| Summary: | Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1999. === Includes bibliographical references (p. 248-254). === The mechanical behavior of elastomeric materials is known to be rate-dependent 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 aspects of response. Based on a detailed experimental investigation a new constitutive model for the time-dependence of unfilled elastomers has been developed. The foundation of the model is that the mechanical behavior can be decomposed into two parts: an equilibrium network corresponding to the 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 further assumed to be governed by the reputational motion of molecules having the ability to significantly change conformation and thereby relaxing the overall stress state. To model the behavior of particle filled elastomers the newly developed constitutive framework is then extended to include filler interactions by amplification of the first strain invariant. In an effort to examine some of the assumptions that are common in the constitutive modeling of particle filled elastomers, a detailed series of micromechanical models were constructed using two- and three-dimensional finite elP.ment simulations. The results indicate that the effect of filler particles can be accurately predicted using stochastic three-dimensional simulations suggesting that successful modeling mainly requires a rigorous treatment of the composite nature of the microstructure and not molecular level concepts such as alteration of mobility or effective crosslinking density in the elastomeric phase of the material. A direct comparison between the new model and experimental data for a number of different elastomers the observed behavior. === by Jörgen S. Bergström. === Ph.D.
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