PSI - Issue 57

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Kalle Lipiäinen et al. / Procedia Structural Integrity 57 (2024) 785–792 Lipiäinen et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Specimens with notched and unnotched geometries were prepared for fatigue testing. Sides of the specimens were machined, and the flat surfaces were left in as-rolled condition. For notched geometries, punching and wire electrical discharge machining (W-EDM) were used to cut the holes on the specimens. The D = 10 mm holes were shear cut on the specimens with a cutting clearance of 11.7%, corresponding to the geometry used in the hole expansion tests according to ISO 16630:2017. The visual surface quality of shear cut edge is illustrated in Fig 2. Both steel grades feature shear and fracture sections approximately with 25 and 75% length, respectively. Nominal net cross section was 35 mm in all specimens. Geometrical notch stress concentration factor (SCF) in comparison to net-cross section was 2.50. The fatigue tests were conducted under uniaxial and sinusoidal constant amplitude loading with 150kN servo-hydraulic test rig using 1 – 2 Hz frequency.

Fig. 2 . (a-b) Specimen dimensions, (c-d) surface quality and (e) test arrangement. Microstructures of base materials transverse to rolling direction after Nital etching are shown in Fig 3a-b. CP had a complex microstructure consisting of ferrite, bainite and small martensitic islands, whereas in SPF the microstructure consisted primarily of quasi-polygonal ferrite. The grain size was considerably finer in CP compared to SPF, and the grain morphology was observed to be slightly elongated in the rolling direction in CP and more equiaxed in SPF. Crack-like features in SAZ are highlighted in Fig. 3c- d, extending 50 μm depth from cut surface. The microstructure has a major contribution in hole expansion test, where more uniform grain structure and low hardness difference between the microstructures can be beneficial. Secondary constituents, such as cementite and inclusions, can lower the hole expansion performance as seen in an earlier study with steels with similar microstructures (Kaijalainen et al., 2017). However, geometrical notches and crack-like defects are more important than microstructure from the fatigue performance viewpoint. As the similar 5 0 μm crack -like features were found for both materials with polished section inspection, it can be expected that fatigue performance of both materials is at similar level.

Fig. 3. Base material and SAZ microstructures and examples of initial crack-like feature due to shear cutting. 3. Fatigue test results Fatigue test results are illustrated as per the nominal stress method (Fig. 4a) and local stress approach (Fig. 4b), considering the geometrical notch SCFs. Fatigue performance of both steel grades were similar and high performance was obtained as expected based on polished section evaluation. In contrast to the un-notched guillotine cut S960 t = 6 specimens tested in (Lipiäinen, Ahola, Virolainen, et al., 2022), the shear cut optimized automotive steels show