The fracture toughness of a rock often varies\ndepending on the specimen shape and the loading type used\nto measure it. To investigate the mode I fracture toughness\nusing semi-circular bend (SCB) specimens, we experimentally\nstudied the fracture toughness using SCB and\nchevron bend (CB) specimens, the latter being one of the\nspecimens used extensively as an International Society for\nRock Mechanics (ISRM) suggested method, for comparison.\nThe mode I fracture toughness measured using SCB\nspecimens is lower than both the level I and level II fracture\ntoughness values measured using CB specimens. A\nnumerical study based on discontinuum mechanics was\nconducted using a two-dimensional distinct element\nmethod (DEM) for evaluating crack propagation in the\nSCB specimen during loading. The numerical results\nindicate subcritical crack growth as well as sudden crack\npropagation when the load reaches the maximum. A Kresistance\ncurve is drawn using the crack extension and the\nload at the point of evaluation. The fracture toughness\nevaluated by the K-resistance curve is in agreement with\nthe level II fracture toughness measured using CB specimens.\nTherefore, the SCB specimen yields an improved\nvalue for fracture toughness when the increase of K-resistance\nwith stable crack propagation is considered.
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