Open Hole Flexural and Izod Impact Strength ofUnidirectional Flax Yarn Reinforced PolypropyleneComposites as a Function of Laminate Lay-Up

T. Gobi Kannan,1 Chang Mou Wu,2 Kuo Bing Cheng1

1Department of Fiber and Composite Materials, Feng Chia University, Taichung 40724, Taiwan, R.O.C.

2Department of Materials Science and Engineering, National Taiwan University of Scienceand Technology, Taipei 10607, Taiwan, R.O.C

An open hole flexural strength and impact energy offlax yarn-reinforced polypropylene (PP) compositeswere studied in this work. Highest flexural strengthand strength retention were observed for axial (06) andcross-ply (0/90/0)s laminates, respectively, while alsoexamining the influence of laminate lay-up and openhole size on flexural strength. It was found that maleicanhydride-grafted polypropylene (MAPP)-treated com-posite laminates achieved marginal improvement onflexural strength for all kinds of laminate lay-up.Off-axial laminates (6456) showed a good strengthretention for open hole laminates after MAPP treat-ment. The fractography study confirmed microbucklingand matrix crack propagation over the compressiveand tensile side of the laminate, respectively. Further-more, severe surface damage was detected over thetensile side of 8-mm hole size laminates. Impact testof the flax/PP laminates showed slight improvement byMAPP treatment. High- and low-impact energy wasexperienced for axial and off-axial laminates. The dam-aged impact sample shows evidence of fiber pull-outfor untreated flax yarn reinforced laminates. POLYM.COMPOS., 34:1912–1920, 2013. VC 2013 Society of Plas-tics Engineers

INTRODUCTION

Natural fiber composites have represented a new mate-

rial category among researches for the last few decades.

This is mainly because of the environmental and econom-

ical advantages of natural fiber composites. Application

of natural fiber composites is limited to the automobile,

construction, and packaging industry. These composites

have not found a high-end structural application because

of poor interfacial bonding between fiber and matrix

which affects mechanical properties of composites.

Research communities are particularly interested in over-

coming the interfacial bonding problem by adding a suita-

ble coupling agent and appropriate surface treatment to

the reinforcement. The surface treatment can be carried

out by cleaning the fiber surface, introducing surface

irregularity, and stopping moisture absorption properties

over the entire surface area of the fiber. Interfacial bond-

ing of the composite material can be enhanced by several

treatments such as alkali, silane, acrylation, acetylation,

maleic anhydride (MA), MA-grafted polypropylene (MAPP)

treatments, and so forth [1–5].

However, many works concluded that the MAPP cou-

pling agent has improved the interface bonding and wett-

ability of flax fiber-reinforced PP composites. This is due

to two reasons: (i) balanced properties of MAPP can ena-

ble the effective bonding between polar and nonpolar

components, and (ii) development of a covalent bond

between fiber and matrix at the interface zone [6,7]. Bia-

giotti et al. [8] devoted their experiments to investigate

the influence of various chemical treatments on the

mechanical properties of flax fiber-reinforced composites.

They concluded that MAPP (5 wt%)-treated flax fiber-

reinforced composites have the highest tensile and flex-

ural properties compared to the various chemical-treated

fiber composites. Cantero et al. [9] have tried to enhance

the wettablity of flax fiber-reinforced PP composites,

where MA, MAPP, and silane chemical agents were used

to treat the flax fiber. Three treatments reduced the polar-

ity of the surface energy of fiber, which may act to

improve the interfacial bonding of the composites. How-

ever, MAPP-treated flax fiber composites achieve the

highest mechanical properties among the three chemical

treatments. Bledzki et al. [11] investigated the effect of

mercerization and MAPP treatment on flax fiber-

reinforced PP composites. It is found that the flexural

strength and tensile strength of flax/PP composites were

improved up to 90 and 50% by mercerized and MAPP

treatment, respectively, as compared to untreated fiber

Correspondence to: K. B. Cheng; e-mail: kbcheng@fcuoa.fcu.edu.tw

DOI 10.1002/pc.22598

Published online in Wiley Online Library (wileyonlinelibrary.com).

