Adjusting initial surface positions and specifying initial clearances for contact pairs in Abaqus/Explicit

Abaqus/Explicit will automatically adjust the positions of surfaces to remove any initial overclosures that exist when a contact pair is defined in the first step of a simulation, except when nodes of a rigid body act as a slave nodes or user subroutine VUINTER is used*CLEARANCE. The adjustments are made with strain-free initial displacements to the slave nodes on the surfaces. Therefore, when a balanced master-slave contact pair is defined, nodes on both surfaces may be adjusted. This automatic adjustment of surface position is intended to correct only minor mismatches associated with mesh generation. You can review the surface adjustments in the status (.sta) file, the message (.msg) file, and the output database (.odb) file; see Contact diagnostics in an Abaqus/Explicit analysis for more information.

Some softened contact models have nonzero contact pressure at zero overclosure (see Contact pressure-overclosure relationships). For these models some initial, nonequilibrated contact pressure may be present at the beginning of an analysis, as the adjustments are made to satisfy zero overclosure rather than zero contact pressure. Large initial contact pressures may cause excessive distortion of elements near the contact surfaces.

Conflicting adjustments from separate contact pairs will cause incomplete resolution of initial overclosures and will lead to a noisy solution or severe distortion of elements. This can occur when a slave node is pinched between two master surfaces.

Because of the lack of a unique outward direction from double-sided facets, the resolution of large initial penetrations for double-sided surfaces can be difficult. Initial penetration will be detected only when a slave node lies within the thickness of the underlying element, and the initial penetration will be resolved by moving the slave node to the nearest free surface as shown in Figure 1.

Figure 1. Correction of initial overclosure for a contact pair involving two double-sided surfaces.

A warning message will be issued to the status (.sta) file if two adjacent slave nodes (connected by a facet edge) are detected on opposite sides of a double-sided master surface involved in contact defined with the contact pair algorithm. No such warning will be issued for node-based surface nodes on opposite sides of a double-sided master surface, because adjacency cannot be determined among the node-based surface nodes. If the master surface is a single-sided surface, initial overclosures will be resolved using the surface normal of the master surface, as shown in Figure 2.

Figure 2. Correction of initial overclosure for a contact pair involving a single-sided and a double-sided surface.

Having slave nodes trapped on opposite sides of a double-sided master surface will often lead to serious problems, which may not became apparent until later in an analysis. Therefore, a data check analysis (see Abaqus/Standard and Abaqus/Explicit execution) is recommended prior to running a large contact pair analysis so that you can check for warning messages in the status file (.sta) and check for mislocated adjacent slave nodes on opposite sides of the master surface.

The adjustments affect only the nodes on the surfaces. Excessive distortion of neighboring elements may result if this feature is used to correct for gross errors in the initial geometry, causing the analysis to end with an error message.

Nodes on a rigid body can act as slave nodes only for penalty contact pairs. Initial penetrations of slave nodes that are part of a rigid body are not resolved with strain-free corrections; i.e., the slave nodes are not adjusted. These penetrations are likely to cause artificially large contact forces in the first increments of an analysis and should, therefore, be avoided in the mesh definition.

If contact pairs are defined in later steps with initially overclosed surfaces, Abaqus/Explicit does not take any special actions to gradually resolve these initial penetrations: contact forces will be applied according to whatever contact constraint enforcement method is being used. These contact forces may be very large, causing large accelerations and velocities and possible distortion of elements. Initial penetrations have the potential to cause problems for contact pairs introduced in any step if a VUINTER user subroutine is used; but in that case you control the application of contact forces.

You can define initial clearances and contact directions precisely for the nodes on the slave surface when they would not be computed accurately enough from the nodal coordinates; for example, if the initial clearance is very small compared to the coordinate values. Initial clearances and contact directions can be defined only in small-sliding contact analyses (Contact formulations for contact pairs in Abaqus/Explicit).

The initial clearance value calculated at every slave node based on the coordinates of the slave node and the master surface is overwritten by the value that you specify. This procedure does not alter the coordinates of the slave nodes.

When the balanced-master slave contact algorithm is invoked for the contact pair, the initial clearance values can be defined on one or both of the surfaces. Initial clearances defined on contact surfaces that act only as master surfaces will be ignored.