# Interface Stiffness

The interface stiffness (`K`) is not constant, it increases with the
penetration. Moreover, there is a viscous damping acting on the rate of penetration.
The contact force is then computed as:

The instantaneous stiffness is then computed as:

Nodal time step can be seriously affected if penetration is large. The stiffness, used to compute the nodal time step takes into account the interface stiffness.

- Increasing the gap
- Increasing the initial stiffness (through the use of the flag
`Stfac`)

Both methods allow absorbing more energy by contact and smoothing the impact. Increasing the gap will allow nodes to slow down over a larger distance, therefore the penetration is reduced.

## Comments

- Even if an elementary time step is chosen for the simulation, a nodal time step is automatically computed if there is an interface TYPE7. The lowest time step is applied for the simulation.
- Contrary to interface TYPE5, a
`Stfac`smaller than 1.0 produces a large penetration at the first touch and results in high interface stiffness and reaction force. To avoid high penetration, a`Stfac`greater than or equal to 1.0 is recommended.Although, increasing the initial stiffness leads to a smaller time step at the beginning of the penetration, it will increase the time step if penetration is large.