Attenuation of ultrasonic Rayleigh–Lamb waves by small horizontal defects in thin aluminium plates

The present paper illustrates how horizontal symmetric crack-like defects can be detected rapidly in thin isotropic plates by using longitudinal ultrasonic waves. The use of such longitudinal waves offers the potential of being significantly faster for non-destructively detecting defects than conventional ultrasonic techniques, which rely on transverse waves propagating through the thickness of a plate. Computational and analytical methods are used to predict reflection coefficients due to the attenuation of a longitudinal ultrasonic wave by physically small defects. It is shown that detectable attenuation (>10%), i.e. reflection coefficients, of the lowest order symmetric Rayleigh–Lamb wave (S0) occurs for a range of small defects (sub-millimeter in length) when using high-frequency waves (MHz range).