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Surface cracks introduced in soda lime glass and MgO single crystal have been examined with a 200‐ and 420‐MHz scanning acoustic microscope. A fringe pattern was observed around the cracking line on the surface which could not be observed with an optical microscope. A simplified model is proposed in which fringe formation is due to the interference between the leaky surface wave reflected by the crack and the specularly reflected wave at the surface or the leaky surface wave that did not reach the crack. By monitoring these fringes, a Hertzian crack in soda lime glass with a depth as small as 25 μm was detected. The direction of crack orientation relative to the surface normal was also obtained from the asymmetrical contrast distribution of the fringes.