Fatigue Behavior of a Nanocomposite Under a Fighter Aircraft Spectrum Load Sequence

C.M. Manjunathaa, Ramesh Bojjab and N. Jagannathanc

Fatigue and Structural Integrity Group, Structural Technologies Division

CSIR-National Aerospace Laboratories, Bangalore 560017, India

amanjucm@nal.res.in, brameshyld@nal.res.in, cnjagan@nal.res.in

[Received: 27 July 2012, revised: November 30, 2012; accepted: January 12, 2013]

Keywords: Fiber reinforced composite, Rubber particle, Silica nanoparticle, FALSTAFF, Spectrum fatigue.

Abstract. Two different E-glass fiber reinforced plastic (GFRP) composite laminates having quasi

isotropic [(+45/-45/0/90)2]S layup sequence were fabricated viz., GFRP with neat epoxy matrix

(GFRP-neat) and GFRP with modified epoxy matrix (GFRP-nano) containing 9 wt. % of CTBN

rubber micro-particles and 10 wt.% of silica nano-particles. Standard fatigue test specimens were

machined from the laminates and end-tabbed. Spectrum fatigue tests under a standard fighter

aircraft load spectrum, mini-FALSTAFF, were conducted on both the composites at various

reference stress levels and the experimental fatigue life expressed as number of blocks to fail, were

determined. The stiffness of the specimen was determined from the load-displacement data

acquired at regular intervals during the fatigue test. The matrix cracks development in the test

specimens with fatigue cycling was determined through optical photographic images. The fatigue

life of GFRP-nano composite under mini-FALSTAFF load sequence was observed to be enhanced

by about four times when compared to that of GFRP-neat composite due to presence of micro- and

nano particles in the matrix. The stiffness degradation rate and matrix crack density was

considerably lower in GFRP-nano composite when compared to that of GFRP-neat composite. The

underlying mechanisms for improved fatigue performance of GFRP-nano composite are discussed.

Introduction

Fiber reinforced plastic (FRP) composite materials find wide usage in various structural

applications such as airframe, wind turbine, ship hull etc., because of their high specific strength

and stiffness in compliance with tailorability. Such composite materials experience variable

amplitude or spectrum fatigue loads in service. Hence, fatigue-durability of the composite materials

under spectrum loads is one of the important concerns in structural applications.

Efforts have been made in recent times to improve the mechanical properties of FRP composites

by incorporation of second phase fillers in the epoxy matrix. Addition of various shaped such as

particulate, fibrous and layered second phase fillers in the epoxy has been shown to improve the

mechanical properties of composites [1-3]. Improvements in strength, stiffness and fracture

toughness [3-5] of polymer composites have been achieved by addition of particles such as

nanosilica. Carbon nano tube (CNT) inclusion in the epoxy shows increase in the matrix-dominated

inter laminar shear strength (ILSS) of FRPs [6], and tensile strength and toughness of epoxies [7].

Beneficial effect of incorporation of SiC particles, carbon nano fibers and nano clay on the

mechanical properties of epoxies, has all been emphasized in recent investigation [8-10].

Significant enhancement in fatigue behavior of rubber particle modified epoxies has been well

established [11-13]. Similarly, epoxy containing silica nanoparticles has also been shown to exhibit

improved fatigue life [14] and reduced fatigue crack growth rate [15]. Influence of carbon

nanotubes and carbon nanofibers on the fatigue behaviour of epoxies has been investigated by

several authors [16,17]. Once again, large increase in fatigue life and reduction of crack speeds has

been observed in these materials due to nanotube/nanofiber pull out and crack bridging.

Journal of Nano Research Vol. 24 (2013) pp 58-66Online available since 2013/Sep/18 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/JNanoR.24.58

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Recent investigations have shown that use of modified epoxies in FRPs exhibit improved fatigue

properties of FRPs as well. Addition of 1wt. % of multi walled carbon nano tubes (MWCNTs) to

the polymer matrix of GFRP composite laminate improves the high-cycle fatigue strength by 60–

250% [18]. Carbon nano fiber (CNF) reinforced composites collectively possess improved fatigue

properties over their unmodified counterparts [19]. Addition of silica nano particles [20,21] and

nanoclay [22] into epoxy matrix has been shown to enhance the fatigue properties of FRP

composites.

