Cracks Faster Than the Shear Wave Speed

Abstract

Classical dynamic fracture theories predict the Rayleigh surface wave speed to be the limiting speed for propagation of in-plane cracks in homogeneous, linear-elastic materials subjected to remote loading. However, in the present study, experimental evidence to the contrary is reported, in which intersonic shear dominated crack growth is seen along weak planes in Homalite-100 under far-field asymmetric loading. It is seen that mode-II (in-plane shear) conditions are essential to attain intersonic crack-tip speeds. The stress field generated by the intersonically propagating crack-tip is recorded using photoelasticity and high speed photography. Intersonic shear cracks, featuring shear shock waves and large scale crack face frictional contact, are initially highly unstable and crack-tip speeds vary from the shear wave speed to the dilatational wave speed of the material. As steady state conditions are achieved, the mode-II intersonic cracks propagate at a constant speed of √2c_s. These observations have potential implications in geological settings where intersonic rupture velocities have been reported for crustal earthquakes.