Engineering Failure Analysis 16 (2009) 2635–2645

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Engineering Failure Analysis

journal homepage: www.elsevier .com/locate /engfai lanal

Cyclic fatigue of polymer nanocomposites

Gong-Tao Wang a, Hong-Yuan Liu a, Nicolas Saintier b, Yiu-Wing Mai a,*

a Centre for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and Mechatronic Engineering J07, University of Sydney,Sydney, NSW 2006, Australiab LAMEFIP-ENSAM ParisTech, Esplanade des Arts et Metiers, 33405 Talence Cedex, France

a r t i c l e i n f o

Article history:Received 23 February 2009Accepted 10 April 2009Available online 8 May 2009

Keywords:Polymer nanocompositesCyclic fatigueStress-life (ra � Nr)Thermal softening

1350-6307/$ - see front matter � 2009 Elsevier Ltddoi:10.1016/j.engfailanal.2009.04.022

* Corresponding author.E-mail address: y.mai@usyd.edu.au (Y.-W. Mai).

a b s t r a c t

This paper presents an experimental study on cyclic fatigue of two polymer nanocompos-ites in two common failure modes: mechanical failure in epoxy nanocomposites and ther-mal softening in polyamide (PA, nylon) 6 nanocomposites. For epoxy nanocomposites, theeffects of hard (silica) and soft (rubber) nano-particles on un-notched samples under con-stant cyclic stress amplitude fatigue were studied. Hard particles were shown to increasebut soft particles decrease the fatigue life of nanocomposites compared to unmodifiedepoxy. At the same stress amplitude, the extent of fatigue crack growth prior to fast frac-ture was largest in rubber nanocomposites and least in pure epoxy, reflecting the differ-ences in their fracture toughness values. Ternary nanocomposites with both hard andsoft (silica and rubber) particles were also investigated and their fatigue performanceswere compared to the binary nanocomposites. Further, the stress (ra) versus life (Nr) testdata of pure epoxy and its binary and ternary nanocomposites are well described by Bas-quin’s law.

PA6 nanocomposites exhibited fatigue failure due to thermal softening when the maxi-mum local temperature of the specimens subjected to cyclic loading reached the glass tran-sition temperature, Tg, of the material. Critical stress (ra) versus frequency (x) envelopesfor design against thermal failure were obtained for PA6/organoclay, PA6/POE-g-MA andPA6/pristine clay. Experimental results compared favorably with theoretical predictions.

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1. Introduction

Widely used as adhesives and matrix for fiber-reinforced composites, epoxies have found unparallel importance in manyindustries such as aerospace, marine, automobile, etc. However, their severe notch sensitivity and brittleness have limitedtheir more extensive applications in load-bearing situations unless the fracture toughness can be significantly enhanced.Well-known to be toughened by either soft or rigid fillers, various toughening mechanisms that are operative in epoxy resinsystems had been thoroughly reviewed in [1]. In general, cavitation of soft particles and debonding of rigid fillers haveprompted massive matrix shear yielding which toughens the epoxy matrix. Compared with micro-fillers, nano-fillers offermore enhancements with lesser loading. Therefore, many efforts [2–4] have been spent on searching systems with sufficientcost/performance ratio. As the size of nano-fillers approaches the chain gyration radius of the matrix polymer molecules, theinterphase plays an increasing important role in the final performance. Naturally, spherical particulates maximize such aneffect.

Not limited to static loading, engineering components and structures experience in-service cyclic loading, whichnecessitates the consideration of fatigue properties of polymers and their composites [5]. Normally, fatigue performance

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2636 G.-T. Wang et al. / Engineering Failure Analysis 16 (2009) 2635–2645

