Heating-Dominated Fracturing of Granite by Open-Ended Microwave: Insights from Acoustic Emission Measurement

Microwave fracturing of hard rocks holds great promise in the civil, mining and tunnelling industries. The role of heating and cooling in the fracturing of rocks and when and where cracks initiate from and propagate to remain unclear and need to be addressed for future field applications of the technology. This study treated an alkali feldspar granite using a 6 kW industrial microwave source and a customised open-ended dielectric-loaded converging waveguide antenna. The real-time acoustic emission (AE) characteristics in the microwave heating and natural cooling phases were recorded and investigated. The surface temperature and P-wave velocity reduction of the specimens were also measured to quantify the thermal damage. The fracturing of granite is found to be tensile failure and is heating-dominated. Although a considerable amount of AE hits and events was detected in the cooling phase, they were of low energy, and therefore, the role of cooling in rock fracturing by open-ended microwave can be neglected. Fractures initiated from the exterior of the antenna and propagated towards the edges and the interior of the specimens. The cracks obtained from the AE localisation were in good agreement with those observed. With the increase of power level, the time for crack initiation was shortened, confirming the high-power effect.