Lamb Waves - Lamb Waves in Ultrasonic Testing

Lamb Waves in Ultrasonic Testing

The purpose of ultrasonic testing is usually to find and characterize individual flaws in the object being tested. Such flaws are detected when they reflect or scatter the impinging wave and the reflected or scattered wave reaches the search unit with sufficient amplitude.

Traditionally, ultrasonic testing has been conducted with waves whose wavelength is very much shorter than the dimension of the part being inspected. In this high-frequency-regime, the ultrasonic inspector uses waves that approximate to the infinite-medium longitudinal and shear wave modes, zig-zagging to and from across the thickness of the plate. Although the lamb wave pioneers worked on nondestructive testing applications and drew attention to the theory, widespread use did not come about until the 1990s when computer programs for calculating dispersion curves and relating them to experimentally observable signals became much more widely available. These computational tools, along with a more widespread understanding of the nature of Lamb waves, made it possible to devise techniques for nondestructive testing using wavelengths that are comparable with or greater than the thickness of the plate. At these longer wavelengths the attenuation of the wave is less, so that flaws can be detected at greater distances.

A major challenge and skill in the use of Lamb waves for ultrasonic testing is the generation of specific modes at specific frequencies that will propagate well and give clean return "echoes". This requires careful control of the excitation. Techniques for this include the use of comb transducers, wedges, waves from liquid media and electro magnetic acoustic transducers (EMAT's).