Effect of Microwave Irradiation on Computed Tomography and Acoustic Emission Characteristics of Hard Rock

Microwave-induced fracturing of hard rock can be used to assist mechanical rock breakage and release stress on rock masses in deep underground engineering operations. This is significant to those aiming to improve the construction efficiency and safety of underground tunnelling and ore extraction operations. The research was devoted to exploring the computed tomography (CT) and acoustic emission (AE) characteristics of hard rock subjected to microwave irradiation. For this purpose, microwave irradiation tests at a given power for different irradiation times were conducted on basalt collected from Chifeng, China by utilising a multimode cavity working at a frequency of 2450 MHz. By applying an infrared camera, the temperature distribution on the surface of samples during microwave irradiation was measured. Three-dimensional (3-d) microscopic CT was conducted on samples treated, and not treated, with microwave irradiation to explore the crack propagation regimes in such samples. Based on CT value and P-wave velocities, the damage characteristics of microwave treatment on basalt samples were evaluated. Afterwards, uniaxial compression strength tests were conducted and AE information in the loading process of samples was collected. After conducting microwave irradiation, internal cracks of samples propagated to generate fracture surfaces and surface temperatures of samples were found to have been non-uniformly distributed. The longer the irradiation time, the higher the degree of crack propagation in samples; P-wave velocities both decreased with increasing irradiation duration. With increasing irradiation time, the brittle failure characteristics of basalt shown in the time-dependent change curve of stress on basalt were gradually weakened while ductile failure characteristics were gradually strengthened. The uniaxial compressive strength, corresponding AE energy and cumulative AE energy at peak strength all gradually decreased with increasing irradiation time. The failure stress, the level of stress drop, AE energy, and cumulative AE energy of basalt after different irradiation times reflected consistent fracturing characteristics of basalt, which were interactively validated.