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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2.3. Microstructure investigations The microstructure was analysed by using a scanning electron microscope (Tescan MIRA3). Therefore, the fatigue specimens were loaded to numbers of cycles N = 10 and N = 10 4 and their cross-sections were analysed for comparison with the initial microstructure. 3. Results and discussion 3.1. Electrical resistance during fatigue Based on the measurement of the voltage drop at the extensometer U Ext and the test current I , the electrical resistance R S in the gauge length was determined according to equation 1. The electrical resistance shows a significant increase during fatigue, see Fig. 2. In order to compensate the influence of geometry, the electrical resistivity was calculated according to equation 2. Therefore, the cross-section A was determined by the diameter and the length of the electrical resistance measurement L DCPD was determined through the total mean strain ε m,t , see e Fig. 3 a). As the length and diameter were considered, the electrical resistivity ρ is no longer dependent on the geometry. As the electrical resistivity ρ remains in the same range during fatigue, see Fig 3 a), the increase of the electrical resistance R S comes through the geometrical changes. In the gauge length, a nearly constant spatial temperature distribution was observed through the high-speed thermography system for different numbers of cycles, see Fig 3 b). Additionally, the temperature evolution during fatigue, measured through the high-speed thermography system is comparable to the results of point measurement using thermocouples, as shown in Fig. 4 a). Accordingly, a reliable temperature measurement can be obtained through the thermocouples. The Matthiesen rule describes the correlation between temperature T and electrical resistivity ρ by dividing the electrical resistivity ρ into a temperature-dependent ρ T and a temperature-independent portion ρ D , Matthiessen (1865). As the temperature-independent portion ρ D dependents on defect density, it was used for the evaluation of damage during fatigue. With the temperature in the range of room temperature ( T R = 20 °C), the temperature-dependent portion ρ T can be described by a linear relation, where ρ 0 is the electrical resistivity at room temperature. The temperature independent portion during fatigue ρ D was calculated through equation 3, see Fig. 4 a).

Fig. 2. Development of voltage drop at extensometer U Ext , test current I and electrical resistance R S versus the number of cycles. = ( $ ·*·+ ( /- . ) ·/012 *,# 3 (2) 4 = − 5 · (1 + α · ( − 5 )) (3) 4,7%8'9&:'9 = 0;< ,-# / . 0 ; /2314,#5-#'6-#5 (4)