A study of mixed-mode dynamic crack initiation in PMMA

The effect of mode-mix on crack initiation (toughness and trajectory) is well documented both theoretically and experimentally for quasi-static fracture. Dynamic fracture (single or mixed-mode) analysis is generally more complicated since inertial effects cannot be neglected. Recent (mode II) shear impact experiments in steel and polycarbonate have shown the existence of fracture mode transitions (brittle to ductile. including shear band formation) as a function of the impact velocity. Here we report experimental results for mixed-mode dynamic crack initiation in commercial PMMA as a model for brittle materials. The two-dimensional combination of a major and a minor mode (1-11 or 11-1 respectively) was systematically achieved using two distinct specimen geometries impacted at various velocities. The results show that, regardless of the dominant loading mode, the initial kink angle between the crack and the original notch line increase with the impact velocity. These results are described and discussed in terms of a simple criterion