The use of additive manufacturing (AM) processes, to produce components with complex geometries has grown significantly over the last decade. Complex 3D-printed structures can take the form of thin shells, plates of varying thicknesses, or having functional graded materials. Improving NDT methods using guided waves is a challenge in particular by local contactless all-optical methods. Elastic waveguides allow waves to propagate with complex dispersion, even if the waveguides have graded elasticity or non-uniform geometrical properties. This can lead to anomalous wave propagation. Several theoretical, numerical, or experimental studies [1] have investigated the guided waves in free elastic plate with different cross-sections, or elastic graded materials (adiabatic modes, Maxwell’s fish-eye, trapped modes, phase conjugation effect, backward propagation modes, etc.) [2]. However, the study of ultrasound propagation in these peculiar waveguides remains original. In this presented work, numerical experiments were carried out using SFE simulation code developed in the Lab [3], which was adjusted to account for laser generation (thermal and optical effects have been neglected). The LPBF process has been used to print multiple aluminium plates with linear thickness losses along the plate h(x, y, z) or locally non-uniform cross-sections (h can increase or decrease in a Gaussian annular shape). Dispersion curves and ultrasonic wavefield were measured by laser ultrasonic technique (LUT) on millimetre inhomogeneous plates. Either with sub-millimetre local Gaussian shape cross-section or with slow linearly thickness variations in one or two directions, waves can propagate in this area due to their width relative to the acoustic wavelengths used. Absolute normal displacements were measured at different points using a broadband laser interferometer and then processed in the spatial and temporal Fourier domain, by filtering the 3D-dispersion curves (kx, ky, ω) of Lamb modes, to determine the thickness variations. Comparatively to thickness measurements using ZGV resonances, we then became interested in the limits and robustness of methods using propagative modes for reconstructing the smooth shape defects along such waveguides. To accomplish this, we used the instantaneous wave number (IWN) method using the A0-mode [4] or we exploited the A1-mode cut-off frequency [5]. By tracking the cut-off frequency of this mode, or local thickness-shear resonance and the associated Airy function, we reconstructed the width of the waveguides. The experimental and numerical results are in good agreement.

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