AN EXPERIMENTAL / NUMERICAL INVESTIGATION OF THE DYNAMIC FRACTURE CHARACTERISTICS OF PMMA

A cohesive zone model (CZM) was used in conjunction with the finite volume method to simulate the dynamic fracture of single edge notched tensile (SENT) specimens of PMMA under essentially static loading conditions. A series of experiments was carried out to study the fracture behaviour of specimens prepared from low, standard and high molecular weight sheets with 0.1 mm, 0.5 mm and 1.0 mm notch depths. The speed of crack propagation through the SENT specimens was measured using an electrical resistance method. It was found that the crack speed and failure load increased with reduced notch depths. For a given notch depth, the failure load was independent of the molecular weight. The surface roughness and subsurface damage were seen to increase substantially as the molecular weight was reduced. Large crack velocity fluctuations were associated with increased surface roughness. In addition, successful crack bifurcation was observed in the low molecular weight specimens containing the shorter notch lengths. In the numerical simulations, global material behaviour was approximated as linear elastic while the cohesive zone model was used to define the local non-linear separation process of the material. Two distinct strategies were investigated. Firstly, the cohesive surfaces were assumed to exist a priori between the continuum cells in a fine mesh region to either side of the initial fracture plane. The cohesive characteristic was assumed to contain both an ascending part and a descending part, following Xu and Needleman [1]. These initially elastic cohesive laws alter the effective elasticity of the material and were found to be unsatisfactory in this regard. An alternative approach was taken whereby cohesive cells were only inserted if a specified failure criterion was satisfied, as employed by Camacho and Ortiz [2]. In this work, the cohesive strength and separation energy were held constant and the shape of the cohesive law was varied. This was found to have a profound effect on the dynamic fracture behaviour of the model.