Maximum strain energy density drives path selection in dynamic cracks

Our fundamental understanding of dynamic ‘simple’ cracks in brittle solids is excellent, yet the criteria for path selection of moving cracks remain unknown. We evaluate the criteria for path selection in dynamic fracture in two ways: first, we deflect dynamic cracks using sparsely implanted defects, and second, we drive them to undergo an intrinsic oscillatory instability in defect-free media. Our experiments cover a wide range of crack velocities, from 10-95% of their limiting velocity within the brittle material. Our high-speed imaging data are used to obtain measurements of the strain fields very near the crack tip - and these measurements reveal that path selection for such rapid and strongly perturbed cracks is determined by the direction of maximal strain energy density rather than by the principle of local symmetry. These results suggest a novel mechanism for material toughening in brittle solids, whereby a crack is steered, deflected and even stopped by embedded inclusions.