Detecting Damage Surfaces in Brittle Materials Using Acoustic Emissions

Abstract

A technique for mapping damage surfaces is developed and applied to Westerly granite. A damage surface for an elastic-brittle material is similar in concept to a yield surface for an elastic-plastic material and may be defined as the locus of stress states where micro-damage begins to occur. Using a generalization of the Kaiser effect, the onset of acoustic emissions (AE) during loading is taken to indicate the initiation of damage and, thus, contact with the damage surface. By following a number of loading paths, the locus of onset stresses can be mapped and identified as a damage surface. To demonstrate the technique, a large block of granite was loaded along one axis, establishing an initial damage state. Subsequent loading of cores taken from the large block at various angles to the initial loading axis, permitted reloading to the damage surface along different paths. The results show that the onset of AE is a strong function of the angle between the original loading axis and the axis of the core, indicating that the damage surface produced by uniaxial loading is highly anisotropic, even though the damage surface for the virgin material is isotropic.