i.e. Four strands of

150 1/0 are twisted together.

2 Shows the number of twisted yarns plied together. By multiplying the two

figures (4 x 2), the total number of basic strands in a plied yarn is obtained.

Moreover, by dividing the basic strand yield with total number of strands in

the yarn, yarn yield can be obtained.

S Designation of twist. Either 'S' or 'Z'.

Table 2.2 Fibre glass filament designations [Vaughan 1998]

Filament

designation

Filament diameter

in × 10-4 µm

B 1.5 3.8

C 1.8 4.5

D 2.1 5

DE 2.5 6

E 2.9 7

G 3.6 9

H 4.2 10

K 5.1 13

Therefore, the above yarn comprises type E-glass, having continuous filaments of 9

micron diameter. The yarn contains 8 (4 x 2) basic 150 strands, having a glass yield

of 1875 (15 000/8) yards/pound and using 'S' twist to create balance [Vaughan 1998].

22

2.3. Fibre reinforced composites

Fibre reinforced composites can be classified according to the form in which the

reinforcement fibre material is used. These are short discontinuous, long

discontinuous and continuous fibre reinforced composites. It can be further classified

according to the structure of the reinforcement such as woven, non-woven, braided,

knitted etc.

The parameters of fibres i.e. length, orientation and volume content dominate the

engineering properties of the composite. Among them, the length of the fibre is very

important and continuous and long discontinuous fibre composites are better in terms

of engineering properties [Reinhart 1998].

2.4. Manufacturing of composites

There are a number of processes used for manufacturing composites depending upon

the type of the end product and the performance required. A brief description of

some of the general composite manufacturing techniques is provided below:

The hand lay-up process is one of the oldest composite manufacturing techniques

and is still widely used for prototype part manufacturing and in the marine industry.

It is a labour intensive process in which the liquid resin is applied to the mould

followed by the placement of the reinforcement. The process of application of resin

and reinforcement layer continued until a suitable thickness is achieved. After fibre

wet-out, the laminate is allowed to cure. The spray-up process is also used as an

alternative to hand lay-up process in which the chopped fibres and resin are

deposited on to the mould by means of a spray gun [Mazumdar 2002, Khan 2010].

The filament winding process is used for making tubular parts and specialised

structures like pressure vessels. The process involves winding the resin impregnated

fibres at the desired angle over a rotating mandrel. Figure 2.1 shows the fibres’

passage moving through the resin bath and after impregnation they move back and

forth by means of the guide while the mandrel rotates at a specified speed. The

desired angle is achieved by controlling the motion of the guide and the mandrel

[Mazumdar 2002].

23

Figure 2.1 Schematic diagram of the filament winding process [Mazumdar 2002]

Pultrusion is a low-cost and a high volume manufacturing process in which the fibre

reinforcement after impregnation with resin is pulled through a heated die to make

the part. Pultrusion is used for the fabrication of composite parts with constant cross-

section profile e.g. rods, beams, channels, tubes, walkways and bridges, handrails,

light poles, etc [Mazumdar 2002].

Resin transfer moulding (RTM) is a closed mould operation in which the

reinforcement material is placed and clamped between two matching mould surfaces.

The resin is injected into the mould cavity through a port or series of ports under

moderate pressure. After curing the part is removed from the mould. Sometimes, for

assisting the resin flow and to remove the air bubbles, a vacuum is also created inside

the mould. The advantages associated with the RTM process are: lower investment

and operating cost, dimensional accuracy, manufacturing of complex parts, good

surface finish, low volatile emission due to closed moulding process. However, the

limitations are complex tooling design and also substantial trial-and-error

experimentation or flow simulation modelling is required for manufacturing the

complex parts [Mazumdar 2002].

The resin infusion process is an alteration to RTM in which only vacuum is used to

drive the resin flow and the laminates are enclosed in a one sided mould covered

with a bag. The resin is introduced inside the bag by means of one set of pipe work

24

while the second set allows the vacuum to be drawn from the bag. This technique is

commonly known as vacuum bagging and is utilised for this project as described in

Section 3.5.1 [Mazumdar 2002, Khan 2010].

The resin infusion technique has several names. Some of them are Vacuum Infusion

(Crystic VI), Co-injection RTM (CIRTM), Liquid resin infusion (LRI), Modified

vacuum infusion (MVI), Vacuum assisted Injection moulding (VAIM), Vacuum

assisted resin injection moulding (VARIM), Vacuum assisted resin transfer moulding

(VARTM), Vacuum infusion moulding process (VIMP) [Summerscales 2010].

2.5. Composite failure

Composite materials have a wide range of applications in structural components

because of their high strength-to-weight and stiffness-to-weight ratios. However, the

problem of delamination is of great concern. Failure caused in laminated composites

is usually by the separation of two laminate layers. Normally impact, shock and

cyclic stresses are responsible for failure. The problem of delamination is due to the

weakness of the composite in the through-the-thickness direction and the reason is

the inherent low adhesion inter-laminar strength [Pekbey and Sayman 2006].

Damage of any composite as a reaction to impact usually appears in the form of one

or more combined failure mechanisms which are matrix cracking, fibre fracture,

fibre-matrix de-bonding and delamination. The most crucial and common life-

restricting crack growth mode in laminated composites is delamination. Apart from

load application, various material properties and geometric parameters also influence

the failure mechanisms. However, whatever the mechanism is, the damage always

causes reduction in the stiffness and strength of the composite structure [Jang et al.

1989, Gweon and Bascom 1992, Pavier and Clarke 1995, Zhou and Davies 1995,

Adanur and Onal 2001, Ray 2005].

Baucom et al [2005a, 2006b] tested the S2-glass and E-glass composites with various

fabric architectures under repeated drop load impact in order to find out the damage

effect. The 4-ply specimens were observed under reflected light photography and

Scanning Electron Microscopy for visualisation of internal damage. It was found that

25

the damage mechanism was dominated by matrix cracking, matrix de-bonding,

delamination of layers and tensile fracture of fibres.

Pekbey and Sayman [2006] indicated that delamination causes serious degradation to

the composite structure. They found experimentally that the compressive strength of

composite materials was reduced with the presence of delamination as it always

weakened the structure.

Kumar et al [2007] investigated the relationship between post-impact compression

strength and the delamination area by performing impact tests on woven E-

glass/epoxy composite laminates. They found an increase in the delamination area

with increasing impact energy levels, which resulted in a decrease of compression

strength after impact. The decrease in load carrying capacity was assumed to be a

response to the degraded cross-sectional area of the sample under the action of

impact damage.

2.5.1. Delamination

Ebeling et al [1997] and Kim and Sham [2000] studied the failure mechanism of

delamination during the double cantilever beam test by the examination of crack

front movement across the width of the woven fabric laminated composite. Figure

2.2 illustrates multiple crack fronts, one for each warp yarn and the progress of crack

propagation between the layers when viewed from the top. Figure 2.2(a) shows

stable crack propagation where the crack front was most advanced in the direction

parallel to the exposed yarn (i.e. warp). However, the crack front lagged where the

yarns were perpendicular to it (i.e. weft) and the overall crack front seemed

discontinuous. Figure 2.2(b) shows unstable crack growth with a sudden load drop.

The entire crack front jumped forward but arrested instantaneously at the next

undulation resulting in a continuous crack front. Figure 2.2(c) shows recurrence of

Figure 2.2(a) for the adjacent cell. The repetition of approximately the same

procedure happened with crack propagation before complete delamination of the

composite laminate. The orientation of the yarn at the crack tip during the stress state

resulted in the change of discontinuous and continuous crack fronts periodically and

hence is responsible for the inter-laminar fracture toughness.

26

Figure 2.2 Schematic diagram of the sequence of delamination crack propagation between the

layer in a woven-fabric laminate as viewed from the top [Kim and Sham 2000]

2.5.2. Importance of filling yarn

Woven fabric laminated composites have an advantage over the unidirectional

layered composites with having a non-planar interply structure which provides

resistance to the growth of the crack. This is because of the interaction of a

delamination crack with the matrix regions and the weave structure during its

propagation. Some other advantages of woven fabrics are easy handling for

automation and conformability for complex shapes [Kotaki and Hamada 1997, Kim

and Sham 2000, Suppakul and Bandyopadhyay 2002].

Sample Width

Direction of delamination propagation

27

The toughness of the matrix is very important in preventing delamination and the

resin-rich areas play a very vital role. Ebeling et al [1997] highlighted two types of

resin-rich areas in glass woven fabric composites and their importance in

delamination. According to them, the first one was a yarn undulation area, where two

yarns intersected each other. The depth of this resin-rich area was half the ply

thickness. The second area was called the interstitial area and was situated at the

junction of four intersecting yarns, having the depth of resin equal to the thickness of

ply as shown in Figure 2.3.

Figure 2.3 Resin rich areas in woven fabric composite

Ebeling et al [1997] experimentally proved that for a brittle matrix, these areas and

especially the interstitial areas, promoted cracking and fracture of composites by

fracturing ahead of the main matrix. However, for stiffer matrices, they acted as

points of increased toughness and momentarily arrested the growth of the crack. The

undulation of the fibres which were perpendicular to the crack direction usually

restricted the crack jump. According to Ebeling et al [1997], delamination started

from the fibre/matrix de-bonding which is the easier path to follow. However, the

presence of filling yarns in the woven fabric forced the crack path to follow the inter-

laminar path and the changing of the crack path caused an increase in the

delamination toughness. They further concluded that composite toughness definitely

increased by increasing the matrix toughness.

Kotaki and Hamada [1997] investigated the fracture toughness of laminated

composites of differently placed satin weave structures. Their experimental results

also showed the highest fracture toughness with the sample which had more

transverse fibre strands.

28

2.5.3. Effect of thickness and number of laminated layers

The thickness of the composite is an essential factor for estimating the structural

damage, absorption of energy and resistance to penetration. Delamination behaviour

was examined by Xiao et al [2007] by making composites of a varying number of

layers. Plain woven S2 glass/SC-15 epoxy composites were manufactured and tested

under quasi-static punch shear apparatus. It was observed that thin laminated

structures had linear failure behaviour, while the thick laminated structures had bi-

linear failure characteristics. The damage sequence reported under action of load was

based on the following steps:

Delamination initiation

Delamination propagation

Fibre compression and shear failure

Fibre tension and shear failure

While examining the bi-linear behaviour, it was observed that the commencement of

delamination took place as a result of transverse shear loading under the application

of punch load. During delamination propagation, a gentler slope of the load-

displacement curve was observed and the flexure and shear stiffness were dropped.

However, the composite continued to carry the load until complete delamination and

the initiation of fibre failure.

Improvement in the load bearing capability and decrease in the amount of deflection

during impact loading was also indicated by Adanur and Onal [2001] for the thick

composite laminates. Aslan et al [2002] performed impact testing on E-glass/epoxy

woven laminated composites to investigate the significance of thickness and

dimensional effects. It was concluded that the peak impact force and the duration of

contact of load were vital factors. Thick composite laminates proved to be stiffer and

possessed high peak forces and smaller contact durations as compared to the thinner

composite laminates. The reason suggested was the increase in flexure and contact

stiffness with the increase in thickness. Therefore, thickness was found to be a

significant and governing factor for dynamic response and damage mechanism under

impact loading.

29

Sutherland and Soares [2004] indicated the difference of delamination damage

modes for thinner and thicker composite laminates of E-glass Polyester/epoxy

composites when subjected to high incident energies. According to them, the thinner

laminates suffered bending and fibre damage whereas indentation damage was found

for the thicker laminates followed by the internal delamination. They also found that

the energy at which the delamination starts increased with the increase in laminate

thickness.

2.5.4. Effect of thermal conditioning on glass composite failure

The exposure to severe thermal conditions of the environment and the effect of

thermal shock on the damage behaviour of glass composites were characterised by

Ray [2005]. The glass-polyester and glass-epoxy woven composites were treated by

varying the holding durations and by altering the number of cycles of high and low

temperatures. It was found that in comparison to glass-polyester, glass-epoxy

composites showed more resistance to thermal shocks because of more cross-linking

and greater adhesion properties. Moreover, improvement was found in inter-laminar

shear strength values with exposure to short holding times and fewer thermal fatigue

cycles. The reason suggested was an improvement in adhesion at the fibre-matrix

interface as an outcome of the surface chemistry mechanism and the post-curing

effect. However, interfacial de-bonding, crack initiation, and reduction in shear

strength values were observed with increasing exposure time to higher and lower

temperature extreme conditions and also with increasing number of cycles. This was

because of the increased residual stresses generated as a result of the difference in

thermal coefficients between the fibre and resin. This was a consequence of the

weakening of the interface and the delamination.

2.5.5. Effect of hygro-thermal exposure on glass composites

Jana and Bhunia [2008] examined the influence of environmental conditions such as

humidity and elevated temperature on the properties of glass composites. S2

Glass/SC-15 epoxy composite was exposed to hygro-thermal ageing conditions and

it was found that the matrix was affected and deteriorated. Inter-laminar shear stress

(ILSS) and delamination damage tolerance (DDT) were used as the tools for

evaluation and DDT was taken as the measure of stress on the onset of delamination.

It was observed that both ILSS and DDT reduced with the increasing exposure cycles

30

of humidity and temperature. It was suggested that hygro-thermal ageing caused

leaching of soluble degradation products which was also indicated by Gu and

Hongxia [2008] and there was a loss of weight. The matrix degradation weakened

the bond between the fibre and matrix and ultimately the failure occurred. The modes

of failure after the hygro-thermal ageing which resulted in delamination were matrix

cracking, fibre breakage to a certain extent and fibre matrix de-bonding.

Studies by Haque and Hossain [2003] also revealed that moisture absorption caused

hydrolysis and leaching effects resulting in diffusion of water into the matrix

materials. They observed micro-structural damage like fibre de-bonding and matrix

cracking due to swelling of the polymer matrix. They also observed that mechanical

properties deteriorated at elevated temperature beyond the glass transition

temperature which was probably due to the increased visco-elastic nature of the

resin. Their study showed that the degradation in strength at elevated temperatures

was more severe than that resulting from moisture absorption.

2.5.6. Effect of water absorption

The effect of water absorption on glass/polyester composites was investigated by Gu

and Hongxia [2008]. They combined two layers of E-glass plain woven fabric with

unsaturated polyester by using the vacuum resin infusion technique. Deterioration of

the composite matrix, reinforcing material, and interface was observed after

prolonged exposure to water (over 21 days) and the peeling strength was decreased.

The reason suggested by the researchers was the dissolution of some matrix elements

with water which percolate out and resulted in weight loss. However, peeling

strength seemed to increase with the exposure to water for 1-14 days. It was assumed

that during a short exposure, water molecules covered the voids of the matrix and

acted as a plasticiser and hence, an increase in weight was also observed. Moreover,

the hydroxyl group developed between the fibres and the matrix provided resistance

to the peeling action.

2.6. Through-the-thickness reinforcement

This project is concerned with improving the lamination strength between the fabric

and resin by modifying the individual fabric surface with the help of the air-jet

texturing process. However, to increase the delamination resistance of composite

31

structures, a common approach is through-the-thickness reinforcement. Growth of

delamination is restricted by bridging the cracks through stitching the laminate layers

in the thickness direction. The Z-fibre pinning process is also an attempt in which

transverse reinforcement is achieved, in the form of small diameter pins. A brief

account of these techniques with their merits and demerits is stated below.

2.6.1. Through-the-thickness stitching

The stitching process consists of sewing a high strength yarn, usually made of

carbon, aramid or glass, through the fabric composite preforms as shown in Figure

2.4. This process, in spite of having a number of advantages in terms of increasing

the laminate strength and resistance to delamination, also causes degradation of the

in-plane mechanical performance. Some of the critical factors are as follows:

Figure 2.4 Schematic diagram of the stitched preform [Nie et al 2008]

Improvement in impact damage resistance through stitching is sensitive to the type of

yarn used for stitching and also to the type and density of the stitching. According to

Kang and Lee [1994], chain stitching caused reduction of in-plane tensile strength

and modulus of S-2 glass/polyester composites with increasing stitching density of

Kevlar fibre. The reason suggested was the damage of some of the reinforcement

fibres during the penetration of the sewing needles.

Velmurugan and Solaimurugan [2007] introduced a number of modifications to the

stitching process of glass/polyester composites stitched with Kevlar. They used

manual plain stitching in place of chain or lock stitch in order to reduce fibre damage

Stitching Yarn

32

during the stitching process. The selection of plain stitch was also to avoid the

formation of thread cross and resin-rich pockets as in the case with lock stitch.

Moreover, instead of using twisted yarns, they utilised untwisted fibre roving and the

reason suggested was the uniform distribution of fibres in the stitches which

consequently increased the absorption of energy. The twisted fibre yarns in contrast,

acted as a whole and resulted in single step de-bonding. With the above

modifications, improved tensile, shear and impact strengths were achieved.

An examination of E-glass plain woven preforms and composites stitched with

Kevlar, using scanning electron microscopy, was carried out by Mouritz [2004] to

identify micro structural damage. Breakage of fibres by the stroke of the sewing

needle and distortion of woven fibres due to the sliding action of the sewing thread

was observed. It was also found that the surface of the preforms suffered from

crimping of the woven fibres as a result of pressing against the stitches which

became a source of distortion. Mouritz [2004] concluded that stitching caused

degradation of tensile fatigue properties in the form of early initiation and growth of

cracks, which happened as a result of crimping and distortion of load bearing fibres.

According to Nie et al [2008] the in-plane tensile strength of stitched composites is

sensitive to the stitch spacing. Small stitch spacing with a higher number of stitches

would effectively suppress the delamination and enhance the load bearing capability

of the composite. However, a higher number of stitches caused more fibre damage

and ultimately reduced the in-plane tensile strength. Nie et al [2008] found 5 mm to

be the optimum stitch spacing for composites of plain weave T300 1 K carbon fibres

with improved inter-laminar in-plane and tensile strengths.

Stitching is more helpful for providing resistance to the crack propagation through

fibre bridging rather than the crack initiation. According to Parlapalli et al [2007],

stitching is effective when the delamination length goes beyond 0.5L where, L is the

length of the specimen of glass/epoxy laminate composite stitched with Kevlar and

Twaron threads. The reason suggested was the possible reduction of composite

stiffness due to stitching. Above the 0.5L delamination length, the stitching started to

become effective.

33

Mouritz [2003] also indicated that improvement in delamination resistance occurred

when crack length grew above 15mm. According to Mouritz [2003], the stitch

bridging zone is not fully developed before the 15mm crack length. Moreover,

because of having very few stitches in a 15 mm length, an insignificant suppression

of the crack growth took place.

According to Yoshimura et al [2008], reinforcement of laminated composites by

using the through-the-thickness stitching technique seemed more promising with

larger impact energy. Yoshimura et al [2008] suggested that with a larger impact

energy level and a larger delamination area, there was an increase in the number of

stitched threads to be strained. The applied energy was then spent more for

increasing the strain energy of threads than spent on crack extensions. However, with

smaller delamination under the impact of low energy, because of the lower number

of available strained threads, the applied work was largely spent on crack growth.

It can be summarised that the in-plane properties of composites may be improved,

degraded or unaffected by the stitching process depending on a large number of

interacting factors. These include the type of laminate, the lamination technique,

stitching conditions i.e. stitch type, density, yarn diameter, orientation and also the

type of loading (Mouritz et al 1997). The major advantage of the stitching process is

that it improves the inter-laminar fracture resistance by resisting the crack growth as

it moves from stitch to stitch [Mouritz et al 1997, Yoshimura 2008, Velmurugan and

Solaimurugan 2007]. However, the drawback of localised damage zones around the

stitches due to needle action, misalignment of fibres by the stitches, formation of

resin rich areas due to spreading of fibres around the stitches and also weak interface

between the stitched yarns and matrix are reported as the major detrimental concerns

[Kang and Lee 1994, Mouritz et al (1996a, 1997b), Beier 2008].

2.6.2. Z-Fibre Pinning

Z-fibre pinning is an alternative technique to the stitching of composite laminates in

the Z-direction. Z-fibres are small diameter rods made up of carbon, titanium,

aluminium, stainless steel, glass etc embedded in resin. The diameter ranges from

0.15 to 1 mm. Insertion of the pins takes place through a specialised ultrasonic

34

insertion gun from a collapsible foam sandwich in which the Z-pins are held as

shown in Figure 2.5. Usually Z-pins are inserted into the prepregs before the resin

curing process [Cartie et al 2004, Partridge and Cartie 2005].

Figure 2.5 Schematic diagram of Z-pinning process Mouritz [2007]

Z-pinning is advantageous in improving the damage tolerance of the laminated

composites by offering resistance to delamination but it has limitations as well.

Zhang et al [2006] demonstrated that Z-pinning was quite effective for delaying the

delamination propagation rather than the damage initiation. The reason suggested

was the weak bond between the pins and the base laminate due to the presence of

resin pockets around the pins. Moreover, the pins were initially placed vertically to

the mode II crack plane and resist less the damage initiation. The pin traction force

increased with the change of angle of the pins during the crack growth and hence

reduction of the delamination area was achieved during the crack growth.

According to Zhang et al [2006], Z-pinning is more effective for thicker laminates

due to the difference in failure mode. For thinner laminates, the dominant failure

mode during transverse impact load is bending which causes matrix cracking.

However, delamination due to inter-laminar shear stresses took place in the thicker

laminates and the Z-pins were found to be helpful in arresting the delamination

cracks for propagation.

35

Allegri and Zhang [2007] stated that Z-fibres were beneficial for improving the

resistance to de-bonding and provided hindrance in delamination growth but the

diameter of the inserted pins was critical. According to them, increasing the pin

diameter would be helpful in increasing the frictional sliding shear and was

advantageous for the joint strength. However, at the same time, it had a detrimental

effect on the in-plane mechanical properties because of the local misalignment of the

in-plane laminates which increased by using the larger diameter pins. Mouritz [2007]

also indicated that the development of resin zones was associated with the amount

and the diameter of Z-pins. Isolation of resin zones from each other took place when

the pins were spaced wide apart. However, with closely spaced or large Z-pins,

continuous resin channels extending in the fibre direction would form which resulted

in decreasing the mechanical properties.

Another problem which is more prominent in Z-pinning is that Z-pinning causes

swelling of laminates. Chang et al [2006] as cited by Mouritz [2007] explained that

the problem of swelling was due to the spreading out of laminates to provide room

for the pins and also by the resistance of Z-pins against the compaction of prepreg

during curing. Swelling causes reduction of the fibre volume content and ultimately

deteriorates the mechanical properties. Stitching, however, raises the fibre volume

content by compacting the laminate preforms [Mouritz 2004a, 2007b].

2.7. Yarn texturing for increasing the bonding strength

The aim of this project is to increase the inter-laminar bond strength between woven

fabric of glass and resin, and between the neighbouring layers. Texturing increases

the bulk of glass yarns and this is expected to improve the adhesion between the

glass yarn and the resin and the resistance to delamination. Although a number of

techniques for producing textured filament yarns have been developed such as gear-

crimping, edge-crimping, stuffer-box, knit-de-knit, false twist and air-jet texturing,

the main techniques used are false twist, stuffer-box and air jet texturing processes.

The stuffer-box method caused buckling of the yarn in a wave form followed by the

heat setting in the crimped state. False twist is the process of twisting, setting and de-

36

twisting thermoplastic filament yarns. Due to the setting, the deformation is

permanently set in the yarn [Hearle et al 2001].

However, for texturing of glass yarn, the false twist and stuffer-box processes are not

practically possible because of the stiff nature of the yarn. In addition, yarns textured

through these processes are very stretchy and only show the texture in the relaxed

state. Therefore, a purely mechanical texturing process by means of an air-jet was

considered the only option for texturing the glass yarn for composite reinforcements.

2.7.1. Air-jet texturing

Air-jet texturing does not require thermoplastic yarn as it works on a purely

mechanical basis. Textured yarns, having an appearance just like spun yarns, can be

produced from thermoplastic, cellulosic or nonorganic filament yarns by the action

of a highly turbulent, non-uniform, supersonic jet of air. Formation of loops takes

place on the surface of the filament yarn, giving it a voluminous character. The

feeding of the yarn leads the delivery or take-up process. A pressurised air jet causes

the filaments of the constituent yarn to texture and blend together as shown in Figure

2.6. The supply yarn is usually wetted by a wetting unit just before feeding into the

texturing nozzle. A wide range of filament yarns can be textured by the air-jet

process [Demir and Behery 1997].

37

Where, L1 = The starting points of the separation of filaments inside the nozzle.

L2 = The starting points of the loop formation process.

L3 = The furthest point of the loops reached outside the nozzle.

Figure 2.6 Mechanism of air-jet texturing [Acar et al 2006] 2.7.1.1. Types of operations in air-jet texturing process There are three types of operations for producing a wide variety of textured yarns

namely

Single-end texturing

Parallel texturing

Core-and-effect texturing

In the single-end process, as the name suggests, a single end of yarn is introduced to

a nozzle with overfeed to produce the resultant yarn. In the parallel texturing process,

two or more yarns are usually fed to the nozzle for blending but have the same

amount of overfeed. The supply yarn may differ in terms of raw material, linear

densities, number of constituent filaments, etc. However, the versatility and

uniqueness of the air-jet process is found in the core-and-effect texturing process. In

38

this process, one or more yarns are supplied to the nozzle with relatively lower

overfeed to form the core and the other group is fed at the same time to the nozzle at

a higher overfeed percentage to create the desired bulk and where relevant a

voluminous effect. For example, a wide variety of fancy yarns is produced through

the core-and-effect process [Demir and Behery 1997].

2.7.1.2. Texturing nozzles The nozzle is the most important component in the line of air-jet texturing and is the

heart of the process. Since the 1950’s, lots of research work has been done to develop

an efficient air texturing nozzle and a number of different designs and shapes have

come into being. However, the purpose of the jet is always to create a supersonic,

turbulent and non-uniform flow to entangle filaments for creating loops and

producing textured yarn [Acar 1989].

Among the number of jets available in the market for producing a variety of textured

yarns, Taslan jets by Du Pont and Hemajet jets by Heberlein have made the most

significant commercial contribution to the field.

