Applied Stress Affecting the Environmentally Assisted Cracking
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IT is recognized that a tensile stress is necessary for SCC to occur.[1] In the absence of any external or internal stress (like residual stress), metals can undergo general corrosion. For a given alloy/microstructure, environmentally assisted cracking (EAC) behavior under an applied tensile stress is commonly discussed in general terms as due to (a) Active path dissolution (APD or AD) involving dissolution of metallic elements, sometimes preferentially at slip bands or at grain boundaries (GBs) or of specific precipitates at the boundaries[2]; (b) Liquid metal embrittlement (LME) involving a liquid metal penetrating into GB or dissolving the host metal to induce embrittlement[3]; (c) Hydrogen-assisted cracking (HAC) involving hydrogen-enhanced decohesion, hydrogen-enhanced localized plasticity (HELP), adsorption-induced dislocation emission or reduction in cohesive energy, particularly, as in Fe-, Ti-, Ni-, and Al-based alloys[4]; In the literature, there has been greater emphasis on the kinetics of crack growth than on the initiation thresholds, even though damage involves both the initiation of a crack and its growth. Not much experimental data are available on the initiation/incubation times for SCC. Incubation time appears to depend on the tensile or compressive stress at the crack tip. In
A.K. VASUDEVAN, Scientific Officer, is with the Office of Naval Research, Code-332, 875, North Randolph Street, Arlington, VA 22203. Contact e-mail: [email protected] Manuscript submitted October 9, 2012. Article published online January 15, 2013 1254—VOLUME 44A, MARCH 2013
particular, under uniaxial tension or compression, the initiation time varies significantly with alloy–environment system. It is observed that SCC initiation times under tensile stress can be one to two orders of magnitude shorter compared to that under compression load. For a crack to initiate under compression, there has to be a local tensile stress at some point away from the notch. Even if this crack forms, it will not grow until the crack-tip driving force becomes tensile. The current article examines the role of applied stress on the EAC crack initiation time in terms of applied stress intensity factor, Kapp, and the factors controlling the crack initiation time.
II.
EARLY EXPERIMENTAL OBSERVATIONS
Early investigators[5] had observed that SCC in stainless steels could occur also under compression load, using the horseshoe-type specimens. They observed that corrosion occurs on the compressive side of a bent sample, albeit at a lower rate than on the tensile side. Since then, there have been several qualitative experimental observations, using bent sheet samples, showing that SCC can occur under compressive stress in 304 stainless steels. Similar results were observed in other steels. For example, SCC occurs under compression in boiling 42 pct MgCl2 solution[6] in 1015 mild steel, and boiling nitrate solution in 1017 mild steels.[7] It was observed that the time to nucleate a SCC crack under far field compression was about one to two orders of magni
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