Acoustic microscopy of surface cracks

Abstract

The uniqueness of the acoustic microscope lies in its ability to image the way in which acoustic waves interact with the elastic properties of a specimen with microscopic resolution. A dominant role in the contrast achieved is played by Rayleigh waves that are excited in the surface of the specimen. They enable very fine surface cracks to be imaged, because they can strike them from the side and be strongly scattered. To account for this theoretically, the imaging theory of the acoustic microscope must be extended to account for non-specular scattering due to the crack. Experiments have been conducted using a cylindrical line-focus lens to measure the signal from a crack in glass, and the results can be compared directly with two-dimensional calculations from the theory. Applications of the ability of the microscope to image microscopic surface cracks include the analysis of metal fatigue and inspection of engineering ceramics, and the theoretical analysis may provide the basis for eventually characterizing surface microcracks quantitatively.