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

1. Introduction Due to efficient material utilization and reproducible high-quality level, metal forming processes such as full-forward rod extrusion, meet the requirements of economical and resource-saving production. In the context of global climate change and worldwide scarcity of resources, the significance of forming processes is increasing. However, the plastic deformation during the forming process resulting in a significant increase in the microstructural ductile-damage like the formation and growth of voids or an increase in the dislocation density, which are often not considered in the selection of the forming process, Tekkaya (2017). Preliminary investigations have demonstrated a substantial impact of ductile-damage on the fatigue behavior, Langenfeld (2023). Therefore, the characterization of the damage mechanisms allows optimized component design that enhances service performance and promotes lightweight construction. For the characterization of the forming-induced damage the electrical resistance measurement is an established method since the microstructure affects the electrical resistance, Omari (2013) and Luecker (2022). Furthermore, the electrical resistance is influenced through the geometry and temperature. Consequently, these factors must be considered. Due to the fact that ex-situ measurements facilitate the quantification of geometry, and the influence of temperature can be disregarded, they are frequently utilized in fatigue tests, Sing (2013) and Nobile (2021). Ex-situ measurements could negatively affect characterization and only provide data at specific points, whereas in-situ measurements enable detailed analysis. However, it is essential to consider the impact of geometric and thermal influences on the electrical resistance, Sauer (2025). Therefore, a complex in-situ experimental setup was employed for the combined measurement of the electrical resistance in combination with the length, the diameter and the temperature in order to compensate and quantify their influence on the electrical resistance. By contacting the electrical resistance measurement through the edges of the extensometer, the strain and electrical resistance measurements were combined, allowing the determination of geometrical influences on the electrical resistance. Additionally, the thermal influence, measured through thermocouples, was compensated. As a result, the temperature independent electrical resistivity was calculated, and used as an indicator for the microstructural damage evolution during fatigue. In addition, the individual geometrical and thermal electrical resistance changes were calculated and compared with the measured electrical resistance change. 2. Materials and experimental procedures 2.1. Material This study examined fatigue specimens made from the case-hardening steel AISI 5115. The chemical composition of the material is shown in Tab 1 . Tab. 1 . Chemical composition of the case-hardening steel AISI 5115 and DIN 1.7139 in wt. %, (Lingnau 2024). C Si Mn P S Cr Fe AISI 5115 0.14 < 0.40 1.10 0.010 0.027 0.80 bal. DIN 1.7139 Min. 0.14 - 1.00 - 0.020 0.80 bal. Max. 0.19 0.40 1.30 0.035 0.035 1.10 bal. The specimens were produced by full-forward rod extrusion, where initial cylinders with a diameter of 30 mm and a length of 71 mm were pressed through a matrix with a shoulder opening angle of 90°. Through the forming process, the diameter was reduced to 23.4 mm, therefore a deformation ratio of φ = 0.5 was achieved. The forming process induced ductile damage in the material, such as an increase in void volume, which was influenced by the shoulder opening angle. Consequently, two different damage states were created with shoulder opening angles of 90° and 30°, respectively. It is expected that the 30° samples will exhibit lower damage, according to Luecker (2022). The work hardening and residual stress levels, on the other hand, are comparable on the center axis. As illustrated in Fig. 1 , the fatigue specimens were produced from the formed components. The test diameter was set at 5 mm to ensure comparable work hardening and residual stress levels. Fig. 1 shows the production of the specimen in detail.