puzzling gravity result

[carolina000]

Dear all,

I have a strange situation here with Abaqus. Hope someone has an answer for this puzzle.

Just imagine you have a very soft rubber ball, in your hand palm down, hanging under gravity. And you want to model how the ball would deform if you turned your hand upside down, palm up. You have the initial geometry of the ball hanging under gravity.
My guess is, as i dont rotate the ball during the analysis, i need a first step, with a gravity load of -g (to reach the reference state of the undeformed ball) and a second step with a gravity load of -g (to model how the ball spreads in my turned hand). I cannot implement a unique step of -2g because the material is non-linear and the deformations are large.
Funny, when i do this in Abaqus, the first step is fine, but the deformations during the second step are 0! Maybe during the first step the ball reaches equilibrium, the point is the deformations during the second step are 0, so not real, right?

Now, If i import the results of the first step (ball in reference state) and apply a -g step, things are fine, that is, the deformations are roughly twice the deformations of the first step. But, why on earth doesnt this work with 2 steps in Abaqus??? In fact, how do i make it work without importing the deformed part???

Looking forward to your feedback!
C.

 

[IceBreakerSours]

In all probability, I may not have understood the problem/analogy of the problem but the two situations (ball in palm-turned-up vs. ball in palm-turned-down) are very different in terms of the boundary conditions. How can you simply apply two steps of the same load of -g? I'd imagine applying same load of -g with appropriate BCs in two situations is a more appropriate way to model the problem.

I cannot implement a unique step of -2g because the material is non-linear and the deformations are large

Have you tried using hybrid elements and a hyperelastic material model with nonlinear geometry (NLGEOM) flag turned on?

http://www.eng-tips.com/faqs.cfm?fid=376

 

[carolina000]

hi IceBreakerSours,

Sure, i have implemented an hyperelastic material model with nonlinear geometry. Just imagine the ball is cubic (!!). I should have said the ball is "stuck" to my hand, that is, one of the surfaces is encastred to my hand. Does this make more sense now?

The boundary conditions are the same in both steps, a surface of the cubic ball encastred to my hand, up side up and up side down.

Thank you for your reply,
Carolina

 

[IceBreakerSours]

Got it.

Now, coming to the step definition, I would've thought applying a +2g load in step 2 (after having applied -g in step 1) would be a more appropriate way to model the problem.

~!ce.

http://www.eng-tips.com/faqs.cfm?fid=376

 

[rstupplebeen]

In the second step modify the load to be +1g. My guess is that the -1g load is being added to the +1g load causing no load. I hope this helps.

Rob Stupplebeen

https://sites.google.com/site/robertkstupplebeen/

 

[IceBreakerSours]

Rob is correct. I don't know why I said +2g (I guess I was watching TV while I left that message )

http://www.eng-tips.com/faqs.cfm?fid=376

 

[carolina000]

hi rstupplebeen and IceBreakerSours,

Now, this is surprising! Does Abaqus normally apply loads from previous steps even if they are not active?? I would have thought steps were completely independant...
Anyhow, thank you for your answers. I will compare the results of what you suggest with the results of deformed parts from odb+ -g.

Thank you!
Carolina