THE EFFECTS OF MOISTURE ON THE FATIGUE CRACK GROWTH BEHAVIOUR OF A LOW ALLOY STEEL NEAR THRESHOLD

Abstract— The influence of moisture on the fatigue crack growth behaviour near threshold of a 2NiCrMoV rotor steel has been investigated. At a high stress ratio ( R = 0.6), moisture enhances fatigue crack growth rates by approximately 60% compared with the growth rate in dry air. The effect would appear to be due to the influence of hydrogen which is confined to a volume of material at the crack tip considerably smaller than the plastic zone. At a low stress ratio ( R = 0.14), the growth rates in moist air can be very much lower than in dry air. This difference is closely associated with the formation of oxides on the fracture surface—moisture modifying the type and extent of oxidation which is observed. Observations of transient crack growth following environmental changes, suggest that fracture surface oxides within approximately 0.3 mm of the crack tip exert a strong retarding influence on crack growth although oxides up to at least 3 mm from the tip may also have some retarding effect.