3D modelling of microwave induced damage and fracturing of hard rocks
A safe, efficient and economical method of rock fragmentation is important and basic stud in mining and tunnelling. It is considered that microwave as a promising technology for rock breakage in mining and tunnelling since some hard rocks (granite, basalt etc.) show good microwave absorption under the irradiation. A 3D numerical model on a meso-scale was proposed in this paper to simulate the damage and fracturing of basalt induced by different power level (up to 3 kW) and irradiation times (1 to 60 seconds). It is a simplified model of a microwave applicator consisted of a multimode resonator and a waveguide with the specimen just below it. Importantly, the model took the inhomogeneity of materials and temperature dependence of thermal conductivity into account was closer to actual condition. Then we verified the rationality of the model. Numerical tests were performed to investigate the effect of microwave irradiation on temperature distribution and strength degradation in several groups of microwave power and exposure time. Uneven distribution of electric field and temperature were showed that they had positive correlation in the numerical simulations. The maximum temperature usually occurs inside the specimen instead of the surface and increases over time. Based on the damage criterion, we studied the form of macroscopic cracks induced by microwave and the results showed tensile failure. In addition, we contend that the specimen is weakening as long as it appears damaged area. The degree of weakening varied with external factors such as microwave irradiation power and internal factors such as tensile strength. Moreover, the macro cracks in specimen at relatively low temperature is mainly due to thermal damage of the extremely uneven temperature distribution caused by microwave irradiation in short time. The simulation results are in good agreement with the experiments results under same power level and irradiation time.