The first British patent [Du Pont 1952] and US patent [Du Pont 1957] was believed

to be the first process of air-jet texturing and was licensed under the brand name

“Taslan” by Du Pont as shown in Figure 2.7. A turbulent region was produced by

passing compressed air through a narrow space. The yarn was fed through the

turbulent zone and the formation of loops took place.

39

Figure 2.7 First Air-Jet Process “Taslan” by Du Pont

According to Demir and Wray [1989], the early jets were developed and modified on

a trial and error basis and there was no understanding of using wet yarn. In the next

modification, as per Figure 2.8a, a venturi, (a short tube with a tapered construction

in the middle that causes an increase in the velocity of flow of a fluid) was used to

speed up the compressed air.

Moreover, the jet was modified by adding a baffle plate and by introducing a screw-

type air channel to produce a spin in the air (shown in Figure 2.8b).

In 1954, Du Pont introduced Taslan Type 9 (Figure 2.8c) as a further amendment of

the texturing nozzle and which stayed longer in the industry. A longitudinal airflow

channel with a venturi was used as a modification and a pre-twisted supplied yarn

was fed at an angle of 45˚ through a stepped, tubular needle [Du Pont 1954, Du Pont

1960].

The major drawback of this jet was the crucial setting of the needle which had to be

done by specially trained operators for a reasonable texturing effect through the

nozzle [Demir and Wray 1989]. Further developments by Du Pont in the field of jet

design came in the form of the Taslan 10 Jet (Figure 2.8d) patented in 1960 [Du Pont

40

1960]. The design concept was altered by using the straight (axial) path for yarn flow

and the air entered at a right angle to the yarn channel. The negative aspect of this

design was the uncontrollable acceleration of the air stream due to the straight exit

tube. The Taslan Type 11 nozzle [Du Pont 1970, Du Pont 1972] (Figure 2.8e) was

the modified version through which this defect was overcome by using a venturi type

channel configuration.

Several versions of the Taslan 11 Jet were also designed by modifying the

compressed air inlet into the turbulence chamber. An advanced development

appeared as Taslan 14 Jet [Du Pont 1976] with a baffle element as shown in Figure

2.8f to deflect the air-jet at the exit of the nozzle. Initially, flat plate-type impact

elements were used but cylindrical bars, conical elements and spherical bodies were

utilised later on [Wickramasinghe 2003].

41

Figure 2.8 Taslan jets (a) Type 7 (b) Type 8 (c) Type 9 (d) Type 10 (e) Type 11 (f) Type 14

With all the previous Taslan Jets, the problem found was the difficulties of setting up

and also inconsistency of product variation among nozzles. This was claimed to be

overcome with the introduction of Taslan 20 Jet as shown in Figure 2.9 [Du Pont

1981].

42

Figure 2.9 Taslan Type 20

The attractive features of Taslan 20 Jets were the shorter channel length and enlarged

yarn inlet at the side of the needle. The nozzle in this type of jet consisted of

cylindrical venturi which could be moved and the position could be adjusted by a

rotating thumb wheel. The venturi could be stimulated from a string-up position to an

operating position by means of a cam, located on a rotatable cylindrical baffle. In

this way, self-stringing of the feed yarn was claimed to be made possible.

With the Taslan 20, the Du Pont nozzle designers had done several modifications by

utilising their experience for achieving maximum output. However, the problem of

pollution of surrounding areas of the texturing nozzle with spin finish and water mist

was still there. The contamination of the venturi resulted in an obstacle for smooth

operation and thus again become the source of jet-to-jet inconsistency [Acar 1989,

Wickramasinghe 2003].

Apart from Taslan jets made by Du Pont, Courtaulds Ltd and Enterprise Machine

and Development (EMAD) Corporation also provided the texturing jets in the market

by filing their patents [Courtaulds ltd 1979, EMAD corp. (1974a, 1976b, 1980c),

Demir and Wray 1989].

In 1978, a new design of air-jet was presented in the texturing market by Heberlein

Maschinenfabrik AG of Switzerland. The name given to this jet was the “Standard-

43

core Hemajet” (Figure 2.10). Heberlein introduced a radial type hemajet core with a

flow of air in the yarn channel by means of three inclined inlet holes. The core had a

wide trumpet shape at the exit side and an adjustable spherical impact baffle element

[Demir and Wray 1989].

Figure 2.10 Standard-core Hemajet [Heberlein guide 1991]

The universal housing Hemajet LB-02 with “T series” jet cores (Figure 2.11) with

one or more aisles for air to flow in the channel of the yarn were begun to be

developed from 1982 [Wickramasinghe 2003, Heberlein guide 1991].

Figure 2.11 (Hemajet LB-02 Universal Housing with T-Series Jet Core) [Heberlein guide 1991]

44

Heberlein developed a quite uncomplicated design unlike Taslan, by minimising

adjustments during the process and good nozzle-to-nozzle production consistency.

Moreover, it was also acknowledged that Heberlein nozzles made possible longer

and smoother running without stoppages, since the straight and simple geometry of

the yarn channel facilitated self-cleaning of the channel [Acar 1989].

Heberlein also become the owner of Taslan as DuPont allocated the trademark and

all of its Taslan air-texturing technology, patents, licences and intellectual property

rights to Heberlein [Maycumber 1997]. The texturing industry accredited Heberlein

jets because of their benefit of freedom from any royalty or licensing fee. The

texturers were free to develop and modify the process after purchasing the nozzle.

The economical consumption of compressed air is also a considerable advantage of

Heberlein nozzles. Furthermore, the standard-core hemajet was not the only jet

manufactured by Heberlein but a number of other jets were also designed with an

abundance of characteristics and features such as single or multiple inlet holes, a

conically enlarged yarn inlet region of the main channel, an exit segment having a

widened trumpet-shape and the development of reduced-wear ceramic nozzles

[Demir and Wray 1989].

Among further jet developments by Heberlein, the Hemajet EO-52 (Figure 2.12)

appeared, claiming to be effective for up to 300% overfeed values of effect yarn and

was applicable for a wide range of yarns [Heberlein guide 1991, Oerlikon 2010].

Figure 2.12 Heberlein Hemajet EO-52 [Oerlikon 2010]

45

After the T-series jet cores, more jet cores were developed including the S-series, A-

Series and then as a further modification T-2, S-2 and A-2 jet cores were introduced

(Figure 2.13). Considering the strength and durability factors, A, S and T jet core

series were made completely of ceramics. The T-2, S-2 and A-2 cores were made

with an additional advantage of a metal outer sleeve which protects the ceramic jet

and makes it almost unbreakable. Coloured rings are present for identification and

avoids the possibility of mixing up the jet cores [Oerlikon 2004a, 2007b].

(a) (b)

Figure 2.13 Hemajet jet cores (a) A and T series, (b) A-2, S-2 and T-2 series [Oerlikon 2004a, 2007b]

Jet housings for texturing were developed in the form of the Hemajet LB-04 and the

Hemajet LB-24 (Figure 2.14). Both jet core series i.e. T, S and A series and T-2, S-2

and A-2 series, can be fitted in these housings. The Hemajet LB-04 has a plastic

body and can be fitted to all types of air texturing machines, having an easy process

for exchanging jet cores, threading the yarn and having higher chemical resistance.

The Hemajet LB-24 is provided with an additional mechanism of rotating jet cores

which enhance cleaning inside the jet and result in increased process efficiency

[Oerlikon 2007a, 2009b].

46

(a) (b)

Figure 2.14 Heberlein Jet Housing (a) Hemajet LB-04, (b) Hemajet LB-24 [Oerlikon 2007a,

2009b]

2.7.2. Key considerations for the air-jet texturing process

2.7.2.1. Wetting of the yarn before entering the jet The wetting of a yarn before subjecting it to the jet has a number of advantages as it

opens up the filaments, washes off the spin finish, lubricates the yarn and reduces its

tension in the jet.

It was understood in the past that the wetting of the yarn was very helpful for an

effective texturing process and it acted as a lubricant; however the mechanism was

not explained [Acar 1989]. Acar et al [2006] explained the wetting mechanism that it

caused a reduction of the inter-filament friction through lubrication, followed by high

friction in the final loop formation stage of the textured yarn. The latter caused

better fixing of loops and ensured a well textured yarn. According to Acar et al

[2006], when the yarn is about to enter the jet and be wetted, there is a reduction in

the friction between the filaments and also among the filament and jet contacting

surfaces which make the relative motion much easier. Meanwhile, after passing

through the wetting unit when the yarn enters the nozzle, it comes into contact with

the secondary flow of the nozzle. During this stage, most of the spin finish is carried

away along with a large quantity of the water. The remaining water particles, usually

trapped in the filaments, come into contact with the super critical jet of air. The

turbulent jet of air converts them into fine mist and they are left by the filaments

during their opening stage. Removal of these mist particles takes place with the

47

primary flow and the filament becomes dry with only small traces of the original spin

finish. There is no longer a relative motion between the constituent filaments of the

yarn, which has just been textured. This results in high static friction between them

which is much higher than in the dry textured yarns ensuring an enhanced loop

formation process.

Kothari et al [1991] also indicated that in the absence of wetting, the inter-filament

friction increased and results in increasing the instability percentage. This is because

the friction created resistance for longitudinal and lateral movement of the

constituent filaments which affect the formation and entanglement of loops.

Therefore, the loops formed were easily pulled out under applied load and instability

increased. The instability as mentioned by Acar and Versteeg [1995] is the extension

percentage of the yarn under the applied load.

The glass yarns are not wetted for texturing. The reason is that since wetting removes

the spin finish from the yarns and it is not desirable to remove the special Silane

finish (size) from glass yarns which is applied by the manufacturer to facilitate the

rapid and complete impregnation of resin. The glass yarns obtained for this project

had the nominal size content of 0.55 % - 0.65 %.

2.7.2.2. Primary flow length A number of nozzles were designed and their performance was analysed by Bilgin et

al [1996]. The relationship between the stabilising zone tension and the primary flow

length were investigated by varying the primary flow lengths, air inlet angles, the

number of air inlet holes and other parameters. Primary flow length is the distance

between the point of contact of compressed air and filaments to the exit of nozzle as

shown in Figure 2.6. Stabilising zone tension is the on-line measure of the tension of

textured yarn between the nozzle and the take-up roller. The increase in the

stabilising zone tension was found with the increase in the primary flow length and

resulted in a well textured yarn. Bilgin et al [1996] proposed that by increasing

primary flow length, the flow was allowed to re-develop progressively more in the

axial direction before exiting from the nozzle and this had a beneficial effect on the

texturing process. The increase in the linear density was also found after the

48

texturing process which indicated good utilisation of overfeed. Scanning electron

microscopy results confirmed that by increasing the primary flow length, the yarn

produced had a greater number of smaller loops which were firmly anchored in the

yarn core.

2.7.2.3. Filament fineness Acar [1989] reported the suitability of finer filaments for the air-jet texturing process

because of their lower bending and torsional stiffness. Formation of loops would be

easier as they deformed easily with lower drag forces. Sengupta et al [1996] stated

that finer filaments produced high bulk and volume as they bent easily under the

action of the jets. However, formation of more neps took place at the same time, as

there were more chances of non-opening due to the lower surface area of the

filaments. Rengasamy et al [2004] indicated that textured yarns made up of finer

filaments have lower instability because the finer filaments bent without difficulty

and formed a larger number of small loops and entanglements.

2.7.2.4. Reduction in strength of textured yarn Rengasamy et al [2004] investigated the effect of the texturing process on the

strength of textured yarn and found that the strength decreased after the texturing

process. This was because after the formation of loops, the parallel arrangement

vanished and hence all the filaments did not contribute to bearing the load stresses.

It was also indicated that tenacity was decreased with increasing the air pressure

since the turbulence by increasing the pressure caused the formation of more loops

and resulted in an enhanced texturing effect. High loop formation decreased the

number of straight filaments responsible for bearing loads and as a result the yarn

suffered loss of tenacity.

2.7.2.5. Overfeeding Overfeed is the positive difference between the input speed of the yarn entering the

jet and the withdrawing speed of the yarn coming out of the jet. According to

Alagirusamy and Ogale [2004] and Sengupta et al [1991], overfeed is one of the

essential requirements of air-jet texturing as it provides additional length of filaments

to facilitate loop formation and the bulk of the yarn. They suggested that the

49

texturing efficiency and the linear density increased with increasing overfeed but the

loop stability reduced. According to Chimeh et al [2005], overfeeding plays a key

role in developing loop size and loop density for air-jet textured yarns and increasing

overfeed up to an optimum level would cause an increase in the linear density.

Moreover, the results of Rengasamy et al [2004] revealed that increasing the

overfeed difference between the core and the effect components resulted in

decreasing tenacity. This is because during loading, the effect filaments having

higher overfeed were less strained and the yarn breaking extension was largely

influenced by the rupture of the core filaments.

2.7.2.6. Filament cross-section Acar [1989] reported that non-circular filament structures are appropriate for the air-

jet process because they have lower torsional and twisting stiffness and therefore,

less drag force required for carrying them. Alagirusamy and Ogale [2004] indicated

that the cross-section of filaments contributed to air texturing performance.

According to their studies, filaments with elliptical, hollow circular and non-circular

cross-sections with a greater surface area are more suitable for air-jet texturing as

compared to solid circular ones of equal linear density.

2.8. Commingling process

Commingling is a common use of air jet technology in composite manufacturing.

Commingling is a process of producing entanglements between the filaments of two

or more constituent yarns by the help of an air jet, usually acting perpendicular to the

yarn flow. The principle of commingling as shown in Figure 2.15 is actually the

formation of the opened sections followed by the nips. Nips are entangled points

between opened sections. The strong jet of air causes the opening of the filaments

and results in an open section whereas, the nips forms on either side [Alagirusamy

and Ogale 2004, Alagirusamy et al 2006].

50

Figure 2.15 Commingling process [Alagirusamy et al 2005]

Miao and Soong [1995] investigated the properties of commingled yarn and found

that they were highly dependent on the process parameters i.e. air pressure, overfeed

ratio and throughput speed. They used Nylon multifilament yarns and their results

illustrated that air pressure is a very important factor and the increase in air pressure

caused enhancement in interlacing. Versteeg et al [1999] also found an increase in

the nip frequency with the increase of supplied air pressure. An increase in overfeed

resulted in a reduction of the nip frequency but the degree of interlacing increased.

Alagirusamy and Ogale [2004] indicated that the requirement of overfeed for the

commingling process is zero or very low, as overfeed reduces yarn tension and hence

affects the nip frequency.

Moreover, the increase in yarn throughput speed decreases both the nip frequency

and the degree of interlacing. Tenacity of the yarn was found to be reduced after

commingling because in flat yarns, filaments were aligned parallel and were

subjected to load together. However after the commingling process, due to varying

angles, the filaments shared the applied stresses unequally, resulting in a reduction of

tenacity [Miao and Soong 1995].

2.8.1. Jet design for the commingling process

Alagirusamy et al [2005] performed a number of experiments to investigate the

consequences of jet design on the process of commingling by changing the number

51

of air inlets, their angles and positions in the jet. Among the three configurations

(shown in Figure 2.16), Configuration 3 with an inclined air jet at a 45° angle

followed by two perpendicular jets on the other side, gave acceptable commingling

results.

Configuration 1 Configuration 2 Configuration 3

Figure 2.16 Air Inlet Configurations for Commingling Process [R. Alagirusamy et al 2005] The design of texturing and commingling jets is differentiated by Alagirusamy and

his co-workers [2004a, 2005b, 2005c] on the basis of the angle and direction of air

flow in the jet. They suggested that air flow in the texturing jet is usually angular in

the direction of the yarn flow. It helps in forwarding the yarn along with inserting

loops in it. Formation of loops takes place due to the impact of shock waves which

cause the opening up of the filaments. Moreover, overfeed of the yarn facilitates

bending of the filaments in reaction to the shock waves. The enhancement of the

texturing process with oblique air inlets at angles of 45 and 60 degrees in the

direction of the yarn flow was also reported by Rwei and Pai [2002]. However, in the

commingling process, blending and homogenous distribution of matrix and

reinforcing filaments is the requirement. Therefore, they suggested that angular jets

against the yarn flow direction (backward commingling jets) are better for the

commingling process as shown in Figure 2.16 (Configuration 3).

2.8.2. Commingled yarns for composites

Composites, having textile-based reinforcement, are usually developed through

impregnating the reinforcement fibres with liquid resin. However, due to the

drawback of higher viscosity of thermoplastic resins, it is difficult to obtain a

homogenous composite structure. Another option for processing thermoplastic

composites is to add the polymer in solid form to the fibres in such a way that

Yarn entry

52

uniform mixing of the two parts of the composite is achieved. Development of

commingled yarns for thermoplastic composites was introduced to overcome the

problem of higher viscosity of thermoplastics and the difficulty in having a

homogenous composite structure. The commingling process is used for mixing the

high performance reinforcement filaments and the thermoplastic resin forming

filaments. Fabrics or other textile forms are developed from these commingled yarns

(hybrid yarns) and impregnation is achieved by application of sufficient heat and

pressure to have small flow paths for the thermoplastic matrix around the fibres

[Alagirusamy and Ogale 2005, Zaixia 2006, Alagirusamy et al 2006, Golzar et al

2007].

The performance of commingled yarns and their effect on the mechanical properties

of composites were reported in the literature. It was indicated that improvement in

the quality of consolidation, mechanical properties and reduction in void content of

thermoplastic composites can be achieved by controlling the parameters i.e. holding

time, tool temperature and pressure [McDonnell 2001, Bernhardsson and Shishoo

2000, Alagirusamy et al 2006].

2.8.3. Glass filament commingling process

A combination of high performance reinforcing filaments with matrix-forming

thermoplastic filaments (glass/polypropylene (GF-PP), glass/polyester (GF-PET),

and glass/nylon (GF-NY)) were developed through the commingling process and

characterised. The effect of air pressure and the volume content of matrix-forming

filaments on the nip frequency and degree of interlacing were investigated

[Alagirusamy and Ogale 2005] and their effects on the tensile properties of yarns

were also studied [Ogale and Alagirusamy 2007]. The nozzle design used for the

above studies was Configuration 3 shown in Figure 2.16.

It was observed that the nip frequency and the degree of interlacement increase with

an increase in the air pressure because of the increase of air flow velocity and yarn

rotation inside the jet. The variation in nip frequency with the change of the volume

content of the matrix forming filaments depends on the number of filaments, nature

and the linear density of filaments [Alagirusamy and Ogale 2005].

53

Moreover, it was observed that tenacity is the property of thermoplastic yarns

whereas the modulus is the property of the high performance yarns. The results

showed that tenacity is unaffected by increasing pressure for all three combinations

but the modulus of GF-PET and GF-NY seems to decrease with increasing pressure.

This was possibly because during high movement of filaments with increasing

pressure, the alignment of glass filaments was effected. GF-PP had no effect on the

modulus because the polypropylene filaments by nature have a higher diameter and

lower density. Therefore, even if the glass filaments moved with increasing air

pressure, in the subsequent winding process they again adjust the structure with the

winding tension.

Reduction in the modulus of commingled yarn was observed with an increase in the

volume content of the matrix forming filaments. This was because of the decrease in

the proportion of glass filaments. Moreover, increase in tenacity of GF-PET and GF-

NY and decrease in tenacity of GF-PP was observed with the increase of the matrix

volume content. The reason suggested was that according to their results, the glass

had lower tenacity than PET and NY but higher than PP. Therefore, increasing the

volume content of the PET and NY resulted in the increase in tenacity of the

commingled yarn. However, an increase in volume content of the polypropylene

resulted in a decrease in the tenacity of the commingled yarn [Ogale and

Alagirusamy 2007].

The commingling process has the ability of distributing the constituent filaments

efficiently over the cross-section. Through microscopic investigation, it was found

that even distribution of filaments over the hybrid yarn cross-section not only

depends on the air pressure but also on the diameter of the reinforcement and matrix

filaments. For uniform distribution of filaments, the diameters should be

approximately equal. Moreover, higher air pressure is not desirable since in addition

to damaging the filaments, it restricts the distribution of filaments by making the

structure more compact [Kang et al 2007, Herath et al 2007].

2.9. Selection criteria for the air-jet texturing process

This project is concerned with producing bulk and loops in the glass yarn to improve

fabric layers adhesion between the glass and the resin. The improvement in bonding

54

by the air-jet texturing process is explained to some extent by Ma et al [2003] in the

context of thermoplastics-based composites. They textured a single glass filament

yarn by varying the overfeed and the air pressure and the coated ratio of yarns was

measured. The coated ratio is the percentage increase in the weight of the yarn due to

the absorption of resin. It was found that the coated ratio increased by increasing the

air pressure and overfeed values. Moreover, an increment was also observed in the

bond tenacity of two layers of the woven fabrics made by using the textured glass

yarns which were impregnated with PVC resin and joined together by means of a

welding torch. The suggested reason was the increase in bulk of the yarn with the

application of air pressure and overfeed. However, the effect of texturing on the yarn

strength was not considered in their study which is likely to be decreased with the

application of excessive pressure and overfeed. This is because the number of the

filaments contributing to the strength of the yarn would decrease with increasing the

bulk content of the yarn.

This project is focused on the utilisation of lower values of overfeed and pressure so

as not to disturb the filament orientation within the textured yarns too greatly and

avoid excessive loss of strength. This approach was shown to have potential by Koc

et al [2008] who found improvement in adhesion of PET yarns to rubber by

incorporating a very small amount of texturing. Although they found a reduction in

yarn strength even with the lower texturing parameters, their results showed

improvement in adhesion between rubber and the PET yarns.

2.10. Summary

The purpose of this research is to improve the bond strength between glass and the

matrix by using air-jet textured yarn. The constituents of composites, delamination

failure, and the preventive measures for increasing the delamination resistance in the

form of through-the-thickness reinforcement were reviewed. Commingling and air-

jet texturing processes were also reviewed. The commingling process is also used for

producing reinforcement yarns but has a different approach as compared to the air-jet

texturing process. Commingling is used for uniformly mixing the thermoplastic yarns

with the reinforcement yarns and thus overcoming the problem of higher flow

viscosity of the thermoplastic resins during manufacturing of composites.

55

To realise the aims of this project, the core-and-effect is the most promising

technique because the core yarn can be potentially processed with minimum overfeed

to maintain the strength while the effect yarn can be subjected to moderate overfeed

for developing the loops and bulk in the resultant textured yarn as will be described

in the following chapters.

56

3. Chapter 3 Glass yarn texturing, weaving and composite

manufacturing process

3.1. Introduction

This chapter describes the equipment and techniques employed for the production of

samples used in this study. The chapter includes the introduction of the air-jet

texturing machine, the texturing process of glass yarns, the development of fabrics

and then finally the fabrication of composite panels. A brief account of each

experimental stage in this research work is provided together with some discussions

of their merits and constraints.

3.2. Air-jet texturing machine

The single head Stähle RMT-D air-jet texturing machine used for this study was

originally for texturing filament yarn (Polyester, Polypropylene, Polyamide, Nylon,

Acetate etc) and can operate at speeds of up to 500 m/min.

3.2.1. Texturing machine components

The Stähle RMT-D air-jet texturing machine mainly comprises a pre-texturing zone

including the drawing and heat setting arrangement followed by the texturing unit

and then finally the winding zone. A brief description of each component is

provided.

3.2.2. Feeder yarn creel

The creel is situated at the back of the machine as shown in Figure 3.1 for holding

the feeder yarn packages. There are 12 slots available to hold the core and effect

supply yarns. After passing from the yarn guides, the feeder yarns were presented to

the feed rollers from the bottom of the machine.

57

Figure 3.1 Creel Section

3.2.3. Rollers arrangement

The machine has two separate sets of rollers for both the core and the effect yarns

before the jet box and one set for the resultant textured yarn after the jet as shown in

Figure 3.2. A range of overfeed and draw ratios can be achieved for both the core

and effect yarns [RMT-D manual 2001].

58

Figure 3.2 Rollers Section

The machine has the ability to heat set the feed yarns before subjecting them to the

jet and also after the texturing process in the form of the heated pins and the godet

rollers. The heat setting process is essential for the synthetic partially-oriented yarns

(POY) however no heat setting was required for texturing the glass yarns so the heat

setting devices were bypassed.

3.2.4. The jet box

The jet box usually consists of two yarn inlets for the core and the effect yarns, a

wetting unit (water applicator), an air-jet with housing, an eyelet for textured yarn

exit and an opening for the water to drain. The water is applied in a controlled

manner to any one of the feeder yarns (usually the effect yarn) to improve the

efficiency of the process. The jet box can be opened by unfastening the two side

screws in order to access especially when threading the machine. Control of the noise

Delivery roller

Core yarn path

Effect yarn path

Godet roller

59

level during operation is also one of the functions of the jet box. Figure 3.3 shows the

jet box on the left and the yarn inlets, jet and water applicator on the right.

Figure 3.3 Jet box and components

3.2.5. Oil application device

The different finishes can be applied to the textured yarn by means of a roller (shown

in Figure 3.4) moving slowly in a fluid reservoir. The textured yarn comes in contact

with the roller just before the package winding process.

Figure 3.4 Oil application roller

3.2.6. Winding unit

The textured yarn after passing through the delivery roller and the yarn guide reaches

the winding unit. The package is negatively driven by means of a rotating cylinder

from underneath. The traverse motion is controlled by the reciprocating yarn guide

moving in a to-and-fro direction as shown in Figure 3.5.

60

Figure 3.5 Winding unit

3.2.7. Suction gun

The machine is equipped with a suction gun (shown in Figure 3.6) which helps in

threading the yarn through the machine. The use of the suction gun for threading the

yarn is always recommended, rather than the use of hands, for operator safety.

Figure 3.6 Suction gun

61

3.2.8. Gearing arrangement

The driving mechanism of the machine depends on the gearing arrangement present

at the back of the machine which receives the drive from an AC motor. Transfer of

the motion from the motor to the gears and among the gears takes place by means of

driving belts and each gear change position is equipped with a belt tensioner as

shown in Figure 3.7. By using different gear ratios, various rollers speeds and a range

of draw ratios and overfeeds can be achieved.

