Lamb wave NDT has successfully been applied to certain isotropic materials and structures, e. g. adhesive bonded aluminum plates. However, Lamb wave excitation in composites is more complicated. This is because of anisotropic properties and a variation of the number of wave modes and mode characteristics in different directions. Also, the transducer characteristics directly affect the mode eceptation and reception. To establish a guideline for the practical applications of Lamb waves in anisotropic composites, theoretical calculation of the acoustic fields in unidirectional and orthotropic plates was carried out. The acoustic field was assumed to be generated by different finite size transducers for a normal incidence setup. A series of boundary value problems have been studied to evaluate the wave propagation behaviour in various composite structures. The solutions were obtained by asymptotic methods for calculating displacements in the far field. It is shown that the amplitude of the propagation waves can significantly vary in certain directions, depending also on transducer size. Modes can be excited with very strong amplitudes by one transducer and with very weak amplitudes by another of different size under the same testing conditions. These results are very useful for analyzing Lamb wave excitation and wave propagation phenomena in composite NDT, and in transducer design for particular composite plate inspection.

Experiments were carried out on unidirectional composite plates. Typical results showing the variation of the Lamb wave modes in different directions with transducers of different sizes were obtained. The practical applications of the theoretical results are discussed.