Defect evaluation using the alternating current field measurement technique

ACFM is often used to search for surface longitudinal cracks in weld seams, for which the technique will have maximum sensitivity to. Attempts are always made where possible to scan with the probe centre over the crack and along the crack for maximum sensitivity to the crack. However, inevitably, this is not always possible with cracks deviating from expected paths. This report details results of the effect of scanning at different angles to the crack (not parallel), scanning not directly over the crack, scanning with lift-off, the effect of probe angulation (ie probe wobble) and also evaluating the accuracy for surface crack depth sizing. Most of the specimens were fabricated from the commonly utilised ASTM-A36 structural steel with accurately fabricated EDM and containing real defects. It was shown that the ACFM technique is generally reliable for detection and accurate sizing of surface defects in air or in water. The essentials of an algorithm which will automate crack detection and sizing is also described.