Influence of crack speed on fracture energy in amorphous and rubber toughened amorphous polymers

To study the influence of the crack speed at initiation on measurements of the toughness of amorphous polymers such as polycarbonate (PC) and rubber t oughened poly(methyl methacrylate) (RTPMMA). 'compact tension' sprecimens w ere tested at a medium loading rate (0.6 m s(-1)). The specimens were equip ped to allow location of the position of the crack tip and to trigger a pho tograph on passage of the crack. The experimental procedure M as validated using PC for which the crack speed was found to change suddenly over very s hort distances. owing to stick slip propagation. Conversely. fracture tests on RTPMMA showed that the acceleration of the crack tip due to the decreas e in fracture energy with crack speed, is controlled by a process zone, the size of which did not allow the systematic use of linear elastic fracture mechanics. The rapid crack propagation regime in RTPMMA corresponds to a pr opagation stabilised at the macroscopic crack branching velocity. This cons tant crack speed corresponds to frustrated microbranching and leads to non- unique values of the fracture energy at the macroscopic branching velocity. (C) 2001 IoM Communications Ltd.