Products: Abaqus/Standard Abaqus/CAE
Abaqus/Standard provides two algorithms for modeling contact and interaction problems: the general contact algorithm and the contact pair algorithm. See “Contact interaction analysis: overview,” Section 31.1.1, for a comparison of the two algorithms. This section describes how to include general contact in an Abaqus/Standard analysis, how to specify the regions of the model that may be involved in general contact interactions, and how to obtain output from a general contact analysis.
The general contact algorithm in Abaqus/Standard:
is specified as part of the model definition;
allows very simple definitions of contact with very few restrictions on the types of surfaces involved;
uses sophisticated tracking algorithms to ensure that proper contact conditions are enforced efficiently;
can be used simultaneously with the contact pair algorithm (i.e., some interactions can be modeled with the general contact algorithm, while others are modeled with the contact pair algorithm);
can be used with two- or three-dimensional surfaces; and
uses the finite-sliding, surface-to-surface contact formulation.
The definition of a general contact interaction consists of specifying:
the general contact algorithm and defining the contact domain (i.e., the surfaces that interact with one another), as described in this section;
the contact surface properties (“Surface properties for general contact in Abaqus/Standard,” Section 31.2.2);
the mechanical contact property models (“Contact properties for general contact in Abaqus/Standard,” Section 31.2.3);
the controls associated with the initial contact state (“Controlling initial contact status in Abaqus/Standard,” Section 31.2.4); and
the algorithmic contact controls (“Numerical controls for general contact in Abaqus/Standard,” Section 31.2.5).
The general contact algorithm in Abaqus/Standard allows for quite general characteristics in the surfaces that it uses, as discussed in “Contact interaction analysis: overview,” Section 31.1.1. For detailed information on defining surfaces in Abaqus/Standard for use with the general contact algorithm, see “Defining element-based surfaces,” Section 2.3.2.
A convenient method of specifying the contact domain is using cropped surfaces. Such surfaces can be used to perform “contact in a box” by using a contact domain that is enclosed in a specified rectangular box in the original configuration. For more information, see “Operating on surfaces,” Section 2.3.6.
In addition, Abaqus/Standard automatically defines an all-inclusive surface that is convenient for prescribing the contact domain, as discussed later in this section. The all-inclusive automatically defined surface includes all element-based surface facets.
The general contact algorithm in Abaqus/Standard uses the surface-to-surface contact formulation. The general contact algorithm does not consider contact involving analytical surfaces or node-based surfaces, although these surface types can be included in contact pairs in analyses that also use general contact.
General contact in Abaqus/Standard is defined at the beginning of an analysis. Only one general contact definition can be specified, and this definition is in effect for every step of the analysis.
| Input File Usage: | Use the following option to indicate the beginning of a general contact definition: |
*CONTACT This option can appear only once in the model definition. |
| Abaqus/CAE Usage: | Interaction module: Create Interaction: Step: Initial, General contact (Standard) |
You specify the regions of the model that can potentially come into contact with each other by defining general contact inclusions and exclusions. Only one contact inclusions definition and one contact exclusions definition are allowed in the model definition.
All contact inclusions in an analysis are applied first, then all contact exclusions are applied, regardless of the order in which they are specified. The contact exclusions take precedence over the contact inclusions. The general contact algorithm will consider only those interactions specified by the contact inclusions definition and not specified by the contact exclusions definition.
General contact interactions typically are defined by specifying self-contact for the default automatically generated surface provided by Abaqus/Standard. All surfaces used in the general contact algorithm can span multiple unattached bodies, so self-contact in this algorithm is not limited to contact of a single body with itself. For example, self-contact of a surface that spans two bodies implies contact between the bodies as well as contact of each body with itself.
Define contact inclusions to specify the regions of the model that should be considered for contact purposes.
You can specify self-contact for a default unnamed, all-inclusive surface defined automatically by Abaqus/Standard. This default surface contains, with the exceptions noted below, all exterior element faces. This is the simplest way to define the contact domain.
