On the nature of shear horizontal wave propagation in elastic plates coated with viscoelastic materials

The possibility of using long–range ultrasonic guided–wave non–destructive testing for the inspection of metallic structures coated with attenuative layers is extremely attractive. However, the material damping may cause severe attenuation of the acoustic signal, resulting in reduction of the test range. This paper addresses the dispersion of shear horizontal waves propagating in metallic plates coated with viscoelastic layers. It is shown that the bilayer modes can be viewed as an interaction between the free elastic plate modes and the modes of the viscoelastic layer if it were clamped on its lower surface. For low–attenuation materials the interaction is strong and the trajectories of the bilayer dispersion curves jump between the two families as the frequency increases. However, as the material damping increases the interaction becomes weak and most of the energy is confined in either the metallic plate or in the attenuative layer, and modes no longer jump. The guided–wave attenuation is related to the strain energy in the viscoelastic layer per unit in–plane power flow by means of an energy–balance argument. The guided–wave attenuation of the modes whose energy travels primarily in the elastic plate exhibits periodic peaks in the frequency domain, which occur at roughly equally spaced critical frequencies. These frequencies are close to the through–thickness resonance frequencies of the clamped–free viscoelastic layer if it is considered to be elastic. Minima of the guided–wave attenuation occur around the Love transition frequencies.