Study of the Choice of Excitation Frequency for Sub Surface Defect Detection in Electrically Thick Conducting Specimen Using Eddy Current Testing

Understanding the scope and limitations of non- destructive testing procedure is essential for selecting the appropriate test parameters for material inspection. This paper presents the scope of material ( ds) and probe dependent ( dt) penetration depths for determining the optimal test frequency ( fopt) for detection of sub surface defects in electrically thick conducting specimens. Numerical modelling is carried out for a pancake coil above an electrically thick aluminium plate, t/ dt > 1, to study the influence of the EC probe and defect location ( td f) on the test frequency for near and deep sub surface defects. The study concludes that the optimal test frequency, fopt for detection of deep sub surface defects ( td f / t 1) is determined by the probe dependent skin depth, dt, and the plate thickness is related to fopt by, t. 1/ fopt. The numerical observations were experimentally validated for machined sub surface notches on a 10 mm thick ( t) aluminium plate.