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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a)

b)

Fig. 3 (a) Electrical resistance R S , length L DCPD , diameter d and resistivity ρ versus the number of cycles; (b) Spatial temperature distribution for different lifetime states N with marked test area.

The deformation results in a transformation of austinite to martensite in the base material. Due to the differing electrical resistivity, a change in electrical resistance occurs, so that a compensation is necessary, Saint-Sulpice et al. (2014). In the case of several conductive phases in a cross-section, the resulting combined electrical resistivity can be calculated form parallel connection. Based on the developed prediction model, the martensite volume fraction ξ fit was reliably predicted during fatigue, see Fig. 4 b). Given that the maximum martensite volume ξ fit fraction is less than 1%, only the temperature independent electrical resistivity of the austenite ρ D, Austenite was considered for further analysis, see Fig. 4 b), as calculated using the equation 4.

Fig. 4 (a) Electrical resistivity ρ , temperature-independent electrical resistivity ρ D and specimen temperature T S versus the number of cycles; (b) Temperature-independent electrical resistivity ρ D , temperature-independent electrical resistivity of the austenite ρ D,Austenite , measured and fitted α´-martensite volume fraction ξ measured , ξ cal versus the number of cycles. The influence of geometry, temperature and martensite on the electrical resistance were considered by the determination the temperature independent resistivity of the austenite ρ D, Austenite . As result, the evolution of damage during fatigue can be evaluated. The increase at the beginning of the fatigue test clearly indicates significant microstructural changes. It is possibly related with an increase in the dislocation density or a deformation of pores due to the overall strong deformation at the first load cycles caused by the low cyclic yield stress and the high ductility of the steel after the brazing process. In the following a decrease can be observed, which could be associated with dislocation movements, before a state is reached in which only minor changes can be observed. 3.2. Microstructure analyses Fig. 5 displays the microstructure of the initial state, after N = 10 and N = 10 4 cycles using backscattered electron (BSE) technique. A notable change in the microstructure can be seen between the initial state and after N = 10 cycles. The clear grain orientation was distorted by the strong plastic deformation in the first cycles, which can be seen from