A mother–daughter mechanism for mode I dominated cracks: Supersonic crack motion along interfaces of dissimilar materials

Abstract

In this paper we summarize our recent progress in applying large-scale atomistic studies of crack dynamics along interfaces of dissimilar materials. We consider two linear-elastic material strips in which atoms interact with harmonic potentials of different spring constants. The two strips are bound together with a weak potential whose bonds snap early upon a critical atomic separation. An initial crack serves as the initiation point for dynamic interfacial fracture. We focus on the maximum speed of tensile dominated cracks along such a bimaterial interface. We observe that upon initiation at a critical load, the crack quickly approaches a velocity a few percent larger than the Rayleigh-wave speed of the soft material. After a critical time, a secondary crack is nucleated a few atomic spacings ahead of the crack. This secondary crack propagates at the Rayleigh-wave speed of the stiff material. If the elastic mismatch is sufficiently large (e.g. ten as in our study), the secondary crack can be faster than the longitudinal wave speed of the soft material, thus propagating supersonically. At this stage, supersonic crack motion is clearly identified by two Mach cones in the soft material. Our study suggests that such mother–daughter transition mechanism, which has been previously reported for mode II crack motion in homogeneous materials, may also play an important role in the dynamics of interfacial cracks under tensile loading. We also include some studies of unconstrained crack motion along interfaces of dissimilar materials, and demonstrate that the crack tends to branch off into the softer region, in agreement with previous study by Xu and Needleman (Int. J. of Fracture 74 (1996)).