Dynamic Creep & Fatigue: Defined & Tested

In summary, dynamic creep and fatigue are both forms of material failure caused by applied loads over time. Creep is characterized by permanent deformation and is tested by measuring elongation over time, while fatigue is characterized by crack propagation and is tested by subjecting the material to repeated cycles until failure occurs. Creep is typically seen in high temperature conditions, while fatigue can occur at any stress level. A fatigue test is appropriate when measuring changes in dimension over a large number of cycles, while a creep test is used for long-term deformation analysis.
  • #1
dental guy
6
0
What is the difference between dynamic creep and fatigue?
How are they tested?
 
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  • #2
Creep is permanent deformation over time; fatigue is crack propagation over time. Both occur due to applied loads and both can lead to failure. Creep is characterized by looking at the elongation of the sample; fatigue, by elongation of the crack.
 
  • #3
In a nutshell...

Creep is a constant stress situation that is below yield. It is extremely prevelant in higher temperature conditions. Creep is usually tested with a wire of a given size that has a constant load and the elongation is measured over time. Be careful not to mistake creep with stress relaxation.

Fatigue is due to cyclic/alternating loading that can be at any stress level. The loading is repeated until the number of cycles imparted causes failure. This is expressed in S-N curves for particular materials. A material is usually said to have an infinite fatigue endunce limit if it can withstand 10^6 cycles at the particular alternating stress level.

I'll have to look up the particular ASTM specs on each test.

Creep:
http://www.engineersedge.com/material_science/creep.htm
http://www.nuc.berkeley.edu/thyd/ne161/jlrhoads/creep.html

Fatigue:
http://www.sv.vt.edu/classes/MSE2094_NoteBook/97ClassProj/anal/kelly/fatigue.html
http://en.wikipedia.org/wiki/Fatigue_(material )
 
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  • #4
Thanks,
If we have to perform experiment on polymers with cyclic loading, then will it be categorised under fatigue and not creep?
 
  • #5
That would be a fatigue type of test. It depends on your outcome and what you are measuring to be able to call it a fatigue test. Are you going to take the samples to failure?
 
  • #6
Thanks fred,
We do not want to take it to failure. We are interested in change in dimension , if it occurs, say after 12-15 millions cyles.
 
  • #7
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1. What is dynamic creep?

Dynamic creep is the gradual deformation or strain that occurs in a material over time when it is subjected to repeated or cyclic loading. This type of creep is typically observed in materials that are under high stress or strain, and it can lead to eventual failure of the material if not properly managed.

2. How is dynamic creep different from static creep?

Static creep occurs when a material is subjected to a constant load over time, while dynamic creep occurs when the load is applied repeatedly or cyclically. Additionally, the rate of deformation in dynamic creep is typically higher than that of static creep, making it a more significant concern in certain applications.

3. What is fatigue in materials testing?

Fatigue is the weakening or failure of a material due to repeated or cyclic loading. This can occur even when the applied load is below the material's yield strength, and it is a common cause of failure in engineering structures and components.

4. How is fatigue testing performed?

Fatigue testing involves subjecting a material to repeated or cyclic loading at various stress levels until failure occurs. This allows researchers to determine the material's fatigue strength and how it will behave under different loading conditions. Different testing methods, such as axial or bending fatigue, can be used depending on the application.

5. What are the practical applications of dynamic creep and fatigue testing?

Dynamic creep and fatigue testing are essential in understanding the behavior of materials under repeated or cyclic loading, which is common in many engineering applications. This information is crucial in designing and selecting materials for structures and components that need to withstand high stress or strain over an extended period of time.

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