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/INTER/TYPE24/inter_ID/unit_ID

inter_title

surf_ID1

surf_ID2

Istf

grnd_IDs

Gap_max_s

Gap_max_m

Stmin

Stmax

Igap0

Ipen0

Ipen_max

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Stfac

Fric

Tstart

Tstop

IBC

Inacti

VISS

Ifric

Ifiltr

Xfreq

sens_ID

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C1

C2

C3

C4

C5

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C6

grnd_IDs > 0 is used to define node to surface contact type, but it may also be used in other contact types. In that case, the node group will be added simply as supplementary slave nodes, which is useful when users want to add spring element nodes, master node of rigid body, etc. into the contact (as slave nodes).

If the surface is defined with shells, two contact segments (shifted by half thickness (t)) with opposite normal directions will be generated:

In case of SPMD, each master segment defined by surf_IDi (i=1,2) must be associated to an element (possibly to a void element).

In cases where quadratic elements are used, it is recommended to define the surfaces by using /SURF/PART/EXT as in that case, middle nodes of quadratic elements are used in the contact treatment.

The surface definition /SURF/PART/ALL is not available with TYPE24.

•

Istf = 2,

•

Istf = 3,

•

Istf = 4,

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Istf = 5,

•	Istf = 6, , Soft stiffness. This option is only available with implicit solution.

For each contact, to make nonlinear iteration convergence easier, smaller initial stiffness is used; for the function of the reaction of the contacting parts (with increasing penetration or rebound), the stiffness will be adjusted, but is always smaller than the input stiffness, then:

For each interface stiffness definition:

Km: master segment stiffness

•	, when the master segment lies on a shell

•	, when master segment is shared by shell and solid

•	, when master segment lies on a solid.

Ks: Slave node stiffness is an equivalent nodal stiffness considered for interface TYPE24, and computed as:

•	, when node is connected to a shell element,

•	, when node is connected to solid element.

While,

S is the segment area,

V is the volume of the solid,

B is the Bulk Modulus,

t is the thickness of the shell

The Stfac value can be larger than 1.0. There is no limitation value to the stiffness factor (a value larger than 1.0 can reduce the initial time step).

If Implicit Analysis is to be considered (defined by /IMPLICIT), the default value is Istf= 4; Istf = 6 is recommended for flexible or bending dominated structures.

While,

•	gm: master element gap:   , with t is the thickness of the master element for shell elements   , for brick elements

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gs: slave node gap:   , if the slave node is not connected to any element or is only connected to brick or spring elements.   , if the slave node is connected to a shell element, with t being the largest thickness of the shell elements connected to the slave node.   , if the slave node is connected to truss or beam elements, with S being the cross section of the 1D element.

If the slave node is connected to multiple shells and/or beams or trusses, the largest computed slave gap is used.

gm and gs are limited separately by Gap_max_m and Gap_max_s before the gap is  computed.

While an adhesion force is computed as follows:

with

is coulomb friction coefficient and it define as below:

with (Coulomb friction)

Where,

p is the pressure of the normal force on the master segment

V is the tangential velocity of the slave node.

Currently, the coefficients C1 ~ C6 are used to define a variable friction coefficient for new friction formulations.

The following formulations are available:

 if

if

 if

Where,

•	First critical velocity must be different to 0 ().

•	First critical velocity must be less than the second critical velocity ().

•	The static friction coefficient and the dynamic friction coefficient , must be less than the maximum friction ( and ).

•	The minimum friction coefficient , must be less than the static friction coefficient and the dynamic friction coefficient ( and ).

If Ifiltr = 1, 2 or 3, the tangential forces are smoothed using a filter:

Where, coefficient is calculated from:

•	If Ifiltr = 1, , simple numerical filter

•	If Ifiltr = 2, , standard -3dB filter, with , and T = filtering period

•	If Ifiltr = 3, , standard -3dB filter, with Xfreq = cutting frequency

The filtering coefficient Xfreq should have a value between 0 and 1.

The master segment position is restored only in case of rebound larger than P0.

In the opposite case, when slave node continues to penetrate, the penetration is computed as:

Shells and thick shells: initial intersections should be avoided, as they will lead to wrong direction of contact force and possible slave nodes anchorage.

Solids: by default, the distance which is considered for searching the initial penetration is compute as:

While for each master segment,

V is the volume of the connected solid element,

A is the segment area,

Ledge (edge= 1 to 4) are the lengths of the edges of the segment.

is the number of master segments

 is an estimation of the depth of the solid element connected to the segment (limited to the size of the segment).

Maximum initial penetration Ipen_max:

When the contact type is asymmetric surface to surface, the output normal contact forces in Time History are calculated correctly, if the two surfaces are well separated.