Subcritical crack propagation

The phenomena of crack initiation and subcritical propagation in structural materials due to the conjoint actions of stress, material microstructure, and environment have been recognized for many years, and the dependencies and mechanisms have been extensively researched. This is especially the case for alloys in relatively low-temperature (<150°C) environments containing high anionic concentrations of, for instance, chlorides, phosphates. 

Elastic energy release due to subcritical crack growth is one recognized source of structure-related AE within its acknowledged lunitations, AEBIL provides a viable means of early on-line deteetion and localization of stable crack propagation. 

Typical subcritical stress-corrosion crack propagation rate ver- sus stress intensity. Source Ref 115... 

One of these failure modes is time-dependent crack propagation under static stress. Structures exposed to hydrogen gas can experience crack propagation at static stresses that are less than the maximum allowable stress for structures in inert environments. This subcritical crack propagation... 

In constant strain experiments, decreases as a function of time, because the effective stress decreases. The crack stops when the stress intensity reaehes the value of iscc (Figure 11.15(b)). A single experiment permits one to measure fiscc nd the subcritical crack propagation rate. On the other hand, a series of tests is needed if is to be determined. 

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