Identifying the ultrasonic inspecting fields that most strongly interact with adhesive bonding defects

Abstract

We propose a systematic modelling procedure to identify the harmonic ultrasonic incident fields that most strongly interact with adhesive bonding defects in submerged multilayer plates. In principle, these are also the optimal choices for the interrogating fields in inverse methods for interfacial stiffness estimation. We model the adhesive bonding with the aid of the spring boundary conditions and model the bonding defects by a reduction in the spring constants. As part of the proposed procedure, we use the invariant embedding technique to compute the reflection coefficient at the top of the immersed laminates. The main advantage of this technique is its unconditional numerical stability even for evanescent waves at high frequencies. The developed algorithm is equally well suited to treating isotropic as well as anisotropic layers. We illustrate the application of the proposed procedure simulating the inspection of a three-layer isotropic plate and a 16-ply anisotropic composite laminate used in the aeronautical industry. In the simulations, we identify the frequencies and angles of incidence that will most strongly interact with bonding defects.