According to different skin effect by eddy current induction, eddy current pulsed thermography involves two heating modes:
(i) Near-surface heating. In this case, the skin depth is much small and can be neglected [20]. For example, ferromagnetic metals with high permeability have a much smaller skin depth (about 0.04 mm at 100 kHz). Thus, surface defect characterization is based on the eddy current interruption [21] while inner defect characterization relies on the heat diffusion [17]; (ii) Volumetric heating. In this case, skin depth is much great. For example, the skin depth in CFRP. with small conductivity (about 50 mm at 100 kHz) is significantly larger than the thickness of most real components [22].