is characterized by either the total life (number of cycles to failure) or the crack growth life approach. Previous studies onmicro-filler reinforced polymers suggested an improved life of un-notched specimens [6] and an increased fatigue crackpropagation (FCP) resistance by tailoring the interface [7,8]. However, for polymer nanocomposites, the results are not al-ways consistent. For example, Bellemare et al. [9] found that nano-clay particles dispersed in PA6 matrix decreased theFCP resistance compared to neat PA6. On the contrary, Wetzel et al. [10] showed that addition of 13 nm alumina particlesshifted the FCP curves towards higher stress intensity amplitude values and less steep slopes relative to pure epoxy. TheirFCP results are supported by recent data reported by Zhao et al. [11] using epoxy/nano-aluminas. In addition, in their studyon epoxy/nano-silica composites, Blackman et al. [12] also confirmed significant improvement in the fatigue thresholdstress-intensity factor and improved FCP resistance compared to neat epoxy. It is obvious that further work is needed to un-ravel this difference in FCP resistance noting that PA6 and epoxy represent typical ductile thermoplastic and brittle thermo-set matrices. Bellemare et al. [9] also conducted an investigation on cyclic fatigue of un-notched specimens of PA6nanocomposites in which they showed the enhanced fatigue life due to these nano-clay particles. Although the crack growthapproach is more suitable to quantitative analysis by fracture mechanics, the total life remains very important in engineeringapplications since the whole life span of a part or a component starts with a well-formed smooth specimen, initiates micro-scopic cracks under cyclic loading, grows into a macroscopic crack, and finally propagates to catastrophic failure. In contrast,FCP covers solely the very last stage of the failure process when the crack propagates each cycle. In an attempt to gain furtherinsight on cyclic fatigue, especially the effects of nano-fillers, a model system of epoxy with nano-silica and nano-rubber,either singly or jointly, was chosen and the fatigue life of un-notched sample evaluated under load-controlled uniaxial cyclicfatigue. Notched FCP studies of the same system will be presented elsewhere.

Besides mechanical failure by crack initiation and growth, thermal softening is another well-known failure mode in poly-mer fatigue. Because of the hysteresis of polymers, cyclic deformation can build up heat leading to temperature rise. If equi-librium is reached before the material glass transition temperature, Tg, true mechanical failure takes place. However, if thetemperature at a critical section exceeds Tg, thermal softening occurs. In this work, thermal softening of PA6-based nanocom-posites was experimentally investigated. Under practical conditions, usable stress-frequency envelopes were obtained. Acomplete theoretical evaluation of the thermal softening effect of PA6-based nanocomposites is given in [13].

2. Cyclic fatigue failure analyses

2.1. The stress-life (S–N) approach

Depending on the controlling variable, the total fatigue life of a specimen can be presented by either the stress-life (S–N)or the strain-life approach. Generally, a typical polymer S–N curve would have three distinct regions [14]. The existence ofregion I is dependent on whether crazes or other deformation mechanisms, which have different manifestations to crackgrowth, form at high values of Dr. In region II, the dependence of total fatigue life on the stress amplitude ra is dominatedby the more and more important role of microscopic cracks with decreasing loads. Region III represents the high-cycle fa-tigue (HCF) behavior which essentially yields the endurance limit of the polymer. As suggested by Basquin [15], ra, stressrange, Dr, and the number of load reversals to failure, Nr, in a fully-reversed, constant amplitude fatigue are related by:

ra ¼Dr2¼ rf Nb

r ð1Þ

where rf is the fatigue strength coefficient and b is the fatigue strength exponent. All tests are performed till the specimenfails (total separation), so the number of reversals to failure Nr is related to the number of cycles Nf (Nf� 1) such that:

Nr ¼ 2ðNf � 1Þ � 2Nf ð2Þ

Correspondingly, for practical components containing blunt notches which undergo constrained plastic straining, the strain-life approach is more appropriate. According to Coffin [16] and Manson [17], the plastic strain amplitude follows a similarpower law as Eq. (1) to Nr for fatigue of metallic materials. Generalized later by Moustafa et al. [18], the same form of equa-tion also applies to total strain amplitude on fatigue of polymer composites.

2.2. Thermal softening modelling

Associated with hysteretic heating and low thermal conductivity, polymers subjected to cyclic loading can have localtemperature rise resulting in thermal softening under certain combinations of stress and strain rates. The typical differencebetween mechanical failure and thermal softening in thermoplastics has been illustrated experimentally in many previousinvestigations [19,20]. This problem was also addressed theoretically in [20] though the heat transfer rate between specimenand environment in the model is only an empirical parameter. A detailed study of this controlling variable is given in [13].Essentially, the failure criterion for the occurrence of thermal softening is given by the condition that:

xr2a P 2bh

E0ð1þ tan2 dÞtan d

ðTg � T1Þ ð3Þ

G.-T. Wang et al. / Engineering Failure Analysis 16 (2009) 2635–2645 2637

where x is fatigue angular frequency, b sample geometrical factor, h average heat transfer rate representing the overall heatexchange between specimen and the environment, E0, tan d, and Tg are material storage modulus, loss tangent and glass tran-sition temperature, respectively, and T1 is environment temperature.

