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Understanding initiation and growth of cracks leads to better evaluation of rock masses behavior. Investigation of crack growth and propagation is noteworthy in geotechnical engineering, petroleum engineering, geology, seismology and many other sciences which encounter with rock fracture mechanics problems. Evaluation of rock slopes stability, design of retaining structures, design of tunnels, and prediction of flow path through rock masses, are some example of application of crack growth study in geotechnical engineering. By evaluating cracks growth, propagation and coalescence, faults seismic behavior can be understood. In general faults are considered as quasi-static cracks, so study of crack propagation under various loading conditions brings important information about faults. Many researchers studied crack growth in Brazilian disk shape specimen under diametrically compression loading. Previous research focused on open crack propagation and closed crack were infrequently included. In this paper using molded gypsum, as a model material, crack growth in Brazilian disk shape specimens with pre-existing open and closed flaws are investigated. The dimension of specimens is 100 mm in diameter and 50 mm in thickness. Inserted open and closed flaws length is 30 mm. Open and closed flaws are inserted using 1 mm thickness steel shims and 0.05 mm thickness silicon tapes, respectively. In this study, the specimens engineering properties were determined by conducting uniaxial compression test and indirect tensile test on Brazilian disks. The experiments were conducted according to ISRM and ASTM standards. Uniaxial compressive strength, Poisson ratio, elastic modulus, and tensile strength of specimens are, respectively, qu=38 MPa, υ=0.25, E=8.5 GPa, and σt=7 MPa. Diametrical compressive loading was applied to the specimens in various inclination angles of flaws with respect to the loading direction. In this paper, influence of open and closed cracks on strength of the specimen is investigated. Fracture toughness of mode I, II and mixed mode I-II, are also determined using analytical solutions. Throughout the tests, slippage between closed crack surfaces is continually monitored by digital microscope. According to the curves of slip distribution along the crack length derived from laboratory data, there is a good agreement between slip distribution along closed flaws and that recorded from real faults by other researchers. Also, results of this study show that open and closed flaws reduce disks strength, significantly (i.e., more than 50%). Inclination angles of the flaw with respect to the loading direction affects crack propagation patterns. Generally, new wing cracks initiate form the flaw tips and then propagate toward the loading direction. Increase in the inclination angle leads to a reduction in the effect of open flaw on crack growth and for angles more than 27.2˚ (i.e., pure shear, mode II) new crack does not initiate from flaw tips. At inclination angle of 90˚, flaw has no effect on crack propagation pattern and the disk fails under tensile splitting condition. In disks with closed flaws, new cracks initiate from flaw tips expect for the case of 90˚ which is the same as open flaws. For the material used in the study, mode I and II fracture toughness are calculated form experimental data and found to be equal to 0.250 and 0.258, respectively. The obtained values for fracture toughness indicate that used material is brittle. Also, the values of mode I fracture toughness for angels above 27.2˚ are negative which point to the fact that flaw tips in these angles are in compression and not in tension.

Keywords: Rock Fracture Mechanics, Fracture Toughness, Brazilian Disk, Crack Growth, Pre-Existing Flaw

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