Active thermography with electromagnetic excitation allows one to detect defects in components non-destructively, reliably and fast by imaging with an infrared camera. In induction thermography a current is contactlessly coupled into the component to be tested by a coil and in conduction thermography by galvanic contacts. In this thesis both techniques are discussed in detail.
For a deeper understanding of the detection mechanism a fundamental knowledge of the excitation, especially of the current density and its direction, is necessary. Based on this knowledge, it can be deduced how much heat is produced locally, how the heat diffuses in the component, and which dynamic temperature distribution arises on its surface. Cracks locally increase the current density and they disturb the heat diffusion. The detection mechanism of defects and the detection limit depends on its type, geometry, and orientation.
In a systematic way, this thesis presents analytical calculations and simulations of the distributions of current density and temperature in the component under test taking into account the disturbance of these distributions by different kinds of defects. Furthermore, it is discussed how these defects can be differentiated enabling one to predict their detectability. Based on this knowledge it is shown on the basis of two systems developed at Siemens during this thesis how an active thermography testing-system should be designed including the necessary algorithms to analyze the data. Finally instructions are given how to test a component.

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