Contact and Convergence in Abaqus/Standard
The really interesting finite-element models, like the one shown in the picture here, exhibit behaviors that can present challenges to the nonlinear solution method employed by Abaqus/Standard. An additional challenge when modeling many of these structures is the presence of contact. When the thin disked is pulled through the ring, for example, it will wrinkle and buckle and eventually come into contact with itself. The Abaqus user may be unable to build a satisfactory Abaqus/Standard model for this case unless they are aware of the techniques for handling convergence issues and for defining effective contact.
Latest and greatest contact in Abaqus/Standard
The recommended contact in Abaqus/Standard today is general contact. General contact uses the surface-to-surface contact discretization to enhance solution quality. This discretization also allows users to avoid issues associated with the node-to surface discretization. Some of this issues are contact snagging, contact chattering, and unwanted contact penetrations. General contact includes the supplemental formulations for edge-toface, edge-to-edge, and vertex-to-face contact which can be very helpful and which are not available with contact pair definitions. General contact has penalty constraint enforcement to improve convergence characteristics. It will use the finite-sliding contact tracking by default but it can be configured to use the small-sliding tracking algorithm.
Why are contact and convergence often linked?
Contact presents challenges to the nonlinear solution method used by Abaqus/Standard. So, any discussion of convergence will naturally lead to a discussion of contact and vice versa. Today in Abaqus/Standard, the best way to avoid the convergence issues associated with contact is to follow the recommendation and use general contact. But, contact is not the only source of convergence problems.
How does Abaqus/Standard solve a nonlinear finite-element model?
There can be no good understanding of convergence without an understanding of the Newton-Raphson method that Abaqus/Standard uses to solve a nonlinear solution analysis. The method uses an incremental approach to applying loads. Each increment requires a number of iterations to produce the solution. And each iteration involves the solution of a linear system of equations. So, anything that might produce a problematic linear system could cause Abaqus/Standard to stop prematurely, or in other words, have a convergence problem.
In this SIMULIA Tech Talk, we will present an overview of the latest contact technology in Abaqus/Standard.
What are some causes of convergence problems?
The most common cause of convergence problems is inadequate modeling by the user. The user may be unaware that penalty contact is better for convergence, for example. Or the user may be using a plain static analysis procedure for a structure which buckles and releases strain energy. There is nowhere for the stored strain energy to go in a plain static analysis. Another common source of convergence problems is the lack of adequate constraints for static equilibrium. Abaqus/Standard has contact stabilization, static stabilization, dynamic procedures, etc. which will help users overcome most common convergence problems.
Want to know more about contact and convergence?
The documentation is a natural place to begin to research the modeling methods for these interesting and potentially troubling models. And there are some training courses available for those who want to know even more about the topics of contact and convergence. The SIMULIA Community is a good source of information on all things SIMULIA.
Click here for the SIMULIA Tech Talk.
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