Thermo-inductive investigations of steel wires for surface cracks

In the case of thermo-inductive probing the material is heated by HF-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. Due to the high excitation frequency (200 kHz) and the magnetic properties of the material, the penetration depth of the current is very small (about 0.03 mm). Therefore the eddy current 'flows around' surface cracks with a depth of 0.1-1 mm. This causes a higher current density and higher temperature around the failures, which are made visible by the infrared camera. Experiments have been carried out on steel wires with a diameter of 4.5-10 mm and with longitudinal surface cracks with a depth of 0.1-0.2 mm. Due to the high heat conductivity of the material, the temperature difference diminishes very quickly. Therefore short heating pulses with duration of 0.1-0.5 sec have been applied. The measurement result shows, that the thermo-inductive method is well suited to detect such shallow flaws. An analytical model has been derived, to calculate the temperature distribution in the wire and around the failure. The model also shows the dependence of the temperature distribution on the parameters of the experiments, as e.g. the length of the heating pulse, which helps to optimize the measurement setup. Additionally, finite element simulations have been carried out. The results of the model-calculations and the simulations are successfully compared with the experimental results.