A matrix-configured multipoint laser doppler vibrometer for high-precision imaging and characterization of metal surface cracks

Nondestructive testing of metallic surface cracks is critical for safeguarding structural reliability in industrial applications. Single-point laser Doppler vibrometers (LDVs) rely on time-consuming scanning, while commercial multipoint systems suffer from limited bandwidth or high costs. Multipoint LDV design aims to reduce the complexity of optical configurations and demodulation algorithms while improving the scalability and flexibility of array-based multipoint vibration measurements. To address these bottlenecks, this study developed a synchronous multipoint LDV based on a 4 × 4 matrix spot configuration, enabling noncontact vibration measurement for laser-induced surface acoustic waves (SAWs) with MHz bandwidth and a noise level below 0.1 nm. By integrating synthetic aperture focusing technology with adaptive beamforming, the system achieved a crack positioning error of less than 5.3%. Simulation results clarified that crack depth (rather than width) dominates SAW propagation characteristics, and provided evidence that imaging artifacts originating from mode-converted waves at the crack bottom allow quantitative crack depth dimension inference. This work presents a cost-effective and low-complexity solution for multipoint nondestructive testing for the metallic component surface, with promising prospects for large-area in-situ detection.