2.3. Materials, characterization and testing

2.3.1. Materials and characterizationsThe raw materials of the resin system include Araldite-F (diglycidyl ether of bisphenol A, DGEBA) and Piperidine, both

obtained from Sigma–Aldrich, 40 wt% nano-silica/bisphenol A (Nanopox� F400, nanoresins AG, Germany), and 25 wt%nano-rubber/bisphenol A (Kaneka Corporation, Japan). To prepare the sample, resin, silica and/or rubber master batcheswere mixed in quantities specified in Table 1a using a laboratory stirrer for 0.5 h at 60 �C, degassed in vacuum of�100 kPa at 80 �C for 2 h, then the vacuum was removed and a stoichiometric amount of hardener was added while stirringslowly. The mixture was at last cast into a preheated stainless steel mould for curing at 120 �C for 16–20 h, as appropriate.Samples were allowed to cool gradually in the oven to room temperature after curing. For comparison, neat resin was pre-pared with the same procedure. Surface grinding and polishing were applied before any mechanical tests to ensure the spec-imen surfaces were parallel and free of macroscopic defects. Further annealing of 2 h at 100 �C was applied to remove anyresidual stresses introduced in the fabrication processes by checking with polarized light microscopy. Before testing, all spec-imens were conditioned in ambient for 2 weeks. The designations of these materials are given in Table 1a.

Nylon 6-based nanocomposites of specified compositions and designations shown in Table 1b were compounded in atwin-screw extruder and injection molded as given in [21]. Because nylon 6 is hygroscopic all specimens were dried for24 h at 80 �C and then stored in a desiccator. Tests were performed within 1 week after drying. Otherwise, the drying processwas repeated. For monotonic tests, the testing time was less than 0.5 h. For fatigue testing, the test duration was kept below0.75 h to avoid any appreciable hygroscopic effects.

Monotonic loading tests were conducted with an Instron 5567 at a strain rate 1.4 � 10�5/s according to ASTM D638-99using type I specimen for nylon 6-based nanocomposites and type IV for epoxy-based nanocomposites. The thermomechan-ical characteristics were recorded with a DMA-2980 analyzer (TA Instruments) under 1 Hz single-cantilever mode and con-stant heating of 2 �C/min. The glass transition temperature Tg was taken at the peak of the loss tangent curve.

Dispersions of nano-particles, namely silica, rubber and clay, were examined on a Philips CM120 biofilter transmissionelectron microscope (TEM) at 120 kV following a standard cryo-microtoming. Further section staining with rutheniumtetroxide (RuO4) was utilized to reveal the rubber particles. Typical micrographs were shown in Fig. 1, from which sphericalparticles of (a) silica �20 nm and (b) rubber �100 nm in epoxy resin can be seen. Fig. 1c–e shows corresponding microstruc-tures of the nylon 6-based nanocomposites. Fracture surfaces of tested specimens of epoxy-based nanocomposites were alsostudied to understand their failure mechanisms.

2.3.2. Cyclic fatigue testingLoad-controlled fatigue was performed under tension–compression (pull–push mode, stress ratio R = �1.0) with a sinu-

soidal waveform and a cyclic frequency 1 Hz for epoxy-based nanocomposites on a MTS 810 machine. To avoid sample buck-ling during compression, care was taken to devise and grip the specimens according to [18] with a geometry given in Fig. 2satisfying ASTM E466-96. A range of ra values was selected for the cyclic fatigue tests.

For nylon 6-based nanocomposites, the same specimen geometry was used in both cyclic and monotonic loading tests.Specimens were tested under tension–tension loading (pull–pull mode) at R = 0.10 over a range of frequencies between 1

Table 1Material compositions.

Material code Epoxy (wt%) Nanosilica (wt%) Nano-CSR (wt%)

(a) Component weight concentrations of epoxy-based nanocompositesE 100 0 0S2 98 2 0S6 94 6 0S10 90 10 0R2 98 0 2R6 94 0 6R10 90 0 10R6S6 88 6 6R6S10 84 6 10R10S10 80 10 10

Nylon 6 (wt%) Organoclay (wt%) Pristine clay (wt%) POE-g-MA (wt%)

(b) Component weight concentrations of nylon 6 and its nanocompositesF0 100 0 0 0F1 90 10 0 0F2 80 0 0 20F4 90 0 10 0

Fig. 1. Dispersion of filler particles. (a) Nano-silica, (b) nano-rubber in epoxy matrix, (c) organoclay C30B, (d) POE-g-MA, and (e) pristine clay in nylon 6matrix.

