Combined effects of microwave irradiation and water cooling on the deformation and failure behaviours of CSTBD granite

Abstract

The low cutting rate and high cutting tool wear rate in hard rock tunnel excavation by a tunnel boring machine (TBM) are critical problems. By integrating microwave technology with water cooling into the TBM cutterhead, microwave-assisted mechanical rock breaking provides a promising solution. To reveal the deformation and failure behaviours of hard rocks after microwave heating and water cooling, this work experimentally conducted a series of tests on cracked straight-through Brazilian disc (CSTBD) granite specimens, where the crack angle was set to 0, 30, 60, and 90° and the microwave heating duration was set to 0.5, 1, 3, 5, and 7 min. During the experiment, digital image correlation (DIC) technology was employed to measure the deformation and failure process. The results show that microwave heating and water-based cooling facilitate the failure of CSTBD granite. The mode-Ⅰ fracture toughness linearly decreases with heating duration for pure tensile failure (crack angle: θ=0°), while it linearly increases with heating duration at other crack angles (30°, 60°, and 90°). The mode-Ⅱ fracture toughness linearly decreases with heating duration in the crack angle range of 0° to 90°. The mode-Ⅰ fracture toughness linearly decreases with the crack angle. The mode-Ⅱ fracture toughness increases with a crack angle of <24° and decreases in the crack angle range of 24° to 90°. The failure patterns are divided into five categories by different crack angles: pure tensile failure (θ = 0°), tensile-shear mixed failure (0° <θ < 24°), shear failure (θ = 24°), compressive-shear mixed failure (24° <θ < 90°), and compressive failure (θ = 90°). The reasons for the main and second cracks during the failure process are revealed. The experimental results have significant implications for microwave-assisted mechanical rock breaking.