Flexural creep of all-polypropylene composites: Model analysis

Flexural Creep of All-Polypropylene Composites:Model AnalysisK. Banik, J. Karger-Kocsis, T. AbrahamMaterials Science, Institute for Composite Materials (Institut für Verbundwerkstoffe GmbH),Technical University of Kaiserslautern, D-67663 Kaiserslautern, GermanyIn this article, simple viscoelastic and empirical modelsare presented to predict and analyze the flexural creepbehavior of all-poly(propylene) (all-PP) composites.Results of the successful application of these modelsto the actual creep behavior of unidirectional (UD) andcross-ply (CP) all-PP composites tested by short-termflexural creep measurements over a temperature rangeof 20–808C are presented. Analysis of the momentarycreep behavior of all-PP composites with different tapelay-ups are made to understand their deformationmechanism. Further to the main theme of the article,an interesting discrepancy while predicting the longtermcreep behavior of these composites based on thetime-temperature superposition (TTS) principle andFindley power law model is visualized which has alsobeen illustrated here. POLYM. ENG. SCI., 48:941–948, 2008.ª 2008 Society of Plastics EngineersINTRODUCTIONPoly(propylene) (PP) is apparently the major polymericconstruction material of future in view of its impressiveconsumption in the past few decades. Fibers are frequentlyused for improving the strength and stiffness ofthis material. Several studies were reported concerningthe reinforcement of PP using different types of fibers toachieve improvement in the thermal, structural, and mechanicalproperties. Among the various fibers, which havebeen used for PP composites, glass fibers are the mostcommon. Although excellent mechanical properties havebeen achieved in this way, but life cycle assessment doesnot yield favorable results for PP composites when theyare traditionally reinforced with glass, carbon, aramidfibers, or fabrics. This is mostly because of the energyintensiveproduction of the above reinforcing fibers andlimited recyclability of the corresponding composites.This laid to the alternative search for recycling friendlyCorrespondence to: K. Banik; e-mail: kaushik.banik@ge.comBanik is currently at GE India Technology Centre, Bangalore, India.Contract grant sponsor: German Research Foundation; contract grantnumber: DFG Ka 1202/17.DOI 10.1002/pen.21041Published online in Wiley InterScience (www.interscience.wiley.com).VC 2008 Society of Plastics Engineersnovel homocomposites. Self-reinforced PP homocompositesrepresent an effective alternative to the traditionalfiber reinforced composites where the matrix and the reinforcementare from the same polymer, thereby supportingthe ease of recyclability. Thus all-PP composites fallunder a new class of material that shows great potential.The group of Peijs [1–6] has recently developed a promisingroute of manufacturing all-PP composites based onthe consolidation of the coextruded fibers/tapes. Thenewly developed all-PP composites make use of highmodulus PP homopolymer tapes, which are coextrudedwith a thin coating of a PP copolymer. The all-PP compositesthus possess a high volume fraction (90%) ofhighly oriented PP reinforcement fraction and 10% volumefraction of copolymer layer which acts as a matrixwhen hot consolidated. By this method a processing temperaturewindow [308C can be achieved [1]. The commercialbreakthrough of this material was achieved(PURE 1 ).The behavior of all-PP composites in a monotonic stateof deformation is an important issue for load-bearingapplications. Generally polymers exhibit elastic action onloading (if loading is rapid enough), and then a slow andcontinuous increase of strain at a decreasing rate can beobserved. When the stress is removed a continuouslydecreasing strain follows an initial elastic recovery. Theyare significantly influenced by the rate of straining orstressing. For example, the longer the time to reach thefinal value of stress at a constant rate of stressing, thelarger is the corresponding strain [7]. Polymers are thereforeregarded as viscoelastic materials. Since time is avery important factor in their behavior, they are alsocalled time-dependent materials. Developments in thetechnology, such as the production of gas turbines, jetengines, nuclear power plants, and space crafts, haveplaced severe demands on high-temperature performanceof materials, including plastics. Consequently, the timedependentbehavior of materials has become of greatimportance. This time-dependent behavior of polymericmaterials under a quasi-static state has been very oftenstudied by creep. Previous research works on all-PP compositeshas shown so far that the creep behavior PP/PPPOLYMER ENGINEERING AND SCIENCE—-2008

