HIGH-RATE COMPRESSION OF POLYPROPYLENE

Abstract

Three grades of polypropylene were tested in compression at room temperature, across an unusually wide range of strain rate: 10−4 to 104 s−1. The quasi‐static testing was done in a Hounsfield machine fitted with a digital image acquisition kit, while tests at the highest strain rates were carried out using a compression split Hopkinson pressure bar. The strain rate dependence of compressive yield stress was compared with the Eyring prediction, and found to be a nonlinear function of log10(strain‐rate). The nonlinearity is attributed to the presence of two relaxation processes in polypropylene, with differing activation volumes: the α‐ and β‐processes. According to the Bauwens two‐process model this would lead naturally to curved Eyring plots, where the apparent activation volume decreases with increasing strain‐rate. Another prominent feature in the experimental results was the increase in magnitude of post‐yield strain‐softening with increase in strain‐rate. This indicates that the dominant structural re...