Figure 3.7 Gearing arrangement

Belt tensioners

62

Figure 3.8 Modified thread line diagram of Stähle RMT-D air-jet texturing machine for glass

yarn

3.2.9. Texturing machine set up for glass yarn

The machine was modified for processing the glass yarn because of the difference in

the nature of the yarn. The modifications were performed in the drawing zone,

winding zone and in the jet box.

3.2.10. Alteration in the drawing zone

There was no requirement for pre- or post-drawing or for heat setting for glass yarns

on the machine. Therefore, the draw ratios were kept to zero by running the input

roller and the feed roller at the same speed. The passage between them was utilised

as a pre-tension zone for stable feeding of the core and the effect yarn to the air jet.

63

The line diagram illustrating the production process of the core-and-effect glass yarn

on the air-jet texturing machine is shown in Figure 3.8 above.

3.2.11. Alteration in the winding zone

A variable speed winding unit was also introduced to the machine by replacing the

original one. The reason was to take up the yarn at a slightly higher speed (5%) than

the delivery roller which helped in tightening up the loops and proper winding of the

resultant textured yarn on the cone for the warping and weaving process.

3.2.12. Type of jet used

The jet and the housing used for texturing the glass yarn in this project had the

following specifications:

Jet housing = Heberlein Hemajet (R) LB-13 (Figure 3.9);

Jet core = T 370 (Figure 3.10).

Figure 3.9 Jet housing (Heberlein hemajet LB-13)

Figure 3.10 Jet core (T-370)

64

The wetting assembly was removed from the jet box as it was not required for

texturing the glass yarn.

After setting up the machine, the next task performed was the selection of two

important process parameters i.e. overfeed and the air pressure for the production of

the core-and-effect yarn.

3.2.13. Selection of the overfeed value

It was considered that a smaller amount of overfeed would be enough for the core

yarn to provide sufficient texture in the structure so as to lose as little as possible

yarn strength. For determining the optimum value of the core yarn overfeed, the core

yarns were textured by varying the overfeed value starting from the smallest value of

2.9%. It was observed that by increasing the overfeed from 2.9% to 5.5%, the loss in

the breaking strength increased from 15% to 40% at 4 bars air pressure. The breaking

strength was tested according to BS ISO 3341 [2000]. Based on the observation

above, 2.9% was taken as the overfeed value of the core yarn for further processing.

For the effect yarn overfeed, a number of trials were conducted. The first trial was

performed with 38% overfeed as that was the minimum value of overfeed which

could be set for the effect yarn in the machine according to the gearing arrangement.

However, it was observed that the yarn was not able to run properly and it became

loose and failed to move forward on the rollers before the jet. The reason for this was

the stiff nature of glass filaments which caused difficulty in the formation of loops

and failed in converting overfeed into loops. The mechanism of loop formation is to

utilise the extra length of yarn fed through overfeed and to convert it into loops with

the action of the air jet. Moreover, the appearance of the loops in the resultant glass

yarn was different to that of the polyester yarn after the texturing process in terms of

tightness of the loops. Before starting to work on glass yarn in this project, the

machine was operated with polyester yarn and it was observed that at 38% effect

yarn overfeed, the loops formed quite easily and were firmly held by the core.

Further trials were conducted by gradually decreasing the overfeed value of the

effect yarn. During these trials, the basic gearing diagram was also altered by

changing some fixed gears along with the normal variable gears. However, an

65

unstable texturing process was observed because the stiffer nature of glass filaments

did not allow the full utilisation of the extra length fed through overfeeding.

Promising results were achieved for the effect yarn at 9.2% overfeed as the yarn ran

properly and the process was found to be stable without any breakage and loosening

of the yarn on the feed rollers before entering the jet.

3.2.14. Selection of the air pressure value

Heberlein, the manufacturer of the air-jet, recommended a moderate range of 3 to 6

bars air pressure for texturing the glass yarn so this range was selected for producing

the core-and-effect yarn. Higher air pressure is also not desirable because, despite the

advantage of development of more loops in the yarn structure, it causes broken

filaments and a reduction in strength of the yarn. Since maintaining the strength of

glass yarn is one of the objectives of this project, it was decided to use lower air

pressures.

During the texturing process, the core-and-effect yarn was produced with visible

loops at pressures of 3, 4, and 5 bars. Smaller and tighter loops were found more in

the structure of the yarns made at 5 bars air pressure followed by 4 bars and then 3

bars as shown in Figure 3.11. The increase in the air pressure from 3 bars to 5 bars

resulted in the formation of small loops which were held firmly in the structure of the

core yarn. Higher pressure helped in opening the core yarn and more rearrangement

of the filaments providing more locking of the loops. The formation of more loops

with increased air pressure was also indicated by Rengasamy et al. [2004].

66

Figure 3.11 Core-and-effect textured glass yarns

Further trials were made by producing the textured yarn at 6 bars air pressure but

deterioration was observed in the structure of the resulting yarn. Although the yarn

possessed smaller loops, at the same time too many broken filaments were observed

in the structure. Also the process was not stable and the effect yarn was out of control

on the rollers on several occasions.

Therefore, the core-and-effect yarn produced for this research work had the

following parameters;

Core yarn linear density = 300 and 600 tex,

Effect yarn linear density = 34 and 68 tex,

Core yarn overfeed % = 2.9 %,

Effect yarn overfeed % = 9.2 %,

Air pressure = 3 to 5 bars.

Refer to Table 5.1 on page 96 for further details of the yarn structure.

67

3.3. Warping process

Warping is an essential preparation process for weaving. The traditional method

involves the use of a considerable number of cones which impose a practical limit on

the number of trials in a research environment due to the preparation time required.

To alleviate this problem, a single-end warping machine, which requires only one

cone, was used in this project. The warping machine, made by Shirley Institute, is

shown in Figure 3.12. This machine was purposely manufactured for making short

length warping beams of maximum 8 metres especially for educational and research

work. The yarn was withdrawn from one side of the machine (shown in Figure 3.13)

and after passing through the tensioning device and the guiding rollers, it was tied on

one edge of the warping wheel.

Figure 3.12 Single end warping machine (made by the Shirley Institute)

The machine was equipped with clutch and breaking devices to control the motion of

the warping wheel. Each rotation of the wheel made up one warp yarn ‘end’ and after

having the desired number of ends, the warp sheet was transferred to the weaving

beam on the beaming unit. The transfer of warp yarns on the weaving beam took

place by slowly rotating the wheel to maintain an even tension with the help of the

brakes. The diameter of the wheel was changeable for adjusting the circumference of

68

the wheel according to the required length of the warp sheet. The gearing

arrangement helped in placing the yarns evenly one after another at an appropriate

distance without overlapping each other and the selection of suitable gears depends

on the linear density of the yarn and the number of ends/cm required in the finished

fabric.

Figure 3.13 Glass yarn warping in process

3.4. Glass fabric production

It was recognised that many types of fabric would have to be produced to achieve the

aims of the project and after initial trials on commercial looms, it was decided to use

a hand loom for this purpose. The advantage of a small hand loom is reduced time of

preparation between production of different samples and greater handling flexibility,

especially as some yarns proved to be difficult to prepare and weave as discussed

later in this thesis. It was recognised, however, that uniformity of weaving

parameters could be potentially more difficult for a hand loom and great care was

taken to minimise this. The hand loom used is shown in Figure 3.14.

Both the glass textured and the non-textured yarns were utilised in fabrics based on

plain (1/1) weave and twill (1/3) weave for comparative purposes. Four harness

69

frames were utilised for lifting and lowering the warp yarns in both types of weave

pattern. The warp beam was prepared by utilising the maximum available

circumference of the single-end warping machine (i.e. 8 metres) and the beam was

installed at the back of the hand loom. The drawing-in process of warp yarns through

the heald wires and the reed was performed carefully to avoid entanglement and

damage to the yarns. The warp yarns were kept in tension by means of the rope and

dead weights as shown in Figure 3.15.

Figure 3.14 Hand loom

70

Figure 3.15 Dead weight for warp yarn tensioning

Apart from the fabrics manufactured using the core-&-effect yarns and the fabrics

made from the non-textured glass yarns, two other categories of textured fabrics were

also produced. The composition of fabrics was changed on the bases of the

constituent yarns. The first one was the core textured fabrics produced by using core

textured yarns and termed “CT fabrics”. The yarn used in warp and weft of the CT

fabrics was the single textured yarn with the absence of the effect yarn. The second

variant was in the form of mixed yarn fabrics termed as “WfW fabrics” having non-

textured glass yarn in the warp and the core-and-effect textured glass yarn in the

weft. The details of both these modified textured fabrics and the properties of their

composites are explained in Chapter 7.

The specifications of the fabrics produced are listed in Table 3.1.

71

Table 3.1 Fabric specifications Fabric types

Fabric specifications 300 plain 300 twill 600 plain 600 twill

No. of ends/cm 6 6 4 4

No. of picks/cm 5.5 6 - 6.5 4 4 – 4.5

Air pressure (bars) 3 – 5 3 - 5 5 5

Core yarn tex 300 300 600 600

Effect yarn tex 34 and 68 34 and 68 34 and 68 34 and 68

The surface structure of the fabrics made by using the textured yarns was different to

the fabrics of the non-textured yarns as shown in Figures 3.16 and 3.17. The textured

yarn fabrics showed a hairy surface with small loops and more bulk in the structure.

Also they were not found to be as shiny as the surfaces of the non-textured fabrics

because of less oriented filaments in the yarn structure. 300 tex non-textured (300+34) tex textured at 5 bars

Figure 3.16 (1/1) Plain weave fabrics

300 tex non-textured (300+34) tex textured at 5 bars

Figure 3.17 (1/3) Twill weave fabrics

3.4.1. Problems during weaving process

There were difficulties in weaving with the yarns textured at 3 bars air pressure

because the loops did not firmly hold in the yarn structure. During the weaving

process, the entanglement of loops among the warp yarns started with the change of

72

the shed and the entanglement clusters formed before and after the heald wires on

several occasions as shown in Figure 3.18. These entanglement clusters caused

resistance in the opening of the shed and created problems for the weft insertion

process.

Figure 3.18 Entanglements during the shedding process

The yarns produced at 4 bars pressure also suffered a few entanglement problems

during the weaving process but the problem was greater with the textured yarns of 3

bars pressure (Figure 3.19), since the loops were more susceptible to being separated

from the core yarn and getting entangled. It was also observed that the problem of

entanglements was lower in the (1/3) twill weave fabric as compared to the plain

weave. This was because only 25% warp yarns were involved in the opening of the

shed and due to the low abrasive action, the formation of entangled clusters was

reduced. The weaving process was therefore carefully handled and the warp yarns

were let off before the fabric take-up process. This helped in reducing the tension in

the yarns when they were pulled through the heald wires and also it helped in

minimising the sliding contact between the yarns. This preventative measure can be

applied to power looms as well for handling the delicate warp yarns by setting up the

let off a little ahead of the fabric take up.

73

Figure 3.19 Entanglements in 300 + 34 tex 3 bars pressure textured warp yarns

Good results for weaving the fabric were obtained with the yarns textured at 5 bars

pressure since the higher texturing air pressure helped in opening the core yarn

structure and caused more rearrangement of the filaments which eventually provided

more locking of the loops. As the loops were small and firmly held in the yarn

structure, there was no entanglement problems like those observed with the textured

yarns at 5 bars air pressure. The fabric production process was smooth in both the

300 and 600 tex yarn categories textured at 5 bars air pressure and both the plain and

twill weave fabrics were produced without any entanglement problems.

3.5. Composite manufacturing

Manufacturing of the glass fabric laminated composites was carried out by infusing

the epoxy resin using the vacuum bagging technique in which 4 layers of the glass

fabrics were laid over one another. A range of composites were produced from the

textured and the non-textured glass fabrics and the classification is based on the

linear density (tex) of the core-and-effect yarns, texturing air pressure and the type of

weave pattern.

3.5.1. Vacuum bagging technique

The vacuum bagging technique is actually an extension of the wet lay-up process in

which the excess resin and entrapped air are extracted to improve the consolidation

of the laminates. The technique employed in this study was by sandwiching the hand

laid laminates with peel-ply, perforated release film and resin infusion mesh and then

74

covered by vacuum bagging film which was sealed to the edges of the tool by means

of tacky tape. Four layers of glass fabrics were taken in each case and were placed

over one another in an identical orientation. The sequence of the vacuum bagging

assembly is shown in Figure 3.20 and the actual vacuum bag with the glass fabric

laminate is shown in Figure 3.21.

Figure 3.20 Configuration diagram of the vacuum bagging process

Spiral cut tube

Tacky tape

Fabric laminate

Resin infusion pipe

Metal tool

Breather cloth

Resin trap

Figure 3.21 Vacuum bag

75

A vacuum pump was used to extract the air from the bag for consolidating the

laminate and the vacuum was maintained by applying pressure up to 1 atmosphere.

The function and importance of each component of the vacuum bagging assembly is

described in Table 3.2. Table 3.2 Consumable materials required for the vacuum bagging [Cripps 2000]

Metal tool To provide a smooth flat base

Peel ply Perforated heat-set nylon ply to provide the clean surface and easy

removal of laminate from the bag.

Perforated

release film

To provide uniform distribution of the resin and also help in

avoiding the adhesion of infusion mesh with the peel ply

Resin infusion

mesh

To provide a path for the resin flow

Release film A PTFE film used to prevent the adhesion of any resin to the metal

tool

Breather cloth A nonwoven porous material used to provide the air flow path over

the laminate for allowing the escape of air and moisture and for

ensuring uniform vacuum pressure across the component. It also

absorbs the excess resin from the laminate.

Breach unit A connector between the bagging film and the suction pipe

Bagging film Membrane which allows a vacuum to be drawn within the bag

Spiral cut tube To flow the resin evenly along the whole width of the laminate

Resin infusion

pipe

To infuse the resin solution into the vacuum bag.

Resin trap The passage made in the vacuum line to collect any excess resin

before it reaches the pump in order to prevent damage to the

vacuum pump

Tacky tape Adhesive strip used for sealing the bag to the tool without any

leakage.

Suction pipe Connector to the vacuum pump

The resin (Araldite LY5052) and hardener (Aradur HY5052) used for making

composites were obtained from Aeropia Ltd. The mixture of resin and hardener was

prepared with a ratio of 100:38. After combining the two components, the vessel was

gently stirred for 4 - 5 minutes for complete mixing. The stirring was performed

76

carefully to avoid the formation of air bubbles. Resin was introduced into the bag

through the infusion pipe already fitted in the assembly as shown in Figure 3.21.

After complete impregnation, the resin supply was cut off although the suction pump

was allowed to maintain the vacuum overnight. The composite panels, after having

cured at room temperature for 24 hours, were then further cured in the oven at 100°C

for 4 hours.

77

4. Chapter 4 Characterisation, equipment and procedures

4.1. Introduction

This chapter describes physical and mechanical test methods and equipment used to

characterise the textured and non-textured glass composites. The composite panels

were developed utilising the vacuum bagging technique and after the curing process

the specimens were cut using a diamond blade cutter according to the respective

standard dimensions for physical and mechanical characterisation. The mechanical

performance was measured using tensile testing, a three point bending test, inter-

laminar shear strength (ILSS) test and mode 1 fracture toughness tests. Before testing

the composites, the breaking strength (tenacity) of glass yarns was also determined.

For physical characterisation, the density was measured by the immersion method

and then the fibre volume and void content of the composites were determined by

calcination or the resin burn out process. Finally the post-fracture analysis was done

using a projection microscope and by using scanning electron microscopy (SEM).

4.2. Breaking strength (tenacity) testing of glass yarns

The tenacity of glass yarns was determined for all the textured yarns produced at

different air pressures and also for the non-textured glass yarns in order to find out

the effects of the texturing process. The standard followed for this test is BS ISO

3341 [2000]. The principle of the test is to determine the breaking force by

elongating the specimen by an appropriate mechanical means until rupture. The

breaking strength or tenacity (cN/tex) was calculated by dividing the value of

breaking force obtained from the test by the density (tex) of the particular glass yarn.

Ten specimens of each type of yarn were tested on an Instron 4411 machine by

gripping them between the pair of clamps as shown in Figure 4.1. The specimen

gauge length was set at 250 mm, a 5 kN load cell was used and the test crosshead

speed was 200 mm/min. The yarns were conditioned before the test according to the

standard BS EN ISO 291 [2008] at (23 ± 2 °C, 50 ± 5 % RH) for 6 hours.

78

Figure 4.1 Glass yarn specimen undergoing breaking strength testing

4.3. Density, Fibre Volume fraction and Void Content

The density of composites made from textured and non-textured glass fabrics was

measured according to the standard BS EN ISO 1183-1 [2004] by using the

immersion method. Five specimens were cut from each panel from different places

and their weight was determined. A fine wire of maximum 0.5 mm diameter was

used for hanging the composite pieces and its weight in air and water was measured.

The weight in air of the specimens was measured by hanging them through a balance

hook by means of a wire. Also the weight of the specimens in water was determined

by immersing them in water in a 100 ml beaker while they were suspended stationary

by the wire. Care was taken to avoid any air bubbles adhering to the specimen or

found in the beaker otherwise the bubbles were removed by means of a fine wire.

The weighing balance used for the measurement of weights had an accuracy of 0.1

mg.

The density of the specimen was calculated using the following formula:

Weight of the specimen in air = mS,A (g) = m3 – m1

Glass yarn specimen clamped in the jaws

79

Weight of the specimen in water = mS,L (g) = m4 – m2

Density of specimen = ρS (g/cm3) = LSAS

WAS

mmm

,,

,

(4.1) [BS ISO 1183-1, 2004]

Where,

m1 is the weight of wire in air;

m2 is the weight of wire in water;

m3 is the weight of (wire + specimen) in air;

m4 is the weight of (wire + specimen) in water;

ρw is the density of water which was taken to be 0.998 g/cm3.

The fibre volume content and void content were determined by using calcination or

the resin burn out process according to standard BS EN ISO 1172 [1999]. After

calculating the density, the specimens were dried in the oven at 100 °C for 2 hours

and then cooled in a dessicator. As a preparation step, the glass crucibles were

cleaned, dried, weighed and then placed in the furnace at 625 °C (the temperature

required for the calcination process) for 10 minutes. After cooling down in the

dessicator, they were verified for any change of weight and if found, the process was

repeated until constant mass for the crucible was reached. For the calcination

process, the weight of the empty crucibles and the weight of the crucibles with the

specimen in them were recorded. The crucibles were then placed in the furnace,

preheated to a temperature of 625 °C and heated to constant mass. Subsequently the

crucibles, together with the residue, were taken out of the furnace and cooled in the

dessicator to ambient temperature and then weighed again.

The fibre volume content (%) was determined by the following equation;

f

Cff WV

(4.2) [Khan 2010]

Where,

Vf is the fibre content as a percentage of the initial volume;

Wf is the fibre content as a percentage of the initial mass;

ρc is the density of composite specimen;

ρf is the density of fibre.

80

Wf was calculated by the following equation:

10012

13

MMMMW f

(4.3) [BS ISO 1172, 1999]

Where,

M1 is the initial mass in grams of the empty crucible;

M2 is the initial mass in grams of the crucible plus specimen;

M3 is the final mass in grams of the crucible plus residue after calcination.

The void content as a percentage of initial volume was determined from the

following equation:

R

Cf

f

Cfo WWV

100100

(4.4) [Khan 2010] Where,

ρR is the density of the resin.

4.4. Tensile testing

The basic purpose of tensile testing is to determine the tensile strength and modulus

of the material. However closer observation provides more information about its

behaviour under the applied load. A composite may split or delaminate, the nature of

the failure may be brittle with no warning, or it may start with visible or audible

signs. All this information is useful and knowledge of the failure mode is vital to

establish the end use of the material [Godwin 2000].

There are varieties of specimen sizes, test piece specifications, and testing procedures

described in a number of published standards. The standard followed for the

determination of tensile properties in this study is BS 2782-10: Method 1003 [1977].

The dimensions of the samples were: overall length = 250 mm, width = 25 mm and

the gauge length = 100 mm. The tests were conducted on an Instron universal testing

machine model 1331 with a 50 kN load cell at an extension rate of 2.0 mm/min. Five

specimens of each type were cut in both the warp and weft directions using a

diamond blade cutter and the dimensions of all the specimens were measured before

the test. Extensometer was attached to the composite specimens as shown in Figure

81

4.2 during testing to acquire data for establishing the values of the Modulus of

Elasticity in megapascals.

Extensometer

Figure 4.2 Composite specimen undergoing tensile testing

Tensile strength at maximum force was determined by using the following equation;

bhF

(4.5) [BS 2782-10: Method 1003 1977]

Where, = Strength at maximum force, in megapascals;

F =Maximum tensile force, in Newtons;

b = Mean initial width of the test specimen, in millimetres;

h = Mean initial thickness of the test specimen, in millimetres.

82

4.5. Flexure testing (Three point bending)

The flexure test is used in industry to determine the mechanical properties of resins

and laminated fibre composites because of the ease of the test method,

instrumentation and the equipment required. This test is also used to determine the

inter-laminar shear strength of a laminate (using a short beam). The process is almost

the same for both the tests (i.e. inter-laminar shear strength (ILSS) and flexure

testing) except for the size of the test specimen. ILSS is usually performed with the

smaller span length/thickness ratio in order to avoid bending and to increase the level

of shear stress [Hodgkinson 2000].

The methods used for determination of flexure properties were three point and four

point bending. Three point bending is the method in which a bars of rectangular

cross-section was loaded from the top while resting on the two supports whereas in

four point bending two loads are symmetrically placed between the two supports as

shown in Figure 4.3 (a and b). The geometry of four point loading provides a

constant bending moment between the central loading members and this causes a

reduction in contact stresses in the beams. Whereas in three point loading

arrangement the stress concentrations exist at the loading point so, four point bending

method is more attractive if the state of stress is of concern but it is easier to perform

the three point bending test [Hodgkinson 2000].

(a)

83

(b)

Figure 4.3 Flexure testing assembly (a) three point bending (b) four point testing [Hodgkinson 2000]

The flexure testing resulted in a wide range of failure modes depending on the

chosen method, type and layup of the materials being tested. The potential failure

modes are tensile fracture, compressive fracture, tensile and compressive fracture

accompanied by inter-laminar shear and inter-laminar shear fracture as shown in

Figure 4.4. All failure modes are not acceptable especially those initiated by inter-

laminar shear. To avoid inter-laminar shear failure, the specimen with large span-to

thickness ratio should be used. The standard for flexure testing BSI 14125 [1998]

recommends a minimum span-to-thickness ratio of 16:1.

Figure 4.4 Potential failure modes for flexure testing [BSI 14125 1998]

84

This test method is not appropriate for the determination of design parameters, but

used as a quality-control test. This is because the specimen is subjected to a

combined stress state and the flexure strength and modulus are combinations of the

subsequent tensile and compressive properties of the material. [BSI 14125 1998 and

Hodgkinson 2000]. During the testing of flexure properties, flexure strength and modulus were

determined for both the textured and non-textured composite samples in the warp

and weft directions according to British standard BS EN ISO 14125 [1998]. Five

specimens of each type were cut in both the warp and weft directions using a

diamond blade cutter. The dimensions of the samples were: specimen length = L =

40 mm, span length = S = 32 mm and width = b = 15 mm. The testing was conducted

on an Instron universal testing machine model 4411 with 5 kN load cell at a constant

crosshead speed of 1.0 mm/min. The radii of loading nose and supports were selected

as 5.0 mm and 2.0 mm respectively.

The flexure strength (σf) including the large displacement correction factor was

calculated using the following equation:

2

2

2 36123

Lsh

LS

bhFL

f (4.6) [BSI 14125 1998]

Where,

σf is the flexure stress, in megapascals (MPa);

F is the load in Newtons (N);

L is the span, in millimetres (mm);

h is the thickness of the specimen, in millimetres (mm);

b is the width of the specimen, in millimetres (mm);

s is the beam mid-point deflection, in millimetres (mm).

Flexure modulus (Ef) was calculated using the following equation:

sF

bhLE f 3

3

4 (4.7) [BSI 14125 1998]

Where (ΔF/Δs) is the slope of the load displacement curve.

85

4.6. Inter-laminar shear strength (ILSS)

The ILSS is measured using the short beam method as it is the simplest test to be

performed and is widely used. The measured strength is called the apparent inter-

laminar shear strength and the method is suitable for use with fibre-reinforced plastic

composites with a thermoset or a thermoplastic matrix. The result obtained for ILSS

is not an absolute value and therefore not used for the determination of design

parameters but can be used for quality control purposes. Due to this fact, the term

‘apparent’ is used for describing the measured quantity. Since the test results are

achieved from different-sized specimens or from specimens tested under dissimilar

conditions, they are not directly comparable. The nature of the test method is similar

to the three-point loading method used to determine the flexure properties but a

smaller span-to-thickness ratio is adopted to increase the level of shear stresses and

to avoid flexure or bending failure [Broughton 2000 and BS ISO 14130 1998].

The development of shear stresses and strain concentrations in the matrix region

between the fibres is generally the cause of shear failure which results in interfacial

failure. The literature [Tanoglu et al. 2001; Paiva et al. 2005; Khan 2010] confirms

that ILSS is a matrix dependent property of any material and is used for

characterising and comparing the bonding strength between fibre and matrix.

Composite samples were tested for ILSS (Figure 4.5) in both the warp and weft

directions. All test dimensions and crosshead speeds were selected and the

calculations were done according to the standard BS EN ISO 14130 [1998] i.e.

specimen length = L = 20 mm, span length = S = 10 mm and width = b =10 mm.

Five specimens of each type were cut in both warp and weft directions using a

diamond blade cutter. The test was conducted on the same Instron universal testing

machine model 4411 used for flexure testing with 5 kN load cell at a constant

crosshead speed of 1.0 mm/min.

86

Figure 4.5 Composite specimen undergoing Inter-laminar shear strength (ILSS) testing

Like flexure testing, a range of failure modes is possible including inter-laminar

shear failure, mixed mode failure of shear + tension or shear + compression, and

non-shear failure mode like tension and compression. However, only inter-laminar

shear failure in the form of single or multiple shears is the acceptable mode.

