Scattering of ultrasonic guided waves by heterogeneous interfaces in elastic multi-layered structures

An analytic-numerical method to simulate the interaction of ultrasonic guided waves with nonuniform interfacial imperfections in elastic multi-layered structures is presented. The primary ingredients in the formulation are the Quasi Static Approximation for the interfaces and the perturbation method to account for nonuniform flaws. The solution algorithm was developed with the aid of the invariant imbedding technique that ensures numerical stability, even for evanescent wave components at high frequencies. The first part of the algorithm is dedicated to the computation of dispersion curves for layered structures and the corresponding modal wave fields. The second part of the algorithm serves essentially to compute the field radiated by interfacial acoustic sources embedded in layered structures. The developed method is equally well suited to treating anisotropic and isotropic layers, and the upper and lower substrates may be either solid, fluid or vacuum. The proposed method application was illustrated by ultrasound inspection simulation of a two-layer structure with an imperfect interface, and the obtained numerical results show that the method is capable of capturing the resulting scattering effect. It is expected that the proposed modeling technique can be used to aid in the design of ultrasound inspecting systems. As an example of that, an empirical rule of thumb for identifying inspecting modes that will strongly interact with interfacial defects was identified. The experience indicates that it works quite well, though more tests still need to be performed.