For a 25 mm thick girth weld, typical TOFD setup uses 5 MHz probes with 60? wedges, spaced to produce a 20 mm PCS (probe centre separation). Scan sensitivity is set on a 3 mm side-drilled hole, and the time-corrected gain (TCG) is applied per ISO 10863. One key advantage is that TOFD provides accurate through-wall sizing of planar flaws, often within ?1 mm, which is critical for fracture mechanics assessments.
However, TOFD has limitations: it has a dead zone near the surface (typically 2-3 mm) and is less sensitive to volumetric defects like porosity. Therefore, industry best practice combines TOFD with phased-array UT (PAUT) for complete coverage, as recommended in API 1104 Annex A for pipeline girth welds. The combination ensures that surface-breaking and near-surface flaws are caught by PAUT, while TOFD screens the full weld volume.
Source: ISO 10863
What is your experience with TOFD for planar defect detection, and how do you integrate it with other NDT methods?