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allows contact between multiple eroding solids to be modeled since a node-based surface does not need to be defined on the eroding solid.
• Contact state information (such as the proper contact normal orientation for double-sided surfaces) is transferred across step boundaries in the general contact algorithm even if the contact domain is modified; in the contact pair algorithm, contact state information is transferred across step boundaries only for contact pairs with no modifications.
• The contact interaction domain, contact properties, and surface attributes are specified independently for the general contact algorithm, offering a more flexible way to add detail incrementally to a model.
• The general contact algorithm does not place any restrictions on the domain decomposition for domain level parallelization (see “Parallel execution in Abaqus/Explicit,” Section 3.5.3).
• The general contact algorithm in Abaqus/Explicit has been developed to minimize the need for algorithmic controls.
See “Knee bolster impact with general contact,” Section 2.1.9 of the Abaqus Example Problems Guide; “Crimp forming with general contact,” Section 2.1.10 of the Abaqus Example Problems Guide; and “Collapse of a stack of blocks with general contact,” Section 2.1.11 of the Abaqus Example Problems Guide, for example analyses that use the general contact algorithm.
Although the general contact algorithm is more powerful and allows for simpler contact definitions, the contact pair algorithm must be used in certain cases where more specialized contact features are desired. The following features are available in Abaqus/Explicit only when the contact pair algorithm is used:
• Two-dimensional surfaces
• Kinematically enforced contact (see “Contact constraint enforcement methods in Abaqus/Explicit,” Section 38.2.3; the general contact algorithm uses only penalty enforcement)
• Small-sliding contact (see “Contact formulations for contact pairs in Abaqus/Explicit,” Section 38.2.2)
• Exponential and no separation contact pressure-overclosure models
• Breakable bonds, such as spot welds (however, mesh-independent spot welds can be used with either contact algorithm; see “Mesh-independent fasteners,” Section 35.3.4)
In addition, the general contact algorithm in Abaqus/Explicit places more restrictions on adaptive meshing than the contact pair algorithm (see “Defining ALE adaptive mesh domains in Abaqus/Explicit,” Section 12.2.2). The choice of contact algorithm may affect the speedup factor if loop-level parallelization is used: the contact pair algorithm includes some loop-level parallelization, while the general contact algorithm has no loop-level parallelization. Contact output is more complete for a contact pair analysis.
The two contact algorithms can be used together in the same Abaqus/Explicit analysis. The general contact algorithm automatically avoids processing interactions that are treated by the contact pair algorithm.
There are fundamental differences in the mechanical contact algorithms in Abaqus/Standard and Abaqus/Explicit even though the input syntax is similar. The main differences are the following:
• Contact pair and general contact definitions in Abaqus/Standard are model definition data (although contact pairs can be removed for a portion of the analysis and added back to the model in a later step of the analysis, as discussed in “Removing and reactivating contact pairs” in “Defining contact pairs in Abaqus/Standard,” Section 36.3.1). In the contact pair algorithm in Abaqus/Explicit contact constraints are history definition data (see “Defining a model in Abaqus,” Section 1.3.1); in the general contact algorithm in Abaqus/Explicit contact definitions can be either model or history data.
• Abaqus/Standard typically uses a pure master-slave relationship for the contact constraints; whereas Abaqus/Explicit typically uses balanced master-slave contact by default. This difference is primarily due to overconstraint issues unique to Abaqus/Standard.
• The contact formulations in Abaqus/Standard and Abaqus/Explicit differ in many respects due to different convergence, performance, and numerical requirements:
– Abaqus/Standard provides surface-to-surface, edge-to-surface, and vertex-to-surface formulations, which Abaqus/Explicit does not;
– Abaqus/Standard and Abaqus/Explicit both provide node-to-surface formulations, but some details associated with surface smoothing, etc. differ in the respective implementations.
• The constraint enforcement methods in Abaqus/Standard and Abaqus/Explicit differ in some respects. For example, both analysis codes provide penalty constraint methods, but the default penalty stiffnesses differ (this is primarily due to the effect of the penalty stiffness on the stable time increment for Abaqus/Explicit).
• The small-sliding contact capability in Abaqus/Standard transfers the load to the master nodes according to the current position of the slave node, but the small-sliding contact capability in Abaqus/Explicit always transfers the load through the anchor point due to a numerical limitation associated with the implementation.
• Abaqus/Explicit can account for the thickness and midsurface offset of shells and membranes in the contact penetration calculations (although in some cases changes in the thickness upon deformation are not accounted for in the contact calculations). Abaqus/Standard cannot account for the thickness and offset of shells and membranes when using the finite-sliding, node-to-surface contact formulation (but can account for the original thickness and offset in all other contact formulations).
