About General Contact in Abaqus/Explicit

The general contact algorithm also considers edge-to-edge contact, which is very effective in enforcing contact that cannot be detected as penetrations of nodes into faces. For example, contact between beam segments and shell perimeter edges (see Figure 1) usually is detected only as edge-to-edge contact.

General contact domain, including edge-to-edge contact.

The terminology “contact edges” refers to feature edges of surface facets (on both shells and solids) as well as to segments representing beam and truss elements. 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 noncircular cross-sections, the radius of the contact edge is equal to the radius of a circumscribed circle around the section. If connected edges have different radii, a nodal radius is first computed as the minimum radius of the adjacent contact edges, and the radius of the edge cross-section is interpolated linearly over the length of the contact edge from the nodal values. Shell element edges reflect the shell thickness in the normal direction and do not extend past the perimeter (similar to shell nodes and facets). Some numerical rounding of features occurs for both node-to-facet and edge-to-edge contact.

To model contact between edges that are not cylindrical in shape, surface elements can be attached to the edge nodes using surface-based tie constraints and node-to-face contact can be defined between the surface elements (see Surface Elements). This technique is useful for modeling geometric details important to the contact definition that are not modeled with the underlying element geometry. Surface elements can also be defined around shell elements in which Abaqus has reduced the contact thickness (that is, if the thickness exceeds the surface facet edge lengths or diagonal lengths) so that the true surface thickness can be modeled. However, using surface elements with general contact requires a physically reasonable mass to be associated with the surface element nodes, and care must be taken not to alter the bulk mass properties when transferring mass to the surface elements from the underlying elements.

By default, when a surface is used in a general contact interaction, all applicable facets, analytical rigid surfaces, nodes, perimeter edges, currently active feature edges, and beam and truss segments are included in the contact definition. You can control which feature edges are considered for edge-to-edge contact, as discussed in Assigning Surface Properties for General Contact in Abaqus/Explicit.

Figure 2 shows examples of beam element profile rendering for portions of beams with circular and rectangular cross-sections. These beams are modeled with several beam elements on either side of a kink. By default, Abaqus/Explicit uses a circular cross-section contact surface representation for beams, regardless of the actual cross-section shape. For noncircular beam element cross-sections, the default circumferential representation for contact encompasses the actual cross-section. Default contact surface representations for both beams of Figure 2 are identical and approximately correspond to the image on the left in Figure 2, except for the addition of a smooth transition at the kink. The default contact representation for beams involves contact nodes co-located with beam nodes and contact edges associated with beam reference edges that pass through nodes of individual beam elements; it does not involve contact faces on beams.

Beam element profile rendering for circular versus rectangular cross-section beams.

An alternative contact surface representation method for beams leads to a more accurate contact representation of noncircular beams. This method uses an automatically generated internal mesh of contact faces, nodes, and edges (such as shown in Figure 3). Beams with circular cross-sections have the same contact surface representation (involving contact nodes and contact edges only) for the default and alternative contact surface representation methods. For the alternative contact surface representation method, internal nodes are positioned at vertices of the cross-section at axial locations of each original beam node. The motion of each of these internal nodes is driven by the corresponding beam node (via a rigid connection). The internal contact faces, nodes, and edges generated by Abaqus/Explicit are not available for postprocessing, but the shape they form closely resembles the beam mesh with beam profile rendering activated.

Internal mesh representations of beam contact surface.

Simulating contact among the beams shown in Figure 4 calls for the alternative contact surface representation. Both node-to-surface and edge-to-edge contact can occur among contact nodes, faces, and edges of such a simulation.

Beams with various cross-sections.

Assigning a nonzero surface thickness for contact (see Surface Thickness) to an alternative beam surface representation has the effect of expanding the contact cross-section and rounding the corners (without influencing the beam stiffness), as shown in Figure 5.

Effect of surface thickness for a rectangular beam cross-section.

The general contact algorithm also enforces contact between Eulerian materials and Lagrangian surfaces. This algorithm automatically compensates for mesh size discrepancies to prevent penetration of Eulerian material through the Lagrangian surface. The all-inclusive surface that is defined by Abaqus/Explicit can be used to enforce contact between all Eulerian materials and all Lagrangian bodies in a model; you can also specify individual Eulerian surfaces in the contact domain (see Eulerian Surface Definition). Eulerian-Lagrangian contact is enforced only for Lagrangian surfaces defined on solid and shell elements. Other surface types, such as beam edges and analytical rigid surfaces, are ignored. Contact interactions between Eulerian materials and interactions due to Eulerian material self-contact are handled naturally by the Eulerian formulation; these interactions do not require a general contact definition. See Interactions for more information.