The default surface does not include faces that belong only to cohesive elements. In fact, the default surface is generated as if cohesive elements were not present. See “Modeling with cohesive elements,” Section 28.5.3, for further discussion of contact modeling issues related to cohesive elements.
| Input File Usage: | Use both of the following options to specify “automatic” contact for the entire model: |
*CONTACT *CONTACT INCLUSIONS, ALL EXTERIOR The *CONTACT INCLUSIONS option should have no data lines when the ALL EXTERIOR parameter is used; any data lines specified will be ignored. |
| Abaqus/CAE Usage: | Interaction module: Create Interaction: General contact (Standard): Included surface pairs: All* with self |
Alternatively, you can define the general contact domain directly by specifying the individual contact surface pairings. Self-contact will be modeled only if the two surfaces specified in a pair overlap (or are identical) and will be modeled only in the overlapping region. In some cases computational performance and robustness can be improved by including only portions of surfaces in the general contact domain that will experience contact during an analysis.
Multiple surface pairings can be included in the contact domain. All of the surfaces specified must be element-based surfaces.
| Input File Usage: | Use both of the following options to specify individual contact interactions: |
*CONTACT *CONTACT INCLUSIONS surface_1, surface_2 At least one data line must be specified when the ALL EXTERIOR parameter is omitted. Either or both of the data line entries can be left blank, but each data line must contain at least a comma; an error message will be issued for empty data lines. If the first surface name is omitted, the default unnamed, all-inclusive, automatically generated surface is assumed. If the second surface name is omitted or is the same as the first surface name, contact between the first surface and itself is assumed. Leaving both data line entries blank is equivalent to using the ALL EXTERIOR parameter. |
| Abaqus/CAE Usage: | Interaction module: Create Interaction: General contact (Standard): Included surface pairs: Selected surface pairs: Edit, select the surfaces in the columns on the left, and click the arrows in the middle to transfer them to the list of included pairs |
The following input specifies that contact should be enforced between the default all-inclusive, automatically generated surface and surface_2, including self-contact in any overlap regions:
*CONTACT *CONTACT INCLUSIONS , surface_2Either of the following methods can be used to define self-contact for surface_1:
*CONTACT *CONTACT INCLUSIONS surface_1,or
*CONTACT *CONTACT INCLUSIONS surface_1, surface_1
You can refine the contact domain definition by specifying the regions of the model to exclude from contact. Possible motivations for specifying contact exclusions include:
avoiding physically unreasonable contact interactions;
improving computational performance by excluding parts of the model that are not likely to interact.
Contact will be ignored for all the surface pairings specified, even if these interactions are specified directly or indirectly in the contact inclusions definition.
Multiple surface pairings can be excluded from the contact domain. All of the surfaces specified must be element-based surfaces. Keep in mind that surfaces can be defined to span multiple unattached bodies, so self-contact exclusions are not limited to exclusions of single-body contact.
| Input File Usage: | Use both of the following options to specify contact exclusions: |
*CONTACT *CONTACT EXCLUSIONS surface_1, surface_2 Either or both of the data line entries can be left blank. If the first surface name is omitted, the default unnamed, all-inclusive, automatically generated surface is assumed. If the second surface name is omitted or is the same as the first surface name, contact between the first surface and itself is excluded from the contact domain. |
| Abaqus/CAE Usage: | Interaction module: Create Interaction: General contact (Standard): Excluded surface pairs: Edit, select the surfaces in the columns on the left, and click the arrows in the middle to transfer them to the list of excluded pairs |
Abaqus/Standard automatically generates contact exclusions for general contact in some situations.
Contact exclusions are generated automatically for interactions that are defined with the contact pair algorithm or surface-based tie constraints to avoid redundant (and possibly inconsistent) enforcement of these interaction constraints. For example, if a contact pair is defined for surface_1 and surface_2 and “automatic” general contact is defined for the entire model, Abaqus/Standard generates a contact exclusion for general contact between surface_1 and surface_2 so that interactions between these surfaces are modeled only with the contact pair algorithm. These automatically generated contact exclusions are in effect throughout the analysis.
