This study presents a newly suggested test method called "Punch through Shear Test" which was originally proposed by Backers and Stephansson. The purpose of this study is to check the validity of the suggested test method by performing Mode II fracture toughness test for Daejeon granite. In addition, the optimal specimen geometry for the testing and the relation between Mode II fracture toughness and confining pressure were also investigated. Fractured surface Was observed to be very smooth which implies that the surface was sheared off and this confirms that Mode II fracturing actually occurred. In addition, continuum analysis and crack propagation simulations were performed. Results of the numerical analyses indicate that the cracks occurred in the specimen are predominantly in Mode II. From this investigation, it can be Concluded that the newly suggested "Punch Through Shear Test" method provides a reliable means of determining the Mode II fracture toughness.
The stress intensity factor K which is dependent on the geometry of a cracked structure represents the strength of stress intensity of a crack tip and has dimension of stress times length ¹/2. For a given cracked body with known loading configuration and magnitude, K and corresponding stresses and displacements can be determined. This implies that there exists a critical value of K, named fracture toughness, which expresses resistance to crack propagation. Crack initiation will take place when the stress intensity factor K exceeds its critical value, Kc. In this study, Mode II fracture toughness test is presented; shear fracture mechanics data such as Mode II fracture toughness, KIIC, and how the toughness varies with confining pressure are presented. In addition, testing method is verified by numerical analyses to check if Mode II fracturing predominantly takes place in a given test geometry. Backers (200 I) and Stephansson et al (2001) suggested a new testing method of which the specimen and loading configuration are shown in Figure I and carried out Mode II fracture toughness tests under various confining pressures to investigate the relationship between the Confining pressure and shear fracture toughness. The objective of this paper is to check the validity of this newly suggested test method.
A specimen is loaded by Hoek cell with a confining pressure and axial force is applied on the inner part of the sample. This leads to concentration of maximum shear stress at the upper and lower notch tip. And a zone of high shear stress is formed between the two notches from which Mode II failure to be generated.
The mechanical properties of the rock specimen used in this study are shown in Table 1. They were obtained from a series of laboratory tests on Daejeon Granite. A rock core of 52 mm in diameter is cut into pieces with length equal to the diameter. End surfaces were ground perpendicular to the lateral surface. The upper and lower notches were drilled into the two end surfaces.