Stress intensity factor evolution considering fracture process zone development of granite under monotonic and stepwise cyclic loading

Abstract

Quasi-brittle materials like rocks may present mechanical non-linearity during cylic loading due to the formation of fracture process zone (FPZ), which poses a challenging task for accurate measurement of stress intensity factor (SIF). Understanding the crack development and SIF evolution under cyclic loading is beneficial for clarifying the mechanism of rock fatigue failure. Crack initiation and propagation are observed in semi-circular bending granite specimens subjected to monotonic and stepwise cyclic loading. Digital image correlation (DIC) and acoustic emission (AE) techniques are used to quantify the fracture behaviors through deformation fields and AE characteristic parameters. With the measured displacements around the notch, a novel J-integral method is coded and adopted to quantify the SIF at the crack tip which inherently takes the FPZ into account, and its validity is verified with synthetic images. Experimental results indicate that the development of the FPZ and SIF under cyclic loading is more complex than that under monotonic loading. The FPZ length, the SIF, and the crack opening displacement increase with the number of cycles in a specific loading level. For the loading level with a smaller upper load limit, the FPZ and SIF increase stably under repeated cycles, and AE events cluster in the first few cycles, exhibiting a significant Kaiser effect. As the upper load limit of the loading level increases, the FPZ and SIF increase linearly with the number of cycles, and the increase in the AE count rate and amplitude indicates the occurrence of abundant energetic microcracks under repeated loads. When the SIF at the crack tip approaches the fracture toughness of granite, the accelerated growth of the SIF in the last few cycles is accompanied by a rapid crack extension, resulting in a sharp rise in cumulative AE counts and energy.