Evolution of dynamic fractures in poly(methyl methacrylate)

The results from single edge notched tension tests support the idea that the nucleation, growth, and coalescence of penny shaped microcracks govern the fracture evolution in poly(methyl methacrylate) (PMMA). The density of the microcracks (seen as parabolic markings on the fracture surface), and therefore the roughness of the fracture surface, increases with the crack velocity. The damage beneath the main fracture surface also increases with crack velocity and the crack length. This damage is formed by attempted and successful crack branches originating from the main fracture and other cracks propagating alongside the main fracture. Microscopy of the virgin material and fractured surfaces showed no consistent evidence of pre-existing flaws, dust particles, or other impurities that would provide nucleation sites for the microcracks. It can therefore be argued that the material microstructure, chain length in particular, must play a significant role in the nucleation of microcracks. This can be supported by an earlier work in which strong dependence of the microcrack density on the molecular weight of PMMA was reported. Crack velocity measurements show rapid initial acceleration of the crack followed by nearly constant mean velocity. The mean velocity is found to increase with decreasing initial notch depth. Oscillations in the crack velocities were also observed and they were more pronounced at higher crack velocities. To a large extent the degree of crack velocity oscillation is dependent on the filtering technique applied to process the raw data. Therefore, no conclusive correlation between the fracture history and surface was obtained.