Comparison of subcritical crack growth and dynamic fracture propagation in rocks under double-torsion tests

Relaxation and rapid displacement loading tests were performed on five types of rocks to gain a deeper understanding of the differences between subcritical crack growth (SCG) and dynamic fractures. The critical surface energy was obtained by fitting a hyperbolic sine function to the SCG velocities and mechanical energy release rates of the five rocks. The specific SCG range and the dimensions of the fracture process zone (FPZ) were determined via the displacement field of the sample bottom surface provided by digital image correlation. The FPZ lengths of the double-torsion rock samples range from 21.03 mm to 34.10 mm, and the lengths increase with the increasing maximum grain size of the rock type. The FPZ width is between 1.63 mm and 3.46 mm and exhibits a similar variation to that of the FPZ length. A three-dimensional optical scanner was used to obtain the morphologies of the subcritical crack surfaces and the dynamic fracture surfaces. The results show that the roughness of the subcritical crack surface is larger than that of the dynamic fracture surface, and both are positively correlated with the maximum grain size of the rock. The scanning electron microscope images demonstrate the change in the roughness of the crack surface; the subcritical cracks preferentially propagate along the grain boundaries while the dynamic fractures directly cut across crystals with some intergranular fractures.