As a result of these differences, contact definitions specified in an Abaqus/Standard analysis cannot be imported into an Abaqus/Explicit analysis and vice versa (see “Transferring results between Abaqus/Explicit and Abaqus/Standard,” Section 9.2.2). However, in many cases you can successfully respecify a contact definition in an import analysis.
36.2 Defining general contact in Abaqus/Standard
• “Defining general contact interactions in Abaqus/Standard,” Section 36.2.1
• “Surface properties for general contact in Abaqus/Standard,” Section 36.2.2
• “Contact properties for general contact in Abaqus/Standard,” Section 36.2.3
• “Controlling initial contact status in Abaqus/Standard,” Section 36.2.4
• “Stabilization for general contact in Abaqus/Standard,” Section 36.2.5
• “Numerical controls for general contact in Abaqus/Standard,” Section 36.2.6
36.2.1 DEFINING GENERAL CONTACT INTERACTIONS IN Abaqus/Standard
Products: Abaqus/Standard Abaqus/CAE
References
• “Contact interaction analysis: overview,” Section 36.1.1
• *CONTACT
• *CONTACT INCLUSIONS
• *CONTACT EXCLUSIONS
• “Defining general contact,” Section 15.13.1 of the Abaqus/CAE User’s Guide, in the HTML version of this guide
Overview
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 36.1.1, 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 as the primary contact formulation, supplemented by the edge-to-surface, edge-to-edge, and vertex-to-surface contact formulations.
Defining a general contact interaction
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 36.2.2);
• the mechanical contact property models (“Contact properties for general contact in Abaqus/Standard,” Section 36.2.3);
• the controls associated with the initial contact state (“Controlling initial contact status in Abaqus/Standard,” Section 36.2.4); and
• the algorithmic contact controls (“Numerical controls for general contact in Abaqus/Standard,” Section 36.2.6).
An example of an analysis that uses general contact to define contact between the various components of an assembly is described in “Impact analysis of a pawl-ratchet device,” Section 2.1.17 of the Abaqus Example Problems Guide.
Surfaces used for general contact
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 36.1.1. For detailed information on defining surfaces in Abaqus/Standard for use with the general contact algorithm, see “Element-based surface definition,” 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 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.
Types of contact formulations within general contact targeting various scenarios
The general contact algorithm in Abaqus/Standard offers capabilities to model surface-to-surface contact, edge-to-surface contact, edge-to-edge contact, and vertex-to-surface contact. The surface-to-surface contact formulation is the primary formulation for general contact. The surface-to-surface contact formulation mainly treats cases with contact over an area of dimensions significant compared to surface facet dimensions, such as the case on the right in Figure 36.2.1–1. General contact uses the other contact formulations as supplementary formulations. For example, the second, third, and fourth cases in Figure 36.2.1–1 would be treated with the edge-to-surface, edge-to-edge, and vertex-to-surface formulations, respectively. The supplementary formulations are also used by general contact to treat contact involving beam and truss elements.
Transitions between the predominant type of contact formulation active in a local region are common. For example, the edge-to-surface contact formulation would be predominant at the stage of the snap-fit simulation shown in Figure 36.2.1–2, because the active contact zone corresponds to a feature edge. Upon further insertion, the surface-to-surface contact formulation would become predominant once the top surface of the darker colored part is in contact with the other part over a significant area. General contact automatically handles transitions between predominant contact formulations as contact conditions evolve. Multiple types of contact constraints will be locally active during transitions. The
natural_image
Four 3D geometric models showing orange and gray cubes on grid backgrounds, no text or symbols present
Figure 36.2.1–1 Contact scenario types for two blocks.
natural_image
3D geometric diagram showing two stacked blocks with one larger and one smaller, rendered in beige and green tones (no text or symbols)
Figure 36.2.1–2 Snap-fit example involving feature edge-to-surface contact with an oblique angle between surface normals in the contact region.
supplementary contact formulations are always enforced with a penalty method, which helps avoid numerical issues with “over-constraints” while multiple constraint types are active.
Additional discussion of the surface-to-surface contact formulation is provided in “Surface-tosurface contact discretization” in “Contact formulations in Abaqus/Standard,” Section 38.1.1. See “Edge-to-surface contact scenarios,” “Edge-to-edge contact scenarios,” and “Vertex-to-surface contact scenarios,” below, for more information on the supplementary contact formulations.
Including general contact in an analysis
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)
Defining the general contact domain
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.
Specifying contact inclusions
Define contact inclusions to specify the regions of the model that should be considered for contact purposes.
Specifying “automatic” contact for the entire model
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 32.5.3, 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.
Abaqus/CAE Usage: Interaction module: Create Interaction: General contact (Standard):
Included surface pairs: All* with self
Specifying individual contact interactions
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
Examples
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_2
Either 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
Specifying contact exclusions
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
Automatically generated contact exclusions
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 36.2.6, for more information on this type of contact exclusion).