POSTER REFERENCE: 0711 FRACTURE TOUGHNESS ENVELOPE OF A LIMESTONE ROCK AT HIGH CONFINING PRESSURE AND TEMPERATURE N. A. Al-Shayea1 and K. Khan2 1 Civil Engineering Department, KFUPM, Dhahran 31261, Saudi Arabia 2 Research Institute, KFUPM, Dhahran 31261, Saudi Arabia ABSTRACT Fracture locus or envelope under a mixed-mode I-II loading can be obtained by plotting the normalized mode-II versus mode-I fracture toughness values. The envelope obtained can be used as a criterion for fracture failure for that material. However, testing conditions have a strong impact on that envelope. The objective of this paper is to present some results of an experimental program that was made to obtain fracture toughness envelope for a limestone rock from Saudi Arabia. Brazilian disks with an inclined central notch were tested under diametral compression, to get variety of mixed mode I-II fracture cases. Tests were conducted using disks with different sizes, and different notch type. Tests were made at different confining pressure from 0 to 28 MPa, and different temperature from 27 to 116oC. Fracture toughness envelopes for the tested rock were obtained for both positive and negative regions (opening and closing of the crack) and at various environmental conditions. A quadratic equation, which fit the experimental results more satisfactory, were proposed as a failure criteria. One of the major contributions of this paper is the effect of high confining pressure and temperature on the fracture toughness envelopes of rocks. KEY WORDS Rock fracture, mixed Mode I-II, fracture envelope, high temperature and pressure. INTRODUCTION Studying the fracture toughness of rocks at elevated temperatures and confining pressures is valuable for a number of practical situations such as hydraulic fracturing used to enhance oil and gas recovery from a reservoir, and the disposal or safe storage of radioactive waste in underground cavities. Based on the loading type, there are three basic crack propagation modes in a fracture process, namely: Mode I (extension, opening), Mode II (shear, sliding), and Mode III (shear, tearing). Any combination of these modes can occur as a mixed-mode. Most, if not all, studies in the past have focused on fracture toughness determination under confining pressures only for Mode-I failure conditions. Nevertheless, due to randomly oriented cracks in rocks and/or in-situ stress conditions, cracks tend to propagate under the influence of a combined action of the basic
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