ABAQUS: There is zero FORCE everywhere ... + Solver porblem: Zero pivot .."

[alpha2012]

I am solving a 2x2x2 model (C4D8 - 8elements) with nodes on the bottom, side and back faces constrained in their respective axis directions for TMF [temp + Mech on the top side/face] load and it solves.

When I change the constraints, by fixing in all directions, only nodes in the bottom face I start getting the warnings below, and the analysis crashes with illegeal floating error.

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1- "There is zero FORCE everywhere in the model based on the default criterion. please check the value of the average FORCE during the current iteration to verify that the FORCE is small enough to be treated as zero. if not, please use the solution controls to reset the criterion for zero FORCE."

FORCE equilibrium accepted using alternative tolerence.

2- "Solver porblem: Zero pivot when processing DOF 1 of 1"

FORCE equilibrium accepted using alternative tolerence.
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How do I over come the warnings, could they be the source of the crash. If not waht should I look for ?

Thank you.

 

[IceBreakerSours]

I believe the model is too "stiff" or, in other words, overconstrained. Open the .msg file and see if there are any "overconstraint" warning messages. If so, you need to re-think the boundary conditions.

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

 

[alpha2012]

there were no negative eigen value or overconstraints warnings. But by changing the BC, the zero pivot warning is gone.

Now the following warning remains.
*************************
1- "There is zero FORCE everywhere in the model based on the default criterion. please check the value of the average FORCE during the current iteration to verify that the FORCE is small enough to be treated as zero. if not, please use the solution controls to reset the criterion for zero FORCE."
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The other info is that I am using a UMAT and it runs fine with free thermal expansion with same BC, but the moment I apply any mechanical load (around .5%) or so; it starts to diverge. The model I am using is an L-bracket structure.

Could it be UMAT stiffness issue, where it is not properly modeled for non-linearity?

 

[IceBreakerSours]

by changing the BC, the zero pivot warning is gone.

Open the ODB, go to Create Display Group and see if there are any warning nodes and highlight them. Also, go to Job Diagnostics and see if any warnings might indicate where the issues might be coming from.

Could it be UMAT stiffness issue, where it is not properly modeled for non-linearity?

It seems so, yes. But may be its a modeling mistake to constrain nodes in all directions for the material you have modeled. I can't say for sure unless I know more about the subroutine. If you wish, you may upload both your INP/CAE and the subroutine.

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