An experimental study of crack growth and its effect on the mechanical and seismic properties of rocks using 3D printing, micro-CT and acoustic emission

Abstract

Rock is heterogeneous and contains numerous discontinuities, e.g., cracks, voids, and joints. The presence of these discontinuities not only governs the mechanical behaviors of rock masses but also significantly affects the rocks' seismic responses. In the past, great efforts have been devoted to studying the effects of discontinuities on the mechanical and seismic behaviors of rocks. Nevertheless, some scientific problems remain. For instance, few studies have been conducted to investigate three-dimensional (3D) internal crack growth and its effects on the mechanical and fracture behaviors of rocks. Thus far, no studies have been reported considering the combined effects of joint geometry (e.g., joint matching coefficient (JMC)) and infilling materials (e.g., infilling types and water content) on wave propagation across joints. Focusing on the remaining problems, experimental research of cracks, especially 3D internal micro and macrocracks, crack growth and the effects on the mechanical and seismic properties of rocks are investigated in this thesis. To replicate naturally brittle and hard rocks, 3D printing (3DP) is introduced to the study of rock mechanics. The translucent resin produced via stereolithography (SLA) is identified as the most suitable 3DP material for replicating brittle and hard rocks based on currently available 3DP techniques. Three methods, including freezing, incorporation of internal macrocracks and the addition of microdefects are adopted to enhance the brittleness of the 3DP resin. To some extent, 3DP application to rock mechanics can solve the problem of preparing artificial rock-like samples with internal structures identical to the prototype rocks. The 3DP technique is adopted to fabricate resin-based artificial rocks containing single and double, pre-existing, penny-shaped 3D internal flaws. Static and dynamic compression tests are performed on these samples to investigate the influence of flaw geometry and loading types on the mechanical and fracturing behaviors of 3DP artificial rocks. For the first time, high-speed cameras are applied to the study of 3D crack growth inside the transparent 3DP resin samples in real-time. Distinctions between the volumetric fracturing of resin-based artificial rocks under static and dynamic loading conditions are compared, and the physical mechanisms behind these types of fractures are proposed. In this research, 3DP is adopted to replicate internal defects and study the mechanical and fracture behaviors of rock in combination with microcomputed tomography (micro-CT). The validity and efficiency of this method is confirmed by comparing the results with the physical tests conducted on the natural prototype volcanic rocks. The advantages and disadvantages as well as potential improvements of the method are compared and discussed. The proposed method provide a promising means to quantify, replicate and visualize the pre-existing defects and microstructures and to understand their influences on the mechanical and fracture behavior of rock under different loading conditions. In addition to mechanical and fracture properties, the damage evolution effects on low-amplitude ultrasonic wave propagation in rock samples are also studied. Acoustic emission activities are employed to quantify damage evolution during uniaxial compression loading. In addition, micro-CT detection is applied to quantitatively determine damage inside the rock sample after the loading and unloading processes. Then, the relationship between damage evolution and ultrasonic wave propagation is quantitatively analyzed. Based on the split Hopkinson rock bar system, stress wave propagation is conducted to study the coupling effects of JMC, loading rate and water contents of the infilling mixture on the mechanical and seismic properties of a single, rough joint. JMC is found to play a dominant role in affecting the mechanical and seismic properties of the joint compared with the infilling mixture and loading rate.