Written by Shaun Bentley, Application Engineer
When trying to simulate complex assemblies with STATIC analysis, things can get tricky. You might encounter various errors when meshing or running the study. Frequently, simplifying the model can help to reduce or eliminate errors, pinpoint the problem, and begin making progress. Here is a check list to help.
WARNING: Since many of these suggestions will compromise the accuracy of your simulation, they typically shouldn’t be used for your FINAL runs and may only be used to as troubleshooting tools.
1.) Remove Sources of Nonlinearity
There are three sources of nonlinearity that you can include in your analysis: geometric, material, and contact. You should try to practically eliminate all of them for your initial troubleshooting runs since nonlinearities can cause complications. Here’s some suggestions for each type:
If you ever got this message when trying to run the study, you most likely should have clicked NO to it.
If you mistakenly selected YES, it would turn on the “Large displacement” option:
And your study might have failed to run as a result. Go to the study properties and make sure it is turned off for now.
Assign “Alloy Steel” to everything and don’t change the material model type to anything other than “Linear Elastic Isotropic”. Custom materials can cause some complications (it’s easy to type a wrong number when creating a custom material) and stiff materials like steel tend to converge more easily.
Do not use any “No Penetration”, “Shrink Fit”, or “Virtual Wall” contacts in your study, since these are all nonlinear contacts. Assume everything is “Bonded” and eliminate “Bolt Connectors” and other connector types that rely on nonlinear contacts for their solution.
2.) Increase Model Stability
Each body in your assembly has six rigid body modes and all six need to be restrained for your model to be stable. The following can be used to reduce rigid body modes and increase the stability of your model:
All of the fixture options shown in the Fixture Property Manager will restrain rigid body modes. Over-apply fixtures just to get your study to run and then start to remove fixtures to see where the problem areas are located.
I only recommend Spring, Pin, Link and Rigid Connection for troubleshooting purposes. Remove/suppress all others for now.
- External Loads
With one exception (Displacement Remote Load), none of the load options remove rigid body modes. However, loads can help stabilize the model for studies that include “No Penetration” contacts as long as the load pushes the bodies in a way that they remain in contact such as with a Bolt Connector pre-load.
- Turn on Soft Springs and Inertial Relief
These two options are discussed briefly here. You can even leave the Soft Springs option turned on for most of your runs, but you likely want to turn off Inertial Relief after you manage to find the problem.
- Direct Sparse Solver
While you’re at it, switch to the Direct Sparse solver since it tends to be more stable (but it can be a lot slower than FFE+).
3.) Clean Up the Geometry and Switch the Mesher
- Eliminate Interferences
Use the interference detection tool with “Treat coincidence as interference” to track down interfering and touching regions.
Many times, “Coincident interferences” are good since these will tend to get bonded together in Simulation (as long as you didn’t change the Global Contact condition), but regions that are actually interfering can cause complications so try to get rid of them. The Cavity tool is one popular method for removing interferences, but it can be a bit messy and cause the mesh to fail.
- Include Only the ESSENTIAL Bodies and Features
Maybe suppress everything except for a very small group of two or three parts in the ESSENTIAL region of your assembly and try to get a simplified load and fixture to run. Then incrementally add more components.
- Clean Up Faces
If you are having some difficulty meshing the geometry, use “Check” to track down tiny edges and invalid faces.
Simulation tries to mesh all of the faces and if the elements don’t fit in some of the tiny faces, then it might fail to mesh.
- Use a Curvature Based Incompatible Mesh
Try one of the curvature based meshers since these tend to be more robust. If you can get a coarse mesh to work successfully, this will make it faster to run the study repeatedly as you troubleshoot. Also, the incompatible mesh will help get it to mesh in the contact regions.
There’s a few other troubleshooting methods I might suggest, but the list above is likely the most popular that I have seen in practice. As you can likely see, troubleshooting Simulations can be laborious. However, once you’ve done it a few times, you’ll start to build an intuition for the things you may need to simplify for future studies, and eventually you may no longer need this checklist.