The general contact algorithm enforces the following types of contact involving DEM or SPH particles:

• contact between DEM or SPH particles and other Lagrangian surfaces; and

• contact among DEM particles.

See Discrete Element Method and Smoothed Particle Hydrodynamics for more information regarding contact involving DEM and SPH particles, respectively.

You can remove the previously specified general contact definition and specify a new one. Contact state information (such as the proper contact normal orientation for double-sided surfaces) is transferred across step boundaries for interactions that are part of both the old and the new contact definitions. Only *CONTACT, OP=NEW can be used to re-introduce interactions that were part of prior contact exclusions.

Alternatively, you can make changes to an existing general contact definition. In this case the existing general contact definition remains active and any additional information specified is appended to the general contact definition. Prior contact exclusions cannot be converted into contact inclusions with this method.

Contact state information (such as the proper contact normal orientation for double-sided surfaces) is transferred across step boundaries even if the contact domain is modified.

CONTACT
CONTACT INCLUSIONS
surf_1, 
CONTACT EXCLUSIONS
surf_a, surf_b

CONTACT, OP=MOD
CONTACT INCLUSIONS
surf_2, surf_3 
CONTACT EXCLUSIONS
surf_a, surf_c

CONTACT, OP=NEW
CONTACT INCLUSIONS
surf_1,
surf_2, surf_3 
CONTACT EXCLUSIONS
surf_a, surf_b 
surf_a, surf_c

Define contact inclusions to specify the regions of the model that should be considered for contact purposes.

CONTACT
CONTACT INCLUSIONS
 , surface_2

CONTACT
CONTACT INCLUSIONS
surface_1, 

CONTACT
CONTACT INCLUSIONS
surface_1, surface_1

CONTACT
CONTACT INCLUSIONS
 ,
 , node_based_surf

You can refine the contact domain definition by specifying the regions of the model to exclude from contact.

The primary motivation for specifying contact exclusions is to avoid physically unreasonable contact interactions. For example, a finite element model might contain multiple forming tools, but not all of the tools participate in the forming process simultaneously; you can specify contact exclusions to prevent certain tools from participating in the contact model in certain steps.

You do not need to be concerned with specifying contact exclusions for parts of the model that are not likely to interact, since these exclusions typically will have minimal effect on computational performance.

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. At least one surface in each pair must be either an element-based surface or an analytical rigid surface. 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.

You cannot exclude only one side of shell-like surfaces. If a side label (SPOS or SNEG) is used in defining an element-based shell-like surface and that surface is excluded from contact, Abaqus/Explicit will exclude all faces associated with these elements.

CONTACT
CONTACT INCLUSIONS, ALL EXTERIOR
CONTACT EXCLUSIONS
 , surface_2

CONTACT EXCLUSIONS
surface_1,

CONTACT EXCLUSIONS
surface_1, surface_1

General contact allows the use of element-based surfaces to model surface erosion for analyses that include material failure. If an appropriate “interior” or “eroding” surface is defined (as discussed in Generating an Interior Surface Automatically), the surface topology evolves to match the exterior of the elements that have not failed. Because of reduced memory usage, eroding surfaces are preferred over interior surfaces. Alternatively, if only one of the bodies can erode, a node-based surface can be used to model surface erosion; this approach can be used with either the general contact or contact pair algorithms. However, even if only one body can erode, it is recommended to define an element-based surface for the eroding body to avoid the usual limitations of node-based surfaces (see Node-Based Surface Definition).

The general contact algorithm modifies the list of contact faces and contact edges that are active in the contact domain based on the failure status of the underlying elements (element failure is discussed in Dynamic Failure Models). General contact considers a face only if its underlying element has not failed and it is not coincident with a face from an adjacent element that has not failed; thus, exterior faces are initially active, and interior faces are initially inactive. Once an element fails, its faces are removed from the contact domain, and any interior faces that have been exposed are activated. A contact edge is removed when all the elements that contain the edge have failed. New contact edges are not created as elements erode. Based on this algorithm, the active contact domain evolves during the analysis as elements fail (see Figure 6 for an example of an eroding solid).

Topology of an eroding contact surface.

You can control whether contact nodes remain in the contact domain after all the surrounding elements have failed. By default, these nodes remain in the contact domain and act as free-floating point masses that can experience contact with faces that are still part of the contact domain. You can specify that nodes of element-based surfaces should erode (that is, be removed from the contact domain) once all contact faces and contact edges to which they are attached have eroded. Further discussion of this technique, including reasons for and against nodal erosion, can be found in Contact Controls for General Contact in Abaqus/Explicit.