About General Contact in Abaqus/Standard

The definition of a general contact interaction consists of specifying:

• the general contact algorithm and defining the contact domain (that is, the surfaces that interact with one another), as described in this section;

• the contact surface properties (Surface Properties for General Contact in Abaqus/Standard);

• the mechanical contact property models (Contact Properties for General Contact in Abaqus/Standard);

• the controls associated with the initial contact state (Contact Initialization for General Contact in Abaqus/Standard); and

• the algorithmic contact controls (Contact Controls Specific to General Contact in Abaqus/Standard).

See Impact analysis of a pawl-ratchet device for an example of an analysis that uses general contact to define contact between the various components of an assembly.

The general contact algorithm in Abaqus/Standard allows for quite general characteristics in the surfaces that it uses, as discussed in About Contact Interactions. For detailed information on defining surfaces in Abaqus/Standard for use with the general contact algorithm, see Element-Based Surface Definition.

A convenient method of specifying the contact domain is using cropped surfaces. You can use such surfaces to perform “contact in a box” using a contact domain enclosed in a specified rectangular box in the original configuration. For more information, see Operating on Surfaces.

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, crack surfaces for enriched elements, and analytical rigid surfaces.

The general contact algorithm does not consider contact involving node-based surfaces, although Abaqus/Standard can include these surfaces in contact pairs in analyses that also use general contact.

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 with the default finite-sliding tracking approach. The surface-to-surface contact formulation is the primary formulation with three-dimensional models and is the only formulation for general contact with two-dimensional and axisymmetric models.

You can specify the small-sliding tracking approach to model surface-to-surface contact, but small sliding is not supported with edges and vertices (see Specifying Small Sliding within General Contact). The finite-sliding tracking approach is the preferred approach due to its generality in handling arbitrary relative surface motions. However, small sliding in general contact provides a highly automated and convenient contact modeling approach in situations where relative surface motions are known to be small.

Pure heat transfer and coupled thermal-electrical contact interactions support surface-to-surface contact as well as edge-to-surface contact with a beam edge acting as the main contact entity.

For three-dimensional models, the surface-to-surface contact formulation primarily treats cases with contact over an area of dimensions significant compared to the surface facet dimensions, such as the case on the left in Figure 5. General contact with finite sliding for three-dimensional models uses the other contact formulations as supplementary formulations. For example, the second, third, and fourth cases in Figure 5 is treated with the edge-to-surface, edge-to-edge, and vertex-to-surface formulations, respectively. General contact also uses the supplementary formulations to treat contact involving three-dimensional beam and truss elements. The surface-to-surface, edge-to-surface, and vertex-to-surface formulations can treat contact interactions in which one surface (which does not represent an edge or a vertex) is an analytical rigid surface. General contact does not consider contact between two analytical rigid surfaces.

Figure 5. Contact scenario types for two blocks.

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 6 because the active contact zone corresponds to a feature edge.

Figure 6. Snap-fit example involving feature edge-to-surface contact with an oblique angle between surface normals in the contact region.

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 are locally active during transitions. The supplementary contact formulations are always enforced with a penalty method, which helps avoid numerical issues with “over-constraints” while multiple constraint types are active.

For two-dimensional and axisymmetric models, surfaces based on solid, shell, beam, and truss elements consist of linear or quadratic segments (sometimes referred to as two-dimensional faces). For three-dimensional models, beam and truss surfaces are comprised of segments (sometimes referred to as "edges"), whereas surfaces based on three-dimensional solid, shell, and membrane elements consist of triangular or quadrilateral faces (and edges of face boundaries). Due to the similarity of two-dimensional surfaces among element types, the two-dimensional surface-to-surface contact formulation can treat surfaces based on solid, shell, beam, and truss elements. The three-dimensional surface-to-surface contact formulation treats surfaces based on solid and shell elements but does not treat beam or truss surfaces.

See Surface-to-Surface Contact Discretization for additional discussion of the surface-to-surface contact formulation. 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.

You can specify an overall general contact definition in multiple portions as model and step data in Abaqus/Standard. However, you must define the envelope of contact interactions eligible to be considered by general contact in one or more of these portions as model data.

