Applying "Size Factor" to FEA Results for Fatigue Analysis

[toothless48]

Hello all,

I have never been able to find a satisfying answer for this question: when determining allowable fatigue stress ranges with Marin factors (e.g. per Shigley), how do you apply the "size factor" to FEA results?

The purpose of the size factor seems to be to take into account material imperfections, which increase with the size of your stressed area. However, in FEA we do not deal with beams in simple bending. Hotspots take complex shapes, and the area cannot be easily or precisely measured. In addition, with complex shapes and loading, hotspots occupy 3D volumes of material, so which dimension would be used for a size factor area?

Does anybody have experience applying Marin factors to FEA for fatigue analysis? Are there better methods that avoid these complications?

Thanks a lot
Mike

 

[realtime2]

Try the explanations offered in N.Dowing's book
"Mechanical Behavior of Materials" 3rd edition
pg 469. Section 10.2.2: "Process Zone Size and
Weakest Link Effects" ISBN 0-13-186312-6.
Its a topic that is difficult to explain in a few
sentences here.
In my own view, unless you have some sort of casting
where the probability of encountering larger flaws goes up with
size, it is probably not a factor that is too important.
Other influencing factors such as accuracy of loads,
FEA stress, material heat-treat. have larger potential
for errors.

 

[toothless48]

Thank you for taking the time to respond! I will certainly take a look at that text.