Damage in brittle materials: experimental methods

Abstract

Acoustic emissions can be used as an indicator of the production of damage in brittle materials. The basic premise of the method is that damage in brittle materials is the result of local, grain-scale cracking which can be detected by using ultrasonic transducers to pick up the resultant bursts of high frequency elastic waves (acoustic emissions or AE). Frequencies used are in the range from .1 to 1 MHz. We hypothesize that there exists a damage surface in stress space, analogous to a yield surface, such that stress states inside the surface produce no damage or AE. The surface is marked by the set of stress states where AE is produced. We have investigated damage surfaces produced by triaxial stress states (sigma/sub 11/ greater than or equal to sigma/sub 22/ = sigma/sub 33/ = confining pressure, where compressive stresses are positive). Our procedure was to use uniaxial loading to induce an anisotropic damage surface and then to use the Kaiser effect to locate points on the damage surface along various triaxial stress paths. In one set of tests, the effects of mean stress were investigated. In a second test series, the principal stress axes were rotated so that a projectionmore » of the full damage surface could be determined. From the effects of mean stress on the ultimate compressive strength of brittle materials, it was expected that the stress required to produce damage would increase strongly with increasing mean stress. An increase was observed which paralleled the increase in strength. An implication of this result is that a good estimate of the dependence of the failure strength on mean stress can be made from a single sample.« less