Evolution of dynamic fractures in PMMA : experimental and numerical investigations

A combined experimental/numerical study has been conducted to investigate dynamic fractures in poly(methyl methacrylate) (PPMA). The results obtained from single-edge-notched-tensile (SENT) fracture tests support the idea that the evolution of fracture in PMMA is governed by nucleation, growth and coalescence of penny-shaped micro-cracks. The density of the microcracks and therefore the roughness of the fracture surface increase with the crack velocity. Both the surface roughness and the size of the process region increase with the crack length for a given specimen. 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 micro-cracks. Instead, it was observed that molecular weight significantly affects the fracture, and therefore must play an important role in the nucleation of micro-cracks. The crack velocity measurements show rapid initial crack acceleration followed by a nearly constant
mean velocity, which was in some cases well above previously reported terminal crack speed. 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 velocities oscillations is dependent on the filtering technique applied to process the raw experimental data. Therefore, no conclusive correlation between the fracture histories and fracture surfaces was obtained. Finite Volume (FV) method was developed for the numerical simulations of the experiments. Global material behaviour was approximated as linear elastic, while a Cohesive Zone Model (CZM) was used for defining the local separation process of the material. Numerical predictions show good agreement with experimentally observed variations of the process region and the crack velocity with initial crack length. Oscillations in the crack speed are also predicted.