PSI - Issue 75

Okan Yılmaz et al. / Procedia Structural Integrity 75 (2025) 435 –441 O. Yılmaz and D. Van Hoecke / Structural Integrity Procedia 00 (2025) 000–000

436

2

Fatigue cracks typically originates from stress raisers. These can be inherent in the microstructure such as non metallic inclusions. Another example would be surface or subsurface defects after the production process. Alterna tively, they can arise from joining, cutting, and forming procedures. While the fatigue strength of welded and bolted joints were studied extensively in the literature, not many researchers considered the e ff ects of cold forming on fa tigue in applications. Talemi et al. (2017) studied the e ff ect of forming on low-cycle fatigue loading conditions using S700MC high-strength steel experimentally and numerically and used lock-in thermography to detect fatigue crack propagation onset. Similar fatigue tests were also performed by Bracke et al. (2017) for S900MC and the e ff ect of nitrogen levels on the fatigue response were studied in detail. Several publications from Gothivarekar et al. (2019, 2020, 2021) considered S500MC and a double bended benchmark specimen to study the e ff ect of di ff erent strain hardening models. Digital image correlation was used for the further validation of the model. Geers et al. (2021) used S355MC and S460MC and focused on di ff erent bending radii and the e ff ects of resulting plastic deformation and internal residual stresses. Yamaguchi et al. (2023) used a hot-rolled UHSS with a tensile strength of 980 MPa to test bended samples with an artificial crack in the bending line. They demonstrated that the internal compressive residual stress introduced by bending arrests crack propagation in the bending direction and propagation life was shortened in the absence of this stress. The present study focuses on the closed profiles manufactured via cold forming and welding. The response under repeated loading is complex due to forming stresses, material hardening behavior, resid ual stresses, and possible microscopic buckling. Fatigue performance of the profiles can be hindered by low bending ratio and high bending angles, increasing stress concentration factor. If the bending equipment is in poor condition, then it could create defects on surface, acting as stress raisers. In addition, forming can enhance the initial micro defects, which are due to suboptimal material quality in flat state. Hence, the material condition of the flat state is also important. In this study, we use an in-house fatigue testing setup to consider the complex e ff ect of cold forming in predicting number of cycles of a formed part. The objective is to use this method to qualify the materials for fatigue-critical applications using cold-formed profiles. A design space is defined to account for the e ff ects of forming parameters such as bending ratio and surface state combining the outputs from experimental and numerical procedures. The accurate quantification of the e ff ect of cold forming on fatigue life is crucial for the applications with fatigue-critical formed sections and to optimize future designs. The chemical composition and mechanical properties of the steel grades considered in this study, denoted as UHSS A and UHSS-B, satisfy the norms in EN10149-2 (2013). These materials were bended in di ff erent bending ratios, r / t, which are given in Table 1. Figure 1 illustrates the sample dimensions of the flat parts, which were V-bended 90° and welded to form a closed profile to be tested under repeated loading. The fatigue tests of the cold formed specimens were carried out in a load control setting on a hydraulic testing machine. Di ff erent test runs had the maximum load levels changing between 20 kN and 70 kN and all tests had a load ratio of 0.1. Test frequency was 2 Hz. To detect the onset of crack, the axial displacement was tracked during the test and additional 0.1 mm of displacement with respect to the test start was defined as the crack initiation criterion. Table 1. Applied bending ratios and mechanical properties of grades UHSS-A and UHSS-B. E: Young’s modulus, UTS: ultimate tensile strength (in transverse direction). Bending ratio (r / t) E (GPa) UTS (MPa) 2. Material and methods

UHSS-A UHSS-B

0.5, 1, 2

207 207

850

3

1050

All samples were bended in a configuration that the bending line is perpendicular to the transverse direction as it is known to be more conservative. To generate a defective surface, the material was deliberately bended below the recommended bending ratio by EN10149-2 (2013). Under severe bending conditions, microscopic cracks may occur in the inner surface of the bended sheet due to microscopic buckling (Yamaguchi et al., 2023). Figure 2 shows that was