Laser ultrasonic detection for strut defects in additively manufactured lattice structure using zero-group-velocity...

Strut defects are prone to occur in additively manufactured lattice structures and can significantly degrade their mechanical performance, while non-destructive testing of such periodically complex architectures remains challenging. This study proposes a laser ultrasonic inspection method based on zero-group-velocity (ZGV) Lamb wave features to detect strut defects in single-layer lattice structures. A numerical framework combining dispersion analysis, single-frequency response analysis, and transient spectral analysis is established to clarify the formation and evolution of ZGV modes in intact and defective structures. The results show that the intact lattice supports a distinct S₀-ZGV mode, whereas a strut defect not only shifts the initial ZGV frequency but also induces a defect-related feature-guided-wave (FGW) branch and the corresponding FGW-ZGV mode. Intact and defective regions exhibit markedly different localized responses at the ZGV frequencies, which form the basis of the proposed detection method. By extracting the spectral energy near the ZGV frequencies, a defect index (DI) is constructed for point-by-point defect detection. Numerical results further show stable and distinguishable DI between defective and intact regions for different defect severities. Laser ultrasonic experiments on additively manufactured Ti-6Al-4V lattice specimens verify that defective and intact regions possess different local ZGV resonance frequencies, and that both reference-frequency strategies based on S₀-ZGV and FGW-ZGV can achieve effective defect visualization. These results demonstrate that the proposed method provides an effective approach for non-contact defect detection in complex lattice structures.