https://www.sciencedaily.com/releases/2021/09/210929142807.htm

One attractive approach to scanning for damages on materials is using "Lamb waves." Named after the British mathematician Sir Horace Lamb, these are elastic waves generated in solid plates following an appropriate mechanical excitation. Because the propagation of Lamb waves is affected by surface damage (such as scratches), they can be leveraged to ensure that the scanned material is free from imperfections. Unfortunately, the generation and subsequent measurement of Lamb waves on transparent materials are not straightforward.

While laser-based techniques exist for generating Lamb waves in a contactless manner, the laser parameters need to be carefully calibrated for each material to avoid causing damage. Moreover, existing approaches do not generate Lamb waves of sufficient amplitude; as such, repeated measurements have to be conducted and averaged to get reliable data, which is time-consuming. As for measuring the generated Lamb waves, no existing technique can quickly detect and use them to look for submillimeter-scale damage on transparent surfaces.