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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1. Introduction Electrical resistance measurements are an established method for the characterization of microstructural damage related to the change in dislocation density, void volume fraction, void formation or deformation, microcracks or grain boundary, due to their influence on the electrical resistance, Polák (1969), Sorger et al. (2019) and Lingnau et al. (2024). In addition, the geometry, temperature and martensite transformation also influence the electrical resistance, which has to be considered for the microstructure characterization. Due to the higher complexity of the measurement setup, ex-situ measurements were frequently used in fatigue tests, Sun et al. (2004) and Lücker et al. (2022). However, these ex-situ measurements have the potential to negatively influence the characterization and provide data only at discrete positions. In contrast, this study presents a method for the in-situ characterization during fatigue tests, were the electrical resistance in combination with the length, the diameter and the temperature were measured through a complex experimental setup, so that their influence on the electrical resistance can be quantified and compensated. Additionally, a predictive model was developed to estimate the change in martensite volume fraction due to deformation-induced phase transformation based on the results of separate fatigue tests. A new developed contacting method was used for electrical resistance measurements by combining strain and electrical voltage measurement at the same time and location. This enables compensation of the measured strain during the electrical resistance measurement. As a result of all these compensations, the temperature and phase-transformation-corrected electrical resistivity could be evaluated as an indicator of microstructural damage. To validate the characterization, the microstructure at different numbers of cycles was analyzed using scanning electron microscopy (SEM) with backscattered electrons (BSE). The method was tested on high-temperature brazed austenitic steel joints. The described topic is of particular interest in this context, as the brazing process causes significant changes to the microstructure, which results in a high ductility of the steel also the brazed joints are often exposed to high operational stresses. 2. Materials and experimental procedures 2.1. Material This paper analyses a brazed butt joint, which was brazed welded with the metastable austenite AISI 304L (X2CrNi18-9, 1.4307) in the form of cold-drawn bar material as base material and an experimental Ni-based filler in form of foil. The chemical composition of the base material and the filler is shown in Table 1. During the high temperature vacuum brazing process, the brazing temperature was at 1160 °C for a total of 60 minutes. The brazing process resulted in a significant change in microstructure, particularly a high reduction in the dislocation density and an increase in grain size. The brazed seam shows two distinct zones, a detailed description of the zones can be found in Otto et al. (2021). The total length of these zones is about 220 µm, Otto et al. (2024) representing about 2% of the initial measurement length L 0 of the extensometer. Therefore, it can be assumed that the extensometer is effectively measuring mostly the reaction of the base material. After the brazing process round fatigue specimen with the geometry shown in Fig. 1 a) were produced. Table 1. Chemical composition (in wt.-%) and solidus and liquidus temperatures (in °C) of AISI 304L steel and Ni-filler. Fe Ni Cr C Si Mn Mo B T S T L AISI 304L Bal. 8.08 18.34 0.02 0.02 1.64 0.33 - 1450 Ni-filler 0 74 15 - 7.5 - 2 1.5 1044 1114 2.2. Experimental procedures For the fatigue tests a servo-hydraulic testing system (Instron 8801) with a maximum load of 100 kN was used. The fatigue tests were performed under uniaxial stress-control in the tension-tension range at a stress ratio of R = 0.1 and a testing frequency of f = 10 Hz. During constant amplitude tests (CAT), a complex in-situ experimental setup was employed. The following section describes the in-situ measurement setup.