Imperial College London University of London Sound Propagation in Lossless Waveguides Coated with Attenuative Materials

Research into ultrasonic guided wave non-destructive testing for the long range inspection of large metallic structures is now well advanced. The main advantage of this method is that a large area can be covered from a given transducer position, so avoiding expensive scanning of all the structure surface. However, in the presence of attenuative coatings the test range can be dramatically reduced. As a consequence, it is of great practical interest to characterise those modes and frequencies which minimise the guided wave attenuation. This thesis investigates the nature of shear horizontal (SH) and Lamb waves propagating in elastic plates coated with viscoelastic layers, this geometry being also representative of coated pipelines with large diameter to wall thickness ratio. For both SH and Lamb waves the mode which exhibits the highest potential for long range inspection purposes is identified and analysed. It is demonstrated that Lamb modes provide longer propagation distance than SH waves. Moreover, it is shown that the acoustic properties of the coating play a major role in the attenuation of the guided waves. In order to measure these properties for a broad variety of viscoelastic materials, two novel techniques are developed. The bulk velocities and attenuation of the coating may be obtained by measuring the phase velocity and attenuation of guided waves propagating in a hollow waveguide filled with the viscoelastic material. This method is feasible when the material flows sufficiently easily for the cylinder to be filled. An alternative, when the material does not flow easily, is to clamp a sample of the coating between two rod waveguides and to measure the reflection and transmission of guided waves across the sample. This has enabled the acoustic properties of the bitumen used to provide corrosion protection on pipes in the chemical industry to be measured both when it is applied in its viscous liquid state and when it has been in place for many years and become solid.