PSI - Issue 77

L.M. Sauer et al. / Procedia Structural Integrity 77 (2026) 34–40

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Author name / Structural Integrity Procedia 00 (2026) 000–000

The electrical resistance of the specimen was measured by using the four-wire configuration, with different cables for current introduction and voltage measurement. This allows line and contact resistances to be neglected, resulting in higher measurement accuracy, Singh (2013). The electrical direct current was applied by a Keithley 2602B system source and introduced through the hydraulic clamps. The electrical voltage was measured through the Keithley 2182A nanovoltmeter. In the following, the system is called nanovoltmeter. To increase the measurement accuracy, the electrical resistance was measured using the current reversal technique, also called delta-mode, in which alternating positive and negative currents are introduced through the source meter. Therefore, thermoelectrical offsets caused by the contacting can be compensated, Nagata (2022). The voltage measurement was contacted through the electrically conductive edges of the extensometer. This setup allows the measurement locally in the test area without brazed or welded contacts. Through the combination of electrical resistance and strain measurement, the measured strain can directly be used to compensate the influence of length change on the electrical resistance. This setup was evaluated in previous works, Sauer (2024). For the measurement of the geometry, the total mean strain ε m,t was measured with an extensometer (Instron 2620-603) with a gauge length of 10 mm. The extensometer was placed in the relevant test area. For the measurement of the diameter d , a 2D-dimensional optical micrometer (Keyence TM-3000) was used, which allows a non-contact measurement in the test area. The temperature was measured with thermocouples type K. The thermocouples were attached to the specimen with rubber band. Due to the limited space of the test area and the necessity of maintaining the contour of the specimen free from adhesive materials for the diameter measurement using the 2D-dimensional optical micrometer, only a single thermocouple was positioned within the test area. A second thermocouple was employed to measure the ambient temperature. By combining the different measurement systems, namely the extensometer, the 2D-dimensional optical micrometer, the thermocouples and the nanovoltmeter all necessary values were measured in order to compensate the geometrical and thermal influence on the electrical

resistance were measured. 3. Results and discussion 3.1. Electrical resistance during fatigue

Fig. 3 shows the measured test current I , the voltage drop at the extensometer U Ext and the calculated electrical resistance of the specimen in the test area R S during fatigue, measured through the nanovoltmeter. The voltage drop and the electrical resistance shows a decrease during fatigue.

Fig. 3 . Measured voltage drop at extensometer U Ext , test current I and electrical resistance R S versus the number of cycles N .

The decrease in the electrical resistance can be explained through the geometrical changes. Since the total mean strain ε m,t , measured through the extensometer, shows a decrease during fatigue, the specimen shows cyclic creeping in compression. This results in a decrease of the length of the electrical resistance measurement L DCPD and an increase of the diameter d , resulting in a decrease of the electrical resistance R S , as shown in Fig. 4 .