Temperature driven real-time weakening and strengthening mechanisms of unconfined granite.

• Micro- and macroscopic behavior of granite under and after heating are compared. • Mechanical difference between RT and AT granite is not related to crack density. • Cooling effect improves the stress resistance and interlocking at the grain size level. • Real-time weakening and strengthening mechanisms of thermal-treated granite is revealed. The distinctive mechanical behaviors exhibited by granite under real-time temperature (RT) and after thermal treatment (AT) highlight a challenge in directly observing real-time cracking phenomena in RT granite within a laboratory setting. To overcome this limitation, a thermo-mechanical coupled grain-based model (GBM) was introduced for numerical investigations. The simulation outcomes reveal that disparities in mechanical responses between AT and RT specimens are attributed not merely to the count of thermally induced cracks, but rather to intricate grain-contact deformations arising from heterogeneous crystal expansions/contractions and the intricate interplay of direction-independent interactions along block edges. During elastic deformation, RT grain contacts with larger deformation reach the peak stress faster than AT contacts. Failed contacts have smaller residual cohesion under temperature with larger deformation due to the stronger slip-weakening effect. Moreover, shear displacements and grain rotations can create new interlocking, increasing granite stiffness after cooling. Consequently, the RT sample is softer to deform and fail, while the overall stress resistance of AT granite is higher due to cooling-induced micro-structure changes. These findings shed light on why the mechanical responses, including strength, stiffness, and plastic strain, of AT and RT granite are different. [ABSTRACT FROM AUTHOR]