PSI - Issue 68

Lukas M. Sauer et al. / Procedia Structural Integrity 68 (2025) 432–438 L. M. Sauer et al. / Structural Integrity Procedia 00 (2025) 000–000

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the different greyscale values within the grains. However, the changes observed between the state after N = 10 cycles and the condition after N = 10 4 cycles are comparatively low but still evident. These results are consistent with those obtained from the electrical resistance evaluation.

Fig. 5. Microstructure of the brazed joint (with porosity) in the initial state, after N = 10 and N = 10 4 cycles using backscattered electron images.

3.3. Comparison of the individual influences on the electrical resistance By performing the in-situ measurements of geometry, temperature and martensite volume fraction, the individual change in the electrical resistance due to the change of geometry ΔR Geometry , temperature ΔR Temperature and martensite volume fraction ΔR Martensite during fatigue was determined and compared with the total change in the electrical resistance ΔR S , Fig. 6. As a result, the geometry shows the highest influence on the electrical resistance, due to the high ductility of the brazed joint and the stress ratio of R = 0.1, while the influence of temperature and martensite formation is considerably lower at approx. 2 µΩ. The determined distribution highly depends upon the selected experimental parameters and the employed materials.

Fig. 6. Comparison of the individual electrical resistance through geometry ΔR Geometry , temperature ΔR Temperature and martensite transformation ΔR Martensite during the fatigue test. 4. Summary and outlook A method for the evaluation of the development of microstructural damage during fatigue is presented in this study. Therefore, a complex experimental setup was used to measure the electrical resistance, geometry, temperature and martensite volume fraction during fatigue through various in-situ measurements. The newly developed contacting method combines electrical resistance and strain measurement, enabling the measured strain to be directly transferred to the length of the electrical resistance measurement. This method allowed the effects of geometry, temperature, and martensite transformation on the electrical resistance to be compensated, in order to evaluate the microstructural damage evolution during fatigue. The method was tested on a Ni-based brazed joint, and an increase at the beginning of the fatigue test indicates a significant microstructure change while only minor changes in the following are obtained.