Weakening Behavior of Sandstone Subjected to Microwave Irradiation: Insights from Real-Time Monitoring of Surface Temperature and Acoustic Emission

Microwave performs great potential for enhancing the efficiency of hard rock breaking. However, the weakening effect and optimal irradiation parameters for energy-efficient rock breaking remain unclear. Therefore, we investigate them by real-time monitoring of surface temperature during microwave irradiation and acoustic emission (AE) signal collected during mechanical experiments. The results show that: (1) Quartz phase transition and high thermal gradient result in microwave-induced macro-cracks. Thermal gradient exhibits a slight variation when irradiation duration is below the critical time ( T ), whereas significant variation when it exceeds T . (2) As irradiation time increases, both strength and elastic modulus display a nonlinear trend: decrease → increase → decrease. High power levels and prolonged duration are not essential for energy-efficient rock breaking; 1/3 T can induce significant strength degradation. Microwave irradiation shifts mechanical failure from shear to tensile and from brittleness to ductility. (3) Compared to untreated specimens, microwave-treated specimens exhibit substantially higher AE counts and energy during crack closure and elastic deformation, but relatively shorter stable crack growth and unstable propagation stages; accumulated AE counts and energy are 3-5 times and 10-15 times greater, respectively. (4) With increasing irradiation duration, the proportion of shear cracks exhibits a nonlinear trend: decrease → increase → decrease, thereby leading to a corresponding nonmonotonic evolution of strength. (5) b -value evolution suggests that microwave irradiation reduces strain energy storage capacity. Prolonged duration (>1/3 T ) may inhibit crack development, thereby enhancing strength. Additionally, the nonmonotonic strength evolution is discussed through temperature assessment, AE analysis, scanning electron microscope (SEM) observations, and thermal-chemical damage.