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

Abstract

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.