/INTER/TYPE20 (Obsolete)
Block Format Keyword This is a general single surface or surface to surface contact interface.
Edge to edge contact is also possible. Penalty stiffness is constant and therefore the time step is not affected (for standard penalty stiffness). This contact interface can replace interface TYPE3, TYPE5, TYPE7, TYPE11 or TYPE19. The interface is basically defined in terms of one or two surfaces. If only one surface is used, this surface is selfimpacting. If two surfaces are defined, nodes of surface two impact surface one. A symmetric treatment can be activated. Edges of surface one and two can be taken into account for the contact. Nodes can be added to surface.
Format
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

/INTER/TYPE20/inter_ID/unit_ID  
inter_title  
surf_ID_{1}  surf_ID_{2}  I_{sym}  I_{edge}  grnd_ID  line_ID_{1}  line_ID_{2}  edge_angle  
I_{gap}  I_{bag}  I_{del}  
Fpenmax  
Blank Format 
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

Stfac  Fric  Gap_{0}  T_{start}  T_{stop}  
I_{BC}  Inacti  VIS_{s}  VIS_{F}  
I_{fric}  I_{filtr}  X_{freq}  I_{form} 
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

C_{1}  C_{2}  C_{3}  C_{4}  C_{5} 
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

C_{6} 
Definitions
Field  Contents  SI Unit Example 

inter_ID  Interface
identifier (Integer, maximum 10 digits) 

unit_ID  Unit Identifier (Integer, maximum 10 digits) 

inter_title  Interface
title (Character, maximum 100 characters) 

surf_ID_{1}  First surface
identifier (Integer) 

surf_ID_{2}  Second surface identifier
(Integer) 

I_{sym}  Symmetric contact
treatment of the nodes and the surface flag
(Integer) 

I_{edge}  Edge definition flag
(Integer) 

grnd_ID  Nodes group identifier used to add nodes to surface
nodes (Integer) 

line_ID_{1}  First line identifier (Integer) 

line_ID_{2}  Second line
identifier (Integer) 

edge_angle  Edges angle Default = 91 (Real) Use only if I_{edge} = 3 If angle between two edges is smaller than edge_angle, the edge is considered. 
$\left[\text{rad}\right]$ 
I_{gap}  Gap/element option flag.
(Integer) 

I_{bag}  Airbag vent holes closure
flag in case of contact.
(Integer) 

I_{del}  Node and segment deletion
flag. 5
(Integer) 

Fpenmax  maximum initial
penetration factor (0 < Fpenmax ≤ 1). 10 Default = 1.0 (Real) 

Stfac  Interface stiffness scale
factor. Default = 1 (Real) 

Fric  Coulomb
friction. (Real) 

Gap_{0}  Minimum gap for impact
activation. 6
If I_{gap} = 0 default is: $\mathit{Ga}{p}_{0}=\mathrm{min}\left(t,\frac{l}{10},\frac{{l}_{\mathrm{min}}}{2}\right)$ (Real) 
$\left[\text{m}\right]$ 
T_{start}  Start
time. (Real) 
$\left[\text{s}\right]$ 
T_{start}  Time for temporary
deactivation. (Real) 
$\left[\text{s}\right]$ 
I_{BC}  Deactivation flag of
boundary conditions at impact. (Boolean) 

Inacti  Deactivation flag of
stiffness in case of initial penetrations. 9
(Integer) 

VIS_{s}  Critical damping
coefficient on interface stiffness. Default = 0.05 (Real) 

VIS_{F}  Critical damping
coefficient on interface friction. Default = 1.0 (Real) 

I_{fric}  Friction formulation flag.
13
14
(Integer) 

I_{filtr}  Friction filtering flag.
15
(Integer) 

X_{freq}  Filtering
coefficient. (Real) 

I_{form}  Friction penalty
formulation type.
(Integer) 

C_{1}  Friction law
coefficient. (Real) 

C_{2}  Friction law
coefficient. (Real) 

C_{3}  Friction law
coefficient. (Real) 

C_{4}  Friction law
coefficient. (Real) 

C_{5}  Friction law
coefficient. (Real) 

C_{6}  Friction law
coefficient. (Real) 
Flags for Deactivation of Boundary Conditions: IBC
(1)1  (1)2  (1)3  (1)4  (1)5  (1)6  (1)7  (1)8 

I_{BCX}  I_{BCY}  I_{BCZ} 
Definitions
Field  Contents  SI Unit Example 

I_{BCX}  Deactivation flag of X
boundary condition at impact.
(Boolean) 