The ILSS in megapascals (MPa) can be determined by the following equation;

bhF

ILSSmax

43

(4.8) [BS ISO 14130 1998]

Where,

Fmax is the failure or maximum load, in Newton;

b is the width, in millimetres, of the test specimen;

h is the thickness, in millimetres, of the test specimen.

4.7. Inter-laminar fracture toughness

Laminated fibre-reinforced composites made of high strength fibres like glass,

carbon, Kevlar etc in a relatively weak matrix material like polyester, epoxy, PEEK

etc are prone to de-lamination which is in the form of separation of the layers. De-

Specimen for ILSS

87

lamination is a common damage form of laminated fibre-reinforced composites

which can be principally detrimental for structural behaviour. Apart from the

external loading conditions, the de-lamination failure also depends on the inherent

properties of the fibre and resin, processing conditions and inter-laminar stresses

induced by temperature and moisture conditions [Khan 2010].

Significant efforts have been made to recognise and improve the de-lamination

resistance of composite materials. The successful methods used to enumerate the de-

lamination resistance of fibre reinforced composites are mode I, mode II and mode

III of inter-laminar fracture toughness tests as shown in Figure 4.6. The critical strain

energy release rate was used to express the inter-laminar fracture toughness of

laminated composites which is the energy consumed by the material as the de-

lamination front proceeds through a unit area and is usually represented by the

symbol Gc. The units commonly used for Gc are Joules per square metre [Robinson

and Hodgkinson 2000].

Figure 4.6 Schematic diagrams of the basic modes of fracture, mode I (opening), mode II

(shear), mode III (tearing) [Robinson and Hodgkinson 2000]

4.7.1. Mode I Inter-laminar fracture toughness

Mode I was selected to measure the Inter-laminar fracture toughness which is the

most commonly used method and is also known as the double cantilever beam

(DCB) test.

88

Figure 4.7 Double cantilever beam (DCB) specimen geometry, (a) end-blocks, (b) piano hinges

[Robinson and Hodgkinson 2000]

Figure 4.7 illustrates two possible types of loading attachments, namely loading

blocks or piano hinges. The parameters are L = total length, b = width and h =

thickness of the beam type specimen. ao is the implanted de-lamination length from

the centre of the hinge and it is produced in the specimen by inserting PTFE film or

other non-adhesives during the processing of the laminated composites.

The piano hinge-type loading attachment was used (as shown in Figure 4.8) for

determining the Mode I fracture toughness of textured and non-textured glass

composites and five specimens were tested for each type. The dimensions of the

samples were: specimen length = L = 140 mm, implanted de-lamination length = ao =

50 mm and width = b = 25 mm. The hinges were bonded using resin containing glass

beads which define the thickness of the layer. The Instron 4411 machine was used

and fracture toughness was measured according to standard ASTM D 5528-01

[2007] at a crosshead speed of 1 mm/min and load cell of 500 N. A 15 μm thick

89

PTFE film was inserted between the centre layers of the laminate prior to the resin

infusion process to produce an initial de-lamination crack.

Figure 4.8 DCB test specimen undergoing fracture toughness testing

Modified beam theory (MBT) was utilised as a data reduction method for the

determination of GIC. The other methods available are the compliance calibration

method (CC) and the modified compliance calibration method (MCC) [ASTM D

5528-01 2007]. The beam theory expression used in this study, for the strain energy

release rate with the correction factors for DCB is as follows;

FPG c

a 2b3

1

(4.9) [ASTM D 5528-01 2007]

Where,

P is the applied load;

is the load point displacement;

b is the specimen width;

a is the de-lamination length;

is the crack-tip rotation correction factor;

DCB specimen in the jaws

90

F is the large displacement effect correction factor.

can be calculated by generating a least squares plot of the cube root of compliance

C1/3 as a function of de-lamination length where compliance is the ratio of the load

point displacement to the applied load. The large displacement effects should be

corrected by the addition of a factor F which can be calculated from the following

equation.

22

31031

2

at

aF

(4.10) [ASTM D 5528-01 2007]

Where, “t” (shown in Figure 4.9) is for piano hinges. This correction factor F

accounts for both the shortening of the moment arm as well as tilting of the end

blocks.

Figure 4.9 Section of DCB with piano hinges indicating “t”

4.8. Scanning electron microscope (SEM)

Observation of composite samples using a scanning electron microscope is very

common since it is capable of generating high resolution images. SEM is a very

significant instrument for analysing the post-fracture surfaces of the fibre reinforced

composites. In this work, SEM was used to examine the fracture surfaces of textured

and non-textured composite samples after subjecting them to ILSS and mode I inter-

laminar fracture toughness testing. The composite samples were inspected using a

Philips model XL30 field emission gun (FEG) SEM microscope. For Mode 1 testing,

small sections of tested samples were cut from the test samples and were stuck to the

metal stubs using double-sided carbon tabs. The whole assembly was then coated

with a very thin layer of carbon using an Edwards S150B sputter coater in order to

improve the conductivity of the surface. For ILSS tested samples, the cross-section

91

of the delaminated surfaces was viewed through a SEM as shown in Figure 4.10. So

the samples were prepared by placing them vertically in a circular plastic cast and the

mixture of polyester resin and hardener was poured in it. The samples were left for a

day to allow the solution to be set and then they were taken out of the cast for

grinding and polishing. The cleaned polished surfaces were presented for viewing to

obtain very fine images.

ILSS Sample Metal Stub

Figure 4.10 Prepared samples for scanning electron microscopy (SEM)

Along with SEM, the textured glass yarn samples and some polished ILSS tested

samples were also examined by using the Projectina Micro Macro Projection

Microscope (MMP-1000) with PIA 4000 software. The variation in arrangement of

loops in different classes of textured yarns and their presence in the textured yarns

structure was verified using the projection microscope.

92

5. Chapter 5 Effect of the texturing process on glass yarn

tenacity

5.1. Introduction

The reason for texturing glass yarn was to produce bulk and loops in the structure but

at the same time the texturing process was expected to reduce the breaking strength

of the yarns due to the disorientation of filaments. The tenacity of the textured and

non-textured yarns was compared in this chapter and the effect of the texturing

process on the breaking strength of the glass yarns was examined. The textured glass

yarns were divided into two categories depending upon the linear density of the core

yarns (i.e. 300 tex and 600 tex).

5.2. Tenacity of the feed yarns

Before analysing the variation in tenacity of the yarns after the texturing process, the

tenacity of the core and effect feed yarns was determined. The testing was performed

on an Instron 4411 according to standard BS ISO 3341 [2000] by taking ten

specimens of glass yarns randomly from the cone. The specimen gauge length was

250 mm, the load cell used was of 5 kN and the test cross head speed was 200

mm/min. The results are shown in Figure 5.1 below.

93

Tenacity of feed yarns

01020304050607080

300 core

600 core

34 effect

68 effect

Feed yarn type

Tena

city

(cN

/tex)

Figure 5.1 Tenacity of the feed yarns

5.3. Tenacity of the 300 tex category

The glass yarns are symbolised on the basis of the thickness of the core and effect

yarns (tex value) and the air pressures used for texturing. For example 300 + 34 5B

means:

Core yarn tex = 300

Effect yarn tex = 34

Air pressure = 5 bars

Tenacity (cN/tex) of the textured and non-textured glass yarns was determined on an

Instron 4411 according to standard BS ISO 3341 [2000]. The yarns were textured by

varying the air pressure from 3 to 5 bars as explained in Section 3.2.14. The results

obtained are shown in Figure 5.2:

94

Tenacity of 300 tex glass yarns after texturing process

0

10

20

30

40

50

60

300 NT

300+34 3B

300+34 4B

300+34 5B

300+68 3B

300+68 4B

300+68 5B

Yarn type

Tena

city

(cN

/tex)

Figure 5.2 Tenacity of textured and non-textured glass yarns of 300 tex category

A significant loss in tenacity was found after texturing the 300 tex core-and-effect

yarns. This loss ranges from 41 % to 47 % in the 300 + 34 tex yarns and from 54 %

to 62% in the 300 + 68 tex yarns respectively. Since the texturing process resulted in

the formation of loops and bulk in the glass yarn, the parallel arrangement of

filaments in the structure was disturbed. However, Figure 5.2 shows that the

texturing air pressure had a statistically insignificant effect on the variation in

tenacity for both the 300 + 34 and 300 + 68 tex yarns. It was found that the loss in

tenacity was higher in the 300 + 68 tex yarns than in 300 + 34 tex textured yarns.

The reason for the lower tenacity of the 300 + 68 tex yarns is the greater proportion

of the effect filaments which contribute less to the strength than the core since the

tenacity is calculated by dividing the breaking force by the total linear density of the

yarn. Yarns textured at 5 bars pressure were also examined under a projection

microscope at 20x magnification to find out if there were any structural differences

between them and the following results were obtained:

95

Figure 5.3 photomicrographs of 300 + 34 tex 5 bars textured yarn structure

Figure 5.4 Photomicrographs of 300 + 68 tex 5 bars textured yarn structure

96

The photomicrographs in Figures 5.3 and 5.4 showed a difference in structure in the

two types of textured yarns. The structure of the 300 + 68 tex textured yarn

possessed more loops and cross filaments which caused more disturbance in the

structure of yarn. The number of filaments in the structure of the feed yarns were

also determined and listed in the following table:

Table 5.1 Number of filaments in glass yarns

Yarn linear

density (tex) No. of filaments

Filament

diameter (μm)

34 211 9

68 341 10

300 767 14

600 1780 13

The filament diameters mentioned in Table 5.1 show that the filaments have different

diameters and hence the doubling of linear density is not just the doubling of yarns. It

can be observed from the above table that the 300 + 68 tex yarn had more filaments

i.e. (767+341=1108) in the structure than the 300 + 34 tex yarn i.e. (767+211=978).

The higher number of filaments usually enhanced the texturing effect. It caused more

mutual entanglement of filaments within the yarn at the same air pressure and hence

decreased the tenacity. Alagirusamy and Ogale [2004] also indicated that

improvement in texturing and an increase in the mutual entanglement of filaments

took place if a higher number of filaments were available in the yarn structure. This

improvement in the texturing process resulted in a decrease in tenacity of the

resultant textured yarn.

Therefore the higher effect yarn linear density and the higher number of filaments in

the structure of the 300 + 68 tex textured yarns were the two possible reasons for the

greater percentage loss in tenacity after the texturing process as compared to the 300

+ 34 tex textured yarns.

97

5.4. Tenacity of the 600 tex category

Textured glass yarns of the 600 tex core category were also examined for change in

tenacity and the following results were obtained:

Tenacity of 600 tex glass yarns after texturing process

05

101520253035404550

600 NT

600+34 5B

600+68 5B

Yarn type

Tena

city

(cN

/tex)

Figure 5.5 Tenacity of textured and non-textured glass yarns of 600 tex category

The same loss in tenacity after the texturing process was found in the 600 tex

category (Figure 5.5) although the percentage loss is smaller than for the 300 tex

yarns i.e. (21% and 24%) in the 600 + 68 and 600 + 34 tex yarns respectively.

However, no significant difference in tenacity was found between the 600 + 34 and

600 + 68 tex yarns textured at 5 bars air pressure. This is likely to be due to the

smaller linear density difference between the 600 + 68 tex yarn and the 600 + 34 tex

yarn. Therefore, the disturbance in the yarn structure after the texturing process was

not very different between the two yarns. This is in contrast to the 300 tex yarn

category where the tenacity of the 300 + 68 tex yarns was significantly lower as

compared to the 300 + 34 tex yarns at 5 bars texturing air pressure (Figure 5.6).

98

Comparison of 300 and 600 tex categories at 5 bar pressure

05

10152025303540

300+34

300+68

600+34

600+68

Yarn type

Tena

city

(cN

/tex)

Figure 5.6 Comparison of tenacity of 300 and 600 tex textured yarns

Moreover, the disturbance during the texturing process is more on the surface.

Therefore, the 600 tex yarns with more core filaments in the structure have a reduced

number of disturbed fibres. This helped in maintaining the tenacity after the texturing

process.

The textured core-and-effect yarns of 600 + 34 and 600 + 68 tex were also observed

under a projection microscope and the following images were obtained:

99

Figure 5.7 Photomicrographs images of 600 + 34 tex 5 bars textured yarn structure

Figure 5.8 Photomicrographs of 600 + 68 tex 5 bars textured yarn structure

100

The photomicrographs in Figures 5.7 and 5.8 also show that the disturbance in the

yarn structure is similar for both types although more loops can be observed on the

surface of the 600 + 68 tex core-and-effect yarn. The tenacity mainly depends on the

core yarns as they were textured with relatively lower overfeed, just enough to

provide some bulk for the intermingling between the core and the effect. In order to

investigate the contribution of the effect yarn to the strength of the resultant core-

and-effect yarn, an experiment was conducted by determining the tenacity of the core

and the effect feed yarns. The details are below.

5.5. Tenacity of combined core-and-effect feed yarns

The tenacity of the non-textured core-and-effect yarns was determined

experimentally by combining them together with the help of minimal twist (one twist

per 250 mm gauge length). The objective was to discover the contribution of the

effect yarn to the strength. Although the behaviour of the effect yarn in the structure

of the textured yarn is different from the non-textured feed yarn due to overfeed and

loops, it provides information on how the texturing process affects the tenacity of the

two yarns differently.

Tenacity of non-textured feed yarns

0

10

20

30

40

50

60

300

334

368

600

634

668

Yarn type

Tena

city

(cN

/tex)

Figure 5.9 Comparison of tenacity of non-textured feed yarns

Figure 5.9 illustrates that the difference in tenacity was statistically insignificant for

both the 300 and 600 tex yarns with the insertion of effect yarns. This shows the

101

contribution of effect yarn in maintaining the overall tenacity of the non-textured

yarns otherwise the tenacity (cN/tex) would be lower.

However, the case is different for the textured core-and-effect yarn and the effect

yarns contributing less in the overall tenacity. The reason is their more disoriented

structure because they were overfed. The effect yarn usually enhances the texturing

effect by the formation of loops and causing disturbance in the yarn structure. This in

turn resulted in a decrease in tenacity of the resultant textured yarn.

5.6. Broken filaments and loss in linear density

The linear density (tex) of the resultant yarn was found to be slightly reduced after

the texturing process as shown in Figure 5.10 although theoretically it was expected

to be increased because of the overfeeding. The reduction in linear density was

calculated against the nominal linear density of the particular yarn which was

determined by incorporating overfeed percentages of the constituent yarns. The

major reason for the reduction in linear density was the modified winding process

which was carried out at a slightly higher speed (5 %) than the speed of the delivery

roller of the machine. This helped in tightening up the loops and proper winding of

the resultant textured yarn on a cone for the subsequent warping and weaving

processes. The other possible reason for this reduction was the broken filaments and

fibre fly generated during the texturing process which settled on different parts of the

machine. A hand blower was used to blow the fibre fly away from the machine

processing line in order to avoid any contamination of the resultant yarn. Choi [1999]

as cited by Alagirusamy and Ogale [2004a, 2006b] reported the effect of different

types of commingling and air texturing nozzles on carbon filaments. They stated that

increase in the air pressure resulted in increasing filament damage and loss of broken

filaments which adversely affected the yarn strength and linear density. Their

experimental results showed that the linear density was decreased in all the cases

with an increase in the texturing air pressure. It was also observed from the results of

this study (Figure 5.10a) that the linear density of the textured yarns decreased with

an increase in the air pressure from 3 bars to 5 bars in the 300 tex category.

102

Variation in linear density of glass yarns (300 tex)

280300320340360380400

Nom

inal lineardensity

300+34 (3 Bar)

300+34 (4 Bar)

300+34 (5 Bar)

Nom

inal lineardensity

300+68 (3 Bar)

300+68 (4 Bar)

300+68 (5 Bar)

Yarn type

Line

ar d

ensi

ty (t

ex)

(a)

Variation in linear density of glass yarns (600 tex)

580600620640660680700

Nom

inal lineardensity

600+34 (5 Bar)

Nom

inal lineardensity

600+68 (5 Bar)

Yarn type

Line

ar d

ensi

ty (t

ex)

(b)

Figure 5.10 Linear density (tex) of textured glass yarns (a) 300 tex (b) 600 tex category

103

5.7. Summary

Significant loss in tenacity was observed after the texturing process in both the 300

tex and 600 tex glass yarn categories. However, the variation in tenacity of the 300

tex textured yarns with the change in texturing air pressure from 3 to 5 bars was

insignificant.

In the 300 tex category, the loss was found to be higher in the 300 + 68 tex yarns as

compared to the 300 + 34 tex yarns. The reason for this is the greater proportion of

the effect filaments which contribute less to the strength than the core since the

tenacity is calculated by dividing the breaking force by the total linear density of the

yarn.

The difference in tenacity of the 600 + 34 and 600 + 68 tex core-and-effect yarns was

found to be statistically insignificant. This is likely due to the smaller linear density

difference between the 600 + 68 tex yarn and the 600 + 34 tex yarn. The disturbance

in the yarn structure after the texturing process was not very different between the

two yarns.

The contribution of the effect yarn in the tenacity of the resultant yarn was

investigated and it was observed that it behaved differently in textured and non-

textured yarn. The effect yarn contributed to the total tenacity in non-textured yarn

but in the textured yarn they contributed less because of more disorientation and

overfeed than expected. However, it enhances the texturing effect by the formation

of loops and causes disturbance in the yarn structure.

It was also observed that the linear density was decreased after the texturing process.

The reason was the installation of a modified winding unit which accounted for the

tightening up of loops by winding the yarn under tension. Moreover, filament

damage during the texturing process which caused a loss of broken filaments in the

form of fibre fly was the other possible reason for the reduction in linear density after

the texturing process.

104

6. Chapter 6 Composites made with textured yarns: mechanical

testing, results and discussion

6.1. Introduction

A common mode of damage in high performance composites is delamination. This

mode of failure depends on the intrinsic properties of the fibre along with the

external loading conditions. The aim of this project is to optimise the lamination

properties of glass composites by using the air-jet texturing process. Composites

were prepared using fabrics made from both textured and non-textured yarns and

their mechanical properties were determined and compared in this chapter to

investigate the effect of the texturing process on these properties.

6.2. Composites nomenclature

The composites are divided into categories based on the linear density of the core

and effect yarns (tex value), the air pressures used for texturing, sample directions

(warp and weft) and the weave structure. For example 300 + 34 tex 5B Pl means:

Core yarn = 300 tex.

Effect yarn = 34 tex.

Air pressure = 5 bars.

Weave type = Plain weave.

The samples were tested in both the warp and weft directions because of the slightly

unbalanced weave structure. Apart from the composites made from the core-and-

effect textured yarns in the warp and weft directions, some other composites were

also produced and these are discussed in detail in the next chapter.

105

6.3. Fibre volume content

The density and fibre volume content are important physical properties of the

composites and are dependent on the amount of reinforcement present in the

structure. The density and fibre volume content were determined using the

immersion and calcination methods respectively for the textured and non-textured

composites. The procedures were explained in detail in Section 4.3 of this thesis. The

following results were obtained: Table 6.1 Fibre volume content of glass composites

Weave Type Composite type Density

(g/cm3) CV% Fibre

Volume Content

CV% Void content

Plain

300 non-textured 1.8628 0.74 52.18 1.85 1.49 300 + 34 3 bars 1.6441 1.42 36.15 3.06 1.28 300 + 34 4 bars 1.6293 1.61 37.69 3.50 4.30 300 + 34 5 bars 1.6278 1.05 38.78 1.71 5.10 300 + 68 4 bars 1.6653 1.39 37.94 1.97 1.57 300 + 68 5 bars 1.5579 0.45 36.56 2.37 7.9

Twill

300 non-textured 1.8361 0.43 48.64 0.2 0.10 300 + 34 3 bars 1.6662 0.25 37.13 0.66 0.53 300 + 34 4 bars 1.6688 0.96 37.84 1.81 1.15 300 + 34 5 bars 1.6928 0.63 38.85 1.27 0.29 300 + 68 3 bars 1.6622 0.19 37.77 1.62 1.65 300 + 68 4 bars 1.6597 1.00 37.59 2.18 1.64 300 + 68 5 bars 1.6314 0.62 38.08 0.88 4.05

Plain 600 non-textured 1.8621 0.75 52.25 1.11 1.63 600 + 34 5 bars 1.6996 1.13 39.10 1.19 0.10 600 + 68 5 bars 1.6665 0.47 38.19 0.85 1.76

Twill 600 non-textured 1.9104 0.35 54.39 0.71 0.01 600 + 34 5 bars 1.7255 0.52 43.96 1.73 3.50 600 + 68 5 bars 1.6510 0.62 36.41 2.01 1.13

The results in Table 6.1 illustrate reductions in the density and fibre volume content

in all the categories of the textured composites as compared to the non-textured

composites. The number of fabric layers and the weave structures are same for the

106

two types of composites (i.e. textured and non-textured) however, because of the

bulkier structure of the constituent textured yarn, the textured composites were found

to be thicker and with lower fibre volume content. Table 6.1 also shows the void

content of composites below 5 % which is an acceptable level for the majority of

applications [Liu et al 2006]. The only exception is the 300 + 68 5 bars plain

composites with approx. 8% void content which might be because of the

experimental error.

6.4. Tensile testing of composites

The basic purpose of tensile testing is to determine the tensile strength and modulus

of the material and to analyse the change in the property due to the texturing process.

The tensile strength and tensile modulus were obtained for both the textured and non-

textured composite samples in the warp and weft directions.

The composites are divided into two groups based on the two weave structures, plain

and twill. In the case of 300 + 68 tex yarns textured at 3 bars pressure, the weaving

process became almost impossible because of the excessive warp yarn entanglement

and resulted in the failure to produce enough fabric to conduct tensile testing. The

same problem happened in the case of the twill fabric of 300 + 34 tex yarns textured

at 4 bars air pressure.

The standard followed for the determination of tensile properties is BS 2782-10

[1977]. Five specimens of each type were cut in both the warp and weft directions

using a diamond blade cutter. Extensometer was applied to the composite specimens

to acquire modulus data. The tests were conducted on an Instron universal testing

machine model 1331 with 50 kN load cell at an extension rate of 2.0 mm/min. The

following results were obtained for tensile properties of the 300 tex composites.

107

6.4.1. Tensile properties of 300 tex plain weave composites

Tensile strength 300 plain

050

100150200250300350400

300 NT

300+34 3B

300+34 4B

300+34 5B

300+68 4B

300+68 5B

300 NT

300+34 3B

300+34 4B

300+34 5B

300+68 4B

300+68 5B

Composite type

Tens

ile s

treng

th (M

Pa)

Figure 6.1 Tensile strength of 300 tex plain weave composites

Tensile modulus 300 plain

0

5000

10000

15000

20000

25000

30000

300 NT

300+34 3B

300+34 4B

300+34 5B

300+68 4B

300+68 5B

300 NT

300+34 3B

300+34 4B

300+34 5B

300+68 4B

300+68 5B

Composite type

Ten

sile

mod

ulus

(MPa

)

Figure 6.2 Tensile modulus of 300 tex plain weave composites

It can be seen from the results of 300 tex plain weave samples (Figures 6.1 and 6.2)

that there was a statistically significant reduction in tensile strength after texturing.

Plain Warp Plain Weft

Plain Warp Plain Weft

108

This is mainly due to the reduction in the breaking strength of the constituent yarns

after the texturing process (as discussed in Chapter 5). There is no obvious trend in

the effects of air pressure on tensile strength and the difference is statistically

insignificant. This is because the tenacity of the constituent yarns was also not

changed considerabily by varying the texturing air pressure as explained in Chapter

5. It can also be seen that the reduction in tensile properties of the textured

composites were smaller when compared to the reduction in the breaking strength of

the contituent yarns after texturing. The reduction in tensile strength ranges from 20

to 32 % after the texturing process whereas the reduction in tenacity of the textured

yarns was in the range of 41 to 62 %. This is due to the contribution of the matrix

which held the filaments together and distributed the load more evenly among the

filaments once they were embedded in the composites. This was also observed by

Langston [2003] in his work on textured Aramid yarns.

The tensile strength values in the weft direction were mostly found to be less than the

warp direction for plain weave because there were slightly fewer yarns (less yarn

density) in the weft direction as shown in Table 3.1.

6.4.2. Tensile properties of 300 tex twill weave composites

The tensile properties of the 300 tex twill weave fabric composites are shown in

Figures 6.3 and 6.4.

Tensile strength 300 twill

050

100150200250300350400

300 NT

300+34 3B

300+34 5B

300+68 4B

300+68 5B

300 NT

300+34 3B

300+34 5B

300+68 4B

300+68 5B

Composite type

Tens

ile s

treng

th (M

Pa)

Figure 6.3 Tensile strength of 300 tex twill weave composites

Twill Warp Twill Weft

109

Tensile modulus 300 twill

0

5000

10000

15000

20000

25000

300 NT

300+34 3B

300+34 5B

300+68 4B

300+68 5B

300 NT

300+34 3B

300+34 5B

300+68 4B

300+68 5B

Composite type

Tens

ile m

odul

us (M

Pa)

Figure 6.4 Tensile modulus of 300 tex twill weave composites

Similarly to the plain weave composites, a statistically significant reduction in tensile

properties was observed in the twill weave composites. It can be seen that the

variation in the tensile properties was statistically insignificant with the change in

texturing air pressure for both 300 + 34 and 300 + 68 tex twill weave composites.

Moreover, the tensile strength in the weft direction of 300 + 34 tex category was

higher than in the warp direction because of the slightly unbalanced weave (having

higher fibre density in the weft direction). This is because of the structure of 1/3 twill

weave which contained less interlacing in the warp and weft directions.

6.4.3. Tensile properties of 600 tex plain and twill composites

600 tex textured composites were made by texturing the glass yarn at 5 bars pressure

only. This was because it was found that for the 300 tex category, the yarn made at 5

bars pressure had the least entanglement problems in weaving. The tensile properties

of 600 tex composites are shown in Figures 6.5 and 6.6.

