Propagation of extended fractures by local nucleation and rapid transverse expansion of crack-front distortion

Fractures are ubiquitous and can lead to catastrophic failure. While fracturing in a two-dimensional plane is well understood, real fractures are three-dimensional and exhibit complex behaviour. Here we show that forward propagation of an extended fracture front occurs via an initial localized rupture followed by very rapid transverse expansion at velocities approaching the Rayleigh-wave speed. Using a circular (penny-shaped) geometry to realize an uninterrupted extended crack front and fluid injection to control loading, we demonstrate that the forward jump amplitude correlates with the transverse velocity. Dynamic rupture simulations reproduce observations only for high transverse velocities. The results highlight the crucial role of transverse dynamics in the forward propagation of extended fractures.