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and 10 Hz. The fatigue machine used was an electromagnetic Bose fatigue machine equipped with a 2 kN load cell. Duringthe experiments, an infrared camera (CEDIP Jade III) was used to record the evolution of the temperature field on the spec-imen surface. The camera characteristics are: spectral camera range 3–5 lm, maximum picture size 320 � 240 pixels, pixelsize of 10�2 mm2, framing rate ranging from 25 to 500 Hz, thermal sensitivity <25 mK at 300 K. For tests shorter than 5000cycles, the specimen temperature evolution was recorded at 25 Hz. For longer fatigue tests, the framing was synchronizedwith the maximum tension. It keeps the file size reasonable (smaller than 3 GB) so that post processing is possible.

3. Results and discussion

3.1. Epoxy-based nanocomposites

3.1.1. Material response under monotonic loadingFig. 3a shows a typical stress–strain curve of pure resin under monotonic loading. It can be seen that there is a large non-

linear response beyond its proportional limit of 21 MPa to the maximum strength of 67.1 MPa. To characterize the cyclic fa-tigue performance, 40–100% of 0.2% offset yield stress r0.2 was used as the stress amplitudes. Silica-filled nanocompositesbehave similarly, as shown in Fig. 3b, to pure epoxy except for a slightly higher elastic modulus. The tensile strength andelongation at break have not been affected up to 10 wt% nano-silica. However, rubber-filled nanocomposites show decreas-ing modulus and strength but increasing elongation to break with rubber content. Values of Young’s modulus (E), maximumload tensile strength (rm), and 0.2% offset yield strength (r0.2) are listed in Table 2a.

Fig. 2. Specimen geometry and grips for fully reversal fatigue test (R = �1.0) taken from [18]. All dimensions in mm.

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3.1.2. Material response under cyclic loadingThe ra values used for cyclic tests in this study were limited by the monotonic r0.2 value of corresponding materials.

Therefore, specimen deformation during testing would be mainly elastic and belong to region II and part of region III fatigue[14]. Detailed results of binary and ternary nanocomposites are given below.

3.1.2.1. (a) Binary nanocomposites. Fig. 4a–c shows log–log plots of experimental results in terms of ra and Nr for neat epoxyresin and its binary nanocomposites. The cyclic fatigue data of all materials apparently can be described by Basquin’s law, Eq.(1), whose fatigue strength coefficient rf and exponent b are given in Table 2a. Note that the Basquin equations obtainedcannot be applied to testing conditions when the maximum cyclic stress is above the yield stress. It is only valid for the rangeof data shown in Fig. 4. Thus, attempts to extend these relations to predict ra and/or Nr beyond the specified ranges will beinaccurate.

It can be seen from Fig. 4b that all silica-filled nanocomposites have a higher fatigue life than neat resin under the sameapplied cyclic stress. This benefit is more obvious for S2 and S6 than S10 which has the highest amount of nano-silica par-ticles. For example, at ra = 30 MPa, Nr = 1.33E4, 3.24E4 and 2.07E4 reversals for E, S2 and S10, respectively. These representfatigue life improvements of 145% and 56% for these two nanocomposites. At a lower cyclic stress, ra = 25 MPa, Nr = 7.92E4,1.74E5, and 1.07E5 reversals for E, S2 and S10. The fatigue lifetimes are still increased by 120% and 35% but are clearly dimin-ished compared to the higher cyclic stress case. This beneficial effect on fatigue life due to the hard silica nano-particles isconsistent with that observed on PA6 due to nano-clay [9].

Results for rubber-filled nanocomposites are shown in Fig. 4c. Above ra = 30 MPa, the data are limited and somewhatcrowded, but the differences in Nr between different materials are not large. Below 30 MPa, it is obvious that pure epoxyoutperforms the rubber nanocomposites. For example, at ra = 20 MPa, Nr = 7.06E5, 1.44E5, 2.13E5 and 4.0E5 for E, R2, R6

Fig. 3. Monotonic tensile stress–strain curves for: (a) pure epoxy and epoxy-based composites with specified amounts of: (b) nano-silica, (c) nano-rubber,and (d) nano-silica/nano-rubber.

