Logarithmic analysis of eddy current thermography based on longitudinal heat conduction for subsurface defect evaluation

•Longitudinal heat conduction induced by eddy current is investigated.•Logarithmic analysis is used to detect and evaluate the subsurface defects in metal.•Separation time is defined as the characteristic feature to measure the defect’s depth.

Longitudinal heat conduction from surface to inside of solid material could be used to evaluate the subsurface defects. Considering that the skin depth of high frequency eddy current in metal is quite small, this paper proposed logarithmic analysis of eddy current thermography (ECT) to quantify the depth of subsurface defects. The proposed method was verified through numerical and experimental studies. In numerical study, ferromagnetic material and non-ferromagnetic material were both considered. Results showed that the temperature–time curve in the logarithm domain could be used to detect subsurface defects. Separation time was defined as the characteristic feature to measure the defect’s depth based on their linear relationships. The thermograms reconstructed by logarithm of temperature can improve defect detectability.