Each general contact surface is identified by a unique section ID, which is typically generated by the GCGEN command. When defining a contact interaction between specific surfaces, you input the section ID numbers (SECT1 and SECT2) on the GCDEF command to indicate the two surfaces for which contact can occur. You must input SECT1 and SECT2. There are no defaults.

Contact Between Two Surfaces

A general contact interaction defined between two surfaces (where SECT1 ≠ SECT2, but both are > 0) is the most specific type of general contact specification (comparable to controlling a specific contact pair):

GCDEF,AUTO,SECT1,SECT2,MATID,REALID

Self Contact

To control self-contact between different portions of a single general contact surface, input the same section ID for both SECT1 and SECT2. In the following example, self contact is defined for the general contact surface identified by section ID = 3:

GCDEF,AUTO,3,3,MATID,REALID

Alternatively, you can set SECT1 or SECT2 = SELF:

GCDEF,AUTO,3,SELF,MATID,REALID

The two types of general contact specifications described above—contact defined between two specific surfaces and self-contact—are the most specific types of general contact interactions that you can define.

Instead of inputting a specific section ID in the SECT1 or SECT2 field of the GCDEF command, you can specify the name of a nodal component that is currently defined within the finite element model. The nodal component must have been previously defined via the CM command and must contain exterior nodes of one or more surfaces. Component names are valid input for SECT1 and SECT2 when Option = (blank), ASYM, AUTO, SYMM, EXCL, DELETE, LIST, TABLE, or TABLESOL.

In the example below, the GCDEF command specifies contact between nodal components representing the pin and rod surfaces of the model discussed in Modeling General Contact - Basic Procedure.

NSEL,S,NODE,....     ! Select a set of nodes on pin surface
CM,PIN,NODE          ! Group selected nodes into component PIN
NSEL,S,NODE,....     ! Select a set of nodes on rod surface
CM,ROD,NODE          ! Group selected nodes into component ROD
!
GCDEF,ASYM,PIN,ROD,BondID,REALID   ! Define asymmetric bonded contact between 
                                   ! components PIN and ROD (BondID is a maerial ID)

The GCDEF command loops through all valid section IDs found in each node component input for SECT1 and SECT2. The node component names are not stored or listed, Instead, the interpreted section IDs are stored and listed as separate GCDEF entries.

Thus, when the node components input for SECT1 and SECT2 contain nodes from multiple general contact surfaces having different section IDs, all possible combinations of SECT1/SECT2 interactions are considered in the general contact definition. As an example, if component 1 (input for SECT1) contains nodes from N1 section IDs and component 2 (input for SECT2) contains nodes from N2 section IDs, the GCDEF command defines contact interactions for N1×N2 surface combinations.

Contact can be defined between general contact surfaces using a combination of section IDs and defined components, as illustrated below:

GCDEF,AUTO,1,COMP2,MATID,REALID       ! Define auto asymetric contact
                                      ! between SECT1 and component COMP2
GCDEF,AUTO,COMP1,SELF,MATID,REALID    ! Define self contact for component COMP1 

The scope of general contact section IDs from a node component can be limited by adding one of the following extensions onto the component name that you input:

If none of these extensions is present, the given GCDEF specification is applied to all types of general contact elements.

The following example demonstrates use of the extensions:

GCDEF,EXCLUDE,7,COMP2_EDGE            ! Exclude contact between SECT7 and component COMP2 edges
GCDEF,SYMM,COMP1_TOP,SELF,MATID       ! Define symmetric self contact for component COMP1 top faces 

Be cautious when using GCDEF,EXCLUDE. If you do not specify an extension on the component name, both faces and edges are excluded by default.

To make global changes for general contact, you can input the label ALL instead of a specific section ID in the SECT1 and SECT2 fields of the GCDEF command.

For example, to override the default frictionless contact among all general contact surfaces with a different interface behavior or contact control, issue:

GCDEF,Option,ALL,ALL,MATID

where MATID corresponds to a behavior other than standard frictionless behavior (for example, always bonded).

To define specifications for self contact for all general contact surfaces, issue:

GCDEF,Option,ALL,SELF,MATID,REALID

or

GCDEF,Option,SELF,ALL,MATID,REALID

Either of these command controls the general contact behavior for self contact for all surfaces (section 1 with section 1, section 2 with section 2, and so on).

To control general contact interaction between a specific surface and any general contact surface (another surface or itself), issue:

GCDEF,Option,SECT1,ALL,MATID,REALID

or

GCDEF,Option,ALL,SECT2,MATID,REALID

The above two commands are equivalent except when Option is set to ASYM (asymmetric contact). For example, the following command treats SECT1 as the contact surface with respect to all other surfaces, which will be target surfaces:

GCDEF,ASYM,SECT1,ALL,MATID,REALID

On the other hand, the following command treats SECT2 as the target surface with respect to all other surfaces, which will be contact surfaces:

GCDEF,ASYM,ALL,SECT2,MATID,REALID

The labels ALL_EDGE, ALL_FACE, ALL_VERT, ALL_TOP, and ALL_BOT are also valid input for SECT1 and SECT2. (See GCDEF for details of the meaning of each label.) These labels apply to all defined general contact element section IDs in the model without regard to the select status of the elements or attached nodes. The labels are immediately converted to section ID numbers, unlike the ALL label which is stored as ALL.

In some coarse mesh situations where there are no nodes internal to a surface, ALL_FACE, ALL_TOP, and ALL_BOT may pick up CONTA172 or CONTA174 section or type IDs that would be missed for a node component containing all defined nodes.

