Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

Fatigue cracks in welded structures are most likely in the longitudinal
direction, and the toe of the weld is the most likely location of fatigue cracking,
although cracking can also occur elsewhere in the weld or adjacent parent
metal. For scanning the heat affected zone and adjacent parent metal, a
zigzag scan at right angles to the weld should be employed. For testing the
weld bead, either a zigzag scan across the bead or several scans along the
weld may be used, depending on the weld condition. If transverse cracking is
considered likely, scanning with the probe transverse to the weld may be
required, although such cracks will give downward signals during scanning for
longitudinal cracks. If the display is set up as indicated above, cracks parallel
to the probe will produce a vertical deflection of the spot. The signal
amplitude is approximately proportional to the depth of the cracks, so crack
depth can be estimated, if required, by comparison of the signal with signals
from the slots in the reference blocks, with a plastic shim of thickness equal to
that of the paint coating at the crack location on the block.
Charlie Chong/ Fion Zhang