CONSTITUTIVE MODELING OF THE LARGE-STRAIN TIME-DEPENDENT BEHAVIOR OF ELASTOMERS

The mechanical behavior of elastomeric materials is known to be rate-d ependent and to exhibit hysteresis upon cyclic loading. Although these features of the rubbery constitutive response are well-recognized and important to its function, few models attempt to quantify these aspec ts of response perhaps due to the complex nature of the behavior and i ts apparent inconsistency with regard to current reasonably successful models of rubber elasticity. In this paper a detailed experimental in vestigation probing the material response of carbon black filled Chlor oprene rubber subjected to different time-dependent strain histories i s presented. Some of the key observations from the experiments are: (1 ) both filled and unfilled elastomers show significant amounts bf hyst eresis during cyclic loading; (2) the amount of carbon black particles does not strongly influence the normalized amount of hysteresis; (3) both filled and unfilled elastomers are strain-rate dependent and the rate dependence is higher during the uploading than during the unloadi ng; (4) at fixed strain, the stress is observed to approach the same e quilibrium level with relaxation time whether loading or unloading. Ba sed on the experimental data a new constitutive model has been develop ed, The foundation of the model is that the mechanical behavior can be decomposed into two parts: an equilibrium network corresponding to th e state that is approached in long time stress relaxation tests; and a second network capturing the non-linear rate-dependent deviation from the equilibrium state. The time-dependence of the second network is f urther assumed to be governed by the reptational motion of molecules h aving the ability to significantly change conformation and thereby rel axing the overall stress state. By comparing the predictions from the proposed three-dimensional constitutive model with experimental data f or uniaxial compression and plane strain compression we conclude that the constitutive model predicts rate-dependence and relaxation behavio r well. (C) 1998 Elsevier Science Ltd. All rights reserved.