Depth Measurements of Short Cracks in Perspex with the Scanning Acoustic Microscope

Abstract

The geometry, such as surface length, depth, shape and orientation, is one of the key factors that dominate the propagation behaviour of short fatigue cracks. Thus, for a quantitative understanding of short crack growth behaviour, it is necessary to monitor the crack geometry throughout the fatigue test. However, a technique for the direct measurement of short crack geometry has been lacking, which, together with the difficulty of short crack detection, is one of the major reasons for the slow progress taking place in the study of short fatigue cracks. The growth of short fatigue cracks is commonly monitored only in its surface length with an optical or scanning electron microscope. The depth determination for the crack is then usually made on the basis of an empirical estimation from the crack surface length. For instance, the penny-shape or semi-elliptical estimation by assuming the profile of the crack as a semi-circle infers the depth of a short crack to be half of its surface length^1,2. Obviously, such an estimation cannot usually provide reliable data on the depth of short cracks in many materials, because of the strong dependence of the short fatigue crack shape on local microstructural characteristics and environment.