Abstract

In Brazilian test, applied diametrical compression stress induces indirect tensile stresses normal to the vertical plane crossing through the rock disc and the ultimate failure occurs at the place where the maximum tensile stress is concentrated. The mechanical behaviour of rock with pre-existing cracks under static loading has been studied widely. In this study, the fracturing behaviour of Brisbane Tuff, under static and cyclic loading has been analysed applying an ISRM standard Cracked Chevron Notched Brazilian Disc (CCNBD) geometry. Specifically, X-ray Computed Tomography (CT) techniques have been used to investigate the fracturing behaviour of rocks under static and cyclic loading. The fracturing behaviour of rocks technically depends on the nature of loading, strength of mineral and text of rocks. Laboratory observations demonstrated that there is a distinct difference in fracturing between the static and cyclic loading. It was found that the cyclic loading had an important effect on microfracture propagation through the Fracture Propagation Zone (FPZ).p>

1. INTRODUCTION

Rock cutting is a common rock breakage, or excavation mechanism in tunneling, mining, well drilling and road construction projects. Understanding of rock brittle failure and propagation of cracks under the applied cutting loads is significant for rock engineers to investigate cuttability, crushing effect and production efficiency in rock fragmentation process. A considerable amount of literature has been published in rock fracture modeling, however, large amounts of the research have been published based on the experimental and mathematical models developed based on the elastic fracture mechanics. The first prediction model was established based on the maximum tensile stress theory by Evans [1]. Later, Roxborough and Philips [2] modified Evans model for various rocks and found correlation between the rolling forces, disc diameter, edge angle and penetration rate. According to the experimental studying, brittleness index was established as a function of uniaxial compressive and tensile stress [3]. Alehossien and Hood [4] proposed a linearized dimensional model to predict rock cutting forces based on the linear model. Non-linear and multi-linear predictive model were suggested by Tiryaki [5]. Amongst the different models of cutter tools, drag picks are more efficient as they attack rock in the tension mode and undercut by the lower magnitudes of forces compared to the shearer or compressive cutters. Rock cutting and fragmentation is essential for rock breakage process and forming different sizes of fragments, chips and particles during the rock excavation. As most rocks are weaker under tensile loading, applying tensile stress for rock breakage is the premise for new cutters. Moreover, rock initial flaws and cracks are locations for stress concentration, and play an important role for further fracturing and damage by coalescence and propagation. In brittle fracture mechanics, stress intensity factor and critical fracture toughness are the most fundamental parameters used for determining fracture propagation, energy release rate, and crack extension driving forces. Typical brittle fracture in rock is characterized by compressive or tensile tests, and post-failure behavior can be explained as the brittleness index for rock. However, under cyclic loading, the fatigue life of rock depends upon both frequency and amplitude of the cyclic loading [6-8]. Crack tip characteristic parameters are generally governed by the crack tip stress components and displacement functions to determine crack propagation analysis. The critical fracture toughness can be obtained experimentally according to the suggested methods of International Society for Rock Mechanics (ISRM) in a different ways that listed and compared in Table 1.

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