Crack Path Selection in Dynamic Fracture

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. We obtain dense measurements of the strain fields surrounding the crack tip via high-speed imaging - 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 a local symmetry criterion. These results suggest a mechanism for material toughening in brittle solids, whereby a crack is steered, deflected and even stopped by embedded inclusions.