Novel techniques for fracture monitoring in crystalline rock
The study of fracture is necessary in geotechnical engineering for the safe design of structures and application of engineering principles. Fracture plays a significant role in the behavior of rock and structures during failure. Often times, fracture processes observed in the laboratory do not accurately represent what occurs in the field due to the effect that specimen size and loading conditions have on fracture properties, namely, fracture toughness and the size of the fracture process zone. The fracture toughness increases with specimen size until it reaches a limiting value when the material is large enough to exhibit brittle behavior. In a similar manner, the size of the fracture process zone is observed to grow with specimen size until a limiting value is reached for a large enough specimen. In this study, experiments were performed on granitic specimens of different sizes and under two different loading conditions, three-point and four-point bending. Digital Image Correlation (DIC) was used to identify and calculate the dimensions of the fracture process zone. The study of fracture and fracture processes in granitic specimens through DIC is challenging due to the extremely small displacements, in the order of microns, that are associated with fracture in granite. The results from experiments performed on Adelaide Black granite under three-point bending are compared to data obtained using acoustic emission on the same rock and were shown to be in good agreement. It was observed that in Charcoal granite specimens of sizes that that are typically used in the laboratory, the fracture process zone is larger under four-point bending compared to three-point bending conditions. However, it is predicted that for a large enough specimen, the dimensions of the fracture process zone are similar for both loading conditions. In addition, in the appendix, the use of ultrasonic measurements to study and determine the geometry of a propagating hydraulic fracture in granite is described.
Digital Image Correlation, Fracture Process Zone, Size Dependence, Rock Mechanics
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign