On the effect of stress amplitude on fracture and energy evolution of pre-flawed granite under uniaxial increasing-amplitude fatigue loads

Rock mass containing discontinues is often subjected to fatigue loads in many mining and civil engineering. The stress amplitude (SA) as an important factor impacts rock fracture evolution during fatigue loading condition. Existing effects to investigate the impacts of SA on rock fatigue behaviors were mainly on constant SA loading condition, the influences of incremental stress amplitude (ISA) for rock subjected to cyclic uniaxial increasing-amplitude conditions is not well understood. In this work, uniaxial increasing-amplitude fatigue loads and post-test 3D CT scanning experiments were conducted on granite containing two pre-existing flaws. Our experimental results reveal the impact of four applied ISA (i.e., 5, 10, 20 and 30 MPa) on rock fracture evolution, energy conversion and crack network pattern at rock bridge segment. The damage accumulation and volumetric deformation increase with the increase of applied ISA. The accelerated damage evolution stage is different for sample with different ISA, and it starts to appear the earliest for a sample subjected to ISA of 30 MPa. Energy conversion analysis reveals that the dissipated energy used to drive the crack propagation is relatively large for a sample with higher ISA. Post-test 3D CT visualization reveals a most striking finding that the crack network pattern becomes complicated as the applied ISA increases. It is suggested that crack coalescence between the flaws is easy to be formed for rock subjected to low ISA, rock bridge segment fracturing is stress amplitude dependent. The testing results are expected to improve the understanding of the influence of stress amplitude on the fatigue behaviors of pre-flawed rock when rock mass is subjected to increasing-amplitude stress disturbance.