A dynamic crack will travel in a straight path up to a material-dependent critical speed beyond which its pathbecomes erratic. Predicting this critical speed and discovering the origin of this instability are two outstanding problems infracture mechanics. We recently discovered a simple scaling model based on an effective elastic modulus that gives successfulpredictions for this critical speed. We now show that a simple atomic picture based on broken-bond relaxation atthe dynamic crack tip provides an explanation for the origin of the effective elastic modulus.
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