Contact elements are constrained against penetrating the target surface. However, target elements can penetrate through the contact surface. For rigid-to-flexible contact, the designation is obvious: the target surface is always the rigid surface and the contact surface is always the deformable surface. For flexible-to-flexible contact, the choice of which surface is designated contact or target can cause a different amount of penetration and therefore affect the solution accuracy. Consider the following guidelines when designating the surfaces:
If a convex surface is expected to come into contact with a flat or concave surface, the flat/concave surface should be the target surface.
If one surface has a fine surface mesh and, in comparison, the other has a coarse mesh, the fine mesh should be the contact surface and the coarse mesh should be the target surface.
If one surface is stiffer than the other, the softer surface should be the contact surface and the stiffer surface should be the target surface.
If higher-order elements underly one of the external surfaces and lower-order elements underly the other surface, the surface with the underlying higher-order elements should be the contact surface and the other surface should be the target. However, for 3D node-to-surface contact, the lower-order elements should be the contact surface. The higher-order elements should be the target surface.
If one surface is markedly larger than the other surface, such as in the instance where one surface surrounds the other surface, the larger surface should be the target surface.
In the case of 3D internal beam-to-beam contact modeled by CONTA177 (a beam or pipe sliding inside another hollow beam or pipe), the inner beam should be considered the contact surface and the outer beam should be the target surface. However, when the inner beam is much stiffer than the outer beam, the inner beam can be the target surface.
These guidelines are true for asymmetric contact. However, asymmetric contact may not perform satisfactorily for your model. The following section details the difference between asymmetric and symmetric contact and outlines some of the situations that require symmetric contact.
Note: The underlying element of contact or target elements can be obtained by the CNCHECK and *GET commands in /PREP7.
CNCHECK *GET,Par, ELEM,N, ATTR, ISOLID
Where:
NThe contact/target element number
ParReturn the underlying element number associated to
N
The following topics related to asymmetric contact vs. symmetric contact are available:
Asymmetric contact is defined as having all contact elements on one surface and all target elements on the other surface. This is sometimes called "one-pass contact." This is usually the most efficient way to model surface-to-surface contact. However, under some circumstances asymmetric contact does not perform satisfactorily. In such cases, you can designate each surface to be both a target and a contact surface. You can then generate two sets of contact pairs between the contacting surfaces (or just one contact pair, for example, a self-contact case). This is known as symmetric contact (or "two-pass contact").
Obviously, symmetric contact is less efficient than asymmetric contact. However, many analyses will require its use (typically to reduce penetration). Specific situations that require symmetric contact include models where
The distinction between the contact and target surfaces is not clear.
Both surfaces have very coarse meshes. The symmetric contact algorithm enforces the contact constraint conditions at more surface locations than the asymmetric contact algorithm.
If the meshes on both surfaces are identical and sufficiently refined, the symmetric contact algorithm may not significantly improve performance and may, in fact, be more "expensive" in CPU time. In such circumstances, pick one surface to be the target and the other the contact surface.
Symmetric Contact
For each symmetric contact definition, there exist two contact pairs: a base pair and a companion pair. By default (KEYOPT(8) = 0), both pairs are active and the program determines different contact pair characteristics for each pair (contact depth, length, pinball radius, contact normal stiffness, contact damping, tolerances, and so on).
In some cases, the reported contact pressure on either side of the contact surfaces may not be accurate. This may occur, for example, when one contact surface has a much finer surface mesh discretization than the other contact surface, or when underlying elements of one contact surface are much stiffer than underlying elements of the other contact surface. Manual averaging of the results is often required. The total contact pressure acting on both sides can be calculated as the average of the contact pressures on each side of the surface. Other quantities such as frictional stress, wear depth, and wear volume must also be manually averaged in this way.
In certain configurations, the program may find one side of a contact surface as closed and the other side of the surface as open. In this case, it can be difficult to interpret the results and manual averaging is not practical.
To alleviate these issues, set KEYOPT(8) = 1. For this setting the program honors both contact pairs in the symmetric contact definition but uses the same contact pair characteristics for both pairs. These characteristics are determined by averaging values from the two pairs. The KEYOPT(8) = 1 setting is particularly useful when modeling contact surface wear and when applying fluid-pressure-penetration loads.
Other Considerations for Symmetric Contact
Keep these points in mind when working with symmetric contact pairs:
The program recognizes symmetric contact definitions only for flexible-to-flexible contact. Rigid-to-rigid contact pairs are always treated as asymmetric pair definitions even though they may follow the symmetric contact pattern of having target and contact elements on both surfaces.
In any contact model, you can mix different types of contact pairs: rigid-to-flexible and flexible-to-flexible contact, symmetric contact and asymmetric contact. However, only one type can exist within a contact pair.
When KEYOPT(8) = 1, the reported contact results are a combination of the results from the two symmetric pairs.
Asymmetric Contact Selection
Sometimes it is more convenient to define symmetric contact (as when graphical picking of contact and target surfaces is difficult), but asymmetric contact is desired for efficiency. In this case, you can define symmetric contact and set KEYOPT(8) = 2 or 3 to have the program internally select which asymmetric pair to activate at the solution stage based on the guidelines mentioned in Designating Contact and Target Surfaces. The other companion pair remains inactive. The two settings differ as follows:
KEYOPT(8) = 2 - For this option, the contact stiffness used for the active contact pair is influenced by the underlying element stiffness of the inactive pair.
KEYOPT(8) = 3 - For this option, the contact stiffness and other contact characteristics used for the active contact pair are completely independent of the inactive pair. It is as if the inactive pair does not exist.
When contact pair-splitting is used at the solution level in a distributed-memory parallel run (CNCHECK,DMP), the KEYOPT(8)= 3 option ensures the same results are obtained with and without the splitting logic.