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. 1. (a) Fatigue specimen geometry, Otto et al. (2021); (b) New developed method for coupled electrical resistance and strain measurement via extensometer.

The total mean strain ε m,t during fatigue was measured by using an extensometer with a measuring length of L 0 = 10 mm. The diameter was measured by using a 2-dimensional optical micrometer (Keyence TM-3000). The system permitted the non-contact measurement within the testing range of the specimen. For the temperature measurement eight thermocouples type K were used. Six thermocouples were applied on the specimen, with two each at the upper and lower shafts and in the centre of the specimen. Two thermocouples were deployed to measure the temperature of the surrounding environment. The spatial temperature distribution during fatigue was determined for two separated fatigue tests by using a high-speed thermography system Image IR 8800 (InfraTec). In order to obtain a constant emissivity, the specimen was coated with a black lacquer. In order to measure electrical resistance, a four-wire configuration was employed, with two separate cables for current introduction and voltage measurement. This approach ensured that the effects of resistance in the wiring and contact points were eliminated, Singh (2013). Accordingly, the electrical resistance of the specimen R S can be calculated in accordance with Ohm's law, based on the test current I and the measured voltage U , Singh (2023). For the measurement of the voltage in the gauge length U Ext a new developed measurement setup was used which combines electrical resistance measurement with the strain measurement via extensometer. Therefore, the electrically conductive extensometer edges were conducted with cables to the voltage measurement system (HBM MX840B QuantumX). The combination of strain and electrical resistance measurement allows local measurement in the gauge length and a direct transfer of the measured total mean strain ε m,t to the length of the electrical resistance measurement. For the direct current input a Sorensen 8AS2 direct current source with a maximum voltage of 100 V and a maximum current of 8.5 A was used. The electrical current was induced via the hydraulic clamps and measured with a second voltage measurement at a shunt, connected in series with a normed electrical resistance of R Shunt = 1.5 mΩ. Fig. 1 b) shows the used experimental setup for the electrical resistance measurement. In accordance with Ohm's law, the electrical resistance of the specimens R S is determined according to equation 1. By combining a 2-dimensional optical micrometer, thermocouples, voltage measurements at the extensometer and at the shunt, all necessary data to compensate for the influence of geometry and temperature can be measured. Given that martensite has a different electrical resistivity than austenite, it is necessary to consider the martensite transformation. In order to measure the martensite volume fraction ξ measured during fatigue, a feritscope (Fischer FMP30) was used. Separate fatigue tests were performed as basis for a predictive model for the martensite volume fraction ξ fit depending on the number of cycles and the maximum stress. ! = " !"# # = " !"# " $%&'# · !$%&' (1)