Micromechanics of cyclic softening in thermoplastic vulcanizates

The strain history dependence of the stress-strain behavior of thermoplastic vulcanizate (TPV) materials is studied through a set of experiments and micromechanical models. Thermoplastic vulcanizates are a class of composite material consisting of a high volume fraction of fully-cured elastomeric particles in a thermoplastic matrix. The stress-strain behavior of TPVs is found to soften after having been subjected to an initial load/unload cycle. rn this paper, the TPV strain history dependence is experimentally documented on a representative TPV material (TPV-R) by subjecting TPV-R to load/unload/reload histories in plane strain compression to various magnitudes of strain. The stress-strain behavior is observed to be more compliant upon reloading, but the tangent modulus is found to increase with strain until the reloading stress-strain curve joins the initial curve. An increase in the magnitude of the initial strain excursion increases the compliance observed during reloading. The unloading behavior following the reload is very similar to the unloading behavior following the initial load. The underlying microscopic mechanisms which govern the strain history effects are investigated using micromechanical modelling of the composite structure and its deformation. The stress-strain behaviors predicted by the simulations are found to be in good agreement with the experimentally observed behavior over the entire strain history for each magnitude of strain considered. The models reveal the softening of the material to result from a reorganization of the particle/matrix microstructural configuration due to plastic stretching of interparticle ligaments during the initial load step followed by ligament bending and rotation during the unloading step. The new microstructural configuration that exists after the first load/unload cycle favors bending and rotation of the (now thinned) matrix ligaments (as opposed to plastic deformation 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 are also captured well by the model. (C) 2001 Elsevier Science Ltd. AII rights reserved.