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

425

6

1000

1000

S960MC, R=-1

S700MC, R=-1

100

100

Laser Punched (low clearance) Punched (high clearance)

Laser Punched (low clearance) Punched (high clearance)

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

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

Stress amplitude (MPa)

Stress amplitude (MPa)

10

10

1.E+04

1.E+05

1.E+06

1.E+07

1.E+04

1.E+05

1.E+06

1.E+07

N (cycles)

N (cycles)

Fig. 4. Fatigue results of axially loaded fatigue tests with a hole of S700MC (left) and S960MC (right). Data points of low clearance (0.05 mm) and high clearance (0.4 mm) punched holes (Debruyne et al., 2019) are plotted with laser-cut holes and compared with relevant detail categories. Arrows denote run-outs.

1400

1400

S960MC (4pb), R=0.1

S700MC (4pb), R=0.1

Laser cut Waterjet Milled

Laser cut Waterjet Milled

1200

1200

1000

1000

Run-outs

Run-outs

800

800

600

600

Maximum bending stress (MPa)

Maximum bending stress (MPa)

400

400

1.E+04

1.E+05

1.E+06

1.E+07

1.E+04

1.E+05

1.E+06

1.E+07

N (cycles)

N (cycles)

Fig. 5. Fatigue results of four-point bending fatigue tests of S700MC (left) and S960MC (right). Cut-edges were achieved by laser cutting, water-jet cutting, and milling. Arrows denote run-outs.

Figure 6a shows a fracture surface obtained after failure of a sample where the hole is manufactured by punching. At the exit side of the punch (bottom side), a burr defect is acting as a crack initiator. Deburring is very likely to improve the fatigue performance although it was not performed for this set of experiments. In Fig. 6b a fracture surface after failure of a sample where the hole is manufactured by laser cutting is seen. The wavy pattern of the surface is also visible here. Some defects were present as artifacts of laser cutting procedure, which might be dominant enough to initiate a crack. Figure 6c provides a typical crack surface for a four-point bending specimen. Irrespective of the cutting method, the cracks initiated at a corner and propagated to approximately half of the total cross-section before stopping the test. The rest of the surface was manually bent open after testing.

4. Conclusions

The objective of this study was to quantify the e ff ect of hole-making processes and edge-preparation on fatigue performance of ultra high-strength steels (UHSS). Figure 7 summarizes the results in combined plots. Bending stress amplitudes in repeated tension were converted by using the Goodman line and addition of a stress concentration factor of 2.35 so all tests can be plotted in a single graph. However, one must be cautious in the interpretation of the converted stress values as the influences of the irregularities in the cross section and di ff erent loading modes are unaccounted for.