A mother-daughter-granddaughter mechanism of shear dominated intersonic crack motion along interfaces of dissimilar materials

In this paper we report recent progress in large-scale atomistic studies of crack propagation along interfaces of dissimilar materials. We consider two linear-elastic material blocks bound together with a weak potential whose bonds snap early upon a critical atomic separation. This approach confines crack motion along the interface. In the two blocks, atoms interact with harmonic potentials with different spring constants adjacent to the interface. An initial crack is introduced along the interface and subjected to shear dominated displacement loading along the upper and lower boundaries of the sample. Upon initiation of the crack, we observe that it quickly approaches a velocity close to the Rayleigh-wave speed of the soft material. After cruising at this speed for some time, a secondary crack is nucleated at a few atomic spacings ahead of the crack. This secondary crack, also referred to as the daughter crack, propagates at the longitudinal-wave speed of the soft material. Shortly after that, a tertiary crack, referred to as the granddaughter crack, is nucleated and begins to move at the longitudinal wave speed of the stiff material. The granddaughter crack is supersonic with respect to the soft material and is clearly identifiable by two Mach cones in the soft material. Our results indicate that the limiting speed of shear dominated cracks along a bi-material interface is the longitudinal wave speed of the stiff material, and that there are two intermediate limiting speeds (Rayleigh and longitudinal wave speeds of the soft material) which can be overcome by the mother-daughter-granddaughter mechanism.