Eddy current (EC) stimulated thermography is an emerging technique for nondestructive testing and evaluation of conductive materials. However, quantitative estimation of the depth of subsurface defects in metallic materials by thermography techniques remains challenging due to significant lateral thermal diffusion. This article presents the application of eddy current (EC) pulse-compression thermography to detect surface and subsurface defects with various depths in an aluminum (AL) sample. Kernel principal component analysis and low rank sparse modeling were used to enhance the defective area, and cross-point feature was exploited to quantitatively evaluate the defects' depth. Based on experimental results, it is shown that the crossing point feature has a monotonic relationship with surface and subsurface defects' depth, and it can also indicate whether the defect is within or beyond the EC skin depth. In addition, the comparison study between AL and composites in terms of impulse response and proposed features are also presented.
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