To model dynamic contact events, two specialized contact methods are provided, the Penalty, dynamic and the Augmented Lagrangian, dynamic methods. Both are based on a viscous formulation that constrains the gap rate to be zero, ensuring that the normal contact is dissipative and does not introduce any spurious energy contribution to the system. Since the methods are dissipative, they are mainly intended for short duration events, such as soft impact between two bodies. For prolonged interaction between two bodies, energy dissipation can become significant, and overclosures can become large, since the gap rate is only approximately zero. Both the dissipation and the accuracy are controlled by a penalty factor that for these two methods conceptually represents a dashpot, rather than a spring. It therefore has a characteristic time user input that sets its magnitude. As a rule-of-thumb, it should be of the order of the contact event duration, but the best choice must be decided on a case-by-case basis.

The Penalty, dynamic method also provides the possibility to combine the stiffness and viscous based penalization of the normal contact. For impact analysis, it is often best to use only the viscous formulation by setting the stiffness Penalty factor control to Viscous only.

When modeling dynamic contact, the main interest is often the kinematics between the contacting bodies. If you rely on the (default) adaptive time-stepping algorithm, the solver typically also tries to resolve the wave propagation in the domains adjacent to the contact pairs. This can cause unnecessarily small time steps and increase the computation time. To avoid this, you can modify the solver to use a manual time-stepping algorithm in the settings for the Time-Dependent Solver in the solver sequence. Make sure to use time steps that are small enough to capture the contact event, otherwise spurious energy contributions can result, and cause the problem to “blow up”.

The time-dependent solvers in COMSOL Multiphysics introduce numerical damping to stabilize the time stepping. This kind of stabilization is often necessary. However, excessive numerical damping runs the risk of removing vital information from the simulation. For this reason, the BDF solvers should be avoided for dynamic contact analyses. The default solver suggests the generalized-α solver when inertial terms are included in the structural mechanics problem.

Regardless of the method that is used, and how the solver is set up, it is good practice to do an a posteriori check of conservation of momentum and energy, to ensure that the solution is acceptable.