Recently, authors have observed that either addition of 9 wt.% rubber micro particles [23] or 10

wt.% silica nano particles [21] in the epoxy matrix enhance the fatigue life of a GFRP composite by

about three to four times. Further, we have shown that [24] the constant amplitude fatigue behavior

of hybrid GFRP composite containing both 9 wt.% micron-rubber and 10 wt.% nano-silica particles

in the epoxy matrix, is also significantly improved.

Most of the fatigue studies on modified epoxies and FRPs with such modified epoxies have been

limited to constant amplitude fatigue behavior. Very few studies have been made on the spectrum

fatigue behavior of composites with modified epoxy matrices. Recently, we have studied the fatigue

behavior of the GFRP hybrid composite under a three-step block load sequence [25]. It was

observed that under both increasing and decreasing block load sequence, the fatigue life of GFRP

composite is enhanced by incorporation of micron-rubber and nano-silica particles in the epoxy

matrix. This study aims at investigating the fatigue behavior of a GFRP-nano composite under a

standard mini-FALSTAFF spectrum load sequence.

Experimental

Materials and Processing. The materials used and the process employed in manufacturing of

GFRP composites are briefly explained in this section. However, detailed description of these

materials and processing can be found in Ref. [24]. The epoxy resin used was LY556. The silica

(SiO2) nano particles were obtained as a colloidal silica sol with a concentration of 40 wt.% in

LY556. The reactive liquid rubber was a carboxyl-terminated butadiene-acrylonitrile (CTBN)

rubber, obtained as a 40 wt.% CTBN- LY556 epoxy adduct. The anhydride curing agent was HE

600. The E-glass fibre cloth was a non-crimp-fabric (NCF) with an aerial weight of 450 g/m2.

The required quantity of the neat epoxy resin, the calculated quantities of silica nano particle-

epoxy resin and CTBN-epoxy adduct, to give the required levels of 10 wt.% added silica and 9

wt.% of CTBN rubber, respectively in the final resin, were all individually weighed and degassed at

50 0C and 0.1 atm. All the resins were then mixed together and the stoichiometric amount of curing

agent was added to the mixture, stirred and degassed once again. The resin mixture was then used

to prepare GFRP composite laminates.

The composite laminates were manufactured by the ‘Resin Infusion under Flexible Tooling’

(RIFT) technique [26]. Glass fibre fabric cloth pieces, about 330 mm square, were cut and laid up

in a quasi-isotropic sequence [(+45/-45/0/90)2]S with a fluid distribution mesh. The resin mixture

was infused into the glass-cloth lay-up at 50 0C and 0.1 atm., cured at 100

0C for 2 hours and post-

cured at 150 0C for 10 hours, maintaining the vacuum throughout the curing cycle. The resulting

2.5-2.7 mm thick GFRP composite laminates had a fiber volume fraction of about 57%. Two types

of composite laminates were prepared following the above procedure viz. GFRP with neat epoxy

matrix (GFRP-neat) and GFRP with hybrid epoxy matrix containing 9 wt.% of rubber micro

particles and 10 wt.% of silica nano particles (GFRP-nano).

Bulk Epoxy Microstructure. The atomic force microscopic (AFM) phase images of the bulk

epoxy polymers are shown in Fig. 1 [24]. The rubber particles were evenly distributed and had an

average size of about 0.5 to 1 µm. The silica particles of about 20 nm in diameter were somewhat

agglomerated to give a ‘necklace-type’ structure with an average width of about 1 µm.

Journal of Nano Research Vol. 24 59

(a) Unmodified epoxy (b) Modified epoxy

Fig. 1. The Atomic Force Microscopic Phase Images of the Bulk Epoxy [24].

Tensile Properties. Tensile properties of both GFRP composites were determined according to

ASTM D3039 test standard [27] specifications. About 250 mm long and constant rectangular cross

sectioned (25 mm x 2.7 mm) specimens were cut from the laminate and end-tabbed. The tensile

tests were performed in a 100 kN computer controlled screw-driven test machine with a constant

crosshead speed of 1 mm/min. Five replicate tests were conducted for both materials and the

average properties determined for GFRP composites investigated are shown in Table 1. The GFRP-

nano composite was observed to exhibit higher tensile strength, by about 6%, but lowered tensile

modulus, by about 9%, when compared to GFRP-neat composite. It has been observed that

addition of 10 wt.% silica nano particles alone increase the tensile properties of GFRP composite

[21] and inclusion of 9 wt.% rubber micro particles alone reduce them [23]. However, presence of

both these partiles appear to contribute in increasing the strength but reducing the modulus.

Table 1. Tensile Properties of the GFRP Composites Investigated.

