How fine a surface crack can you see in a scanning acoustic microscope?

SUMMARY Surface cracks can be detected with great sensitivity in an acoustic microscope, even when the width of the crack is much less than the acoustic wavelength. This is because of the strong excitation and scattering of Rayleigh waves. Cracks in a variety of materials, including cracks from indents in several different ceramics, have been studied by acoustic microscopy, and the results compared with images from light and scanning electron microscopes. No crack or part of a crack that was seen by the other two techniques failed to be seen by acoustic microscopy. The contrast of a crack in an acoustic microscope showed no sign of fading as the crack became thinner towards its tip, and diffraction from tips suggests that they are acoustically well‐defined points on the surface. Although theoretically the acoustic contrast from a crack might be expected to diminish when the crack width is smaller than the shear‐wave decay length in the fluid, in practice this is so small that for many purposes it may be considered that there is almost no limit to how fine a crack can be and still be detected in an acoustic microscope.