# Objective:

Learn what methods are available for connecting FE meshes.

# Applies to:

- Design analysis
- Topology Optimization (only tie constraints supported)

# Procedure:

**Method 1: Tie Constraint**

A Tie Constraint is a rigid connection that ties two nodes together. Tie constraints make the displacement of the selected nodes equivalent, effectively removing one degree of freedom from the system.

Choose the Independent nodes and the Dependent nodes using a Boundary Selection block. The Independent boundary looks for the Dependent boundary within a specified tolerance. If the tolerance is too low to find the boundary, an error will occur. This error can be fixed by increasing the tolerance or editing the boundary. The Rotation checkbox toggles the option to tie rotational degrees of freedom.

Go here for an example of using a Tie Constraint.

**Method 2: Structural Bonded Contact**

A structural bonded contact acts as an elastic connection between the two models that allows relative motion with a defined stiffnesse. It's usually used if you have two surfaces that are glued together with some material that has a vastly different stiffness than the connected surfaces and that in turn allows the connected surfaces to not move completely in sync but have certain resistance between them. It's often used for the calculation of contact failure and to more accurately model connected/welded surfaces, but generally for any connection that is dictated by a finite stiffness.

Choose the Independent nodes and the Dependent nodes using a Boundary Selection block. The Independent boundary looks for the Dependent boundary within a specified tolerance. If the tolerance is too low to find the boundary, an error will occur. This error can be fixed by increasing the tolerance or editing the boundary.

The contact stiffness is suggested to be 0.5 * (Young's Modulus of Part 1 + Young's Modulus of Part 2).

Another method of determining the contact stiffness is k = E * A/d (where A is the contact area and d is the contact layer thickness).

**Method 3: Thermal Bonded Contact**

Creates a bonded contact between two FE boundaries which allows for heat flux between the two components. Contact resistance can be defined to account for a thin membrane material at the boundary, such as thermal paste. The contact resistance between two materials is often calculated from experimental tests of contact conductance.

Click here for an example using thermal bonded contact.

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