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 toughened poly(methyl methacrylate) (RTPMMA), ʹcompact tensionʹ specimens were tested at a medium loading rate (0.6 m s^-1). The specimens were equipped to allow location of the position of the crack tip and to trigger a photograph on passage of the crack. The experimental procedure was validated using PC for which the crack speed was found to change suddenly over very short distances, owing to stick-slip propagation. Conversely, fracture tests on RTPMMA showed that the acceleration of the crack tip, due to the decrease 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 propagation stabilised at the macroscopic crack branching velocity. This constant crack speed corresponds to frustrated microbranching and leads to non-unique values of the fracture energy at the macroscopic branching velocity.