Microcracking Process as a Base of Shear Rupture Mechanism of Extreme Dynamics

Dynamic shear ruptures (up to intersonic velocities) have been observed in brittle materials from micro to macro scales including earthquakes. Earthquake laboratory experiments have revealed two key features of extreme ruptures: 1) intensive microcracking in the rupture tip and 2) dramatic strength weakening of the rupture head. Although essential for understanding earthquakes, rock mechanics, tribology, and fractures, the relation between the intensive microcracking and strong interface weakening remains enigmatic despite continuous studies over decades. The fact that rupture propagation is always accompanied by intensive microcracking allows the hypothesis that the microstructure generated by this process plays an essential role in determining the relative displacement, speed and resistance of the rupture faces. This paper proposes further development of a recently identified shear rupture mechanism according to which the shearing rupture faces can create, under certain conditions, a fan-shaped microstructure in the rupture head, representing a mechanical system with extremely low shear resistance (up to an order of magnitude less than the frictional strength). The fan-structure represents also a self-sustaining mechanism of stress intensification. These two features provide driving power for extreme ruptures. The paper discusses the role of the fan-structure in creation of extreme ruptures propagating with the pulse-like mode.