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The layer-wise manufacturing process, i.e., Additive Manufacturing (AM), of structural components is widely used in various industries, including the aerospace and automotive sectors. This enables the engineers to design a smart structure by providing different materials to different layers based on their required functionality. AM also allows the manufacturing of components with complex geometry. Due to the layer-wise manufacturing, the components are prone to process-induced defects, including inter-layer delamination, cracks, and porosity. These defects could potentially lead to premature failure due to critical loads experienced by the components in service. Therefore, the early detection of these defects is necessary. Various Non-Destructive Inspection (NDI) techniques, such as ultrasonic testing, x-ray tomography, eddy current testing, etc., are conventionally used for defect detection in various industries. The ultrasonic-guided wave-based NDIs are quite popular as they have increased sensitivity to smaller defects and can travel long distances with minimal loss. We have performed the numerical simulation of guided wave propagation in the multi-layered structural waveguide using Fourier transform-based Spectral Finite Element (FSFE) method. The analysis of multi-layered structures is done in two ways, i.e., assuming interface bonding to be perfect (also known as classical laminate theory) and allowing the different levels of interface bonding strength. We have discussed the later case in this paper, in which the interlayer interface bonding layer is replaced by the distributed spring-dashpot systems to represent its viscoelastic behavior. By providing different values to spring and damping constants, we have simulated different levels of interface bonding strength. This problem can be solved via two approaches. In the first approach, the governing differential equations for all the n-layers are solved simultaneously for the dispersion relation. The other approach is based on the transfer matrix method, where the structural waveguide is assumed to be periodic in the length direction with the period as a unit cell. The dispersion relations are obtained from the spectral analysis, and the FSFE is formulated to get the time-domain responses for various excitation. These responses can be used as the base diagnostics signal for inspecting the AM components during manufacturing and in-service.
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