VC 2013 Society of Plastics Engineers

POLYMER COMPOSITES—2013

composites. Aranberri-Askargorta et al. [10] examined the

wetting behavior of PP with flax fiber, where MAPP is

used as coupling agent for treating the flax fiber surface.

This work concludes that the grafting small amounts of

MA onto the PP surfaces did not find any significant

influence on wetting behavior of PP. It is also recom-

mended that the large amount of MA content for grafting

PP which helps to increase the surface tension of PP and

also improve the wettabilty with flax fiber. Mohanty

et al. [11] conducted the experiment by using MAPP as a

coupling agent for the surface treatment of jute fibers. It

is found that the flexural strength of jute/PP composites

was improved around 72% after MAPP treatment.

Several researchers have addressed the characteristics

of unidirectional (UD) flax fiber-reinforced composites in

the past few decades. For example, Zhang and Miao [12]

made an effort to examine the flexural behavior of UD

flax fiber-reinforced composites with woven fabric struc-

ture, where conventional twisted/wrap flax yarn and PP

filament are used as a weft and warp yarn, respectively.

The results concluded that flexural modulus of wrapped

yarn composites showed reasonable improvement than

conventional twisted yarn composites. Miao and Shan

[13] examined the mechanical properties of the compo-

sites made from highly aligned flax/PP nonwoven fabric

preform. The work demonstrates that the tensile and flex-

ural results of highly aligned nonwoven fabric reinforced

composites follows the similar results as UD woven fab-

ric composites. Velde and Kiekens [14] have studied the

flexural properties of UD and multidirectional flax fiber-

reinforced thermoplastic composites. Boiled flax fiber and

MA-modified PP were used as reinforcement and matrix

for UD composite production. Multidirectional composites

were made from needle-punched hybrid boiled flax/PP

nonwovens. Highest flexural properties were noted for

UD-boiled composites. Flexural properties of all compo-

sites produced with MA-modified PP showed the good

improvement in the flexural properties.

The stress concentration development is one of the pri-

mary concerns for composite material. It occurred due to

the discontinuity formed by a hole during use of compos-

ite in structural applications. Hole size effect, which

explains the strength reduction characteristics of the com-

posites while scaling up the hole size, has significant

influence on the mechanical behavior of the composite

material [15]. Some of the researchers have analyzed the

effect of open hole structure on the tensile properties of

synthetic fiber-reinforced composites [15–17]. It is also

possible to have different types of mechanical properties

and damage behavior by open hole structures on compos-

ite materials when subjected to the flexural loading during

their end application. On the other hand, different fiber

orientation also has a significant influence on the mechan-

ical properties of composites. Many works revealed that

the change of reinforcement orientation from 0 to 90�

reduces the tensile strength of composites consecutively

[18,19]. But, there is no scientific publication for under-

standing the effect of open hole and laminate lay-up on

the flexural behavior of laminated composites.

This work made an attempt to reveal the effect of dif-

ferent laminate lay-up, open hole size, and coupling agent

on the open hole flexural properties of UD flax yarn-

reinforced PP composites. The impact properties of

untreated and MAPP-treated laminates for different lami-

nate lay-up will be explained in detail. Efforts have also

been made to examine the different types of damage

behavior of open hole flexural and impact laminates.

EXPERIMENTAL

Materials

Flax and PP interwoven fabrics were made by a Dor-

nier rapier weaving machine, where 105.4 tex and 72.2

tex fineness flax ring spun and PP draw textured yarn

were used as a weft and a warp, respectively. Melt flow

index (MFI) of yarn grade PP was 34 g/10 min. (at

230�C and 2.16-kg load).The flax and PP yarns were pro-

vided by the Tai Yuang Textile and Tri Ocean Textile

Company, respectively. Interwoven fabric was constructed

with 13 picks (weft)/cm and 22 ends (warp)/cm, which

enabled the achievement of 40/60% (flax/PP) volume

fraction in the final composite laminates. Interwoven fab-

ric load breaking along weft and warp direction was

found around 532 and 970 N, respectively.

