ABSTRACT:

The resistance to fracture propagation under shear loading can be described by the Mode II fracture toughness, and can be expected to be dependent on a stress acting perpendicular to the direction of fracture propagation. The Punch-Through Shear with Confining Pressure (PTS/CP-) test is introduced as a method to determine the Mode II fracture toughness of rock material. Its unique feature is the ability to apply a confining pressure independent of the shear load. With increasing confining pressure (0.1MPa<P<70MPa) KIIC increases non-linear. The micro structural breakdown process involved in the formation of the macroscopic shear fracture changes with increase of P. Tensile (Mode I) components are increasingly suppressed with the increase of the normal load on the fracture trace. A mechanical analysis of the breakdown process supports the fractographic descriptions. Cyclic loading with successively increasing load of the PTS/CP- specimens under the influence of confining pressure shows constant slope of reloading loops suggesting that the material properties in the plane between the notches do not change. Preliminary results from ongoing campaigns show an increase of KIIC with temperature (20-250°C).

1. INTRODUCTION

Fracturing is a process in brittle materials that has great importance to the performance of structures. Rapid and violent failures of large-scale geotechnical, mining or civil engineering structures cause significant safety hazards, material damage, and interruption to or even cessation of mining or building activities. Geomechanics and related faulting is of major importance in structural geology and seismology also [1]. Ability to recognise pre-failure rock mass behaviour may result in predicting or averting the potential for geotechnical and geological failure [2]. Rock fracture mechanics is one approach to resolve this task. Rock fracture mechanics (RFM) can be employed not only to improve safety, but also enhance performance and economy of rock engineering structures. Examples are the geological disposal of radioactive waste, terrestrial sequestration of carbon dioxide to ease prejudicial effects on the environment, efficient underground storage of oil, gas or air, enhanced recovery of hydrocarbons and underground constructions at increasing overburden pressure for infrastructure or transport. With increasing capability of today?s computers the rock fracture mechanical analysis of underground structures becomes increasingly convenient. RFM makes use of the physical breakdown process of the rock in the formation of connected fracture networks, rather than using empirical relations. One of the parameters needed by computer codes using RFM, e.g. Fracod2D, is the Mode II fracture toughness. Mode II fracture toughness is a measure of the resistance of a material to the propagation of a fracture subject to a shear load. At the surface or shallow depth, the dominant mode of failure is tensile, but at elevated overburden pressure this mechanism of failure becomes less important. As rock in nature is subjected to high confining pressures, KIIC at elevated confining pressure is of interest in earthquake research, structural engineering and mining at larger depth and in computer modelling. Fracture toughness describes the critical stress concentration at a fracture tip necessary to initiate crack propagation. The factor is a parameter dependent on the geometry of the fracture, applied stress and initial fracture length.

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