ABSTRACT: Crack initiation and crack damage strength thresholds are key parameters to describe the brittle behavior of rock and are used as input values for numerical models. A set of 15 Quintner limestone samples, with calcite veining, were tested in uniaxial compression. The strength thresholds were determined by acoustic emission and strain methods. Crack initiation values were less variable than crack damage and peak strength. The calcite vein orientation was determined to influence the strength thresholds and stiffness of the samples, which generally increased as the vein angle off the loading direction was increased. The influence was most pronounced on the stiffness and less so on the crack initiation threshold. An influence on crack damage and peak strength is indistinguishable with the small set of samples; however an visual observation suggests that when thick calcite veins were present, the samples tended to be stronger.
1. INTRODUCTION The strength of solids is typically based on the maximum stress which the solid can with stand before cracks accumulate to render the solid unable to carry load. One of the earliest studies, prior to the concept of stress, on breaking iron wires was documented by Leonardo da Vinci [1]. Griffith’s experimental work substantiated this early work [2] and also showed that the strength was dependent on the surface quality of the test specimens. Work by Griffith, [2] and [3], was fundamental to developments which demonstrated that crack initiation in brittle rock is manifested by crack growth sub-parallel to the loading direction in unconfined tests. The stress where cracks begin to grow or new mcro-cracks are initiated is called the crack initiation stress (sCI) and below this threshold Tapponier and Brace [4] found that the natural micro-cracks were mostly limited to the grain scale for crystalline rocks.