Effect of Thermal Shock on Interlaminar Strength ofThermally Aged Glass Fiber-Reinforced Epoxy Composites

Bankim Chandra Ray

Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela 769008, India

Received 9 August 2003; accepted 18 May 2005DOI 10.1002/app.23019Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Glass fiber/epoxy composites were ther-mally conditioned at 50, 100, 150, 200, and 250°C for differ-ent periods of time and then immediately quenched directlyin ice-cold water from each stage of conditioning tempera-ture. The polymerization or depolymerization by thermalconditioning and the debonding effect by concurrently fol-lowing thermal shock in polymer composites are assessed inthe present study. The short-beam shear tests were per-formed at room temperature on the quenched samples toevaluate the value of interlaminar shear strength of the

composites. The short conditioning time followed by ther-mal shock resulted in reduction of shear strength of thecomposites. The strength started regaining its original valuewith longer conditioning time. Conditioning at 250°C andthereafter quenching yielded a sharp and continuous fall inthe shear strength. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci100: 2062–2066, 2006

Key words: composites; mechanical properties; interfaces;adhesion; ageing

INTRODUCTION

Differential coefficient of thermal expansion betweenfiber and matrix resin in a composite may lead toresidual thermal stresses at the fiber/matrix interfaceand is a prime cause of thermal shock. Thermal ex-pansion coefficients in polymers are considerablyhigh. Thus the interfacial debonding may occur underextremes of temperature.1,2 Thermal shock also pro-duces a large thermal gradient in a composite. Thequenching from high temperature to low temperaturegenerally produces tensile stresses at the surface andcompressive stresses in the interior.3 Susceptibility todelamination are one of the inherent weaknesses oflaminated composite structures.4 They are also sus-ceptible to crack initiation and propagation along thelaminar interfaces in various failure modes.5 The fi-ber/matrix interface has always been considered as acrucial aspect of polymer composites. It is at the inter-face where stress concentration develops because ofdifferences between the reinforcement and matrixphase thermal expansion coefficients. The interfacemay also serve as a locus of chemical reaction. Thematrix of composite after curing and cooling is subjectto a triaxial stress state.6 The mismatch of the thermalexpansion coefficient between fiber and matrix is themost important reason for residual stress in polymercomposite. The fiber has a lower coefficient of thermal

expansion than has the polymer matrix. The resultingthermal residual stresses are of compressive nature inthe fiber and tensile nature in the matrix.7 A recentstudy8 showed that the thermal residual stress in glassfiber/epoxy system strongly reduces the maximumbearable load of composites.9 The hydrostatic tensilestress severely reduces strain to failure of epoxyresin.10,11 The objective of the present study is to as-sess the interlaminar shear strength (ILSS) of ther-mally conditioned glass/epoxy laminates followed byice-cold water quenching from the conditioning tem-perature. The higher conditioning temperature notonly raises the degree of thermal shock but also actslike an activator12 of the diffusion of water moleculesthrough the composites from quenching medium. Theshort beam shear (SBS) test was performed here toinvestigate the interfacial bond strength. This charac-terization is valid where only the bonding level is avariable.13

The interactions between the fiber and matrix resinduring thermal treatment are complex but importantphenomena. These interactions may often lead to theformation of interphase. This interphase most proba-bly has composition, microstructure, and propertiesdifferent than those of the bulk resin. The adhesivestrength in amorphous materials is controlled by theresultant interatomic bonds as well as depends on thestresses by the reaction. The interfacial bond strengthmay be raised by localized chemical reactions. But it isalso observed that a progressive reaction may result inthe formation of a brittle reaction product. The behav-ior of the interfacial contact between fiber and matrix

Correspondence to: B. C. Ray (bcray@nitrkl.ac.in)

Journal of Applied Polymer Science, Vol. 100, 2062–2066 (2006)© 2006 Wiley Periodicals, Inc.

is strongly influenced by the presence and nature ofresidual stresses.6

The thermal ageing behavior of epoxy matrix com-posites is of particular interest because of their ex-panding use for structural applications in automotiveand aircraft sectors where increased temperatures arevery common service conditions.

