PSI - Issue 4

Stefan Kolitsch et al. / Procedia Structural Integrity 4 (2017) 95–105 Stefan Kolitsch/ Structural Integrity Procedia 00 (2017) 000 – 000

103 9

Fig. 5. Comparison of different materials in the Kitagawa-Takahashi diagram at stress ratios R of 0.1 and 0.7, for near surface cracks with Y = 0.8 and fatigue strength experiments.

It can be seen that endurance limits and the long crack thresholds for the pearlitic and fine-pearlitic materials are almost equal. The endurance limit for the ferrite-martensite material is the highest of all considered materials, whereas for crack lengths a > 0.1 mm the endurable stress range is smaller compared to the others. For higher stress ratios, e.g. R = 0.7, shown by the dashed lines, the long crack threshold is roughly equal for all materials because closure is almost vanishing. Furthermore the experimental data of the endurance limit under bending with surface roughness are displayed by crosses in Fig 5. For this the fracture surfaces, exemplarily shown in Fig 6 a) and b) were analyzed using an optical microscope. The initial notch, considered as a crack, is measured and approximated by a semi-elliptical shape (Fig 6 c)). a) b) c)

Fig. 6. Flaw size determination from the fatigue strength experiments: a) view of the total fracture surface, b) detailed view of the failure initiation point and c) schematic sketch of the measured, idealized flaw geometry