Stress corrosion and acoustic emission during tensile crack propagation in Whin Sill dolerite and other basic rocks

Abstract

Double torsion tests in ambient air and liquid water have been used to establish critical stress intensity factors (KIc) and stress intensity factor (KI)-crack velocity (v) diagrams for propagation of single tensile cracks in Whin Sill dolerite and Ralston Intrusive. KIc for Whin Sill dolerite was 3.28 MN m−3/2 and for Ralston Intrusive was 2.58 MN m−3/2. No stress corrosion limit was encountered. Acoustic response was monitored during experiments on Whin Sill dolerite which spanned a velocity range from 5 × 10−4 to 5 × 10−9m s−1. The rate of acoustic emission was an indirect measure of crack velocity. The slopes of KI-v diagrams and KI-acoustic emission rate diagrams had similar slopes of c. 30 in double logarithmic coordinate frames. Slopes of the KI-v curves for Ralston Intrusive fell from 43.4 in air to 23.3 in water, during tests which spanned a crack velocity range from 5 × 10−4 to 10−8 m s−1. The activation enthalpy for dolerite between 20° and 75°C was determined by two complementary methods to be 30.4 × 1.9 and 33-46 kJ mol−1. Amplitude distributions of acoustic emission events depend strongly on KI and the ‘humidity’ at the crack tip. As KIc is approached, b-values for dolerite in air decrease from 1.8 to 1.25, and for dolerite in water a decrease occurs in b-value from 3.5 to 1.25. At KI greater than 0.8 KIc the b-values for tests in water and in air are virtually identical. A discussion is given of these results in terms of their implications for earthquake mechanics.