Contact Initialization for General Contact in Abaqus/Explicit

Abaqus/Explicit automatically adjusts the positions of surfaces or optionally stores penetration offsets (as discussed in Resolving Discrepancies between Mesh Geometry and Desired Initial Clearance or Penetration State) to remove small initial overclosures. In addition, if the search zone for contact initialization is extended to the gap region (as discussed in Contact Initialization Search Criteria), Abaqus/Explicit removes small gaps existing in the general contact domain in the first step of a simulation. Position adjustments are made with strain-free initial displacements. Figure 1 shows an example of repositioning nodes to resolve initial overclosures. This automatic adjustment of the initial penetration or gap state is intended to correct only minor mismatches associated with mesh generation and is done even if the interaction is defined through user subroutine VUINTERACTION.

Configuration of contact surfaces after strain-free adjustments to resolve overclosure.

By default, adjustments to the initial penetration or gap state adjust initial nodal positions such that surfaces are “just touching” (with zero penetration/separation). Alternatively, you can specify a single non-negative value representing a uniform clearance or a nodal distribution to define a clearance per node (see Distribution Definition). If a distribution is defined and the clearance is omitted for a secondary node, the clearance value is interpolated from the values at the main nodes. The secondary node is ignored if clearance values are omitted from the distribution for the secondary node and all of the nodes of the nearest main face. The adjustments occur only for regions that satisfy the search zone tolerances, as discussed in Contact Initialization Search Criteria. Mesh distortion can occur if large strain-free adjustments occur to achieve the specified initial clearance distance. Figure 2 shows an example in which a uniform initial gap is desired between concentric surfaces.

Specifying a uniform initial gap between concentric circular surfaces.

The general contact algorithm in Abaqus/Explicit can optionally treat initial overclosures as interference fits. In this case the interference distance usually corresponds to the original mesh overclosure distance although exceptions are discussed in Specifying an Interference Distance. Subsequent resolution of this interference is effectively a form of loading within the first step and typically generates stresses and strains.

The general contact algorithm uses a shrink-fit method to gradually resolve the interference distance over the first step of the analysis as shown in Figure 3. Stresses and strains are generated as the interference is resolved. Large velocities can develop if the interference fit is resolved too rapidly. You should follow these general recommendations:

• Do not apply other loads while the interference fit is being resolved.
• Resolve the interference fit over a large enough number of increments such that the kinetic energy remains small.

Gradual resolution of contact interference fit.

By default, Abaqus/Explicit automatically adjusts positions of surfaces to resolve discrepancies between gap or overclosure of the original mesh geometry and the desired initial gap or penetration state among general contact interactions of the first step. These adjustments are made with strain-free initial displacements to secondary node positions. This automatic adjustment of nodal positions is intended to correct only minor mismatches associated with mesh generation.

Output variable STRAINFREE (see Abaqus/Explicit Output Variable Identifiers) can be used to review strain-free adjustments. This output variable contains nodal vectors representing initial strain-free adjustments. By default, STRAINFREE is written to the output database (.odb) file for the original field output frame at zero time if any strain-free adjustments are made by Abaqus/Explicit.

Discrepancies between gap or overclosure of the original mesh geometry and a desired initial clearance state specified can optionally be resolved with contact offsets instead of strain-free adjustments. This option is not applicable to contact initialization methods involving interference fit. Contact offsets effectively modify penetration calculations for individual secondary nodes, such that the contact force is computed as $f=k\left(d_{cur}-d_{offset}\right)$ ; where k is the penalty stiffness, $d_{offset}$ is the contact offset distance, and $d_{cur}$ is the current penetration distance. The contact offset distance remains constant for a given secondary node if you deactivate strain-free adjustments.

The use of contact offsets instead of strain-free adjustments (or repositioning parts in a preprocessor to avoid overclosures) can lead to confusion and nonphysical behavior for the following reasons:

• The contact force is proportional to the modified penetration $d_{\mathrm{mod}}=d_{cur}-d_{offset}$, and this modified penetration differs from the penetration inferred by visual inspection of the current configuration of the model.
• The use of offsets, especially large offsets, tends to degrade robustness of the contact algorithms. You can request initial overclosure and crossed surface diagnostics to diagnose some of these problems (see Contact Diagnostics in an Abaqus/Explicit Analysis).

Contact offsets are used by default to treat discrepancies between the original mesh geometry and the desired initial contact state in the following cases:

• Discrepancies remaining after strain-free adjustments, which may be due to conflicting adjustments from separate contact (such as when a secondary node that is part of a shell is pinched between two main facets), boundary conditions, tie constraints, coupling constraints, and rigid body constraints.
• Discrepancies associated with contact interactions newly included in general contact after the first step if no contact was defined in the previous step.
• Discrepancies associated with contact interactions for an Abaqus/Standard analysis imported into Abaqus/Explicit if the contact interaction is not defined with user subroutine VUINTERACTION.

In these cases, contact force is computed with $f=k\left(d_{cur}-d_{offset}\right)$, as discussed in Achieving a Desired Initial Zero or Positive Clearance State with Contact Offsets Instead of Strain-Free Adjustments, but the contact offset distance, $d_{offset}$, is reduced as surfaces separate. If $d_{cur}$ ever decreases below $d_{offset}$, $d_{offset}$ is reset to $d_{cur}$.

If the general contact domain is extended after the first step, Abaqus/Explicit takes no special actions, as part of initialization for the new step, to adjust nodal positions, store penetration offsets, or gradually resolve initial penetrations for the newly introduced interactions. Penalty contact forces are applied proportional to the current penetration, or the penetration may be ignored. Furthermore, contact initialization diagnostics are not available for these new interactions.

As an alternative to specifying search distances, you can specify a search node set containing the secondary nodes for which nondefault contact initialization instructions are meant to apply. However, the nondefault contact initialization instructions apply only for nodes of this set within a fixed distance from the main surface. The fixed distance is the maximum element size for solid elements or the thickness for structural elements (for example, shell elements) associated with the nodes.

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

In general, the concepts of specifying an initial clearance distance with the alternative interface are the same as with the preferred interface (see Specifying an Initial Clearance Distance); however, the syntax for specifying the initial clearance distance is different.

In general, the concepts of achieving an initial clearance distance with strain-free adjustments using the alternative interface are the same as with the preferred interface (see Achieving Desired Initial Clearance or Penetration State with Strain-Free Adjustments); however, the syntax for specifying this method is different.

In general, the concepts of achieving an initial clearance distance with offsets using the alternative interface are the same as with the preferred interface (see Achieving a Desired Initial Zero or Positive Clearance State with Contact Offsets Instead of Strain-Free Adjustments); however, the syntax for specifying this method is different.

In general, the concepts of modifying contact initialization search criteria using the alternative interface are the same as with the preferred interface (see Contact Initialization Search Criteria); however, the syntax for specifying this method is different.