Simulation of dynamic fracture of an impact-loaded brittle solid: Microcracked and polycrystalline solids

A modified molecular dynamics method is introduced to describe the dynamic fracture and damage accumulation of impact-loaded brittle solids with realistic microstructural features. We examine the influence of pre-existing microcracks and grain boundaries on dynamic fracture. Pre-existing microcracks were found to increase the total damage (pre-existing damage plus accumulated damage due to impact) within a material for all combinations of impact pulse height and microcrack concentration studied. The damage accumulated due to the impact itself, however, is found to increase with increasing concentration of pre-existing microcracks at low impact pulse heights and to decrease with increasing concentration at higher impact levels. Therefore, microcracks may either enhance cracking by providing weaker crack paths or retard it by reflecting and dissipating tensile elastic waves. The presence of weak grain boundaries in a polycrystalline material enhances intergranular fracture while diverting cracks away from grain interiors. Weak grain boundaries lead to the fragmentation of the polycrystalline material into grain size powders. The number of broken grain boundary bonds increases and the number of broken matrix bonds decreases as the grain boundary bond strength is reduced. At low impact pulse strength the total number of broken bonds increases as the grain boundary bonds are weakened, but at high pulse strengths, the total number of broken bonds is reduced as the grain boundary bond strength is reduced. Increasing impact strength shifts the fracture mode from intergranular to transgranular. The principal influence of both microcracks and grain boundaries - appears to be their ability to scatter elastic waves propagating through the material.