Mechanical Property Material %

change GFRP-neat GFRP-nano

Ultimate tensile strength, σUTS [MPa] 365 ± 13 386 ± 11 + 5.75

Tensile modulus, ET [GPa] 17.5 ± 0.1 15.9 ± 1.1 - 9.15

Fatigue Testing. Fatigue tests on both GFRP-neat and GFRP-nano composites were conducted

under a standard spectrum load sequence. The load sequence considered in this investigation was a

fighter aircraft loading standard for fatigue evaluation, mini-FALSTAFF [28-30] shown in Fig. 2.

It is a short version of standard FALSTAFF load spectrum, which is a standardized variable-

amplitude test load sequence developed for the fatigue analysis of materials used for fighter aircraft.

In Fig. 2, the normalized stress is plotted against peak/trough points of load sequence. One block of

this load sequence consists of 18,012 reversals at 32 different stress levels and represents loading

equivalent of 200 flights. The stress sequence for our experiments was obtained by multiplying

with a constant reference stress value, σref for all the peak/trough points in the entire block.

Spectrum fatigue tests with various reference stress levels were conducted on both GFRP

composites. The geometry and dimensions of the test specimens employed for spectrum fatigue

tests are shown in Fig. 3. Tests were conducted in a computer controlled 50 kN servo-hydraulic test

machine. Sinusoidal waveform with an average frequency of 3 Hz (6 reversals per second) was

employed in the tests. For any given reference stress, the number of load blocks required to fail the

test specimen, Nb was determined. Whenever a specimen failed in-between a full block, it was

rounded-off to the nearest complete block number.

CTBN

SiO2

1 µµµµm 1 µµµµm

60 Journal of Nano Research Vol. 24

Fig. 2. The Mini-FALSTAFF Spectrum Load Sequence [30].

Fig. 3. Schematic Diagram Showing the Dimensions of the Fatigue Test Specimens.

The stiffness variation of the specimen subjected to spectrum fatigue loads was determined as a

function of applied load blocks. At three equal distances in the mini-FALSTAFF load block, an

additional load cycle with maximum stress well below the reference stress level of spectrum was

applied on the specimen to measure the stiffness using load-displacement data. For the purpose of

comparison, the normalised stiffness of the specimen was defined as the ratio of measured stiffness

to the initial stiffness (obtained before application of first spectrum load block) at the end of any

given load block.

For the purpose of estimating matrix crack density, in the test conducted with σ ref = 250 MPa,

the test specimen was dismounted after the application of complete 3 load blocks. Since GFRP is

translucent, the matrix cracks developed during fatigue testing were clearly visible under

background light. The matrix cracks were photographed under an illuminated background light.

The crack density expressed as number of cracks per unit length was determined from the

photographs. Several straight lines were drawn on the photographs and the number of cracks

intersected with the line was counted. In this way, several lines were drawn and the average matrix

crack density was determined for both the composites.

Results and Discussion

The spectrum fatigue life expressed as number of blocks required for the specimen to fail, for

both GFRP-neat and GFRP-nano composites under mini-FALSTAFF load sequence at various

reference stresses determined is shown in Fig. 4. Fatigue life was observed to increase with reduced

reference stress in both GFRP composites, a similar trend observed by Philippidis et al [31] in

GFRP under WISPERX load spectra. However, it may be clearly seen that, for a given reference

50 mm 35 mm 50 mm

12.5 mm

6 mm

~2.7 mm thick laminate

One block

Journal of Nano Research Vol. 24 61

stress, the GFRP-nano composite exhibit enhanced fatigue life when compared to GFRP-neat

composite. The fatigue life was observed to be enhanced over entire range of reference stress levels

investigated, by about four times.

Fig. 4. The Experimental Fatigue Lives of GFRP Composites Under

Mini-FALSTAFF Spectrum Load Sequence.

The normalized stiffness variation with spectrum load blocks, evaluated for the fatigue test with

σref = 250 MPa for both GFRP composites is shown in Fig. 5. In general, both GFRP-neat and

GFRP-nano composites exhibit a typical stiffness reduction trend as observed in FRP composites

[32-36]. The three regions of the stiffness reduction curve are clearly identifiable. It may be noted

that the stiffness reduction in region I and region II was quite steep and significant in GFRP-neat

composite when compared to GFRP-nano composite.

Fig. 5. Normalised Stiffness Variation Curves for GFRP Composites Determined

Under Mini-FALSTAFF Spectrum Load Sequence, σ ref = 250 Mpa.

