Support for an environmental effect on fatigue mechanisms in the long life regime

Recent studies in the Giga Cycle Regime show that the internal crack initiation is largely promoted, compared to the High Cycle Regime. This change of mechanism is still under wide discussion. The purpose of this study is to highlight the influence of the environment on both internal and surface fatigue failure. In that way, experimental results from the literature concerning Giga Cycle Fatigue are presented and crack initiation mechanisms are discussed. High Cycle Fatigue results on nodular cast iron containing casting defects are presented. The position of the defect at the origin of the failure is analysed, considering the environmental effect. Giga Cycle results from the literature are then analysed using simple fracture mechanics assessment. It is shown that the environment has to be considered when computing fatigue life assessment for materials containing surface or internal defects. Our results also indicate that, even if there is a change of mechanism between HCF and GCF, the initiation stage is still predominant in the Giga Cycle Regime.