Experimental study on effects of cyclic loading paths on cracking behavior and fracture characteristics of granite

To investigate the impact of cyclic loading paths on the Mode I fracture characteristics of granite, a series of fracture tests were conducted on semi-circular bend (SCB) specimens using acoustic emission (AE), digital image correlation (DIC) and 3D scanning. The tests included monotonic loading, variable amplitude (VA) cyclic loading, and tiered constant amplitude (TCA) cyclic loading. The results demonstrated that VA cyclic loading leads to an average enhancement of 5.6% in fracture toughness, whereas TCA cyclic loading results in an average reduction of 3.7% in fracture toughness. During cyclic loading and unloading, the TCA loading path exhibits a higher rate of cumulative plastic deformation growth and greater magnitude cumulative plastic deformation compared to the VA loading path. Additionally, detailed observations from 3D scanning revealed that under cyclic loading, the fracture trajectories and surface morphology exhibit more tortuosity and roughness compared to monotonic loading, particularly in the case of TCA cyclic loading. Furthermore, according to the AE and DIC analysis, the proportion of microcrack initiation and stable propagation stage, cumulative AE counts and fracture process zone (FPZ) size generated under VA cyclic loading are greater than those generated under TCA cyclic loading, displaying a consistent variation pattern with fracture toughness. The changes of fracture properties can be attributed to a competition between the compaction-induced hardening effect and the cracking-induced damage effect. A higher cyclic upper limit and incomplete unloading are more likely to accelerate the growth of the irreversible crack damage and rapid crack extension, resulting in weakened rock fracture properties. These findings provide important references for rock instability control in underground rock engineering.