Best Practices for Specifying Contact Conditions

This section describes some of the practices you should try to keep in mind while defining the properties of the contact conditions for your model.

Mesh Requirements

Defining a proper mesh is critical to contact conditions. A well-defined mesh ensures accurate stress measurements at a contact region. Furthermore, a quality mesh is essential for nonlinear contact conditions in order to obtain an accurate solution. This is especially true for curved surfaces. Use local Mesh Controls, such as Proximity Controls and Contact Sizing controls to better ensure mesh quality. Review the Apply Mesh Controls and Preview Mesh section of the Help for more information on this topic.

Selecting Contact Formulation

Mechanical provides the following options for the Formulation property:

Formulation methods work in combination with the specified contact Types (Bonded, No Separation, Frictionless, Rough, Frictional, and Forced Frictional Sliding). The Augmented Lagrange method is the default Formulation property for all contact types.

However, you can use the Bonded and No Separation contact types with the Multi-Point Constraint (MPC) Formulation method. The examples listed below outline cases when this option is useful. Please see the Selecting a Contact Algorithm (KEYOPT(2)) section of the Mechanical APDL Contact Technology Guide for additional technical information about choosing contact formulations.

Overlapping Contact Conditions and Boundary Conditions

To avoid contact conditions that overlap constraints, use the Bonded or No Separation contact types because you will see an overall correct solution, however, the reported reactions will be inaccurate.

This same phenomenon occurs in a less obvious way when you attempt to apply a Remote Displacement to a rigid body that also has bonded contact using a penalty based formulation.

The example illustrated below shows a remote constraint applied to a rigid body that is also has No Separation contact using a penalty formulation. In this example, the solution is correct, however, inaccurate reactions are obtained on the Remote Displacements because it is connected to the contact region via the MPC equations created. Using a remote displacement causes the solver to reorder the CE’s such that constrained node shares a CE with the bonded contact. This results in inaccurate reactions.

Using a General Joint instead of a remote displacement avoids the issue.

Regardless of the MPC formulation selection, MPC-based contact is used for Remote Boundary conditions. It is good practice to avoid having two or more MPC-based boundary conditions overlap. The solver does however attempt to negotiate and resolve the overconstraint conditions. The application issues a warning in this situation.

Intelligent use of Contact Trimming as well as the Pinball setting on remote boundary conditions can also be effective tools to mitigate this behavior.

In addition, MPC as well as other FE connections can be viewed via the Solution Information feature to help you graphically view the distribution of MPC equations in a model. These equations are generated from the MAPDL contact elements. See the Using Finite Element Access to Resolve Overconstraint tutorial for an example of an overconstraint situation along with steps to identify and correct it.

Contact Behavior

Properly choosing your source and target topology is also important. See the specific guidelines outlined in the MAPDL contact documentation. The default behavior is auto-asymmetric wherein the MAPDL solver determines the optimal source/target. Using a pure asymmetric behavior is suggested only for users willing to closely review each contact pair and able to determine the proper configuration.

Tip:  Using the Initial Contact tool can help you determine which side the MAPDL solver chooses to keep in the analysis.

Initial Contact Tool

The Initial Contact Tool can be invaluable in determining that the contact is properly defined. It is also useful to determine the proper side for the source/target. Further the Initial Contact Tool can be useful to:

Diagnostic Tools, NR Residuals, and Contact Result Trackers

You can use NR residuals and result trackers to help obtain a fully converged analysis. For example:

Contact Tool Results

Following the solution process, it is strongly recommend that you insert a Contact Tool to check penetration. Penetration units are the same as that of displacement - compared with displacements in local area.

For example, if local displacements are 2mm but penetration is 0.02mm, would a change in displacements by +/- 0.02mm influence overall results (including local stresses)? By comparing penetration to the results in local area (not maximum deformations of entire model), you can determine if penetration values are acceptable or not.

Caution:  Do not assume that penetration values are always negligible because your solution converged. You need to verify this after the solution.

If you believe that penetration is excessive, modify the Penetration Tolerance (Augmented Lagrange), Normal Stiffness (Penalty or Augmented Lagrange), or use the Pure Lagrange formulation to reduce the penetration.

Release 15.0 - © SAS IP, Inc. All rights reserved.