# To MPC or Not to MPC: ANSYS Contact Formulations

MPC contact is a fancy term for Multi Point constraint contact. It’s one of the many ways to define contact in ANSYS for situations where the surfaces are bonded and can’t separate.

MPC contact is usually the best choice when it comes to bonded and non-separating surfaces. However, there are times when it can cause over constraints (which we’ll explain later).

### The advantages of MPC contact

1. The MPC contact algorithm is not based on penalty or Lagrange methods (which you can read more about elsewhere).
2. There’s no need to worry about contact stiffness.
3. MPC adds internal equations to connect the movements of the surfaces in contact.
4. The DOF (degrees of freedom) of each contact node are eliminated. Instead, all DOF have to match the pilot node, which is defined by the MPC equation.
5. Because of the above, MPC is a highly efficient contact method.
6. If there aren’t any material or geometric non-linearities, MPC contact represents a truly linear situation. This means you only need one equilibrium iteration to solve the system of equations, resulting in faster run times compared to other contact methods.
7. For large displacement models, the MPC equations will update at each equilibrium iteration, overcoming the limitations of conventional constraint equations.
8. MPC contact can be used in many scenarios where conventional algorithms wouldn’t work.
9. It supports several types of contacts: Solid-Solid, Shell-Shell, Shell-Solid, Beam-Shell, and Beam-Solid.

MPC contact also lets you ignore unwanted gaps in CAD geometry without having to modify the geometry itself.

### The Limitations of MPC contact

1. Over constraint can occur. You might see a warning message like this:

“One or more MPC contact regions or remote boundary conditions may have conflicts with other applied boundary conditions or other contact or symmetry regions. This may reduce solution accuracy.”

This means that one degree of freedom is subjected to multiple constraints. Sometimes, the solver will try to remove some constraints, but in other cases, it may not run at all. Overconstraint can happen in these situations:

• A displacement or temperature boundary condition is applied to the MPC contact surface nodes.
• Multiple constraint equations are defined for the same surfaces.
• Rigid bodies and joints are connected via MPC contact.

2. MPC contact cannot link rigid-flexible bodies if the rigid body is modeled via a primitive segment (circle, cylinder, cone, or sphere).

3. There are several other specific cases where MPC contacts may not work or are not ideal. You can find more details under ANSYS Help section 10.6.

Good Luck!