SOLVER PROBLEM. ZERO PIVOT WHEN PROCESSING A (TIED) CONTACT

[maiden666]

Hi,
Good day.
I am a beginner to Abaqus.I am (have to)trying a Transient dynamic loading over an assembly.The analysis is nonlinear with NLGEOM and contact.

The problem goes as follows:

Step 1 : Pre-tension on the bolt due to which two surfaces come in contact ( housing and a base plate ).

Step 2 : Acceleration pulse application to the base plate.

I get a Zero pivot error at the mid of the dynamic load step.My dynamic loading is for 10 secs.I have opted for 0.001 as intial time step , 0.01 as maximum time step and 1.0e-15 as minimum time step.
The analysis goes fine till 1 sec with 0.01 sec time incriment and then it jumps back to a lesser time incriment ( 1 e-15 ) with the following msg.
***NOTE: REPEATING INCREMENT WITH TIME INCREMENT OF 1.00000E-15 BECAUSE 0
POINTS HIT AND/OR 1 POINTS SEPARATE

INCREMENT 64 STARTS. ATTEMPT NUMBER 2, TIME INCREMENT 1.000E-15
***WARNING: SOLVER PROBLEM. ZERO PIVOT WHEN PROCESSING A (TIED) CONTACT
CONSTRAINT D.O.F. 1 FOR SLAVE NODE 50937. THIS MAY BE DUE TO THE
UNIT USED FOR THE DISPLACEMENT DIFFERS BY MORE THAN 10 ORDER OF
MAGNITUDE THAN THE UNIT USED FOR THE MATERIAL STIFFNESS. IF THIS
IS THE CASE, PLEASE USE A DIFFERENT UNIT SYSTEM.

OVERCONSTRAINT CHECKS:
An overconstraint was detected at one of the Lagrange multipliers associated
with contact slave node 50937. There are multiple constraints applied directly
or chained constraints that are applied indirectly to this node.
The following is a list of nodes and chained constraints between these nodes
that most likely lead to the detected overconstraint.
How to read this information:
(This explanation is printed once per analysis.
More details given in the OVERCONSTRAINT CHECKS section of the User's Manual.)
A sample output is shown below:
A (or LAGRANGE MULTIPLIER)->B,E: constraint 1
..B->C: constraint 2

My boundary conditions are as follows :
STEP 1 : pretension load on the screw using *PRETENSION option.
Base plate fixed in all DOF's.
*BOUNDARY
Nodes_for_bc,1,3,0.0 --------- Nodes_for_bc:node set which contains base plate bottom side nodes.
*CLOAD
53297,1,16000.0---- 53297 ---Pretension node.

Step 2 : Fixed boundary condition on the pretension node and prescribed Accln pulse application

*BOUNDARY, FIXED
53297,1
*CLOAD ,OP = NEW
*BOUNDARY,AMPLITUDE = curve1, TYPE = ACCELERATION,OP=MOD --------- curve 1 contains the accln vs time history.
Nodes_for_bc,1,,100.0
Nodes_for_bc,2,,0.0
Nodes_for_bc,3,,0.0.

Is this problem which i face is because of my BC's ?
Kindly help me .

666.

 

[psantunes]

I 666!

First of all, I must know if you are using the frictionless tangencial behaviour between the bolts heads and your upper part?

First , try to use in the *NO SEPARATION in the normal interaction behaviour definition in the bolt heads contact properties!

The second option you have is to define in the upper plate BC that dont allow the in-plane movement of your part.

I dont´t know very well your problem but it seems that your upper plate is not well constrained!

Psantunes

 

[pilsung42]

Hi 666,

I have exactly the same problem as you described. Instead of a plate, I am modeling a column with contact interfact between RC column and footing. The column was pre-tensioned in Step 1 and subjected to ground acceleration in Step 2 of the analysis.

In Step 2, I have the same setting as you did (initial time = 0.001 secs, total time = 10 secs, minimum time step = 1E-15 and maximum time step = 0.01 secs). The analysis went well until it hits 1 sec of ground acceleration and it cut back to 1E-15 time step with the following warning message.

***NOTE: REPEATING INCREMENT WITH TIME INCREMENT OF 1.00000E-15 BECAUSE 0 POINTS HIT AND/OR 1 POINTS SEPARATE

Then it keeps iterating until the analysis terminated.

I tried to change a few parameters including damping coefficient in "Interaction Properties". Have you tried to use "damping" in your problem? Give it a try and see if it helps.

If anybody else has any suggestions, please let us know.

Best,
pilsung42