Principal Stress, VonMises Stress , Fatigue

[harsunra]

1. I didnt understand why some people extracting Principal stress for aluminum material

which is subjected to dynamic loads(acceleration).Why not Von Mises?

2. Which has more advantages in predicting Fatigue Life and how?

3. How will you distinguish between tensile and compressive stress, when extracting

Principal & Von Mises.

[DanStro]

Sorry if this is stating things that you already know.

The difference (from the way I understand it) is under what conditions the analyst thinks the part will fail. I don't think it changes just because it is a fatigue study.

For ductile materials Von Mises is usually used (in school we used this for fatigue) and for brittle materials Principal stresses are normally used.

If you think the part will fail when the tensile (or compressive) stress goes above the materials yield stress then you would look at the Principal stresses. You would use Von Mises if you think the part will fail when the strain energy goes above the yield stress of the material.

I don't know a way of extracting the Principal stresses if you only have the resulting Von Mises.

HTH,
Dan

[john.phillip]

Choosing which stress vector will be applied is not only a matter of ductility-brittleness. In ductile materials fatigue, the crack's growth is governed by the direction of the principal stress and whether this principal stress is tensile or compressive. So, for example, in fatigue for weldings, you should verify which principal stress vector is more perpendicular to the path of the growing crack, and whether its fluctuation is tensile or compressive, as only the tensile stress cycle will make the crack grow.

Sometimes, the design rules do not allow you to apply von Mises stress (or the Principal stress components) for fatigue analysis, and you have to stick with one or the other. This is more for safety reasons than to be cost effective.

Compressive or tensile information is not kept after computing the von Mises equivalent stress, as it is always positive.

[stewartcs]

what is VonMises Stress?
i don't know the exact Definition of it.

The von Mises criterion states that failure occurs when the energy of distortion reaches the same energy for yield/failure in uniaxial tension.

Source: http://www.efunda.com/formulae/solid_mechanics/failure_criteria/failure_criteria_ductile.cfm

CS