Thermo-Inductive Surface Crack Detection in Metallic Materials

Abstract

Abstract. In the case of thermo-inductive testing the material is heated by induced eddy currents and the emission from the material surface is detected by an infrared camera. Anomalies in the surface temperature correspond to in-homogeneities in the material. The penetration depth of the induced eddy current depends on the induction-frequency and on the material properties as e.g. on the relative permeability value. In the case of magnetic materials, like steel, and a high excitation frequency, the penetration depth of the current is very small. Therefore, the eddy current 'flows around ' surface cracks with a depth of 0.1-1mm. In contrast, in stainless steel the eddy current at the same excitation frequency has a much larger penetration depth and therefore leads to a different behaviour around a crack of similar depth and one observes lower temperatures around the crack. Finite element simulations have been carried out, in order to model the temperature distribution caused by the eddy currents. It is shown, that depending on the material properties, on the excitation frequency and on the duration of the heating pulse one gets higher or lower temperatures around the surface crack. The results of the simulations are compared with the experimental results.