Table 2Material properties.

E S2 S6 S10 R2 R6 R10 R6S6 R6S10 R10S10

(a) Thermal and mechanical properties of epoxy-based compositesYoung’s modulus E (GPa) 2.86

(0.11)a2.90(0.06)

2.98(0.08)

3.14(0.14)

2.67(0.02)

2.52(0.03)

2.25(0.10)

2.80(0.02)

2.90(0.3)

2.81(0.08)

Tensile strength rm (MPa) 67.1 (0.6) 66.2 (0.2) 64.3 (0.5) 65.5 (0.2) 62.9 (0.3) 58.1 (0.1) 54.3 (0.1) 57.1 (0.4) 60.9(0.8)

53.0 (0.3)

Yield stress r0.2 (MPa) 41.0 45.4 40.4 51.2 44.5 41.2 38.0 40.0 41.1 37.8Fatigue strength coefficient rf

(MPa)79.0 92.0 98.9 90.4 125 107 79.5 88.9 57.0 62.0

Fatigue strength exponent b �0.102 �0.108 �0.115 �0.111 �0.154 �0.137 �0.107 �0.140 �0.074 �0.080F0 F1 F2 F4

(b) Monotonic properties of nylon 6 and its nanocompositesYoung’s modulus E (GPa) 2.55 (0.02)a 4.1 (0.4) 1.24 (0.08) 2.47 (0.3)Tensile strength rm (MPa) 59.9 (0.4) 68.3 (0.6) 36.1 (0.2) 63.2 (0.3)Yield stress r0.2 (MPa) 33.7 60.1 27.3 50.2Glass transition temperature Tg (K) 332.1 329.5 329.9 320.5

a Values given in brackets are ± one standard deviation of corresponding measurements.

2640 G.-T. Wang et al. / Engineering Failure Analysis 16 (2009) 2635–2645

and R10, respectively, representing reductions of 80%, 70% and 43% in fatigue life for these three nanocomposites. Thus, add-ing soft nano rubber particles to epoxy does not seem to improve the fatigue performance of epoxy.

Fig. 5a–c shows the fracture surfaces of S10, R2 and E, respectively, under nearly the same cyclic stress amplitude of 23.5–24.5 MPa. There are typical failure regions of fatigue initiation from a corner defect (S10 and R2) or surface flaw (E), followedby slow crack growth (characterized by fatigue striations) and finally fast fracture. Stress-whitening in the fatigue growth

Fig. 4. Relationships between applied stress amplitude ra and fatigue life Nr for: (a) pure epoxy resin and epoxy-based composites with specified amountsof: (b) nano-silica; (c) nano-rubber, and (d) nano-silica/nano-rubber.

G.-T. Wang et al. / Engineering Failure Analysis 16 (2009) 2635–2645 2641

zone is prominent in R2 due to rubber cavitation. The amount of fatigue crack growth prior to fast fracture as measured fromthese micrographs are: 5.2 mm for R2, 3.4 mm for S10 and 1.5 mm for E, and Nr = 5.03E4, 1.43E5 and 9.66E4 reversals,respectively. Since Nr is the sum of reversals required for fatigue crack initiation Ni and fatigue crack propagation Np, for abasic understanding of cyclic fatigue and lifetime prediction, we need to determine the FCP of these materials under thesame testing conditions. However, it may be surmised that fatigue initiation life Ni is dominant in S10 (filled with hardnano-silica particles); and FCP hence Np controls the fatigue life of R2 (filled with soft nano-rubber particles).

3.1.2.2. (b) Ternary nanocomposites. The ra versus Nr results for three ternary composites R6S6, R6S10 and R10S10 are shownin Fig. 4d together with the data for pure epoxy plotted for comparison. Recall that rubber nanocomposites R2, R6 and R10(ra below 30 MPa) have lower and silica nanocomposites S2, S6 and S10 (all ra values) have higher fatigue lifetimes than pureresin E. By adding both nano-silica and nano-rubber to epoxy, the cyclic fatigue performance can be raised by comparing thedata for R6S10 and R10S10 with E. But the compensating effect of the hard nano-particles is insufficient to uplift the fatigueperformance of the soft nano-particles to above pure epoxy. This is very obvious for R6S6 whose results remain much lowerthan E.