composites depends strongly on stress, temperature, voidcontent, fiber loading, and fiber dimensions [8–11]. Oneof the greatest constraints in studying the creep behavioris the relatively long time required for performing a test.Hence, time-temperature superposition (TTS) methodsthat are able to predict the long-term data have gainedconsiderable attention [12–15]. Also many viscoelasticand empirical models have been proposed to predict thelong-term creep behavior of polymers and their composites[7, 9].In this article, the creep behavior of all-PP compositesproduced by different tape lay-ups is analyzed using simpleviscoelastic and empirical models to understand theirdeformation mechanism. Also, it has been illustratedusing the Findley equation that the applicability of themodel to predict the long-term creep depends, apart fromthe test temperature, on the material property, viz. crystallinitythat changes during the course of creep when thethermal or mechanical conditioning of the test specimensis ignored.CREEP AND CREEP MODELSCreep is a slow, progressive deformation of a materialunder constant stress. When a plastic material is subjectedto a constant load, it deforms continuously with time.This time-dependent behavior of materials, referred to asviscoelasticity, is also an important characteristic of reinforcedplastics. In general, for a constant stress, r, thecreep compliance J(t) can be written as the ratio of thestrain (e) to stress at a certain time (t) as follows:JT; ð tÞ ¼ eðT;tÞ: (1)sBesides experimental observation, highly developed creepmodeling in traditional composites can also be applied tothe polymer homocomposites to develop comprehensiveunderstanding of the creep deformation. In the past halfcentury, numerous creep models have been proposed andapplied to describe the creep behavior of viscoelasticmaterials [7, 12, 13, 16–21]. Various materials of thesame geometry may respond differently under identicalexternal effects. Such differences in response are oftenattributed to the inherent constitution of the materials.Consequently, the response behavior of a particular material,or a class of materials, is described mathematicallyby the so-called constitutive relations [22]. Thus, thecreep behavior of an isotropic linear viscoelastic systemcan be represented byZeðÞ¼s t 0 f ðÞexp t ð1t=tÞdt; (2)0where e(t) is the creep strain, t is the time, s 0 is theapplied stress, t is the retardation time, and f(t) is the distributionof the retardation time. Although it is assumedthat such an expression should represent an experimentalcreep, it is actually not so because accurate informationabout f(t) is rather elusive [23]. Several models weredeveloped using the constitutive relation to explain thecreep behavior of polymeric materials using very simpleelastic or viscous system and a complex combination ofboth. The basic constitutive equation for creep models hasthe following qualitative form:e ¼ e 0 þ e c ; (3)where e is the total strain at some time, t, after a stressapplication, e c , time-dependent creep component of strainat t, and e 0 is the instantaneous strain after stress application.The instantaneous strain component (e 0 ) is elasticand the creep component, e c , is a simple arithmetic functionof time (linear, logarithm, exponential, power law),either alone or in combination, to define the basic behaviorof this aspect. All linear viscoelastic models are madeup of linear springs and linear dashpot. Inertia effects areneglected in such models [7]. Also, there have been severalattempts to develop constitutive models to describetheir nonlinear creep behavior. The most widely usednonlinear viscoelastic models are the Schapery model andits modifications, which are based on irreversible thermodynamicsprinciples and have shown to accuratelydescribe creep performance [24, 25]. More recently, Deanand Broughton [26] modeled the nonlinear creep behaviorof