The influence of environment and stress ratio on fatigue crack growth at near threshold stress intensities in low-alloy steels

Abstract

Fatigue crack propagation at low stress intensities has been studied in two low alloy steels in a variety of environments with particular emphasis being placed on the influence of stress ratio and strength level. It was found that fatigue crack growth rates are lower and threshold stress intensities ( ΔK 0 ) are higher in vacuum than in humid, laboratory air but, in dry gaseous environments (argon, hydrogen and air) and at low stress ratio ( R ~ 0.1), crack growth rates are faster and ΔK 0 values are lower than in laboratory air. However, the influence of stress ratio is considerably greater in laboratory air than in dry gaseous environments with the result that, at high stress ratio ( R ~ 0.8) ΔK 0 values are similar in all environments examined. Increasing material strength level resulted in higher, near-threshold crack growth rates and a reduction in ΔK 0 in both dry and humid air environments. The results are discussed in terms of the influence of crack closure and environmental effects on fatigue crack growth behaviour. The importance of corrosion debris produced in fatigue cracks at low stress intensities is also discussed.