Twill Warp Twill Weft

110

Tensile strength 600 plain & twill

050

100150200250300350400

600 NT

600+34 5B

600+68 5B

600 NT

600+34 5B

600+68 5B

600 NT

600+34 5B

600+68 5B

600 NT

600+34 5B

600+68 5B

Composite type

Ten

sile

stre

ngth

(MPa

)

Figure 6.5 Tensile strength of 600 tex plain & twill weave composites

Tensile modulus 600 tex plain & twill

05000

1000015000200002500030000

600 NT

600+34 5B

600+68 5B

600 NT

600+34 5B

600+68 5B

600 NT

600+34 5B

600+68 5B

600 NT

600+34 5B

600+68 5B

Composite type

Tens

ile m

odul

us (M

Pa)

Figure 6.6 Tensile modulus of 600 tex plain & twill weave composites

The results show a reduction in tensile strength after the texturing process but the

percentage of the reduction was smaller compared with the 300 tex composites. This

is due to the fact that the 600 tex glass yarns showed smaller reductions in tenacity

Plain Warp Plain Weft Twill Warp Twill Weft

Plain Warp Plain Weft Twill Warp Twill Weft

111

than the 300 tex yarns after texturing. The decrease in tensile properties was found to

be statistically significant in most of the plain and twill weave composites of the 600

+ 34 and 600 + 68 tex categories. It was observed that the tensile properties of the

twill weave composites in the weft direction were higher than or similar to the warp

direction for both the textured and non-textured composites. This is because of the

slightly unbalanced weave and higher fibre density in the weft direction as shown in

Table 3.1.

The failure mechanism of textured and non-textured composites was also observed to

be different. Delamination in the testing area of the specimen accompanied by an

audible cracking sound was observed before failure in the non-textured composite

samples whereas the textured composites failed without excessive delamination. It

can be observed through the images (Figures 6.7 and 6.8) taken after the tensile

testing that de-lamination surrounding the area of failure was higher in the non-

textured 600 tex plain composites followed by delamination marks in the whole

testing area. However, the textured composites of 600+ 68 tex 5 bars plain weave did

not show any major sign of delamination in the testing area. Moreover, the

delamination at the point of failure was also found to be small for textured

composites. This illustrated the difference in the laminate bonding between the two

types of composites. The textured composites showed better results as the loopy and

bulkier yarn structure promoted bonding between the laminate layers.

112

Figure 6.7 Tensile tested samples of 600 tex non-textured plain weave composites

Delamination surrounding the area of failure

Delamination marks in the testing area

113

Figure 6.8 Tensile tested samples of 600 + 68 tex 5 bars textured plain weave composites

6.5. Flexure testing of composites

The flexure test is used in the industry to determine the resistance of laminated

reinforced fibre composites when they are subjected to bending loads. The flexure

test is also used to determine the inter-laminar shear strength (ILSS) of a laminate by

using a short beam [Hodgkinson (2000)]. The process is almost the same for the two

tests except for the size of the test specimen (explained in Section 4.5).

Delamination surrounding the area of failure

114

The flexure properties were determined according to British Standard BS EN ISO

14125 [1998] and the following results were obtained for the 300 tex plain weave

category.

6.5.1. Flexure properties of 300 tex plain weave composites

Flexure strength 300 plain

0

100

200

300

400

500

300 NT

300+34 3B

300+34 4B

300+34 5B

300+68 4B

300+68 5B

300 NT

300+34 3B

300+34 4B

300+34 5B

300+68 4B

300+68 5B

Composite type

Flex

ure

stre

ngth

(MPa

)

Figure 6.9 Flexure strength of 300 tex plain weave composites

Flexure modulus 300 plain

0

4000

8000

12000

16000

300 NT

300+34 3B

300+34 4B

300+34 5B

300+68 4B

300+68 5B

300 NT

300+34 3B

300+34 4B

300+34 5B

300+68 4B

300+68 5B

Composite type

Flex

ure

mod

ulus

(M

Pa)

Figure 6.10 Flexure modulus of 300 tex plain weave composites

Plain Warp Plain Weft

Plain Warp Plain Weft

115

It can be seen from Figures 6.9 and 6.10 that the flexure strength of the 300 tex plain

weave composites decreased significantly after the texturing process with both 34

and 68 tex effect yarns. The only exception is for the plain woven composite of 300

+ 34 tex yarn textured at 5 bars pressure which showed a flexure strength equal to the

300 tex non-textured composite. The flexure modulus showed a significant reduction

after the texturing process in the weft direction of all the 300 tex composites.

However, the reduction in the warp direction was small and statistically insignificant.

No particular trend can be seen for the textured composites with the change of

texturing air pressure from 3 to 5 bars. In the 300 + 68 tex composites, no significant

variation in the flexure properties is observed with the change of texturing air

pressure.

Sudarisman [2008] reported that the flexure properties also depend on the fibre

volume content of the composite structure along with the other parameters.

Therefore, it can be assumed that the decrease in flexure properties of the textured

composites is because of their lower fibre volume content.

6.5.2. Flexure properties of 300 tex twill weave composites

The flexure properties of the twill weave composites from the 300 tex category are

shown in Figures 6.11 and 6.12:

Flexure strength 300 twill

0

100

200

300

400

500

300 NT

300+34 3B

300+34 4B

300+34 5B

300+68 3B

300+68 4B

300+68 5B

300 NT

300+34 3B

300+34 4B

300+34 5B

300+68 3B

300+68 4B

300+68 5B

Composite type

Flex

ure

stre

ngth

(MPa

)

Figure 6.11 Flexure strength of 300 tex twill weave composites

Twill Warp Twill Weft

116

Flexure modulus 300 twill

0

4000

8000

12000

16000

300 NT

300+34 3B

300+34 4B

300+34 5B

300+68 3B

300+68 4B

300+68 5B

300 NT

300+34 3B

300+34 4B

300+34 5B

300+68 3B

300+68 4B

300+68 5B

Composite type

Flex

ure

mod

ulus

(MP

a)

Figure 6.12 Flexure modulus of 300 tex twill weave composites

The twill weave composites of the 300 tex textured yarns showed slightly better

results as compared to the plain weave composites of the same yarns. This is because

twill weave has less interlacing and more float yarns in the fabric structure as

compared to the plain weave which provided more contact surface between the

layers and therefore improve bonding.

It can be seen that although the flexure strength was reduced significantly in most

cases of textured composites, the modulus showed statistically insignificant

differences after texturing. Moreover, no significant variation was observed in the

flexure properties of the 300 + 34 and 300 + 68 tex categories with the change of

texturing air pressure in both the warp and weft directions.

6.5.3. Flexure properties of 600 tex composites

Flexure properties of the 600 tex textured composites are shown in Figures 6.13 and

6.14:

Twill Warp Twill Weft

117

Flexure strength 600 tex plain and twill

0

100

200

300

400

500

600 NT

600+34 5B

600+68 5B

600 NT

600+34 5B

600+68 5B

600 NT

600+34 5B

600+68 5B

600 NT

600+34 5B

600+68 5B

Composite type

Flex

ure

stre

ngth

(MPa

)

Figure 6.13 Flexure strength of 600 tex plain & twill weave composites

Flexure modulus 600 tex plain and twill

0

4000

8000

12000

16000

600 NT

600+34 5B

600+68 5B

600 NT

600+34 5B

600+68 5B

600 NT

600+34 5B

600+68 5B

600 NT

600+34 5B

600+68 5B

Composite type

Flex

ure

mod

ulus

(MPa

)

Figure 6.14 Flexure modulus of 600 tex plain & twill weave composites

It was observed using the t-test (Figures 6.13 and 6.14) that the flexure strength and

modulus of the 600 tex composites in both plain and twill weave structures were not

affected by the texturing process. Although the fibre volume content of the

Plain Warp Plain Weft Twill Warp Twill Weft

Plain Warp Plain Weft Twill Warp Twill Weft

118

composites was found to be lower after the texturing process, the introduction of

loops and bulk in the yarn helped in improving the fibre-matrix bonding and hence

enhanced the transfer of load between the laminates. Moreover, the lower

deterioration of 600 tex yarn after the texturing process also assisted in maintaining

the flexure properties. Therefore, it is concluded that the 600 tex composites were

better than the 300 tex composites in maintaining flexure properties after the

texturing process.

6.6. Inter-laminar shear strength (ILSS) testing

The ILSS was tested according to the BS EN ISO 14130 [1998] by using the three

point bending short beam method. The testing procedure was the same as the one

used for determining the flexure properties but the difference was the size of the

sample. ILSS is usually performed with a smaller span length/thickness ratio in order

to avoid bending and to increase the level of shear stress as discussed in Section 4.6.

The test was conducted on an Instron universal testing machine model 4411 with a 5

kN load cell at a constant crosshead speed of 1.0 mm/min.

6.6.1. ILSS of 300 tex plain and twill weave composites

The results of plain weave composites in the 300 tex category are as follows:

Inter-laminar shear strength 300 tex plain

0

5

10

15

20

25

30

35

40

45

300 NT

300+34 3B

300+34 4B

300+34 5B

300+68 4B

300+68 5B

300 NT

300+34 3B

300+34 4B

300+34 5B

300+68 4B

300+68 5B

Composite type

ILS

S (M

Pa)

Figure 6.15 ILSS of 300 tex plain weave composites

Plain Warp Plain Weft

119

After the texturing process, the ILSS of the 300 tex plain composites (Figure 6.15)

increased in both the warp and weft directions. The loops and bulky structure of the

textured yarns provided more contact area for the resin and increased the laminate

bonding strength. The increase in ILSS after the texturing process was statistically

significant in most cases. However, the effect of air pressure on the ILSS shows no

particular trend.

The inter-laminar shear strength in the twill weave 300 tex textured and non-textured

composites is shown in Figure 6.16.

Inter-laminar shear strength 300 tex twill

0

5

10

15

20

25

30

35

40

45

300 NT

300+34 3B

300+34 4B

300+34 5B

300+68 3B

300+68 4B

300+68 5B

300 NT

300+34 3B

300+34 4B

300+34 5B

300+68 3B

300+68 4B

300+68 5B

Composite type

ILS

S (M

Pa)

Figure 6.16 ILSS of 300 tex twill weave composites

The t-test confirms that the ILSS of the twill weave composites increased after the

texturing process (Figure 6.16). It can also be observed that the ILSS in the twill

weave composites was slightly higher than the plain weave composites in both the

warp and weft directions. The reason is the difference in structure of the two types of

weaves as explained in Section 6.4.2. The highest ILSS of 40 MPa was found in the

warp direction of the composite of 300 + 34 tex yarn textured at 5 bars pressure

followed by 39 MPa in both the warp and weft directions of the composite of 300 +

68 yarns textured at 5 bars pressure. The textured structures with bulk and loops

Twill Warp Twill Weft

120

offered more contact between fibre and resin as the loops provide anchoring and

bridging between the layers which resulted in better bonding.

6.6.2. ILSS of 600 tex plain and twill composites

The inter-laminar shear strength of the 600 tex composites are shown in Figure 6.17:

Inter-laminar shear strength 600 plain & twill

05

101520253035404550

600 NT

600+34 5B

600+68 5B

600 NT

600+34 5B

600+68 5B

600 NT

600+34 5B

600+68 5B

600 NT

600+34 5B

600+68 5B

Composite type

ILS

S (M

Pa)

Figure 6.17 ILSS of 600 tex plain & twill weave composites

The results of the 600 tex composites illustrate a significant increase in inter-laminar

shear strength in both warp and weft directions of the plain and twill weave

composite structures. The texturing process helped to increase the bonding strength

between the composite laminates. It was also observed that the percentage

improvement of ILSS in the 600 tex category was higher than in the 300 tex

category.

In order to know the failure mechanism of composites after the ILSS testing, the

specimens were prepared for examination using a projection microscope and a

scanning electron microscope (SEM) as described in Section 4.8.

Plain Warp Plain Weft Twill Warp Twill Weft

121

6.6.3. Microscope and SEM Analysis

The microscope images of the samples after the inter-laminar shear strength test are

shown in Figures 6.18 to 6.22. The pictures show a bulkier structure of the yarns

with a greater yarn diameter in the textured fabric providing more contact area for

the resin. The decrease in fibre volume content and increase in thickness also

provided evidence of this bulkier structure. Figure 6.18 demonstrates a clear, smooth

delamination in non-textured samples whereas Figure 6.19 of the textured samples

shows the crack growth was in a zigzag fashion. This was possibly because of the

improved laminate bonding and the resistance provided by the loops through fibre

bridging between the laminate layers.

122

Figure 6.18 600 tex non-textured twill weave composite

Smooth Delamination

123

Figure 6.19 600 + 34 tex 5 bars twill weave composite

In Figure 6.20 some weft filaments of the upper layer can be seen below the crack

after delamination. This is an evidence of improvement in bonding between the

layers which force some filaments to remain attached with the lower layer after

delamination.

Delamination in a zigzag manner

124

Figure 6.20 SEM images 600 + 34 tex 5 bars twill weave composite

The improvement in bonding strength between the fibres and the resin helped in

transferring the load from the matrix to the fibres, which resulted in an increase in

ILSS up to 40 % (300 + 34 tex and 600 + 34 tex, 5 bars twill weave composites). In

contrast, fibre matrix debonding can be clearly observed without any significant fibre

damage in the composite specimen of the 600 tex without textured glass filaments

(Figure 6.18).

Figure 6.21 shows that there were no clear separation lines between the layers of the

composite made by using textured glass fabrics. The loops and bulkier structure of

the textured yarn caused the mixing of filaments from adjacent layers. However,

Figure 6.22 shows that for composites from the non-textured yarns, the separation

line can be easily seen due to a relatively straight filament structure.

Crack

125

Figure 6.21 SEM image 600 + 34 tex 5 bars plain weave composites

Figure 6.22 SEM image 600 without textured plain weave composite

6.7. Fracture toughness (Mode I) testing

This test method is usually used to find quantitatively the effect of fibre surface

treatment, local variations in fibre volume, processing conditions and environmental

variables on G1c (Strain energy release rate) of a composite material. It can also be

utilised to compare quantitatively the relative G1c values of differently constituted

composite materials. In addition, the method can help in determining the

Separation lines are not noticeable

Clear separation lines

126

delamination failure criteria for composite damage tolerance and durability analyses

[ASTM D 5528-01, 2007].

The Mode I fracture toughness of the textured and non-textured glass composites

was tested on an Instron 4411 machine according to standard ASTM D 5528-01

[2007]. The procedures, the equations used to determine the critical strain energy

release rate (G1c) and the testing jigs employed for the measurement of delamination

resistance are explained in detail in Section 4.7.1. The yarn used for fabricating twill

weave textured samples was 600 + 68, textured at 5 bars air pressure and the non-

textured twill samples were developed using the 600 tex non-textured yarn. Twill

weave fabrics were selected for determining the effect of texturing on fracture

toughness properties because Suppakul and Bandyopadhyay [2002] reported higher

fracture energies for these structures. They investigated the effect of different weave

patterns of E-glass fabric composites on the inter-laminar fracture toughness property

and found the highest energy values were from the twill weave composites.

Figure 6.23 (a & b) shows typical load versus crosshead displacement curves for the

double cantilever beam (DCB) tests performed on specimens made of textured and

non-textured glass fabrics. Initially, the applied load increased linearly with

extension due to the implanted delamination (developed by introducing the PTFE

film between the middle layers as stated earlier in Section 4.7.1) for both types until

the commencement of the crack propagation. With the growth of the crack, several

load drops were experienced until the desired delamination of the DCB specimen

was reached. The critical load (N) and the crack length (mm) were recorded at the

onset of the crack propagation and at intervals during crack propagation.

127

Figure 6.23 Typical load versus crosshead displacement curves for mode I specimens of the 600

non-textured twill weave and the 600 + 68 tex 5 bars twill weave composites The resistance curve (R-curve) in Figure 6.24 shows mode I crack initiation and

propagation energies of the textured and non-textured glass composite samples. The

initiation values of G1c can be evaluated in three forms as stated below by using the

load displacement value.

At the point of deviation from linearity in the load-displacement curve and

symbolised as NL and is usually the lowest of all the three G1c initiation

values.

By visually observing the point on the edge at which the delamination starts

with the help of a magnification device symbolized as VIS.

The 5% offset which is measured at the point at which the compliance has

increased by 5%.

All three G1c initiation values were calculated for both the textured and non-textured

samples. It was found from the R-curve that the textured composite samples have

higher initiation and propagation G1c values than the non-textured samples. The

improvement in G1c values in the textured composites was due to enhanced bonding

between the layers since the bulkier loopy structure of textured yarns offered more

contact area for resin to adhere. Deng and Ye [1999] reported that the values of G1c

relied on the mechanisms of delamination growth and were affected by several

factors such as the inter-laminar bonding strength, fibre/matrix adhesion and the

128

degree of fibre bridging. Therefore, the higher G1c value of the textured composites

provided more evidence of improvement in laminate bonding strength after the

texturing process. Figure 6.25 shows mean values of the initiation and propagation

data along with the error bars and it was found that the increase in G1c of textured

composites was statistically significant.

0

0.5

1

1.5

2

2.5

50 55 60 65 70 75 80 85 90 95 100

Delamination length, a (mm)

GIc

(kJ/

m2)

600+68 textured Prop600 Non-textured PropNL texturedNL non-textured5% offset textured5% offset non-textured

Figure 6.24 Initiation and propagation values for mode I testing of 600 + 68 tex 5 bars textured

and 600 non-textured twill weave composites

Visual Initiation values

129

0.0

0.5

1.0

1.5

2.0

NL N

ontextured

NL Textured

5% offset

Non textured

5% offset

Textured

VIS

Non

textured

VIS

Textured

Non texturedP

rop.GIC

TexturedP

rop.GIC

GIc

(kJ/

m2)

Figure 6.25 Comparison of the mean values of G1c (visual, 5 % offset and propagation) for mode

I DCB testing of 600 + 68 tex 5 bars textured and 600 non-textured twill weave composites

It can be observed from Figure 6.24 that the value of G1c was increased with

propagation of cracking in both the textured and non-textured samples. This fact was

also evident from the mean data shown in Figure 6.25 where the mean G1c values of

crack propagation for both textured and non-textured samples were higher than the

crack initiation values. It was reported that since the G1c value depends on the

delamination crack length and laminate thickness, the initiation values of G1c are

usually much lower than the values measured during stable propagation. Also the

fracture toughness properties depend on a number of factors such as the complex

interaction of fibres, fibre geometry, resin type and properties but fibre bridging is

the dominant factor in contributing to fracture toughness of composites [ASTM D

5528-01, 2007].

The SEM micrographs for the DCB fracture surfaces of the textured and non-

textured samples are presented in Figures 6.26 and 6.27. It can be observed that in

the non-textured composites, the fracture occurred predominantly at the fibre/matrix

interface as reflected by the barse filaments and filament imprints or impressions left

by them on the fracture surfaces indicating poor interfacial bonding. Although some

Crack Initiation

Crack Propagation

130

broken filaments can be observed on the surfaces, the common phenomenon of

fractured de-bonded fibres or fibre bridging was not commonly found. However, the

examination of fractured surfaces of textured samples (Figure 6.27) show debonded

and fractured filaments and evidence of fibre bridging which results in higher

fracture toughness. It was reported that pull-out and breakage of bridged fibres are

dissipative processes that require input of work and thus result in high fracture

resistance [Suppakul and Bandyopadhyay 2002]. It can be seen from the images of

the textured samples that the filaments have lower alignment than the non-textured

samples and cross filaments can be observed which demonstrate the loopy and

bulkier yarn structure.

Figure 6.26 SEM micrographs of fracture surfaces of 600 tex twill weave non-textured

composite

131

Figure 6.27 SEM micrographs of fracture surfaces of 600 + 68 tex 5 bars twill weave textured

composite

6.8. Summary

This chapter presents the effect of the texturing process on the mechanical properties

of glass fabric composites. The composites were divided into two categories

depending upon the linear density of the constituent yarn i.e. the 300 tex and 600 tex

category. The mechanical properties were tested for both the textured and non-

textured samples and the results were compared.

The tensile properties were significantly reduced after the texturing process in both

the composites of the 300 tex (14% to 32%) and the 600 tex (10% to 25%)

categories. However, the composites of the 600 tex category showed a lower

reduction in tensile properties after texturing than the 300 tex category. This is due to

the fact that the deterioration in breaking strength of the 600 tex glass yarn after

texturing is smaller than that of the yarns of 300 tex category. The flexure properties

of the 300 tex composites were reduced (i.e. 14% to 30%) after texturing but

remained unchanged for 600 tex composites.

132

Improvement was observed in the inter-laminar shear strength (i.e. 15% to 33% in

the 300 tex and 35% to 45% in the 600 tex category) and also in the fracture

toughness Mode-1 (50%) of the composites after texturing. This is due to the fact

that the loops and bulkier structure of the textured fabric offered more contact

between the fibre and resin and enhanced fibre-matrix adhesion. The fibre bridging

due to the textured loops also resulted in higher fracture toughness for the textured

composites.

The effect of texturing air pressure on the mechanical properties was unclear. The

performance of composites was also compared on the bases of the fabric weave

structure. Twill weave has less interlacing and more float yarns in the fabric structure

as compared to the plain weave, which provided more contact surface and therefore

improved bonding between the layers.

133

7. Chapter 7 Composites with textured and non-textured core

yarns

7.1. Introduction

This chapter is concerned with some of the other types of textured composites which

can be produced in addition to those made from the core-&-effect textured yarns in

both the warp and weft directions as described in Chapter 6. The composition of

fabrics was changed by using the core textured yarns or using the non-textured core

yarn in the warp and core-&-effect yarn in the weft. These composites are

represented by “CT” and “WfW” respectively as mentioned in Section 3.4. The aim

of developing these composites especially the WfW composites was to improve

weaving performance and the tensile properties without losing too much of the

advantage of the improved inter-laminar shear strength. This chapter explains in

detail these modified composites and the effect of texturing on their mechanical

properties.

7.2. Core textured yarn composites

The core textured (CT) composites were developed using single core textured yarn.

The yarn for these composites was made by texturing only the 600 tex yarn at 5 bars

pressure. The resulting yarn had a bulkier structure but did not possess any loops

because of the absence of finer effect yarn. This was found to be a relatively simpler

texturing process in terms of yarn handling and the overfeed percentage for the yarn

was kept the same as for the other textured yarns, i.e. 2.9 %. The yarn is coded as

“CT yarn” which means core textured yarn.

Two different types of composites were prepared using the CT yarn by varying the

weft. In Type 1, the plain and twill weave fabrics were made using 600 tex CT yarn

for both the warp and weft. In Type 2, 600 tex CT yarn was used in the warp and the

core-&-effect yarn of 600 + 68 tex was used in the weft.

7.2.1. Fibre volume content of CT composites

Table 7.1 shows the density and fibre volume content of the CT composites

determined by using the immersion and calcination methods respectively. The

134

procedures were explained in detail in Section 4.3 and the following results were

obtained.

Table 7.1 Fibre volume content of CT composites

Weave Type Composite type Density

(g/cm3) CV%

Fibre Volume Content

CV%

Void content

Plain

600 non textured 1.8621 0.75 52.25 1.11 1.63 600 CT 5 bars (Type 1) 1.6960 0.56 39.91 4.75 1.27 600 CT 668 5 bars (Type 2)

1.6550 0.80 38.83 1.32 0.80

Twill

600 non textured 1.9104 0.35 54.39 0.71 0.01 600 CT 5 bars (Type 1) 1.6870 0.14 36.16 2.52 0.86 600 CT 668 5 bars (Type 2)

1.6640 0.62 36.61 1.71 1.02

Table 7.1 shows reductions in the density and fibre volume content of the CT

textured composites after texturing similar to the composites of the core-&-effect

textured yarns (Section 6.3). The number of fabric layers and the weave structure is

the same for the textured and non-textured composites. However, the bulkier

structure of the constituent textured yarn resulted in thicker textured composites with

lower fibre volume content. Table 7.1 also shows the void content of composites

below 2 % which is well within the acceptable level for critical applications [Liu et al

2006].

7.3. Mechanical properties of CT composites

The mechanical properties of these composites were determined and compared with

the properties of the non-textured composites, similar to the work done with core-&-

effect textured yarn based composites.

7.3.1. Tensile properties of 600 tex CT composites

The tensile properties of the 600 tex CT composites are shown in Figures 7.1 and

7.2.

135

Tensile strength 600 tex core textured plain & twill

050

100150200250300350400

600 NT

600 CT 5B

600 CT 668 5B

600 NT

600 CT 5B

600 CT 668 5B

600 NT

600 CT 5B

600 CT 668 5B

600 NT

600 CT 5B

600 CT 668 5B

Composite type

Ten

sile

stre

ngth

(MP

a)

Figure 7.1 Tensile strength of 600 tex CT plain & twill weave composites

Tensile modulus 600 tex core textured plain & twill

0

5000

10000

15000

20000

25000

30000

600 NT

600 CT 5B

600 CT 668 5B

600 NT

600 CT 5B

600 CT 668 5B

600 NT

600 CT 5B

600 CT 668 5B

600 NT

600 CT 5B

600 CT 668 5B

Composite type

Tens

ile m

odul

us (M

Pa)

Figure 7.2 Tensile modulus of 600 tex CT plain & twill weave composites

Similarly to the case of core-&-effect textured yarn composites, a reduction in tensile

properties was observed after the texturing process in both types of CT composites.

Tensile strength and modulus were significantly reduced in both the warp and weft

directions of the plain and twill weave structures. This is due to the fact that the

tenacity of CT yarns also decreased significantly (31.2 %) after the texturing process.

Plain Warp Plain Weft Twill Warp Twill Weft

Plain Warp Plain Weft Twill Warp Twill Weft

136

It can also be observed that there was no considerable difference in the tensile

properties of the two types of CT composites with the change of weft yarn.

7.3.2. Flexure properties of 600 tex CT composites

The flexure properties of the 600 tex CT composites are shown in Figure 7.3 and 7.4.

