The Role of Dilatation in the Nonlinearly Viscoelastic Behavior of PMMA under Multiaxial Stress States

In order to better understand phenomena related to "yield-like" behavior of polymers, the nonlinear thermo-mechanical behavior of Polymethyl Methacrylate (PMMA) under combined axial (tension, compression) and shear stress states (torsion) is investigated on thin walled cylindrical specimens at temperatures between 22 and 110°C. In contrast to the mutual independence of shear and dilatational response under conditions appropriate for linearized viscoelasticity, one observes an increasingly strong influence of the first stress or strain invariant on shear creep at shear strains in excess of 0.5%. While shear stresses alone elicit nonlinear response in creep (rates) as "intrinsically nonlinear shear response", the superposition of small positive dilatation accelerates shear deformations while negative dilatation retards it in qualitative agreement with free volume arguments when comparison is effected via maximum shear. In addition, an isochronal representation of the "intrinsically nonlinear shear response" demonstrates that the nonlinear behavior becomes more pronounced the closer one approaches the glass transition temperature from below.