STRUCTURAL REARRANGEMENTS DURING YIELDING OF GLASSY-POLYMERS

The tensile yield and post yield behavior of polycarbonate (PC) are ex amined by means of a new experimental device, based on a noncontact me asurement of the deformation. This method has the advantage of a detai led measurement of the deformation along the gauge length, which leads to the construction of the true stress-strain curves of the material tested. Especially in the case of nonuniform deformation, where neck i nitiation and neck propagation occurs, the correct description of the intrinsic response of the material requires true stress-strain plots. A viscoplastic model based on the fundamental assumptions of Eyring is used to describe the yield and post-yield behavior of PC. The strain softening is attributed to a relaxation mechanism that was introduced in previous treatments, and this effect is supported by DSC tests. The theoretical results are compared with the results obtained by an elas to-plastic model, developed elsewhere, and it is shown that the materi al follows a viscoplastic path in this deformation procedure.