Effect of microwave heating on shear behaviour and AE characteristics of intact granite under variable angle shear test

As a clean energy source, microwaves have significant potential to enhance the efficiency of breaking hard rock for mechanical excavation in tunnelling engineering. In this study, the variable angle shear tests were carried out on granite specimens after microwave irradiation. Two-dimensional digital image correlation (2D-DIC) and acoustic emission (AE) technologies were used to monitor the mechanical behavior change during loading. The results indicate as the microwave heating time increased from 0 to 5 min and then to 10 min, the cohesion of the granite specimen decreased from 32.603 to 29.355 MPa and then to 24.666 MPa. But there is only a slight increase in the internal friction angle. As the shear angle and microwave irradiation duration increase, the cumulative AE counts and AE energy significantly decrease, indicating that microwaves can effectively reduce the energy needed in hard rock breaking. The change in AE b-value during loading can be divided into five stages: a rapid rise period, a stable fluctuation period, a rapid downward period, a small rising period, and a final downward period. Notably, the small rising period of the b-value between two rapid downward periods can be used as a precursor to failure. However, microwave irradiation can shorten or even eliminate this period. As observed through 2D-DIC, the strain evolution and crack propagation process of granite specimens are significantly influenced by both the shear angle and microwave irradiation. Decreasing shear angle and increasing heating time leads to a more dispersed distribution of the maximum principal strain concentration zone during loading, and the final failure pattern of the sample becomes more complex. Through the quantitative calculation of the normal force on the hob, it is found that microwave pretreatment can effectively enhance excavation rates. The maximum penetration increased to 15.67 mm/r for the specimens after 10 min irradiation, representing a 46.18% improvement compared to untreated specimens.