Corrosion Fatigue Cracking in High-Strength Steels: Effects of Cycle Frequency, Waveform, and Potential

Knop, Mark; Lynch, S.P.; Birbilis, Nick

doi:10.1115/1.860298_ch31

Cited by 2 publications

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“…For increasing levels of cathodic protection at constant ∆K, da/dN increased in the transition and upper plateau regimes as more hydrogen became available for adsorption at crack/void tips [11]. For coated specimens, longer rise-times in the transition regime (at −1050 mV(SCE)) were necessary to achieve the same crack growth rate as for bare specimens, showing that hydrogen diffusion from the sides (as well as crack tips) played a role [11]. …”

Section: Advanced Materials Research Vols 891-892mentioning

confidence: 99%

“…Before presenting and discussing new results concerning the effects of hold times at maximum/minimum loads and effects of fall time (for given rise times), it is worth briefly reviewing previous work [11,12] on the effects of rise time, ∆K, electrode potential, and coatings. 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.…”

Section: Corrosion-fatigue Crack-growth In Steels: Effects Of Rise Timentioning

confidence: 99%

“…Tests were carried out using the same experimental procedure as in previous work [11,12]. They were carried out under cathodic protection (−1050 mV(SCE)) in aqueous 3.5% NaCl on bare HY80 steel (~600 MPa strength) at a ∆K of 30 MPa√m and an R ratio of 0.5 for different waveforms, but with the same rise-times of 0.5 s and 2.5 s -corresponding to the lower plateau region and the transition region, respectively, of the da/dN versus rise-time plot for triangular waveforms with no hold times.…”

Section: Corrosion-fatigue Crack-growth Rates In Steels: Effects Of Hmentioning

confidence: 99%

“…For cleavage-like fractures and intergranular fractures associated with high localised strains, such as occur for corrosion fatigue of cathodically protected steels, an adsorption-induced dislocation-emission (AIDE)/nanovoid-coalescence mechanism probably predominates, with possible minor contributions from a (solute) hydrogen-enhanced localised plasticity (HELP) mechanism and a hydrogen-enhanced decohesion (HEDE) mechanism (Fig. 4) [11]. The similarity in the fracturesurface appearance for fatigue in hydrogen-bearing and in liquid-metal environments ( Fig.…”

mentioning

confidence: 99%

“…Detailed discussion of AIDE, HELP, HEDE, and other mechanisms, and evidence supporting them, can be found in recent reviews [2,6,10]. (b) Adsorption-induced dislocation-emission (AIDE) from crack tips due to hydrogen on the surface and at surface interstitial lattice sites (SILS) between the first and second atomic layers; (c) Hydrogen diffusion from crack tips and side surfaces to region ahead of cracks; (d) Hydrogenenhanced localised plasticity (HELP) facilitating movement of emitted dislocations away from cracks; (e) Hydrogen diffusion/dislocation transport to nanovoids, and condensation of H 2 in nanovoids; (f) AIDE facilitating nanovoid growth; and (g) Nucleation of nanovoids by hydrogeninduced decohesion at particle/matrix interfaces, at intersections of planar slip bands, or at hydrogen-stabilized vacancy clusters [11]. Molecular hydrogen (H 2 ) in nanovoids is deeply trapped, will not diffuse away during unloading and, for materials such as steels, will be available for redissociation/adsorption/AIDE during subsequent loading.…”

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Effects of Waveform and Cycle Period on Corrosion-Fatigue Crack Growth in Cathodically Protected High-Strength Steels

Knop

Birbilis

Lynch

2014

AMR

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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' ∆K 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 K max 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. Review of Mechanisms and Some General Aspects of Corrosion FatigueFor 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. In addition, the fracture path and fracturesurface appearance can be markedly different in inert and embrittling environments [1,2]. Fatigue in inert environments generally produces transgranular fractures exhibiting 'ductile' striations whereas intergranular or cleavage-like fractures with 'brittle' striations are observed after fatigue in embrittling environments (although brittle striations are sometimes difficult to resolve owing to their shallow profile and post-fracture corrosion/film formation (Fig. 1).Fatigue crack growth at intermediate ∆K, for both inert and embrittling environments, generally involves plastic blunting and crack growth during the rising load part of each stress cycle, with resharpening of the crack tip occurring by deformation just behind crack tips during unloading. 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. Nanovoid formation at intermediate ∆K levels is often overlooked because the presence of nano-scale dimples on fracture surfaces is clearly resolved only when 'state-of-the-art' high-resolution SEM is used [5] or, even better, when TEM is used to examine well-shadowed replicas under ideal conditions [6] (Fig. 3).The localisation of plasticity and increased crack growth rates in both aqueous and gaseous hydrogen environments for many materials (e.g. Fe, Mg, Ni, Ti) are genera...

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Section: Advanced Materials Research Vols 891-892mentioning

confidence: 99%

Section: Corrosion-fatigue Crack-growth In Steels: Effects Of Rise Timentioning

confidence: 99%

Section: Corrosion-fatigue Crack-growth Rates In Steels: Effects Of Hmentioning

confidence: 99%

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confidence: 99%

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