Principle of local symmetry in mixed-mode fracture

Abstract When brittle materials are subjected to uniaxial tensile stress, we observe planar crack propagation perpendicular to the loading direction. However, when additionally subjected to an out-of-plane shear stress, the crack tip/front deviates, leading to fragmentation in the form of twisting facets. Conventional linear fracture mechanics does not adequately describe this effect unless additional criteria are assumed. Among these, the principle of local symmetry, which states that fractures propagate in shear-free directions, is prominent but lacks robust experimental validation. Here we study fractures in hydrogels under mixed load. From the geometry of the evolving crack, we deduce the tension field near the fracture tips and find clear evidence that the principle of local symmetry governs the fragmentation. Our results indicate that the finite size of the fragmentation pattern’s facets and their elastic interactions account for deviations from the principle of local symmetry, providing new insights into fracture mechanics.