The Influence of the Heating Rate and Thermal Energy on Crack Detection by Eddy Current Thermography

The capability of eddy current thermography in detecting cracks is investigated numerically and experimentally in relation to the crack orientation, the heating rate and the excitation period. The numerical investigation shows that, for cracks parallel to the heat flow the detection region increases with the increase of the heating rate, while for cracks perpendicular to the heat flow, it increases with the increase of the excitation period. The experimental results confirm that the detection of cracks parallel or perpendicular to the heat flow is improved by increasing the heating rate or the excitation period, respectively. The optimum time period for the detection of a crack depends on the crack orientation: For cracks parallel to the heat flow (i.e. perpendicular to the current flow), the best results are obtained at the beginning of the heating period. For cracks perpendicular to the heat flow, the optimum detection period is delayed with the distance of the crack from the heated area. If the crack is very close to the edge of the plate, both the detection period and the sharpness of the crack are reduced. The experimental results are compared to data obtained by identical experiments, where the use of a lower performance camera was combined with data processing techniques. The comparison indicates that a higher performance camera is more effective and may compensate for the improvements achieved in the detection of a crack through data processing techniques.