Supershear cracks appear along weak interface in semi-regular lattices under pre-tension

Rapid fracture of three semi-regular lattices (kagome, snub-square, elongated-triangular) is studied with pre-stretched strip models of finite element. Considering inherent geometric nonlinearity, the dynamic crack propagation is triggered by suddenly introducing an edge crack along the middle weak interface. We observed that the speed of mode I crack exceeds the shear wave speed of lattices, and tensile crack in the kagome lattice even travels faster than the pressure wave, which shatters the prediction of classical fracture theory. Pre-stretch level dominates the supershear fracture of lattices. As the pre-strain exceeds the critical value of lattice geometry, supershear propagation occurs, which is confirmed by theoretical prediction of the crack speed in lattices. As the crack speed increases further, the oblique shear shock front and pressure shock front form around the crack tip. Moreover, the energy near the crack tip flows toward the crack wake to form shock wave fronts. This study may deepen the understanding of supershear fracture in lattice metamaterials.