29.3.6 Contact controls for general contact

Product: ABAQUS/Explicit  



Contact controls for the general contact algorithm:

  • can be used to selectively scale the default penalty stiffness for particular regions within a general contact domain; and

  • can be used to control whether nodes are removed from the general contact domain once all of the faces and edges to which they are attached have eroded.

Scaling default penalty stiffnesses

The general contact algorithm uses a penalty method to enforce the contact constraints (see Contact formulation for general contact, Section 29.3.4, for more information). The “spring” stiffness that relates the contact force to the penetration distance is chosen automatically by ABAQUS/Explicit, such that the effect on the time increment is minimal yet the allowed penetration is not significant in most analyses. Significant penetrations may develop in an analysis if any of the following factors are present:

  • Displacement-controlled loading

  • Materials at the contact interface that are purely elastic or stiffen with deformation

  • Rigid bodies or deformable elements (especially membrane and surface elements) that have relatively little mass of their own and are constrained via methods other than boundary conditions (for example, connectors) involved in contact

See The Hertz contact problem, Section 1.1.11 of the ABAQUS Benchmarks Manual, for an example in which the first two of these factors combine such that the contact penetrations with the default penalty stiffness are significant.

You can specify a scale factor by which to modify penalty stiffnesses for specified interactions within the general contact domain. This scaling may affect the automatic time incrementation. Use of a large scale factor is likely to increase the computational time required for an analysis because of the reduction in the time increment that is necessary to maintain numerical stability (see Table 29.3.6–1).

Table 29.3.6–1 Effect of scale factor on time increment.

Penalty scale factorLower bound to ratio of the time increment with contact divided by the time increment without contact

The surface names used to specify the regions where nondefault penalty stiffness should be assigned do not have to correspond to the surface names used to specify the general contact domain. In many cases the contact interaction will be defined for a large domain, while a nondefault penalty stiffness will be assigned to a subset of this domain. If the surfaces to which a nondefault penalty stiffness is assigned fall outside the general contact domain, the controls assignment will be ignored. The last assignment will take precedence if the specified regions overlap.

Input File Usage:           
surface_1, surface_2, scale_factor

This option must be used in conjunction with the *CONTACT option. It should appear at most once per step; the data line can be repeated as often as necessary to assign penalty stiffness scale factors to different regions. If the first surface name is omitted, a default surface that encompasses the entire general contact domain is assumed. If the second surface name is omitted or is the same as the first surface name, the specified contact controls are assigned to contact interactions between the first surface and itself. Keep in mind that surfaces can be defined to span multiple unattached bodies, so self-contact is not limited to contact of a single body with itself.

Control of nodal erosion

You can control whether contact nodes remain in the contact domain after all the surrounding faces and edges have eroded due to element failure. 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 (i.e., be removed from the contact domain) once all contact faces and contact edges to which they are attached have eroded. Nodes that you include in the contact domain only with node-based surfaces are never removed from the contact domain.

Computational cost can increase as a result of free-flying nodes if nodal erosion is not specified, particularly for analyses conducted in parallel. The increased computational cost is related to the likelihood of free-flying nodes moving far away from the elements that remain active, which stretches the volume of the contact domain and thereby tends to increase contact search costs as well as the cost of communication between processors in parallel analysis. However, contact involving free-flying nodes can contribute significant momentum transfer in some cases, which will not be accounted for if nodal erosion is specified.

Input File Usage:           

This option must be used in conjunction with the *CONTACT option. This parameter setting applies to the entire general contact domain.