Noncontact pulsed laser-scanning laser Doppler vibrometer (PL-SLDV) phased array imaging for damage detection in composites.

Guided wave phased arrays, which use multiple sensors in compact patterns to perform damage imaging through phase delays, have garnered significant interest for the rapid inspection of large composite panels. Previous phased arrays typically used large, wired ultrasonic transducers attached to composites, limiting array reconfigurability and preventing contactless inspection from a distance. This study presents a fully noncontact guided wave phased array imaging approach, which utilizes a dual laser-based guided wave generation and sensing system, namely a pulsed laser-scanning laser Doppler vibrometer (PL-SLDV) system, along with synthetic phased array beamforming and wavefield analysis. The PL-SLDV system employs a Q-switched PL module to generate nanosecond laser pulses that excite ultrasonic guided waves through the thermoelastic effect. To ensure consistent laser-to-ultrasound energy conversion across different composites and prevent potential thermal damage to composites, the laser pulses are directed onto a thin aluminum patch bonded on the composite. The SLDV acquires guided wave signals based on the Doppler effect, and its integrated galvo mirrors can quickly steer laser beam directions to scan a composite plate, thereby acquiring guided wave signals at various array points. Time/phase delays are then applied to the acquired signals through post-processing for synthetic phased array beamforming. To generate inspection images using the acquired wave signals, an improved delay-and-sum (DAS) imaging algorithm is introduced. It uses adaptive weighting factors and incorporates phase delay and back-propagation phase shift, accounting for the frequency- and direction-dependent dispersion relation, to overcome the dispersion effect and directional dependency of waves in anisotropic materials. Moreover, the fusion of phased array imaging and a wavefield analysis approach, which can extract frequency-wavenumber dispersion relations from experimental wavefields, enables our phased array method to perform damage imaging without requiring prior knowledge of composite properties, such as mechanical properties or theoretical dispersion curves. Additionally, the noncontact wave generation/acquisition feature of our PL-SLDV system allows for inspecting composites from a distance and easily constructing phased arrays with different patterns. Proof-of-concept experiments demonstrate that multiple defects in different directions can be successfully detected. Additionally, this study reveals that PL-generated guided waves can contain multiple modes, such as A, S, SH, A, S, and SH modes, offering valuable insights for researchers interested in using PL-generated guided waves.