Prediction of threshould for very high cycle fatigue (N>107 cycles)

M.D. CHAPETTI

Praga

Congreso; 10th International Fatigue Congress (Fatigue 2010); 2010

Institute of Physics of Materials, Academy of Sciences of the Czech Republic

This work deal with the very high cycle fatigue behavior of high strength steels in which the fracture origins are mostly at non-metallic inclusions. The estimation of the total fatigue life for these materials could only success if the fatigue crack initiation life could be properly quantified. This is a very hard task because the phenomenous involved in the fatigue crack nucleation and initiation processes are not well understood yet. This stage is not uniquely governed by fatigue, but also by other processes that are usually active helping the pure fatigue process. From an analysis of several features involved in the crack initiation and propagation processes and taking into account that the fatigue limit usually observed for metals is mainly given for a microstructural threshold for fatigue crack propagation, an expression was deduced to estimate the internal fatigue limit, that is, the stress level below which fracture produced by cracks initiated from an internal inclusion is not found after 1010 cycles. Such estimation becomes very important when it is necessary to assess fatigue resistance of components subjected to very high cycle fatigue, for which the experimental measure of the fatigue limit could require very long time by using traditional fatigue testing machines.