Extending Main Surfaces and Slide Lines

Extending the main surface or a slide line:

  • can prevent nodes from “falling off” or getting trapped behind the main surface (or slide line) in finite-sliding problems;

  • allows the secondary node to find a main surface when the secondary node has no intersection with the main surface at the start of the analysis in small- and infinitesimal-sliding problems;

  • can avoid numerical roundoff difficulties associated with contact modeling;

  • should not be used in lieu of proper contact modeling techniques;

  • should not be used to reduce the number of underlying elements of a contact surface;

  • applies only at the perimeter of a main surface in three dimensions and at the ends of a main surface in two dimensions; and

  • applies only to contact pairs that use a node-to-surface discretization.

This page discusses:

 

See Also
About Contact Pairs in Abaqus/Standard
Common Difficulties Associated with Contact Modeling in Abaqus/Standard
*CONTACT PAIR
*SLIDE LINE

Extending the Main Surface for Small-Sliding, Node-to-Surface Contact

If a secondary node cannot find an intersection with the main surface at the start of the analysis, it will be free to penetrate the main surface because no local tangent plane will be formed. This type of problem, which typically occurs for node-to-surface contact when the secondary node is aligned with the end or perimeter of the main surface (which does not wrap around the corner of the rectangular body), is illustrated in Figure 1 and may be caused by numerical roundoff errors when a preprocessor is used to generate the nodal coordinates. There are no extensions to main faces in the interior of a surface. If the main surface in Figure 1 were defined such that it wrapped around the corner of the body, no extensions to the main surface would be required because the secondary node would project onto the main surface using the projection method discussed in Using the Small-Sliding Tracking Approach. Cases such as that shown in Figure 1 are not problematic for the small-sliding, surface-to-surface formulation because the constraint formulation considers the region of the secondary surface near a secondary node.

Secondary node fails to find an intersection with the main surface for small-sliding, node-to-surface contact if e=0.

For node-to-surface contact you can specify the size of the extension zone, e, as a fraction of the end segment or facet edge length (see Figure 2). If e is set to zero, Abaqus will not extend the ends. The value given must lie between 0.0 and 0.2. The default value is 0.1 for node-to-surface contact; surface extensions are not available for surface-to-surface contact.

Definition of size of extension zone.

Extending the Main Surface or Slide Line in Finite-Sliding, Node-to-Surface Contact

To prevent secondary nodes from “falling off” or getting trapped behind a main surface, an open surface or slide line can be extended beyond its perimeter edges (in three dimensions) or end nodes (in two dimensions) for finite-sliding, node-to-surface contact.

You can specify the size of the extension zone, e, as a fraction of the end segment or facet edge length (see Figure 2). The geometry in the extension zone is extrapolated from the end segment or facet edge. If e is set to zero, Abaqus/Standard will not extend the ends. The value given must lie between 0.0 and 0.2. The default value is 0.1 for node-to-surface contact. Surface extensions are not available for surface-to-surface contact; for finite-sliding, surface-to-surface contact, constraints are located within secondary faces, and “falling off” will not occur until nearly the entire secondary facet slides off the main surface. Extensions for finite-sliding, node-to-surface contact should be considered only if other modeling techniques to prevent “falling off” are not feasible and when the secondary node is expected to travel in the extended zone for a short period of the solution phase or during nonconverged iterations.