EXPERIMENTAL

Glass fibers of random orientation and epoxy resin(Ciba-Geigy, India LY 556) with hardener (HY 951)were used for the fabrication of laminates. The SBSspecimens from the laminated composite were ex-posed to 50°C temperature. The holding times werevaried (5, 10, 15, 20, and 25 min) at the conditioningtemperature. The samples were quenched immedi-ately in ice-cold water from each stage of conditioningtemperature. The experiment was carried out at 100,150, 200, and 250°C temperatures. The holding timeswere maintained at those temperatures (like at a 50°Ctemperature exposure). The SBS tests were performedon the as-quenched samples at room temperature toevaluate the value of ILSS. It is a 3-point bend test,which generally promotes failure by interlaminarshear. The SBS test was conducted as per ASTM stan-

dard (D2344–84). The tests were carried out almostinstantaneously after quenching to minimize any re-versible recovery to occur in the composites.

The ILSS values were calculated as follows,

ILSS�0.75p/bt

where p is maximum load, b the width of specimen,and t the thickness of specimen.

RESULTS AND DISCUSSION

Figure 1 shows the variation of ILSS value with theconditioning time for the as-quenched glass/epoxycomposites. The specimens were conditioned first at50°C and 100°C. There is a reduction in ILSS value forless conditioning time and then increments in shearvalues are observed with increasing conditioningtime. The effect of thermal shock is debonding and itmay result in fall of ILSS value. Less conditioning timeyields low degree of postcuring strengthening effect.The rise in ILSS value for greater conditioning timemay possibly be due to the higher order of furtherpolymerization. These could dominate over thedebonding effect of thermal shock. The same trend inthe variation of ILSS values is observed for the condi-

Figure 1 Effect of thermal shock on ILSS of thermally conditioned (at 50°C [ ] and at 100°C [�]) glass/epoxy composites.

THERMAL SHOCK AFFECTS SHEAR STRENGTH OF COMPOSITES 2063

tioning at 100°C. The fall in ILSS value at the initiallevel of conditioning time is more noticeable herebecause of greater degree of thermal shock. The higherorder of thermal shock may initiate more misfit strainat the interface.

The effects of quenching on the variation of ILSSvalues of thermally conditioned (at 150°C and 200°C)temperatures for different conditioning times areshown in Figure 2. The degradative effects of thermalshock are evident in both the conditioning tempera-tures. The reason could be the high order of thermalshock. There may also be a chance of development ofgreater hygrothermic stresses (fast moisture absorp-tion by specimens from the quenching medium due tohigh thermal gradient) during cooling from such ahigh temperature. The debonding effect of thermalshock results in the continuous reduction of ILSSvalue because of the damage at the fiber/matrix inter-face. A slight absorption of moisture in the compositeand the differences in coefficients of thermal expan-sion between the fiber and the epoxy resin may lead tohigher residual thermal stresses at the interfaces.These could reduce the threshold stress for the inter-facial debonding. Thus, this conditioning may initiatethe nucleation for delamination. The rise in shear val-ues is observed for longer conditioning times. Thiscould be attributed to the development of strongerbond at the fiber/matrix interface due to such type of

thermal exposure. Epoxy resin may be forming aninterpenetrating network or further crosslinking net-work2 at these conditioning temperatures and times. Itis also possible for the molecules of one surface todiffuse fast into the other, which may result in greaterinterdiffusion at high conditioning temperature andfor longer conditioning time.14

The change in ILSS values of as-quenched glassfiber/epoxy laminates with conditioning time isshown in Figure 3. Here the specimens were firstconditioned at 250°C for different periods of time. Thecontinuous phenomenal fall in ILSS value with theconditioning time is reflected here. The dominatingweakening effects of higher degree of thermal shockand thermal spike are observed in the test data. Epoxyresin may degrade either by chain breaks at the lowerenergy bond and/or by release of monomers at achain end. Thermal degradation often leads to chainscission by depolymerization.15

The fiber/matrix interfacial behavior is based onmechanical principles with the assumption made ei-ther at the level of adhesion theories or by using thesurface chemistry approach.16 The matrix shear yield-ing, interfacial debonding or a combination of bothmay be reflected in the SBS test. The recent studiesrevealed that the effect of thermal shock on interfacialdamage of thermally and cryogenically conditionedKevlar/epoxy and Kevlar/polyester composites is not

Figure 2 Effect of thermal shock on ILSS of thermally conditioned (at 150°C [ ] and 200°C [�]) glass/epoxy composites.

