Energy Evolution and Fracture Behavior of Sandstone Under the Coupling Action of Freeze–Thaw Cycles and Fatigue Load

A series of freeze–thaw (F–T) cycle and multilevel fatigue loading tests are carried out on sandstone samples to explore rock mass's fracture behavior and energy evolution characteristics under the coupling action of F–T cycles and fatigue loads. First, the energy evolution characteristics of the sample are analyzed by the image integration method, and the law of energy storage and energy dissipation of the sample are further discussed. Subsequently, a coupled damage model is established based on the Lemaitre strain equivalence hypothesis. Finally, based on the b value and AF-RA waveform theory, the sample's crack evolution process and failure mode are analyzed using acoustic emission (AE) technology. The results show that the specimen's elastic energy and total energy density under the coupling action increase step-like with increasing the upper limit stress. The dissipated energy density decreases rapidly and stabilizes after the first cycle of each stage. The energy evolution process of the sample obeys the linear energy storage law and the two-stage energy dissipation law, in which the energy dissipation law is transformed from linear to exponential in the accelerated energy release stage. The coupling damage of the sample increases exponentially with the number of cycles, and the damage growth rate is slow at first and then fast. In addition, the crack propagation process of the specimen exhibits a 3-stage characteristic. As the number of F–T cycles increases, the proportion of shear cracks in the sample increases significantly, the failure mode transitions from X-conjugate failure to shear failure with a single oblique section, and the fatigue-softening effect is enhanced. Highlights Freeze-thaw sandstone sample is tested under multi-level fatigue loads. The linear energy storage law and two-stage energy dissipation law are proposed. The sample has obvious accelerated energy release before failure. A coupling damage model under F–T cycles and fatigue load is established. Facture behavior of the sample is analyzed by b value and AF-RA waveform theory.