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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discussed tension-tension fatigue experiments. A slight increase in the segmented manganese sulfide proportion f S is observed over time. This variation is primarily attributed to small differences in the working distance during image acquisition. Assuming that the actual manganese sulfide proportion within the analyzed region remains constant, these fluctuations are considered to be measurement artifacts. Consequently, as in the preceding investigations, the ratio f DS is primarily used for assessing damage evolution, as it provides a more robust and reliable metric than the absolute damage proportion f D alone. The data indicate that f DS increases steadily with each cycle during tension phase of loading, whereas it remains nearly constant or slightly decreases during compression loading phases. The formation of damage in and around manganese sulfides can generally be attributed to two primary mechanisms, as also described in studies by Lingnau et al. (2024b). The first mechanism is the fracturing of manganese sulfides due to their more brittle nature compared to the surrounding steel matrix, which can occur both during forming processes and under cyclic loading. The resulting voids and cracks are not completely closed by plastic deformation of the matrix, leading to permanent damage. The second mechanism is decohesion between the manganese sulfides and the matrix, where partial or complete separation occurs at the interface. This interfacial decohesion results in localized damage around the periphery of the manganese sulfides. The fracture of manganese sulfides was observed in multiple experiments conducted within the scope of this study. To illustrate this phenomenon, Fig. 7a shows the segmented image of the initial state, while Fig. 7b and Fig. 7c display the segmented micrographs captured at a nominal stress of 450 MPa after the first and third load cycle. The detected damage parameter f DS increased from 0.195 in the initial state to 0.320 after the third cycle, corresponding to a total damage increase of 64 %. Void formation is clearly visible as a result of the fracture of the manganese sulfide. In addition, interfacial decohesion between the manganese sulfide and the surrounding steel matrix can be observed at the edges of the sulfide comparable to the investigations of Lingnau et al. (2024a). It is hypothesized that such decohesion occurs primarily under compressive loading conditions, both during cyclic fatigue and during forming processes. However, this mechanism could not be conclusively verified in the present study, especially considering that the analysis was limited to surface observations. As a result, no definitive conclusions can be drawn regarding damage evolution within the material volume. At the specimen surface, manganese sulfides may detach or break out, which complicates the reliable detection of decohesion-related damage. For a more comprehensive understanding of decohesion phenomena, investigations that enable volumetric analysis of the material are recommended.

Fig. 7. Representation of the corresponding segmented image sections for a) the initial state, b) for a stress of 450 MPa in the first loading cycle and the final state. 4. Summary and outlook This study provides a significant contribution to the characterization of damage initiation and evolution in the case-hardening steel 16MnCrS5 under both elastic and plastic deformation, as well as to the understanding of the load path-dependent damage evolution. Comparable specimens were tested under different loading paths to study damage development under tension–tension, compression–compression and combined tension-compression loading over multiple load cycles. Continuous acquisition of mechanical test data, combined with intermittent SEM imaging, enabled direct correlation between the recorded mechanical response and microstructural observations. The use of advanced AI-based image segmentation software facilitated the quantification of load path-dependent damage accumulation.