Theoretical and Experimental Analysis of the Thermal Response in Induction Thermography in the Frequency Range of 2.5 Hz to 20 kHz

The one-dimensional propagation of electromagnetic waves and the propagation of the resulting thermal waves in conducting material are analysed in a coherent way. The heat release due to resistive losses has a static and an oscillating part. Both are considered as heat source terms for the thermal diffusion equation. The time dependence of the temperature is described by analytical solutions. Electrically and thermally conducting materials are classified by the ratio of thermal penetration depth to the skin depth. Experiments performed on ferritic steel, stainless steel and carbon-fibre-reinforced polymer show the time dependence of the thermal signal after heating begins, as described by the theory. At low induction frequencies, an oscillating part of the surface temperature at the double of the induction frequency is detected in accordance with the theory. The results point out new opportunities for induction thermography.