I_{BCY}  Deactivation flag of Y
boundary condition at impact.
(Boolean) 

I_{BCZ}  Deactivation flag of Z
boundary condition at impact.
(Boolean) 
Comments
 The simplest input is to enter only one selfimpacting surface surf_ID_{1}. Symmetric treatment flag I_{sym} is used for node to surface contact but edge to edge contact is always symmetric.
 To emulate an interface TYPE7 or
TYPE11 input:
TYPE20 to Emulate TYPE7 Input TYPE20 to Emulate TYPE11 Input (TYPE20) surf_ID_{1} = surf_ID_{m} (TYPE7) (TYPE20) grnd_ID = grnd_ID_{s} (TYPE7)
(TYPE20) line_ID_{1} = line_ID_{s} (TYPE11) (TYPE20) line_ID_{2} = line_ID_{m} (TYPE11)
(TYPE20) line_ID_{1} = 0
(TYPE20) line_ID_{2} = 0
(TYPE20) surf_ID_{1} = 0 (TYPE20) grnd_ID = 0
(TYPE20) surf_ID_{2} = 0 (TYPE20) I_{sym} = 2
(TYPE20) I_{edge} = 0
(TYPE20) surf_ID_{2} = 0 (TYPE20) I_{sym} = 0
(TYPE20) I_{edge} = 0
 In case of SPMD, each main segment defined by surf_ID_{m} must be associated to an element (possibly to a void element).
 For the flag I_{bag}, refer to the monitored volume option (Monitored Volumes (Airbags)).
 Flag I_{del} = 1 has a CPU cost higher than I_{del} = 2.
 If I_{gap} = 0, a default
value used for Gap_{0,} which is
computed as:
(1) $$Ga{p}_{0}=min\left(t,\frac{l}{10},\frac{{l}_{min}}{2}\right)$$With, t
 Average thickness of the main shell elements
 l
 Average side length of the main brick elements
 l_{min}
 Smallest side length of all main segments (shell or brick)
 If I_{gap} = 1, the gap is
computed for each impact as:
(2) $${g}_{s}+{g}_{m}$$With, g_{m}: main element gap:
${g}_{m}=\frac{t}{2}$ , with t is the thickness of the main element for shell elements
g_{m} = 0 for brick elements
 g_{s}: secondary node
gap:
g_{s} = 0 if the secondary node is not connected to any element or is only connected to brick or spring elements.
${g}_{s}=\frac{t}{2}$ , with t is the largest thickness of the shell elements connected to the secondary node.
${g}_{s}=\frac{1}{2}\sqrt{S}$ for truss and beam elements, with S being the cross section of the element.
If the secondary node is connected to multiple shells and/or beams or trusses, the largest computed secondary gap is used.
If the free edge of a shell element is in contact, then I_{gap} can shift the gap of the free edges border shells, as:
 g_{m}: main element gap:
 Deactivation of the boundary condition is applied to secondary nodes group (surf_ID_{s}).
 Inacti = 3 may create
initial energy if the node belongs to a spring element.Inacti = 5:
 Maximum penetration value is set
as a fraction of the actual gap (including variable gap):
$Penetration\ge Fpenmax\cdot Gap$
If the initial penetration of a secondary node is greater than the calculated maximum value (Fpenmax), the node will be deactivated from the interface (node stiffness deactivation).
 One node can belong to the two surfaces at the same time.
 There is no limitation value to the stiffness factor (but a value can be greater than 1.0 can reduce the initial time step).
 For Friction Formulation:
 If the friction flag is 0 (default), the old static
friction formulation is used:
${F}_{t}\le \mu \cdot {F}_{n}$ with $\mu =\mathit{Fric}$ ( $\mu $ is Coulomb friction coefficient)
 For flag I_{fric} > 0, new
friction models are introduced. In this case, the friction coefficient is
set by a function
$\mu =\text{\mu}(\rho ,V)$
,Where,
 $\rho $
