Comparing FEA vs beam theory in bones: von mises vs. max. principal
Comparing FEA vs beam theory in bones: von mises vs. max. principal
(OP)
Hi, I'm running some FEA models of long bones in Abaqus.
I've extracted the cross-sectional properties at midshaft, and I've calculated what the predicted bending stress would be according to Euler beam theory applying the same bending load as FEA.
When I extract stress values at midshaft from my FEA models, I get values ~20% higher for von mises, and ~30% for max. principal than predicted by beam theory.
I understand that Euler Bernoulli ignores the effects of shear, which could be quite significant, as these bones have a fairly low l/d.
My question is: is it typical for Von Mises stress to be lower than Max. Principal stress?
And wouldn't we predict Max. Principal Stress to be closer to the values predicted by beam theory than Von Mises?
Appreciate the help
Cbrassey
I've extracted the cross-sectional properties at midshaft, and I've calculated what the predicted bending stress would be according to Euler beam theory applying the same bending load as FEA.
When I extract stress values at midshaft from my FEA models, I get values ~20% higher for von mises, and ~30% for max. principal than predicted by beam theory.
I understand that Euler Bernoulli ignores the effects of shear, which could be quite significant, as these bones have a fairly low l/d.
My question is: is it typical for Von Mises stress to be lower than Max. Principal stress?
And wouldn't we predict Max. Principal Stress to be closer to the values predicted by beam theory than Von Mises?
Appreciate the help
Cbrassey






RE: Comparing FEA vs beam theory in bones: von mises vs. max. principal
One important thing to realize is that shear deformation will likely be low due to your l/d ratio. However, the shear stress may still be significant.... especially if this beam / bone is experiencing some torsional loading.
Lastly, the FEM model could be showing you some "warping" of the cross section that simple beam theory would not show.
RE: Comparing FEA vs beam theory in bones: von mises vs. max. principal
Yes, I would expect there to be significant shearing. I'm not surprised the FEA values are higher than the simple beam predictions. I'm just unsure why von mises stress is closer to the predictions than max. principal stress.
Cheers
RE: Comparing FEA vs beam theory in bones: von mises vs. max. principal
Also, if you're trying to validate your model I wouldn't look at just a single point. Instead, I'd look at the variation of stress along the length of the member. If your maximum and minimum values occur at the right locations, then that may validate the general model behavior even if in some locations your hand calculations are off by as much as 20%.... Remember FEM does tend to show localized stress risers that may be unrealistic or which are usually ignored for hand calcs.