Objective:
Learn what methods are available for connecting FE meshes.
Applies to:
 Design Analysis
 Topology Optimization (only Tie Constraint and Rigid Connector are supported)
 Rigid Connector
 Tie Constraint
 Structural Bonded Contact
 Thermal Bonded Contact
Procedure:
Method 1: Rigid Connector
The Rigid Connector block can be used to define a connection between a point and the surface of a component in a simulation or optimization study. This is typically used in multicomponent simulations where only the mass of the auxiliary components affects the analysis, while the geometry of these components can be neglected. These auxiliary components are modeled as Point FE Component in nTop. This can be used to reduce the size of the finite element model, improving the simulation or optimization solver's performance without compromising the results' accuracy.
This Rigid Connector system, in combination with point masses, can be used in studies where the mass participation of the components is essential, like a Modal Analysis, a Buckling Analysis, or an Optimization study (Topology or Field Optimization) with a Frequency Response as a constraint or an objective.
The block has the following inputs :
 Point: This is where the block will create a remote point. This is typically at the center of mass of the component being represented. To learn more about the attributes that can be applied to the point, please look at the Point Attribute Learn More link.
 Boundary: This defines the boundaries (typically nodes on a surface) that will be connected to the point with rigid elements

Connector: This defines the behavior of the nodes on the boundary linked to the motion of the point and has two options 
 Rigid: The rigid option constrains the nodes on the boundary to move with the point. It prevents relative motion between the nodes on the boundary and behaves like a rigid entity. This is popularly known as the RBE2 connector that connects an independent node (green) to a set of dependent nodes (yellow)

 Flexible: This flexible option links the motion of the point to the nodes on the boundary and enables relative motion between these nodes. This distributes the mass between the nodes on the boundary and avoids locally stiffening the model (unlike the Rigid connector). This is popularly known as the RBE3 connector that connects a dependent node (yellow) to a set of independent nodes (green)
You can read about an example of using a Rigid Connector here.
Method 2: 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. An error will occur if the tolerance is too low to find the boundary. 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.
You can read here for an example of using a Tie Constraint.
Method 2: Structural Bonded Contact
A Structural Bonded Contact block is an elastic connection between the two meshes, allowing relative motion with a defined stiffness. This is 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 calculating contact failure and to more accurately model connected/welded surfaces, but generally for any connection dictated by a finite stiffness.
The nodes are connected as if a spring of contact stiffness defined is attached between them.
Choose the Independent and Dependent nodes using a Boundary Selection block (How to select boundaries of an FE Mesh  FE Boundary by Body and How to select boundaries of an FE Mesh  FE Boundary by Flood Fill). The Independent boundary looks for the Dependent boundary within a specified tolerance. An error will occur if the tolerance is too low to find the boundary. This error can be fixed by increasing the tolerance or editing the boundary.
The contact stiffness is suggested to be the average of two Young's moduli.
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, allowing 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.