TYPE7

When defining an interface TYPE7 with solid element parts on the master side, is it necessary to put shells on the external surface of the part?

No, only the external surface of the solid should be defined.

As of Radioss V9.0, it is possible to define the surface of an external skin of a solid part (or a subset, a material, a property, and so on) with the option /SURF/PART/EXT.

With older Radioss versions, this surface should be built using segments (a surface type /SURF/SEG will be created but no additional element added).

When defining an interface TYPE7, which is neither self-impacting nor symmetric, how are the master side and the slave side chosen?

It is recommended to choose the coarse mesh on the master side for better contact detection.

In the following example, contact is not detected if the fine mesh is chosen on the master side.

In case both meshes are of the same size, the stiffer structure on the master side should generally be chosen.

How is the initial interface stiffness computed for an interface TYPE7?

The interface stiffness is a nonlinear function of the penetration p and goes to an infinite value when the slave node gets very close to the master segment in order to prevent the node from crossing the master segment and hence, better represent contact.

The initial stiffness K0 (that is to say the stiffness when the node enters the gap), computed by Radioss takes into account either both master and slave stiffness (flag I_stf =2 to 5) or only master stiffness (old way corresponding to I_stf =0).

The stiffness is computed according to geometrical and material characteristics of the elements on master side and/or on slave side (depending on the value of I_stf).

For shell elements:(1)

K_{0} = \frac{1}{2} S t f a c \cdot E \cdot t

Where,

$E$: Young modulus of the material
$t$: Thickness of the shell

For solid elements:(2)

K_{0} = \frac{1}{2} S t f a c \cdot B \cdot \frac{A^{2}}{V}

Where,

$B$: Bulk modulus of the material
$A$: Area of the segment
$V$: Volume of the solid element
Stfac: Scale factor which can be modified by the user with a default value equal to 1

For an interface TYPE7 with no initial penetration, there are problems during the computation: the time step on nodes which are in contact drops and the computation is unstable (some energy is created); why?

First check the gap that is used for this interface:

The value must be physically realistic and based on shell thicknesses in case of contact between shells.

In case of constant gap (I_gap =0) with no input gap (Gap_min=0), Radioss determines a default value for Gap_min and you can check this value in the output file (Runname_0000.out).

Also check stiffness' on slave and master side. If they vary by a ratio greater than 100, the stiffer structure must be on the master side for I_stf=0 so that the interface initial stiffness will be greater and even though the interface initial time step will be lower this will increase the time step if penetration is large. Indeed the penetration will be lower for the same energy absorption and thus for “stopping the slave node”.

The following formula estimate the stiffness on slave and master side:

	Shells	Solids
Stiffness on Slave side $K_{s}$	$K_{s} = \frac{1}{2} \cdot E \cdot t$	$K_{s} = B \cdot \sqrt[3]{V}$
Stiffness on Master side $K_{m}$	$K_{m} = S t f a c \cdot \frac{1}{2} \cdot E \cdot t$	$K_{m} = S t f a c \cdot B \cdot \frac{S^{2}}{V}$

Where,

$S$: Segment area
$V$: Volume of the solid
$B$: Bulk modulus
$E$: Young modulus of the material
$t$: Thickness of the shell

If it is necessary to keep the less stiff structure on the master side (for instance because its mesh is coarser), the value for the interface stiffness factor Stfac can be changed to increase the interface initial stiffness (otherwise the default value 1 for Stfac is generally well-suited).

For shell Stfac can then be set to the following value:(3)

S t f a c = \frac{K_{s}}{K_{m}}

The previous method of choosing the maximum stiffness between master side and slave side would be automatically set up with I_stf =3; but it may still be necessary to use other ways of setting the interface stiffness in case the stiffness of master and slave side are very different.

If some elements fail on the master side or on the slave side of the interface, it is important to set I_del =2 (or I_del =1) for this interface. This will prevent nodes connected to deleted elements whose stresses are released to impact with a possible high velocity.