Multi-scale study on the fatigue mechanical properties and energy laws of thermal-damage granite under fatigue loading

To reveal the mechanical properties and energy laws of high-temperature rock mass engineering under fatigue disturbance, this paper conducted a multi-scale study on thermally damaged granite. First, the macroscopic mechanical properties of the samples were studied. Secondly, the law of energy evolution was summarized based on thermodynamic theory. Then, a rockburst index was introduced, and NMR and SEM technologies were used to conduct a multi-scale discussion on the mechanism of influence on temperature. Finally, an improved nonlinear continuous damage model (INCDM) was established, and a hardening index and damage growth rate of low-cycle fatigue were defined. The result shows that temperature first strengthens and then weakens the fatigue mechanical properties of the sample, with a threshold temperature of 225°C. Temperatures below the threshold cause uneven expansion of mineral particles to squeeze natural pores, reduce the porosity of the sample, and thus increase the fatigue life and strength of the sample. Temperatures above the threshold cause dehydration and phase change of the minerals such as quartz, feldspar, and mica, forming transgranular/intergranular cracks, parallel cleavage and stratification, thus reducing the fatigue strength of the sample. In addition, the total energy, elastic energy and dissipated energy density of the sample all show a step-like increasing trend with the normalized cycle index. Energy storage satisfies a linear law. Affected by accelerated energy release, energy dissipation changes from linear to nonlinear law. As the temperature increases, the rockburst tendency first increases and then decreases. The fatigue failure changes from sudden instability to progressive instability mode. The fatigue-thermal damage of the sample satisfies a power law, and increases as a compound power function with the normalized cycle index.