Flexure strength 600 tex core textured plain and twill

0

100

200

300

400

500

600 NT

600 CT 5B

600 CT 668 5B

600 NT

600 CT 5B

600 CT 668 5B

600 NT

600 CT 5B

600 CT 668 5B

600 NT

600 CT 5B

600 CT 668 5B

Composite type

Flex

ure

stre

ngth

(MPa

)

Figure 7.3 Flexure strength of 600 tex CT plain & twill weave composites

Flexure modulus 600 tex core textured plain and twill

0

4000

8000

12000

16000

20000

600 NT

600 CT 5B

600 CT 668 5B

600 NT

600 CT 5B

600 CT 668 5B

600 NT

600 CT 5B

600 CT 668 5B

600 NT

600 CT 5B

600 CT 668 5B

Composite type

Flex

ure

mod

ulus

(MPa

)

Figure 7.4 Flexure modulus of 600 tex CT plain & twill weave composites

Plain Warp Plain Weft Twill Warp Twill Weft

Plain Warp Plain Weft Twill Warp Twill Weft

137

Figures 7.3 and 7.4 show that the flexure properties of CT composites were either

similar or increased after the texturing process. It can also be observed that the

flexure properties of the twill weave composites were slightly higher than the plain

weave composites in both types of CT composites.The reason is probably due to the

fact that the twill weave has less interlacing and more float yarns which provided

more contact surface for the resin and improved the bonding between the layers.

7.3.3. ILSS of 600 tex CT plain and twill composites

The inter-laminar shear strength (ILSS) of the CT composites are shown in Figure

7.5.

Inter-laminar shear strength 600 plain & twill

05

1015202530354045

600 NT

600 5 bar CT

600 CT+668 5B

600 NT

600 5 bar CT

600 CT+668 5B

600 NT

600 5 bar CT

600 CT+668 5B

600 NT

600 5 bar CT

600 CT+668 5B

Composite type

ILSS

(MP

a)

Figure 7.5 ILSS of 600 tex CT plain & twill weave composites

It can be seen that the ILSS increased significantly after texturing in both types of the

600 tex CT composites similar to the ILSS of the core-and-effect textured yarn

composites. The bulkier structure of the CT composites provided more contact

surfaces between the fibre and resin and helped in improving the bonding between

the layers of laminated composites. However, the differences in ILSS between the

two types of CT composites were statistically insignificant.

Plain Warp Plain Weft Twill Warp Twill Weft

138

7.4. Mixed yarn composites

Mixed yarn composites were made by using the fabric which had 600 tex non-

textured yarn in the warp direction and the 600 tex based core-&-effect textured yarn

in the weft direction (WfW).

7.4.1. Fibre volume content of WfW composites

Table 7.2 shows the density and fibre volume content of the WfW composites

determined by using the immersion and calcination methods respectively.

Table 7.2 Fibre volume content of WfW composites

Weave Type Composite type Density

(g/cm3) CV%

Fibre Volume Content

CV%

Void content

Plain 600 non textured 1.8621 0.75 52.25 1.11 1.63 600 + 34 5 bars WfW 1.7356 0.42 42.82 2.13 1.32 600 + 68 5 bars WfW 1.7676 1.05 43.62 2.28 0.65

Twill 600 non textured 1.9104 0.35 54.39 0.71 0.01 600 + 34 5 bars WfW 1.7621 1.00 44.46 2.31 0.99 600 + 68 5 bars WfW 1.7987 0.08 45.79 0.46 0.21

A decrease in the density and fibre volume content of the WfW composites was

found as compared to the non-textured composites but the decrease is smaller than

between the core-and-effect textured and the CT composites. This is due to the fact

that the bulkier textured yarn was only used in the weft direction. The void content of

the WfW composites was found to be low and below the 2% level.

7.5. Mechanical properties of WfW composites

7.5.1. Tensile properties of 600 tex WfW composites

The tensile properties of the 600 tex WfW composites are shown in Figures 7.6 and

7.7.

139

Tensile Strength 600 (mixed yarn composites) plain & twill

050

100150200250300350400

600 NT

600+34 5B (W

fW)

600+68 5B (W

fW)

600 NT

600+34 5B (W

fW)

600+68 5B (W

fW)

600 NT

600+34 5B (W

fW)

600+68 5B (W

fW)

600 NT

600+34 5B (W

fW)

600+68 5B (W

fW)

Composite type

Ten

sile

stre

ngth

(MPa

)

Figure 7.6 Tensile strength of 600 tex plain & twill weave WfW composites

Tensile Modulus 600 (mixed yarn composites) plain & twill

05000

1000015000200002500030000

600 NT

600+34 5B (W

fW)

600+68 5B (W

fW)

600 NT

600+34 5B (W

fW)

600+68 5B (W

fW)

600 NT

600+34 5B (W

fW)

600+68 5B (W

fW)

600 NT

600+34 5B (W

fW)

600+68 5B (W

fW)

Composite Type

Tens

ile M

odul

us (M

Pa)

Figure 7.7 Tensile modulus of 600 tex plain & twill weave WfW composites

It can be seen that the tensile strength of the WfW composites was unchanged after

texturing especially in the warp direction. The reason for this is the presence of non-

textured glass yarn in the warp. Although the cross-sectional area of the samples was

slightly different because of the increase in thickness, it does not affect the tensile

strength. Moreover, the difference in tensile modulus was also mostly insignificant.

Plain Warp Plain Weft Twill Warp Twill Weft

Plain Warp Plain Weft Twill Warp Twill Weft

140

7.5.2. Flexure properties of 600 tex WfW composites

The flexure properties of the 600 tex WfW composites are shown in Figures 7.8 and

7.9.

Flexure Strength 600 (mixed yarn composites) Plain and Twill

0100200300400500600

600 NT

600+34 5B W

fW

600+68 5B W

fW

600 NT

600+34 5B W

fW

600+68 5B W

fW

600 NT

600+34 5B W

fW

600+68 5B W

fW

600 NT

600+34 5B W

fW

600+68 5B W

fW

Composite type

Flex

ure

Stre

ngth

(MPa

)

Figure 7.8 Flexure strength of 600 tex plain & twill weave WfW composites

Flexure Modulus 600 (mixed yarn composites) Plain and Twill

0

4000

8000

12000

16000

20000

600 NT

600+34 5B W

fW

600+68 5B W

fW

600 NT

600+34 5B W

fW

600+68 5B W

fW

600 NT

600+34 5B W

fW

600+68 5B W

fW

600 NT

600+34 5B W

fW

600+68 5B W

fW

Composite type

Flex

ure

Mod

ulus

(MPa

)

Figure 7.9 Flexure modulus of 600 tex plain & twill weave WfW composites

It can be observed from Figure 7.8 and 7.9 that the flexure properties of the WfW

composites were either similar or increased considerably. The reason is that the fibre

volume content did not decrease as much as for the core-and-effect textured

Plain Warp Plain Weft Twill Warp Twill Weft

Plain Warp Plain Weft Twill Warp Twill Weft

141

composites. It was reported that the flexure properties depend on the fibre volume

content of the composite structure along with the other parameters [Sudarisman

2008]. Moreover, the textured yarns in the weft with loops and bulk enhanced the

fibre-matrix bonding and resulted in improved flexure properties of the WfW

composites.

7.5.3. ILSS of 600 tex WfW composites

The inter-laminar shear strength (ILSS) of 600 tex WfW composites are shown in Figure 7.10.

Inter-laminar shear strength 600 (mixed yarn composites) Plain & Twill Composite

01020304050

600 NT

600+34 5B W

fW

600+68 5B W

fW

600 NT

600+34 5B W

fW

600+68 5B W

fW

600 NT

600+34 5B W

fW

600+68 5B W

fW

600 NT

600+34 5B W

fW

600+68 5B W

fW

Composite type

ILS

S (M

Pa)

Figure 7.10 ILSS of 600 tex plain & twill weave WfW composites

Figure 7.10 illustrates a significant increase in the inter-laminar shear strength (ILSS)

of the WfW composites after texturing in the warp and weft of both weave structures.

The presence of textured yarn in the weft helped in improving the bonding between

the laminates and resulted in increasing the ILSS.

The mixed yarn composites have the advantage of a relatively simpler weaving

process because the non-textured warp yarns prevent entanglements during the

change of shed. Moreover, the deterioration in the tensile properties after texuring is

less significant than the core-and-effect textured composites and the CT composites.

Plain Warp Plain Weft Twill Warp Twill Weft

142

7.6. Comparison of mechanical properties

The tensile properties of all the textured and non-textured composites of 600 tex

categories are shown in Figures 7.11 and 7.12. It can be observed that the properties

of the WfW composites were higher than the core-and-effect textured and the CT

composites. Moreover, the difference between the tensile properties before and after

texturing is also statistically insignificant. The reason for this is the presence of non-

textured yarn in the warp direction which helped in maintaining the tensile

properties.

Comparison of tensile strength 600 tex

0

100

200

300

400

600 NT

600+34 5B600+68 5B600 C

T 5B600 C

T 668 5B600+34 5B

(WfW

)600+68 5B

(WfW

)

600 NT

600+34 5B600+68 5B600 C

T 5B600 C

T 668 5B600+34 5B

(WfW

)600+68 5B

(WfW

)

600 NT

600+34 5B600+68 5B600 C

T 5B600 C

T 668 5B600+34 5B

(WfW

)600+68 5B

(WfW

)

600 NT

600+34 5B600+68 5B600 C

T 5B600 C

T 668 5B600+34 5B

(WfW

)600+68 5B

(WfW

)

Composite type

Ten

sile

stre

ngth

(MPa

)

Figure 7.11 Tensile strength of 600 tex plain & twill weave composites

Plain Warp Plain Weft Twill Warp Twill Weft

143

Comparison of tensile modulus 600 tex

0

5000

10000

15000

20000

25000

30000

600 NT

600+34 5B600+68 5B600 C

T 5B600 C

T 668 5B600+34 5B (W

fW)

600+68 5B (WfW

)

600 NT

600+34 5B600+68 5B600 C

T 5B600 C

T 668 5B600+34 5B (W

fW)

600+68 5B (WfW

)

600 NT

600+34 5B600+68 5B600 C

T 5B600 C

T 668 5B600+34 5B (W

fW)

600+68 5B (WfW

)

600 NT

600+34 5B600+68 5B600 C

T 5B600 C

T 668 5B600+34 5B (W

fW)

600+68 5B (WfW

)

Composite type

Tens

ile m

odul

us (M

Pa)

Figure 7.12 Tensile modulus of 600 tex plain & twill weave composites

Figures 7.13 and 7.14 show the flexure properties of all the textured and non-textured

composites of the 600 tex categories. It can be seen that the flexure properties of all

the textured composites are mostly similar to each other; however, slightly higher

values are observed for the WfW composites. A possible reason for this is the

relatively smaller decrease in the fibre volume content of the WfW composites after

texturing. Although the texturing process reduces the fibre volume content, the

bulkier structure of the textured yarns enhanced the fibre-matrix bonding and

maintained the flexure properties.

Comparison of flexure strength 600 tex

0

100

200

300

400

500

600

600 NT

600+34 5B600+68 5B600 C

T 5B600 C

T 668 5B600+34 5B

WfW

600+68 5B W

fW

600 NT

600+34 5B600+68 5B600 C

T 5B600 C

T 668 5B600+34 5B

WfW

600+68 5B W

fW

600 NT

600+34 5B600+68 5B600 C

T 5B600 C

T 668 5B600+34 5B

WfW

600+68 5B W

fW

600 NT

600+34 5B600+68 5B600 C

T 5B600 C

T 668 5B600+34 5B

WfW

600+68 5B W

fW

Composite type

Flex

ure

stre

ngth

(MPa

)

`

Figure 7.13 Flexure strength of 600 tex plain & twill weave composites

Plain Warp Plain Weft Twill Warp Twill Weft

Plain Warp Plain Weft Twill Warp Twill Weft

144

Comparison of flexure modulus 600 tex

0300060009000

12000150001800021000

600 NT

600+34 5B600+68 5B600 C

T 5B600 C

T 668 5B600+34 5B

WfW

600+68 5B W

fW

600 NT

600+34 5B600+68 5B600 C

T 5B600 C

T 668 5B600+34 5B

WfW

600+68 5B W

fW

600 NT

600+34 5B600+68 5B600 C

T 5B600 C

T 668 5B600+34 5B

WfW

600+68 5B W

fW

600 NT

600+34 5B600+68 5B600 C

T 5B600 C

T 668 5B600+34 5B

WfW

600+68 5B W

fW

Composite type

Flex

ure

mod

ulus

(MPa

)

Figure 7.14 Flexure modulus of 600 tex plain & twill weave composites

Figure 7.15 illustrates the inter-laminar shear strength of the textured and non-

textured composites of the 600 tex categories. It can be observed that the ILSS of all

the categories of textured composites is significantly higher than the non-textured

composites. The open, bulkier structure and the loops of the textured yarns provided

more contact surface between the fibre and the resin and improved the inter-laminar

shear strength. Moreover, the variation of ILSS among the textured composites is

insignificant.

Plain Warp Plain Weft Twill Warp Twill Weft

145

Comparison of inter-laminar shear strength 600 tex

05

101520253035404550

600 NT

600+34 5B600+68 5B600 5 bar C

T600 C

T+668 5B600+34 5B

WfW

600+68 5B W

fW

600 NT

600+34 5B600+68 5B600 5 bar C

T600 C

T+668 5B600+34 5B

WfW

600+68 5B W

fW

600 NT

600+34 5B600+68 5B600 5 bar C

T600 C

T+668 5B600+34 5B

WfW

600+68 5B W

fW

600 NT

600+34 5B600+68 5B600 5 bar C

T600 C

T+668 5B600+34 5B

WfW

600+68 5B W

fW

Composite type

ILS

S (M

Pa)

Figure 7.15 Inter-laminar shear strength of 600 tex plain & twill weave composites

The above comparison of mechanical properties reveals the advantage of the WfW

composites; They maintain the tensile and flexure properties of the composites from

non-textured yarns but have significantly higher inter-laminar shear strength than the

non-textured specimens. Moreover, the weaving of the WfW fabrics was found to be

easier without any entanglements.

The next task was to explore the possibility of the development of textured yarn

fabrics on a power loom so as to confirm that they can be produced using industrial

practice in the future. The following experiment was conducted in this respect.

7.7. Production of mixed yarn fabric on a power loom

A trial was performed to weave WfW fabric on a rapier loom. The following are the

details of the experiment.

The loom was a dornier rapier loom model PTVH4/J operated with the Staubli’s

Jacquard shedding mechanism. It had 700 tex warp yarns already installed. The 600

+ 68 tex 5 bars core-and-effect textured yarn was used in the weft and was supplied

from a cone as shown in Figure 7.16. The warp yarns also come directly from the

cones. A creel was placed behind the loom to hold the warp cones and the yarn

Plain Warp Plain Weft Twill Warp Twill Weft

146

tension was maintained by means of the roller arrangement. The loom operated at 82

picks/min and had the warp and weft density of 15 ends/cm and 12 picks/cm

respectively. The loom ran smoothly without any breakages and approximately one

metre of fabric was successfully produced.

Figure 7.16 Production of mixed yarn fabric on a power loom

Cone carrying textured yarn

147

7.8. Summary

This chapter covers the effect of texturing on the mechanical properties of 600 tex

mixed yarns (WfW) and core textured (CT) composites. The WfW composites were

made using the non-textured yarn in the warp and the core-and-effect textured yarn

in the weft. The CT composites were composed of a single textured glass yarn

without any effect yarn. The CT composites were further divided into two types:

Type 1 consisted of core textured yarns in both the warp and weft whereas Type 2

had core textured yarns in the warp and core-and-effect textured yarns in the weft.

The tensile properties of CT composites were significantly reduced after the

texturing process (16% to 30%); however the WfW composites had similar tensile

properties to the non-textured composites. The reason for this is that the textured

yarn is only used in the weft direction in the WfW composites. The flexure properties

were not changed considerably for both the CT and WfW composites. Although the

textured composites had proportionally lower fibre volume content than the non-

textured composites, the bulkier yarn structure provided better fibre-matrix bonding

and hence enhanced the transfer of load between the laminates. Improvement was

observed in the inter-laminar shear strength of both the CT (32 % to 46 %) and the

WfW composites (35 % to 49 %).

It can be concluded from the comparison of the mechanical properties of the 600 tex

composites that the WfW composites are the most advantageous; they maintained the

tensile and flexure properties but have significantly higher inter-laminar shear

strength (35 % to 49 %). Moreover, the weaving of WfW composites is also easier

without entanglements as demonstrated by weaving it successfully on a commercial

rapier loom.

148

8. Chapter 8 Conclusions and Recommendations for future

work

8.1. Conclusions

The aim of this project was to optimise the lamination properties of glass composites

by using the air jet texturing process and to minimise the problem of delamination.

Core-and-effect textured glass yarns were developed in this respect and the optimum

texturing parameters were investigated. The glass yarns were divided into two

categories depending on the core yarn linear density. These were 300 tex and 600 tex

yarns. It was found that the textured yarns in the 300 tex category with visible loops

could be produced at 3 to 5 bars air pressure, 2.9 % core yarn overfeed and 9.2 %

effect yarn overfeed. However, the weaving process became very difficult for the

yarns textured at 3 and 4 bars air pressure because of the excessive warp yarn

entanglement during the change of shed. Therefore, the yarns for the 600 tex

category were textured at 5 bars pressure only. Following are the main conclusions

of this project.

8.1.1. Tenacity of yarn after texturing

Before the development of the fabrics and composites, the effect of texturing on the

yarn tenacity was investigated. The tenacity of the core-and-effect glass yarn

decreased significantly after the texturing process in both the 300 tex and 600 tex

glass yarn categories. In 300 tex yarns the decrease in tenacity was 40 % to 60 %

whereas, the decrease in tenacity of 600 tex yarns ranged from 21 % to 24 %.

However, the air pressure did not show a significant effect on the yarn tenacity. A

higher proportion of effect filaments produced final yarns with lower tenacity as the

more textured effect-filaments contributed less to the strength than the core.

Similarly, higher core yarn linear density resulted in higher final yarn tenacity. Some

decrease in linear density after texturing was also observed. The post-texturing

stretching and to a lesser extent loss of broken filaments in the form of fibre fly

during texturing were the possible reasons for this reduction in linear density.

149

8.1.2. Tensile properties of composites

The effect of texturing on the mechanical properties of the composites was

investigated and it was observed that the tensile properties were significantly reduced

after texturing. The reduction was observed to be 14% to 32% in the 300 tex

category and 10% to 25% in the 600 tex category. The percentage decrease was

smaller for the 600 tex yarn composites due to the fact that the deterioration in

tenacity of the 600 tex glass yarns after texturing was also smaller than the 300 tex

yarns.

8.1.3. Flexure properties of composites

The flexure properties were mostly found to be unaffected by the texturing process.

Although the fibre volume content of the composites was found to be lower after the

texturing process, the introduction of loops and bulk in the yarn helped to improve

the fibre-matrix bonding and hence enhanced the transfer of load between the

laminates. It was also observed that the 600 tex yarn composites were better than the

300 tex yarn composites in maintaining the flexure properties after texturing and the

reason is the lower deterioration of 600 tex yarn after the texturing process.

8.1.4. Inter-laminar shear strength and fracture toughness of composites

The inter-laminar shear strength and the fracture toughness Mode-1 increased

significantly after the texturing process. The increase in inter-laminar shear strength

was found to be 15 % to 33 % in the 300 tex category and 35 % to 45 % in the 600

tex category. The fracture toughness (mode-1) was observed to be increased by 50 %

after texturing. This was because the loops and bulkier structure of the textured

fabric offered more contact between the fibre and resin and enhanced the fibre-

matrix adhesion. Although the textured composites had proportionally lower fibre

volume content than the non-textured composites, the bulkier yarn structure provided

better fibre-matrix bonding and hence enhanced the transfer of load between the

laminates. The data in this project did not show any clear trend for the effect of

texturing air pressure on the mechanical properties.

150

8.1.5. Weave structure

Among the plain and twill weave structures, the twill weave provided more surface

contact and therefore possessed better mechanical properties. This is because in twill

weave, there is less interlacing and more float yarns.

8.1.6. Composites with combination of textured and non-textured yarns

Composites with textured and non-textured core yarns were also produced and the

effect of texturing on their mechanical properties was determined. The fabrics were

made by using the core textured yarns in the warp and weft, or by using non-textured

core yarn in the warp and core-&-effect textured yarn in the weft. It was observed

that the composites made from fabrics having the non-textured yarn in the warp and

core-&-effect textured yarn in the weft had the best combination of mechanical

properties among all the textured composites. They maintained the tensile and

flexure properties after texturing and had significantly higher inter-laminar shear

strength, ranging from 35 % to 49 %, similar to the other textured composites.

Moreover, weaving of these fabrics was also relatively easier and was successfully

done on a Rapier Loom without any loop entanglement and yarn breakage.

Based on the results found in this project, the bonding strength of the laminated

composite structures could be improved by using air jet textured yarns. The bulkier

and loopy structure of textured reinforcement yarn can be utilised to provide more

surface contact between the fibre and resin and the problem of delamination can be

reduced provided the production parameters of both yarn and fabrics are optimised.

The best combination of mechanical properties can be achieved by using the twill

weave fabrics of mixed 600 tex textured and non-textured yarns having textured

yarns in the weft and non-textured yarns in the warp. However, the panel thickness

will be increased by about 5 % and the fibre volume content will reduced by a

similar figure.

8.2. Recommendations for future work

The glass yarns used to develop the core-and-effect textured yarns in this project

have the linear densities of 300 tex and 600 tex for core yarns and 34 tex and 68 tex

for effect yarns. Other yarn configurations i.e. linear densities, core-effect ratios and

151

their interaction with the texturing parameters could be further analysed. Moreover,

glass yarns are used in this project because of the convenience but the possibility of

texturing other high performance fibres like Carbon, Kevlar, etc and the effect of

texturing on their properties could be part of future work.

Further development of textured yarn fabric on a power loom could be carried out to

optimise parameters so as to achieve practical industrial production. Further

attention could be made to produce yarns which improve weaving efficiency.

Only composites of plain and twill weave structures have been analysed in this

research work. However, other important weave structures, especially satin weave,

could also be utilised in future and the effect of texturing on their mechanical

properties could be investigated.

The polymer matrix composites may be exposed to environments involving elevated

temperatures and humidity in certain applications. The absorption of moisture and

thermal fatigue cycles strongly affects the physical and mechanical properties of

composites [Ray 2005, Jana and Bhunia 2008]. Therefore, determination of the

effects of hygro-thermal conditioning on the mechanical performance of textured

composites could be part of future work.

Moreover, the effect of water absorption on the mechanical performance of textured

composites could also be analysed in future. This is because glass composites are

extensively employed in a wide variety of marine applications and degradation of

mechanical properties, especially in the through-the-thickness direction, which

usually happens with prolonged exposure to water [Gu and Hongxia 2008].

152

References

1. Acar, M and Versteeg, H. K. (1995) Reply to "Comments on 'Effects of

Geometry on the Flow Characteristics and Texturing Performance of Air-Jet

Texturing Nozzles'". Textile Research Journal, 65(9), pp. 556.

2. Acar, M et al. (2006) The Mechanism of the Air-Jet Texturing: The Role of

Wetting, Spin Finish and Friction in Forming and Fixing Loops. Textile

Research Journal, 76(2), pp. 116-125.

3. Acar, M. (1989) Basic principles of Air-jet Texturing and Mingling/Interlacing

Processes. In: Air-jet texturing and Mingling/Interlacing; Proceedings of the

International Conference. September 1989. Loughborough University of

Technology, pp. 01-17.

4. Acar, M. (1989) Trends in Air-jet Texturing. In: Air-jet texturing and

Mingling/Interlacing; Proceedings of the International Conference. September

1989. Loughborough University of Technology, pp. 217-226.

5. Adanur, S and Onal, L. (2001) Factors Affecting the Mechanical Properties of

Laminated Glass/Graphite-Epoxy Hybrid Composites. Journal of Industrial

Textiles, 31(2), pp. 123-133.

6. Alagirusamy, R. and Ogale, V. (2004) Commingled and Air Jet-textured

Hybrid Yarns for Thermoplastic Composites. Journal of Industrial Textiles,

33(4), pp. 223-243.

7. Alagirusamy, R. and Ogale, V. (2005) Development and Characterisation of

GF/PET, GF/Nylon, and GF/PP Commingled Yarns for Thermoplastic

Composites. Journal of Thermoplastic Composite Materials, 18, pp. 269-285.

8. Alagirusamy, R. et al. (2005) Effect of Jet Design on Commingling of

Glass/Nylon Filaments. Journal of Thermoplastic Composite Materials, 18, pp.

255-268.

9. Alagirusamy, R. et al. (2006) Hybrid Yarns and Textile Preforming for

Thermoplastic Composites. Textile Progress, 38(4), pp. 1-71.

10. Allegri, G. and Zhang, X. (2007) On the delamination and debond suppression

in structural joints by Z-fibre pinning. Composites: Part A, 38, pp. 1107–1115.

11. Arbelaiz, A. et al. (2005) Mechanical properties of flax fibre/polypropylene

composites. Influence of fibre/matrix modification and glass fibre

153

hybridization. Composites: Part A (Applied science and manufacturing), 36,

pp. 1637–1644.

12. Aslan, Z., Karakuzu, R. and Sayman, O. (2002) Dynamic Characteristics of

Laminated Woven E-Glass–Epoxy Composite Plates Subjected to Lowvelocity

Heavy Mass Impact. Journal of Composite Materials, 36(21), pp. 2421-2442.

13. ASTM (2007) D 5528-01, Standard test method for Mode I Inter-laminar

fracture toughness of unidirectional fibre-reinforced polymer matrix

composites. West Conshohocken, United States: ASTM International.

14. Baucom, J. N. and Zikry, M. A. (2005) Low-velocity impact damage

progression in woven E-glass composite systems. Composites: Part A, 36, pp.

658-664.

15. Baucom, J. N., Zikry, M. A. and Rajendran, A. M. (2006) Low-velocity impact

damage accumulation in woven S2-glass composite systems. Composites

Science and Technology, 66, pp. 1229-1238.

16. Beier, U. et al. (2008) Evaluation of preforms stitched with a low melting-

temperature thermoplastic yarn in carbon fibre-reinforced composites.

Composite Part A, 39, pp. 705-711.

17. Bernhardsson, J. and Shishoo, R. (2000) Effect of Processing Parameters on

Consolidation Quality of GF/PP Commingled Yarn Based Composites. Journal

of Thermoplastic Composite Materials, 13, pp. 292-313.

