PSI - Issue 57

Okan Yılmaz et al. / Procedia Structural Integrity 57 (2024) 420 – 427

423

4

Yılmaz and van Hoecke / Structural Integrity Procedia 00 (2023) 000–000

1000

EN13001-3-1 hole detail for UHSS (YS > 650 MPa), following quality req. (no punching)

EN13001-3-1 punched hole detail

1

100

m

Eurocode 3 punched hole detail

FAT-class = detail category

Stress amplitude (MPa)

FAT80 (m=5, corrected for R=-1) FAT90 (m=3, corrected for R=-1) FAT160 (m=5, corrected for R=-1)

Normal/shear stress range (MPa)

2 ⋅ 10 6

10

1.E+04

1.E+05

1.E+06

1.E+07

Number of cycles

N (cycles)

Fig. 2. Definition of FAT classes (detail categories) (left) and typical detail categories provided by EN13001-3-1 (2018) and EN1993-1-8 (2005) (Eurocode 3) for structural elements with holes (right).

stresses experienced within a repeated loading cycle. The fatigue strength of a connection is demonstrated using the detail categories, also called FAT classes. The fatigue strength in terms of stress range at two million cycles determines the detail category as illustrated in Fig. 2. From the two-million-cycle point, the S-N curve is constructed as a line with a slope − 1 / m (also provided by the detail category) in double logarithmic scale, also called Basquin model (Basquin, 1910). EN1993-1-8 (2005) suggests FAT90 with a slope term m = 3 for the detail “Structural element with holes subject to bending and axial forces” regardless of the hole-making procedure. The stress range is to be calculated on the net cross-section. No additional requirement is given on the manufacturing method or the material. EN1993-1-8 (2005) specifies the highest attainable level as FAT160 and by that definition, no detail can reach a better fatigue strength at any number of cycles. In non-welded details or stress-relieved welded details, the mean stress influence on the fatigue strength may be considered by determining a reduced e ff ective stress range when part or all of the stress cycle is compressive. This e ff ective stress range is calculated by adding the tensile portion of the stress range and 60% of the magnitude of the compressive portion of the stress range. For a fair comparison of tension-compression (R = − 1) results with tests / curves at repeated tension (R > 0), the stress range should be converted to the e ff ective stress range. FAT classes are denoted as corrected in upcoming figures for tests at (R = − 1). EN13001-3-1 (2018), a European standard for crane design, provides more information about the manufacturing methods and details as a function of material yield strength in its Detail 1.3. The stress range is again to be calculated on the net cross-section. Di ff erent from EN1993-1-8 (2005), this standard defines requirements for di ff erent manu facturing methods, surface conditions and edge treatment. For all design curves on this detail, the defined slope is m = 5 instead of 3. If the hole is punched, the proposed design curve is FAT80 irrespective of yield strength. For di ff erent manufacturing methods, it is possible to go up to FAT160 if surface quality and burr removal requirements are respected. Contrary to EN1993-1-8 (2005), this standard does not instruct any mean stress correction.

3. Test results and discussion

Before initiating the fatigue tests, hole profiles were measured at di ff erent depths to evaluate the cut quality of the hole. Figure 3 shows the hole profiles compared to the theoretical perfect hole profiles. 3D profile measurements are given within the plots as additional visual. We observed that laser-cut sides show more variance compared to punched holes with lower clearance of c = 0 . 05 mm. They also produced higher roughness values and a distinctively wavy shape compared to punched holes. The figure also illustrates the holes obtained in the project DURAMECH (Debruyne et al., 2019) where the variation in punched edges from entry to exit side were larger following the clearance value c = 0 . 4mm.