Von Mises Stress and Endurance Limit 2

[BrianE22]

Back when I was in school no one talked about Von Mises. I'd like to learn more about it. Whereas for simple bending or axial loading you can start out with the Endurance limit = .5 * Ultimate stress, is there a way to use Von Mises stress to predict endurance limit? Say, Endurance limit = .5 * Ultimate stress * ?.

I'm not looking for in-depth, "aircraft design" type design precedures. Just wondering if there was a good starting point for a "not super accurate" prediction of fatigue life.

 

[SwinnyGG]

For any stress calculation, there are tradeoffs between all of the common failure criterion methods. Depending on the particular stress state you are analyzing, Tresca/Von Mises/Principal stress methods will have differing outcomes and differing levels of conservative or non-conservatism.

In short, it's not a simple answer, but in general using principal stresses is the most conservative (i.e. safest) method. If you're not trying to design things on the razor's edge (as the do in aircraft where grams count) then Von Mises or Tresca don't really need to be applied.

There's a couple of good threads on this topic already, here's one:

http://www.eng-tips.com/viewthread.cfm?qid=123897

 

[TGS4]

Just to make sure that you understand, von Mises is a way of converting a multi-axial stress state (tensor stress state) into a single number. From the perspective of crack initiation (which is essentially what "fatigue" is), there re a number of ways to correlate that stress tensor to a single number which correlates to a uniaxial or rotating bending fatigue tests.

Once you have initiated a crack, however, you enter the realm of fracture mechanics. The crack will grow normal to the orientation of the maximum principal stress, and can change (abruptly, occasionally) as the crack grows if the orientation of the maximum principal stress changes. This has no relation to any invariant (von Mises, Tresca, etc).

 

[BrianE22]

Thanks guys - I've been playing around with it in our FEA program and am feeling more comfortable with it. Good link above and several good points in your posts.