Laboratory study on acoustic emission signals and damage mechanism of rock with thermal storage potential under fatigue loading

This study selected granite with thermal storage potential for heat treatment and fatigue loading tests. First, the differences in fatigue mechanical properties of the samples were compared. Then, based on the b-value theory and AF-RA classification model, the kernel density estimation and gradient descent algorithm were used to analyze the growth scale and the proportion change of tensile-shear cracks. Based on the critical slowing-down theory, an early warning index of fatigue failure was proposed. Finally, the thermal damage mechanism was discussed on multiple scales by using a polarizing microscope, scanning electron microscopy and thermogravimetric analysis technology, and a fatigue damage model was established. Our laboratory work shows that temperature has a staged influence mechanism on granite samples under fatigue loading. As the temperature increases, the strength, modulus, and fatigue life of heat-treated samples will show a staged evolution law of strengthening-weakening-accelerated weakening. The proportion of shear cracks inside the sample decreases first and then increases, and the development time of fatigue failure shortens first and then prolongs. The staged thermal decomposition process and microstructural changes of minerals such as quartz, feldspar, and mica are the main reasons for the staged influence mechanism of temperature. In addition, the fatigue damage curve of the heat-treated granite sample is an inverted S-shaped, which can be divided into three stages: initial damage-stable damage-accelerated damage. The hysteresis curves formed by the samples with a larger proportion of shear cracks are sparser. The appearance of a large number of shear cracks can be used as a precursor indicator of rock fatigue failure. The early warning index proposed in this paper provides new ideas for understanding and predicting the disaster behavior of thermal storage rock mass engineering under fatigue disturbance.