## von Mises or principle stress for fatigue analysis?

## von Mises or principle stress for fatigue analysis?

(OP)

When the fatigue life is considered. Mean Stress and stress amplitude governs according to Goldman’s theory. Does the stress refer to von Mises or principle stress?

Thanks,

Thanks,

## RE: von Mises or principle stress for fatigue analysis?

Neither. Fatigue calculations are *usually* based on either PRINCIPAL stresses (not PRINCIPLE) or STRESS INTENSITY (first principal minus third principal), depending on what you're looking at, the code you're dealing with or potentially lots of other things.

Cheers,

-- drej --

## RE: von Mises or principle stress for fatigue analysis?

## RE: von Mises or principle stress for fatigue analysis?

## RE: von Mises or principle stress for fatigue analysis?

for -1<alpha<0 Signed Tresca is very conservative

Signed von Mises is conservative

for alpha=0 (uniaxial) both are O.K.

for 0<alpha<1 Signed Tresca is O.K.

Signed von Mises is non-conservative

for alpha=1 (equibiaxial) both are O.K.

Ref. MSC.Fatigue

http://www.geocities.com/fea_tek/

A.A.Y.

## RE: von Mises or principle stress for fatigue analysis?

i'd prefer max. principal

## RE: von Mises or principle stress for fatigue analysis?

feajob> A quick Q. How is alpha defined?

Thank u.

## RE: von Mises or principle stress for fatigue analysis?

## RE: von Mises or principle stress for fatigue analysis?

I had experience with Signed Von Mises, in my cases it predict (about) 5 percent longer life than Signed Tresca.

## RE: von Mises or principle stress for fatigue analysis?

But is O.K. when -1 < alpha < 0

AAY

## RE: von Mises or principle stress for fatigue analysis?

## RE: von Mises or principle stress for fatigue analysis?

i have an ANSYS rst-file from a PSD analysis and have to calculate the fatigue life of the component using nCode´s FE-Fatigue. Do you know if the program (FE-Fatigue) takes in account the sigma value, as the stresses from the Ansys calculation are 1 sigma values.

Thanks

## RE: von Mises or principle stress for fatigue analysis?

IMHO you can't say "a priori" which criteria you should follow, because any criteria encomprises "safe" stress states that are good for some applications and bad for others. In other terms, depending on the stres state a criteria can be over- or under-conservative. The best example is if you compare VonMises with Trescà-Guest, as it has been pointed out by Feajob.

In the USA the stress intensity is commonly used (SINT in Ansys), while in Europe the convention is to use VonMises (SEQV in Ansys), as you can see if you compare ASME norms to EN. ASME are known to be extremely conservative overall (i.e., on the other hand, extremely safe) because they use both conservative criteria (Trescà-Guest "everywhere"!) and conservative limits.

Just my two-pence thoughts...

Regards

## RE: von Mises or principle stress for fatigue analysis?

I imagine the von mises stress could be a concern for ductile materials during cyclical loading. For instance, 1100 soft aluminum will exhibit plastic deformation during fatigue loading at low frequencies. In this case, the Von Mises stress can be a concern. However, for brittle materials the principal stresses should be used since tensile forces propagate the fatigue crack.

Regards

modey

## RE: von Mises or principle stress for fatigue analysis?

Use worst principal at each loading condition to be conservative or look at stress ranges for individual stress components, and even biaxial effects if you are near to the line.

## RE: von Mises or principle stress for fatigue analysis?

I think that it depends on the biaxiality ratio in your specific problem. We cannot say that signed Von Mises is never correct for fatigue analysis. Please see my previous posts.

AAY

## RE: von Mises or principle stress for fatigue analysis?

## RE: von Mises or principle stress for fatigue analysis?

The failure criterion depends on many factors: material, loading conditions, and environment. For instance high frequncy cyclic loading rates can cause a fatigue crack to propagate brittlely. While low frequecies can cause a ductile crack to propagate. To sum this up when trying to design against fatigue crack propagation in FEA modeling the criterion you use depends on various factors.

Modey

## RE: von Mises or principle stress for fatigue analysis?

and if you're using non-linear fracture mechanics then you'll be using J-integrals ... no ?

particularly if we're splitting hairs (or heirs) about which stress to apply (max. principal or von mises).

the logic about using max. principal is that it "best" represents mode 1 (tension) crack growth. using von mises (as a failure criterion) includes shear into the equation and if there's much difference (between vM and maxP) then you're in a mixed mode crack scenario, and i really don't know where you'll get the data for this (crack geometry models, material crack growth, residual strength) ... but then maybe you auto guys are concerned with different aspects of FM (materials, geometry, design constraints, regulations) than i've experienced in aero.

## RE: von Mises or principle stress for fatigue analysis?

ZCP

www.phoenix-engineer.com

## RE: von Mises or principle stress for fatigue analysis?

## RE: von Mises or principle stress for fatigue analysis?

I think you're both right. as I said before, VonMises, MaxPrincipal, Stress Intensity, etc, are all based upon ASSUMPTIONS (as every analytical theory does, anyway), and neither can completely capture the behaviour of "real" materials over all the possible stress fields: there are cases where MP becomes totally unrealistic, others where VM will under-estimate the equivalent stress. Because the crucial point is this: we always want to reconduct a stress field in a point to a single "sigma" value in this point...

Finally, I think it's important to refer to the Norms that are valid for the country you work in (or that are requested by the customer, it depends): for ex., if you refer to ASME then you will be obliged to carry all the calculations with Stress Intensity, no matter if you disagree or not in some points...

Regards