Computational ultrasound testing for non-destructive evaluation of multilayered structures: 3D modeling of longitudinal wave propagation signal responses

Non-destructive evaluation (NDE) using ultrasonic imaging (UI) is essential for detecting defects in complex multilayered structures, which are commonly encountered in aerospace, automotive, and medical fields. A thorough understanding of material properties, wave propagation, and dispersion behavior is crucial for accurate defect detection. The Transfer Matrix Method (TMM) provides a comprehensive approach by modeling the entire UI process, including the control system, ultrasonic transmitter, wave propagation (both bulk and guided modes), and receiver response. This study focuses on predicting the backscattered ultrasonic signal during longitudinal wave propagation in a multilayer structure immersed in water, considering normal incidence and specific frequencies. TMM, employing a quadrupole formalism, integrates stress and velocities at layer boundaries and models the multilayer structure as a transfer matrix derived from individual layer matrices. This approach allows for the calculation of reflection coefficients across a wide frequency spectrum. TMM generates detailed distance-time and distance-frequency representations that illustrate the propagation of various longitudinal modes in configurations, such as plexiglass/water/glass under direct (PWG) and reverse (GWP) insonation. Comparisons between PWG and GWP distance-time planes may be affected by layer arrangement and properties, which influence the reflection coefficient, highlighting the system's sensitivity to layer order even with similar thicknesses and materials.