 Pressure of the normal force on the main segment
 $V$
 Tangential velocity of the secondary node
 If the friction flag is 0 (default), the old static
friction formulation is used:
 Currently, the coefficients C_{1} through C_{6} are used to define a variable
friction coefficient
$\mu $
for new friction formulations.The following formulations are available:
 I_{fric} = 1
(Generalized viscous friction law):
(3) $$\mu =\mathit{Fric}+{C}_{1}\cdot p+{C}_{2}\cdot V+{C}_{3}\cdot p\cdot V+{C}_{4}\cdot {p}^{2}+{C}_{5}\cdot {V}^{2}$$  I_{fric} = 2
(Modified Darmstad law):
(4) $$\mu =Fric+{C}_{1}\cdot {e}^{\left({C}_{2}V\right)}\cdot {p}^{2}+{C}_{3}\cdot {e}^{\left({C}_{4}V\right)}\cdot p+{C}_{5}\cdot {e}^{\left({C}_{6}V\right)}$$  I_{fric} = 3
(Renard law):
$\mu ={C}_{1}+\left({C}_{3}{C}_{1}\right)\cdot \frac{V}{{C}_{5}}\cdot \left(2\frac{V}{{C}_{5}}\right)$ if $V\in \left[0,{C}_{5}\right]$
$\mu ={C}_{3}\left(\left({C}_{3}{C}_{4}\right)\cdot {\left(\frac{V{C}_{5}}{{C}_{6}{C}_{5}}\right)}^{2}\cdot \left(32\cdot \frac{V{C}_{5}}{{C}_{6}{C}_{5}}\right)\right)$ if $V\in \left[{C}_{5},{C}_{6}\right]$
$\mu ={C}_{2}\frac{1}{\frac{1}{{C}_{2}{C}_{4}}+{\left(V{C}_{6}\right)}^{2}}$ if $V\ge {C}_{6}$
Where,
${C}_{1}={\mu}_{s}$ ${C}_{2}={\mu}_{d}$
${C}_{3}={\mu}_{\mathrm{max}}$ ${C}_{4}={\mu}_{\mathrm{min}}$
${C}_{5}={V}_{\mathit{cr}1}$ ${C}_{6}={V}_{cr2}$
 First critical velocity ${V}_{cr1}={C}_{5}$ must be different to 0 ( ${C}_{5}\ne 0$ ).
 First critical velocity ${V}_{cr1}={C}_{5}$ must be lower than the second critical velocity ${V}_{cr2}={C}_{6}$ ( ${C}_{5}<{C}_{6}$ ).
 The static friction coefficient ${C}_{1}$ and the dynamic friction coefficient ${C}_{2}$ , must be lower than the maximum friction ${C}_{3}$ ( ${C}_{1}\le {C}_{3}$ and ${C}_{2}\le {C}_{3}$ ).
 The minimum friction coefficient ${C}_{4}$ , must be lower than the static friction coefficient ${C}_{1}$ and the dynamic friction coefficient ${C}_{2}$ ( ${C}_{4}\le {C}_{1}$ and ${C}_{4}\le {C}_{2}$ )
 I_{fric} = 1
(Generalized viscous friction law):
 Friction filtering:If I_{filtr} ≠ 0, the tangential forces are smoothed using a filter:
(5) $${F}_{t}=\alpha \cdot {{F}^{\prime}}_{t}+\left(1\alpha \right)\cdot {{F}^{\prime}}_{t}{}^{1}$$Where, α coefficient is calculated from: If I_{filtr}= 1 $\alpha ={X}_{\mathit{freq}}$ , simple numerical filter
 If I_{filtr} = 2 $\alpha =\frac{2\cdot \pi}{{X}_{\mathit{freq}}}$ , standard 3dB filter, with ${X}_{\mathit{freq}}=\frac{dt}{T}$ , and T is filtering period
 If I_{filtr} = 3
$\alpha =2\cdot \pi \cdot {X}_{freq}\cdot dt$
, standard 3dB filter, with X_{freq} is cutting frequency
The filtering coefficient X_{freq} should have a value between 0 and 1.
 Friction penalty formulation I_{form}
 If I_{form} = 1,
(default) viscous formulation, the friction forces are:
(6) $${F}_{t}=\mathrm{min}\left(\mu {F}_{n},{F}_{adh}\right)$$While an adhesion force is computed as:
${F}_{adh}=C\cdot {V}_{t}$ with $C=VI{S}_{F}\cdot \sqrt{2Km}$
 If I_{form} = 2,
stiffness formulation, the friction forces are:
(7) $${F}_{t}^{new}=\mathrm{min}\left(\mu {F}_{n},{F}_{adh}\right)$$While an adhesion force is computed as:
${F}_{adh}={F}_{t}^{old}+\text{\Delta}{F}_{t}$ with $\text{\Delta}{F}_{t}=K\cdot {V}_{t}\cdot dt$
Where, V_{t} is contact tangential velocity.
 If I_{form} = 1,
(default) viscous formulation, the friction forces are: