Increment dependency in non-linear static analysis

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Increment dependency in non-linear static analysis

André-20
Hey all!

I really need some help!

I modelled a thick cylindrical shell with:
lenght: 5mm
diameter: 5mm
thickness: 0.2mm
Material: Al (elasto-plastic)
and compressed it by applying a displacement of 1mm to the upper
nodes. I found out that when I change the size of the time
increments ABAQUS should use, the results are very different.
Unfortunately, they are not converging (i.e. the smaller the
increments the earlier buckling occurs or sth. like that).

Does anyone know why the results (even using *RIKS)depend so much on
the number/size of the increments? Additionally I found out, that
when I us the *STABILIZE function, that there is no dependency. But
I don't know really how this stabilization/damping term influences
the correctness of my results. There is also no increment dependency
when I set the BC's to clamped-clamped instead of free movement in
the lateral directions.

Thank you very much in advance!
Andre









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Re: Increment dependency in non-linear static analysis

suhoi222
You don't mention anything about any imperfection in your model. It may help you trigger the post-critical regime, just before which you might actually be. In the F-delta nonlinear response, in the vicinity of the critical point, a smaller increment actually does make a difference, since the Kt (i.e., stiffness matrix) becomes singular (the non-Riks case). *Stabilize card introduces some viscous forces into your model, but that's recommended in case of collapse. The choice the elements makes also a big difference. There's a cylindrical shell example in the manual you might want to check out.
   
  have fun
   
  S
 
André <[hidden email]> wrote:
  Hey all!

I really need some help!

I modelled a thick cylindrical shell with:
lenght: 5mm
diameter: 5mm
thickness: 0.2mm
Material: Al (elasto-plastic)
and compressed it by applying a displacement of 1mm to the upper
nodes. I found out that when I change the size of the time
increments ABAQUS should use, the results are very different.
Unfortunately, they are not converging (i.e. the smaller the
increments the earlier buckling occurs or sth. like that).

Does anyone know why the results (even using *RIKS)depend so much on
the number/size of the increments? Additionally I found out, that
when I us the *STABILIZE function, that there is no dependency. But
I don't know really how this stabilization/damping term influences
the correctness of my results. There is also no increment dependency
when I set the BC's to clamped-clamped instead of free movement in
the lateral directions.

Thank you very much in advance!
Andre









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Re: Increment dependency in non-linear static analysis

wisonyang
In reply to this post by André-20
I did the buckling analysis of nonlinear material cyliderical tube.
I used *buckle followed by a static step. I increased the
displacement in the first step until it converged at the buckling
pressure. I didnot find the increment-dependence in any step. I
would like to know what analysis type you used? Did you use *Riks?
In my opinion it should be used for post-buckling analysis and it is
better to introduce a small imperfection to assist the analysis.

Wison

 --- In [hidden email], André <akripo@m...> wrote:

>
> Hey all!
>
> I really need some help!
>
> I modelled a thick cylindrical shell with:
> lenght: 5mm
> diameter: 5mm
> thickness: 0.2mm
> Material: Al (elasto-plastic)
> and compressed it by applying a displacement of 1mm to the upper
> nodes. I found out that when I change the size of the time
> increments ABAQUS should use, the results are very different.
> Unfortunately, they are not converging (i.e. the smaller the
> increments the earlier buckling occurs or sth. like that).
>
> Does anyone know why the results (even using *RIKS)depend so much
on
> the number/size of the increments? Additionally I found out, that
> when I us the *STABILIZE function, that there is no dependency. But
> I don't know really how this stabilization/damping term influences
> the correctness of my results. There is also no increment
dependency
> when I set the BC's to clamped-clamped instead of free movement in
> the lateral directions.
>
> Thank you very much in advance!
> Andre
>









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