Discrete element analysis of crack propagation, energy evolution, and failure mechanisms of red sandstone under pulsating loading

To clarify the damage evolution and failure mechanisms of red sandstone under pulsating loading, laboratory experiments and PFC numerical simulations were combined to investigate the mechanical response, acoustic emission (AE) characteristics, crack propagation, and energy evolution under different loading frequencies and stress amplitudes. Laboratory tests were conducted at 5 Hz with three stress amplitudes (0.05σc, 0.10σc, and 0.15σc), while the simulations extended the frequencies to 2, 5, and 8 Hz, yielding nine loading conditions. The results show that with increasing stress amplitude, the average failure strength first decreased from 0.933σc to 0.900σc and then increased back to 0.933σc, exhibiting a distinctly non-monotonic trend, with the shortest average loading life occurring at 0.10σc. Loading frequency and stress amplitude had a coupled effect on damage. The LM condition (2 Hz, 0.10σc) most strongly promoted damage, with failure at approximately 0.80σc and an average loading life of about 3.60 × 103 cycles. AE activity was stage-dependent, and crack evolution involved scattered initiation, local clustering, and rapid coalescence. Tensile cracks initiated earlier than shear cracks, indicating mixed tensile–shear failure dominated by tensile cracking. Energy analysis showed gradual accumulation of external input energy and internal elastic strain energy, with a marked increase in damping energy dissipation before failure under some conditions. These results indicate that the damage and failure of red sandstone under pulsating loading are jointly influenced by loading frequency, stress amplitude, and crack interactions; within the investigated range, low loading frequency combined with moderate stress amplitude more readily promotes damage and failure.