Subcritical crack growth in rocks under shear loading

In this study, the subcritical crack growth parameters for Coconino sandstone under modes II and III loading were determined experimentally by using the constant stress rate test. We extend the constant stress rate test technique to modes II and III subcritical crack growth in rocks. The experimental results of the modes II and III tests, combined with mode I results published elsewhere, show that the values of the modes I, II, and III subcritical crack growth parameters are very similar to each other regardless of the loading configuration and the specimen geometries. The main reason for this is thought to be that subcritical crack growth is environmentally induced crack growth rather than the mechanical rupture of bonds. The effect of confining stress, specimen size, and water saturation on subcritical crack growth under mode II loading has also been investigated. The parameter n linearly increases with increasing confining stress and the parameter A exponentially decreases with increasing confining stress. Increasing the specimen size results in a linear increase in the subcritical crack growth parameter A. But, the parameter n is independent of the specimen size. The parameter n is almost constant regardless of water saturation, and the parameter A is found to increase slightly when the specimen is fully saturated. These results suggest that the subcritical crack growth parameter n can be considered a material constant for a given rock type.