PSI - Issue 57

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

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weight of suspension and cross member applications. Shear cutting is used in automotive industry to cut formed parts in a very fast and productive way as described by Rosenthal et al. (2020). Larour et al. (2022) studied AHSS automotive grades shear cutting, hole expansion ratio (HER) and its relation to cutting and material parameters. Shear cutting results in formation of shear-cut affected zone (SAZ) and induces cracks to cut edges together with high tensile residual stresses which influence fatigue performance and formability of the edges. Cut edge properties and fatigue performance have been studied e.g., by Stahl et al. (2020) and Paetzold et al. (2017). Kesti et al. (2022) studied performance and edge ductility of commercial Docol 800 HHE steel grade and discussed design guidelines of formed automotive components. Lipiäinen, Ahola, Kaijalainen, et al. (2022) introduced local cyclic behaviour -based parametric fatigue strength assessment method which utilizes a theory of critical distances (TCD) for obtaining local stresses. The method is evaluated with experimental fatigue test results and fractography obtained equivalent crack size for finite element analysis (FEA). The parametric methods aim to connect quality influenced by manufacturing parameters. Consequently, the need for experimental testing to optimizing manufacturing quality decreases as local parameters obtained by simple analysis are used to evaluate fatigue performance. Björk et al. (2023) discussed on modelling the production chain from material to service life with recognizing the needed performance level and local material and manufacturing characteristics to fulfil the performance with efficient production from optimized steel grade which can be considered as a backwards design. Multiparametric model based on measurable local properties and component geometries is needed to implement backward design concept throughout the design and production chain. In this study, fatigue performance of two hot-rolled (HR) automotive 800 MPa high-strength steel grades is studied with unnotched specimens and notched specimens with circular holes prepared with shear cutting and electrical discharge machining. The aim of the study is to connect local properties with fatigue strength. Polished sections and fractography are used to characterize the input parameters for fatigue strength assessment, and the approach is demonstrated in Section 5. 2. Methods and specimens Investigated materials in this study were two industrially hot-rolled strip steels with 800 MPa tensile strength grade intended for automotive applications: SPF800 and CP800, in which SPF stands for single-phase ferritic and CP for complex-phase steel, respectively. For both steels, thickness was 3 mm, and the materials were investigated in as received condition. Tensile properties were tested in the rolling direction according to ISO 6892-1:2019 utilizing flat specimens with 120 mm parallel length and 20 mm width. Tensile test results for base materials are illustrated in Fig. 1b. Yield strengths of the steels were between 700 and 750 MPa and a minor difference in the behavior near yield point is visible. The ultimate tensile strengths of the studied materials were between 800 and 850 MPa. Base material hardness was measured with HV3-method (Struers Durascan 70). The average hardness for measurements were 269 and 265 HV for SPF800 and CP800 materials, respectively.

0 100 200 300 400 500 600 700 800 900

a

b

SPF

CP R p 0.2 = 700 MPa and R m = 812 MPa R p 0.2 = 745 MPa and R m = 827 MPa

Stress [MPa]

0 2 4 6 8 10121416

Strain (A 80 ) [%]

Fig. 1. Base material tensile test (a) arrangement and (b) results.