PSI - Issue 38

W. Radlof et al. / Procedia Structural Integrity 38 (2022) 50–59 W. Radlof et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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statements can be made about the failure behavior of torsion loaded specimens. However, as the previous investigations have shown that an increase in potential correlates with structural failure, it is possible to deduce failure by the temperature increase at the end of the fatigue test. Nevertheless, the potential drop method is to be preferred. One possible reason, why the hotspots are not visible in the torsion tests, is the distance of the thermography camera to the sample due to space related circumstances. Here, the distance is approx. 5 times larger than in the bending tests, where cracks and immediate strut failures could be detected in the temperature curve and images.

4. Conclusion In this study local image-based and in-situ measurement techniques were successfully implemented into the fatigue tests on additively manufactured lattice structures to characterize their damage behavior. In detail, different types of lattice structures with open porosities between 50% and 70% were designed, additively manufactured by means of electron beam melting and tested under cyclic bending and torsion loading. For the characterization of the failure behavior and the identification of local phenomena, such as strain hotspots and crack growth, the digital image correlation technique (DIC) as well as temperature field and potential drop measurements are used. Thereby, the direct current potential drop method has proven to be the most sensitive method to examine the damage behavior of lattice structures with 50% und 60% porosity under both, bending and torsional fatigue loading. For the 70% lattice structure, there is a load-level dependency, which should be investigated in further experiments. However, strut and structural failure can be identified by changes in the potential drop. Moreover, the identification of crack initiation and strut failure using the potential drop method could be verified by micrograph analyses and DIC. In micrograph analyses, cracks were found in multiple struts, which most of them were additionally seen in analyses with DIC. The temperature field measurement can also be used to identify points of local damage, whereby a maximum distance of the camera to the sample must be considered. However, the potential drop method and analyses with DIC are preferable to characterize the damage behavior of additively manufactured lattice structures under fatigue loading. Acknowledgements The research was funded by Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – SFB 1270/1 – 299150580. The authors would like to thank the Chair of Microfluidics for the manufacturing of the specimens. Fig. 8: Fatigue behavior of an exemplarily 50% porosity lattice structure tested under torsion loading. (a) Cyclic deformation behavior based on the change of the potential and temperature of the analyzed strut-lines with (b) with temperature field images of the lattice structure at different states of the fatigue life.