Fatigue characteristics and fracture behaviour of sandstone under discontinuous multilevel constant-amplitude fatigue disturbance

Discontinuous fatigue disturbances are common in rock mass engineering, but the corresponding mechanical behaviour of rock is unclear. In this study, discontinuous multilevel constantamplitude fatigue tests of sandstone containing pre-existing cracks with different angles were performed, and the fatigue characteristics and fracture behaviour were analysed using acoustic emission (AE) and digital image correlation (DIC) techniques. The results showed that the peak strength and strain, fatigue life and modulus decreased with increasing crack angle. The dissipated energy first decreased and subsequently increased during loading, and the relationship between the three strain energy density parameters and the stress level was a quadratic polynomial. Meanwhile, there was a significant accelerating energy release (AER) phenomenon in the AE energy, and the AE energy release of the 0 degrees and 30 degrees samples was more rapid. A fatigue damage model considering the initial damage was established based on the strain energy density, and the initial damage and cumulative rate of fatigue damage were proportional to the crack angle. In terms of fracture behaviour, the samples developed wing cracks, anti-wing cracks, secondary shear cracks, and out-of-plane shear or tensile crack propagation in sequence. The change in bvalue indicated that the load holding at a higher stress level would increase the propagation scale of rock fatigue cracks. The AF and RA values indicated that the 0 degrees and 30 degrees samples were dominated by tensile fracture, while the 60 degrees and 90 degrees samples were dominated by shear fracture. The shear cracks were concentrated in the unstable fracture stage, and the ratio was proportional to the crack angle.