This section describes useful techniques for making sure that you establish robust contact settings and conditions.
Mesh Quality
Poor mesh quality can cause convergence problems, especially when nonlinear contact is applied on it. Examples of mesh quality are illustrated below. Understanding the use of Local Mesh Controls can help you refine the mesh on your contact conditions.
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The contact surface shown here has a mesh that could be improved. |
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This contact surface shown here has a very good mesh quality. |
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Tip: For nonlinear contact models, you can either set Physics Preference to Mechanical and Error Limits to Aggressive Mechanical, or set Physics Preference to Nonlinear Mechanical. See Ansys Workbench and Mechanical APDL Application Meshing Differences for details.
Mesh Sizing
In order to create smooth results, you need to make sure that the elements closely follow the curvature and that your model has sufficient contact elements on curved surfaces, including straight surfaces that may become curved as the analysis proceeds. This is especially true for nonlinear contact conditions. For the best results, use similar mesh densities on both sides of the contact pair. To improve element size and density, modify the mesh using Sizing Controls.
This example shown below illustrates a curve with not enough elements.
The parts are now shown below with improved element sizes and density for each contact side.
Understanding Contact Formulation
Formulation options allow you to specify which algorithm the software uses for a particular Contact pair computation. Formulations define the solution method used. See the Contact Formulation Theory section for additional information.
(Default): this setting is suitable for most problems.
: this setting is suited to contact occurring only on an edge or vertex.
(Multi-Point Constraint): this setting is ideal for all linear contacts when there is no over-constraint.
: this Formulation option:
provides the highest accuracy.
works well with material nonlinearities.
works well with shells or thin layers.
enables interference fit
allows large sliding
: this setting is ideal for linear contact when there may be over-constraint.
Advantages of MPC Contact
The MPC Formulation option does not provide stiffness settings. It is a purely linear way to connect contacting bodies. This setting is ideal for shell-solid, shell-shell, and beam-shell contacts. MPC prevents artificial stiffness when gaps exist between curved surfaces (another alternative is to use fixed Joints).
One caveat is that MPC is the most sensitive contact type to overconstraint so it should be avoided when there are other contacts or boundary conditions that overlap.
Following a completed solution, you can view MPC equations and other FE Connections graphically.
Contact Stiffness
Using the Normal Stiffness property, you can manually increase the stiffness associated with a contact pair. A high stiffness setting can lead to reduced penetration and an increase in accuracy. However, it can also lead to ill-conditioning and divergence.
For contacts that lead to convergence difficulty, you should try lowering the stiffness.
For pretension problems, increase the stiffness, because penetration can strongly influence the pretension forces.
And, when there is difficulty converging due to high penetration, you may need to increase the stiffness.
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Here is an example of poor convergence. 122 iterations were required and there are too many bisection instances. |
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This example shows very good convergence as a result of the stiffness being reduced. No bisections have occurred and only 30 iterations were required. |
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Overlapping Contact and Boundary Conditions
Care should be taken when a contacts and constraint type boundary conditions have overlapping topology. In this case, you can use modeling techniques to eliminate the overconstraint, including:
Modifying the Contact Trimming.
Modifying the Pinball Radius property on the remote boundary conditions. This property specifies a radius (length unit) that the boundary condition is applied to the elements that are within the region.
Implementing node-based scoping on the boundary conditions.
If you cannot eliminate an overcontraint situation, the use of the MPC Formulation should be avoided. The Beam Formulation option can be a good alternative in this case.
Initial Gaps and Rigid Body Motion
There are times when parts (that have exterior constraints placed on them) are dependent upon contact to prevent rigid body motion. In addition, when small gaps exist (especially for curved contact conditions), rigid body motion can occur at the beginning of the solution before contact engages.
When nonlinear contact is present, small gaps that are initially open can lead to rigid body motion (as shown below). Changing the Interface Treatment property to can be an effective means to resolve this gap. Mathematical adjustment to close the gap causes an open region to be visible when post-processing. This gap is simply offset by the contact elements. Note that for concentric cylinders, is not recommended. Instead the offset should be manually entered.
An alternative is to employ the Stabilization Damping Factor property. Rather than ignoring the gap by offsetting the contact elements, damping is applied and makes the approach of the bodies towards each other less difficult for the solver to handle, leading eventually to closed contact. An example is shown below. Note that you need to use care and verify that the damping does not negatively affect the accuracy of your analysis.
