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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with minimal artifacts was selected to generate a 3D model with dimensions of 8.54 × 8.77 × 10.18 µm³. Image segmentation was automated using the open source software FIJI ImageJ. Voids were then segmented from the image stacks using Zen Intellesis and the pixel data was converted to voxels using FIJI ImageJ. Several small mesh models were created and saved in .obj format. These mesh structures were then converted to a solid model using a mesh activator, and all the parts were assembled into a final model using Autodesk Inventor, which was rendered to enhance the visual representation. The same workflow was used to develop a 3D model of the MnS inclusion distribution (Fig. 5b)). To analyze the influence of voids and MnS inclusions on fatigue behavior and to understand the associated damage mechanisms, a rendered 3D CAD model of the voids and MnS inclusion distribution was created (Fig. 5c)). This model was generated from 510 SE and BSE images. Contrary to previous assumptions (Hering and Tekkaya (2020)), the results show that the voids are distributed throughout the representative volume and are not exclusively located in the MnS inclusions. The MnS inclusions appear as plate-like formations resulting from the full forward rod extrusion process, leading to significant microstructural notches that contribute to crack initiation and propagation. The brittle phases limit deformation and prevent compensation by the matrix, leading to fracture of the MnS inclusions during both the full forward rod extrusion process and subsequent fatigue loading. This results in the formation of large voids in and around the MnS inclusions, which can accelerate crack growth. The volume fraction of MnS inclusions is about 0.27% and the void volume is 0.13%.

Fig. 5. (a) Processing of the FIB-SEM Inlens images (b) and a rendered 3D model of the MnS inclusion (c) as well as combined 3D model of the voids and the MnS inclusion. 4. Conclusion and outlook The present study has shown that phase shift has a significant effect on the fatigue properties of forward rod extruded 16MnCrS5. This was demonstrated for a phase shift of d = 90° in axial-torsional total strain-controlled fatigue tests. The damage mechanisms were validated by metallographic methods to better understand the underlying damage mechanisms and to correlate them with the fatigue data. Under axial-torsional loading, cyclic softening and tension-compression asymmetry were observed, which can be attributed to the Bauschinger effect and occur under both synchronous and asynchronous loading. Under cyclic loading, the dislocation density decreases due to mechanisms such as dislocation annihilation, which is reflected in the cyclic softening of the material. It has also been shown that a phase shift of d = 90° leads to an improvement in the fatigue behavior of the specimens, regardless of the initial damage state, which is influenced by the shoulder opening angle. Compared to fatigue tests without a phase shift, specimens with a phase shift of d = 90° showed a factor of 1.25 higher number of cycles to failure. The influence of forming-induced ductile damage on fatigue