Source of convergence difficulty in the first increment of an Abaqus/Standard contact analysis

QUESTION

My Abaqus/Standard contact analysis terminates in the first increment after too many severe discontinuity iterations. What might be causing this?

ANSWER

Topics:

Abaqus

→ Abaqus/Standard Analysis Techniques

→ Contact

(The following applies to any version.)

One or more of the contact pairs in the analysis may have an excessive initial overclosure. The overclosure may be a feature of the model, or it may be the result of a modeling error. In either case, an excessive overclosure may cause the analysis to terminate prematurely. In this section some common sources of excessive initial overclosures are discussed.

1. In the case of an overclosure that is a feature of the model, as with an interference fit analysis, the overclosure may be too large to resolve in one increment.

To help with the diagnosis, print information about the overclosures during the datacheck and solution phases:

• Abaqus/CAE: Job Module : Job Manager : Edit ® General ® Preprocessor printout ® Print contact constraint data
• Step Module : Output ® Diagnostic Print... ® Contact
• Input file: *PREPRINT, CONTACT=YES
• *PRINT, CONTACT=YES

The solution in this case is to use the *CONTACT INTERFERENCE option to remove the overclosure. Sometimes resolving the overclosure over several increments is required. Adjusting the solution controls to increase the maximum number of severe discontinuity iterations allowed in an increment may also be needed to overcome this difficulty.

Sometimes small overclosures (relative to element dimensions) can be removed by adjusting the slave nodes in the contact pair:

• Abaqus/CAE: Interaction Module : Interaction Manager : Edit ® Slave node adjustment
• Input file: *CONTACT PAIR, ADJUST=[Node set label | Adjustment value]

2. The master surface normals may be pointing away from the slave surface.

Use Abaqus/Viewer to check the normals on the master surface.

The solution in this case is to redefine the master surface so the normals point towards the slave surface, assuming the surfaces are open.

3 . The model may be permitting a rigid body motion (VERY large overclosures detected; sometimes on the order of 10**15), typically due to loading a body with forces when there are insufficient boundary conditions or insufficient active contact constraints to remove rigid body motion.

You will, in most cases, also see messages about NUMERICAL SINGULARITIES in the message (.msg) file. Print overclosure information during the solution (as described in Item 1) to diagnose the problem.

This case can be resolved with different approaches:

• Use boundary conditions to move the bodies until they are just in contact (do this in a "dummy" step). In the next step remove these boundary conditions and replace them with forces that maintain the contact (this is the recommended technique).
• 
  
• Add soft springs to the model in the directions of the rigid body motion. The degrees of freedom (dofs) requiring springs can be seen from the dofs associated with the numerical singularity messages. The stiffness of the springs must be small enough that the forces in the springs are negligible compared to typical forces in the problem.
• 
  
• Use contact stabilization (*CONTACT CONTROLS, APPROACH (Version 6.1 or later) or *CONTACT CONTROLS, STABILIZE (Version 6.5 or later)) to stabilize the motion (using damping effects) when, at some point in the analysis, contact is meant to prevent rigid body motion. (If this is not done carefully, the viscous forces can dominate the solution. Print out the forces due to viscous effects applied with these options using the VF output variable. Additionally, output variable ALLSD measures the energy dissipated by viscous damping; the ratio of this quantity to the elastic strain energy or other appropriate general energy measure should be small.)

If the analysis converges in an iteration during which NUMERICAL SINGULARITIES appear, check the solution carefully. Abaqus/Standard attempts to fix the solution, but sometimes this fix will not give a correct solution. You should remove the cause of the NUMERICAL SINGULARITIES instead. If numerical singularities only occur in iterations prior to the iteration which converges, the solution should be correct; however, you should always check that this is so.

4. Contact chattering: the contact state changes from one iteration to the next so that the maximum number of SDIs (severe discontinuity iterations) is reached. Read Causes of contact chatter for additional information.