This paper describes the development of a new test for measuring the mode II fracture toughness for rock, KIIC. The test is called the Modified Punch Through Shear test (MPTS), which is a modification of the PTS test developed by . Laboratory MPTS tests on Columbia granite were conducted. The lab tests and finite element modeling reveal that a valid shear crack is produced. Tests conducted at different confining stresses show that the shear cracking strength is a function of confining stress. Tests at each confinement were repeated 7 times, and even though the scatter in the results is large, best-fit results for shear cracking strength as a function of confining stress were determined. PFC modeling has also been conducted to simulate in detail the development of tensile and shear cracks in the MPTS test. A clump model rather than the standard Circular Particle Interaction (CPI) model was used to improve the strength envelope.
The determination of material properties associated with shear crack growth in rocks, also called mode II crack growth, is of great interest in the geomechanics community. There has been a large amount of literature written on the complex micromechanics of shear crack growth in rocks [e.g., 1], and here we focus on the development of a test that can be used to estimate the Mode II fracture toughness, KIIC. Valid tests to determine KIIC must, first of all, produce a shear crack. In many instances only a macro tension crack is produced under Mode II loading. Secondly, because of the frictional nature of shear crack growth in rocks, KIIC is strongly dependent on confining stress [2, 3] and a valid KIIC test must be able to determine KIIC as a function of confining stress. Finally, a valid KIIC test must be simple enough and the results repeatable. Stephansson et al.  developed a corebased test for determining KIIC that involves the development of a valid shear crack and also determines KIIC as a function of confining stress. Their test is called the punch-through shear test (PTS-test) and utilizes cylindrical core samples with circular notches. One of the drawbacks of this test is that cylindrical shaped shear cracks are produced, unlike the planar shear cracks that are observed in the field. In this paper, the authors introduce a modified version of the PTS test, called the Modified Punch Through Shear test (MPTS). This test is similar in many respects to the PTS test but utilizes a rectangular rather than cylindrical geometry and biaxial rather than triaxial loading. The shear cracks that are produced in the MPTS test are planar due to the two dimensional geometry. The MPTS test involves additional sample preparation from core, which is a drawback. On the other hand, tensional and shear crack growth in the MPTS test can be monitored with a video camera. This paper describes the results of MPTS tests conducted on Columbia granite.