Static FEA Shaft Analysis: ignore stress riser effects?
Static FEA Shaft Analysis: ignore stress riser effects?
(OP)
Hi All--this is my first time posting on this site. I need some advice on shaft design & interpretation of FEA results.
I'm working an linear static FEA model of a small wind turbine shaft (AIS 1026 steel) using COSMOS Adv. Professional 2008. The shaft is subject to a bending moment, axial & torsional loading--see attached photo. It also has a couple of step featues, but the small step feature (45.9 mm to 45.01 mm with a 1 mm radius fillet) is causing me trouble. I am using 3 mm solid elements with much finer mesh around the fillets (I have varied the mesh in this area from 1 mm to .1 mm). The mesh aspect ratio is quite good, as well.
This feature is causing very high stresses in the area of the 1mm fillet. I have performed a series of successively finer mesh scenarios around this feature, and the stress continues to rise. The stresses in the rest of the shaft are well below yielding, though. My first inclination was to do a quick hand calculation to check the results, but all of the S.C.F. formulae & charts for shafts with a filleted shoulder do not apply to such a small step.
It seems to me that the mesh is well-refined in the area of the fillet. My questions are as follows: first, how should I interpret the FE model? Refine more? Refine less? Second, any thoughts on the proper K_t for hand calc verification?
Thanks very much for your help!
I'm working an linear static FEA model of a small wind turbine shaft (AIS 1026 steel) using COSMOS Adv. Professional 2008. The shaft is subject to a bending moment, axial & torsional loading--see attached photo. It also has a couple of step featues, but the small step feature (45.9 mm to 45.01 mm with a 1 mm radius fillet) is causing me trouble. I am using 3 mm solid elements with much finer mesh around the fillets (I have varied the mesh in this area from 1 mm to .1 mm). The mesh aspect ratio is quite good, as well.
This feature is causing very high stresses in the area of the 1mm fillet. I have performed a series of successively finer mesh scenarios around this feature, and the stress continues to rise. The stresses in the rest of the shaft are well below yielding, though. My first inclination was to do a quick hand calculation to check the results, but all of the S.C.F. formulae & charts for shafts with a filleted shoulder do not apply to such a small step.
It seems to me that the mesh is well-refined in the area of the fillet. My questions are as follows: first, how should I interpret the FE model? Refine more? Refine less? Second, any thoughts on the proper K_t for hand calc verification?
Thanks very much for your help!





RE: Static FEA Shaft Analysis: ignore stress riser effects?
Or just test the shaft and correlate the results to the FE model.
RE: Static FEA Shaft Analysis: ignore stress riser effects?
Ed.R.
RE: Static FEA Shaft Analysis: ignore stress riser effects?
corus
RE: Static FEA Shaft Analysis: ignore stress riser effects?
For reference, I didn't want to change the fillet radius value, since this part is being tested for increased loads "as-is." In other words, this shaft is already in production and we'd like to use it with increased loads w/o modification. I may end up changing a few values to ensure my model is reasonable...
I'm in the process of simplifying the model even further so that I can actually use a real SCF value to compare the values. Corus, thanks for the elements versus degrees recommendation....I may also modify the way I applied my loads & restraints.
RE: Static FEA Shaft Analysis: ignore stress riser effects?
Peterson's book is the best source for SCF info. If your model predicts close to a value that you get from there I'd say you have fairly good convergence. That being the case your FS < 1 which might present a bit of a problem.
Good luck.
RE: Static FEA Shaft Analysis: ignore stress riser effects?
corus
RE: Static FEA Shaft Analysis: ignore stress riser effects?
RE: Static FEA Shaft Analysis: ignore stress riser effects?
My goals for the analysis of this shaft were twofold: first, I wanted do a simple linear static analysis to ensure the shaft could handle the new loads. Second, if the static analysis looked OK, I would proceed to fatigue analysis, which is the more relevant one for rotating shafts.
The show stopper occurred when the linear static showed the FS < 1. I want to be sure that those peak stresses were correct, and not just an artifact of improper FE modeling. Is the localized yielding (plasticity) around the small fillet indeed a show stopper (see photo)? As rb1957 mentioned, there are only a few areas of apparent yielding. Perhaps I should move on to fatigue analysis?
RE: Static FEA Shaft Analysis: ignore stress riser effects?
can you section the model (to see the stress distribution thru the thickness) ?
if your original post is still good (FoSmin = 0.1) i'd be nervous about the fatigue results.
you mentioned this to allow for increased loads ... would you be able to spon peen the fillet ? this'll help your fatigue life significantly.
RE: Static FEA Shaft Analysis: ignore stress riser effects?
Did you mean shot peen the surface? Yes....that would be an excellent option for increasing the fatigue life.
Thanks!