PSI - Issue 18

Matthias Hell et al. / Procedia Structural Integrity 18 (2019) 823–836 Hell et al. / Structural Integrity Procedia 00 (2019) 000–000

829

7

3. Influences on the fatigue behaviour

Different influences on the fatigue behaviour have to be considered during the fatigue life assessment. Fig. 7 shows an overview of different possible influencing factors. Due to the manufacturing process, the microstructure and the load-time history, i.e. load sequence and load magnitude, the stress-strain behaviour will be time or damage and location dependent. In addition to the intrinsic distribution of the cyclic stress-strain and fatigue behaviour, a gradient load-time history, consisting of mechanical and thermal loads acts on the component. Due to stress gradients, which may be caused by loading, the geometry or the metallurgical features also the component size with respect to the extension of highly stressed regions has a significant influence on the fatigue strength. Nearly all manufacturing processes alter the cyclic material behaviour compared to the virgin state of the material. Anisotropy or inhomogeneous material behaviour for example may be resulting from forging, additive manufacturing processes like Selective Laser Melting or heat treatments.

Fig. 7. Different influences on the fatigue behaviour

3.1. Location and size dependence of the material behaviour

Forging of components usually leads to anisotropy and inhomogeneous property distributions. Assuming a homogeneous material behaviour may therefore lead to an misevaluation of the actual stress-strain state and the fatigue properties. In order to investigate the effect of the forging process and the additional heat treatment on the fatigue strength on a local basis, small scale specimen were extracted by wire electrical discharge machining from a axle stub made of Precipitation Hardening Ferritic Pearlitic Steel (PFH) with a carbon concentration of 0.38 wt%, see Fig. 8. With the specimens, strain-controlled tests with constant amplitude loading have been conducted in order to evaluate the cyclic stress-strain behaviour as well as the strain life relation.