Vos Mises vs Max/Min Stress 1

[Henry26]

Hello guys,

Please, could somebody help with a doubt?

I don't understand exactly in what situation I really should use Von Mises as criterium of evaluation of stresses.
I found some studies that used von mises because the material is ductile, but I don't understand the reason.
I my mind I should use max and min stress and just use von mises for an overview of stress.

Somebody knows the reason of using von mises as criterium to evaluate stress in some cases?

Thank you,
Henry

 

[FEA way]

It depends which failure criterion you want to use. Usually von Mises criterion is used and, as you said, it applies to ductile metals. For brittle materials maximum (principal) stress is used. This type of stress is also needed for fatigue analyses.

 

[Agent666]

It's because min and max stresses compared to a material strength limit do not capture the critical combination of principal stresses.

For example von mises has a lower failure limit when one of the principal stresses is in tension and the other in compression. Compared with both principal stresses of the same sign. This can be seen quite easily in the elliptical shape of the failure surface.

 

[rb1957]

Note von Mises is strictly a yield criterion, not a failure criterion. However it is an awfully convenient expression for combining multiple principal stresses.

another day in paradise, or is paradise one day closer ?

 

[Trenno]

I stumbled across this video recently that may be of use to you:

Understanding Failure Theories (Tresca, von Mises etc...)