CONTACT

Interaction module: Create Interaction: Step: Initial, General contact (Standard)

The general contact domain is the envelope of potential interactions for a simulation and the associated surface entities (nodes, faces, etc.). You determine the general contact domain based on general contact inclusions and exclusions that you specify at the model level and implied contact exclusions based on other model-level features such as contact pairs and surface-based tie constraints. If you specify an all-inclusive contact inclusion at the model level together with a contact exclusion between nonoverlapping surfaces A and B, the general contact domain includes potential interactions between all surface entities of the model (even for surfaces that you have not explicitly defined and named) except for potential interactions between surfaces A and B. Entities of surfaces A and B can still experience self-contact and contact with surfaces other than A and B. Exclusions take precedence over inclusions at the model level regardless of which definition appears first.

Named surfaces used in the definition of general contact can span multiple unattached bodies. For example, if you allow a named surface spanning multiple bodies to possibly contact itself, contact will be enforced between the entities of that surface from the same body or different bodies.

You can deactivate interactions that you specify at the model level and subsequently reactivate them across steps. Define contact exclusions at the step level to deactivate certain interactions of the contact domain. Define contact inclusions to reactivate the interactions in a subsequent step (see Deactivating and Reactivating Contact Interactions across Steps).

You can deactivate general contact interactions established at the model level and subsequently reactivate them across steps using contact exclusions and contact inclusions, respectively. Abaqus/Standard issues an error message if you attempt to activate an interaction in a step that is not eligible for contact according to the model-level general contact definition. The model-level specification establishes the envelope of eligible contact interactions.

Consider the following example: A particular forming tool is relevant only to contact starting in Step 3 of a simulation. For convenience, set the original position of this tool where it will eventually be introduced in the simulation. Exclude this part from contact interactions for the first two steps to avoid undesired interference. You will:

• Include the interactions involving this tool at the model level.
• Specify a contact exclusion for this part with the entire model in Step 1 to deactivate these contact interactions.
• Specify a contact inclusion for this part with the entire (or desired subset) of the model in Step 3 to reactivate these contact interactions.
• Specify any needed contact initialization instructions for the newly reactivated contact interactions in Step 3.

Contact deactivation and subsequent reactivation in analysis steps do not require use of the same surface names (nor the same extents of the surfaces) that are used to define the model-level contact inclusions that establish the envelope of eligible contacts interactions.

If contact deactivation occurs for contact interactions that were actively in contact at the end of the previous step, contact forces (and heat or electrical fluxes in the case of additional thermal and electrical interactions) are ramped down for contact constraints that were active at the end of the previous general step.

Any contact state associated with friction, cohesive contact, etc., is initialized on contact reactivation for interactions that were previously deactivated. Specifying a contact inclusion in a step has no effect on the contact state for contacts that were not previously deactivated.

Some steps might involve deactivation of some contact interactions and reactivation of other contact interactions. In case of overlapping interactions between contact inclusions and contact exclusions specified within a step, the contact exclusions take precedence.

Step-dependent activation and deactivation are not supported for heat transfer, coupled thermal-electrical, and coupled thermal-electrochemical procedures. Similarly, step-dependent activation and deactivation are not allowed for general contact with XFEM surfaces anywhere in the model. For the small-sliding tracking approach in general contact, portions of the model where small-sliding is specified also cannot participate in step-dependent activation and deactivation.

The general contact algorithm can consider three-dimensional edge-to-surface contact. In addition to modeling contact between segments of beam or truss elements and faceted surfaces, it is more effective at resolving some interactions than the surface-to-surface contact formulation. Figure 6 and Figure 7 show examples in which the edge-to-surface contact formulation is most effective for resolving contact.

Figure 7. Edge-to-surface contact examples.

The bottom-right example in Figure 7 shows a feature edge of an element-based surface in contact with an analytical rigid surface. The feature edges of an analytical rigid surface do not act as edges in the edge-to-surface formulation. You can enhance convergence behavior by avoiding feature edges and corners in analytical rigid surfaces used for contact: Instead, use smooth analytical rigid surfaces with continuously varying surface normal directions for contact.

