Thermographical investigations of inductively heated metallic surfaces

Abstract

Thermo-inductive investigations can be well used for the detection of surface cracks in metallic materials. The workpiece is heated by a short inductive pulse and an infrared camera is recording the temperature distribution of the surface. Irregularities and failures in the surface cause anomalies in the temperature distribution, making the failures visible and detectable in the infrared images. Results of experiments show that magnetic and non-magnetic materials have very different behavior: surface cracks in magnetic materials are heated stronger than the failure-free surface. On the other hand, in non-magnetic materials cracks are less heated than the surface itself and become visible through lower temperature values. These different behaviors can be well explained by the different penetration depth of the eddy current, mainly influenced by the magnetic permeability of the material. Model calculations have been carried out in order to describe the distribution of the eddy current around a surface crack and to calculate the resulting temperature profile around it. The time-dependent evaluation of the temperature changes provides results which are independent of the emissivity differences and therefore shows also very well-defined results in the case of grinded or scratched surfaces. This technique has been used for a couple of different work-pieces presenting its advantages. The experimental and calculated results are compared, showing a very good agreement.