Micromechanics of cyclic softening in thermoplastic vulcanizates

The strain history dependence of the stress-strain behavior of thermoplasti c vulcanizate (TPV) materials is studied through a set of experiments and m icromechanical models. Thermoplastic vulcanizates are a class of composite material consisting of a high volume fraction of fully-cured elastomeric pa rticles in a thermoplastic matrix. The stress-strain behavior of TPVs is fo und to soften after having been subjected to an initial load/unload cycle. rn this paper, the TPV strain history dependence is experimentally document ed on a representative TPV material (TPV-R) by subjecting TPV-R to load/unl oad/reload histories in plane strain compression to various magnitudes of s train. The stress-strain behavior is observed to be more compliant upon rel oading, but the tangent modulus is found to increase with strain until the reloading stress-strain curve joins the initial curve. An increase in the m agnitude of the initial strain excursion increases the compliance observed during reloading. The unloading behavior following the reload is very simil ar to the unloading behavior following the initial load. The underlying mic roscopic mechanisms which govern the strain history effects are investigate d using micromechanical modelling of the composite structure and its deform ation. The stress-strain behaviors predicted by the simulations are found t o be in good agreement with the experimentally observed behavior over the e ntire strain history for each magnitude of strain considered. The models re veal the softening of the material to result from a reorganization of the p article/matrix microstructural configuration due to plastic stretching of i nterparticle ligaments during the initial load step followed by ligament be nding and rotation during the unloading step. The new microstructural confi guration that exists after the first load/unload cycle favors bending and r otation of the (now thinned) matrix ligaments (as opposed to plastic deform ation of the ligaments) during reloading; the ligament bending and rotation occur under low stress levels which results in the more compliant response . The additional features of the stress-strain behavior during reloading ar e also captured well by the model. (C) 2001 Elsevier Science Ltd. AII right s reserved.