LAUSR

This study investigates the tensile properties of granite subjected to cyclic thermal treatment under cyclic loading-unloading conditions, which is of great significance for the modification of hot dry rock reservoirs. Brazilian splitting tests under cyclic loading-unloading were conducted on granite samples exposed to 400 °C cyclic water-cooling shock (applied for 1, 3, 5, and 7 cycles) at different preset load upper limits (65%, 70%, 75%, and 80% of the peak load). The experimental results reveal the evolution of the tensile properties of granite under the combined effects of 400 °C cyclic water-cooling shock and cyclic loading-unloading. The findings indicate that the tensile strength of granite decreases with an increasing number of cyclic water-cooling shocks and further declines as the preset load upper limit decreases. Under typical conditions, the peak displacement of granite exhibits three distinct stages with increasing loading-unloading cycles: rapid increase, slow increase, and eventual failure. During the slow increase stage, peak displacement decreases due to an increase in elastic stiffness. Initially, elastic stiffness increases with the number of cycles, followed by a stabilization phase, and subsequently declines. After granite failure, macroscopic failure cracks gradually deviate from the center as additional cyclic water-cooling shocks are applied. In contrast, cyclic loading-unloading has a minimal effect on macroscopic cracks. Furthermore, as the number of cycles increases, microcrack evolution transitions from intergranular to transgranular cracking. Under cyclic loading-unloading conditions, these cracks continue to propagate, ultimately forming a fracture network. The findings of this study provide a theoretical foundation for the fracturing and modification of hot dry rock reservoirs.