Contact Initialization for General Contact in Abaqus/Standard

As discussed above in Default Contact Initialization Method, initial gaps and large initial overclosures between surfaces are not adjusted by the default contact initialization methods. You can optionally specify nondefault search distances both above and below the surfaces in an interaction; secondary surfaces that lie within these search distances are repositioned directly onto their associated main surface using strain-free nodal adjustments. Abaqus/Standard takes shell thickness into account when calculating these search distances.

For new activations at the step level, strain-free nodal adjustments are not allowed and nodes on secondary surfaces are not repositioned. Instead, initial overclosures might have to be reconciled by precise positioning before activation while initial gaps might be resolved by establishing contact on activation with boundary motions to avoid rigid body modes. The subsequent discussion involving strain-free nodal adjustments applies only to the initial configuration.

Specifying a search distance above a surface is used to close small initial gaps between surfaces. Specifying a search distance below a surface is used to increase the default overclosure tolerance that Abaqus/Standard uses when performing strain-free adjustments; if you specify a search distance smaller than the default overclosure tolerance, Abaqus/Standard uses the default tolerance instead. As with the default initialization behavior, contact exclusions are created for initial overclosures that are larger than the specified search zone.

Increasing the extent of the search zones for strain-free adjustments can potentially increase the computational cost of an analysis. It is not generally recommended that you specify a large search zone since this may cause mesh distortion when nodes are repositioned over large distances.

CONTACT INITIALIZATION DATA, SEARCH ABOVE=a, SEARCH BELOW=b

By default, the strain-free adjustments discussed above adjust initial nodal positions such that surfaces are “just-touching” (with zero penetration/separation). Alternatively, Abaqus/Standard can make the adjustments to achieve an initial clearance distance that you specify. The adjustments occur only for regions that satisfy the search zone tolerances, as discussed above. Mesh distortion can occur if large strain-free adjustments are required to achieve the specified initial clearance distance. Because nodal adjustments are not allowed for new activations in a step, this specification is ignored at the step level.

CONTACT INITIALIZATION DATA, INITIAL CLEARANCE=h

You can specify an initial clearance to apply to general contact interactions in portions of the model where the small-sliding tracking approach is active or to interactions in heat transfer and coupled electrical-thermal procedures that have no mechanical degrees of freedom. Abaqus/Standard stores the difference between the specified clearance and the actual clearance (with or without nodal adjustments) as offsets. In addition, for small sliding you can also specify applicable contact directions during initialization.

CONTACT INITIALIZATION DATA, ADJUST=NO, SEARCH ABOVE=a, SEARCH BELOW=b

CONTACT INITIALIZATION DATA, INITIAL CLEARANCE=clearanceName, ADJUST=NO
*CLEARANCE, NAME=clearanceName, TABULAR

Optionally, the general contact algorithm in Abaqus/Standard can treat overclosures as interference fits. The general contact algorithm uses a shrink-fit method to gradually resolve the interference distance over the step of the analysis (if multiple load increments are used for the step) as shown in Figure 2, such that the fraction of the interference resolved up to and including a particular increment approximately corresponds to the fraction of the step completed. Stresses and strains are generated as the interference is resolved. Gradually resolving interference over several increments improves robustness (compared to always resolving the full interference in the first increment, which is the default for contact pairs) for cases in which a nonlinear response occurs for “interference-fit loading.” It is generally recommended that you do not apply other loads while the interference fit is being resolved.

Figure 2. Gradual resolution of contact interference fit.

Because contact conditions are enforced in an average sense in a region around each constraint location for the surface-to-surface contact formulation used by general contact in Abaqus/Standard, penetrations or gaps may be observed at secondary nodes when surface-to-surface constraints are in a zero-penetration state.

CONTACT INITIALIZATION DATA, INTERFERENCE FIT

CONTACT INITIALIZATION DATA, INTERFERENCE FIT, 
SEARCH BELOW=b

Specifying the Interference Distance

By default, the interference distance is implied by the initial overclosure of the mesh in the initial configuration or when surfaces are newly activated in a step; alternatively, you can specify the interference distance in the initial configuration. In this case Abaqus/Standard first makes strain-free adjustments of nodal positions such that the initial overclosure in the adjusted configuration corresponds to the specified interference distance and then invokes the shrink fit method discussed above, as depicted in Figure 3. Mesh distortion can occur if large strain-free adjustments are required to achieve the specified interference distance. Because strain-free nodal adjustments are not allowed for new contact activations at the step level, the interference distance specification is ignored; instead, the physical overclosure is resolved.

