Application of the plane simple shear test for determination of the plastic behaviour of solid polymers at large strains

Abstract

A simple shear test apparatus was designed and used in experiments for determining the plastic behaviour of various amorphous and semicrystalline polymers at large shear strains. The geometry and dimensions of the specimens were determined after a critical evaluation of the test conditions, so as to avoid plastic buckling of the specimens, and to minimize undesirable stresses at the specimen ends. Using the present technique, it was possible to conduct tests at room temperature up to strains of 200% for polymethyl methacrylate (PMMA) and 1000% for polyethylene, without extensive crazing. Optimum precision and homogeneity of strains within the samples could be achieved because of firm guiding of the gripped specimen heads during the tests. A systematic study of the influence of shear strain rate and temperature on the plastic behaviour was made particularly for the polyethylene samples. The kinematics of large deformation simple shear is discussed and relations between the stress and the finite-strain tensors is presented, with particular attention being paid to the development of normal stresses. The problem of end effects is also investigated. Finally, it is shown that the strain hardening of polyethylene under simple shear is much smaller than under uniaxial tension. A possible interpretation of this behaviour is proposed in terms of the uniqueness versus multiplicity of slip systems.