SUBCRITICAL FRACTURE PROPAGATION IN ROCKS: AN EXAMINATION USING THE METHODS OF FRACTURE MECHANICS AND NON-DESTRUCTIVE TESTING

Time- and environment-sensitive crack growth occurs in a wide variety of materials and results from a stress-sensitive chemical reaction between the solid and its environment. Quantification of this process allows one to predict, or exert control over, long-term fracture stability. This thesis documents an experimental investigation of tensile rock fracture with an emphasis on characterizing time-dependent crack growth using the methods of fracture mechanics.; Subcritical fracture experiments were performed in moist air on glass and five different rock types at crack velocities ranging from 10('-2) to 10('-7) meters per second using the double-torsion technique. The experimental results, combined with a survey of the ceramics literature, suggest that subcritical-fracture resistance in polycrystals is dominated by microstructural effects that operate independent of the specific chemical reaction of the stress-corrosion process.; Evidence for gross violations of the assumptions of linear elastic fracture mechanics and double-torsion theory was found in the tests on rocks. The large data scatter did not warrant further crack-growth tests under varying environmental conditions as was possible for glass.; In an effort to obtain a better understanding of the physical breakdown processes associated with rock fracture, a series of non-destructive evaluation (NDE) tests were performed during subcritical-fracture experiments on glass and granite. The NDE tests provide an estimate of the size and shape of the zone in which the inelastic fracture processes operate. Zone-size estimates for Westerly granite were based on the spatial distribution of holographically-measured surface displacements, acoustic-emission locations, and the spatial variation of ultrasonic-wave transmission and exceed dimensions of 10 mm under the various test conditions employed.; Comparison of the observed process-zone shape with that expected on the basis of a critical-normal-principal-tensile-stress criterion shows that the zone is much more elongated in the crack-propagation direction than predicted by the continuum-based microcracking model alone. In situ microscope observations of the fracture process show that frictional interlocking between partially-separated crack surfaces is largely responsible for the elongated nature of the process zone. Crack-plane friction and friction-induced microcracking are identified as mechanisms contributing to R-curve behavior and the crack-velocity sensitivity of fracture morphology.