Figure 3. Treatment of a specified interference distance that differs from the interference implied by the original mesh.

The search region for the strain-free adjustments and subsequent shrink fit resolution is at least as large as the search region for the case discussed previously in which the interference distance is not specified. The search region will include overclosures at least as large as the specified interference fit and openings at least as large as the optionally specified search distance above a surface.

Input File Usage

CONTACT INITIALIZATION DATA, INTERFERENCE FIT=h, SEARCH ABOVE=a, SEARCH BELOW=b

Cases in Which Interference Fit Resolution with Contact Pairs Is Preferred

Large interferences may be difficult to resolve with the surface-to-surface formulation. Using this formulation, overclosures tend to be resolved along the secondary facet normal directions; using the node-to-surface formulation, which is available only with the contact pair algorithm, overclosures tend to be resolved along the main surface normal directions. Figure 4 illustrates a case where differing normal directions lead to undesirable tangential motion during an interference fit. In some cases it may be preferable to resolve large initial overclosures with node-to-surface discretization using the contact pair algorithm (see Modeling Contact Interference Fits in Abaqus/Standard).

Figure 4. Comparison of contact formulations in an example with a large interference fit.

The surfaces in a contact initialization assignment can be either single- or double-sided. Single-sided surfaces must have consistent surface normal orientations for adjacent faces. Strain-free adjustments will not move surface nodes past the reference surface of the opposing surface if the assignment of a contact initialization method is made with double-sided surfaces.

Using single-sided surfaces in the assignment of a contact initialization method for shells or membranes provides enhanced control over contact initialization for cases in which shell or membrane reference surfaces are initially crossed or are initially on the wrong side of each other. Figure 5 shows examples of adjustments for nearby segments of shell surfaces. For the case shown on the left it is assumed that single-sided surfaces with normal directions pointing away from each other are used in the assignment of the contact initialization method. In this case nodes are moved across the opposing reference surface during the strain-free adjustments.

For the case shown on the right in Figure 5 it is assumed that single-sided surfaces with normal directions pointing toward each other are used in the assignment of the contact initialization method. In this case an initial gap is observed between the single-sided surfaces (which is also the case if double-sided surfaces are used in the contact initialization assignment). No strain-free adjustments will be made by default for openings such as this; however, if a nondefault contact initialization method is specified with an initial opening search tolerance set to a value exceeding the initial separation distance, strain-free adjustments will close the gap as shown in the figure (without moving nodes past the opposing reference surface).

The following contact initialization assignments are specified below as model or step data in a general contact analysis with the corresponding contact initialization data specified once at the model level:

• a global assignment of shrink_fit to the entire general contact domain;

• a local assignment of shrink_fit_local to contact between surfaces surface_A and surface_B—the search zone is specified explicitly to increase the default overclosure tolerance;

• a local assignment of the default Abaqus contact initialization method to contact between surface_C and surface_D; and

• a local assignment of sfa_pickside to contact between double-sided surfaces surface_1 and surface_2 by specifying one side of each surface, surface_1_TOP and surface_2_BOTTOM, in the data lines (see bottom left of Figure 5).

  Figure 5. Strain-free adjustments during contact initialization for single-sided shell surfaces.
  
  

CONTACT INITIALIZATION DATA, NAME=shrink_fit, INTERFERENCE FIT
CONTACT INITIALIZATION DATA, NAME=shrink_fit_local,
   INTERFERENCE FIT, SEARCH BELOW = 15.0
CONTACT INITIALIZATION DATA, NAME=sfa_pickside, 
   SEARCH BELOW = 10.0
…
CONTACT
CONTACT INCLUSIONS, ALL EXTERIOR
CONTACT INITIALIZATION ASSIGNMENT
 , , shrink_fit 
surface_A, surface_B, shrink_fit_local
surface_C, surface_D, 
surface_1_TOP, surface_2_BOTTOM, sfa_pickside