Research on Ultrasonic TOFD Imaging Inspection for Heavy-walled Weld Based on Phase Coherence Characteristics

Abstract

To improve the signal to noise ratio (SNR) and lateral resolution, phase coherence imaging (PCI) post-processing algorithm is presented and applied in ultrasonic time-of-flight diffraction (TOFD) inspection for heavy-walled welds. Firstly, delay and sum operation for the aperture data of TOFD-B image is implemented by synthetic aperture focusing technique (SAFT). Secondly, phases of aperture data were used to establish phase coherence factor which represented the phase dispersion for each pixel in the B-SAFT image. Finally, the B-SAFT image is dynamically weighted by phase coherence factor. The results show the PCI is capable of suppressing structure noise and enhancing SNR by amplifying the contribution of phase information. For austenitic weld with 78 mm thickness, the average SNR of three Φ3 side drilled hole were more than 30 dB, higher 20 dB than TOFD-B image. Furthermore, the PCI could improve the lateral resolution by enhancing aperture beam directivity. Compared to TOFD-B image, the half lateral width of defect in processed images were decreased by more than 70 percent for a 48mm thickness CV weld.