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

422

3

(a)

Test loading direction

Φ13 or Φ14

TD

40

250

Cut-edge (in tension)

(b)

TD

76

8

t

200 152

Fig. 1. (a) Axially loaded fatigue sample with test loading direction and (b) four-point bending fatigue samples with cut-face in tension highlighted. All dimensions are in mm, t is the sheet thickness.

together with the mechanical properties. It must be noted that S700MC material is of 8-mm thickness while S960MC has the thickness of 6 mm.

Table 1. Chemical composition and mechanical properties of the UHSS grades S700MC and S960MC. YS: yield strength, UTS: ultimate tensile strength, A g : uniform elongation, A: fracture elongation. Chem. comp. (wt%)

C

Mn

Si

P

S

Al

Nb

V

Ti

Mo

B

S700MC S960MC

≤ 0 . 12 ≤ 2 . 10 ≤ 0 . 60 ≤ 0 . 025 ≤ 0 . 015 ≥ 0 . 015 ≤ 0 . 09 ≤ 0 . 20 ≤ 0 . 22 ≤ 0 . 50 ≤ 0 . 005 ≤ 0 . 20 ≤ 2 . 20 ≤ 0 . 60 ≤ 0 . 025 ≤ 0 . 010 ≥ 0 . 015 ≤ 0 . 09 ≤ 0 . 20 ≤ 0 . 25 ≤ 1 . 00 ≤ 0 . 005

Average mechanical properties

A g (%)

A(%)

Direction

YS (MPa)

UTS (MPa)

S700MC (8 mm)

Rolling

704 793 947

805 837

9 . 1 7 . 7 5 . 2 3 . 4

20 . 5 17 . 7 13 . 1 10 . 0

Transverse

S960MC (6 mm)

Rolling

1032 1081

Transverse

1048

Two types of fatigue tests were carried out in this study, axial-loading tests with a hole and four-point bending tests of cut edges. Figure 1 shows the sample geometries for both tests. For axial fatigue tests, a stress controlled at constant amplitude at frequencies between 10 and 20 Hz. The tests were carried out in tension-compression, i.e., at load ratio of R = − 1. Holes of 13-mm-diameter were manufactured by punching with a clearance value of c = 0 . 05 mm by an in house punch-die assembly that simulated fine-blanking conditions. Rather low clearance values allowed us to compare with the test results of Debruyne et al. (2019) where holes were punched with a clearance of c = 0 . 4 mm. Laser-cut holes were of 14-mm-diameter instead of 13 mm, a minor di ff erence that should be negligible when comparing fatigue levels to detail categories. Four-point bending fatigue tests were also force controlled at constant amplitude at frequency of 20 Hz. These tests were carried in repeated tension with a load ratio of R = 0 . 1. The cut edge on the tensile sides were achieved by milling, laser cutting, and water-jet cutting. The standards EN1993-1-8 (2005) and EN13001-3-1 (2018) provide information to treat di ff erent manufacturing methods and cut edge conditions. They do it by introducing fatigue level classes, shortly called a FAT class or FAT level. The fatigue strength is represented using a double-logarithm plot called an S-N curve, that is the log(stress range) vs. log(number of cycles) curve. The stress range is defined as the di ff erence between the maximum and minimum