Static fatigue

Static fatigue describes the fracture that happens at a stress level that is less than that required to cause an ordinary tensile fracture.[1] It is a manifestation of the possible adverse effects of the environment on the behaviour of materials. This term highlights the contribution of the environment to the crack propagation in materials that are under applied or residual stress, which leads to stress concentration and thus stress 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 can be 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 higher melting point metal, making the latter embrittled. It is often manifested by static fatigue. For example, as shown in Figure 1,[2] 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 "flawless", its static fatigue limit serves as a design parameter in a hostile environment.

Stress-Corrosion Cracking (SCC)
Main article: Stress corrosion cracking

SCC is the unexpected sudden failure of a stressed material exposed to an aqueous, corrosive fluid. Static fatigue is also found in this form of embrittlement, such as the moisture-enhanced static fatigue of glass,[3] hydrogen embrittlement,[4] embrittlement of some polymers in adverse environmental effects,[5] 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.[6] For different exposure times in air, the static fatigue is temperature dependent, indicating that kinetic considerations can explain the phenomenon. At low temperatures, the static fatigue is not obvious because of limited atomic mobility. In this case, below certain temperatures, embrittlement is not observed. However, at higher temperatures, the static fatigue is also not as pronounced, due to increased crack-tip viscous deformation or lesser surface adsorption of the embrittling species.

References
  1. Courtney, Thomas H. (2005-12-16). Mechanical Behavior of Materials: Second Edition. Waveland Press. ISBN 9781478608387.
  2. Rostoker, W. (1960). Embrittlement by Liquid Metals. New York: Reinhold.
  3. Wiederhorn, S. M.; Bolz, L. H. (1970-10-01). "Stress Corrosion and Static Fatigue of Glass". Journal of the American Ceramic Society. 53 (10): 543–548. doi:10.1111/j.1151-2916.1970.tb15962.x. ISSN 1551-2916.
  4. Lou than, M.R. "Hydrogen Embrittlement - Office of Scientific and Technical Information" (PDF).
  5. Brown, Norman; Parrish, Mark F. (1974). Bishay, Adli (ed.). Recent Advances in Science and Technology of Materials. Springer US. pp. 1–13. doi:10.1007/978-1-4613-4538-1_1. ISBN 9781461345404.
  6. Kingery, W.D. (1976). Introduction to ceramics. New York: Wiley. ISBN 978-0471478607.
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