Flexural creep of all-polypropylene composites: Model analysis

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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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Flexural creep of all-polypropylene composites: Model analysis

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