Constitutive equations for the viscoplastic-damage behaviour of a rubber-modified polymer

In this work, experimental tests in tension on a RT-PMMA material have been achieved under various constant true strain rates and at room temperature. The volumetric dilatation was determined in real time during the deformation by videomeasurements. The experimental results have revealed the presence of both the nucleation and growth deformation mechanisms. Modified viscoplastic constitutive equations for homogeneous glassy polymers at isothermal loading, including strain softening and strain hardening, are proposed. Such a model is based upon an approach originally developed for metals at high temperature. Next, this modified model is coupled with a micromechanics formulation, using the Gurson-Tvergaard model, to investigate the macroscopic mechanical response of the RT-PMMA. The constitutive relation includes strain softening, strain hardening, strain rate sensitivity and void evolution. In the test conditions, the modified viscoplastic model coupled with the original Gurson-Tvergaard model produce quantitative agreement with experimental observations.