The rod doesn't want to fit in place

[Anderl]

Hello all,

I'm wondering why this problem doesn't want to converge. I'm trying to find out the force necessary to be applied to lock the barbell. (Please, see the picture).

I'm performing a nonlinear study, but the material remain isotropic, elastic and lineal, and is the same for both workpieces.

For simplicity, I'm running a 2D simplification. The contact pairs are defined as showed in picture.

If instead of applying a force, I solve the problem by applying advanced restrictions and impose the displacement necessary to bring the rod to end position, then the problem converges without mayor problems. The problem in this case is, that I don't trust the results, thus I want to solve this another way.

Looking the problem closely, if I try to solve this as a linear problem, then in the 2D aproximation I obtain a very curious "solution": There is a moment in which the sheet opens too much and loses contact with the bar....

I found a correspondence between both situations: the solution in the non-linear problem stops always when the same deflection point is achieved. So, there is a point where the rod and the sheet lose the contact, which causes the non convergence of the solution... ??

I tried all: restarting the solution varying all posible factors, I redo the contact pair...

Any idea?

Thanks for watching.

 

[academix]

Hello:

Remember, you are using force convergence at every time step.

This problem may arise out of any one or combination of the following:

1. Contact stiffness is inappropriately defined
2. Contact surfaces, if left without friction, are defining the mating surfaces as polished. The cylindrical part may try to rotate about its own self-equilibrating axis (may be an explanation as to why enforced displacement provides answers while force application does not)
3. Once the static equilibrium iterations reach a point and try to expand the clip, it may need to be solved using non-linear dynamic option with inertial contributions being accounted for.

There is a similar problem in tutorials (with enforced displacement though) that you may want to experiment with before trying your own problems.

Best regards


Natarajan Ramamoorthy
Design Engineering Consultant

http://www.egsindia.com

 

[Anderl]

Academix, thank you very much for your answer,

You hit the nail on the head with this: I think friction is the answer. I'll try to solve it again with friction.

anyway, -> I am not sure how to do it... Could you be more specific? thanks in advance.

-> Yes. I know this. I already did it some time ago...

Thanks again.

 

[academix]

Hello:
When you create the study as Non-linear dynamic, the inertial contributions are accounted for. Remember, all non-linear simulations are trying to mimic natural phenomena. So, our input, boundary conditions including contact, need to be closer to reality,

Best Regards


Natarajan Ramamoorthy
Design Engineering Consultant

http://www.egsindia.com

 

[Anderl]

Academix, thank you so much for your time.

I'll try to solve the problem with friction over the night.
I'll try it first with enforced displacement: I think it's the best approach because, by applying a force, once exceeded the "midpoint" (let us say, it is the point when the force turns from positive to negative) I think the problem won't to converge no way.

-> Thanks: although it should be obvious, those little details are really important.

 

[Anderl]

Academix,

I was able to solve the problem by applying a force. Anyway, the solution's convergences seems a little erratic... I must to increase the convergente tolerance from the default value = 0.001 till 0.05, and the solution doesn´t "moves" toward the convergence... I mean with this, that in step X the rod still didn't move and in step X+1 the rod has moved full down.

Anyway, it seems like the friction was the key.

The solution with an enforced displacement and friction now seems much more coherent also.

Thanks.