PSI - Issue 75

Okan Yılmaz et al. / Procedia Structural Integrity 75 (2025) 435 –441 O. Yılmaz and D. Van Hoecke / Structural Integrity Procedia 00 (2025) 000–000

437

3

After bending

R30

F

320

30

64

TD

TD

80

t

80

Two formed parts welded together

Fig. 1. Initially flat samples bended and welded together to form a closed profile for fatigue testing. All dimensions are in mm, t is the sheet thickness.

Defective bend

Smooth bend

r/t=0.5

r/t=1

r/t=2

*Scale bar of 100 µm in all figures

Fig. 2. Scanning electron microscope cross-sectional images showing the surface condition of defective and smooth bends of UHSS-A. Defective surfaces had microcracks that can go as deep as several hundred micrometers.

the case as the defective inner bending surfaces had microcracks that can go as deep as several hundred micrometers. Alternatively, smooth bending surfaces were obtained when the EN10149-2 (2013) recommendations were respected.

3. Calculation of fatigue initiation life

The non-linear FKM guideline (Fiedler et al., 2019) was used for calculation of the damage and fatigue initiation life. The guideline o ff ers a practical method using local strain approach, focusing on a critical notched area. To calculate the stresses and strains in the notched area, a linear elastic analysis is performed and elastoplastic stresses and strains are approximated from the flow curves using Neuber’s method (Neuber, 1961). The cyclic stress-strain curves are described using the Ramberg-Osgood equation (Ramberg and Osgood, 1943). For more details the reader is referred to Fiedler and Vormwald (2018); Fiedler et al. (2019); Yadegari et al. (2023).