polypropylene under uniaxial tension using a stretchedexponential function with four parameters. Nonlinearbehavior arises because one of the parameters, related toa mean retardation time for the relaxation process responsiblefor creep, is dependent on stress. There is no doubtthat nonlinear viscoelastic behavior predominates over amajor portion of the whole response interval of most polymericmaterials and plays the key role in most applications[16, 17]. On the other hand, if the material is testedin the linear viscoelastic range, its creep behavior can berepresented by simple rheological models. The Burgermodel, which is a combination of elastic and viscous elements,has been reported to give a satisfactory representationof the creep compliance [7, 9, 27].Burger model is a series of combinations of Maxwelland Kelvin-Voigt models [7, 9, 27]. Figure 1 shows ascheme of four-element Burger model. According to thismodel, the total strain is given by the general equatione ¼ e 1 þ e 2 þ e 3 ; (4)e ¼ s þ s ð1 expðtE 2 =ZE 1 E 2 ÞÞþ ts ; (5)2 Z 1where e 1 and e 2 are the elastic and viscous strain representedby the Maxwell model and e 3 is the viscoelasticstrain represented by Kelvin-Voigt model, E 1 and E 2 representthe moduli of the Maxwell and the Kelvin spring,Z 1 and Z 2 represent the viscosity of the Maxwell and942 POLYMER ENGINEERING AND SCIENCE—-2008 DOI 10.1002/pen

The long-term creep behavior may be predicted fromshort-term creep data by applying the time-temperaturesuperposition (TTS) principle. The premise of TTS is thattemperature accelerates the creep deformation kinetics.Hence creep curves at different temperatures may beshifted along the logarithmic time axis until they superimposeto form creep master curve extending to much longertimes than the short term data. The validity of TTSmust be established by long-term tests but is nonethelessuseful for obtaining a general description of the material’slong-term behavior [29, 30].FIG. 1.Schematic representation of the four-element Burger model.Kelvin dashpot respectively, s is the applied stress andt is the creep time.Besides the constitutive models, empirical or mathematicaldescriptions have also been widely studied [7]due to their simple expression and satisfactory simulationor prediction capability. Among these, the Findley powerlaw model is commonly accepted. The constitutive equationproposed by Findley is simplified in the followingform [7, 28]:e F ¼ e 0 þ e c t n : (6)Here n is a dimensionless material parameter, e 0 is the instantaneousor time-independent strain (functions of stressand environment variables including temperature, moisture,etc.), and e c is the time-dependent term. The symbolt may be taken to represent a dimensionless time ratiot/t 0 , where t 0 is the unit time. e 0 , e c , and n are creepparameters and each can be assigned a numerical valueby experimentally measuring the instantaneous and timedependentcompliance of a viscoelastic material and thencalculating the values from Eq. 6. Because of the oversimplificationof this model (it does not take into considerationthe changes in the shape of the material), thevalues calculated from the above equation cannot beuniversally applied. The shape of a material depends onmolecular mobility and is a function of the microstructureand molecular alignments during the processing of materialsand the shape from sample to sample can be influencedby other external factors such as moisture content,humidity, and temperature. Since all-PP composites likeother viscoelastic materials are very much influenced bythe factors as detailed above, the parameters obtainableby the power law model can only give an approximationof the real behavior. Nevertheless, it is expected that thismodel can be used as a first hand tool to predict the longtermdeformation behavior of all-PP composites.EXPERIMENTALIn this study, commercially available all-PP (PURE 1 )coextruded tapes, obtained from Lankhorst-Indutech B.V.