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 (δ_s) and probe dependent (δ_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/δ_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 ≈ 1) is determined by the probe dependent skin depth, δ_t, and the plate thickness is related to f_opt by, t ∝ 1 / √f_opt. The numerical observations were experimentally validated for machined sub surface notches on a 10 mm thick (t) aluminium plate.