In a pair-based contact definition, the contact interaction is defined between a contact surface consisting of contact elements (CONTA172, CONTA174, CONTA175, or CONTA177) and a target surface consisting of target elements (TARGE169, TARGE170). Contact elements and target elements from these two surfaces are assigned the same real constant set ID number, which must be greater than zero. The program looks for contact interaction only between surfaces with the same real constant set ID. The material ID of the contact elements is used to specify interaction properties (such as friction coefficient) via MP or TB commands.
In a general contact definition, general contact surfaces are defined. The contact elements (CONTA172, CONTA174) are overlaid on the exterior surfaces of deformable bodies, and the target elements (TARGE169, TARGE170) are used to cover standalone rigid bodies. The 3D line contact elements (CONTA177) are overlaid on 3D beams and on feature edges of 3D deformable bodies and/or perimeter edges of shell structures. The contact vertex elements (CONTA175) are overlaid on convex corners of 2D or 3D solid bodies and/or shell structures and on endpoints of beam structures. A zero real constant ID and a zero material ID are assigned to the contact and target elements. However, different section IDs and contact/target element type IDs are assigned to different general contact surfaces. In other words, each surface consists of contact or target elements that have a unique section ID. The program looks for contact interaction among all surfaces, including self-contact (contact within a surface).
The table below compares characteristics of pair-based contact and general contact definitions.
| Contact Characteristic | Pair-Based Contact Definition | General Contact Definition |
|---|---|---|
|
Contact Element Real Constant ID |
Real ID > 0 for contact pair identification |
Real ID = 0 for general contact identification |
|
Contact Element Section ID |
Arbitrary Section ID, used as-needed for geometry corrections |
Unique Section ID for each general contact surface (for surface identification) |
|
Contact Element Material ID |
Used for interaction definition. Mat ID > 0 assigned as an element attribute |
Mat ID = 0 (not used by contact element) |
|
Target Element |
Real ID > 0 for contact pair identification Rigid or deformable surface |
Real ID = 0 (not used by target element) Unique Section ID > 0 for each rigid surface |
|
Interactions and Contact Controls |
Based on Mat ID and Real ID of contact element |
Defined via GCDEF command between two general contact surfaces (identified by Section IDs). The material ID and real constant ID supplied on the GCDEF command identify properties associated with the interaction but are not assigned directly as element attributes. |
|
Symmetric Contact Behavior |
KEYOPT(8) |
GCDEF, |
|
Interface Behaviors and Treatment |
KEYOPT(12) KEYOPT(9) |
TB,INTER,,, TBDATA,, |
|
Contact/Target Radii |
Real constants R1, R2 |
SECTYPE,,CONTACT,RADIUS SECDATA,R1,R2 |
|
Contact Search |
Within each pair |
Among all general contact surfaces |
|
Contact Surface Generation Tool |
ESURF command |
GCGEN command |
General contact and pair-based contact definitions can coexist in a finite element model. For example, you may choose to use pair-based contact to model contact interactions involving cohesive debonding behavior, but model other contact interactions with a general contact definition.
If both pair-based contact and general contact are defined in a model, the pair-based contact definitions are always preserved, and the general contact definition automatically excludes overlapping interactions wherever pair-based contact exists.
The pair-based contact definition is usually more efficient and robust than the general contact definition. Some contact capabilities are not available with the general contact definition (see Features Not Supported by General Contact).
Following are advantages of general contact over pair-based contact:
The general contact surfaces are formed automatically (see the GCGEN command) based on physical parts and geometric shapes in the model. General contact fully identifies all possible contact (including self-contact) and minimizes user intervention. This alleviates some of the problems you my encounter when using pair-based contact (for example, the contact and target surfaces may not be extended enough to include all potential contact regions, such as corners and edges, during deformations).
Contact searching takes place among all general contact surfaces so that contact between multiple, very thin layers is accounted for. For general contact, pinball radius is critical in some situations such as bonded contact and initial interference fit.
General contact, by default, automatically designates contact and target pairing for contact interactions. The automatic asymmetric pairing logic is based mainly on the relative mesh refinement and material rigidity. The logic is also applied to self-contact within a general contact surface. For example when a user-defined surface overlays several unconnected parts, the self-contact implies contact between the parts as well as contact of each part with itself.
In a 3D general contact definition, the surface-to-surface contact formulation (CONTA174) may be combined with the 3D edge-to-surface formulation (CONTA177) and the 3D vertex-to-surface formulation CONTA175. CONTA174 is used as the primary contact constraint and CONTA177 and CONTA175 are used as supplemental constraints. The additional 3D edge-to-surface contact can more accurately resolve contact involving feature edges of solid bodies and perimeter edges of shell structures penetrating into other surfaces. The 3D vertex-to-surface contact formulation is intended to prevent penetrations between convex corners of solid bodies and shell surfaces, as well as the endpoints of beam structures, into other surfaces.
Most contact models can employ either the pair-based contact method or the general contact method. It is up to you do decide which contact method suits your purposes. The main difference between the two methods is in the input syntaxes. However, both definitions share the same contact algorithms.
Pair-based contact usually provides a more efficient and robust solution compared to the general contact definition. General contact offers a highly-automated contact definition procedure with limited user intervention. Therefore, the choice is a trade-off between ease of defining contact and solution robustness and performance.
General contact is mainly useful for models in which determining contact pairs between many parts is difficult or even impossible. However, if the identification of all potential contact interfaces is obvious, using contact pairs is recommended.
Notable differences in default element settings (key options) between pair-based contact and general contact are listed below. The general contact defaults apply when the GCGEN command is used to create the general contact elements.
| Control Parameter | Pair-Based Contact Default | General Contact Default |
|---|---|---|
| Degree of freedom selection: KEYOPT(1) | User-specified | Automatic based on the underlying elements |
| Contact algorithm: KEYOPT(2) | Augmented Lagrange method | Penalty method |
| Symmetric contact behavior: KEYOPT(8) | No automatic pairing by default | Automatic assignment of contact/target pairing for any two surfaces coming into contact |
| Effect if initial penetration/gap KEYOPT(9) | Included (controlled by KEYOPT(9)) | Excluded (controlled by
TBDATA,,,C1 in
conjunction with TB,INTER) |
| Beam/shell thickness effect: KEYOPT(11) | Excluded | Automatically included |
For more information on default element key options for pair-based and general contact, see Defining Non-Default Contact Settings.
The following contact features are not available in a general contact definition:
Debonding and cohesive contact behavior (TB,CZM)
Contact surface wear (TB,WEAR)
Impact constraints (KEYOPY(7) = 4)
Brake squeal analysis (CMROTATE)
Fluid pressure penetration loading (SFE,,,PRES).
3D thermal contact involving TBOT and TTOP degrees of freedom (KEYOPT(13))
Rezoning
Contact modeling for general axisymmetric solids and superelements
Contact pair-splitting in a distributed-memory parallel processing run (CNCHECK,SPLIT and CNCHECK,DMP)
In addition, the Contact Wizard and the Contact Manager do not support general contact.