Determination of Short Crack Depth with an Acoustic Microscope

Abstract

For the prediction of the lifetime of any component, subjected to alternating stresses, the knowledge of the growth behavior of defects is essential. Most methods of monitoring the propagation of short cracks are confined to measuring the length of the crack on the surface [1]. The depth of the crack must be determined indirectly, assuming the shape of the crack. Acoustic waves, on the other hand, offer the possibility of measuring the depth directly, since acoustic waves can penetrate into the material. This allows the measurement not only of the growth behavior of fatigue cracks on the surface, but also changes of the crack geometry inside the specimen. Current applications of direct acoustic monitoring of crack growth have been developed for cracks of the order of millimeters. One acoustic depth measurement technique is the Time-of-Flight-Diffraction (TOFD) technique [2–4], which is based on timing measurements of the scattered signals from the defect. Our investigations are concerned with the application of TOFD technique for the depth measurement of short cracks (70–200 μm in surface length) using a scanning acoustic microscope (SAM) [5–6]. Depth measurements were first carried out on cracks in the transparent material polystyrene. This allows a direct comparison between acoustic and optical depth measurements. Subsequently, the depth of fatigue cracks in an A1 alloy were measured, and the acoustic measurements were compared with direct measurements of the crack geometry by sectioning the crack.