Troubleshooting stability issues in an analysis

Contact and stabilization – Dealing with unstable models

Most of the analysis work you perform will likely be done as some sort of static analysis (linear static or nonlinear static), which requires a statically stable solution. Parts can deform and accommodate some small relative displacements and controlled displacements; however, problems like fatal analysis errors or convergence failures can arise when parts are free to experience rigid body motion (e.g., like a pin rotating in a hole). As some contact types allow for sliding and even separation, you may need to assist in making the geometry stable. Consider the following:

• Add constraints to remove unwanted degrees of freedom. For example, if you have a part normally held in by a track or keyway and that restricting geometry has not been modeled, add constraints to the body to limit the possible movements to the same as the physical model.
• If criteria are met, use a half or quarter symmetry model. The constraint you add to the cut surfaces will add stability to the geometry without adding any artificial constraints in the solution. Reference Loads and constraints in FEA for additional details on use of symmetry.
• Add additional parts back in. Sometimes if the model is stripped down too far, the part(s) it relies on for stability may not exist any longer in your simulation. It may be the fastest path to include those parts. Alternative to the immediately preceding suggestion, the addition of weak springs to oppose the motion can help to stabilize the model. Springs is discussed further in Connectors in FEA.
  
  

Contact troubleshooting

Stability is one item to keep in mind when using contact – however, there are a few other items to be aware of when utilizing contact in an analysis.

• If you have an assembly, some contact needs to be defined for the interaction. If a body is free to move in the assembly (un-statically stable), this will likely lead to a fatal error, such as "E5004: Stiffness matrix singular or non-positive definite" with the Autodesk Nastran solver. 
• If you have contact defined and the contact doesn’t seem to be being made, check to see if the model has gaps between parts. You might want to manually define max activation distance to ensure this is larger than the length of the gap. 
• Unsymmetric penetration types should create fewer contact elements (and so solve quicker); however, the symmetric option makes it less possible that parts penetrate one another. The symmetric option should always be used with shell elements. 
• A relatively common question comes up when reviewing results. If one part is very rigid and fixed and another part closes, say, a 0.001" gap to make contact, the displacements are exaggerated in the results view. So, if we scale up the displaced geometry’s movement 10x, it will look like it has moved 0.010" and has interfered. To remove such questions, examine the results of the displacements at actual value, or 1x. 

If contact issues arise, consider stability of the parts, ensure contact has been defined, if there are gaps, what penetration type is used, and whether the results are being exaggerated.