The Explicit Dynamics () study step is a special case of the Time Dependent study step. It only solves for physics interfaces and multiphysics couplings that are set up for explicit dynamics analysis, for example the Solid Mechanics, Explicit Dynamics interface. The study steps configures the Time-Dependent Solver to use an explicit solver type for time stepping.

See Time Dependent for all other settings.

Select the type of Mass Matrix to use, Lumped or Consistent. Using a lumped (diagonal) mass matrix with explicit time stepping leads an efficient update of each time step since no inversion of the mass matrix is necessary. In contrast, using a consistent mass matrix require the inversion of the mass matrix by a linear solver which will add cost to each time step. The type of mass matrix used also impacts the dynamic properties and stability of the model equations. Hence, it can be motivated to use a consistent mass matrix in some cases, but for the majority of problems a lumped mass matrix is the preferred option.

The physics interfaces suggest an adaptive time step within the stability limit of the explicit solver. Select the Time step safety factor checkbox and enter a value in the field to scale the suggested time step. It is sometimes necessary reduce the time step since the suggestion by the physics interface is always an estimate of the true stable time step of the model, that does not consider all possible mass and stiffness contributions. For example, the stiffness added boundary conditions like contact implemented by penalty methods is not accounted for.

Computing the time step can be expensive. Therefore, this is by default only done every 50th time step. To change the frequency by which the time step is updated, select the Time step update frequency checkbox and enter a value in the associated field.

A large part of the computational cost for many explicit dynamics simulations relates to the contact search performed by Contact Pair and General Contact Pair nodes in the model. The implementation of the hierarchical search method allows for some data structures and results of the contact search to be kept between time steps. Considering the large number of steps used by an explicit solver, only updating such data and results at selected intervals can significantly reduce the computational cost. The loss in accuracy is often negligible thanks to the small time steps, and that the fine search and final mapping is always updated if the mesh has moved between steps. Select the Contact update frequency checkbox to manually control how frequently the contact updates are done.