Section 6: Selected Applications & Techniques

■
Benefit of TOFD Parallel Scanning
Although perpendicular TOFD scanning down the weld can give highly
accurate depth measurement, generally speaking a parallel scan will give
more accurate depth information as well as flaw information, and location in
the index position in the weld. With perpendicular scanning, no index position
is possible without multiple offset scans being performed or complimentary
NDT techniques to position the flaw. In parallel scanning Index position is
ascertained by locating the minimum time peak, which corresponds to when
the indication is centered between the two probes. For these reasons this
technique is often used in critical crack sizing inspections, as well as change
monitoring, in other words, monitoring a crack or other defect for growth until
it reaches a critical level at which time it is repaired or replaced. For these
reasons the technique is often performed on critical components that are
costly to shut down for repair, often in the Power Generation industry. More
information is often gathered from the flaw as diffraction occurs across the
flaw instead of just down the flaw.