PSI - Issue 57

Kalle Lipiäinen et al. / Procedia Structural Integrity 57 (2024) 785–792 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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significantly higher fatigue performance when geometrical SCFs are considered and also crack-like features in more narrow area from the cut surface. Geometric stresses (Fig. 4b) enable comparison on shear cut edge and rolled surfaces. Scatter can be seen when unnotched and shear cut holes are compared when only geometrical stresses are considered neglecting residual stresses, quality and local cyclic behavior. Fatigue performance considering geometrical SCFs exceeds FAT 375 MPa m = 5 curve similar to laser cut specimens studied in (Lipiäinen, Ahola, Kaijalainen, et al., 2022).

a

b

1000

2000

1000

375 MPa m = 5

375 MPa m = 5

SPF Un-notched SPF Shear cut incl. SCF SPF W-EDM incl. SCF CP Un-notched CP Shear cut incl. SCF CP W-EDM incl. SCF

SPF Un-notched SPF Shear cut SPF W-EDM CP Un-notched

Local stress Range [MPa]

Nominal stress Range [MPa]

125 MPa m = 3

125 MPa m = 3

CP Shear cut CP W-EDM

100

100

10000

100000

1000000

10000

100000

1000000

Fatigue cycles [-]

Fatigue cycles [-]

Fig. 4. Fatigue test results (a) nominal stress approach and (b) considering local stresses. Fractography with SEM is used to evaluate the influence of local quality and imperfections on fatigue performance. Based on the size of features close to fatigue crack initiation site, a numerical quality parameter can be used in fatigue strength assessment. In the unnotched specimens, the fatigue failures originated from surface imperfections and non metallic inclusions (NMIs) located on surface or in the vicinity of surface with interaction on surface features (Figs 5a and 5b). Surface quality of rolled surface is connected with fatigue performance and e.g. ref. (SSAB, 2017) illustrates FAT values as a function of material strength and surface roughness. The fracture surfaces of the notched specimens are illustrated in Figs 5c-5g. Based on the fractography, internal cracking due to shear cut induced local plasticity has significant effect on fatigue performance. Iron oxides (rust) was found inside W-EDM cut specimens indicating presence of initial cracks which is typical for the underwater process causing fatigue failures.

Fig. 5. Fractography for (a-b) unnotched specimen, (c) W-EDM cut edge, (d-e) Shear cut CP specimen and (f-g) Shear cut SPF specimen.