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very conclusive.17,18 The probable reasons for the in-conceivable and inconsistent behavior of polymericcomposites could be attributed to the postcuring hard-ening effects of thermal conditioning, the develop-ment of compressive stresses by quenching and alsothe debonding effect of thermal shock. The resultantresidual stresses are the manifestation of those phe-nomena.

An interfacial reaction may impart different mor-phological modifications to the matrix resin micro-structure in proximity to the fiber surface. The inter-actions between fiber and polymer matrix during ther-mal treatment are important phenomena. Theexistence of a weak boundary layer in glass/epoxycomposites may be interpreted by the migration ofcuring agent to the interface. The layer has a lowermolecular mobility compared with the bulk resin.19–21

The microstructural gradient may promote crack ini-tiation and propagation through this layer.22–24 Thislayer is believed to be more brittle than the bulkmatrix having stoichiometric ratio between epoxy andcuring agent.25

Environmental exposure results in reduced interfa-cial stress transmissibility because of matrix polymerplasticization, chemical degradation, and mechanicaldamage.26–28 Matrix plasticization reduces matrixmodulus. Chemical degradation is the result of weak-ening of the bonds at the fiber/matrix interface. Me-

chanical degradation here is a function of thermalshock misfit strain at the interface. The strain pulls theepoxy away from the fiber.

A significant chemical and structural change usu-ally may take place during thermal ageing. These

Figure 4 Scanning electron micrograph shows matrix anddebonded areas of the treated samples at a magnification of�750.

Figure 3 Effect of thermal shock on ILSS of thermally conditioned (at 250°C) glass/epoxy composites.

THERMAL SHOCK AFFECTS SHEAR STRENGTH OF COMPOSITES 2065

changes in epoxy matrix can exert an influence onmechanical properties of a fiber-reinforced compos-ite.29 The adhesive bond strength represents the cu-mulative effect of multiple weak bonds acting in con-cert. The successive opening of intrachain loops orfolded domains under stress in a crosslinked mul-tichain matrix avoids the breaking of strong bonduntil all domains are unfolded or opened.30 High tem-perature thermal ageing and subsequent hydrother-mal ageing with a temperature gradient may promoteirreversible effects of the epoxy resin, especially in thevicinity of the substrate.31 All these factors may mod-ify the local stress threshold required for breaking ofstrong bond of adhesion at the fiber/polymer inter-face. That eventually could initiate additional matrixcracking21 and also other damages in the interfacialregion of a composite. Micro- and macro-damages areevident in the scanning electron micrograph of thetreated specimen in Figure 4. A SEM micrograph withthe same magnification of fractured surface for un-treated composites is shown in Figure 5. It possiblyreveals a large area of resin matrix with comparativelyvery less matrix and interfacial damages.

CONCLUSIONS

It may be reasonable to state that at low thermalconditioning temperature, the debonding effect ofthermal shock is not noticeable except for the lessconditioning time. There is a possibility of an im-provement in ILSS value because of dominating post-curing phenomena. The higher conditioning tempera-

ture is adversely affecting the interfacial propertiesbecause of the higher degree of thermal shock. Thevery strong weakening effect of thermal degradationand thermal shock is evident in glass/epoxy compos-ites for the conditioning at 250°C and thereafter, fol-lowing thermal shock treatment. Many factors arecontributing for the nature of interfacial behavior ofcomposite materials in such active and complex envi-ronment.

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Figure 5 Scanning electron micrograph of the untreatedspecimens at a magnification of �750.

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