von Mises stress in fatigue

[izax1]

I have a structure with combined mechanical and thernal loads. My thermal loads give raise to small von Mises stresses but large principal stresses. (almost hydrostatic) Will my large principal stresses give me fatigue problems, or can I forget it due to small von Mises stress?????

Thanks for any hints

 

[FEMSOL]

In fatigue analysis usually shear stress criteria(Not Von mises stress criteria ) is used. For shear stress criteria
maximum principal and minimum principal stresses are used. Therefore you need to look at the both of the principal stress , not to one of them(as long as you are not using maximum principal stress criteria , where maximum stress is the stress you need to use to go to the S-N curve)

 

[GregLocock]

But a truly hydrostatic situation would cause no shear loading, and provides no mechanism for crack growth or dislocation motion. I think. Cheers

Greg Locock

 

[PW]

Fatigue is only a problem if there are cyclic loads present. If the loads are constant static loads then fatigue will not be an issue.
If the loads do cycle then the principal stress ranges need to be determined and the fatigue life can then be calculated.
A good example is in appendix C of BS5500.

pw

 

[FEMSOL]

PW, YOU ARE RIGTH ABOUT FATIGUE PROBLEM. IF YOU DO NOT HAVE AN CYCLIC LOADING FATIGUE IS NOT AN ISSUE, CREEP MIGTH BE AN ISSUE THOUGH IF THE TEMPERATURE IS TOO HIGH.


GREGLOCOCK IS RIGTH ABOUT HYDROSTATIC LOADING CAUSE NO SHEAR
BUT IT DOES NOT MEAN NO MECHANISM FOR CRACK GROWTH. AS LONG AS YOU HAVE CYCLIC LOADING REGARDLESS OF YOU HAVE SHEAR OR NOT FATIGUE IS AN ISSUE.