Modelling microwave fracturing of rocks: A continuum-discontinuum numerical approach

Abstract

Existing numerical models cannot well reproduce the fracturing process and reveal the underlying mechanisms of rocks under microwave irradiation. In this work, the electromagnetic-thermal-mechanical multiphysics is decoupled into microwave-induced heating (continuum-based) and thermally-driven fracturing (discontinuum-based), with temperature serving as the key interlink. The rigid-body spring-subset network (RBSSN) model is proposed to calculate the progressive fracturing of rocks under open-ended microwave irradiation, where the individual contacts between adjacent tetrahedral blocks are disassembled into three hypothetical spring-subsets. To depict failure characteristics of large-scale rocks under microwave irradiation, a variable-sized block model is developed by densifying the rigid-blocks near the irradiation. This electromagnetic-thermal-mechanical decoupling framework effectively captures the microwave fracturing process, revealing that microwave irradiation induces tensile-dominant progressive failure and regionalized deterioration (localized damage and macroscopic radial fissure). The fracturing rate of rocks is time-dependent, progressing through silent, violent and slowdown periods of rupturing with extended exposure time. The reason why high-power microwave is more effective in promoting visible fractures under the identical input energy is analyzed by combining the thermal deformation theory and RBSSN simulation. It is found that, power levels should be kept within reasonable scopes to maximize fracturing effects as excessive power densities lead to initiation of numerous microcracks around the high temperature zone and susceptibility to spalling.