TOFD (time-of-flight-diffraction) is proposed as an option to Pulse-echo methods by some practitioners. It suffers from shortcomings that can limit its effectiveness.
- Near surface defects are either not detected or not possible to size due to a dead zone formed by a lateral wave. The lateral wave has the typical ringdown associated with pulse length at any interface. Factors affecting ring time include probe damping, element nominal frequency, and angle of incidence. Advocates of the method point out that a disruption of the lateral wave can be seen where near surface confusion and undercut are present, however, the advantage of precise depth assessment associated with TOFD advantages is not seen here.
- In a situation where a high-low (poor fit up or mismatch) exists between two welded pieces, the transmitted pulse forms two signals off the opposite wall, one from the higher side arriving prior to, the one from the lower. The amount of high-low can be accurately assessed but if a defect occurred in the lower of the two sides the back wall signal from the higher side could obscure the defect.