Reducing Dead Zone in Ultrasonic Time-of-Flight Diffraction (TOFD): A Review

Ultrasonic time-of-flight diffraction (TOFD) technique is a means of defect detection adopting the diffracted longitudinal waves from flaw tips. However, the dead zone is induced by the coupling between tip-diffracted longitudinal wave and lateral wave or longitudinal back-wall wave. In this paper, the methods for reducing dead zone are reviewed from parameter optimization, alternative TOFD techniques and signal processing techniques. The principles, effects and influencing factors of different methods are analyzed and compared in sequence. Finally, the future challenges focus on the multiple overlapped signals (N ≥ 3) from shallow subsurface defects in thin plates and the change of dead zone induced by irregular structure.