Properties of guided waves in composite plates, and implications for NDE

Guided waves are potentially very useful for the rapid NDE of plate structures. In the aerospace industry in particular, they have been proposed for the structural health monitoring of carbon fibre skin panels, in either an active or a passive configuration. In an active configuration a guided wave is deliberately generated and then later received after it has travelled through the structure; in a passive configuration an acoustic emission sensor is used to listen to sound created by the structure itself. The successful development of these ideas depends on a good knowledge of the properties of the guided waves, yet it has been found that the properties of guided waves in such materials are very much more complicated than those in a simple metal skin. The complications include steering of the direction of the group velocity by the anisotropy of the plate, and attenuation because of damping of the matrix material and scattering by the fibres. The authors have studied these phenomena both analytically and experimentally. This paper presents an over-view of the findings and identifies the key implications for practical developments of structural health monitoring techniques.