Multiscale mechanical responses and damage evolution of granite under heating-LN2 cooling cycles

As a typical hot dry rock (HDR) reservoir rock, the mechanical damage and failure behaviour of granite are strongly influenced by thermal–stress coupling. In this study, macroscopic, mesoscopic, and microscopic experiments were conducted on 40 granite samples subjected to cyclic heating to 300 °C followed by LN2 quenching (−196 °C). The aim was to investigate the evolution of the microstructure and meso-damage variables in granite and to establish their relationship with the deterioration of macroscopic mechanical properties and damage accumulation. The results showed that mechanical properties decreased exponentially with increasing cycle number. The most pronounced degradation occurred during the first cycle, with the wave velocity (V) deteriorating by 61.22%. As the number of cycles increased, microcracks propagated from grain boundaries into the mineral grains, while the macroscopic failure mode gradually shifted from pure tensile failure to mixed tensile–shear failure. After 12 cycles, both the mechanical properties and damage evolution approached a steady state. This behaviour is primarily governed by the combined effects of thermal shock and fatigue damage, whereas variations in the mineral structure and crack evolution contributed to the observed deterioration in mechanical properties. A major contribution of this study is the development of an elastic modulus (E) prediction model based on the Mori–Tanaka method, which integrates the cooperative calibration of micro- and macroscale parameters. This approach overcomes the limitations of traditional homogenisation methods under cyclic damage accumulation conditions and significantly improves the accuracy and reliability of E prediction, yielding an average relative error of only 3.46%.