Abstract
The processes involved in corrosion fatigue in general are briefly outlined, followed by a brief review of recent studies on the effects of cycle frequency (rise times) and electrode potential on crack-growth rates at intermediate ΔK levels for cathodically protected high-strength steels. New studies concerning the effects of fall times and hold times at maximum and minimum loads on crack-growth rates (for Kmax values below the sustained-load SCC threshold) are presented and discussed. Fractographic observations and the data indicate that corrosion-fatigue crack-growth rates in aqueous environments depend on the concentration of hydrogen adsorbed at crack tips and at tips of nanovoids ahead of cracks. Potential-dependent electrochemical reaction rates, crack-tip strain rates, and hydrogen transport to nanovoids are therefore critical parameters. The observations are best explained by an adsorption-induced dislocation-emission (AIDE) mechanism of hydrogen embrittlement.
Highlights
IntroductionFor many materials, embrittling environments (e.g. aqueous, gaseous hydrogen, liquid metals) can increase rates of fatigue crack growth by up to several orders of magnitude compared with the rate in inert environments, with the degree of embrittlement depending on variables such as ∆K, cycle frequency, and the ‘potency’ of the environment
For many materials, embrittling environments can increase rates of fatigue crack growth by up to several orders of magnitude compared with the rate in inert environments, with the degree of embrittlement depending on variables such as ∆K, cycle frequency, and the ‘potency’ of the environment
Upper plateau da/dN values were observed for all waveforms, but were significantly higher for the Kmax-hold waveforms than the negative-sawtooth and Kmin-hold waveforms, with this difference decreasing when rise times were longer (Fig. 6)
Summary
For many materials, embrittling environments (e.g. aqueous, gaseous hydrogen, liquid metals) can increase rates of fatigue crack growth by up to several orders of magnitude compared with the rate in inert environments, with the degree of embrittlement depending on variables such as ∆K, cycle frequency, and the ‘potency’ of the environment. The difference between crack growth in inert and embrittling environments is that slip is more localised for the latter, resulting in less blunting and greater crack-growth increments for a given crack-tipopening displacement (Fig. 2). This figure, and others like it in the literature [e.g. 3,4], omits an important feature, namely, the formation of nanovoids ahead of cracks. Plots of da/dN versus rise time for a given ∆K (for positive sawtooth waveforms) show a sigmoidal relationship, with sigmoids displaced upwards and to the right for higher ∆K values such that the inflection points of the sigmoids fall on a common transition line (Fig. 5(a)). The sigmoidal da/dN versus rise-time plots were explained on the basis that (i) for short
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