Numerical Finite Element Modeling of Lamb Wave-Defect Interactions in Bonded Metal Structures: Benchmarking Mode Selection Criteria

This study investigates the interaction of guided ultrasonic Lamb waves with both internal and surface defects in aluminum/epoxy/aluminum bonded structures. The three-layer assembly consists of two 0.9398 mm-thick aluminum adherends bonded with a 0.25 mm epoxy adhesive core. Three defect configurations are examined: a centered hidden notch (CHN), a single external notch (1EN) located on the upper surface, and two symmetric external notches (2SEN) positioned on opposite faces. Displacement fields acquired from the top surface are processed using a two-dimensional fast Fourier transform (2D-FFT) to decompose the wavefield and identify incident, reflected, and transmitted modes. By calculating acoustic power, we can quantify how energy is distributed across different Lamb modes using specific energy coefficients. This helps pinpoint which modes are best for detecting defects. To figure out the most effective inspection strategy for each type of defect, we carried out a detailed multi-criteria assessment based on energy balance and 13 quantitative indicators. The findings show that both the type and size of a defect can be reliably extracted from these modal energy signatures. Overall, this work supports structural health monitoring (SHM) and nondestructive evaluation (NDE) by offering a rigorous yet practical framework for mode selection that responds well to defects in adhesive-bonded assemblies. The methodology holds significant potential for aerospace and automotive applications, where bond-line integrity is paramount, particularly for ensuring the safety and reliability of critical components such as aircraft structures and automotive chassis. The partial validity of our numerical approach is supported by the experimental study of Santos, whose results are consistent with those obtained in this study.