(Sneek, The Netherlands), was used to make the unidirectional(UD) and cross-ply (CP) laminates. The PP tapeshave a skin-core-skin (A-B-A) morphology with PPhomopolymer as the core and a random PP copolymer asthe skin layer. The other characteristics of these coextrudedtapes are available in the literatures published earlier[31, 32].The manufacture of all-PP composites involved a twostageprocess—winding of the PP tapes, [both unidirectional(08) and cross-ply (08/908)], and consolidation ofthe fabric plies at a suitable temperature and pressure inan autoclave. For winding the tapes, a filament windingtechnique was adopted. A detailed description of the compositemanufacture is available in the references [31, 32].Flexural creep tests were performed using three-pointbending mode at temperatures ranging from 208C to808C, in the same DMA Q800 apparatus. Isothermal testswere run on the specimens by increasing the temperaturestepwise by 108C. Before the creep measurement, thespecimens were equilibrated for 5 min at each temperatureand then the flexural creep behavior was tested for30 min. All the tests were performed under a constantload of 10 MPa. Test specimens of dimensions 60 3 153 1.8 mm 3 (length 3 width 3 thickness) were employedfor creep studies and the average of three statisticallyrelevant creep data has been reported for each kind ofspecimen.To determine the crystallinity of the all-PP composites,differential scanning calorimetric (DSC) investigationswere performed before and after the creep tests using aMettler-Toledo DSC 821 instrument (Greifensee, Switzerland).The crystallinity contents of the composites werecharacterized from the enthalpy of fusion associated inthe first heating run. Heating scans were performed from258C to 2008C at a heating rate of 108C/min with nitrogenblanketing inside the sample chamber. An average ofthree specimens was taken for each of the specimens.The crystallinity was then determined by taking the ratioof the enthalpy of fusion of the all-PP composite to theenthalpy of fusion of 100% crystalline PP taken as207.1 J/g [33].DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2008 943

Isochronous strain-stress plots for CP all-PP composite speci-FIG. 2.mens.RESULTS AND DISCUSSIONDetermination of Linear Viscoelastic RegionTo ensure working in the linear viscoelastic region,isochronous experiments were conducted for the all-PPcomposites. Short-term creep experiments lasting for30 min was performed for a series of stresses increasedstepwise from 2 MPa to 10 MPa. From the series ofapplied stresses, strain–stress plots were constructed. Figure2 represents the strain–stress results for the CP all-PPcomposite specimens conducted at room temperature(23 6 1)8C. From analysis of the isochronous tests forthe CP all-PP composites (the specimens showing thelargest creep deformation), a stress of 10 MPa wasselected for testing all the samples.Experimental Creep ResultsThe experimental data for the creep strain versus timeat different temperatures ranging from 208C to808C areshown in Fig. 3 for all-PP composites with UD and CPlay-ups. As expected, variation of the test temperatureshows a remarkable effect on the creep behavior of theall-PP composites with enhanced creep tendency as thetemperature is increased. This effect can be recognized asthe effect of temperature on the mobility of the macromolecularchains. With the increase in temperature, mobilityof the macromolecules increases which leads to higherdeformation of the composites. It is also clear from theresults that the CP laminates show higher creep strainthan the UD ones at all the temperatures indicating higherresistance to creep in the case of later. This could beexplained from the difference in the composite morphology,namely crystallinity and void content as well as thereinforcement architecture [32]. In the case of UD composites,in addition to the lower amount of voids as well ashigher percentage of crystallinity, higher volume fractionof the tapes in the longitudinal direction remarkablyincreased the strength and stiffness of the all-PP compositesand thereby a reduction in creep strain. Figure 3 alsoindicates that the creep rate increases with the increase intemperature as the slope of the creep curves tends toincrease with temperature.From Fig. 3, it appears that the shapes of the variouscreep curves are amenable to the well-known reductionscheme of time–temperature superposition. Coincidenceof the data is good and the master curves obtained byTTS principle (Fig. 4) show higher creep resistance forUD all-PP composites as anticipated from the short-termcreep data. Generally, the master curves are importantsince the polymer’s behavior can be traced over muchgreater periods of time than those determined experimentally.The shift factors, a T , necessary to generate the mastercurves was obtained directly from the experimentalcreep curves plotted against time by measuring theamount of shift along the time scale necessary to superimposethe curves on the reference. The reference temperaturewas chosen as 308C for both the composite types.The log a T values thus obtained by shifting the creepcurves are listed in Table 1. It shows that log a T decreasesFIG. 