Thermal pattern reconstruction of surface condition on freeform-surface using eddy current pulsed thermography

Measuring and evaluating surface conditions of a freeform surface are difficult means of active thermography due to the varied curvatures. In this paper, we propose a method for obtaining the thermal pattern of the entire freeform surface of a propeller using eddy current pulsed thermography (ECPT). To understand the electromagnetic field and heating distributions on propeller, the induction fields of two induction coil types (line conductor and Helmholtz coil), one in air space and the other with a propeller placed are simulated and analysed through multiphysics-based models. It shows that the relatively uniform electromagnetic field induced between Helmholtz coil pair in air space becomes non-uniform with a propeller existing, as it is distorted by propeller freeform surface. Therefore, it is difficult to obtain precise thermal pattern to profile the surface condition. To overcome these challenges, segment heating is applied using a line conductor type induction coil and the thermal image stitching is proposed for showing the entire thermal pattern on propeller surface. Both finite element simulation and experimental study are implemented. The results show that more clearly thermal distributions on propeller freeform surface can be obtained using the proposed method.