Solver problem.Zero pivot...

[MirBG]

Hi all,
I have a problem with coupled pore fluid pressure/stress analysis during the simulation of the construction of an embankment, in particular using the clay plasticity model.
Could you suggest me a solution?

The warnings message is:


***WARNING: THE SYSTEM MATRIX HAS 562 NEGATIVE EIGENVALUES.


***WARNING: Solver problem. Zero pivot when processing D.O.F. 1 of 21 nodes.
The nodes have been identified in node set
WarnNodeSolvProbZeroPiv_1_1_1_1_1.

***WARNING: Solver problem. Zero pivot when processing D.O.F. 2 of 19 nodes.
The nodes have been identified in node set
WarnNodeSolvProbZeroPiv_2_1_1_1_1.

***WARNING: Solver problem. Zero pivot when processing D.O.F. 8 of 8 nodes.
The nodes have been identified in node set
WarnNodeSolvProbZeroPiv_8_1_1_1_1.

***WARNING: Solver problem. Zero pivot when processing D.O.F. 8 at one or more
of the internal nodes of 1 elements. The elements have been
identified in element set WarnElemSolvProbZeroPiv_8_1_1_1_1.



***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.
***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.
***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.
***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.
***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.
***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.
***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.
***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.
***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.
***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.
***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.
***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.
***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.
***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.
***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.
***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.
***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.
***WARNING: THE SYSTEM MATRIX HAS 107 NEGATIVE EIGENVALUES.

 

[xerf]

1. Check the model for sufficient constraints against rigid body motion.

2. Try add/increase hardening.

 

[MirBG]

Thanks a lot. I'll try to solve the problem.

 

[MirBG]

I've tried to use rigid body in my model and the job has been finally completed, but I cannot visualise strains and deformation results that I need. I think this solution cannot be used in a cam-clay model.
What can I do to solve the problem in another way?

 

[hamadadf]

Hi
i had a similar problem before also in a fluid/stress analysis.In my case, my problem was fixed by changing the element used and checking the units. (thats ofcourse if you are sure your model is not overor under constrained).wha type of element are you using?

 

[lisw]

As what I had found in the ABAQUS/Standard example: forming a channel, they talked about the zero pivot in the Boundary Conditions for Step 1.

If a contact region is constrained by a boundary condition in the direction that contact occurs, there will be two constraints applied to a single degree of freedom. And this can cause numerical problems, and ABAQUS /Standard may issue a zero pivot warning message in the message file.

I hope this is what your problem is.

 

[rtmpxr]

if you use rigid body elements how can you expect to have stresses ?? Your problem is to impose constraints in a such manner to avoid rigid body DISPLACEMENTS! Check again your case.

 

[johnsmith2]

Lisw,

As you mention the problem of applying two constraints to the single degree of freedom, I have the question on the contact analysis, but not on the coupled pore fluid pressure/stress analysis, as mentioned above.

If my model is cyclic symmetrical with two cut surfaces of the sector. Of course, I need to apply the symmetrical boundary conditions, but at the same time, these two "cut surface" will intersect with my contact surface in the assembly. How abaqus can analyze?