Low-cycle fatigue failure mechanisms of rock-like materials with crack-pore flaws: A phase-field modeling study

Crack-pore flaws ubiquitously exist in rock-like materials. To investigate the low-cycle fatigue failure mechanisms of rock-like materials containing crack-pore flaws, this study developed a fatigue phase-field model accounting for cyclic damage accumulation. Two reported experimental cases, namely the three-point bending fatigue fracture tests on the rectangular concrete beams and the semicircular sandstone specimens, were employed to validate the effectiveness of the model. On this basis, taking the semicircular specimens containing the prefabricated cracks and random polygonal pores as examples, multicondition three-point bending fatigue fracture simulations were carried out to systematically reveal the regulatory effects of prefabricated crack inclination angle, porosity, mean pore diameter, and cyclic loading level on the fatigue damage evolution. The results indicate that the fatigue damage evolution of rock-like materials containing crack-pore flaws consistently exhibits a three-stage characteristic: initial damage, slow development, and accelerated failure. The fatigue fracture mode transitions from pure mode I tensile fracture to mixed I–II tensile-shear fracture with increasing crack inclination angle. The fatigue life first decreases and then increases with the inclination angle, and the lower the loading level, the more pronounced the life difference caused by the prefabricated crack inclination angle. Stress concentration induced by pores is the core driver of damage acceleration. Governed by stress concentration, the fatigue life exhibits a piecewise decline with increasing porosity and a U-shaped evolution with increasing mean pore diameter. Damage localization within pore clusters ultimately dominates the macroscopic instability path. The findings offer theoretical basis for the fatigue assessment of inherently flawed rock-like materials.