Extending Main Surfaces and Slide Lines

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.

Figure 1. 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.

Figure 2. Definition of size of extension zone.

CONTACT PAIR, SMALL SLIDING, EXTENSION ZONE=e 

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.

CONTACT PAIR, EXTENSION ZONE=e 
SLIDE LINE, ELSET=element_set_name, EXTENSION ZONE=e