Will a load below the fatigue limit cause breakage on a piece of immobile iron?

Replies

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  Ramani Aswath

  Not likely. However, stress corrosion cracking is possible in such cases.

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  Stress Corrosion Cracking

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  Amilcare Schenone

  Thanks, i’ve read the article but is not easy for me to understand every part since are in english. From what I read, everything should be kept in a non-aggressive environment and if the surface of the frame that holds the motor does not have scratches or rust spots, no cracks should form.  Some things are not clear to me though, if we think of a spring, if we leave it compressed for so long it will lose its ability to flex, it will not come back as before (i guess), isn't it the same for a frame that has to hold up an engine for years?  perhaps there is a load limit below which the piece will return as before also being compressed for years?  What happens to the metal microstructure in these cases?  Correct me if I'm wrong. 

  if the weight of the motor has been designed to keep the deformation of the underlying frame always in the "elastic" and not "plastic" area, how can it damage the metal in the absence of external environmental attacks?

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  Ramani Aswath

  Creep is another process that can cause problems in certain metals even when they are stressed well below their yield strength. In the case of iron alloys this cannot cause problem at  ambient temperatures.

  On the whole, there should be no problem in a non-corrosive environment.

  #-Link-Snipped-#

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  Amilcare Schenone

  Thanks, but i cannot open the file 

  Ok , opened 

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  Amilcare Schenone

  Thanks, I read the article, but it is not clear to me if it refers to metals in operation or with a static weight applied.  In the case of a car held at room temperature I believe that the only significant variable is the temperature.  If the dislocation of iron crystals happens simply with the passage of time, shouldn't we see all or almost all of the things made of iron breaking?

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  Ramani Aswath

  Creep happens at a higher temperature. Nothing much happens at ambient temperatures.

  Here is a description of the famous iron pillar at Delhi built in 402 AD, weighs about six tonnes and standing unaffected.

  Iron Pillar Of Delhi

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  Amilcare Schenone

  Interesting, i think is a bit different anyway, I believe that the column does not have an elastic deformation caused by a weight above it, can we say that it is "at rest" or perhaps the weight itself leads it to always be in the field of deformation?  I get lost in this step ...

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  Ramani Aswath

  Consider this. The bottom portion of the column has been under more than 5 tonnes compressive load since 402 AD. However, it is only 5 kg/sqcm, which is nothing for steel.

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  Amilcare Schenone

  the topic of stress relaxation, is applicable to every level of strain or even here as for fatigue, is there a limit below which there will be no relaxation over time?  the article also refers to low temperatures.

  I mean, ther’s a limit below which does stress relaxation not occur or is it stopped

• 

  Ramani Aswath

  At ambient temperatures and stress levels well below the yield stress there should be no relaxation in a truly elastic material.

  In some old churches in Europe it has been found that vertical glass panes have become thicker at the bottom than at the top by self weight. This is because glass is actually a super cooled liquid and flows with time.