• Source: Static fatigue

Static fatigue describes how prolonged and constant cyclic stress weakens a material until it breaks apart, which is called failure. Static fatigue is sometimes called "delayed fracture". The damage occurs at a lower stress level than the stress level needed to create a normal tensile fracture. Static fatigue can involve plastic deformation or crack growth. For example, repeated stress can create small cracks that grow and eventually break apart plastic, glass, or ceramic materials. The material reaches failure faster by increasing cyclic stress. Static fatigue varies with material type and environmental factors, such as moisture presence and temperature.




Applications


Static fatigue tests can estimate a material’s lifetime and hardness to different environments. However, measuring a static fatigue limit can take a long time, and it is hard to measure a material’s true static fatigue limit with full certainty.




Typical occurrence






= Stress corrosion cracking

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Stress corrosion cracking (SCC) happens when a stressed material is in a corrosive (chemically destructive) environment. One example of SSC embrittlement is when moisture increases static fatigue effects in glass. SCC is also seen in hydrogen embrittlement, embrittlement of some polymers, and more.




= Plastic Deformation (Plastic Flow)

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Plastic deformation happens when stresses flatten, bend, or twist a material until it cannot return to its original shape. This can create cracks in the material and decrease its lifetime ultimately leading to the deformation of plastic.




Examples of Static Fatigue and Stresses on Materials


Plastic pipes under water or other fluids experience hydrodynamic forces that can result in fatigue. The pipes reach failure sooner as temperatures and exposure to aggressive substances increase. For static fatigue tests, rotating machines apply weight on the material under study causing it to bend in different directions, which weakens the material overtime.




References

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