EFFECTS OF CRACK FLANK OXIDE DEBRIS AND FRACTURE SURFACE ROUGHNESS ON NEAR-THRESHOLD CORROSION FATIGUE

Over the years, many models have been proposed for the environmental contribution to cracking associated with sub-critical flaw growth by corrosion fatigue and stress corrosion. In general, these models are based on mechanisms involving hydrogen embrittlement and active path corrosion (i.e. metal dissolution) processes. At ultralow growth rates in fatigue (<10⁻⁶ mm/cycle), however, approaching the cyclic stress intensity threshold ΔK_o for no detectable growth, recent studies in steels have revealed behavior totally inconsistent with such mechanisms. For example, near-threshold growth rates in ultrahigh strength steels are found to be decelerated in hydrogen gas compared to air, whereas in lower strength steels growth rates are accelerated in helium gas and decelerated in water, again compared to air. To provide alternative descriptions of environmentally-affected fatigue crack growth at such near-threshold levels, new models have recently been proposed based on the role of oxide deposits, formed within the crack, in promoting crack closure and crack tip blunting in moist, as opposed to dry, environments. Where thicknesses of the crack flank corrosion deposits are comparable with crack tip opening displacements, e.g. at near-threshold levels, such deposits have been shown to markedly affect crack growth behavior, consistent with observed effects of load ratio, strength level, and environment.