Flexible Ultrasonic Arrays with Phase Calibration for Nondestructive Evaluation

In critical industrial sectors such as aerospace and the oil and gas industry, nondestructive testing (NDT) of curved components faces significant challenges owing to poor acoustic coupling when conventional rigid ultrasonic arrays are employed. This study presents an innovative solution through the design and fabrication of a flexible ultrasonic transducer (FUT) based on a 1-3 type piezoelectric composite. The array conforms well to irregular surfaces, thereby ensuring efficient acoustic energy transmission. However, the flexible deformation of the array introduces unknown phase errors among the elements, which can severely deteriorate imaging quality. To address this issue, this work makes a core contribution by proposing and validating a boundary-condition-based phase self-calibration algorithm. The algorithm exploits known geometric features of the test specimen as references and computes the normalized cross-correlation between the signals received by each array element and the reference signal, thereby accurately estimating and compensating for phase delays induced by array deformation. To validate the effectiveness of the proposed method, imaging experiments were conducted with the total focusing method (TFM) on a semi-cylindrical aluminum block containing two known through-hole defects. Quantitative analysis of the experimental results demonstrates that, after phase calibration, defect localization accuracy was significantly improved—particularly in the axial direction—where the error was reduced from 9.65 mm to 1.22 mm, corresponding to an 87% improvement in accuracy. This study demonstrates that the proposed self-calibration technique can substantially enhance the accuracy and reliability of flexible arrays for quantitative NDT of complex curved components, highlighting its potential for practical applicability in industrial inspection.