3. Creep response of UD and CP all-PP composites at different temperatures (n 208C, l 308C,~ 408C, ! 508C, ^ 608C, 3 708C, " 808C); solid lines represent Burger fit.944 POLYMER ENGINEERING AND SCIENCE—-2008 DOI 10.1002/pen

TABLE 2.Parameters obtained from the Burger model.SpecimenTemperature(8C)E 1(MPa)E 2(MPa)g 1(GPa-s)g 2(GPa-s)FIG. 4. Creep strain master curves generated for all-PP compositeswith unidirectional (UD) and cross-ply (CP) lay-ups; T ref is the referencetemperature.with increasing temperature. The negative sign indicates ashift to the right of the creep curves, that is, toward thereference at lower temperature. A higher shift value thusindicates a larger shifting of the creep curve to the referencecurve. The shifting scheme indicates that as temperatureincreases, molecular relaxation accumulate at a constantrate and that the underlying mechanism of creepremains unchanged.Creep Modeling AnalysisSimulated creep curves using the Burger model arealso shown in Fig. 1 with the solid lines. It can be seenthat they show a satisfactory agreement with the experimentaldata at each temperature. The first instantaneousdeformation arises from the spring or the elastic element(E 1 ) and later time-dependent deformation comes fromthe parallel spring and dashpot (g 2 ) and from the viscousdashpot flow (g 1 ). The model parameters obtained for theall-PP composites produced by unidirectional and crossplylay-ups are shown in Table 2. According to theseresults, viscosity decreased with increase in the creep testtemperature and higher flow occurred in the dashpot andpermanent deformation increased. In the viscous part, g 2TABLE 1.Temperature (8C)Shift factor (log a T ) for the UD and CP all-PP composites.UDlog a T20 1.16 1.06730 0 040 21.215 21.14850 22.304 21.95160 23.151 22.51270 23.786 22.99880 24.426 23.501CPUD 20 1.22 6.37 12.11 0.4230 1.10 4.30 10.00 0.2540 0.94 2.67 5.00 0.1550 0.78 1.74 2.50 0.1060 0.66 1.40 2.00 0.0770 0.55 1.20 1.67 0.0780 0.48 1.09 1.11 0.06CP 20 0.49 2.39 3.33 0.1530 0.44 1.57 2.00 0.1040 0.38 0.91 1.25 0.0650 0.33 0.68 1.11 0.0360 0.29 0.63 1.10 0.0270 0.25 0.62 1.07 0.0280 0.23 0.62 1.00 0.02tends to decrease with the increase in the temperatureconfirming that the rise in temperature increases the mobilityof the polymer chains. The modulus (E 1 ) of theMaxwell spring showed a decreasing function with temperature,which can be attributed to the softening of thematerial at elevated temperature and the stiffness thusdecreased with diminished instantaneous modulus. Thisindicates that the plastic deformation becomes severe atelevated temperature. The retardant elasticity (E 2 ) and viscosity(g 2 ) showed a similar dependency on temperature,thus decreasing with increasing temperature. It could bealternatively stated that the deformation of the Kelvin-Voigt unit increased with increasing temperature. InterestinglyTable 2 also shows that the variation in the tapelay-up has a significant effect on the viscous and elasticcomponents of the Burger model as they were found tobe higher for UD than for the CP composite specimens.This arises from the difference in the microstructures andthe tape lay-up. It strongly suggests that the variationin the reinforcement architecture has a significant effect onthe viscoelastic properties. Because of the higher viscousflow and lower elastic modulus, CP composite specimensdeform more than that of the UD counterpart under thesame loading condition.The results mentioned earlier thus shows that the Burgermodel provides a constitutive representation of creepof the all-PP composites and from the modeling parameterstheir structure-to-property relationship can be betterrealized. Additionally, an empirical equation, Findleypower law, is frequently applied to simulate and predictthe long-term creep properties due to its simple expressionand satisfactory applicability [7]. In this case, themethod was adopted to provide a broad understanding ofthe all-PP composites. The simulated curves (representedby solid lines) of the all-PP composites produced by differenttape lay-ups are shown in Fig. 5. It can be seen thatthey agree well with the experimental data at each temperatureand the power law can be used as a predictivemodel for creep. The complete modeling parameters e 0 ,DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2008 945

