Automatic scanning with multi‐frequency eddy current on multi‐layered structures

Eddy current testing is a frequently used NDT method at Saab/CSM but recently only single frequency testing has been used. The purpose of our work was to increase both testing speed and sensitivity by using multi‐frequency eddy current testing combined with a scanning system for corrosion detection in multi‐layered structures. The wing of the Saab 2000 aircraft was one specific example for which several samples were manufactured with both artificial (chemically etched) corrosion of various severity and cracks. Using previously determined optimal single frequency as a start, frequency combinations were determined to give increased detectability for the different structures and defects. The influence of different disturbing signals, e.g. signals from rivets, thickness variations, noise, and how to reduce them using the multi‐frequency technique was studied. Tests were also made in “field” conditions to evaluate the system.