Near-Surface Defects Identification in water immersion ultrasonic testing based on FFT and Phase Spectrum Difference

Ultrasonic pulse-echo testing always exist a near-surface blind zone, because the defect echo overlaps with the interface echo, it is difficult to accurately identify the near-surface defect. This paper presents an investigation with fast Fourier transform (FFT) combined with phase spectrum difference (PSD) for detecting near-surface defects in metal materials. Firstly, the phase spectrum of defect signal and free-defect signal are obtained by FFT, and the PSD between them is calculated. The FFT is utilized again to acquire the main frequency and corresponding period of PSD. Then, the offset time is proposed to represent the location of the near-surface defects. The theoretical correctness of the proposed method is validated by the ultrasonic simulation signals of single defect, multi-defects and noise interference in the near-surface blind zone. Finally, the water immersion ultrasonic testing experiment is carried out to detect the near-surface artificial defects of test specimens from a large ring forging. The experimental result revealed that the calculation of near-surface defects depth by offset time has high accuracy.