The GCDEF command allows you to exclude contact between specified surfaces. Use Option = EXCL and specify the surfaces SECT1 and SECT2:

GCDEF,EXCL,SECT1,SECT2

There are two possible reasons to define a contact exclusion:

To exclude self-contact within a surface, set SECT1 = SECT2. For example, the following command excludes self contact for the surface identified by section ID number 4:

GCDEF,EXCL,4,4

To exclude self-contact for all general contact surfaces, issue:

GCDEF,EXCL,All,SELF

or

GCDEF,EXCL,SELF,ALL

For many assembly models, self-contact is unlikely to occur. Using one of the above commands to exclude self-contact prevents unreasonable interactions within each part, in particular for assemblies with very thin parts.

To exclude contact between a particular surface and all other general contact surfaces (partial general contact exclusion), issue:

GCDEF,EXCL,SECT1,ALL

or

GCDEF,EXCL,ALL,SECT2

Alternatively, you may delete general contact elements on surfaces that should be excluded. This results in an even more efficient solution.

To exclude all general contact interactions, which completely overrides the default frictionless contact, issue:

GCDEF,EXCL,ALL,ALL

The labels ALL_EDGE, ALL_FACE, ALL_VERT, ALL_TOP, and ALL_BOT are also valid input for SECT1 and SECT2. (See GCDEF for details of the meaning of each label.) For example

GCDEF,EXCL,4,ALL_EDGE            ! Exclude contact between section 4 and all edges

Example 8.1: Excluding Spurious Contact

Consider the model below in which one shell box (node component BI) is fitted inside another shell box (node component BE). For this shell-to-shell contact example, some of the shell elements of the inner and outer boxes overlap, and the general contact solution detects two possible contact interactions:

1. The exterior surface of the inner shell box contacting the interior surface of the outer shell box

2. The interior surface of the inner shell box contacting the exterior surface of the outer shell box

The second interaction is obviously spurious contact, and it should be excluded manually by issuing:

GCDEF,EXCL,BE_TOP,BI_BOT

Figure 8.14: Contact Between Two Shell Boxes

The Option field on the GCDEF command accepts four input labels that enable you to define the interface pairing option for general contact surfaces identified by SECT1 and SECT2.

GCDEF,AUTO,SECT1,SECT2,MATID,REALID

GCDEF,ASYM,SECT1,SECT2,MATID,REALID

GCDEF,SYMM,SECT1,SECT2,MATID,REALID

GCDEF,EXCL,SECT1,SECT2

The automatic asymmetric pairing option (Option = AUTO) is based mainly on relative mesh refinement and material rigidity. This logic is also used for self-contact within a surface (when SECT1 = SECT2). For example when a user-defined surface overlays several unconnected parts, contact between parts having the same section ID as well as contact of each part with itself are considered to be self-contact. Self-contact within a physical part always uses symmetric contact.

The automatic asymmetric pairing logic, which is set by default for surface-to-surface contact (CONTA172 and CONTA174), works well in most cases.

For the asymmetric pairing option (Option = ASYM), the guidelines for assigning contact-target pairing rules for pair-based contact discussed in Designating Contact and Target Surfaces are also applicable for situations in which you manually define the asymmetric logic for the general contact.

The symmetric pairing option (Option = SYMM) is often used when the interface mesh from both sides is relatively coarse. Symmetric contact enforces the contact constraint conditions at more surface locations than asymmetric contact. The limitations of symmetric logic for pair-based contact discussed in Asymmetric Contact vs. Symmetric Contact are also applicable for situations in which you manually specify symmetric general contact.

The symmetric pairing logic is set by default for 3D beam-to-beam or edge-to-edge contact (CONTA177).

Examples: Self-Contact

The following table presents examples of how to use the contact pairing options for self-contact.

GCDEF,AUTO,5,5,MATID,REALID

GCDEF,SYMM,6,6,MATID,REALID

GCDEF,AUTO,COMP1,COMP2,MATID,REALID

GCDEF,AUTO,ALL,SELF,MATID,REALID

GCDEF,SYMM,ALL,SELF,MATID,REALID

Examples: Contact Between One Surface and All Other General Contact Surfaces

The following table presents examples of how to use the contact pairing options to define interactions between one specific surface and all general contact surfaces.

GCDEF,AUTO,SECT1,ALL,MATID,REALID

GCDEF,AUTO,ALL,SECT2,MATID,REALID

GCDEF,Option1,6,ALL,MATID1,REALID1
GCDEF,Option2,7,ALL,MATID2,REALID2

GCDEF,Option,6,7,MATID,REALID

GCDEF,SYMM,SECT1,ALL,MATID,REALID

GCDEF,SYMM,ALL,SECT2,MATID,REALID

GCDEF,ASYM,SECT1,ALL,MATID,REALID

GCDEF,ASYM,ALL,SECT2,MATID,REALID

GCDEF,AUTO,COMP1,ALL,MATID,REALID

Examples: Contact Among All General Contact Surfaces

The following table presents examples of how to use the contact pairing options to define contact among all general contact surfaces.

GCDEF,AUTO,ALL,ALL,MATID,REALID

GCDEF,SYMM,ALL,ALL,MATID,REALID

If you need to remove a particular general contact specification that was previously defined, use Option = DELETE on the GCDEF command:

GCDEF,DELETE,SECT1,SECT2

To remove general contact interactions previously defined by “ALL,ALL”, issue:

GCDEF,DELETE,ALL,ALL

Note that GCDEF,DELETE,ALL,ALL does not remove the entire GCDEF table. It merely removes any existing GCDEF,,ALL,ALL definitions, while leaving intact any existing GCDEF definitions that are more specific.

To remove the entire GCDEF table, issue:

GCDEF,DELETE,TOTAL

It is good practice to list all interactions (GCDEF,LIST) before and after a GCDEF,DELETE command.