Contact edges representing three-dimensional beam and truss elements have a circular cross-section (regardless of the actual cross-section of the beam or truss element), unless you enable actual beam cross-section representation for beams with noncircular cross-sections (see Contact Surface Representation for Beams). The radius of a contact edge representing a three-dimensional truss element is derived from the cross-sectional area specified on the truss section definition (it is equal to the radius of a solid circular section with an equivalent cross-sectional area). For three-dimensional beams with circular cross-sections, the radius of the contact edge is equivalent to the section radius. For three-dimensional beams with noncircular cross-sections, the radius of the contact edge is equal to the radius of a circumscribed circle around the section. Edge-to-surface contact for three-dimensional beam or truss elements is activated by including the associated surfaces into the general contact domain. By default, the all-inclusive surface contains surfaces based on beam or truss elements.

The surface thickness for two-dimensional beam segments corresponds to the in-plane thickness of the beam section. The surface thickness for two-dimensional truss segments corresponds to the radius of a solid circular section with an equivalent cross-sectional area as specified for the truss section. Beams and trusses are treated only with the surface-to-surface contact formulation in general contact, as discussed in Types of Contact Formulations within General Contact Targeting Various Scenarios.

Beam section offsets, specified either as part of the beam cross section definition or directly, are neglected for contact interactions in Abaqus/Standard.

The edge-to-surface contact formulation is commonly used to resist penetrations of feature edges of one surface into a relatively smooth portion of another surface (which might be an analytical rigid surface), with the contact normal direction based on the relatively smooth surface. The main and secondary roles of surfaces in the edge-to-surface contact formulation are reversed for some situations involving large-diameter beams. By default, if half of the beam radius exceeds the facet dimensions of the other surface, the beam acts as the main surface such that the edge-to-surface contact formulation resists penetrations of a smooth portion of a neighboring solid or shell surface into a beam, with the contact normal direction based on the radial direction of the beam. The bottom-left example in Figure 7 corresponds to such a case with a relatively large diameter beam (see Main and Secondary Surface Roles of a Contact Formulation for details about how to control main and secondary assignment).

In pure heat transfer and coupled thermal-electrical analyses, only the edge-to-surface formulation with the one-dimensional line ("beam") element–based surfaces acting as the main is supported. Consequently, these line element–based surfaces act as the main by default. The line elements are intended to model higher dimensional physics with a one-dimensional idealization. They are expected to have a cross-sectional (beam) radius that is comparable to the facet dimensions of the surrounding surfaces to obtain accurate results for thermal/electrical interactions.

The edge-to-surface contact formulation considers twisting of beams only for cases in which the contact normal is based on the radial direction of the beam. When considered, beam twisting influences the calculation of incremental slip.

By default, when a surface is used in a general contact interaction, all applicable facets are included in the contact definition along with edges of solid and shell elements with feature angles of at least 45°. See Feature Edges for a discussion of controls related to which feature edges are considered for edge-to-surface contact. Edge-to-surface contact constraints never participate in thermal, electrical, or pore pressure contact properties. For example, in a coupled temperature-displacement analysis, surface-to-surface constraints can influence mechanical and thermal interactions; but, if edge-to-surface constraints are included, they will only help resist penetrations.

The contact area associated with a feature edge depends on the mesh size; therefore, contact pressures (in units of force per area) associated with edge-to-surface contact are mesh dependent.

The general contact algorithm can optionally consider three-dimensional edge-to-edge contact except on crack surfaces for enriched elements. Feature edges on solid and shell-like surfaces, shell perimeter edges, and edges representing beams (and trusses) can be included. Figure 8 shows examples in which the edge-to-edge contact formulation is most effective for resolving contact.

Figure 8. Edge-to-edge contact examples.

Two edge-to-edge contact formulations are available. One formulation bases the contact normal direction on the cross product between the two respective edges considered for contact, and the other formulation uses a radial direction of one of the beams as the contact direction (similar to what is done for tube-to-tube contact elements, which are discussed in Tube-to-Tube Contact Elements). Four of the examples in Figure 8 rely on the formulation with the cross product normal to resist penetrations, and the example on the lower right of Figure 8 relies on the formulation with the radial normal. The edge-to-edge contact formulation with the radial normal is applicable only to cases with some thickness contributing to the contact calculations.

