# Initial Penetrations

In an FE model, initial penetrations are very common, are unavoidable and result from the discretization during the meshing process (Figure 1).

## Inacti

Special treatment for initial penetrations can be accomplished through the use of the Inacti flag. It is possible to remove penetrated nodes from the interface or to remove the main segments relating to the penetrated nodes. Both treatments allow getting rid of initial penetrations very easily, but they may lead to poor results if the number of penetrated nodes is large.

Setting Inacti to 3 allows Radioss Starter to automatically modify the coordinates of penetrated nodes to avoid initial penetrations. Special care must be taken when doing so, since this operation can lead to initially constrained springs.

It is also possible to obtain a variable gap in time by setting Inacti to 5. The illustration Figure 2 explains how the effective gap is updated taking into account the previous penetrations.
At t=0, if a node is initially penetrated, its gap is automatically corrected. Then this "initial corrected gap" will be increased every time the node is moving away from the main segment. This option is mainly used for unfolding the airbag, it allows a decent time step at the beginning of the unfolding, whereas nodes are all highly penetrated.

To avoid high frequency effects Inacti = 6 is recommended instead of Inacti =5.

## Fpenmax

Fpenmax (maximum fraction of initial penetration), is used to deal with big initial penetration. Node stiffness will be deactivated, if $Penetration\ge Fpenmax\cdot Gap$ , whatever the value of Inacti.

## Igap3 + %mesh_size

With Igap= 3 and %mesh_size, the size of the mesh can be taken into account to avoid initial penetrations. In this case, the variable gap is computed as:(1)
$\mathrm{max}\left\{Ga{p}_{\mathrm{min}},\mathrm{min}\left[Fscal{e}_{gap}\cdot \left({g}_{s}+{g}_{m}\right),%mesh_size\cdot \left({g}_{s_l}+{g}_{m_l}\right),Ga{p}_{\mathrm{max}}\right]\right\}$
Where,
${g}_{m_1}$
Length of the smaller edge of element
${g}_{s_1}$
Length of the smaller edge of elements connected to the secondary node

## Irem_gap

The option Irem_gap is used to deactivate secondary nodes which close (Curvilinear $Distance<\sqrt{2}\cdot Gap$ ) to elements. This option is useful for self-impact contact when mesh size is very small.
Note: When dealing with initial penetrations, it is strongly advised to remove initial penetrations during the creation of the FE model, using pre-processing tools like HyperMesh and HyperCrash depenetrators.