PSI - Issue 57

Okan Yılmaz et al. / Procedia Structural Integrity 57 (2024) 420 – 427 Yılmaz and van Hoecke / Structural Integrity Procedia 00 (2023) 000–000

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1. Introduction

Ultra high strength steels provide opportunities for reduced-weight designs since they have higher tensile and fatigue strengths than the conventional mild steel grades. For typical applications such as telescopic cranes, tippers, chassis, and agricultural machinery, lower weight leads to lower fuel consumption and better road homologation. However, before the start of their service life, steel parts of designed components must be machined to their final shape as they are supplied in standardized shapes and sizes. Di ff erent metal cutting methods used to obtain the desired final part induce various physical phenomena within the cut material a ff ecting the in-use properties. These methods may be divided into three main groups: (i) mechanical, e.g., shearing, punching, fine-blanking, drilling, water-jet, and saw cutting; (ii) thermal, e.g., laser, plasma, and oxy-fuel cutting; and (iii) electrical cutting such as electrical discharge machining. This study attempts to link several of these methods, punching, laser cutting, milling, and water-jet cutting to an important in-use property: fatigue performance. A handful of studies focusing on the e ff ects of manufacturing processes on fatigue are available in the literature albeit they di ff er greatly in terms of studied materials and process parameters. Alegre et al. (2004); Sanchez et al. (2004); Bannister et al. (2006); Brown et al. (2007) studied punched and drilled holes in structural steels using fatigue samples similar to the present work. Shiozaki et al. (2015) focused on the e ff ects of heat treatment and clearance on fatigue performance of the punched holes while Wang et al. (2016) analyzed laser-cut holes. Stahl et al. (2020) studied thedi ff erence between one- and two-stage shear cutting on the hole quality of components made of high-strength steel. In addition, Jime´nez-Pen˜a et al. (2019, 2020) analyzed hole-making e ff ects on ultra-high strength steels using punched, drilled, water-jet cut, laser-cut, and plasma-cut specimens following an extensive test and characterization campaign within the project DURAMECH (Debruyne et al., 2019). To reassess the fatigue detail categories of plain members and mechanically fastened joints in EN1993-1-8 (2005), Bartsch and Feldmann (2021) compiled published fatigue test data. A dependence of fatigue strength on stress ratio and yield strength as well as fabrications properties was shown. In accordance with past studies, specimens with drilled holes showed a higher fatigue strength than punched holes while hot-dip galvanizing had a negative e ff ect on the fatigue strength of drilled sheets. Besides the studies focusing on hole-making, plenty of authors concentrated on the cut-edge by using classical shaped fatigue samples with edges prepared via various manufacturing processes. For example, Sperle (2008) studied machined, laser-cut, plasma-cut, and oxygen-gas-cut edges of di ff erent steel grades with yield strength from 240 to 900 MPa. Thomas et al. (2011); Thomas (2012) showed that fatigue performance of blanked edges was generally equal or better than laser-cut ones for tested steels. Lara et al. (2013) studied laser cutting and di ff erent shearing cases with specific emphasis on Al-Si coating. HIPERCUT project (Aldazabal-Mensa et al., 2016; Cicero et al., 2017) analyzed steels of yield strength from 355 to 890 MPa with thicknesses between 8 and 25 mm cut by laser, plasma, and oxy-fuel methods. S355MC structural steel sheets of 5 and 15 mm were tested by Bursi et al. (2017) for drilled, laser, plasma, and oxy-fuel cut cases. Lillema¨e-Avi et al. (2018) tested steels that had undergone the normal shipyard production process of plasma cutting, grinding, and sandblasting for cut-edge e ff ects. Quality improvement methods, such as shot peening, grinding, and cutting speed reduction were analyzed by Diekho ff et al. (2020) with S355M and S690Q steels cut by oxygen, plasma, laser, and water-jet. Lipia¨inen et al. (2021) investigated fatigue strength characteristics of CO 2 laser-, fiber laser-, and plasma-cut edges of ultra-high strength steel grades. The objective of this study is to quantify the e ff ect of hole-making processes and edge-preparation on fatigue performance of ultra high strength steels. A series of fatigue experiments were conducted on steel grades with yield strengths from 700 to 960 MPa using axially loaded coupon samples with a hole manufactured by punching and laser cutting. Test data is compared to the detail categories provided by design standards. Additional tests under four-point bending loading were conducted to focus on flat edges machined by laser cutting, milling, and water-jet cutting. Material properties and test conditions are described in Sec. 2. Test results are presented and discussed in Sec. 3 and conclusions are summarized in Sec. 4.

2. Material and methods

The steel grades considered in this study, S700MC and S960MC are thermomechanically rolled (M) structural steels (S) suitable for cold forming (C) while the numbers denote the minimum yield strength in MPa at ambient temperature as denoted in EN10149-2 (2013). The chemical compositions of the steels are summarized in Table 1