Stress cracking corrosion elasticity

Erlings JG, de Groot HW, Nauta J. The effect of slow plastic and elastic straining on sidphide stress cracking and hydrogen embrittlement of 3.5% Ni steel and API 5L X60 pipeline steel. Corrosion Science 1987 27 1153-67. 

Erlings, J. G., de Groot, H. W., and Nauta J., The Effect of Slow Plastic and Elastic Straining on Sulphide Stress Cracking and Hydrogen Embrittlement of 3.5% Ni Steel and API 5L X60 Pipeline Steel, Corrosion Science, 27 1153-1167 (1987). 

Most ceramics (as we have seen) contain flaws holes and cracks left by processing, cracks caused by thermal stress, corrosion or abrasion. Even if there are no cracks to start with, differences in elastic moduli between phases will nucleate cracks on loading. And most of these flaws have a size which is roughly that of the powder particles from which the ceramic was made. If the flaw size can be reduced, or if samples containing abnormally large flaws can be detected and rejected, the mean strength of the ceramic component is increased. 

Under development are intelligent vehicles for crack detection. An elastic-wave version (developed by British Gas and the Harwell Laboratory) is currently being evaluated in a test-loop. This vehicle has successfully detected stress-corrosion cracks in the test-loop. The Gas Research Institute (USA) is sponsoring development work with intelligent vehicles at the Battelle Columbus Division (Ohio). Facilities for testing vehicles were commissioned in 1991... 

Constant-load SCC tests have been shown to be more severe than constant-deflection tests. Under a constant load, stress increases as the cross-section is reduced by cracking or corrosion. However, this condition produces decreasing stress when deflection is fixed. It has been suggested that SCC threshold stress is associated with the onset of plastic deformation, that is, the elastic limit of the alloy. The elastic limit is difficult to measure unambiguously, however, the stress at which 0.2% plastic deformation occurs is generally used. 

In other words, a K". Equation (17) was derived by the elastic stress field, Eqn. (3), and the fracture criteria, Eqns. (13) and (14). This analogy between a and Kj is similar to the ambient temperature stress-corrosion cracking situation wherein the elastic -field governs the crack tip stresses. 

Sensitivity to stress corrosion cracking can be determined with elastically or plastically prestressed specimens, snch as those of the Jones, U-bent, and C-ring types. Specimens sensitive to stress corrosion cracking can also be produced by introducing heavy tensile stresses into the surface by means of stress-indncing grinding. 

Various efforts have been made to estimate the amount of corrosion that will cause spalling. It has been shown that cracking is induced by less than 0.1 mm of steel section loss, but in some cases far less that 0.1 mm have been needed. This is a function of the way that the oxide is distributed (i.e. how efficiently it stresses the concrete), the ability of the concrete to accommodate the stress (by creep, plastic or elastic deformation) and the geometry of rebar distribution that may encourage crack propagation by concentrating, stresses, etc., for example, in a closely spaced series of bars near the surface, or at a corner where there is less confinement of the concrete to restrain cracking. 

Stress-corrosion cracking of steel was first encountered in a practical way in riveted steam boilers. Stresses at rivets always exceed the elastic Unfit, and boiler waters are normally treated with alkalies to minimize corrosion. Crevices between rivets and boiler plate allow boiler water to concentrate, until the concentration of alkali suffices to induce S.C.C., sometimes accompanied by explosion of the boiler. Because alkalies were recognized as one of the causes, failures of this kind were first called caustic embrittlement. With the advent of welded boilers and with improved boiler-water treatment, S.C.C. of boilers has become less common. Its occurrence has not been eliminated entirely, however, because significant stresses, for example, may be established at welded sections of boilers or in tanks used for storing concentrated alkalies. 

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