Stress-corrosion cracking critical factors

General description. Incomplete penetration describes the condition in which the weld fails to reach the bottom of the weld joint, resulting in a notch located at the root of the weld (Fig. 15.12). This critical defect can substantially reduce the intrinsic mechanical strength of the joint and can combine with environmental factors to produce corrosion fatigue (Chap. 10), stress-corrosion cracking (Chap. 9), or crevice corrosion (Chap. 2). 

Eliminate unfavorable environments. The presence of oxygen and other oxidizers is a critical factor in stress corrosion cracking. For example, the cracking of austenitic stainless steel in chloride solutions can be reduced or completely eliminated if oxygen is removed. 

A plate with width 300 mm and thickness 30 mm is made of steel with yield strength 900 MPa and fracture toughness 90 MPa. In a certain corrosive environment a critical stress intensity factor for stress corrosion cracking is found to be Kiscc = 70 MPa. 

STiscc MPa mm stress intensity factor, critical value at initiation of stress corrosion cracking (SCC)... 

The chapter builds on our critical reviews on Mg corrosion [1-4] and Mg SCC [5,6]. SCC [5-8] involves (1) a stress, (2) a susceptible alloy and (3) an environment. SCC is related to hydrogen embrittlement (HE). HE is SCC that is caused by hydrogen (H), which can be gaseous, can come from corrosion, or can be internal from prior processing. HE is often postulated as the SCC mechanism. SCC can be extremely dangerous. Under safe loading conditions, SCC causes slow crack growth. Fast fracture occurs when the crack reaches a critical size. SCC, for any alloy + environment combination, can be characterised by [7,8] the threshold stress, ctscc> threshold stress intensity factor, iscc> the stress corrosion crack velocity. 

KISCC Abbreviation for the critical value of the plane strain stress-intensity factor that will produce crack propagation by stress corrosion cracking of a given material in a given environment. 

Kjsq- Threshold stress-intensity factor for stress-corrosion cracking. The critical plane-strain stress intensity at the onset of stress-corrosion cracking under specified conditions. 

The stress corrosion resistance of maraging steel has been evaluated both by the use of smooth specimens loaded to some fraction of the yield strength and taking the time to failure as an indication of resistance, and by the fracture mechanics approach which involves the use of specimens with a pre-existing crack. Using the latter approach it is possible to obtain crack propagation rates at known stress intensity factors (K) and to determine critical stress intensity factors (A iscc) below which a crack will not propagate (see Section 8.9). 

To this point, it has been assumed that failure occurs when K =T (or G=R) but, in studies of fracture, it is sometimes found that crack growth can occur at lower values of or G. Thus, kinetic effects must be included in any general formalism. There are several mechanisms that can give rise to sub-critical crack growth, but most attention has been directed to stress corrosion. This behavior has been extensively studied in silicate glasses but it can also occur in many polycrystalline ceramics. Figure 8.72 shows a typical response of ceramics to stress corrosion, with crack velocity v plotted as a function of K (or G). At low values of K, there often appears to be a threshold value of the stress intensity factor below which... 

Fatigue results from the fact that cracks propagate and develop although the stress intensity factor (SIF) is less than the toughness (so-called stress corrosion for a review see Freiman, Wiederhorn and Mecholsky, 2009 Ciccotti, 2009). Accordingly, SIF increases progressively and may eventually reach the critical value for breakage. 

Exposure to seawater results in decrease in critical stress intensity factor and the susceptibility to SCC68 0.2% Fe improves the resistance to SCC presence of >5 wt percent of A1 increases the velocity of cracking Sn in the alloy decreases SCC resistance chloride bromide and iodide induce or accelerate SCC69 Occurs by trangranular cleavage of a-phase in which a-phase controls the crack propagation rate Intergranular corrosion due to formation of titanium methoxide... 

Under conditions of SCC the crack will grow at lower stress intensities than die critical value under non-corrosive conditions, i.e. we have a lower critical stress intensity factor, which we denote by Kiscc. When Equations (7.12) (with Kiscc instead of Kic) are satisfied, Kiscc is a very useful quantity, much more generally relevant than a threshold value of nominal stress. 

In the absence of corrosive agents, a crack will not propagate unless the stress intensity factor exceeds a critical value, ATjc, called the plain strain fracture toughness. 

A critical research gap in corrosion science is the absence of the corrosion equivalent for the stress intensity factor (K) that has been the mainstay of structural mechanics for the past several decades. The stress intensity factor was developed to predict the behavior of pre-existing flaws in structural materials and the eventual life of a component under conditions in which the flaw develops into stable cracks. The power of K is in the concept of similitude well-defined cracks and crack tips that are different in size or shape but possess the same K (as determined by geometry, loading, and the theories of linear-elastic fracture mechanics) will experience the same mechanical driving force for crack growth. Thus, similitude allows small, well-defined samples to be tested in the laboratory to determine the conditions of crack growth and fracture and the results to be quantitatively extended to more complicated real-world structures containing cracks. Virtually... 

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