FIG. 5. Creep response of UD and CP all-PP composites at different temperatures (n 208C, l 308C,~ 408C, ! 508C, ^ 608C, 3 708C, " 808C); solid lines represent Findley fit.e c , and n under each condition are listed in Table 3, fromwhich an explicit dependency of the modeling parameterson temperature becomes evident. Since n is a material parameterindependent of the stress or a state of combinedstress, it was considered constant. The time-independent(instantaneous) strain, e 0 increased rapidly with temperature.Also the time-dependent, e c , showed an increase withthe increase in temperature. Additionally Table 3 showshigher increment in e 0 and e c with temperature for CPthan UD all-PP composites as anticipated from their creepbehavior. The reinforcement architecture showed no effecton the power n.Next, the predictive ability of the Findley model forthe long-term creep performance was verified and comparedwith the master curves generated at T ref ¼ 308C.Thus, the Findley power law model was used to simulatethe creep performance over an extended time scale of10 6 –10 7 s (11–115 days) using the model parametersavailable from the short-term creep performance at 308C.This has been shown in Fig. 6 for the UD and CP all-PPcomposites. Interestingly it shows that (i) the power lawholds good only at the initial stages when compared withthe master curves (ii) then the simulated creep curvesshowed a significant deviation from the TTS mastercurves as the creep time was extended to days andmonths. This was unexpected as both the Findley modeland the TTS principle are widely used in engineeringapplications for the prediction of long-term creep performance[7] and is therefore expected to predict similarlong-term performance. However, a primary reason forthis unexpected result could be accomplished to themicrostructural change, viz. crystallinity of the all-PPcomposites during the short-term creep tests at highertemperatures. It was detected during the thermal analysisof the same specimen tested for creep that the crystallinityof the material increased significantly than before conductingthe creep tests. Figure 7 thus represents the DSCthermograms of UD all-PP composites before and afterthe creep tests were performed at different temperatures.Similar effect was also observed in the case of the CPall-PP composites and the results are summarized in Table4. It shows that the crystallinity increases from 51% to56.5% for UD composites and from 49.5% to 54.5% forCP composites respectively. During creep test, the specimenswere kept isothermal for 5 min at each temperaturebefore they are tested. This process acted as an annealingTABLE 3.The simulated parameters of the Findley power law model.SpecimenTemperature(8C)e 0(%)e c(10 22 s 2n ) n r 2UD 20 0.076 0.004 0.277 0.98130 0.083 0.006 0.274 0.9840 0.095 0.01 0.29 0.98550 0.11 0.016 0.283 0.98460 0.13 0.021 0.277 0.98370 0.153 0.024 0.292 0.98880 0.179 0.025 0.31 0.992CP 20 0.189 0.012 0.279 0.98530 0.198 0.021 0.272 0.99240 0.217 0.033 0.286 0.99550 0.264 0.038 0.294 0.98560 0.299 0.051 0.262 0.97770 0.342 0.058 0.245 0.97280 0.39 0.062 0.239 0.971FIG. 6. TTS principle and Findley power law model applied to the all-PP composites; both experimental and reference temperature is 308C.946 POLYMER ENGINEERING AND SCIENCE—-2008 DOI 10.1002/pen

