Contact Initialization for Contact Pairs in Abaqus/Standard

Initial overclosures in an Abaqus/Standard contact pair are treated as interference fits by default. Alternatively, initial overclosures can be treated as unintended and resolved by either adjusting the position of the surfaces automatically or, for small-sliding and tied contact pairs, introducing an offset distance term in the contact penetration calculations.

Options for modeling interference fits are discussed in Modeling Contact Interference Fits in Abaqus/Standard. You can also invoke an algorithm to adjust initial surface positions to close small gaps or to achieve a specified initial clearance.

Adjusting the position of surfaces in an Abaqus/Standard contact pair:

can be performed only at the start of a simulation;
causes Abaqus/Standard to move the nodes of the secondary surface so that they precisely contact the main surface (with some exceptions for surface-to-surface discretization and overlapping interaction definitions);
does not create any strain in the model;
can eliminate small gaps or penetrations caused by numerical roundoff when a graphical preprocessor such as Abaqus/CAE is used and, thus, prevent possible convergence problems;
is required when two surfaces are tied together for the duration of the analysis;
should not be used to correct gross errors in the mesh design;
cannot be used with symmetric main-secondary contact; and
will account for shell and membrane thicknesses and shell offsets (these factors are accounted for in the adjustment zone and in the adjustments) for contact formulations other than the default finite-sliding, node-to-surface contact formulation (see Contact Formulations in Abaqus/Standard).

This page discusses:

Treatment of Initial Overclosures and Unintended Initial Gaps
Adjusting Initial Surface Positions to Resolve Small Initial Gaps or Overclosures
Storing Penetration Offsets for Noncompliant Surfaces to Be Initially in Contact
Initial Clearance or Overclosure for Small-Sliding Contact
Alternative Methods for Specifying Precise Initial Clearances or Overclosures

Treatment of Initial Overclosures and Unintended Initial Gaps

By default, initial overclosures for Abaqus/Standard contact pairs are treated as interference fits (see Modeling Contact Interference Fits in Abaqus/Standard). Optionally, initial overclosures can be treated as initially in contact and just touching the main surface. Situations in which regions with small initial overclosures or gaps should be treated as initially in contact and just touching include:

Regions meant to fit together (be in conforming contact) in the original configuration.
Regions meant to be tied together for the duration of the analysis (see Defining Tied Contact in Abaqus/Standard).

Small penetrations or gaps for nodes to treat as initially in contact can be resolved by either adjusting the position of the surfaces automatically (see Adjusting Initial Surface Positions to Resolve Small Initial Gaps or Overclosures) or, for small-sliding and tied contact pairs, introducing an offset distance term in the contact penetration calculations (see Storing Penetration Offsets for Noncompliant Surfaces to Be Initially in Contact).

Adjusting Initial Surface Positions to Resolve Small Initial Gaps or Overclosures

You can control which secondary nodes can have their initial position adjusted, as necessary, such that the initial penetration becomes zero by specifying an "adjustment zone" distance or a node set label.

Using an “Adjustment Zone” to Control Which Secondary Nodes Have Initial Gaps and Overclosures Resolved

When you specify a, the depth of the “adjustment zone,” Abaqus/Standard forms an adjustment zone extending a distance a from the main surface in the gap direction and deeply into the penetration region. Abaqus/Standard measures the distance along the main surface normals that pass through the nodes of the secondary surface. Any nodes on the secondary surface that are within the “adjustment zone” in the initial geometry of the model are moved precisely onto the main surface. The motion of these secondary nodes does not create any strain in the model; it is treated as a change in the model definition. An example of adjusting the surfaces of a contact pair is shown in Figure 1 and Figure 2. If you specify a negative value for a, Abaqus/Standard issues an error message.

Input File Usage

Use one of the following options to specify the adjustment zone:

CONTACT PAIR, ADJUST=a
secondary_surface, main_surface
...

CONTACT PAIR, ADJUST=YES, POSITION TOLERANCE=a
secondary_surface, main_surface
...

Abaqus/CAE Usage

Interaction module: contact interaction editor: Specify tolerance for adjustment zone: a

Special Case of Adjusting Only Overclosed Secondary Nodes

When you specify the depth of the adjustment zone, Abaqus/Standard moves any secondary nodes penetrating the main surface in the initial configuration so that they just contact the main surface. Specifying a value of 0.0 for a causes Abaqus/Standard to adjust only those secondary nodes that are penetrating the main surface. Figure 3 shows the effect of specifying a=0.0 in the example shown in Figure 1. Specifying a=0.0 is not the same as omitting the adjustment zone parameter. If you omit the adjustment zone parameter, overclosures are treated as interference fits.

Using a Node Set Label When Adjusting Surfaces

You can specify a node set label instead of an adjustment zone depth when only a subset of the secondary nodes should be adjusted and specifying a may cause the inappropriate adjustment of other secondary nodes. Abaqus/Standard adjusts only those nodes on the secondary surface belonging to the node set. The node set can contain nodes that are not on the secondary surface at all: Abaqus/Standard will ignore them and adjust only the nodes in the node set that are part of the secondary surface.

Abaqus/Standard moves any secondary nodes in the specified node set regardless of how far they are from the main surface. The adjustments of the nodes from their initial configurations do not create strains in the elements forming the secondary surface. If Abaqus/Standard adjusts secondary nodes that are far from the main surface, the elements may become poorly shaped, which can cause convergence difficulties.

Input File Usage

CONTACT PAIR, ADJUST=node_set_label
secondary_surface, main_surface
...

