Stress corrosion cracking loading

If the casting is loaded or stressed externally in service, prior thermal stress treatment is of Htde value. Stress-corrosion cracking does not distinguish between residual or appHed stresses. 

Figure 15.19 shows various crack orientations that can occur in connection and attachment welds. Applied stresses from external loading of these components can add to the residual weld stresses, producing still higher stress loads. This can increase the susceptibility to stress-corrosion cracking and can affect orientation and location of crack paths. 

Figure 4-441. Rate of stress—corrosion crack propagation as a function of crack depth during tensile loading. (From Ref. [183].)...

Fig. 8.2S Long-time constant-load tests demonstrating a distinct stress-corrosion cracking threshold stress in the case of a straight l3Cr martensitic SS as opposed to a nickel-bearing SS...

Crooker T. W. and Hauser, J. A., II, A Literature Review on the Influence of Smaii-Ampiitude Cyciic Loading on Stress Corrosion Crack Growth in Aiioys, NRL Memorandum Report 5763, Naval Research Laboratory, Washington DC, April 3 (1988)... 

Fig. 8.54 Susceptibility of Ti-8Al-lMo-lV to stress-corrosion cracking in 3.5% NaCl under both tensile and torsional loading, corresponding to mode I and mode 111, respectively. The ordinate consists of the ratio of failure value in solution to failure value in air °...

In more recent work embrittlement in water vapour-saturated air and in various aqueous solutions has been systematically examined together with the influence of strain rate, alloy composition and loading mode, all in conjunction with various metallographic techniques. The general conclusion is that stress-corrosion crack propagation in aluminium alloys under open circuit conditions is mainly caused by hydrogen embrittlement, but that there is a component of the fracture process that is caused by dissolution. The relative importance of these two processes may well vary between alloys of different composition or even between specimens of an alloy that have been heat treated differently. 

Early use of the test was in providing data whereby the effects of such variables as alloy composition and structure or inhibitive additions to cracking environments could be compared, and also for promoting stress-corrosion cracking in combinations of alloy and environment that could not be caused to fail in the laboratory under conditions of constant load or... 

Certain types of corrosion are selective. Thus, corrosion cracking is observed primarily in the case of alloys and only when these are in contact with particular media. Corrosion is often enhanced by various extraneous effects. Stress corrosion cracking can occur under appreciable mechanical loads or internal stresses corrosion fatigue develops under prolonged cyclic mechanical loads (i.e., loads alternating in sign). 

Structural failure may occur when the overall structural cross-section cannot support the applied load or, when the critical flaw size ac is exceeded by preexisting discontinuity or by reaching the critical crack size through fatigue, stress corrosion cracking or creep mechanisms. Using fracture mechanics the stress at a crack tip can be calculated by a stress-intensity parameter K as,... 

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