Flexural creep of all-polypropylene composites: Model analysis

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