PSI - Issue 77

L.A. Lingnau et al. / Procedia Structural Integrity 77 (2026) 26–33 Author name / Structural Integrity Procedia 00 (2026) 000–000

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1. Introduction In the face of advancing climate change and limited resources, the demands for energy efficiency, emission reduction and sustainable use of resources are increasing. In this context, forming technology offers considerable potential for lightweight construction and efficient use of materials and costs. The performance of components manufactured through forming processes is significantly influenced by their material properties (Tekkaya et al., 2017). In contrast to machining, which primarily affects surface and edge zones, forming processes alter the entire volume of the material. The stress conditions occurring during forming lead to microstructural transformations that impact key material properties such as yield strength, tensile strength, hardness, residual stresses, and, in particular, forming induced ductile damage. In forming, ductile damage refers to the initiation, growth, and coalescence of microvoids during plastic deformation, which results in a reduction of the material’s mechanical integrity. Process parameters, such as the shoulder opening angle in full-forward rod extrusion, can be adjusted to minimize damage and improve mechanical properties (Hering & Tekkaya, 2020). This method is commonly used to manufacture rotationally symmetric components, such as shafts. However, this process can introduce local microstructural inhomogeneities that, under certain loading conditions, may develop into critical defects that affect component performance (Ehle et al., 2020). A profound understanding of the interaction between microstructure, material properties and performance is essential for the targeted optimization of component design, functionality, service life, and resistance to operational loading. This is particularly relevant in the context of energy absorption under impact loading and fatigue strength (Gerin et al., 2016). Previous studies have primarily focused on the fatigue behavior and its interaction with forming-induced ductile damage. In the low cycle fatigue (LCF) regime, repeated stresses exceeding the yield strength result in irreversible plastic deformation, which is a common damage mechanism in metallic materials (Callaghan et al., 2010). Key parameters such as strain rate and strain amplitude strongly impact fatigue life (Shankar et al., 2016). In the low cycle fatigue (LCF) regime, surface-near defects are particularly relevant for crack initiation (Lingnau et al., 2024a). Forming-induced microvoids at the interfaces of the matrix and inclusions act as local stress concentrators and affect LCF behavior (Lingnau et al., 2025). These microstructural heterogeneities often represent preferred initiation sites for fatigue failure under service loading (Zapara et al., 2014). Numerous studies have demonstrated a direct correlation between fatigue strength and defect characteristics, as these define the threshold for crack initiation (Beretta et al., 2011). For the case-hardened steel 16MnCrS5, Moehring and Walther (2020) investigated the effect of forming-induced ductile damage on LCF behavior under torsional loading, while Langenfeld et al. (2023) identified comparable effects under axial loading. Further studies confirmed these findings for combined axial-torsional loading conditions (Lingnau & Walther, 2023). However, the influence of forming-induced ductile damage and its progression under mechanical loading remains insufficiently understood. Addressing this research gap requires a detailed analysis of the evolving damage mechanisms under in-situ conditions. Therefore, microstructural investigations are conducted using in-situ tensile testing within a scanning electron microscope (SEM). In addition, AI-based image segmentation methods are employed (Lingnau et al., 2024b), providing new insights into the interactions between mechanical loading, load path dependency, and forming-induced initial damage on damage accumulation.

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

The specimens were produced from cylindrical billets of case-hardening steel 16MnCrS5 (DIN 1.7139, AISI 5115). The material, provided by Georgsmarienhuette, consisted of rolled and drawn cylindrical billets with a ferrite-pearlite microstructure. The chemical composition is shown in Tab. 1 . No additional heat treatment was applied to the material. In order to investigate the void formation and growth and non-metallic inclusions such as manganese sulfides, the steel was examined in its non-deformed, as-delivered condition. This approach allows for the assessment of the influence of inclusions both during the forming processes and in later service conditions, particularly with regard to fatigue damage mechanisms.