## 11 Conclusions and Summary  
It has been shown that induction thermography can be used to detect surface cracks and to estimate crack depth. It is possible to use it for ferro-magnetic and for non-magnetic materials, but the measurement circumstances such as excitation frequency and pulse length have to be adapted to the material.  
Figure 33: Phase images for a non-magnetic steel sample with five artificial defects, each of them with a 1 mm depth, and inclination angles of \(75^{\circ}\), \(60^{\circ}\), \(45^{\circ}\), \(30^{\circ}\) and \(0^{\circ}\) from left to right, respectively; (**a**) \(t_{pulse}=0.1\) s; (**b**) \(t_{pulse}=0.6\) s; (**c**) profiles for different heating pulses.  
Figure 32: Phase difference (**a**) and phase contrasts (**b**) for an asymmetrical phase distribution depending on the inclination angle of the crack with a 1 mm depth. For the definition of the displayed values see Figure 31a.  
If the crack is perpendicular to the surface, then two ratios determine the results. One of them is the ratio of the crack depth to the penetration depth of the eddy current, and the second one is the ratio of the crack depth to the thermal diffusion length. Using finite element simulations, the eddy current and temperature distributions were calculated and phase images determined. The calculated normalized phase difference curves (Figures 24b and 25b) can be applied for any metal, and provide a good approximation of the crack depth under given measurement parameters.  
The situation is more complex for inclined cracks. The simulations and measured results show that short pulses produce an asymmetrical phase distribution, from which not only the crack depth but also the inclination angle can be estimated. In specific cases, it can be very useful to carry out the measurements with several pulse lengths and combine their results to more precisely estimate the crack depth.  
In this paper, the influence of the measurement conditions, the material parameters, the crack depth, and its inclination angle on the temperature and phase contrast were investigated to provide a guide for optimizing the experimental setup. The goal is not only to detect a surface crack but also to be able to estimate its depth.  
Additionally, for the detectability of the cracks, the signal-to-noise ratio (SNR) plays a very important role. If the heating power is low, especially for non-magnetic materials with a short heating pulse duration, then the noise may become significant and hamper detection. Further investigations are in progress to investigate the influence of the SNR on detectability also by applying multiple pulses as a lock-in technique.  
In this study, cracks that have a length of at least 10 mm at the surface were investigated. For shorter cracks, the hot spots around their crack tips dominate the infrared images. Further 3D numerical simulations and experiments are planned to investigate this case.  
**Acknowledgments:** Financial support by the Austrian Federal Government (in particular from Bundesministerium fur Verkehr, Innovation und Technologie, and Bundesministerium fur Wissenschaft, Forschung und Wirtschaft) represented by Osterreichische Forschungsforderungsgesellschaft mbH and the Styrian and the Tyrolean Provincial Government, represented by Steirische Wirtschaftsforderungsgesellschaft mbH and Standortagentur Tirol, within the framework of the COMET Funding Program is gratefully acknowledged.  
**Author Contributions:** The author claims to have developed all the methods presented here.  
**Conflicts of Interest:** The author declares no conflict of interest.