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 (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);

• the mechanical contact property models (Contact properties for general contact in Abaqus/Standard);

• the controls associated with the initial contact state (Controlling initial contact status in Abaqus/Standard); and

• the algorithmic contact controls (Numerical controls for general contact in Abaqus/Standard).

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.

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. 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.

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 and crack surfaces for enriched elements.

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.

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 1. General contact uses the other contact formulations as supplementary formulations. For example, the second, third, and fourth cases in Figure 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.

Figure 1. 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 2 because the active contact zone corresponds to a feature edge.

Figure 2. 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 will be 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.

Additional discussion of the surface-to-surface contact formulation is provided in Surface-to-surface contact discretization. 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.

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.

CONTACT

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.

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 2 and Figure 3 show examples in which the edge-to-surface contact formulation is most effective for resolving contact.

Figure 3. Edge-to-surface contact examples.

The contact edges representing beam and truss elements have a circular cross-section, regardless of the actual cross-section of the beam or truss element. The radius of a contact edge representing a 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 beams with circular cross-sections, the radius of the contact edge is equivalent to the section radius. For beams with non-circular 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 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 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, with the contact normal direction based on the relatively smooth surface. The master and slaves 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 will act as the master 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 rightmost example in Figure 3 corresponds to such a case with a relatively large diameter beam (see Master and slave roles for details about how to control master and slave assignment).

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 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 4 shows examples in which the edge-to-edge contact formulation is most effective for resolving contact.

Figure 4. 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 4 rely on the formulation with the cross product normal to resist penetrations, and the example on the bottom right of Figure 4 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 5 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 5. 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 6. 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 6. 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 6 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 6. 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 7. 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

The general contact algorithm can consider vertex-to-surface contact except on crack surfaces for enriched elements. Figure 8 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. 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 8. Vertex-to-surface contact examples.