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 (( delta_{s} )) and probe dependent (( delta_{t} )) penetration depths for determining the optimal test frequency (( f_{opt} ) ) 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/delta_{t} )?>?1, to study the influence of the EC probe and defect location (( t_{df} )) on the test frequency for near and deep sub surface defects. The study concludes that the optimal test frequency, ( f_{opt} ) for detection of deep sub surface defects (( t_{df} /t approx 1 )) is determined by the probe dependent skin depth, ( delta_{t} ), and the plate thickness is related to ( f_{opt} ) by, ( t propto 1/sqrt {f_{opt} } ). The numerical observations were experimentally validated for machined sub surface notches on a 10 mm thick (( t )) aluminium plate.