Creep and Static Fatigue: What's the Distinction?

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Discussion Overview

The discussion centers on the distinction between creep and static fatigue, particularly in the context of materials such as ceramics and glasses. Participants explore definitions, mechanisms of failure, and the conditions under which these phenomena occur.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants question whether creep and static fatigue are the same phenomenon or if they are distinct, noting that both terms are used in different contexts.
  • One participant asserts that static fatigue is specific to ceramics and glasses, highlighting that its failure mechanism is influenced by the presence of water vapor and is akin to stress corrosion cracking in metals.
  • Another participant clarifies that static fatigue in ceramics refers to cracking due to residual or constant stresses, suggesting that plastic deformation is not a factor in this context.
  • It is noted that glasses and ceramics do exhibit creep, albeit at very slow rates, with historical examples of old glass showing a ripple effect attributed to this slow flow over time.
  • A participant mentions that concrete also exhibits creep under compression, indicating that this behavior is not limited to ceramics and glasses.
  • One participant challenges the explanation of the ripple effect in old glass, suggesting it is more related to manufacturing processes rather than creep.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and mechanisms of creep and static fatigue, indicating that multiple competing perspectives remain unresolved.

Contextual Notes

The discussion highlights the dependence on specific material properties and environmental conditions, such as the presence of water vapor, which may influence the failure mechanisms being discussed.

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What is the difference between these two phenomena? Or are they the same thing?

I have heard both terms used in different classes, but they seem to be the same or similar.
 
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Static fatigue is a term limited to ceramics and glasses. The loading is similar to a standard creep test i.e. static tensile load, but the failure is usually dependent on water vapor being present in the air.

According to my trusty materials handbook, the mechanism for failure is similar to stress corrosion cracking of metals. It also states that this mode of failure will not occur in a vacuum.
 
Ok, then static fatigue in ceramics is actually referring to failure by cracking due to residual/constant stresses. Then plastic deformation is not a consideration here.

Do glasses and ceramics exhibit creep at all?
 
Glasses and ceramics (e.g. concrete) do creep. The creep rate is very, very slow.

If one looks at old glass from the 1700's and 1800's, one may notice a ripple effect. This is due to very slow flow - mm/decades or mm/centuries.

Concrete under compression will flow.
 
Fatigue also occurs in metals such as aluminium. Cracking is mostly caused by the loading and unloading of forces.
 
actually it the rate is much slower, old glass ripples is almost always due to manufacture/nonsupercooled fluidity related phenomena
 

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