Fig. 2.

Failure crack morphologies in glass/sapphire/polycarbonate trilayers, from contact with WC spheres of radius r = 5.0 mm. (a) Core radial cracking in the bottom-surface abraded sapphire at P_m = 400 N and n_F = 1014 cycles. For this crack mode, initiation and penetration occur simultaneously. (b) Veneer cone cracking in the top-surface abraded glass layer, showing dominant I cracks at P_m = 300 N and n_F = 43613 cycles. For this mode, initiation generally occurs at a relatively low number of cycles, and the crack propagates stably prior to failure. Interfaces are accentuated by superimposed white lines, for clarification.