Additional Contact Initialization Options for Small-Sliding Contact in Abaqus/Standard

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.

The initial clearance or overclosure value calculated at every secondary node (based on the coordinates of the secondary node and the main surface) is overwritten by the value that you specify. This procedure is performed internally, and it does not affect the coordinates of the secondary nodes. If you define a clearance, Abaqus/Standard will treat the two surfaces as not being in contact, regardless of their nodal coordinates. If you define an overclosure, Abaqus/Standard will treat the two surfaces as an interference fit and attempt to resolve the overclosure in the first increment. If the defined overclosure is large, you may need to specify an allowable interference that is ramped off over several increments. See Modeling Contact Interference Fits in Abaqus/Standard for further discussion of interference fits.

You can define initial clearance or overclosure values only for small-sliding contact (Contact Formulations in Abaqus/Standard) whether contact is modeled with contact pairs or with general contact as a nondefault option. 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.

You can specify a uniform clearance or overclosure for a contact pair by identifying the main and secondary surfaces of the contact pair and the desired initial clearance, $h_0$ (positive for a clearance; negative for an overclosure). No other data are needed.

For general contact, you can specify a named clearance to associate with contact initialization for portions of the general contact domain where small sliding is active.

Alternatively, you can specify spatially varying clearances or overclosures by providing a table of data specifying the clearance at a single node or a set of nodes belonging to the secondary surface. Any secondary surface node that is not identified will use the clearance that Abaqus/Standard calculates from the initial geometry of the contacting surfaces. For contact pairs, identify the main and secondary surfaces along with clearance data.

For general contact, you can create a named clearance to associate with contact initialization for portions of the general contact domain where small sliding is active. General contact assigns implicit main-secondary roles by default. Therefore, it is important to check that the node set or the node labels actually belong to the surface that acts as the secondary while assigning the contact initialization to a pair of surfaces. If the implicit secondary-main roles are opposite of what you expect from the clearance specification, you may want to specify the correct secondary-main roles (Main and Secondary Surface Roles of a Contact Formulation).

This modeling approach provides a simple way to approximate effects of threads without directly including threads in the mesh geometry. The meshed parts typically have cylindrical surfaces at the interface with this approach, such that default contact normal directions are approximately radial. This capability adjusts contact normal directions to be normal to faces of reference threads. The thread face normal directions have large components in the radial and axial directions and (for three-dimensional models only) a small component in the circumferential direction due to the spiral nature of the threads. Either the bolt or bolt hole can act as the secondary surface.

The capability to adjust contact normal directions based on reference thread geometry is available only for small-sliding contact formulations, so it will not provide accurate results after relative twisting motion between a bolt and hole. For simulations involving relative twisting motion, you can consider the following alternative modeling approach:

1. Create nominal meshes without threads for the bolts and parts with bolt holes.
2. Create surface element meshes to capture the bolt thread geometry and bolt hole thread geometry.
3. Specify surface-based tie constraints to constrain each bolt thread surface to a bolt and each hole thread surface to a hole.
4. Specify finite-sliding contact (with penalty enforcement of contact constraints) between the respective thread surfaces.

Uniform Association with Top or Bottom Thread Face

By default with this capability based on reference thread geometry, all secondary nodes are assumed to consistently correspond to the "top" or "bottom" thread face, such that adjusted contact normal directions for all secondary nodes have the same axial component and the same (small) circumferential component. In this case, the overall contact interface provides only "one-way" resistance to relative axial motion between the bolt and bolt hole.

You specify the thread geometry parameters along with two points "a" and "b", as shown in Figure 1, defining the axis of the bolt/bolt hole. The contact interface supports tension in a bolt if point "a" is near the bolt tip and point "b" is near the bolt head (as in Figure 1), and it supports compression in the bolt if points "a" and "b" have the opposite orientation. If a negative half-thread angle is specified, the opposite convention for supporting tension or compression occurs.

For general contact, instead of specifying the secondary and main, the clearance definition is identified by a name label to be associated with a contact initialization definition as in earlier cases. Because node sets and node labels are not specified explicitly in this case, implicit main-secondary role assignments internally identify the list of secondary nodes to apply the clearances.

Reference thread geometry.