Post-failure behaviour of brittle rock

Abstract: The study of the post-failure behaviour of brittle rocks in the laboratory needs stiff and servo-controlled testing machines. Yet, the problem is not completely solved unless a suitable feedback signal is created for the servo-loop of the testing machine. A technique has been developed, with the aid of which a suitable feedback signal may be provided for the testing machine. A pair of resistance strain gauges, each 5 an in length were embedded in a 2 mm thick Cil Monothane sleeve. Positioned circumferentially and at the mid-height of the specimen, the long strain gauges provide a reliable feedback signal that makes the controlled loading possible. The technique is particularly suitable for controlled triaxial testing. With the aid of gauged sleeves, we have performed series of controlled uniaxial and triaxial experiments on different brittle rocks. Based on the shape of the experimental curves obtained, post-failure response of the tested rocks has been described, placing much emphasize on the role of the testing machine. Trying to simulate the prevailing state of stress in the Earth's crust, where a shear fracture may propagate, we have carried out nonfixed, controlled confining pressure triaxial experiments. In such an experiment the confining pressure does not remain constant, but rises in opposition to the dilational behaviour of rock sample in both pre and post-peak phases of deformation. Under such circumstance, the pressure medium bears a level of stiffness. Using the experimental results of non-fixed confining pressure experiments, stress drop and energy release rate, pertaining to the shear fracture propagation have been determined. Along with the evaluation of these two fracture parameters, relevant theoretical bases have been outlined. It has been tried to place the estimates of the fracture parameters in their right perspective, by resorting to the previously existing estimates obtained in different ways, by examining the influential aspects of triaxial testing details and through explanations relating the conditions of the propagated shear fracture surfaces to the different levels of the normal stress, acting across the fracture surfaces.

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