The MAPP (POLYBOND 3002) polymer was obtained

from Chemtura Corporation, USA. MAPP with 9.8 g/10

min MFI (at 230�C and 2.16-kg load) was used to make a

thin film, which enabled its use as a coupling agent for

composite production.

METHODS

Lamina Preparation

Figure 1 illustrates the schematic diagram of flax/PP

composites production process. Heat setting is the pri-

mary step for single-lamina preparation, where the inter-

woven fabric was subjected to the preheating process

under 160�C for 30 min in a hot air oven. It helped to

prevent the fabric shrinkage and keep the original position

of the fabric in hot pressing during single lamina prepara-

tion [20,21]. Fabric heating temperature and period are

well-established from our previous research experience,

which helps to improve the flax orientation as well as

mechanical properties of composites [21]. Previously

heated woven fabric was processed in the hot pressing

machine at 170�C for 1 min., where MAPP film [5 wt%

on the weight of materials] was applied and spread over

one side of the fabric. Lamina preparation was performed

to keep the MAPP between two layers during the final

stage of composite preparation. Untreated lamina was

obtained by following the similar condition to above,

where the MAPP film was not incorporated on the fabric.

DOI 10.1002/pc POLYMER COMPOSITES—2013 1913

Composite Production

The better stacking sequence such as axial (06), cross-

ply (6456), and off-axial (0/90/0)S laminate lay-up was

used for composite preparation, which was estimated

from the result of our previous research work [21].

MAPP-treated single laminas were well-stacked by fol-

lowing a stacking technique [22,23] and pressed at 190�Cfor 3 min under a pressure of 10 MPa by the hot pressing

machine. The hot press mold was cooled to reach a tem-

perature of about 50�C, which enabled conversion of the

melted lamina into the solidified composite laminates.

Open hole Sample Preparation

Open hole flexural test laminates were produced by

using a drill machine, where the open hole was drilled

over the center position of the sample with 4, 6, and

8-mm diameter.

Matrix Rheology Test

A parallel plate (AR 2000 EX) rheometer was used to

estimate the rheological behavior of the PP matrix for dif-

ferent temperature range. The rheology test results can be

used to understand the PP flow condition and also help to

prepare the optimum process condition such as tempera-

ture for composite laminate production [24]. The average

viscosities (Pa s) of the PP matrix were recorded as

1000 6 5, 728 6 3, 600 6 2, and 324 6 1 for the corre-

sponding temperature 170, 180, 190 and 200�C. The

PP matrix viscosity showed a sudden drop at 190�Ctemperature. So, it was decided as a suitable temperature

for composite preparation to avoid the degradation of nat-

ural fiber.

Flexural Test

Four point flexural test was conducted to understand

the damage propagation behavior of open hole laminates.

This was performed by a Universal testing machine, Tra-

pezium X (AG-100 KNX). The span to depth ratio used

was 1:16, which would provide a support span length of

32 mm and loading span length of 10.6 mm. The sample

was prepared with the dimension of 120 3 25 3 2 mm3.

The complete test was carried out as per ASTM D6272

standard, where the distance between the loading noses

was used as one third of the support span. The cross-head

speed was used 2 mm/min and result of each sample con-

cluded from five tests. A schematic representation of

open hole flexural test is shown in Fig. 2.

Izod Impact Test

The impact energy of composites was investigated by

an Izod impact tester (QC 639 F-Com tech testing

FIG. 1. A schematic diagram of flax/PP composites production process (not to the scale). [Color figure can

be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIG. 2. A schematic diagram of four point open hole flexural test (not

to the scale). [Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]

1914 POLYMER COMPOSITES—2013 DOI 10.1002/pc

machines, Taiwan), according to the ASTM D256 stand-

ard. A V-notch sample was used to conduct the experi-

ment, where the sample dimension was 63.5 3 12.7 3

2 mm3. An average test result was tabulated by assessing

five laminates.

