High-rate compression of polypropylene

Three grades of polypropylene were tested in compression at room temperature, across an unusually wide range of strain rate: 10-4 to 104s-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 logio(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 relaxation time exceeds the experimental time-scale at the highest strain-rates, but lies below It for the quasi-static tests. © 2008 American Institute of Physics