ABSTRACT

The Punch-Through Shear with Confining Pressure (PTS/CP-) experiment has been proven to be a reliable testing method for the determination of Mode II fracture toughness. The Mode II fracture toughness, KIIC, is a measure of the resistance of a material to the propagation of a shear loaded fracture. Due to a simple but effective specimen geometry, the PTS/CP- experiment is the only available experiment that is able to apply a confining pressure independent from the shear load. In this contribution the influence of temperature on fracture toughness is presented and discussed. Several studies have dealt so far with the influence of temperature on Mode I (tensile) fracture toughness, but there is little to no data available on KIIC. The temperature was varied between 25°C to 250°C and experiments were carried out on a fine-grained granite. Experimental data suggests that KIIC remains constant up to 150°C and increases at elevated temperature. Ultrasonic measurements, prior and after heating, provided a basis to link the development of thermally induced cracks to the data of KIIC.

1 INTRODUCTION

A commonly employed way to analyse stability of constructions in geomaterials is to combine strength and stress state. If the acting stresses exceed the strength, failure is expected. This phenomenological approach bares several limitations. For example, if determining the strength parameters of rock material, one faces the problem that the values are valid strictly for the applied boundary conditions only. Hence, an increase in volume of the tested material usually results in a change, i.e. a reduction of strength. This limits the applicability of such data.

An alternative to the empirical continuum mechanics strength criteria are fracture mechanics based approaches. Linear fracture mechanics in general assumes preexisting discontinuities in a material that act as stress concentrators. The magnitude of the stress concentration governs the brittle fracture process. If pre-existing cracks or flaws are propagated by the stresses and coalesce to form larger discontinuities, the structures may loose integrity and fail. The mechanistic criteria try to mirror the physical origin of the processes and are therefore more exact.

Based on the principles of fracture mechanics, it is possible to not only asses the stability and safety of underground constructions, like caverns, tunnels or boreholes, but also to simulate – based on physical principles – the development of fractures in the vicinity of such openings. From the simulations the geometry of fracture patterns might be derived and used for different aspects, like fluid flow simulations. Some software packages are already available, e.g. Fracod2D, or under development.

Linear fracture mechanics provides the tools to estimate the stress and displacement fields around the tip of a discontinuity. Cracks or fractures are usually subdivided into three basic types, namely Mode I, Mode II and Mode III, based on the crack surface displacement (Lawn 1993; Fig. 1A). In Mode I, the tensile mode, the crack tip is subject to displacements perpendicular to the crack plane. In Mode II the crack faces move relatively to each other in the crack plane.

This content is only available via PDF.
You can access this article if you purchase or spend a download.