Threshold conditions for dynamic fragmentation of ceramic particles

A framework of a particle-impact experiment was developed and used to study the dynamic fragmentation of brittle materials. In the experiment a small, spherical particle of a brittle material impacts against a thick, hard anvil. Unlike conventional particle-impact experiments, observations and measurements are focused on dynamic failure processes in the particle, minimizing, if not completely eliminating, damage to the target during the impact. The impact process is observed using high-speed photography. Results are presented for aluminum oxide and silicon nitride particles striking a titanium diboride anvil. The radius of the particles ranged from 0.40 to 3.18 mm. It is observed that above a certain threshold velocity the particle undergoes fragmentation upon impact. This threshold velocity depends on the particle material properties, and decreases with increasing particle radius. An elasto-dynamic finite element simulation of the particle-impact experiment was carried out. The finite element simulations showed that for these experimental conditions the stress amplitude within the particle, for a given impact velocity, is independent of the size of the particle. The size of the particle influences only the contact time, and thus the duration of the stress pulse applied to the particle. The size dependence of the threshold velocity was explained by using a cohesive zone model to represent the dynamics of the failure process. Thus, the experimental results allow the dynamic cohesive strength and the dynamic toughness of the material to be determined. In addition to reporting measurements of the threshold velocity, the observed pattern of particle fragmentation is described. This experiment provides a means of studying the threshold conditions for dynamic fragmentation of brittle materials, and illustrates some of the issues involved in the application of cohesive zone models to problems involving impact and dynamic fragmentation in brittle materials.