3.2. Thermal softening of polyamide 6-based nanocomposites

The monotonic properties of nylon 6-based nanocomposites, F1, F2 and F4 and their glass transition temperatures aregiven in Table 2b. Thermomechanical characteristics determined by DMA tests, in terms of storage modulus and loss tangent,versus sample temperature data are plotted in Fig. 6. For comparison, similar results for neat nylon 6, F0, are superimposed inthis figure. A typical surface temperature rise pattern leading to thermal softening is shown in Fig. 7 for a F1 specimen at10 Hz, stress ratio 0.10 and cyclic stress 80% yield stress. The temperature continuously increases at a relative constant rateuntil a critical point where there is an abrupt increase. Fig. 8 shows the temperature evolution measured at surface positions(see Fig. 9) along the specimen length. The temperature at the specimen surface is found to be homogeneous along the spec-imen axis until close to Tg. Upon reaching Tg, strain localization occurs (see localized temperature rise in Fig. 9c) so that the

Fig. 5. Fractographs of: (a) silica nanocomposite S10, (b) rubber nanocomposite R2, and (c) pure epoxy subjected to cyclic fatigue at nearly same ra (23.5–24.5 MPa). Black arrows indicate fatigue crack growth direction. Zones of fatigue crack initiation, fatigue crack growth and fast fracture are clearly shown.Stress-whitening is obvious in R2 material.

Fig. 6. Storage modulus and loss tangent versus sample temperature plots for: (a) PA6/organoclay (F1); (b) PA6/POE-g-MA (F2); and (c) PA6/pristine clay(F4) nylon 6-based nanocomposites.

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specimen fails within a few more cycles. For specimen F1, strain localization starts once the temperature reaches 328.5 Kwhile the Tg of this material is 329.5 K. Once localization starts, the temperature continues to increase to �345 K far abovethe Tg. Similar observations are noted on F2 and F4 materials.

Fig. 10a shows the critical cyclic stress versus frequency loci obtained from experiments for F1, F2 and F4. Predictionsbased on Eq. (3) taken from [13] for F1 are also superposed in this figure with an average heat transfer rate of 12.8 W/

0

10

20

30

40

50

60

70

80

0 200 400 600 800 1000 1200 1400 1600

Number of cycles

Tem

pera

ture

(°C

)

Fig. 7. Temperature evolution of the critical point on the surface of nylon 6/organoclay (F1) specimen subject to testing conditions: stress ratio R = 0.10,frequency 10 Hz, and ra equals 80% of 0.2% offset yield stress.

Fig. 8. Temperature distribution along the 35 mm green line (zero position starts from the top in Fig. 9) at different fatigue stage for F1, same testingconditions as for Fig. 7.

Fig. 9. 2D spatial temperature distribution on specimen surface at different cycles: (a) 1152, (b) 1390, and (c) 1398 cycles for F1, same conditions as Fig. 8.

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Fig. 10. (a) Maximum allowable stress versus frequency plots for nylon 6-based nanocomposites: F1, F2 and F4 against predicted curve based on Eq. (3) forF1. (b) Normalized stress amplitude versus frequency envelopes for F1, F2 and F4 against fatigue failure by thermal softening showing insensitivity on typesof nano-particles.

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(m2 K). The agreement is good though predictions are consistently higher than experiments. Failure by thermal softeningoccurs above the locus or curve for each material. At a fixed frequency and increasing stress, thermal softening happens firstwith F2, then F4 and finally F1. Nano-organoclay performs better than pristine clay (which is not exfoliated and usually clus-tered); but rubber, as expected, is the worst performer. It is noted that this material ranking follows the 0.2% offset yieldstress ranking in Table 2b. To establish the thermal softening sensitivity, ra is normalized by r0.2 and re-plotted inFig. 10b. It can be seen that all the data for three materials fall within the same scatter band so that the thermal softeningsensitivity is independent of microstructure.

4. Conclusions

Failure of un-notched epoxy-based nanocomposites with hard silica and soft rubber nano-particles under cyclic fatiguefollows Basquin’s equation very well. Within the range of testing conditions, for the binary nanocomposites, silica particlesincrease but rubber fillers decrease the fatigue life at the same stress amplitude. Mechanisms leading to these results needfurther investigations. For the ternary nanocomposites with silica and rubber nano-particles, the fatigue life is not betterthan the pure epoxy matrix.

