Static fatigue

Static fatigue describes the fracture happening at a stress level less than the value required to cause ordinary tensile fracture. It is a manifestation of the possibly adverse effect of environment on materials behavior. This term highlights the contribution of environment to the crack propagation in materials under applied or residual stress, which leads to stress concentration and thus fatigue. It is also called “delayed fracture”, referring to the long period of time the crack takes to grow large enough to cause spontaneous failure. It is a form of material embrittlement, and occurs in various materials and diverse environments.
Typical occurrence.
As a common phenomenon, static fatigue is manifested in many kinds of embrittlement, of which the mechanisms are closely related to the nucleation and growth of cracks. Two typical situations are listed here for reference.
Metal Embrittlement (ME).
ME happens when a low-melting-point metal is placed in contact with a metal with higher melting point, so the latter is embrittled. It is often manifested by static fatigue. For example, as shown in Figure 1, in a test of the static fatigue of a 2024 aluminum coated with mercury, the alloy is subject to a stress level less than the value causing plastic flow, and the time it takes to fracture is measured. Usually a stress called static fatigue limit is present, representing the boundary below which the material does not fracture, no matter how long the test duration is. In this scenario, the static fatigue often depends on the presence of initial flaws. Also, if the material is “flaw-free”, its static fatigue limit serves as a design parameter in a hostile environment.
Stress-Corrosion Cracking (SCC).
SCC is the unexpected sudden failure of a stressed material exposed to an aqueous, corrosive fluid. Static fatigue is also found in this embrittlement form, such as the moisture-enhanced static fatigue of glass, hydrogen embrittlement, embrittlement of some polymers in adverse environmental effect, etc. The static fatigue is also manifested similarly to the one described in ME. The static stress where a material failure can be prevented is reduced by adverse environmental effects. Furthermore, the static fatigue limit is observed.
Kinetics.
The strength vs. temperature plot of glass exposed in air is displayed in Figure 2. For different exposure times in air, the static fatigue is temperature dependence, indicating kinetic considerations can be taken to explain the phenomenon. At low temperature, the static fatigue is not obvious because of limited atomic mobility. In this case, below certain temperature, embrittlement is not observed. However, at higher temperature, the static fatigue is also not as pronounced, due to increased crack-tip viscous deformation or lesser surface adsorption of the embrittling species.