Optical Microscopy

Damaged surface examination was performed by an

Optical microscope (NOKITA NKT-A20 Ng). It was

done under 7x–45x magnifications. A high-quality digital

camera was also used to capture the image of fractured

composite laminates.

RESULTS AND DISCUSSION

Effect of Laminate Lay-Up on the Open Hole FlexuralProperties

Table 1 shows the flexural strength of undrilled and

open hole flax/PP laminates. Different laminate lay-up

has significant influence on open hole flexural strength of

the composites. Axial laminates exhibited the highest

flexural strength for undrilled and open hole samples.

However, the flexural test result of undrilled axial

laminate showed the maximum flexural strength

(69.7 6 3.4MPa). It happened due to the presence of more

number of fibers in the axial direction, which enabled it

to hold the flexural stress developed over the surface of

the material. Moreover, the undrilled laminate had a less

chance for stress concentration. But, the axial laminate

experienced the reduction of flexural strength while intro-

ducing an open hole among them. Even though the open

hole laminate experienced pure shear stress from the four

point bending test, it could not find any damage like the

brittle-type laminates. It is mainly because of the ductile

nature of the laminate and also unexplained reasons con-

verts four point shear into tensile and compressive force.

The tensile side of damage of an axial laminate found

slight crack propagation toward the lateral direction of

the hole. But, it was not extending the crack after particu-

lar region of the sample. Meanwhile, the compressive

side of the axial laminate did not entertain any severe

damage except some microbuckling, which confirms

material stress holding ability against compressive force.

However, the laminate failed as it was unable to with-

stand the compressive and tensile force induced by shear

force.

Cross-ply laminates showed intermediate flexural

strength for undrilled and open hole laminates. Undrilled

laminate showed 54.5 6 1.3 MPa flexural strength from

four point bending test. Figure 3 shows the open hole

flexural strength for different laminate lay-up. Undrilled

and open hole cross-ply-untreated laminates followed the

same trend of as axial laminates, when subjected to the

flexural test. It is confirmed that the transverse directional

lamina contribution to the sample failure are very limited

than axial directional lamina, which may be the reason

for obtaining moderate flexural strength to the cross-ply

laminates. The tensile side damage of the cross-ply lami-

nate fractography showed severe damage growth over the

lateral direction of the hole. Microbuckling and mild sur-

face damage was observed over the compressive side of

the cross-ply laminate.

Off-axial laminates ended with very low flexural

strength than other laminate lay-up, where undrilled lami-

nate flexural strength noticed around 34.4 6 1.3 MPa.

This occurred because of the off-axial arrangement of

flax yarn, which may not provide their inherent properties

to the flexural test. However, the compressive and tensile

side of the off-axial laminate did not show any damage.

This is mainly because of the high extensibility nature of

the off-axial laminates and does not offer any damage to

the sample when maximum flexural load is applied on it.

The laminate shows the clear surface without any dam-

age, which is shown in Fig. 4.

TABLE 1. Flexural strength of undrilled and open hole flax/PP composite laminates.

Type of laminate

lay-up

Untreated sample MAPP-treated sample

UDFa

(rUDF) (MPa)

Hole

diameter

(mm)

OHFb

(rOHF)

(MPa)

Strength retention

rOHF/rUDF (%)

UDFa

(rUDF) (MPa)

Hole

diameter

(mm)

OHFb

(rOHF) (MPa)

Strength

retention

rOHF/rUDF (%)

Axial 69.7 6 3.4 4 62.9 6 2.5 90 73.1 6 2.2 4 62.7 6 1.5 86

6 60.6 6 2.6 87 6 62.0 6 3.6 85

8 58.5 6 1.7 84 8 55.0 6 1.9 75

Cross-ply 54.5 6 1.3 4 51.0 6 2.5 94 65.7 6 2.9 4 57.9 6 1.7 88

6 50.0 6 1.5 92 6 52.1 6 2.4 79

8 42.9 6 1.5 79 8 44.8 6 1.4 68

Off-axial 34.4 6 1.3 4 30.3 6 1.2 88 37.0 6 2.9 4 36.3 6 0.6 98

6 28.0 6 1.4 81 6 30.6 6 1.1 83

8 25.1 6 0.9 73 8 26.9 6 0.8 73

aUndrilled sample flexural strength.bOpen hole sample flexural strength.

