Guided A0 wave mode interaction with interfacial disbonds in an elastic-viscoelastic bilayer structure

The study reports on the characterization of interfacial disbonds in an elastic-viscoelastic (steel-rubber) adhe-sively bonded bilayer structure where the elastic material (steel) has a significantly higher impedance than the viscoelastic material (rubber). The study uses the A0 mode guided wave at a low frequency of 100 kHz, which was generated and detected using non-contact air-coupled ultrasound. The A0 mode could be generated and detected from both steel and rubber sides, despite the high viscoelasticity of the rubber layer. However, the A0 mode guided wave propagation is sustained only in the steel layer and not in the rubber layer. The mode travelling along the steel layer leaks into the rubber layer and hence can be detected from the rubber side in the bonded region. When the disbond is below either the transmitter or the receiver, a better sensitivity in the disbond detection was achieved from the rubber side rather than the steel side. Using the experimental and 2D frequency-domain finite element simulation results, a linear correlation between the extent of disbond and the amplitude, time-of-flight (ToF) difference of the A0 mode arrival in the disbond region on the steel side was also established. The results show the potential to characterize interfacial disbonds regardless of the position with respect to the transmitter or receiver in elastic-viscoelastic multilayered structures using air-coupled guided wave inspection from single-side access.