Abaqus/Standard automatically generates contact exclusions for self-contact of each rigid body in the model, because it is not possible for a rigid body to contact itself.
When you specify pure master-slave contact surface weighting for a particular general contact surface pair, contact exclusions are generated automatically for the master-slave orientation opposite to that specified (see “Numerical controls for general contact in Abaqus/Standard,” Section 31.2.5, for more information on this type of contact exclusion).
Abaqus/Standard assigns default pure master-slave roles for contact involving disconnected bodies within the general contact domain, and contact exclusions are generated by default for the opposite master-slave orientations. Options to override the default pure master-slave assignments with alternative pure master-slave assignments or balanced master-slave assignments are discussed in “Numerical controls for general contact in Abaqus/Standard,” Section 31.2.5.
Contact exclusions are generated automatically for portions of surfaces that are severely overclosed in the initial configuration of the model. See “Controlling initial contact status in Abaqus/Standard,” Section 31.2.4, for more information.
The following input specifies that the contact domain is based on self-contact of an all-inclusive, automatically generated surface but that contact (including self-contact in any overlap regions) should be ignored between the all-inclusive, automatically generated surface and surface_2:
*CONTACT *CONTACT INCLUSIONS, ALL EXTERIOR *CONTACT EXCLUSIONS , surface_2
Either of the following methods can be used to exclude self-contact for surface_1 from the contact domain:
*CONTACT EXCLUSIONS surface_1,or
*CONTACT EXCLUSIONS surface_1, surface_1
All of the variables available for output with Abaqus/Standard contact pairs are also available for general contact. Nodal contact variables include contact pressure and force, frictional shear stress and force, relative tangential motion (slip) of two surfaces during contact, clearance between surfaces, and heat flux per unit area; all of these variable can be contoured on contact surfaces in the Visualization module of Abaqus/CAE. History output variables record composite values for the entire contact domain, including the total force and moment due to contact pressure and frictional stress, the center of pressure and frictional stress, and the total contact area (defined as the sum of all the facets where there is contact force). Conventions and limitations associated with these variables are discussed in more detail in “Defining contact pairs in Abaqus/Standard,” Section 31.3.1.
Instructions on forming contact output requests are available in the following sections:
To request output to the data (.dat) file, see “Surface output from Abaqus/Standard” in “Output to the data and results files,” Section 4.1.2.
To request output to the output database (.odb) file, see “Surface output” in “Output to the output database,” Section 4.1.3.
Additional output that is specific to general contact is also available.
Abaqus/Standard generates the following internal surfaces when a general contact domain is defined: General_Contact_Faces and General_Contact_Component_k, where k corresponds to the component number. The automatically generated “component” surfaces are related to the default assignment of master-slave roles for contact between disconnected surface regions: the lowered-number component surfaces will act as master surfaces to the higher-numbered component surfaces. General_Contact_Faces contains all surface faces that are included in the general contact domain. These internal surfaces can be viewed using display groups in the Visualization module of Abaqus/CAE. The internal surface names generated by Abaqus/Standard should not be used in model definitions.
The generic variables CSTRESS and CFORCE are valid field output requests for general contact in Abaqus/Standard. If CSTRESS is requested for the general contact domain, the variable CPRESS (contact pressure) can be contoured in Abaqus/CAE. If CFORCE is requested for the general contact domain, the variables CNORMF (normal contact force) and CSHEARF (shear contact force) can be plotted as vectors in a symbol plot in Abaqus/CAE.
For general contact CPRESS is calculated as the magnitude of the net contact normal force (the CNORMF vector) per unit area (it is an unsigned value). This convention for reporting contact pressure is different from the convention used for contact pairs. The direction of action of the net contact pressure for general contact can be determined by examining a plot of CNORMF.
CNORMF and CSHEARF are resultant force quantities. If a double-sided surface is contacted on both sides, the resultant force is a vector sum of the force from each side of the surface (for example, the contact normal force will be zero for a double-sided surface that is pinched with equal and opposite forces on each side of the surface).