Practical test methods for validating FEA and classical design calculations
Practical test methods for validating FEA and classical design calculations
(OP)
We have what is probably a common problem: we have a line of components that have been designed according to classical stress calculations (stress riser K factors, von mises calcs, etc). When we get beyond that envelope we use FEA. The problem is that when we use both methods to design a component that valid for both methods, we get a different result. The FEA results in a heavier/thicker component, presumably because the stress rises in corners and transitions that the classical stress checks ignore. Our FEA analysis have never been calibrated by real-world failure testing - the feedback is only real-world lack of failures.
Our components are designed for "infinite" life and in practice operate for decades in continuous service - therefore the classical methods are adequate and the FEA is conservative. The materials are ductile steel and stainless steels. Our stress limits are much less than yield point, to give you an idea.
How might we approach the question of making the two analysis methods more equivalent?
I was thinking maybe we can make test samples and (over)load them to the point of reaching yield? Then adjust the methods to ensure they calculate yield effectively, and from there, lower stress levels should also be consistent.
Thanks,
David
Our components are designed for "infinite" life and in practice operate for decades in continuous service - therefore the classical methods are adequate and the FEA is conservative. The materials are ductile steel and stainless steels. Our stress limits are much less than yield point, to give you an idea.
How might we approach the question of making the two analysis methods more equivalent?
I was thinking maybe we can make test samples and (over)load them to the point of reaching yield? Then adjust the methods to ensure they calculate yield effectively, and from there, lower stress levels should also be consistent.
Thanks,
David





RE: Practical test methods for validating FEA and classical design calculations
Have you done a mesh study? To be clear, have you doubled and tripled the mesh densities and compared the results in identical locations between all three models (the third being your routine mesh density). I was surprised by the results the first time I did one.
RE: Practical test methods for validating FEA and classical design calculations
I guess my point is we need to figure a way to set up stress limits or FEA practices that agree with the highly successful components that have been designed with classical methods. Perhaps we should establish a stress limit for areas where the classical calcs apply, and a higher stress limit for local stress concentrations. There is plenty of room between our normal stress limit and yield to do that.
RE: Practical test methods for validating FEA and classical design calculations
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Greg Locock
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RE: Practical test methods for validating FEA and classical design calculations
RE: Practical test methods for validating FEA and classical design calculations
I believe the problem is that we apply the same stress limits for FEA as we do for classical stress calculations. Are you suggesting that a fully-detailed FEA justifies a higher stress limit?
David
RE: Practical test methods for validating FEA and classical design calculations
Refine the mesh there, use the actual stress strain curve, nonlinear static analysis.
If you can get mesh convergence within 1% to 3% of the previous value, then you will know the realistic zone of plastic strain there. If the plastic strain area is really localized and small, then you might use the stress levels 1 to 2 elements away as your amplitude to compare against your HCF limit.
Just my thoughts.
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RE: Practical test methods for validating FEA and classical design calculations
RE: Practical test methods for validating FEA and classical design calculations
well, you've got a classically designed component that has worked well (not failed). therefore an FEA of this component, whatever it shows, is acceptable; no?
of course, you started your problem statement with "When we get beyond that envelope (supported by classical calcs) we use FEA", so how do you compare the FEA of a new part with this FEA of an existing part ? maybe ...
1) the highest stress on the new component is no higher than that demonstrated on the existing part ? (but this might bring mesh sensitivity into the discussion)
2) compare the amount of the structures at "elevated" stresses ? (how localized are the stress peaks)
3) maybe stick to doing classical calcs ? but you'll probably bump into customers wanting FEA !
another day in paradise, or is paradise one day closer ?
RE: Practical test methods for validating FEA and classical design calculations
I had considered performing a few well-converged FEA's of designs considered "good" that were originally based on classical calcs, to see if there was any consistent limit to stresses or areas of stress concentration.
We use FEA where classical calcs become very difficult to apply. Where the joints are simple in cross-section we are fine but we have better optimized section shapes that do not lend well to classical analysis. And yes, a few customers want FEA.
David
RE: Practical test methods for validating FEA and classical design calculations
Then, use your measured stresses in a fatigue calculation, paying close attention to selecting proper fatigue details that do not effectively stack-up safety factors.
The advantage here is that your fatigue calc is not dependant on the accuracy of your FE model.
tg
RE: Practical test methods for validating FEA and classical design calculations
Fortunately this is not a reversing fatigue loading situation. Life here is 30-50 years of continuous service as a rotating machine, which is "infinite" compared to any low-cycle or even high-cycle fatigue example.
RE: Practical test methods for validating FEA and classical design calculations
if testing, just static test (to ultimate) to show it good.
another day in paradise, or is paradise one day closer ?
RE: Practical test methods for validating FEA and classical design calculations
Geesamand
FEM tools(Ansys,Nastran) are developed based on classical theory. You can check this with simple cantilever beam with concentrated load at its free end. Your results will match 99.99% with the theoretical calculations. But for the larger models FEM is a better option. Because there are no standard formulae are readily availabe. In calssical approach calculations are done based on considering factors like notch radius, key way compliance etc. For big and complicated model it is very tough to bring in exact geometry of podcut for calculation purpose.
With a good practice in FE modeling and Analysis you will get best results with a little variation of 8-10% of the theoretical value.
More over with FEA you can easily change your design criteria like topology,material,structure strengthening etc.
All this can easily done ,which results in quicker product life cycle.
There are many FEM tools are available like : Nastran, Ansys, LS dyna