Threshold conditions for dynamic fragmentation of glass particles

A particle impact experiment was used to study dynamic fragmentation of glass particles. In the experiment a small spherical particle of glass impacts against a thick, hard anvil. Observations and measurements are focused on dynamic failure in the particle, minimizing, if not completely eliminating, damage to the target during the impact. The impact and fragmentation of the particle is observed using high-speed photography. Results are presented for soda-lime glass particles striking a titanium diboride anvil. The radius of the particles ranged from 0.60 to 2.50 mm. It was observed that above a certain threshold velocity the particle undergoes fragmentation upon impact. The threshold velocity was observed to decrease with increasing particle size. Elasto-dynamic finite element simulations show 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. In addition to reporting measurements of the threshold velocity, the observed pattern of particle fragmentation is described. It was observed that above the threshold velocity the glass failed by shattering into numerous fragments, ranging in size from dust-like particles to larger debris. It appeared that failure initiated at the contact point. The finite element simulations showed that the center of the contact region is the point of maximum Mises stress in the particle, and the Mises stress at this point is sensitive to the friction coefficient used in the simulations. Using the results from the simulations, the measured threshold velocity at a given particle size was converted to a critical value of the Mises stress and a contact time. The critical Mises stress is found to be hardly dependent on the contact time. Based on these observations, it is proposed that fragmentation of the particle is governed by a peak Mises stress condition and the contact time plays a minor role. (C) 1999 Elsevier Science Ltd. All rights reserved.