Scaling of fatigue crack growth in rock

This paper presents a comprehensive set of size effect experiments on fatigue crack kinetics for Berea sandstone. It is observed that for all specimens the Paris-Erdogan law is applicable for a wide range of amplitudes of the stress intensity factor. The fatigue tests also indicate that there is a small-crack growth regime at the beginning stage of cyclic loading, where the growth rate decreases as the crack initiates from the notch tip. It is shown that the fracture kinetics for both the small-crack growth and the Paris regimes is subjected to a strong size effect. Meanwhile, a series of size effect tests on monotonic strength of Berea sandstone is also performed in order to investigate the difference in fracture process between fatigue and monotonic loading scenarios. By using the digital image correlation method, it is found that the fracture process zone (FPZ) length under cyclic loading is about 60% larger than that under monotonic loading. In parallel with the experimental investigation, it is shown that the observed effect of the specimen size on the fracture kinetics can be explained by a size effect model for the critical energy dissipation for fatigue crack growth, which is analogous to the size effect on the apparent fracture energy for monotonic loading.