Unsteady Dynamic Crack Propagation in a Brittle Polymer

Dynamic crack propagation in a brittle polymer, poly(methyl-methacrylate) (PMMA), was studied using the method of caustics in combination with a Cranz–Schardin high-speed camera. Four different types of specimen geometry and loading method were employed to achieve the crack acceleration, deceleration, and/or reacceleration processes in one fracture event. The dynamic stress intensity factor KID and crack velocity were obtained in the course of the crack propagation and the corresponding relationship was determined. The effect of the crack acceleration and deceleration on the KID-velocity relationships was as follows: (1) the variations of KID and the velocity were strongly influenced by the specimen geometry and loading method; (2) the velocity change was qualitatively in accord with KID; (3) KID for a constant crack velocity was larger when the crack decelerated than it was when the crack accelerated or reaccelerated; (4) KID for an acceleration-free crack was uniquely related to the velocity; and (5) KID could be expressed as two parametric functions of the velocity and acceleration.