Non-destructive testing using electromagnetic impedance spectroscopy

Abstract Eddy current testing, which involves the use of alternating currents (a.c.), is widely used as a non-destructive testing method for remote surface analysis. The depth into the material at which the measurements are made depends upon the energising frequency used. This work considers a technique in which a wide frequency range is used. The eddy current probe is held stationary above a surface whilst the frequency is scanned from near d.c. to up to 20 MHz. This technique should be useful where the depth of the feature under investigation is either variable or unknown. It is hoped that it will lead to a method for the detection of corrosion phenomena such as de-alloying, pitting and exfoliation. This paper discusses the results obtained from scanning pitting corrosion on aluminium and exfoliation of 7075-T6 alloy. The results indicate that these surface changes can be detected, but further work is required if the changes are to be quantified.