This paper presents an experimental investigation on both monotonic strength and fatigue crack kinetics for Berea sandstone. It is found that for all specimens the Paris-Erdogan law is applicable for a wide range of the amplitudes of the stress intensity factor. The fatigue tests also indicate that there is a small-crack growth regime at the beginning stage, where the growth rate decreases as the crack propagates. The fracture kinetics for both the small-crack growth and the Paris regimes is subjected to a strong size effect. During the strength and fatigue tests, the damage process is examined by the digital image correlation method, and it is shown that the length of the fracture process zone under cyclic loading is about 60% larger than that under monotonic loading. In parallel with the experiments, a theoretical model is developed to explain the observed size effect on fatigue crack kinetics.

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