Thin metal circular disk-FEM results reliability
Thin metal circular disk-FEM results reliability
(OP)






Hello,
I use ANSYS Workbench for mechanical stress analysis of a thin metal circular disk (Diameter=700mm, Thickness=10mm) with some air gaps and a hole in the center (please see pics) subjected to bending. The results that I get for the equivalent von-Misses stress show me that in a very small region near the air gaps' corners the stresses are much larger than the yield strength( 235MPa) I tried to change the mesh (size and element type), and the boundary conditions but the results do not change significantly. The disk has been constructed and works properly with no sign of failure.
So the problem is: Is my model reliable? Is it possible the constructed disk to encounter such big stresses in such small regions without breaking? Can I trust the results of Ansys? Or there is something wrong I have done when I constructed the mesh..?
Sorry for the bad structure of the post, I am new to the forum..
Thank you in advance!





RE: Thin metal circular disk-FEM results reliability
RE: Thin metal circular disk-FEM results reliability
Do you have any idea of which code is appropriate for such an application?
P.S. The disk is actually the rotor of an AFPM electric generator of a smal wind turbine.
RE: Thin metal circular disk-FEM results reliability
another day in paradise, or is paradise one day closer ?
RE: Thin metal circular disk-FEM results reliability
1)What is the meaning of ''fatigue'' analysis as long as I have a static load, not a dynamic one?
2) What do you mean exactly by ''artifice of the linear FEA''
RE: Thin metal circular disk-FEM results reliability
Comment for 1 only):
Your "service/fatigue" and your "ultimate (abuse)" loads are generally different in structures. For instance, you might have a 100lbs of fatigue loading for a structure - whereas your ultimate/abuse loading may be 250lbs.
If you already have already applied your fatigue loading in your analysis, then you might have a "surface finish factor" that you need to take into account. I'm not a fatigue expert but surface finish factors may affect the fatigue analysis results significantly. So, your actual physical model may not be failing due to this effect.
You may also want to check your surface finish factor while performing a "fatigue analysis" with your service/fatigue loading. (ie. 100 lbs I mentioned above - depending on your own structure's requirements)
Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer
RE: Thin metal circular disk-FEM results reliability
The loading on the structure doesn't necessarily have to be a dynamic load, but a repeated 'static' load that causes failure.
In general these codes use the calculated linear stress rather than stresses calculated using non-linear plastic analysis.
Surface finish does have an effect on fatigue damage as well as environment, and in other cases the proximity to welds.
RE: Thin metal circular disk-FEM results reliability
What would happen is the stress would redistribute over a larger area in those corners as they passed the yield strength.
RE: Thin metal circular disk-FEM results reliability
1) rerun with material non-linearity (elastic/plastic material stress/strain), or
2) if this is a fatigue analysis, maybe use the stress peak to infer a stress concentration factor, or
3) ignore it ... it is a result of the running a highly non-linear problem (a stress concentration) with a linear FEA.
You might investigate how much material is expected to yield ? Plot the stress results, then cut-off at yield stress, so you have truncated the stresses at yield. Now the area in the stress results curve above yield needs to get reflected in this truncated analysis. You can increase the stress near the edge of the truncated stress, so that you add the same area as was removed with the truncation ... clear as mud ?
another day in paradise, or is paradise one day closer ?
RE: Thin metal circular disk-FEM results reliability
1)I runed a non-linear analysis and I got a contour with the plastic strain one with the stresses. But how can I now decide if the plastic area is big enough to cause the failure of the disc? Is there an upper allowable limit for the plastic strain for example?
2)I have a static constant load, neither a dynamic nor a repeated one. So I think there is no meaning of fatigue analysis.
3) That's what I will probably do at the end.
Thnx for your answers!
RE: Thin metal circular disk-FEM results reliability
Of course is failure is related to some sort of geometric change, then you would also have to decide if the permanent deformation caused by the plastic strain produces operational issues for this particular item.