## General Mechanical

Topics relate to Mechanical Enterprise, Motion, Additive Print and more

#### Fatigue Analysis — Which stress?

• albin.linderstam
Subscriber

Hello!

I'm doing a fatigue analysis, not the Fatigue Tool in Ansys, but a predefined fatigue method ("hand calculations") where we use stresses from the static analysis in Ansys as input. It is ductile materials (aluminium alloys and stainless steels). I'm wondering which stress that is the most correct to examine since it's not defined in the fatigue method.

I've read in other forums that Von Mises Stress be used for static analysis of ductile materials in general and that max principal stress can be used for brittle materials and for example only torque load. But regarding Fatigue Analysis it seems that the opinions differ, even though it seems that most recommend Max Principal Stress, for example:

http://fatiguetoolbox.org/fatlab/documentation/theory-reference "This is probably the most commonly used fatigue effective stress and is therefore used as default. It is not fool-proof for multiaxial loading though"

So the question is Which stress should be examined for a Fatigue Analysis? Von Mises, Max Principal Stress, other or the favorite one "it depends"?

• Rev0
Subscriber
Hallo Sorry but the best answer is: It depends!
First of all you should decide for a proper method, guideline or code for your application. These define if you need to evaluate nominal, structural or local stress. This decision has the highest influence to your FEA because it could change a simple beam structure up to a solid structure with modeled welds and nonlinear bolt connections.
Some (european) examples why it depends:
Pressure vessels prefere stress intensity
Bolts are evaluate with nominal stress (tension, bending, shear)
Welds need stresses in longitudinal, parallel and shear
So if you want to identify high stressed regions you might use von Mises or on the more conservative side the stress intensity. But if this applies to your "predefined fatigue method" I can't tell.
And please never forget to use the correct stress amplitude or range for fatigue!
Regards JB