effect and increased the total crystallinity of the testspecimens by secondary crystallization. The creep resistanceof the all-PP composites at higher temperature wastherefore supposed to be higher than would have beenexpected without a change in material property. Hencethe master curves, generated by superimposing the creepdata obtained at different temperatures to the reference,predicted lower creep strains or higher creep resistancefor the all-PP composites compared with the Findleypower law prediction. Note that Fig. 7 also shows that themelting temperature increased from 1658C to 1678C forUD all-PP composites as a result of secondary crystallization.This also supports our explanation that additionalcrystallization occurred while conducting short-term creeptests at different temperatures. However it raises a questionwhether the TTS principle or the Findley equationcan be used universally to successfully predict the longtermcreep performance of semicrystalline thermoplastics,like the all-PP composites, whose T g (about 08C) issignificantly lower than the test temperatures. It can beargued here that the Findley power law is an oversimplifiedcreep model that does not take into account of thechange in material property during the test. On the otherhand TTS can be considered as more precise since it is asuperposition of the creep data obtained at different temperatures.The debate will, however, continue as towhether Findley or TTS can be considered reliable insuch cases. Nevertheless the predicted creep strain wasfound to be obviously lower for the UD composites,which once more showed the effectiveness of the reinforcingarchitecture on the creep behavior. Thus from themodel analysis of the creep curves it appears that the basicproperty that changes due to the change in the reinforcementarchitecture is the viscoelasticity. When the all-PP tapes were consolidated unidirectionally, the elastic aswell as the retardant modulus of the composites increased;correspondingly their viscosity also became higher whichlead to a lower instantaneous creep strain followed bydecreasing coefficient of the time-dependent term, e F .SpecimenTABLE 4.Summary of the results of DSC analysis.Crystallinity (%)T m (8C)Before After Before AfterUD 51 56.5 165 167CP 49.5 54.5 161 162CONCLUSIONThe experimental results showed a dependency of thecreep behavior on the reinforcement architecture of all-PPcomposites with higher creep resistance achievable whenthe all-PP tapes are reinforced unidirectionally. Both theBurger and Findley power law model could be satisfactorilyapplied to simulate the short-term creep behavior andthe modeling results provided a comprehensive understandingof the deformation mechanism for these compositesystems. The parameter analysis based on the Burgermodel showed higher elastic and viscous componentsassociated with the all-PP composites when the PP tapeswere consolidated unidirectionally. Analysis of the Findleymodel showed lower instantaneous creep strain and adecreasing coefficient of the time-dependent creep termfor unidirectionally laid up all-PP tapes. The viscoelasticmodels thus indicated that viscoelastic property changedconsiderably by changing the reinforcement architecture.It could also be concluded from the above results that thecreep mechanism of the all-PP composites is not alteredby the reinforcement architecture but their creep behavioris affected significantly. A comparison between the Findleyprediction based on the model parameters at 308C andthe TTS principle (obtained at T ref ¼ 308C) showed thatthe later predicted a lower creep strain at larger timescales. This unexpected behavior could be explained bythe change in crystallinity of the material while performingthe short-term creep at higher temperatures. Howevera question still remains whether the TTS principle or theFindley equation can be successfully applied to predictthe long-term creep for material systems like all-PP compositeswhose glass transition temperature is well belowthe test temperature and the microstructural change canoccur during the test. Nevertheless it has been suggestedhere that TTS can provide a more precise prediction insuch cases since it involves a superposition of the creepdata at different temperatures.REFERENCESFIG. 7. DSC thermograms of the UD all-PP composites before andafter performing the creep test.1. T. Peijs, Mater. Today, 6, 30 (2003).2. N. Cabrera, B. Alcock, J. Loos, and T. Peijs, Proc. Inst.Mechanical Eng. J. Mater.: Design Appl., 218, 145 (2004).3. B. Alcock, N.O. Cabrera, N.-M. Barkoula, J. Loos, and T.Peijs, Compos. A, 37, 716 (2006).4. B. Alcock, N.O. Cabrera, N.-M. Barkoula, J. Loos, and T.Peijs, Compos. A, 38, 147 (2007).DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2008 947

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