Dynamic Creep & Fatigue: Defined & Tested

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    Creep Fatigue
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Discussion Overview

The discussion focuses on the differences between dynamic creep and fatigue in materials, including their definitions, characteristics, and testing methods. It explores the implications of these phenomena in the context of experimental testing, particularly for polymers under cyclic loading.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Some participants define creep as permanent deformation over time due to constant stress below yield, while fatigue is characterized by crack propagation due to cyclic loading.
  • It is noted that creep is prevalent at higher temperatures and is tested by applying a constant load to a sample and measuring elongation over time.
  • Fatigue is described as resulting from repeated loading until failure occurs, typically represented in S-N curves, with an infinite fatigue endurance limit defined for materials that can withstand a certain number of cycles without failure.
  • One participant questions whether an experiment on polymers with cyclic loading would be categorized as fatigue or creep, suggesting it would be a fatigue test if it involves cyclic loading.
  • Another participant clarifies that their interest lies in measuring dimensional changes after a significant number of cycles, without taking the samples to failure.

Areas of Agreement / Disagreement

Participants generally agree on the definitions and characteristics of creep and fatigue, but there is some uncertainty regarding the classification of tests based on the specific conditions and outcomes being measured. The discussion remains unresolved regarding the implications of testing parameters on categorization.

Contextual Notes

The discussion does not resolve the specific ASTM specifications for testing creep and fatigue, nor does it clarify the assumptions underlying the definitions provided. There is also a lack of consensus on the implications of testing parameters for categorizing experiments.

Who May Find This Useful

This discussion may be useful for materials scientists, engineers, and researchers interested in the mechanical properties of materials, particularly in relation to creep and fatigue testing methodologies.

dental guy
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What is the difference between dynamic creep and fatigue?
How are they tested?
 
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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.
 
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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Thanks,
If we have to perform experiment on polymers with cyclic loading, then will it be categorised under fatigue and not creep?
 
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?
 
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.
 
http://sfbay.craigslist.org/forums/?ID=145261281
 

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