DOI 10.1002/pc POLYMER COMPOSITES—2013 1915

Strength retention (%) is the value calculated for com-

paring the retention strength of an open hole sample with

an undrilled laminate. Highest strength retention was

obtained for cross-ply laminates, which was 94% for

4-mm hole size laminates. Similarly, the intermediate

hole size (6 mm) made a good performance (92%) with

cross-ply laminates. But, 8-mm hole size performed well

only with the axial laminates, which was 84%.

Effect of Open Hole Size on the Flexural Properties

Figure 5 shows the flexural strength of axial laminates

without hole and with different hole sizes. The open hole

laminate was subjected to the flexural force on both sides

of the hole by using a four point rectangular fixture.

Open hole size was found to have significant influence on

the flexural properties of flax yarn-reinforced composites.

Axial open hole laminates exhibited the highest flexural

strength than cross-ply and off-axial laminates from small

to large hole size. However, the highest strength retention

was found for 4-mm open holes in different laminate lay-

up. The hole with 4 mm size has observed low notch sen-

sitivity. Increasing hole size has reduced the strength

retention of the laminate irrespective of laminate lay-up.

It is mainly because of the high-stress concentration

around notch area. Similarly, the highest notch sensitivity

was observed for 8-mm hole where strength retention was

about 84, 79, and 73% for, respective, laminate lay-up

such as (06),(0/90/0)S, and (6456).

On the other hand, microbuckling was only observed

on the compressive side of open hole flax yarn-reinforced

PP composites. It is clearly illustrated in Fig. 6. The ten-

sile side of the laminate could experience damage for the

6 and 8-mm hole size. The laminate that contained a 6-

mm hole size showed a matrix crack growth at the edge

of the hole but not propagating throughout the laminate.

However, the 8-mm hole laminate was affected by severe

damage, which formed a matrix crack at the hole edge

FIG. 3. Open hole flexural strength of different laminate lay-up flax/

PP laminates for 6-mm hole size.

FIG. 4. Photograph of flexural damage for different laminate lay-up samples with 6 mm hole size (a) com-

pressive side, (b) tensile side. [Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

FIG. 5. Flexural strength of axial flax/PP laminates without hole and

with different hole sizes.

1916 POLYMER COMPOSITES—2013 DOI 10.1002/pc

and extended laterally to the edge of the specimen. The

off-axial laminate did not experience any crack damage

for different hole size in both tensile and compressive

side. But, the off-axial laminate found only a small

microbuckling either on tensile or compressive side.

Effect of MAPP Coupling Agent on the Open HoleFlexural Properties

Figure 7 shows the open hole flexural strength of

untreated and MAPP-treated laminates for different lami-

nate lay-up with 4-mm hole size. The MAPP-treated axial

laminate flexural strength was improved by 5% than the

untreated laminate. It is evidence that the MAPP treat-

ment has improved the interfacial bonding between flax

and PP matrix. But, the improvement of the MAPP

treated axial laminate flexural strength was not significant

compared to other laminate lay-up. Axial laminates with

4 and 6-mm hole size almost followed the same flexural

strength after MAPP treatment. The laminate with an 8-

mm hole size experienced a sudden reduction of flexural

strength compared to the untreated laminate. It may be

due to the ineffectiveness of MAPP treatment compared

to the open hole size effect.

Cross-ply MAPP-treated laminates achieved significant

improvement by MAPP treatment, where the undrilled lami-

nate showed flexural strength of about 65.7 6 2.9 MPa.