The photographs of matrix cracks observed on the surface of the composites subjected to three

complete spectrum load blocks, at a reference stress, σref = 250 MPa is shown in Fig. 6. As observed

in our earlier constant amplitude fatigue studies [24], the GFRP-neat composite exhibited severe

200

225

250

275

300

0,1 1 10 100 1000

Ref

eren

ce S

tres

s, σ

ref

(MP

a)

Number of Blocks, Nb

GFRP-Neat

GFRP-Nano

0,6

0,7

0,8

0,9

1,0

0 3 6 9 12 15

Norm

alis

ed S

tiff

nes

s

Number of Blocks, Nb

GFRP-Nano

GFRP-NeatStage I

Stage II Stage III

62 Journal of Nano Research Vol. 24

cracking than that by GFRP-nano composite. The crack density, expressed as number of cracks per

unit length was about 0.95/mm and 0.43/mm in GFRP-neat and GFRP-nano composite,

respectively. Thus, suppressed matrix cracking was clearly observed in GFRP-nano composite

under mini-FALSTAFF load sequence.

(a) GFRP-neat (b) GFRP-nano

Fig. 6. Photographs showing matrix crack in GFRP-composites

subjected to three complete mini-FALSTAFF spectrum load

block with σref = 250 Mpa.

The progressive fatigue damage accumulation leading to final failure, under cyclic loads in

polymer composites has been well documented [32-37]. The complete damage progress involving

(i) initiation and growth of matrix cracks, (ii) initiation of disbonds and delaminations due to

coalescence of primary and secondary matrix cracks, and (iii) subsequent growth of cracks/

delaminations to lead to final failure, have been observed. In an earlier investigation [37] we have

observed that fatigue crack growth rates of hybrid bulk epoxy (containing both micron-rubber and

nano-silica particles) is over an order of magnitude lower than that of neat epoxy. Further, we have

observed that use of such hybrid matrix in GFRP enhance the constant amplitude fatigue life due to

suppressed matrix cracking, delayed initiation of delamination and reduced crack / delamination

growth rate [24]. Similar mechanisms being operative and contribute to improved fatigue life under

block loads has also been observed by the authors [25].

The stiffness loss behaviour shown in Fig. 5 is an indication of the underlying mechanisms being

operative in the composite material [24]. The stiffness loss in stage I and stage II primarily results

due to matrix cracking [32-37]. Once the matrix crack density saturate and attain characteristic

damage state (CDS) [ 32,35], the disbond and delaminations created due to coalescence of primary

and secondary matrix cracks grow and lead to further loss in stiffness. The present results show that

when both composites are subjected to same number of spectrum load blocks, the crack density is

lower in GFRP-nano compared to GFRP-neat composite (see Fig. 6). Thus, stiffness loss curves

shown in Fig. 5 clearly indicate the underlying mechanisms i.e., suppressed matrix cracking,

delayed initiation of delamination and reduced crack / delamination growth rate [37], for improved

spectrum fatigue behaviour in GFRP-nano composite.

Conclusions

The results obtained in this investigation clearly show that the addition of 9 wt.% rubber micro-

particles and 10 wt.% of silica nano-particles to the epoxy matrix of a GFRP composite enhance the

fatigue life under mini-FALSTAFF spectrum load sequence by about four times. The stiffness

degradation rate and matrix cracking was observed to be more severe in GFRP-neat composite

when compared to that of the GFRP-nano composite. Suppressed matrix cracking, delayed initiation

of delamination and reduced crack / delamination growth rate appear to contribute for the observed

enhancement in spectrum fatigue life of GFRP-nano composite.

4 mm

Journal of Nano Research Vol. 24 63

Acknowledgements

Authors wish to thank Mr. Shyam Chetty, Director and Dr. Satish Chandra, Head, Structural

Technologies Division, CSIR-National Aerospace Laboratories, Bangalore, India for their constant

support and encouragement during this work. Part of this work, including fabrication of material,

was carried out in the Dept. of Mechanical Engineering, Imperial College, London, UK. Authors

thank Prof. AJ Kinloch and Dr. AC Taylor, Imperial College, London, UK for their support and

encouragement. The technical support staff members of the Department of Mechanical Engineering

and the Composite Centre of Aerospace Department, Imperial College, London, UK and the

Materials Evaluation Lab, FSIG, STTD, CSIR-NAL, Bangalore are thanked for their assistance in

experimental work.

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