G.-T. Wang et al. / Engineering Failure Analysis 16 (2009) 2635–2645 2645

Under cyclic loading, un-notched thermoplastic nylon 6-based nanocomposites fail by thermal softening when the localsurface temperature equals or exceeds their glass transition temperature. Nano-clay, especially exfoliated, outperforms rub-ber and has a larger critical stress-frequency space for safe design. Theory and experiments agree well for a nylon 6/organo-clay nanocomposite.

Acknowledgements

GTW acknowledges a Faculty Scholarship from School of AMME, University of Sydney. We also thank the partial supportfrom the CRC–Advanced Composite Structures and the Australia–France Bilateral Program (FR060129). Nanoresins AG kindlyprovided the nano-silica master batch for this study.

References

[1] Garg AC, Mai Y-W. Failure mechanisms in toughened epoxy resins—a review. Compos Sci Technol 1988;31:179–223.[2] Boo WJ, Sun L, Warren GL, Moghbelli E, Pham H, Clearfield A, et al. Effect of nanoplatelet aspect ratio on mechanical properties of epoxy

nanocomposites. Polymer 2007;48:1075–82.[3] Sinha Ray S, Okamoto M. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog Polym Sci 2003;28:1539–641.[4] Tjong SC. Structural and mechanical properties of polymer nanocomposites. Mater Sci Eng: R: Rep 2006;53:73–197.[5] Parsons M, Stepanov EV, Hiltner A, Baer E. Correlation of fatigue and creep slow crack growth in a medium density polyethylene pipe material. J Mater

Sci 2000;35:2659–74.[6] Horst J, Spoormaker J. Fatigue fracture mechanisms and fractography of short-glassfibre-reinforced polyamide 6. J Mater Sci 1997;32:3641–51.[7] Karger-Kocsis J. Effects of processing induced microstructure on the fatigue crack propagation of unfilled and short fibre-reinforced PA-6. Composites

1990;21:243–54.[8] Azimi HR, Pearson RA, Hertzberg RW. Fatigue of rubber-modified epoxies: effect of particle size and volume fraction. J Mater Sci 1996;31:3777–89.[9] Bellemare SC, Bureau MN, Denault J, Dickson JI. Fatigue crack initiation and propagation in polyamide-6 and in polyamide-6 nanocomposites. Polym

Compos 2004;25:433–41.[10] Wetzel B, Rosso P, Haupert F, Friedrich K. Epoxy nanocomposites – fracture and toughening mechanisms. Eng Fract Mech 2006;73:2375–98.[11] Zhao S, Schadler LS, Duncan R, Hillborg H, Auletta T. Mechanism leading to improved mechanical performance in nanoscale alumina filled epoxy.

Compos Sci Technol 2008;68:2965–75.[12] Blackman B, Kinloch A, Sohn Lee J, Taylor A, Agarwal R, Schueneman G, et al. The fracture and fatigue behaviour of nano-modified epoxy polymers. J

Mater Sci 2007;42:7049–51.[13] Wang G-T. Fracture toughness and fatigue resistance of polymer nanocomposites. PhD Thesis, The University of Sydney; 2009.[14] Suresh S. Fatigue of materials. Cambridge; New York: Cambridge University Press; 1998.[15] Basquin OH. The exponential law of endurance tests. In: Proceedings of the American Society for Testing and Materials, vol. 10; 1910. p. 625–30.[16] Coffin LF. A study of the effects of cyclic thermal stresses on a ductile metal. Trans Am Soc Mech Eng 1954;76:931–50.[17] Manson SS. Behavior of materials under conditions of thermal stress. National advisory commission on aeronautics: Report 1170 1954.[18] Moustafa A-H, Mai Y-W, Lowe A. On the low endurance fatigue of rubber-toughened adhesives and its implication on characterization of damage. Int J

Damage Mech 1993;2:177–98.[19] Mai Y-W. Cyclic fatigue and thermal softening of polypropylene. J Appl Polym Sci 1981;26:3947–53.[20] Constable I, Williams JG, Burns DJ. Fatigue and cyclic thermal softening of thermoplastics. J Mech Eng Sci 1970;12:20–9.[21] Lim S-H, Dasari A, Wang G-T, Yu Z-Z, Mai Y-W, Yuan Q, et al. Impact fracture behaviour of nylon 6-based ternary nanocomposites. Compos: Part B

2009; doi:10.1016/j.compositeb.2009.03.006.