Fracturing mechanism of rock under variable amplitude cyclic loading and implications for design of lightweight rock cutting machines
Cutting of abrasive, high strength and stiffness rocks is difficult using mechanical tools. A way of weakening these rocks is by fatigue loading. However, a fatigue loading takes a large number of cycles, of the order of 103 (Hardy and Chugh, 1970) to break the rock. Consequently this process is slow. In order to reduce time and take advantage of the fatigue phenomenon of rock, a variable amplitude cyclic loading method of breaking the rock was adopted. In each cycle of loading, the amplitude (magnitude) of loading is increased cyclically until the rock fractures. A research program was designed to gain a better understanding of the micromechanics of rock fracture under variable amplitude cyclic loading. When a cutting tool contacts a rock, the rock is subjected to triaxial stress field near the cutter tip. Therefore it is necessary to understand its constitutive behaviour under triaxial stress conditions. In the oscillating disc cutting system, the cutter oscillates at a certain fixed frequency. The combination of oscillation of the cutter and linear motion (velocity) of the cutting system results in variable amplitude cyclic loading and unloading condition onto the rock. Therefore, a series of experiments was designed to investigate the constitutive behavior of rock in triaxial stress condition under monotonic loading, five-cycle and ten-cycle variable amplitude loading and unloading, using acoustic emission (AE) hypocenter distribution and surface strain mapping. Also, several indentation experiments were performed using the acoustic emission monitoring technique to understand the mechanism of fracturing under direct indentation loading and unloading
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