Assigning surface properties for contact pairs in Abaqus/Explicit

Accounting for thickness in the contact pair algorithm will cause the surface to extend past the parent element boundary in the plane of the element by an amount equal to one-half its thickness. For example, this surface extension, which is semi-circular in shape, will cause contact to be established between the edge of a shell and an opposing surface before the node on the shell boundary reaches the opposing surface. The extension is present for both single-sided and double-sided surfaces. Figure 3 demonstrates this concept. Such “bull-nose” extensions are avoided when the general contact algorithm (About general contact in Abaqus/Explicit) is used. The effect of a shell or rigid offset on a surface is shown in Figure 4. Poorly defined surfaces can result near corners if large offsets are present, as shown in Figure 5. You should consider this when defining a model. A warning message will be issued if the offset magnitude is greater than one-half of any of the parent shell element edge lengths. However, at acute corners it is possible for an offset less than one-half of the parent element size to result in a poorly defined contact surface (and in this case no warning will be given).

Figure 3. Extension of contact surface for edge contact without zero surface thickness.

Figure 4. Extension of contact surface if a shell offset is present.

Figure 5. Example of a poorly defined surface near a corner when a large shell offset is present.

You can control the thickness and offset used in the contact calculations only; they do not affect surface-based constraints. These settings are intended primarily for self-contact surfaces since you cannot force zero thickness for these surfaces, as described below.

Self-contact surfaces should not contain facets that are thicker than their edge or diagonal lengths. Extremely large thicknesses will cause nodes to appear to be penetrating nearby facets in even a flat self-contact surface due to the algorithmic use of a semi-circular tube with a radius of half the contact thickness around the edge of each facet (see Figure 6).

Figure 6. Undesired penetration resulting from a large thickness in a self-contact surface.

You can scale the effective thickness used for all of the facets on a surface by a single factor, f. Alternatively, you can adjust only the thicknesses for surface facets in which the thickness to minimum edge or diagonal length ratio exceeds a specified value, r; the amount by which a facet thickness is adjusted may vary during an analysis because of changes in the facet size. If the thickness to element size ratio exceeds 1.0 in the initial configuration for a self-contact surface, an error message recommending that you adjust the thickness will be issued.

You should not specify extremely small values for f or r for double-sided surfaces or surfaces that will be involved in self-contact since these surfaces must have a contact thickness that is significant compared to the facet size. For surfaces involved only in two-surface contact it is acceptable to set f=0.0; however, it is computationally more efficient to use the method described below to force a zero surface thickness. It is also possible to enforce the offset but not the thickness in the surface model by setting the scale factor, f, equal to zero.

SURFACE, NAME=name, SCALE THICK=f

SURFACE, NAME=name, MAX RATIO=r

You can force the surface thickness and offset to be zero, rather than inherit the thickness and offset of underlying shell, membrane, or rigid elements. In this case the contact surface is taken as the reference surface (see Figure 7).

Figure 7. Contact surface with zero thickness and offset.

You cannot ignore the thickness for a surface that is used as a contact surface for single-surface (self) contact. If one of the surfaces in a contact pair is a double-sided surface, zero thickness can be forced on only one of the two surfaces: at least one surface in a contact pair involving double-sided surfaces must have a nonzero thickness. The ability to force zero surface thickness is useful for performing parameter studies on the thickness or offset of a model since you can change the thickness and offset without also having to move the mesh to control the initial separation between the surfaces.

SURFACE, NAME=name, NO THICK

Example

Contact calculations are generally most accurate with the default treatment of thickness and offset. However, when a shell offset of half the original shell thickness has been specified, forcing zero surface thickness will give an accurate representation of one side of the surface. This approach can be more accurate near a corner (especially on the exterior side of a corner) than if the offset and thickness are enforced for the surface, as shown in Figure 8.

Figure 8. Forcing zero surface thickness when the shell offset is half the original shell thickness.

For situations in which it is desirable to ignore the effect of the offset but when it is still necessary to model the thickness in the contact calculations, you can force only the surface offset to be zero without affecting the surface thickness. In this case the contact surface is the outside surface of an imaginary shell, membrane, or rigid element whose midsurface is at the reference surface (see Figure 9).

Figure 9. Contact surface with zero offset.

This method could be used for a self-contact surface that would be poorly defined if the offset were enforced (thickness must be enforced for self-contact surfaces).

SURFACE, NAME=name, NO OFFSET