Force output for push button

[Janus3]

Hello,
I have modeled axisymmetric analysis of sheet dome (simple push button).
I need to get the force response of the sheet metal. According to experimental results the force increases until certain level then the "spring" action occurs and the force drops a little bit and if you keep "pushing the button", the force increases very much. I am concerned about the force drop range.
However I keep getting steadily increasing force curve from the surface nodes (NFORC). Actually except one node all indicates the behavior similar to the experimental one (drop occurs), but the one that behaves differently indicates so high force comparing to the others, that when I summarize the NFORCes, I get the increasing tendency.
I have used quadratic 8 node elements for this FEA.
I think the model itself is somehow OK, I probably do something wrong when evaluating the results. I would welcome any suggestion.
Thank you very much.

 

[Janus3]

The model looks like half lens (deformable shell) with punch (rigid) above it. I tried to apply both force and displacement every time with similar results. I am out of any ideas. Please, anyone, help!

 

[Janus3]

It seems that I may be able find the solution after some more research and trying. I have used now the riks analysis for the buckling and the model seems to work OK. Now the main problem is to get the force output. I am still confused about it.
I would welcome your kind consideration.

 

[bassmanjax]

If you want the punch force, you can plot the reaction force (RF) at the punch reference node?

Regards

Martin

 

[bassmanjax]

And one other thing....do you have NLGEOM set to yes? You'll need that if this is (as I suspect) a snap-through type problem.

Regards

Martin

 

[Janus3]

Hello Martin,
thank you for your advice.
I think, I am doing some non-sense. I do need to get the punch force but even when getting RF from the punch reference node still the results are not good. I am still a beginner with ABQ so my questions are probably stupid enough, I am sorry.
NLGEOM are on.
Is CAX8R element type right for this analysis? Punch has RAX2 elements.
Is better to use displacement than force as the initial load?
Is it necessary to use riks after the load step?
instead of applying BC to the edge of the dome, I have just added rigid support with non friction contact properties under it and left the edge otherwise free. Now the model really looks like 1/2 lentil placed on a table and being pushed from above.
I am using CAE as pre-processor.
I would really appreciate anyone's kind opinion on this matter.
Jan

 

[Janus3]

However the snap-through analysis works well for 2D plates, I can not get it. I have even tried the MAX1 elements and modeled the dome as a membrane, still nothing.

 

[Janus3]

And yet one more question: The material is stainless steel and I have both the elastic and plastic data. For the buckling analysis though only the elastic properties are necessary, is that right?
I could not get the "snap" for this material even for the 2D analysis as in the example manual...

 

[bassmanjax]

You could try modelling with axisymmetric shells for the dome - SAX1 are the linear axisymmetric shell elements.

Also, is your punch geometry simple? If so, could you model it as an analytical rigid surface rather than meshing it with rigids?

I'd suggest driving the punch with a displacement rather than a load, especially if you are using Riks. With Riks, you might find that you have to reduce the minimum time increment to a small value in order for it to 'drive' through the snap-through point.

I'm not sure I understand how you've done your boundary conditions on the dome either. Can you not just apply displacement/rotation constraints to the edge nodes?

Regards

Martin

 

[Janus3]

Thank you very much for your advices.
finally I went step-by -step through the example in the ABQ example manual. And now I am fiddling with damping coefficients in static analysis instead of using riks.
The punch is now analytical rigid.
The edge of the dome is pinned.
I will post my progress here.
Probably I am doing something very stupid without realizing it :-(.

 

[bassmanjax]

Hi Janus3

You're not doing anything stupid, you're learning by trying stuff out and there's no harm in that .

Martin

 

[Janus3]

Thank you very much for your support.
Recent progress is actually good.
I have very small arch that is very sensitive to geometry changes.
But the main point turned out to be the damping coefficient and to choose a proper loading level. I have too much carefully used smaller displacement then was needed for the snap-through.
See very easy )).
I am so relieved that it finally works.
Thank you again for your patience.

 

[Yoman228]

I would use a displacement control method as it tend to be more stable though snap though if the scope allow you to. My guesses is that riks is may not be necessary when you use displacement control method.

If you using damping within the model. ensure to check the ratio of the damping you put in the system is small compared to your total energy within the system.

If you using the elastic model only, check you maximum stress though the analysis. And ensure it does not pass the yield of the material. I believe using elastic only property is fine in your analysis as a push bottom will not last only if it have been plasticity.