Modified PTS test and clump modeling on mixed mode crack growth in granite

This paper describes the development of a new test for measuring the mode II fracture toughness for rock, K_IIC. The test is called the Modified Punch Through Shear test (MPTS), which is a modification of the PTS test developed by [4]. 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.