Influence of surface condition on the fatigue behaviour of specimens made of a SAE 5115 case-hardened steel

The effect of different surface conditions on the fatigue properties of cyclically loaded bending specimens of the case‐hardened steel SAE 5115 (Material Number 1.7131) was investigated. The aim of the investigations was to achieve further knowledge of the processes that finally lead to the failure of case‐hardened and cyclically loaded bending specimens. Surface roughness, microstructure and residual stress distribution were regarded as parameters that govern the fatigue process. On the basis of a well‐defined adjustment of different surface conditions the effect of internal oxidation, surface roughness and residual stresses on fatigue crack initiation and growth were assessed. The effect of different parameters on the endurance limit has been quantified by the application of a fracture mechanics and a weakest‐link approach. The calculations contributed to a deeper insight into the complex interaction between the parameters governing the fatigue process.