Crack propagation toughness of rock for the range of low to very high crack speeds

A fracture mechanics characterization is performed of mode-I crack propagation processes in Solnhofen limestone. High levels of crack propagation velocity in the rock are generated by providing high levels of elastic energy that are stored at the tip of the initial notch at the moment of initiation. This is experimentally achieved by subjecting the initial notch to impact loading conditions and/or by initiating the mode-I crack from a shear (mode-II) loaded starter notch. The dynamic stress intensity factors and the velocity of the propagating cracks are measured by a chain of strain gauges positioned along the prospective crack propagation path. The strain gauge measuring technique is used in an optimized form to reduce the influence of higher order terms of the crack tip stress field. Cracks are accelerated to velocities up to about 2000 m/s. The measured fracture toughnesses are significantly (almost a factor of ten) higher than the crack initiation toughness. Thus, the energy dissipated by a crack propagating at a high velocity is almost two orders of magnitude larger than the energy to initiate a crack. Furthermore, the data show a sharp increase of the fracture toughness in the regime of slow crack propagation velocities. The fracture toughness crack velocity dependence shows a R-curve behaviour as typically found for ductile steels. The results explain characteristic peculiarities of the crack propagation behaviour of brittle fracture in rock.