PSI - Issue 68

Lars A. Lingnau et al. / Procedia Structural Integrity 68 (2025) 303–309 L. A. Lingnau et al. / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction and state of the art The performance of components manufactured using forming technology is largely dependent on their material properties (Tekkaya et al. (2017)). While processes such as machining mainly affect the surface and edge zone of the component, forming affects the entire volume of the material. The stress state during forming induces microstructural changes, which in turn affect properties such as yield strength, tensile strength, hardness, residual stresses and ductile damage. Forming-induced ductile damage refers to the formation, growth and coalescence of voids during plastic deformation (Koplik and Needleman (1988)), which leads to a deterioration in the mechanical properties of the material. By changing process parameters during the forming process, such as adjusting the shoulder opening angle during cold extrusion (Hering and Tekkaya (2020)), damage can be reduced and mechanical properties improved. Full forward rod extrusion is a forming process commonly used to produce mechanical components such as shafts. Depending on the stress level, these local material inhomogeneities can occur both during the various forming stages and during operation and can reach failure-relevant lengths. Understanding the relationships between material properties, microstructure and operational behavior is therefore of great importance. This knowledge enables the optimization of process parameters to positively influence component performance, considering design and functional requirements, service life, and adaptability to operational loads, which also impacts energy absorption in Charpy impact tests and fatigue resistance. However, previous studies have mainly focused on fatigue behavior after hot forming or synchronous axial-torsional loading. The influence of forming-induced ductile damage on the low cycle fatigue (LCF) properties is of considerable economic and environmental importance, as many component failures are due to LCF damage, where cyclically induced stresses exceed the yield strength and lead to irreversible plastic deformation in steel (Callaghan et al. (2010)). Various parameters such as strain rate and strain amplitude have a significant influence on the LCF behavior (Shankar et al. (2016)). In addition, defects and especially the near-surface microstructure have a strong influence on the fatigue properties in the LCF range. Forming-induced ductile damage occurs in the form of voids in the matrix material or at non-metallic inclusions and leads to local stress increases (Langenfeld et al. (2023)). The resulting material or component failure under service conditions can be attributed to these inhomogeneities (Zapara et al. (2014)). Numerous studies have investigated the relationship between fatigue strength and defects, as these defects determine the threshold condition for crack initiation at the defect tip (Beretta et al. (2011)). With regard to the influence of forming-induced ductile damage on the fatigue behavior of the case-hardened steel 16MnCrS5, Moehring (Moehring and Walther (2020)) investigated the influence of forming-induced ductile damage in the LCF range under torsional loading, while Langenfeld (Langenfeld et al. (2023)) investigated its effect under axial loading and demonstrated a significant influence on the fatigue behavior. This influence has been confirmed in other studies for axial-torsional loading (Lingnau and Walther (2023)). The influence of forming-induced ductile damage in the LCF range under asynchronous axial-torsional fatigue loading has not yet been sufficiently investigated. Therefore, it is important to examine and quantify the changing damage mechanisms and fatigue performance under asynchronous load paths. This includes microstructural investigations and an innovative 3D modeling approach using a Focused Ion Beam Scanning Electron Microscope (FIB-SEM) to make statements about the distribution of forming-induced ductile damage in the material (Lingnau et al. (2024)). This provided new insights into the defect-dependent damage mechanisms.

2. Material and experimental procedures 2.1. Material and specimen manufacturing

The fatigue specimens were produced from cylindrical billets of case-hardening steel 16MnCrS5 (DIN 1.7139, AISI 5115) using full forward rod extrusion. The base material, provided by Georgsmarienhuette, consisted of rolled and drawn cylindrical rods with a ferrite-pearlite microstructure. The chemical composition is shown in Table 1. No additional heat treatment was applied to the material. During the extrusion process, cylindrical billets with an initial diameter (d 0 ) of 30 mm and a length (l 0 ) of 71 mm were forced through a die, reducing their diameter and resulting in an extrusion strain (ε extr ) of 0.5. This extrusion