Residual wall thickness profile measurement of thin plates based on phased array zero-group-velocity mode

Plate-like structures in harsh environments often experience wall thickness loss, making accurate residual thickness assessment critical for timely maintenance and failure prevention. This article presents a phased-array zero-group-velocity (PA-ZGV) method for profiling the residual thickness of thin plates by combining the material-property sensitivity of zero-group-velocity (ZGV) Lamb modes with the wave-steering capabilities of phased arrays (PAs). Two key enhancements were introduced: (1) optimized phased-array excitation to amplify the intensity of the S1-ZGV mode, and (2) time-domain summation (TDS) processing to eliminate time-truncation limitations. As a result, four ZGV-based methods for thickness profile reconstruction were developed and validated through finite element simulations. Experimental validation using a linear PA transducer on aluminum plates with grooved defects demonstrated excellent agreement with high-frequency pulse-echo and line-laser scanning methods, achieving an average root-mean-square error below 0.03 mm. All four ZGV-based methods achieve high accuracy in central-region measurements. Under reflection-prone edge-region conditions, however, the PA-assisted and TDS-assisted strategies, particularly PA-ZGV-TDS, provided more robust ZGV frequency extraction and lower reconstruction errors than the conventional single-element ZGV method. These findings establish PA-assisted ZGV approaches as a low-frequency, high-precision, and robust solution for residual thickness mapping in industrial thin-walled structures.