Abaqus/CAE Usage

Interaction module: contact interaction editor: Adjust secondary nodes in set: node_set_label

Adjusting Overclosed Secondary Nodes Using a Node Set Label

Because Abaqus/Standard adjusts only the secondary nodes in the specified node set, any overclosed secondary nodes not in the specified node set remain overclosed at the start of the simulation. Using a node set label may, therefore, cause convergence problems if severely overclosed secondary nodes, which need to be adjusted, are not included in the node set. This behavior is different from that seen if a is specified, in which case Abaqus/Standard adjusts all of the overclosed nodes on the secondary surface.

Comments Unique to Surface-to-Surface Contact

The following points apply to the surface-to-surface discretization (see Contact Formulations in Abaqus/Standard for further discussion of surface-to-surface discretization):

Strain-free adjustments to secondary node positions may not result in exactly zero gap with respect to the main surface as measured at a secondary node. The adjustments are made to achieve zero gap between the surfaces in an average sense in a region near each secondary node within the adjustment zone.
Strain-free adjustments will occur for some secondary nodes outside the adjustment zone if a significant portion of a secondary face (or segment in two dimensions) to which it is attached is within the adjustment zone.

Adjustments for Overlapping Contact Pairs

Nodal adjustment definitions are processed sequentially at the start of an analysis. If different constraint or contact definitions involve the same nodes, some adjustments may cause lack of compliance for contact or constraint definitions that were previously processed. These conflicts can be avoided in some cases by changing the processing order of constraint and contact definitions: nodes in common between different contact or constraint definitions should be processed first as secondary nodes and later as main nodes.

Input File Usage

To change the processing order of constraint and contact definitions, change the order of the definitions in the input file. Constraint and contact definitions are processed in the order in which they appear.

Abaqus/CAE Usage

To change the processing order of constraint and contact definitions, change the names of the constraints and interactions in the model. Constraints and interactions are processed alphabetically according to their name.

Storing Penetration Offsets for Noncompliant Surfaces to Be Initially in Contact

In some cases adjustments to the initial positions of surfaces to achieve zero initial penetration cause element distortion issues. It is generally recommended that you resolve the clash in the initial mesh by repositioning parts or making other changes to the model. However, for small-sliding and tied contact pairs, another modeling technique is available. You can specify that Abaqus/Standard introduce offset distance terms in the contact penetration calculations such that the modified initial penetration is zero even though the configuration appears to have an initial penetration or gap. This contact offset persists throughout the simulation and can cause confusion with respect to interpreting configurations.

This method involves explicitly setting the "adjustment zone" parameter to indicate that no initial surface position adjustments are to be made and specifying a position tolerance, a. Abaqus/Standard creates a penetration offset for secondary nodes within a gap distance, a, or overclosed with respect to the main surface.

Input File Usage

CONTACT PAIR, ADJUST=NO, POSITION TOLERANCE=a
secondary_surface, main_surface
...

Abaqus/CAE Usage

Storing penetration offsets in the contact penetration calculations is not supported in Abaqus/CAE.

Initial Clearance or Overclosure for Small-Sliding Contact

You can define precise initial clearance or overclosure values and contact directions for the nodes on the secondary 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. For more information, see Additional Contact Initialization Options for Small-Sliding Contact in Abaqus/Standard.

You can define initial clearance or overclosure values only for small-sliding contact (Contact Formulations in Abaqus/Standard). For a technique that can be used to model clearances or overclosures between finite-sliding contact pairs, see Alternative Methods for Specifying Precise Initial Clearances or Overclosures below.

Alternative Methods for Specifying Precise Initial Clearances or Overclosures

Abaqus/Standard offers an alternative method of defining precise initial clearances or overclosures that is applicable to both small-sliding and finite-sliding contact pairs. In this method you specify an adjustment zone depth for the contact pair (as described above in Adjusting Initial Surface Positions to Resolve Small Initial Gaps or Overclosures) to move the surfaces forming the contact pair exactly into contact at the start of the analysis. Then, in the first step of the simulation you specify an allowable contact interference, $v$ , for the contact pair (see Modeling Contact Interference Fits in Abaqus/Standard). The contact interference definition must refer to an amplitude curve; the form of the amplitude curve depends on whether a clearance or an overclosure is being defined and is described below. The clearance or overclosure will be uniform across the surfaces.

Input File Usage

Use all of the following options:

CONTACT PAIR, ADJUST=a 
secondary_surface, main_surface
AMPLITUDE, NAME=amplitude_name
CONTACT INTERFERENCE, AMPLITUDE=amplitude_name
secondary_surface, main_surface,  $v$

Abaqus/CAE Usage

Interaction module: contact interaction editor: Specify tolerance for adjustment zone: a, Interference Fit: toggle on Uniform allowable interference, Amplitude: amplitude_name, Magnitude at start of step:  $v$

Specifying a Precise Clearance by Defining an Allowable Contact Interference

To specify a precise clearance by defining an allowable contact interference, the amplitude curve should have a constant magnitude for the duration of the step. A positive value should be given as the allowable interference, $v$ . When viewed in Abaqus/CAE, these surfaces will appear to penetrate each other when they are in contact. The surfaces start the simulation with coordinates that have them exactly touching, but the specified interference $v$ makes them behave as if they have a clearance between them.

Specifying a Precise Overclosure by Defining an Allowable Contact Interference

To specify a precise overclosure by defining an allowable contact interference, the amplitude curve should ramp from zero to unity over the duration of the step to allow Abaqus/Standard to resolve the overclosure gradually. A negative value should be given as the allowable interference, $v$ . When viewed in Abaqus/CAE, the surfaces start the simulation with coordinates that have them exactly touching, but the specified interference $v$ makes them behave as if they are overclosed. As Abaqus/Standard resolves the overclosure, these surfaces will appear to separate from each other. When the gap between the two surfaces is equal to a distance of $∣ v ∣$ , the surfaces will behave as if they are precisely in contact.