Post-failure behavior of a granite and diabase

Abstract

Post-Failure Behavior of a Granite and Diabase Compression experiments have been performed on Westerly granite and Frederick diabase at a mean strain rate of 0 (10−5) sec.−1. In such tests Westerly granite and Frederick diabase exhibit Class II failure behavior between zero and 22000 psi and between 500 psi and 2200 psi, confining pressure respectively. In order to control brittle failure in these rocks, therefore, energy must be extracted from the samples. Complete stress-strain curves for Westerly granite can be divided into a number of characteristic regions. Each of these regions can be described in terms of distinct fracture patterns. In Westerly granite in uniaxial compression and at less than 2 900 psi confining pressure, the fracture patterns consist of local cracks predominantly parallel to the direction of greatest compression, slabs and narrow intensely crushed shear zones. At greater 2 900 psi confining pressure the fracture patterns consist of local cracks predominantly inclined to the direction of greatest compression and of macroscopic shear fractures. At high confining pressure, local cracks tend to propagate in their initial plane. The fracture patterns in Frederick diabase are sensibly the same as those in Westerly granite at 22000 psi confining pressure. The ultimate strength of Westerly granite above 22000 psi confining pressure and in Frederick diabase is controlled by faulting. At lower confining pressure faulting in granite occurs only in the post-failure region beyond the peak of the stress-strain curves. Preliminary experiments suggest that creep in Westerly granite under high and constant stress produces the same fracture patterns that occur in the post-failure region in continuous-deformation tests. In uniaxial compression creep terminates in macroscopic fracture near the intersection of the constant stress-strain path of a creep sample with the post-failure branch of the stress-strain curve of a quasi-statically loaded specimen. A comparison of all fracture patterns suggests that the fracture mechanisms in rock depend upon confining pressure, mineral composition and grain size of the rocks tested. It is unlikely, therefore, that rock failure in compression can in general be described by a single failure criterion.