18. Bilgin, S., Versteeg, H. K. and Acar, M. (1996) Effect of Nozzle Geometry on

Air-Jet Texturing Performance. Textile Research Journal, 66(2), pp. 83-90.

19. British Standards Institute (1977) 2782-10: Methods of testing Plastics —Part

10: Glass reinforced plastics — Method 1003: Determination of tensile

properties. London: British Standards Institute.

20. British Standards Institute (1998) 14125: Fibre-reinforced plastic composites

—Determination of flexural properties. London: British Standards Institute.

21. British Standards Institute (1998) 14130: Fibre-reinforced plastic composites

— Determination of apparent inter-laminar shear strength by short-beam

method. London: British Standards Institute.

22. British Standards Institute (1999) 1172: Textile-glass-reinforced plastics —

Prepregs, moulding compounds and laminates — Determination of the textile-

glass and mineral-filler content — Calcination methods. London: British

Standards Institute.

154

23. British Standards Institute (2000) 3341: Textile glass—Yarns—Determination

of breaking force and breaking elongation. London: British Standards Institute.

24. British Standards Institute (2004) 1183-1: Plastics - Methods for determining

the density of non-cellular plastics - Part 1: Immersion method, liquid

pyknometer method and titration method. London: British Standards Institute.

25. British Standards Institute (2008) 291: Plastics — Standard atmospheres for

conditioning and testing. London: British Standards Institute.

26. Broughton, W.R. (2000) Through-thickness testing. In: Hodgkinson, J.M. (ed.)

Mechanical testing of advanced fibre composites. Cambridge, England:

Woodhead Publishing Limited.

27. Cartie, D. D. R., Cox, B. N. and Fleck, N. A. (2004) Mechanisms of crack

bridging by composite and metallic rods. Composites: Part A, 35, pp. 1325–

1336.

28. Chimeh, M. Y. et al. (2005) Characterizing bulkiness and hairiness of air-jet

textured yarn using imaging techniques. Journal of the Textile Institute, 96(4),

pp. 251-255.

29. Courtaulds Ltd., Yarn Processing, Patent BP1554572, 24th October 1979.

30. Cripps, D., Searle, T. J. and Summerscales, J. (2000) Open Mold Techniques

for Thermoset Composites. In: Kelly, A. and Zweben, C. (eds.) Comprehensive

Composite Materials. Vol 2. Elsevier Science Ltd.

31. Demir, A. and Behery, H. M. (1997) Synthetic filament yarn: texturing

technology. Upper Saddle River, NJ : Prentice Hall.

32. Demir, A. and Wray, G. R. (1989) Evaluation of Air-jet Texturing Process – A

Historical Review. In: Air-jet texturing and Mingling/Interlacing; Proceedings

of the International Conference. September 1989. Loughborough University of

Technology, pp. 19-37.

33. Deng, S. and Ye, L. (1999) Influence of fibre-Matrix Adhesion on Mechanical

Properties of Graphite/Epoxy Composites: II. Inter-laminar fracture and In-

plane shear behaviour. Journal of Reinforced Plastics and Composites, 18 (11),

pp. 1041-1057.

34. Du Pont de Nemours & Co. Inc, Bulky Continuous Filament Yarn, Patent

US2783609, 5th March 1957.

155

35. Du Pont de Nemours & Co. Inc, Bulky Yarns and Methods and Apparatus for

the Production Thereof, Patent GB732929, 15th December 1952.

36. Du Pont de Nemours & Co. Inc, Improvement in Apparatus for Production of

Voluminous Yarns and Products Produced Thereby, Patent GB893020, 20th

May 1960.

37. Du Pont de Nemours & Co. Inc, Method and Apparatus for Texturing Yarn,

Patent GB1448100, 2nd September 1976.

38. Du Pont de Nemours & Co. Inc, New Devices for Texturing Yarn, Patent

GB1282148, 19th July 1972.

39. Du Pont de Nemours & Co. Inc, Yarn Texturing Jet, Patent US4259768, 7th

April 1981.

40. Du Pont de Nemours & Co. Inc, Yarn Treating Apparatus, Patent GB762630,

19th July 1954.

41. Du Pont de Nemours & Co. Inc, Yarn Treating Apparatus, Patent US2958112,

1st November 1960.

42. Du Pont de Nemours & Co. Inc, Yarn Treating Apparatus, Patent US3545057,

8th December 1970.

43. Ebeling, T. et al (1997) Delamination Failure of a Woven Glass Fibre

Composite. Journal of Composite Materials, 31(13), pp. 1318-1333.

44. Enterprises Machine and Development Corporation, Resiliently supported

Baffle for Yarn Texturing Air-jet, Patent US3979805, 14th September 1976.

45. Enterprises Machine and Development Corporation, Resonant Baffle for Yarn

Texturing Air-jet, Patent US3881232, 6th May 1975.

46. Enterprises Machine and Development Corporation, Yarn Texturing Air-jet

with Cylindrical and Planar Baffles, Patent US4187593, 12th February 1980.

47. Godwin, E.W. (2000) Tension. In: Hodgkinson, J.M. (ed.) Mechanical testing

of advanced fibre composites. Cambridge, England: Woodhead Publishing

Limited.

48. Golzar, M., Brunig, H. and Mader, E. (2007) Commingled Hybrid Yarn

Diameter Ratio in Continuous Fibre-reinforced Thermoplastic Composites.

Journal of Thermoplastic Composite Materials, 20, pp. 17-26.

49. Gu, H. and Hongxia, S. (2008) Delamination behaviour of glass/polyester

composites after water absorption. Materials and Design, 29, pp. 262-264.

156

50. Gweon, S. Y. and Bascom, W. D. (1992) Damage in carbon fibre composites

due to repetitive low-velocity impact loads. Journal of Materials Science 27,

pp. 2035-2047.

51. Haque, A. and Hossain, M. K. (2003) Effects of Moisture and Temperature on

High Strain Rate Behavior of S2-Glass–Vinyl Ester Woven Composites.

Journal of Composite Materials, 37, pp. 627-647.

52. Hearle, J. W. S., Hollick, L and Wilson, D. K. (2001) Yarn Texturing

Technology. Woodhead publishing limited in association with The Textile

Institute.

53. Heberlein Guide (1991) “Air Texturing”. Position within the Production

Process of Textiles. 327/01. pp. 4.1-4.3.

54. Herath, C. N. et al. (2007) An Analysis on the Tensile Strength of Hybridized

Reinforcement Filament Yarns by Commingling Process. Materials Science

Forum, 539-543, pp. 974-978.

55. Hodgkinson, J.M. (2000) Flexure. In: Hodgkinson, J.M. (ed.) Mechanical

testing of advanced fibre composites. Cambridge, England: Woodhead

Publishing Limited.

56. Jana, R. N. and Bhunia, H. (2008) Hygrothermal Degradation of the Composite

Laminates From Woven Carbon/SC-15 Epoxy Resin and Woven Glass/SC-15

Epoxy Resin. Polymer Composites, pp. 664-669.

57. Jang, B. Z. et al. (1989) Impact Resistance and Energy Absorption Mechanisms

in Hybrid Composites. Composites Science and Technology, 34, pp. 305-335.

58. Kang, B. C. et al. (2007) Microscopic Evaluation of Commingling-Hybrid

Yarns. Materials Science Forum, 539-543, pp. 992-996.

59. Kang, T. J. and Lee, S. H. (1994) Effect of Stitching on the Mechanical and

Impact Properties of Woven Laminate Composite. Journal of Composite

Materials, 28(16), pp. 1574-1587.

60. Khan, L.A. (2010) Cure Optimization of 977-2A Carbon/epoxy Composites for

Quickstep Processing. Thesis (PhD), University of Manchester.

61. Kim, J. K. and Sham, M. L. (2000) Impact and delamination failure of woven-

fabric composites. Composites Science and Technology, 60, pp. 745-761.

62. Koc, S. K., Hockenberger, A. S. and Wei, Q. (2008) Effect of air-jet texturing

on adhesion behaviour of polyester yarns to rubber. Applied Surface Science,

254, pp. 7049-7055.

157

63. Kotaki, M. and Hamada, H (1997) Effect of interfacial properties and weave

structure on mode I interlaminar fracture behaviour of glass satin woven fabric

composites. Composites: Part A, 28A, pp. 257-266.

64. Kothari, V. K., Sengupta, A. K. and Rengasamy, R. S. (1991) Role of Water in

Air-Jet Texturing: Part I: Polyester Filament Feeder Yarns with Different

Frictional Characteristics. Textile Research Journal, 61(9), pp. 495-502.

65. Kumar, R. L. V. et al (2007) Post Impact Compression Strength (PICS)

Evaluation of Glass/Epoxy Composite Using a Novel Approach. Effect of

Delamination Area. Journal of Reinforced Plastics and Composites, 26(11),

pp. 1101-1109.

66. Langston, T. B, (2003). The mechanical behaviour of air textured aramid yarns

in thermoset. Thesis (MSc), NCSU.

67. Liu, L. et al (2006) Effects of cure cycles on void content and mechanical

properties of composite laminates. Composite Structures, 73, pp. 303-309.

68. Ma, H. Y., Sui, Q. X. and Chou, T. W. (2003) Absorption and Permeability of

Air-Jet Textured Glass Fiber Yarn and its Fabric for Resin. Journal of Donghua

University, 20(2), pp. 122-124.

69. Maycumber, S. G. (1997) DuPont Taslan technology, trademark assigned to

Heberlein In Daily News Record, HighBeam Research Available from:

www.highbeam.com.

70. Mazumdar, S. K. (2002) Composites Manufacturing. Materials, Product, and

Process Engineering. CRC Press.

71. Mcdonnell, P. et al. (2001) Processing and mechanical properties evaluation of

a commingled carbon-fibre PA-12 composites. Composites: Part A, 32, pp.

925-932.

72. Miao, M. and Soong, M. C. C. (1995) Air Interlaced Yarn Structure and

Properties. Textile Research Journal, 65(8), pp. 433-440.

73. Mouritz, A. P. (2003) Comment on the impact damage tolerance of stitched

composites. Journal of Materials Science Letters, 22, pp. 519-521.

74. Mouritz, A. P. (2004) Fracture and tensile fatigue properties of stitched

fibreglass composites. Proc. Instn Mech. Engrs Part L: J. Materials: Design

and Applications (Special Issue Paper), 218, pp. 87-93.

75. Mouritz, A. P. (2007) Review of z-pinned composite laminates. Composites:

Part A, 2007. 38, pp. 2383–2397.

158

76. Mouritz, A. P., Gallagher, J. and Goodwin, A. A. (1996) Flexural strength and

interlaminar shear strength of stitched GRP laminates following repeated

impacts. Composite Science and technology, 57, pp. 509-522.

77. Mouritz, A. P., Leong, K. H, and Herszberg, I. (1997) A review of the effect of

stitching on the in-plane mechanical properties of fibre-reinforced polymer

composites. Composite Part A, 28A, pp. 979-991.

78. Naganuma, T. et al. (2009) Influence of prepreg conditions on the void

occurrence and tensile properties of woven glass fiber-reinforced polyimide

composites. Composites Science and Technology, 69, pp. 2428–2433.

79. Nei, J. et al. (2008) Effect of stitch spacing on mechanical properties of

carbon/silicon carbide composites. Composites Science and Technology, 68, pp.

2425–2432.

80. Oerlikon textile components (2004) Heberlein HemaJet-Jet Cores, Heberlein,

Available from: www.components.oerlikontextile.com.

81. Oerlikon textile components (2007) Leaflet - HemaJet Jet Cores Series 2,

Heberlein, www.components.oerlikontextile.com.

82. Oerlikon textile components (2007) Leaflet - HemaJet-LB24, Heberlein,

www.components.oerlikontextile.com.

83. Oerlikon textile components (2009) Leaflet - HemaJet-LB04, Heberlein,

www.components.oerlikontextile.com.

84. Oerlikon textile components (2010) Leaflet - HemaJet-EO52, Heberlein,

www.components.oerlikontextile.com.

85. Ogale, V. and Alagirusamy, R. (2007) Tensile properties of GF-polyester, GF-

nylon, and GF-polypropylene commingled yarns. Journal of the Textile

Institute, 98(1), pp. 37-45.

86. Operating Manual (2001) Edition, Operating Manual for Air-Jet Texturing

Machine RMT-D. Stähle SSM Switzerland.

87. Paiva, J.M.F.d., Mayer, S and Rezende, M.C. (2005) Evaluation of mechanical

properties of four different carbon/epoxy composites used in aeronautical field.

Materials Research, 8 (1), pp. 91-97.

88. Parlapalli, M. R. et al. (2007) Experimental investigation of delamination

buckling of stitched composite laminates. Composites: Part A, 38, pp. 2024–

2033.

159

89. Partridge, I. K. and Cartie, D. D. R. (2005) Delamination resistant laminates by

Z-Fiber pinning: Part I manufacture and fracture performance. Composites:

Part A, 36, pp. 55–64.

90. Pavier, M. J. and Clarke, M. P. (1995) Experimental techniques for the

investigation of the effects of impact damage on carbon fibre composites.

Composites Science and Technology, 55, pp. 157-169.

91. Pekbey, Y. and Sayman, O. (2006) A Numerical and Experimental

Investigation of Critical Buckling Load of Rectangular Laminated Composite

Plates with Strip Delamination. Journal of Reinforced Plastics and Composites,

25, pp. 685-697.

92. Penn, L. S. and Wang, H. (1998) Epoxy resins. In: Peters, S.T. (ed.) Handbook

of Composites, second edition. London, England: Chapman & Hall.

93. Peters, S. T. (1998) Introduction, Composite basics and Roadmap. In: Peters,

S.T. (ed.) Handbook of Composites, second edition. London, England:

Chapman & Hall.

94. Ray, B. C. (2005) Thermal Shock and Thermal Fatigue on Delamination of

Glass-fiber-reinforced Polymeric Composites. Journal of Reinforced Plastics

and Composites, 24(01), pp. 111-116.

95. Reinhart, T. J. (1998) Overview of Composite. In: Peters, S.T. (ed.) Handbook

of Composites, second edition. London, England: Chapman & Hall.

96. Rengasamy, R. S., Kothari, V. K. and Patnaik, A. (2004) Effect of Process

Variables and Feeder Yarn Properties on the Properties of Core-and-Effect and

Normal Air-Jet Textured Yarns. Textile Research Journal, 74(3), pp. 259-264.

97. Robinson, P. and Hodgkinson, J.M. (2000) Interlaminar fracture toughness. In:

Hodgkinson, J.M. (ed.) Mechanical testing of advanced fibre composites.

Cambridge, England: Woodhead Publishing Limited.

98. Rwei, S. P. and Pai, H. I. (2002) Fluid Simulation of the Airflow in Texturing

Jets. Textile Research Journal, 72, pp. 520-525.

99. Sengupta, A. K., Kothari, V. K. and Jsensarma, J. K. (1996) Effects of Filament

Modulus and Linear Density on the Properties of Air-Jet Textured Yarns.

Textile Research Journal, 66(7), pp. 452-455.

100. Sengupta, A. K., Kothari, V. K and Srinivasan, J. (1991) Effect of Process

Variables in Air-Jet Texturing on the Properties of Spun Yarns with Different

Structures. Textile Research Journal, 61(12), p. 729-735.

160

101. Sims, G. D. and Broughton, W. R. (2000) Glass Fibre Reinforced Plastics—

Properties In: Kelly, A. and Zweben, C. (eds.) Comprehensive Composite

Materials. Elsevier Ltd.

102. Sudarisman and Davies, I. J. (2008) The effect of processing parameters on

the flexural properties of unidirectional carbon fibre-reinforced polymer

(CFRP) composites. Materials Science and Engineering A, 498, pp. 65-68.

103. Summerscales, J. (2010) A taxonomy for resin infusion processes, Marine

and Offshore Composites, The Royal Institution of Naval Architects.

104. Suppakul, P. and Bandyopadhyay, S. (2002) The effect of weave pattern on

the mode-I inter-laminar fracture energy of E-glass/vinyl easter composites.

Composite Science and Technology, 62 (5), pp. 709-717.

105. Sutherland, L. S. and Soares, C. G. (2004) Effect of laminate thickness and of

matrix resin on the impact of low fibre-volume, woven roving E-glass

composites. Composites Science and Technology, 64, pp. 1691–1700.

106. Tanoglu, M. et al. (2001) Effects of thermoplastic preforming binder on the

properties of S2-glass fabric reinforced epoxy composites. International

Journal of Adhesion and Adhesives, 21 (3), pp. 187-195.

107. Varma, I. K. and Gupta, V. B. (2000) thermosetting resin - properties. In:

Kelly, A. and Zweben, C. (eds.) Comprehensive composite materials. Vol 2.

Elsevier Science Ltd.

108. Vaughan, D. J. (1998) Fiberglass reinforcements. In: Peters, S.T. (ed.)

Handbook of Composites, second edition. London, England: Chapman & Hall.

109. Velmurugan, R. and Solaimurugan, S. (2007) Improvements in Mode I

interlaminar fracture toughness and in-plane mechanical properties of stitched

glass/polyester composites. Composites Science and Technology, 67, pp. 61-69.

110. Versteeg, H. K., Acar, M. and Bilgin, S. (1999) Effect of Geometry on the

Performance of Intermingling Nozzles. Textile Research Journal, 69(8), pp.

545-551.

111. Wickramasinghe, G. L. D. (2003) Steam-jet Intermingled Sewing Threads.

Thesis (PhD), UMIST.

112. Xiao, J. R., Gama, B. A. and Gillespie Jr., J. W. (2007) Progressive damage

and delamination in plain weave S-2 glass/SC-15 composites under quasi-static

punch-shear loading. Composite Structures, 78, pp. 182-196.

161

113. Yoshimura, A. et al. (2008) Improvement on out-of-plane impact resistance

of CFRP laminates due to through-the-thickness stitching. Composites: Part A,

39, pp. 1370–1379.

114. Zaixia, F. et al. (2006) Investigation on the Tensile Properties of Knitted

Fabric Reinforced Composites made from GF–PP Commingled Yarn Preforms

with Different Loop Densities. Journal of Thermoplastic Composite Materials,

19, pp. 113 – 126.

115. Zhang, X., Hounslow, L. and Grassi, M. (2006) Improvement of low-velocity

impact and compression-after-impact performance by z-fibre pinning.

Composites Science and Technology, 66, pp. 2785–2794.

116. Zhou, G. and Davies, G. A. O. (1995) Characterisation of thick glass woven

roving/polyester laminates: 1. Tension, compression and shear. Composites,

26(8), pp. 579-586.

162

Appendix A: Calculations for draw ratio and overfeed

The parameters i.e. draw ratios and the overfeed percentage of both the core and

effect yarns; can be calculated with the help of the diameters of the rollers and the

gear teeth ratios. The calculations shown in Appendix A were based on Figure A.1

which illustrates the system of gears of the air-jet texturing machine.

Figure A.1 Gearing diagram of Stähle RMT-D air-jet texturing machine

Where,

Z1 to Z16 are gears,

Z7 is the changeable gear for the core yarn draw ratio,

163

Z13 is the changeable gear for the effect yarn draw ratio,

Z5 is the changeable gear for the core yarn overfeed percentage,

Z11 is the changeable gear for the effect yarn overfeed percentage.

Draw ratio for core yarn (DRc)

The draw ratio of the core yarn depends on the selection of the variable gear Z7

while assuming all the other parameters as constant. The equation is as follows;

67

43

GG

DDDRc (A.1)

Where,

D4 is the diameter of the core yarn input roller 4 (Figure A.1);

D3 is the diameter of the core yarn feed roller 3 (Figure A.1);

G6 and G7 are the number of teeth of gears Z6 and Z7 respectively.

By keeping D4 = 80 mm, D3 = 126 mm and G6 = 28 constant and by varying the

teeth of gear Z7 (G7) the following draw ratios for the core yarn can be obtained.

Table A.1 Core yarn draw ratios

No. of Teeth (G7) DRc

40 2.25

39 2.19

38 2.14

37 2.08

36 2.03

Draw ratio for effect yarn (DRe)

The draw ratio of the effect yarn depends on the selection of the variable gear Z13

assuming all the other parameters as constant. The equation is as follows;

1213

68Re

GG

DDD (A.1)

Where,

D6 is the diameter of the effect yarn input roller 6 (Figure A.1);

D8 is the diameter of the effect yarn feed roller 8 (Figure A.1);

G12 and G13 are the number of teeth of gears Z12 and Z13 respectively.

164

By keeping D6 = 80 mm, D8 = 126 mm and G12 = 33 constant and by varying the

teeth of gear Z13 (G13), the following draw ratios for the effect yarn can be

obtained.

Table A.2 Effect yarn draw ratios

No. of Teeth (G13) DRe

40 1.91

39 1.86

38 1.81

37 1.76

36 1.72

Overfeed percentage for core yarn (OFc)

Core yarn overfeed is the positive difference in the speed between the core yarn

entering the jet and the textured yarn coming out of the jet. The core yarn overfeed

percentage derived from the gearing arrangement is as follows;

100154

31

1416

125

GG

GG

GG

DDOFc (A.3)

Where,

D5 is the diameter of the core yarn feed roller 5 (Figure A.3);

D12 is the diameter of the textured yarn delivery roller 12 (Figure A.3);

G1, G3, G4, G5, G14 and G16 are the number of teeth of gears Z1, Z3, Z4,

Z5, Z14 and Z16 respectively.

The parameters taken as constant for the above equation were D5 = 124.5 mm, D12

= 126 mm, G16 = 40, G14 = 40, G1 = 40, G4 = 100 and G5 = 96 and the number of

teeth (G3) of the variable gear Z3 can be changed to achieve following values of the

core yarn overfeed.

165

Table A.3 Core yarn overfeed percentage

No. of Teeth (G3) OFc

40 2.9

39 5.5

38 8.3

37 11.3

36 14.4

Overfeed percentage for effect yarn (OFe)

Effect yarn overfeed is also calculated through the gearing arrangement as per the

calculation of overfeed for the core yarn and following equation was established.

10011110

21

1416

128

GG

GG

GG

DDOFe (A.4)

Where,

D8 is the diameter of the effect yarn feed roller 8 (Figure A.1);

D12 is the diameter of the textured yarn delivery roller 12 (Figure A.1);

G1, G2, G10, G11, G14 and G16 are the number of teeth of gears Z1, Z2,

Z10, Z11, Z14 and Z16 respectively.

The parameters taken as constant for the above equation were D8 = 126 mm, D12 =

126 mm, G16 = 40, G14 = 40, G1 = 40, G10 = 40 and G11 = 29 and the number of

teeth (G2) of the variable gear Z2 can be changed to achieve the following values of

effect yarn overfeed.

Table A.4 Effect yarn overfeed percentage

No. of Teeth (G2) OFe

40 37.9

39 41.5

38 45.2

37 49.1

36 53.2

166

Appendix B: Mechanical properties Note: (S) means statistically significant and (NS) means not significant Table B.1 Tenacity of textured and non-textured glass yarns of 300 tex category

Yarn type Tenacity

(cN/tex) S.D C.V %

confidence

95%

%

Reduction

Significance

300 NT 47.76 3.82 8.00 2.36

300+34 3B 25.33 3.99 15.73 2.47 46.96 < 0.05 (S)

300+34 4B 27.67 1.90 6.88 1.18 42.06 < 0.05 (S)

300+34 5B 28.13 4.74 16.84 2.94 41.10 < 0.05 (S)

300+68 3B 21.73 3.52 16.20 2.18 54.50 < 0.05 (S)

300+68 4B 18.11 4.00 22.10 2.48 62.08 < 0.05 (S)

300+68 5B 20.98 5.29 25.24 3.28 56.07 < 0.05 (S)

Table B.2 Tenacity of textured and non-textured glass yarns of 600 tex category

Yarn type Tenacity

(cN/tex) S.D C.V %

confidence

95%

%

Reduction

Significance

600 NT 40.81 4.80 11.76 2.97

600+34 5B 31.14 4.05 13.01 2.51 23.70 < 0.05 (S)

600+68 5B 32.30 5.97 18.49 3.70 20.85 < 0.05 (S)

Table B.3 Tenacity of non-textured feed yarns

Yarn type Tenacity

(cN/tex) S.D C.V %

confidence

95%

%

change

Significance

300 core 47.76 3.82 8.00 2.36

334 47.82 4.95 10.34 3.07 0.13 0.49 (NS)

368 49.82 4.34 8.71 2.69 4.13 0.14 (NS)

600 core 40.81 4.80 11.76 2.97

634 42.88 3.27 7.62 2.03 4.83 0.14 (NS)

668 42.54 4.75 11.16 2.94 4.07 0.21 (NS)

34 effect 55.02 5.15 9.36 3.20

68 effect 65.68 6.42 9.77 3.98

167

Table B.4 Variation in linear density (tex) of textured glass yarns

Yarn type Linear Density (Tex) S.D. CV

%

Confidence

95 %

%

reduction

346 nominal L.D

300+34 (3 Bar) 331.6 1.14 0.34 1.00 4.11

300+34 (4 Bar) 330.2 2.49 0.75 2.18 4.52

300+34 (5 Bar) 326.8 3.90 1.19 3.42 5.50

383 nominal L.D

300+68 (3 Bar) 364.8 5.59 1.53 4.90 4.74

300+68 (4 Bar) 364.4 1.95 0.53 1.71 4.85

300+68 (5 Bar) 362.6 3.44 0.95 3.01 5.32

655 nominal L.D

600+34 (5 Bar) 621.4 3.29 0.53 2.88 5.06

692 nominal L.D

600+68 (5 Bar) 661 2.46 0.37 2.16 4.45

Table B.5 Tensile strength of 300 tex Plain weave composites

Weave/

direction

Composite

type

Tensile

strength

(MPa)

Std-

dev

C.V

%

95% Conf.

Interval

%

change Signif.