Undrilled, 4, 6, and 8-mm hole cross-ply samples showed

enhancement in flexural strength of about 20, 13, 4, and

4%, respectively, by MAPP treatment. This result shows

the improvement of interfacial bonding between fiber and

matrix. However, the 8-mm hole size laminate followed

the same trend of increment like the 6-mm laminate after

MAPP treatment. It is also confirming that the hole size

has a more vital role than MAPP function.

The off-axial laminate also recorded significant

improvement in flexural strength by MAPP treatment,

where undrilled laminate was improved by up to 7%

than the untreated laminate. Similarly, the off-axial

MAPP-treated open hole laminates such as 4, 6, and 8

mm shows increase in flexural strength by 19, 9, and 7%,

respectively, from corresponding untreated laminates.

This result confirms that the off-axial laminate has a

good adhesion between fiber and matrix.

Although considering the strength retention of different

laminate lay-up laminates, the MAPP-treated off-axial

laminate had the highest value of about 98%. It concludes

that the MAPP treatment has reduced the notch sensitivity

effect of the off-axial laminates. This is due to the good

interface bonding which may restrict the stress concentra-

tion development by notch.

The open hole size effect principle was completely fol-

lowed by MAPP treated laminates, where the increasing

hole size has reduced the flexural strength of the compo-

sites. High and low notch sensitivity was admitted for 8

and 4-mm hole size laminates, respectively. However,

strength retention of cross-ply and off-axial laminates

found a wide interval between 4 and 6-mm hole size in

comparison with untreated laminates. It confirmed a

FIG. 6. Photograph of axial flexural damaged samples for different hole size (a) compressive side, (b) ten-

sile side. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIG. 7. Open hole flexural strength of untreated and MAPP treated

laminates for different laminate lay-up with 4-mm hole size.

DOI 10.1002/pc POLYMER COMPOSITES—2013 1917

different notch sensitivity level between 4 and 6-mm

MAPP-treated laminates. Similarly, axial laminates con-

taining 6 and 8-mm hole size were also showing a wide

difference in strength retention compared to the untreated

laminate. This significant change of the MAPP treated

laminate is used to identify the notch sensitivity for dif-

ferent hole size and also had a chance to find out the

wide range of applications.

Figure 8 illustrates the tensile and compressive side of

untreated and MAPP-treated flexural damage laminates

for different laminate lay-up (4-mm hole size). The com-

pressive side of flexural damage laminates observed

micro buckling for different orientations and it is clearly

indicated in Fig. 9. But, the tensile side of laminates

obtained visible crack in the lateral direction of the hole.

Crack growth was mostly dominated for cross-ply lami-

nates and where it extended from hole edge to the sample

edge. The crack growth was also visible for the axial

untreated laminate. However, the tensile side of MAPP

treated axial laminate did not experience any severe dam-

age on both sides except for some micro buckling. It is

one of the evidence that supports good interface bonding

of the MAPP treated laminate, which has also been

observed for different hole size.

Figure 10 shows the schematic diagram of tensile side

of cross-ply laminates compared with actual failed lami-

nate. It confirms matrix crack over the surface of the

composites with minimum damage over the flax yarn.

The overall flexural test results and damage behavior of

FIG. 8. Photograph of a flexural damage of untreated and MAPP-treated laminates for different laminate

lay-up (4-mm hole size) (a) compressive side, (b) tensile side. [Color figure can be viewed in the online

issue, which is available at wileyonlinelibrary.com.]

FIG. 9. Schematic diagram of compressive side damage of axial lami-

nate and its comparison with actual failed sample (8-mm hole size).

[Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

1918 POLYMER COMPOSITES—2013 DOI 10.1002/pc

MAPP-treated laminate concluded that the sample held

up to the long period from compressive and tensile force

compared to the untreated laminate. It also offered higher

flexural strength to the laminate, which may be due to the

good interface bonding of MAPP-treated laminates.

