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Section 8.1: Rigid Body Constraints Up Chapter 8: Contact and Constraints Subsection 8.2.1: Rigid Interfaces

## 8.2 Contact

FEBioStudio can be used to set up several types of contact conditions. These contact conditions allow the connection of non-conforming meshes to each other or the specification of non-penetration constraints. FEBioStudio supports the following contact interface categories.
 Interface Description Rigid Connect a deformable mesh to a rigid body Sliding Enforce a non-penetration constraint between two bodies Tied Tie two non-conforming surfaces together
To add a contact definition, select the Physics/Add Contact menu. A dialog box shows up that allows you to select the step for which the contact definition is to be active (or select Initial if the definition is to be active during all steps). To add a particular contact condition, select an option from the list and click on the Add button.
In FEBio, contact constraints are enforced using an Augmented Lagrangian approach. This implies that the Lagrange multipliers are only approximated to a user-specified tolerance. The following parameters will appear in nearly all contact interfaces.
• augmented Lagrangian: Turn the augmented Lagrangian method on or off. When off, a penalty method is used for constraint enforcement.
• augmentation tolerance: Set the convergence tolerance for the Lagrange multipliers.
• Penalty factor: The penalty factor controls the rate of convergence. A high penalty factor will try to reach the tolerance quickly, but if chosen too high might introduce instability into the system. If it is too low, convergence to within the specified tolerance might not be reached.
For sliding interfaces, there will usually be an option for two-pass option. In a usual contact implementation, the required integrations are only performed over one of the surfaces, usually referred to as the slave surface. If the contacting surfaces are perfectly smooth, it does not matter which surface acts as the slave or master surface. However, in an FE simulation the surfaces are discretized and are most likely non-conforming. Thus, the choice of master and slave surfaces is important and will introduce bias in the solution. It is usually advisable to select the more finely meshed surface as the slave. In the two pass algorithm, an attempt is made to reduce the bias by performing the contact calculations twice, with the roles of slave and master surface switched for the second pass. Although it may appear that the two pass algorithm is always the best choice, this is not always so. Certain contact applications perform better using a single pass. See the FEBio Theory Manual for a more detailed description of the contact model.
After the contact parameters are entered the user needs to define the master and slave surfaces. To do this, first close the contact interface dialog and select the contact interface in the Model Editor. In the Model Editor you will now notice two selection boxes, one for the master surface and one for the slave surface. The boxes work similarly as for boundary conditions. For instance, to add a surface of your model to the master surface, select the surface in the Graphics View and press the '' button in the master's surface selection box.
A description of the different contact definitions follows, although you should consult the FEBio User's Manual for a detailed overview of the parameters and use of the various contact interfaces.
Section 8.1: Rigid Body Constraints Up Chapter 8: Contact and Constraints Subsection 8.2.1: Rigid Interfaces