16 Jul Solving convergence problems with Abaqus/Standard

There are various reasons why a Abaqus/Standard finite element simulation may not converge. This post provides a description of the most common convergence problems and an explanation what might be the cause of these occurrences.

When convergence problems appear, one should review the output files of Abaqus.
The symptoms of almost all convergence problems can be found in the message file (msg). The printed output (.dat) and status (.sta) files may also contain symptoms of a problem.

Contact info@4realsim.com to obtain more information on Abaqus and/or FEA services.

Let’s look at the most common convergence problems:

Numerical singularities

Numerical singularities may indicate that the rigid body motions of the simulation model are not fully constrained.
A possible check is to use a Frequency (eigenmodes) analysis to verify that a model is fully constrained. Modes with a frequency near zero are indications of an underconstrained model. Animate these modes to visualize the problem.

Zero pivots

Zero pivots errors occur when there is an force term but no corresponding stiffness term.
A common cause is both the lack of constraints and the overconstraints of some degrees of freedom.

Negative eigenvalues

Negative eigenvalues indicate that the stiffness matrix is not positive definite.
It is normally associated with a loss of stiffness or a solution that is not unique, such as, for example, that there is instability in the material because it has reached the ultimate stress point or because it has exceeded the maximum load it can withstand, producing buckling, for example.

Excessive yielding

Excessive yielding indicates relatively large deformations in a problem where the material model has some form of plasticity.
This is usually related to:
Loads: Is the load value realistic
Material data: Perfect plastic deformation
Mesh refinement: Meshes that are suitable for elastic linear analysis may not be suitable for problems with plasticity and large deformations.

Element distortion

These messages appear when the volume at the integration point of an element becomes negative.

Hourglassing or shear locking

This will be handled in a future post.

4RealSim Abaqus customers can contact the 4RealSim support line when facing these issues, and together we can examine the error, the cause of the error and find a solution to resolve the convergence problem.

Contact info@4realsim.com to obtain more information on Abaqus and/or FEA services.