Impact Properties

Table 2 shows the impact energy of flax yarn-

reinforced composites for different laminate lay-up. The

impact energy obtained around 340 6 0.4 and 360 6 0.3 J/m

for untreated and MAPP-treated axial laminates, respec-

tively. Highest impact energy is noted for the axial lami-

nate than other laminate lay-up, which is mainly because

of the presence of more number of fibers in the axial

direction as well as requirement of highest energy for

breaking axially oriented fiber. The MAPP-treated axial

laminate was only improved up to 5%. Figure 11 shows

the damage behavior of untreated and MAPP-treated

impact laminates. The following energy dissipation mech-

anisms were observed for axial-untreated laminates during

impact failure of the sample: fiber and matrix fracture,

fiber/matrix debonding, and fiber pull-out. The final dam-

age of fiber pull-out can be detected from axial-untreated

laminate.

Cross-ply laminates ended with the impact energy

value of 148 6 0.2 and 160 6 0.2 J/m for corresponding

untreated and MAPP-treated laminates. The main reason

for the lowest impact energy of the cross-ply laminate is

that the sample can easily form the crack through the

matrix. Low-impact energy value indicates impact on a

matrix rich zone, which ensures the presence of low-fiber

content in the damage propagation path of composites.

Damaged laminate surface also had evidence of fiber

breakage and pull-out, which may not be found over the

cross-section of the MAPP-treated laminate.

The off-axial MAPP-treated laminates show the incre-

ment above 11% from the untreated laminate, where the

impact energy of the untreated and MAPP-treated lami-

nate was 177 6 0.2 and 197 6 0.2 J/m. The intermediate

value of the off-axial laminate is mainly because of the

balanced lamina arrangement in positive and negative

direction. The laminate experienced failure around the

inter fiber cleavage area as well as the fiber orientation

direction, where the untreated laminate found more fiber

breakage than the MAPP-treated laminates.

CONCLUSIONS

Flax yarn-reinforced PP composites with axial, cross-

ply, and off-axial laminate lay-up were used to analyze

the open hole flexural properties and Izod impact proper-

ties, and the effect of MAPP treatment were also demon-

strated successfully. Flexural test results concluded that

the highest strength was to the axial laminates. However,

cross-ply open hole laminates observed high strength

retention when compared to other laminate lay-up open

hole laminates.

MAPP-treated laminates experienced marginal improve-

ment (4–20%) than untreated laminates on the open hole

flexural value of flax-reinforced PP composites. High

strength retention was claimed for off-axial MAPP-treated

laminates. Fractography analysis mostly ended with micro-

buckling failure on the compressive side of the flexural

test laminates. The tensile side of open hole flexural lami-

nate had a matrix crack on both sides of the hole and it

extended up to the nearest edge of the sample.

TABLE 2. Impact energy of flax yarn reinforced composites.

Type of laminate

lay-up

Impact energy (J/m)

Improvement

(%)

Untreated

laminate

MAPP-treated

laminate

Axial 340 6 0.4 360 6 0.4 5

Cross-ply 148 6 0.3 160 6 0.2 8

Off-axial 177 6 0.2 197 6 0.2 11

FIG. 10. Schematic diagram of tensile side damage of cross-ply lami-

nate and its comparison with actual failed sample (8-mm hole size).

[Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

FIG. 11. Comparison of untreated and MAPP-treated impact damaged

samples. [Color figure can be viewed in the online issue, which is avail-

able at wileyonlinelibrary.com.]

DOI 10.1002/pc POLYMER COMPOSITES—2013 1919

The impact strength test evaluated highest impact

energy to the axial laminates compared to other laminate

lay-up. The impact energy was enhanced by about 5, 8,

and 11% by MAPP treatment for axial, cross-ply, and

off-axial laminates, respectively. Fractography analysis

found more fiber pull-out for untreated laminates com-

pared to the MAPP-treated impact laminates.

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1920 POLYMER COMPOSITES—2013 DOI 10.1002/pc