History- and temperature-dependent yield phenomena of polycarbonate related to its rate of physical ageing

The yield force (or stress) of polycarbonate in simple extension was studied to determine its dependence on deformation history, including the stretching rate, and temperature. At different temperatures, the stretching of a specimen at a constant rate was interrupted both before yielding and during cold drawing to allow the occurrence of stress relaxation and the accompanying physical ageing (hardening). When a test was resumed, progressive erasure of ageing occurred, leading to a yield force whose value compared to that obtained by continuous stretching can be explained qualitatively in terms of the rate of physical ageing, determined earlier at an extension of 2.6% over a wide temperature range. In addition, the present data support the conclusion reached previously that physical ageing occurs during stress relaxation at a fixed deformation. Also determined were the yield and drawing stresses and the yield strain from -90 to 100°C. © 1991.