Plain

Warp

300 NT 360.70 21.34 5.90 18.70

300+34 3B 243.30 25.35 10.42 22.22 -32.55 <0.05

(S)

300+34 4B 253.30 20.77 8.20 18.21 -29.77 <0.05

(S)

300+34 5B 288.91 13.13 4.54 11.51 -19.90 <0.05

(S)

300+68 4B 266.94 12.82 4.80 11.23 -25.99 <0.05

(S)

300+68 5B 251.61 20.80 8.26 18.22 -30.24 <0.05

(S)

Plain

Weft

300 NT 279.00 17.54 6.29 15.37

300+34 3B 264.34 18.73 7.09 16.42 -5.26 0.12

168

(NS)

300+34 4B 207.51 14.19 6.84 12.44 -25.62 <0.05

(S)

300+34 5B 240.82 10.39 4.32 9.11 -13.68 <0.05

(S)

300+68 4B 208.01 11.44 5.50 11.21 -25.44 <0.05

(S)

300+68 5B 200.01 22.66 11.33 22.21 -28.31 <0.05

(S)

Table B.6 Tensile modulus of 300 tex Plain weave composites

Weave/

direction Comp. type

Tensile

modulus

(MPa)

Std-dev C.V %

95%

Conf.

Interval

% change Signif.

Plain

Warp

300 NT 21470.00 3684.00 17.16 3229.40

300+34 3B 17140.00 1320.23 7.70 1157.21 -20.17 <0.05

(S)

300+34 4B 19800.00 738.24 3.73 647.09 -7.78 0.19

(NS)

300+34 5B 18311.00 1314.00 7.18 1151.72 -14.71 0.065

(NS)

300+68 4B 16100.00 4984.70 30.96 4369.19 -25.01 0.07

(NS)

300+68 5B 16680.00 1042.59 6.25 913.85 -22.31 <0.05

(S)

Plain

Weft

300 NT 19585.00 1839.40 9.39 1612.30

300+34 3B 18520.00 813.63 4.39 713.17 -5.44 0.14

(NS)

300+34 4B 18980.00 4727.26 24.91 4143.55 -3.09 0.4

(NS)

300+34 5B 15665.20 1154.93 7.37 1012.32 -20.01 <0.05

(S)

169

300+68 4B 14845.00 656.03 4.42 642.90 -24.20 <0.05

(S)

300+68 5B 17575.00 3287.73 18.70 3221.92 -10.26 0.17

(NS)

Table B.7 Tensile strength of 300 tex Twill weave composites

Weave/

direction Comp. type

Tensile

strength

(MPa)

Std-

dev

C.V

%

95%

Conf.

Interval

%

change Signif.

Twill

Warp

300 NT 341.53 40.87 11.96 35.83

300+34 3B 240.67 8.54 3.55 7.49 -29.53 <0.05 (S)

300+34 5B 255.98 19.04 7.44 16.69 -25.05 <0.05 (S)

300+68 4B 268.50 23.80 8.86 20.86 -21.38 <0.05 (S)

300+68 5B 258.52 9.84 3.80 8.62 -24.31 <0.05 (S)

Twill

Weft

300 NT 318.98 23.86 7.48 20.92

300+34 3B 272.54 22.21 8.15 19.46 -14.56 <0.05 (S)

300+34 5B 265.40 16.45 6.20 14.42 -16.80 <0.05 (S)

300+68 4B 248.57 11.52 4.63 10.10 -22.07 <0.05 (S)

300+68 5B 242.68 12.87 5.31 11.29 -23.92 <0.05 (S)

Table B.8 Tensile modulus of 300 tex Twill weave composites

Weave/

direction Comp. Type

Tensile

modulus

(MPa)

Std-dev C.V

%

95%

Conf.

Interval

%

change Signif.

Twill

Warp

300 NT 21987.80 1169.92 5.32 1025.46

300+34 3B 14928.20 818.48 5.48 717.42 -32.11 <0.05 (S)

300+34 5B 16020.75 400.54 2.50 392.52 -27.14 <0.05 (S)

300+68 4B 17242.00 917.95 5.32 899.57 -21.58 <0.05 (S)

170

300+68 5B 16340.00 716.24 4.38 627.80 -25.69 <0.05 (S)

Twill

Weft

300 NT 20314.80 509.27 2.51 446.39

300+34 3B 16576.20 922.24 5.56 808.36 -18.40 <0.05 (S)

300+34 5B 16388.40 997.68 6.08 874.50 -19.33 <0.05 (S)

300+68 4B 15522.00 766.80 4.94 672.12 -23.59 <0.05 (S)

300+68 5B 17020.00 630.07 3.70 552.28 -16.22 <0.05 (S)

Table B.9 Flexure strength of 300 tex plain weave composites

Weave/

direction Composite type

Flex str

(MPa)

Std-

dev C.V %

95%

Conf.

Interval

%

change Signif.

Plain

Warp

300 NT 419.78 37.49 8.93 32.86

300+34 3B 305.68 18.43 6.03 16.15 -27.18 <0.05 (S)

300+34 4B 307.16 31.72 10.33 27.80 -26.83 <0.05 (S)

300+34 5B 431.80 6.72 1.56 5.89 2.86 0.26

(NS)

300+68 4B 348.63 52.15 14.96 45.72 -16.95 <0.05 (S)

300+68 5B 268.00 18.90 7.05 16.56 -36.16 <0.05 (S)

Plain

Weft

300 NT 388.34 11.91 3.07 10.44

300+34 3B 277.27 44.75 16.14 39.22 -28.60 <0.05 (S)

300+34 4B 247.00 33.54 13.57 29.39 -36.40 <0.05 (S)

300+34 5B 337.00 26.26 7.80 23.02 -13.22 <0.05 (S)

300+68 4B 301.57 43.09 14.29 37.76 -22.34 <0.05 (S)

300+68 5B 255.00 33.04 12.96 29.00 -34.34 <0.05 (S)

171

Table B.10 Flexure modulus of 300 tex plain weave composites

Weave/

direction

Composite

type

Flexure

Modulus

(Mpa)

Std-dev C.V %

95%

Conf.

Interval

%

change Signif.

Plain

Warp

300 NT 11883.12 2309.51 19.44 2024.33

300+34 3B 7859.14 768.53 9.78 673.63 -33.86 <0.05

(S)

300+34 4B 9728.62 1034.12 10.63 906.43 -18.13 0.05

(NS)

300+34 5B 11318.60 1654.90 14.62 1450.54 -4.75 0.34

(NS)

300+68 4B 10517.62 1372.13 13.05 1202.70 -11.49 0.15

(NS)

300+68 5B 9803.55 1059.16 10.80 928.38 -17.50 0.06

(NS)

Plain

Weft

300 NT 11184.33 474.03 4.24 415.50

300+34 3B 7896.82 1800.48 22.80 1578.16 -29.39 <0.05

(S)

300+34 4B 8430.30 765.38 9.10 670.87 -24.62 <0.05

(S)

300+34 5B 7697.96 417.10 5.42 365.60 -31.17 <0.05

(S)

300+68 4B 9015.93 1411.83 15.66 1237.50 -19.39 <0.05

(S)

300+68 5B 8324.44 1057.01 12.70 926.50 -25.57 <0.05

(S)

Table B.11 Flexure strength of 300 tex twill weave composites

Weave/

direction

Composite

type

Flex str

(MPa)

Std-

dev C.V %

95%

Conf.

Interval

%

change Signif.

Twill

Warp

300 NT 428.37 24.48 5.72 21.46

300+34 3B 358.14 27.01 7.54 23.67 -16.39 <0.05 (S)

172

300+34 4B 407.50 38.20 9.37 33.49 -4.87 0.17 (NS)

300+34 5B 353.83 25.44 7.19 22.30 -17.40 <0.05 (S)

300+68 3B 337.74 17.41 5.16 15.26 -21.16 <0.05 (S)

300+68 4B 375.12 18.47 4.92 16.19 -12.43 <0.05 (S)

300+68 5B 365.36 24.18 6.62 21.20 -14.71 <0.05 (S)

Twill

Weft

300 NT 412.65 36.83 8.93 32.28

300+34 3B 385.03 28.12 7.30 24.65 -6.69 0.11 (NS)

300+34 4B 381.13 58.88 15.45 51.61 -7.64 0.17 (NS)

300+34 5B 369.79 21.82 5.90 19.13 -10.39 <0.05 (S)

300+68 3B 365.92 25.06 6.85 21.97 -11.32 <0.05 (S)

300+68 4B 373.52 33.60 8.99 29.45 -9.48 0.06 (NS)

300+68 5B 350.90 17.15 4.89 15.03 -14.96 <0.05 (S)

Table B.12 Flexure modulus of 300 tex twill weave composites

Weave/

direction

Composite

type

Flexure

Modulus

(MPa)

Std-dev C.V %

95%

Conf.

Interval

%

change Signif.

Twill

Warp

300 NT 11262.20 1137.9 10.10 997.40

300+34 3B 11537.03 985.57 8.54 863.87 2.44 0.35

(NS)

300+34 4B 10967.55 1546.6 14.10 1355.6

6 -2.62

0.37

(NS)

300+34 5B 12151.40 789.20 6.50 691.72 7.90 0.1 (NS)

300+68 3B 11459.64 415.43 3.63 364.13 1.75 0.37

(NS)

300+68 4B 11988.10 1061.1 8.85 930.14 6.45 0.16

(NS)

300+68 5B 11857.09 825.10 6.96 723.21 5.28 0.19

173

(NS)

Twill

Weft

300 NT 12445.70 1657.9 13.32 1453.2

300+34 3B 10993.80 735.94 6.70 645.07 -11.67 0.06

(NS)

300+34 4B 11425.32 1589.4 13.91 1393.2 -8.20 0.17

(NS)

300+34 5B 11939.48 604.52 5.06 529.90 -4.07 0.27

(NS)

300+68 3B 10568.72 1072.2 10.14 939.87 -15.08 <0.05 (S)

300+68 4B 10874.12 876.96 8.06 768.67 -12.63 0.06

(NS)

300+68 5B 10487.89 574.68 5.48 503.72 -15.73 <0.05 (S)

Table B.13 ILSS of 300 tex plain weave composites

Weave/

direction

Comp.

type

ILSS

(MPa)

Std-

dev C.V %

95%

Conf.

Interval

%

increase

ILSS

Signif.

Plain

Warp

300 NT 31.556 2 6.34 1.753

300+34 3B 35.206 1.76 5.00 1.543 11.57 <0.05 (S)

300+34 4B 35.204 4.38 12.44 3.839 11.56 0.07 (NS)

300+34 5B 39.159 3.04 7.76 2.665 24.09 <0.05 (S)

300+68 4B 36.16 4.11 11.36 3.601 14.59 <0.05 (S)

300+68 5B 34.35 1.5 4.35 1.310 8.85 <0.05 (S)

Plain

Weft

300 NT 26.433 2.27 8.59 1.990

300+34 3B 30.77 2.71 8.81 2.375 16.41 <0.05 (S)

300+34 4B 27.978 2.62 9.36 2.296 5.84 0.17 (NS)

300+34 5B 31.044 2.58 8.32 2.260 17.44 <0.05 (S)

300+68 4B 28.52 3.46 12.13 3.033 7.90 0.15 (NS)

300+68 5B 29.63 5.08 17.17 4.460 12.09 0.12 (NS)

Table B.14 ILSS of 300 tex twill weave composites

Weave/

direction

Composite

type

ILSS

(MPa)

Std-

dev C.V %

95%

Conf.

%

increase Signif.

174

Interval ILSS

Twill

Warp

300 NT 30.11 2.08 6.91 1.823

300+34 3B 35.66 2.96 8.30 2.595 18.43 <0.05 (S)

300+34 4B 33.88 1.58 4.66 1.385 12.52 <0.05 (S)

300+34 5B 40.12 1.11 2.77 0.973 33.24 <0.05 (S)

300+68 3B 34.97 4.51 12.90 3.953 16.14 <0.05 (S)

300+68 4B 37.25 1.51 4.05 1.324 23.71 <0.05 (S)

300+68 5B 38.77 1.79 4.62 1.569 28.76 <0.05 (S)

Twill

Weft

300 NT 27.85 3.42 12.28 2.998

300+34 3B 33.57 3.26 9.71 2.857 20.54 <0.05 (S)

300+34 4B 34.27 2.24 6.54 1.963 23.05 <0.05 (S)

300+34 5B 36.97 5.12 13.86 4.490 32.75 <0.05 (S)

300+68 3B 34.35 4.32 12.58 3.787 23.34 <0.05 (S)

300+68 4B 35.2 2.063 5.86 1.808 26.39 <0.05 (S)

300+68 5B 38.65 4.61 11.91 4.040 38.78 <0.05 (S)

Table B.15 Tensile strength of 600 tex plain and twill weave composites

Dir. Type Tensile

strength (MPa)

Std-dev

C.V % 95%

Conf. Interval

% change

Signif.

Plain

Warp

600 NT 310.80 54.20 17.44 47.51

600+34 5B 278.28 28.28 10.16 24.79 -10.46 0.139 (NS)

600+68 5B 264.21 13.60 5.15 10.88 -14.99 0.07 (NS)

600 CT 5B 259.26 19.34 7.46 16.95 -16.58 0.051 (NS)

600 CT 668

5B 260.10 16.64 6.40 14.59 -16.31 0.052 (NS)

600+34 5B

(WfW) 280.00 25.89 9.25 22.70 -9.91 0.147 (NS)

600+68 5B

(WfW) 288.30 17.65 6.12 15.48 -7.24 0.21 (NS)

Plain 600 NT 292.24 8.64 2.96 7.57

175

Weft 600+34 5B 214.42 11.52 5.37 10.10 -26.63 <0.05 (S)

600+68 5B 252.28 6.42 2.55 5.63 -13.67 <0.05 (S)

600 CT 5B 229.80 10.73 4.67 9.40 -21.37 <0.05 (S)

600 CT 668

5B 218.50 9.04 4.14 7.93 -25.23 <0.05 (S)

600+34 5B

(WfW) 292.78 6.88 2.35 6.03 0.18 0.458 (NS)

600+68 5B

(WfW) 261.70 20.67 7.89 18.12 -10.45 <0.05 (S)

Twill

Warp

600 NT 320.68 32.63 10.17 28.60

600+34 5B 266.89 7.36 2.76 6.45 -16.77 <0.05 (S)

600+68 5B 250.74 22.55 8.99 19.76 -21.81 <0.05 (S)

600 CT 5B 259.17 12.97 5.00 11.37 -19.18 <0.05 (S)

600 CT 668

5B 243.30 14.58 5.99 12.80 -24.13 <0.05 (S)

600+34 5B

(WfW) 294.16 18.39 6.25 16.12 -8.27 0.08 (NS)

600+68 5B

(WfW) 298.67 28.34 9.49 24.84 -6.86 0.14 (NS)

Twill

Weft

600 NT 335.70 38.71 11.53 33.93

600+34 5B 308.22 15.29 4.96 14.98 -8.19 0.103 (NS)

600+68 5B 248.30 6.62 2.67 5.80 -26.04 <0.05 (S)

600 CT 5B 235.50 23.12 9.82 20.27 -29.85 <0.05 (S)

600 CT 668

5B 236.30 25.25 10.70 22.13 -29.61 <0.05 (S)

600+34 5B

(WfW) 317.92 23.05 7.25 20.20 -5.30 0.203 (NS)

600+68 5B

(WfW) 254.55 14.05 5.52 12.30 -24.17 <0.05 (S)

176

Table B.16 Tensile modulus of 600 tex plain and twill weave composites

Dir. Type Modulus (MPa) Std-dev C.V %

95% Conf.

Interval %

change Signif.

Plain Warp

600 NT 23080.80 2171.76 9.41 1903.60 600+34

5B 18627.40 1231.92 6.61 1079.80 -19.29 <0.05 (S)

600+68

5B 16511.33 1570.60 9.51 1256.74 -28.46 <0.05 (S)

600 CT

5B 15198.60 386.10 2.54 338.40 -34.15 <0.05 (S)

600 CT

668 5B 16431.00 1867.00 11.36 1636.00 -28.81 <0.05 (S)

600+34

5B

(WfW) 20200.00 821.58 4.07 720.14 -12.48 <0.05 (S)

600+68

5B

(WfW) 19260.00 3007.52 15.62 2636.16 -16.55 <0.05 (S)

Plain Weft

600 NT 19751.20 1073.56 5.44 941.00 600+34

5B 18237.20 4671.64 25.62 4094.80 -7.67 0.259 (NS)

600+68

5B 16008.00 2089.00 13.04 1831.00 -18.95 <0.05 (S)

600 CT

5B 16081.80 2362.10 14.69 2070.50 -18.58 <0.05 (S)

600 CT

668 5B 14546.00 1509.00 10.38 1323.00 -26.35 <0.05 (S)

600+34

5B

(WfW) 19800.00 1505.00 7.60 1319.16 0.25 0.477

(NS)

600+68

5B 19309.00 2775.95 14.38 2433.18 -2.24 0.37 (NS)

177

(WfW)

Twill Warp

600 NT 22679.00 1530.35 6.75 1341.14 600+34

5B 17998.40 1802.75 10.02 1580.15 -20.64 <0.05 (S)

600+68

5B 17912.00 1693.70 9.46 1484.60 -21.02 <0.05 (S)

600 CT

5B 15542.00 1137.00 7.32 997.00 -31.47 <0.05 (S)

600 CT

668 5B 16236.00 2689.00 16.56 2357.00 -28.41 <0.05 (S)

600+34

5B

(WfW) 17520.00 939.15 5.36 823.18 -22.75 <0.05 (S)

600+68

5B

(WfW) 17777.00 2218.80 12.48 1944.82 -21.61 <0.05 (S)

Twill Weft

600 NT 22526.80 3149.15 13.98 2760.30 600+34

5B 18173.25 2663.35 14.66 2610.04 -19.33 <0.05 (S)

600+68

5B 16749.80 566.97 3.38 496.96 -25.65 <0.05 (S)

600 CT

5B 17321.00 2197.30 12.69 1926.00 -23.11 <0.05 (S)

600 CT

668 5B 16278.00 1341.00 8.24 1176.00 -27.74 <0.05 (S)

600+34

5B

(WfW) 20520.00 1416.69 6.90 1241.76 -8.91 0.121

(NS)

600+68

5B

(WfW) 18385.00 2077.20 11.30 1820.70 -18.39 <0.05 (S)

178

Table B.17 Flexure strength of 600 tex plain and twill weave composites

Dir. Type Flex str (MPa) Std-dev C.V %

95% Conf.

Interval

% change Signif.

Plain Warp

600 NT 381.11 26.15 6.86 22.92

600+34 5B 370.20 28.98 7.83 25.40 -2.86 0.27 (NS)

600+68 5B 354.21 11.11 3.14 9.74 -7.06 <0.05 (S)

600 CT 5B 372.22 25.08 6.74 21.98 -2.33 0.3 (NS)

600 CT 668 5B 390.68 14.93 3.82 13.09 2.51 0.25 (NS)

600+34 5B WfW 365.51 10.13 2.77 8.88 -4.09 0.13 (NS)

600+68 5B WfW 386.65 20.89 5.40 18.31 1.45 0.36 (NS)

Plain Weft

600 NT 331.56 33.30 10.04 29.19

600+34 5B 337.32 14.36 4.26 12.59 1.74 0.37 (NS)

600+68 5B 337.86 22.85 6.76 20.03 1.90 0.37 (NS)

600 CT 5B 359.55 15.73 4.38 13.79 8.44 0.07 (NS)

600 CT 668 5B 358.03 24.84 6.94 21.77 7.98 0.1 (NS)

600+34 5B WfW 346.52 27.84 8.04 24.41 4.51 0.23 (NS)

600+68 5B WfW 387.99 39.18 10.10 34.34 17.02 <0.05 (S)

Twill Warp

600 NT 351.53 46.33 13.18 40.61

600+34 5B 320.15 15.30 4.78 13.41 -8.93 0.104 (NS)

600+68 5B 394.80 5.32 1.35 4.70 12.31 0.053 (NS)

600 CT 5B 409.18 13.73 3.35 12.03 16.40 <0.05 (S)

600 CT 668 5B 420.20 14.69 3.50 12.88 19.53 <0.05 (S)

600+34 5B WfW 453.21 29.26 6.46 25.65 28.92 <0.05 (S)

600+68 5B WfW 440.95 48.48 10.99 42.50 25.44 <0.05 (S)

Twill Weft 600 NT 379.21 19.52 5.15 17.11

179

600+34 5B 355.82 31.15 8.76 27.31 -6.17 0.1 (NS)

600+68 5B 386.36 39.98 10.35 35.05 1.89 0.36 (NS)

600 CT 5B 396.52 49.05 12.37 42.99 4.57 0.24 (NS)

600 CT 668 5B 423.76 45.07 10.64 39.50 11.75 0.05 (NS)

600+34 5B WfW 408.15 26.17 6.41 22.93 7.63 <0.05 (S)

600+68 5B WfW 400.00 41.80 10.45 36.65 5.48 0.18 (NS)

Table B.18 Flexure modulus of 600 tex plain and twill weave composites

Dir. Type Flexure modulus

(Mpa)

Std-dev

C.V %

95% Conf. Interval

% change Signif.

Plain Warp

600 NT 11995.37 2040.7 17.0 1788.79

600+34 5B 11644.72 1093.9 9.39 958.89 -2.9 0.37 (NS)

600+68 5B 12072.49 846.67 7.01 742.12 0.6 0.47 (NS)

600 CT 5B 13611.00 1173.4 8.62 1028.6 13.4 0.088 (NS)

600 CT 668 5B 12861.00 344.50 2.68 301.9 7.2 0.2

(NS) 600+34 5B

WfW 13529.63 915.07 6.76 802.08 12.7 0.088 (NS)

600+68 5B WfW 13938.40 310.60 2.23 272.24 16.1 0.051

(NS)

Plain Weft

600 NT 9071.25 1101.7 12.1 965.66

600+34 5B 9634.94 389.10 4.04 341.06 6.2 0.16 (NS)

600+68 5B 11390.12 961.66 8.44 842.91 25.5 <0.05 (S)

600 CT 5B 13157.00 924.00 7.02 809.7 45.0 <0.05 (S)

600 CT 668 5B 11817.00 359.00 3.03 314.34 30.2 <0.05

(S) 600+34 5B

WfW 12471.31 479.76 3.85 420.52 37.4 <0.05 (S)

600+68 5B WfW 12914.44 1045.1 8.09 916.06 42.3 <0.05

(S) Twill Warp 600 NT 12679.67 2479.7 19.5 2173.53

180

600+34 5B 11500.75 406.57 3.54 356.37 -9.1 0.18 (NS)

600+68 5B 12153.26 610.86 5.03 535.43 -4.15 0.33 (NS)

600 CT 5B 14098.10 733.70 5.2 643.1 11.1 0.14 (NS)

600 CT 668 5B 14296.00 458.32 3.21 401.73 12.7 0.11

(NS) 600+34 5B

WfW 14172.02 762.42 5.38 668.28 11.7 0.127 (NS)

600+68 5B WfW 16385.40 2072.1 12.6 1816.31 29.2 <0.05

(S)

Twill Weft

600 NT 10176.13 1021.0 10.0 895.00

600+34 5B 11829.66 900.93 7.62 789.69 16.2 <0.05 (S)

600+68 5B 11282.74 663.91 5.88 581.93 10.8 <0.05 (S)

600 CT 5B 13320.00 367.00 2.75 321.56 30.8 <0.05 (S)

600 CT 668 5B 15003.00 952.00 6.34 834.25 47.4 <0.05

(S) 600+34 5B

WfW 12297.01 707.66 5.75 620.28 20.8 <0.05 (S)

600+68 5B WfW 15308.46 1268.9 8.29 1112.23 50.4 <0.05

(S)

Table B.19 ILSS of 600 tex plain and twill weave composites

Dir. Type ILSS (MPa)

Std-dev C.V % 95% Conf.

Interval

% increase

ILSS Signif

Plain Warp

600 NT 28.45 2.49 8.75 2.18

600+34 5B 38.77 2.64 6.81 2.31 36.25 <0.05 (S)

600+68 5B 39.99 2.04 5.10 1.79 40.55 <0.05 (S)

600 5 bar CT 38 1.93 5.07 1.69 33.53 <0.05 (S)

600 CT+668 5B 39.53 2.53 6.40 2.22 38.91 <0.05

(S) 600+34 5B

WfW 38.28 3.29 8.59 2.88 34.53 <0.05 S

600+68 5B WfW 37.33 2.38 6.38 2.08 31.18 <0.05

S

Plain 600 NT 24.16 2.45 10.16 2.15

181

Weft 600+34 5B 32.57 1.58 4.85 1.38 34.80 <0.05 (S)

600+68 5B 34.68 0.65 1.86 0.57 43.55 <0.05 (S)

600 5 bar CT 33.24 3.22 9.68 2.82 37.57 <0.05 (S)

600 CT+668 5B 34.28 3.15 9.19 2.76 41.88 <0.05

(S) 600+34 5B

WfW 35.87 3.6 10.04 2.23 48.47 <0.05 S

600+68 5B WfW 35.95 1.02 2.85 0.89 48.79 <0.05

S

Twill Warp

600 NT 27.63 0.76 2.77 0.67

600+34 5B 39.90 7.92 19.85 6.94 44.39 <0.05 (S)

600+68 5B 41.35 1.39 3.36 1.22 49.62 <0.05 (S)

600 5 bar CT 36.47 3.21 8.8 2.81 31.97 <0.05 (S)

600 CT+668 5B 40.06 1.06 2.63 0.93 44.96 <0.05

(S) 600+34 5B

WfW 39.47 6.73 17.04 4.17 42.84 <0.05 S

600+68 5B WfW 39.27 1.28 3.25 0.79 42.10 <0.05

S

Twill Weft

600 NT 24.09 0.24 0.99 0.21

600+34 5B 33.27 1.68 5.05 1.47 38.06 <0.05 (S)

600+68 5B 34.886 1.89 5.42 1.66 44.76 <0.05 (S)

600 5 bar CT 33.994 3.13 9.2 2.74 41.06 <0.05 (S)

600 CT+668 5B 35.76 2.91 8.14 2.55 48.39 <0.05

(S) 600+34 5B

WfW 35.867 3.93 10.95 2.43 48.83 <0.05 S

600+68 5B WfW 36.466 3.45 9.47 2.14 51.32 <0.05

S