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Multilayer plates are widely used as structural components, especially in high-tech industries. However, compared to much simpler homogeneous plates, they pose unique challenges to nondestructive testing (NDT) techniques. The inspections of multilayer plates using traditional ultrasonic (US) testing modalities can be challenging and time-consuming. Faster inspection can be achieved with the use of full-field techniques based on guided elastic waves. In particular, the local wavenumber estimation (LWE) of Lamb waves has been proven effective for damage detection in various plate-like structures. However, the effectiveness of LWE depends on multiple parameters, such as the excitation frequency, the choice of a Lamb wave mode, or the inspection side, among others. This is especially significant in the case of multilayer plates, where different types of damage result in a nontrivial behavior of Lamb waves. In this work, we present a framework for the evaluation of disbonds in adhesively bonded multilayer plates through local wavenumber estimation. The efficacy of the proposed approach is demonstrated on a test sample made of three layers of aluminum with different thicknesses bonded together with an epoxy adhesive. The sample contains artificial defects of various sizes introduced at different adhesive interfaces. First, we present a theoretical analysis of the dispersion curves in the test sample to determine the optimal LWE inspection parameters for particular types of defects. Next, the approach is validated experimentally based on a set of full-wavefield datasets acquired with a scanning laser Doppler vibrometer (SLDV). We demonstrate that the proposed methodology allows obtaining clear images of defects and determining their through-thickness locations in multilayer structures. Based on the obtained results, we believe that the proposed framework can also be effectively applied to different structures and damage types. guided waves, damage imaging, adhesively bonded plates, local wavenumber estimation |
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