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Material question
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Posted by: bandar

10/30/2003, 23:18:11

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What are the characteristics of an ideal engineering Material?
What are the difintions of design stress and safe working stress?






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Re: Material question
Re: Material question -- bandar Post Reply Top of thread Forum
Posted by: fvierheller

10/31/2003, 17:40:23

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The maximum von Mises stress criterion is based on the von Mises-Hencky theory, also known as the Shear-energy theory or the Maximum distortion energy theory.

In terms of the principal stresses s1, s2, and s3, the von Mises stress is expressed as:

svonMises  = {[(s1 - s2)2 + (s2 - s3)2 + (s1 - s3)2]/2}(1/2)

 

The theory states that a ductile material starts to yield at a location when the von Mises stress becomes equal to the stress limit. In most cases, the yield strength is used as the stress limit. However,  the ultimate tensile or set your own stress limit.

svonMisesslimit

 

Yield strength is a temperature-dependent property. This specified value of the yield strength should consider the temperature of the component. The factor of safety at a location is calculated from:

 

Factor of Safety (FOS) = slimit / svonMises

 

Large factors of safety in a region indicate that you can save material from that region.

  • A factor of safety less than 1.0 at a location indicates that the material at that location has failed.

  • A factor of safety of 1.0 at a location indicates that the material at that location has just started to fail.

  • A factor of safety larger than 1.0 at a location indicates that the material at that location is safe.

  • The material at a location will start to fail if you apply new loads equal to the current loads multiplied by the resulting factor of safety.







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