Controlling initial contact status for general contact in Abaqus/Explicit

Abaqus/Explicit automatically adjusts the positions of surfaces to remove small initial overclosures that exist in the general contact domain in the first step of a simulation. The adjustments are made with strain-free initial displacements. This automatic adjustment of surface position is intended to correct only minor mismatches associated with mesh generation and is done even when the interaction is defined through user subroutine VUINTERACTION.

Conflicting adjustments from separate contacts, boundary conditions, tie constraints, coupling constraints, and rigid body constraints can cause incomplete resolution of initial overclosures. This can occur, for example, when a slave node is pinched between two master facets. Initial overclosures that are not resolved by repositioning nodes are stored as temporary contact offsets to avoid large contact forces at the beginning of an analysis. The penalty contact force is computed as $f=k\left(d_{cur}-d_0\right)$; where k is the penalty stiffness, $d_0$ is the initial unresolved penetration distance, and $d_{cur}$ is the current penetration distance. If $d_{cur}$ ever decreases below $d_0$, $d_0$ is reset to $d_{cur}$.

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 contact involving two double-sided surfaces.

Slave nodes that are trapped on opposite sides of a double-sided master surface will often lead to serious problems, which may not become apparent until later in the analysis. Surfaces that are initially crossed often indicate a modeling problem for single-sided surfaces as well, because the initial search for slave nodes in the interior of solids is limited to a distance of about 15% of the facet dimensions; slave nodes more deeply penetrated than this are ignored by the algorithm to adjust initial overclosures.

Initial overclosure information—including node adjustment data, contact offsets, crossed surfaces, nodes that could not be corrected, and any warnings—is written to the status (.sta) file, the message (.msg) file, and the output database (.odb) file. The default tolerance used to report gross initial penetrations, which could indicate an error with your model definition, depends on the contact type. Node-to-surface contact uses the characteristic length of the contact facet, edge-to-edge contact uses the length of the tracked edge, and the typical element dimension is used for node-to-analytical rigid surface contact. For more information on the overclosure warnings, see Contact diagnostics in an Abaqus/Explicit analysis, and Viewing diagnostic output.

Initial penetrations are stored as temporary contact offsets that do not generate contact forces in the following cases:

• If the general contact domain is created in steps other than the first step (i.e., the contact definition follows a step in which no contact was defined) or

• if an Abaqus/Standard analysis is imported into Abaqus/Explicit and the contact interaction is not defined with user subroutine VUINTERACTION.

However, deep penetrations may not be treated correctly; they may be ignored or, in the case of penetrations past the midsurface of shells, the wrong contact directions may be used. Initial overclosure and crossed surface diagnostics can be requested to diagnose these problems (see Contact diagnostics in an Abaqus/Explicit analysis).

If the general contact domain is extended after the first step, Abaqus/Explicit does not take any special actions to gradually resolve initial penetrations for the newly introduced interactions: penalty contact forces will be applied proportional to the penetration, or the penetration may be ignored. In addition, initial overclosure and crossed surface diagnostics are not available for these new interactions.

In some cases the default algorithm will not correctly resolve initial overclosures, or a precise initial gap (i.e., a positive clearance) between surfaces may need to be modeled. Specifically, deep penetrations may be ignored, tangled double-sided surfaces may not be separated correctly (see Figure 1), and gaps between curved surfaces in the discretized model may be inconsistent with the non-discretized model. To resolve these issues, you can define contact clearances and assign them to contact interactions. Examples are given below.

You can specify a search node set to identify which slave nodes will be tagged as initially bonded in a VCCT crack propagation analysis. See Crack propagation analysis for more details.

CONTACT CLEARANCE, NAME=clearance_name,
SEARCH NSET=node set name
CONTACT CLEARANCE ASSIGNMENT
surface_1, surface_2, clearance_name