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 (δs) and probe dependent (δt) 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/ δt > 1, to study the influence of the EC probe and defect location (tdf) 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 (tdf/ t≈ 1) is determined by the probe dependent skin depth, δt, 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.