Mechanical properties and fracture damage behavior of thermal storage rocks under constant amplitude low cycle fatigue loading

This paper studied the mechanical properties and fracture damage behavior of granite with thermal storage potential under constant amplitude low cycle fatigue loading. Based on the distribution evolution characteristics of acoustic emission (AE) ringing counts, b-values, and AF-RA, the crack growth process, crack growth scale, and the change in the ratio of tension-shear cracks in the samples were analyzed. Mineral scale evidence of thermal damage of the sample was given through mineral thin section analysis, and a fatigue damage model was established using the dissipated potential function. The results show that with the increase of the lower limit stress level, the fatigue deformation and fatigue damage of the sample increase in a step-like manner, while the loading and unloading response ratio and crack evolution process have staged evolution characteristics. Temperature has two mechanisms for the fatigue mechanical properties of the sample: strengthening and weakening. In the temperature strengthening stage, the fatigue strength of the sample increased by about 8% compared with the control group, showing a sudden instability failure mode dominated by tensile cracks. In the temperature weakening stage, the fatigue strength of the sample decreased by about 50%, and the failure mode changed to progressive instability failure. The mechanism of temperature influence is mainly controlled by factors such as thermal expansion of mineral particles, phase change, dehydration decomposition reaction, and evaporation of free water. Transgranular cracks, parallel cleavage, and stratification are mineral-scale evidence of thermal damage. The nonlinear relationship between fatigue damage and cycle number can be converted into a linear relationship between stage cumulative damage and stress ratio. The stage cumulative damage curve of the sample is inverted S shape, which can be divided into three stages: initial damage, stable damage, and accelerated damage.