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Geophysical signatures of crack initiation and growth in rocks under uniaxial compression
ZAFAR, SANA
ZAFAR, SANA
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2023
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2024-11-29
Abstract
Fracturing processes in rocks is a complex phenomenon. Understanding the crack nucleation, propagation, and mechanisms involved during rock fracture plays an important role in the design of engineering structures, in the field of seismology, in natural resource extraction, different rock-breaking processes such as drilling and blasting, and in geological investigations. In order to investigate and predict the rock behavior in the above-mentioned practical situations, it is essential to understand the micromechanics of the fractures produced in rocks specifically under the effect of time. In this research, acoustic emission (AE) monitoring in combination with the non-contact 2-dimensional Digital Image Correlation (2D-DIC) technique has been implemented to investigate the damage processes in rock under time-independent and time-dependent loading conditions. Prismatic Barre granite specimens were used for testing to ensure a planar surface for accurate in-plane 2D-DIC measurements.
In the first part of the thesis (Chapters 2 and 3), the AE dataset was collected for short-term and long-term (stress relaxation and creep) rock fracturing experiments. Conventional AE analysis results in conjunction with the 2D-DIC strain-based measurements were used to compare the rock behavior in the two different loading conditions, specifically, in terms of damage localization and associated fracturing mechanisms. It was observed that the AE signals and the strain-based measurements directly reflected the state of damage in the rock specimen and can be used to identify the cracking levels i.e., the crack initiation (CI) and crack damage (CD), and the mode of deformation. The frequency-magnitude distribution (?- value) of the AE events at different cracking levels was used to identify the degree of damage for short and long-term loading experiments.
In the second part of the thesis (Chapter 4), a calibration apparatus was used to successfully calibrate Nano 30 AE sensors. These calibration results were then applied to the AE datasets for the stress relaxation and creep experiments to characterize the AE source parameters. Using the calibration information, the magnitude, source dimension, stress drop, and radiated seismic energy of the fractures were successfully estimated for the relaxation and creep experiments. Magnitudes of the AE events were in the range of -9 < ?? < -5, consistent with other laboratory observations, and the radiation efficiency was << 1%. These results were further used to relate the laboratory observations with the large magnitude natural seismic hazards and induced seismicity from mining, civil, and petroleum engineering applications.
In the third part of the thesis (Chapters 5 and 6), with the ability to characterize the AE sources, the method was applied to the conventional creep experiments to understand the micromechanical behavior of rocks during the primary, secondary, and tertiary stages of creep. The creep experiments were also conducted at different stress levels of the unconfined compressive strength to analyze the effect of stress on the creep behavior of rocks. The temporal evolution of the source parameters of the AE events showed significant variation in the three stages of creep. This is perhaps the only study that tracks the spatial and temporal changes in the seismic source parameters for the time-dependent loading conditions. The source mechanisms were characterized during the brittle creep deformation using various approaches based on the moment tensor decomposition components and the polarity of the AE signals. Tensile cracking was identified as the dominant mode of deformation during brittle creep experiments regardless of the applied stress. These observations are very useful to understand the underlying mechanisms of deformation caused by the progressive damage in rocks, which can be further employed for damage-related failure under time-dependent loading.
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