Pure shear fracture of brittle rock a theoretical and laboratory study

University dissertation from Luleå : Luleå tekniska universitet

Abstract: This doctoral thesis deals with determination of Mode II fracture toughness, KIIc, for rock. Three types of specimens, antisymmetric four-point bending, direct shearing and compression-shear specimens, are experimentally and numerically investigated. Mode II fracture can be sucessfully created in compression-shear specimens as well as in four-point bending and the direct shearing specimens with guiding-grooves. The obtained KIIc are 2-3 times as large as Mode I fracture toughness KIc. The size effect on KIIc is also studied. Since the compression-shear test has the advantages of simple specimen geometry, simple loading device and favourable stress condition for creating Mode II fracture, it is suggested as a standard method for determining KIIc for rock. Finally, a new criterion is established for both Mode I and II fractures. The criterion says that Mode II fracture occurs when KIIOmax reaches KIIc while KIOmax is less than KIc.

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