The example shown in Figure 9 involves compression of a spring modeled with beam elements. This example relies on the edge-to-edge contact formulation with a radial normal direction to resolve contact between adjacent spring coils, and it relies on the edge-to-surface contact formulation to resolve contact between the spring and other surfaces.

Figure 9. Compressing a spring modeled with beam elements.

The edge-to-edge contact formulation with a radial normal can involve the “exterior” of beam, shell, and solid feature edges and the “interior” of hollow beams, as shown in the example in Figure 10. This example involves a wire modeled with beam elements being wound onto a cylinder modeled with solid elements. The wire passes through a hollow cylindrical guide before coming onto the cylinder. The “radial” edge-to-edge formulation resolves contact between adjacent coils of the wire and also resolves contact between the wire and the interior of the hollow beam representing the guide. The edge-to-surface contact formulation resolves contact between the wire and the cylinder.

Figure 10. Winding a wire onto a cylinder.

The edge-to-edge contact formulation with a contact normal direction based on the cross product of the edge directions is applicable only while edges are not nearly parallel. The edge-to-edge contact formulation with a radial contact normal direction is typically most applicable while contact edges are nearly parallel, but Figure 11 shows an exception. The hollow beam is simultaneously in contact with the two other beams. The cross product version of the edge-to-edge contact formulation resolves contact between the exterior of the hollow beam and the beam that is near the top of Figure 11. The radial version of the edge-to-edge contact formulation resolves contact between the interior of the hollow beam and the spiral-shaped beam, with the contact direction corresponding to the interior radial direction of the hollow beam. The radial version of the edge-to-edge contact formulation is effective in this case because individual segments of the spiral-shaped beam span relatively small arcs of the hollow tube.

Figure 11. Edge-to-edge contact example with an internal beam spiral.

In addition to choosing to activate one or both types of edge-to-edge contact formulations, you must specify a feature angle criterion to activate feature and perimeter edges to participate in edge-to-edge contact. See Feature Edges for a discussion of controls related to which feature edges are considered for edge-to-edge contact. If only beam edges are present, specifying the contact formulation alone is sufficient.

Edge-to-edge contact formulations do not consider twisting of the beams. Beam-to-beam contact cannot be used to model contact between beam-like elements that share nodes with underlying solid or shell elements (for example, beam elements that are used to model stringers).

CONTACT FORMULATION, TYPE=EDGE TO EDGE, FORMULATION=BOTH

CONTACT FORMULATION, TYPE=EDGE TO EDGE, FORMULATION=NO (default)

CONTACT FORMULATION, TYPE=EDGE TO EDGE, FORMULATION=RADIAL

CONTACT FORMULATION, TYPE=EDGE TO EDGE, FORMULATION=CROSS

Interaction module: Create Interaction: General contact (Standard): Contact Formulation: toggle on Edge-to-edge formulation assignment, Formulation: select Both, No, Radial, or Cross

The general contact algorithm can consider three-dimensional vertex-to-surface contact except on crack surfaces for enriched elements. Figure 12 shows examples in which the vertex-to-surface contact formulation is most effective for resolving contact. The vertex-to-surface contact formulation is intended to avoid localized penetration of a node at a convex corner of a solid or shell/membrane surface or at an end point or kink of a beam/truss into a relatively smooth portion of another surface (which may be an analytical rigid surface). Most vertex nodes are along feature edges, although, for example, a node at the tip of a cone may satisfy the vertex node criteria. See Vertex Nodes for a discussion of the vertex node criteria. Vertex nodes are effectively treated as spherical in the vertex-to-surface formulation. The spherical radius of the contact vertex corresponds to the surface thickness at the node.

Figure 12. Vertex-to-surface contact examples.

The bottom-right example in Figure 12 shows a vertex node of an element-based surface in contact with an analytical rigid surface. The corners of an analytical rigid surface do not act as vertices in the vertex-to-surface formulation. Convergence behavior can be enhanced by avoiding feature edges and corners in analytical rigid surfaces used for contact and instead using smooth analytical rigid surfaces with continuously varying surface normal directions for contact.