Lamb waves are guided acoustic waves which propagate in the plane of a plate and, like bulk acoustic waves, their interaction with defects means that they can be used for inspection purposes. Because they propagate in the plane of a plate, a reasonably large area of a suitable structure can potentially be inspected from a single location although with some reduction in resolution and sensitivity compared to the conventional single point ultrasonic inspection described above.

In unbounded isotropic solids longitudinal and transverse modes of acoustic wave propagation are possible and these are generally non-dispersive. However, in the case of Lamb waves an infinite number of modes can exist, all of which are dispersive, hence the use of Lamb waves for inspection purposes is, in general, not as straightforward as the use of bulk waves.

Although Lamb produced his famous dispersion equations for acoustic wave propagation in plates in 1917, it was not until Worlton (1961) provided experimental confirmation of Lamb waves at megacycle frequencies that the possibility of using Lamb waves for nondestructive testing began to be investigated in earnest. Since then, much work has been done on the use of Lamb waves and other guided waves for inspection purposes.

Very broadly speaking, the use of Lamb waves for non-destructive inspection purposes falls into two categories depending on the distance of propagation. Firstly, there are short range applications, where Lamb waves and other guided waves are used to obtain information which cannot be obtained by more conventional means. These areas include the determination of the elastic properties of materials , the detection of defects near to interfaces such as in the inspection of adhesive joints , and temperature and thickness measurement of thin films. In these cases, sensitivity is of key importance and generally this is the main selection criterion for a suitable Lamb wave mode. Effects such as dispersion are not so important as the propagation distances are small.

The second area which is of greater relevance here is in applications where the propagation distance is large. Applications of long range Lamb wave inspection include the detection of delaminations in rolled steel and composites, and pipeline inspection. In long range applications, the aim is to inspect large areas rapidly.