Ultrasonic detection of wrinkles in composites with gradual phase shift migration

Fiber reinforced polymer composites (FRPs) are essential for various industrial fields, but wrinkles inside will greatly reduce their mechanical properties. Full-matrix capture (FMC) is a popular data structure for ultrasonic phased array imaging in composites. However, such structure may lead to data redundancy and noise interference. In this paper, a gradual phase shift migration (GPSM) is proposed to characterize wrinkles accurately. The gradual matrix is formed from the expansion along principal diagonal of FMC data with equal transmitter-receiver spacing. The dilemma between lateral resolution and sidelobe interference intensity is resolved to obtain the best imaging resolution by selecting an appropriate data structure. Moreover, to address the inconsistency of ultrasound velocities at different propagation directions caused by anisotropy of composites, the angle-dependant velocity is corrected by backwall reflection method (BRM). Based on gradual matrix data, the velocity-corrected phase shift factor is applied in the GPSM algorithm to obtain the wavefield at different depths through a layer-by-layer wavefield extrapolation. The experimental results indicate that four wrinkles can be detected in thick hybrid carbon-glass FRPs based on GPSM, with angle detection errors less than 6%. Furthermore, the GPSM method combining partial diagonal data takes only 0.5 s, achieving 60% improvement in computational efficiency compared to that with all gradual matrix data. The proposed method can be applied for high-resolution imaging of various multilayered medium in real-time.