Microscopic Fracture Modes of Brittle Polymers in Dynamic Crack Propagation

Fracture process in the dynamic crack propagation is studied at microscopic level based on experiments of brittle fracture of PMMA plate in fixed sided condition. The feature of fracture surface changes with the crack propagation velocity; parabolic line pattern in low velocity region, crazing and micro-branching surfaces in high velocity region, and the transition to bifurcations of the crack path at high loading energy case. The microscopic fracture surface patterns are caused by the growth and coalescence of microcracks ahead of the running main crack. The critical stress condition of the microcrack growth initiation is examined from the fracture surface analysis and the dynamic stress analysis of the crack. With use of the condition, the formation mechanism of parabolic pattern is numerically simulated. The fracture surface formation energy is calculated for the propagation velocity from the experimental result and related with the mode transition mechanism.