Failure mode transitions in polymers under high strain rate loading

A rather unusual failure mode transition from brittle to ductile at high strain rates occurs under a combined pressure and shear loading. This transition also represents a change in the failure mode from a normal stress dominated fracture mode at low loading rates to a shear stress dominated shear banding failure at high strain rates. While most such observations have been in metallic materials, where such transitions are attributed to thermal softening caused by adiabatic heating, in this paper we present evidence of such mode transitions in a polymer. Experimental observations of the pressure-shear loading experiments are reported in two polymers; polycarbonate (PC) and polymethylmethacrylate (PMMA). Dynamic photomechanics techniques were used in obtaining information regarding the crack tip state in these experiments. While PC exhibits a failure mode transition to shear banding, PMMA changes to a shear mode of fracture; dynamic shear fracture has been observed in real-time using high speed photography for the first time. A numerical simulation of the experiment using a simple constitutive description of the material is performed in order to gain an understanding of the evolution of the crack tip fields that generate the observed mode transitions. The results of the simulation suggest that thermal softening may not play a significant role in the failure mode transitions in polymers. On the other hand, it is shown that the competition between shear yielding and normal stress dominated fracture mechanisms is the key to the failure mode transitions in these polymers.