Stress corrosion cracking propagation models

By means of experimental data on welded pipes made from SAE 304 (1.4301) a model for intergranular stress corrosion cracking in high purity high temperature water was derived. Crack initiation- as well as crack propagation-phases were treated. 

SCC occurs when the mechanical stress is high enough. According to several SCC models (Sec. 5.5), the macroscopic stress level is not necessarily the mechanical parameter that controls the cracking process. However, from an engineering point of view, it is accepted that the stress corrosion cracks initiate when the stress exceeds a threshold Oj, and propagate when the stress intensity factor is in excess of a threshold i-scc-... 

Crack propagation models are then presented for both short and long cracks. Anodic dissolution and hydrogen effects at the crack tip are analyzed. Finally, the possible relation between stress corrosion cracking and CF is shown for crack initiation and crack propagation processes. 

Figure 4.45 Schematic of film rupture model (a) illustrates the main feature of the model—the protective film is ruptured at the crack tip by localized slip, permitting propagation by anodic dissolution. The crack tip is depicted in greater details in (b) which illustrates the view that crack advance results from a number of independent film rupture and transient dissolution events. (From Staehle, R.W. (1979). Stress Corrosion Cracking in Alloys, NATO, p. 223)...

Stress corrosion by water at the crack tip is the likely mechanism responsible for crack propagation in ceramics. In this respect, it is interesting to develop briefly the model proposed by Lawn [16] for single crystals or glasses. This model is based on a thermally activated reaction of water and ceramic to form complex. For low crack velocity (stage I), V is given by ... 

As approaches the crack can also propagate by hydrogen embrittlement processes during the higher load parts of the stress cycle. This forms the basis of various models which have been developed to describe corrosion fatigue, probably the best-known of which are the superposition models due to Wei"". In its most recent version this model takes the form ... 

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