Environmentally Affected Near-Threshold Fatigue Crack Growth in Steels

Abstract

Environmentally affected fatigue crack propagation in steels is described for ultralow growth rates (below 10−6 mm/cycle) in terms of the role of crack surface corrosion debris in promoting crack closure. It is shown that the reported effects of gaseous and aqueous environments (air, water, hydrogen, helium, etc.) on near-threshold crack growth in lower strength steels are consistent primarily with an oxide-induced crack closure mechanism. Moist atmospheres, such as humid air and water, are shown to promote the formation of oxide deposits within the crack, which at low load ratios are thickened by fretting-assisted oxidation to maximum thicknesses comparable with cracktip opening displacements. Using ultrasonic techniques, this is shown to increase closure loads and to lower effective alternating stress intensities at the crack tip. Observations that near-threshold growth in dry helium is similar to that in dry hydrogen gas and faster than in air, are shown to be consistent with such concepts since both environments provide a dry atmosphere limiting oxide formation. Extensive data on near-threshold corrosion-fatigue crack growth in ultrahigh-strength (300-M) and lower-strength (2¼Cr-1Mo and SA516) steels are examined in the context of this mechanism, and it is found that the threshold for no crack growth (ΔKo) is consistent with a maximum excess oxide thickness approximately equal to the pulsating crack-tip displacement (ΔCTOD). The implications of this and other microscopic mechanisms of closure are discussed in the light of microstructural and environmental influences on near-threshold fatigue.