Novel interventional electromagnetic thermography for subsurface defect detection

For low-emissivity metal materials, reducing the influence of non-uniform emission in detecting subsurface defects remains a challenge. This paper proposes a novel interventional electromagnetic thermography technique. In particular, a thin water film medium is involved in the detection which enhances the detectability of subsurface defects by utilizing the thermal radiation characteristic and thermophysical parameters of the participating medium. The physical mechanism of the proposed technique is analyzed in solid aspect. Static experiments have been carried out on the ferromagnetic plate with subsurface notches. The obtained results have shown that water film can eliminate the inhomogeneous emission of the detection target while the high emissivity of the water film corrects the temperature distribution. Moreover, dynamic scanning experiments have been conducted on the ferromagnetic plate containing artificial subsurface defects and the non-austenitic stainless-steel plate containing natural subsurface intergranular corrosion defect. The commonly used black paint is added to the dynamic experiment for performance comparison. The comparative experimental results have demonstrated that the proposed technique can effectively identify subsurface defects with burial depth up to 1.0 mm in the ferromagnetic plate. The results are especially more pronounced when the subsurface defects occur at buried depths ranging from 0.4 mm to 1.0 mm where higher thermal contrast and signal-to-noise ratio have been attained. The technique has shown superior detection performance for detecting natural subsurface microcrack.