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

In Abaqus/Standard you can define precise initial clearance or overclosure values and contact directions for contact initialization in small-sliding contact for both contact pairs and general contact.

This page discusses:

See Also
About Contact Pairs in Abaqus/Standard
About General Contact in Abaqus/Standard

Products Abaqus/Standard

Defining Precise Initial Clearance or Overclosure Values

You can define precise initial clearance or overclosure distances and contact directions when they would not be computed accurately enough from the surface geometry. 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. 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.

The initial clearance or overclosure value calculated for a potential contact constraint based on surface geometry is overwritten by the value that you specify. This procedure is performed internally, and it does not affect nodal positions. If you define positive clearance, Abaqus/Standard treats the two surfaces as not being in contact, regardless of their nodal coordinates. If you define an overclosure, Abaqus/Standard treats the two surfaces as an interference fit and attempts to resolve the overclosure in the first increment. If the defined overclosure is large, you might 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.

Specifying a Uniform Clearance or Overclosure for the Surfaces

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.

Input File Usage

Use the following option to specify a uniform clearance or overclosure for a contact pair:

CLEARANCE, SECONDARY=surface_name, MAIN=surface_name, VALUE=
            
              h
              0

Use the following options to specify a uniform clearance or overclosure for general contact:

CLEARANCE, NAME=clrName, VALUE=
            
              h
              0
            
          
CONTACT INITIALIZATION DATA, INITIAL CLEARANCE=clrName

Abaqus/CAE Usage

Interaction module: contact interaction editor: Clearance: Initial clearance: Uniform value across secondary surface: 
            
              h
              0

Specifying initial clearance or overclosure for small-sliding contact in general contact is not supported in Abaqus/CAE.

Specifying Spatially Varying Clearances or Overclosures for the Surfaces

Alternatively, you can specify spatially varying clearances or overclosures by providing a table of data specifying nodal clearance values. You can specify these clearances for nodes of either surface of a small-sliding contact interaction within general contact; for small-sliding contact pairs, you must specify these clearances on nodes belonging to the secondary surface. General contact assigns implicit main-secondary roles by default. If you specify clearance values on nodes that act as main nodes, Abaqus/Standard interpolates the clearance used for an individual contact constraint from the clearance values of the main nodes participating in the contact constraint. If user-specified clearances do not apply to a particular contact constraint, Abaqus/Standard calculates the initial clearance from the initial geometry of the contacting surfaces.

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. For contact pairs, identify the main and secondary surfaces along with clearance data.

Input File Usage

Use the following option to specify spatially varying clearances or overclosures for contact pairs:

CLEARANCE, SECONDARY=surface_name, MAIN=surface_name, TABULAR
node number or node set label, clearance value

Repeat the data line as often as necessary.

Use the following options to specify spatially varying clearances or overclosures for general contact:

CLEARANCE, NAME=clrName, TABULAR
node number or node set label, clearance value

Repeat the data line as often as necessary.

CONTACT INITIALIZATION DATA, INITIAL CLEARANCE=clrName, NAME=C1

The following options assign the surface surf_1 a secondary role assuming the nodes with specified clearances belong to it before the surface pair surf_1 and surf_2 are assigned the named clearance during contact initialization.

CONTACT
CONTACT FORMULATION, TYPE=MAIN SECONDARY ROLES
surf_1, surf_2, SECONDARY
CONTACT INITIALIZATION ASSIGNMENT
surf_1, surf_2, C1

Abaqus/CAE Usage

You cannot specify initial clearance or overclosure values using a table of data in Abaqus/CAE.

Reading Spatially Varying Clearances or Overclosures from an External File

Abaqus/Standard can read the spatially varying clearances or overclosures for a contact pair or general contact from an external file.

Input File Usage

Use the following option to specify spatially varying clearances or overclosures from an external file for contact pairs:

CLEARANCE, SECONDARY=surface_name, MAIN=surface_name, TABULAR, INPUT=file_name

Use the following options to specify spatially varying clearances or overclosures from an external file for general contact:

CLEARANCE, NAME=clrName, TABULAR, INPUT=file_name
CONTACT INITIALIZATION DATA, INITIAL CLEARANCE=clrName, NAME=C1

Abaqus/CAE Usage

You cannot specify initial clearance or overclosure values using an external input file in Abaqus/CAE.

Specifying the Surface Normal for the Contact Calculations

Normally Abaqus/Standard calculates the surface normal used for the contact calculations from the geometry of the discretized surfaces, using the algorithms described in Contact Formulations in Abaqus/Standard. When specifying spatially varying clearances or overclosures, you can redefine the contact direction that Abaqus/Standard uses with each secondary node by specifying the components of this vector. The vector must be defined in the global Cartesian coordinate system, and it should define the main surface's desired outward normal direction.

For general contact, instead of specifying the secondary and main roles, the clearance definition is identified by a name which is then associated with a contact initialization definition.

Input File Usage

Use the following option to specify the surface normal for the contact calculations for contact pairs:

CLEARANCE, SECONDARY=surface_name, MAIN=surface_name, TABULAR
node number or node set label, clearance value, first normal component, second normal component,
third normal component

Repeat the data line as often as necessary.

Use the following options to specify the surface normal for the contact calculations for general contact:

CLEARANCE, NAME=clrName, TABULAR, INPUT=file_name
CONTACT INITIALIZATION DATA, INITIAL CLEARANCE=clrName, NAME=C1

Abaqus/CAE Usage

You cannot redefine contact directions in Abaqus/CAE, except for threaded bolt connections (see Generating Contact Normal Directions Based on a Reference Thread Geometry below).

Generating Contact Normal Directions Based on a Reference Thread Geometry

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 the 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 roles, the clearance definition is identified by a name label to be associated with a contact initialization definition (as in earlier cases). Nodes specified for the applicable region of the reference thread geometry can correspond to nodes of either surface of small-sliding contact interactions.

Figure 1. Reference thread geometry.

Input File Usage

Use the following option to specify a uniform association with the top or bottom thread face for contact pairs:

CLEARANCE, SECONDARY=surface_name, MAIN=surface_name, TABULAR, BOLT, 
NORMAL ADJUSTMENT=UNIFORM AXIAL COMPONENT
half-thread angle, pitch, major bolt diameter, mean bolt diameter
node number or node set label, clearance value, coordinates of points a and b on the axis of the bolt/bolt hole

Repeat the second data line as often as necessary.

Use the following options to specify a uniform association with the top or bottom thread face for general contact:

CLEARANCE, NAME=clrName, TABULAR, BOLT, NORMAL ADJUSTMENT=UNIFORM AXIAL COMPONENT
half-thread angle, pitch, major bolt diameter, mean bolt diameter
node number or node set label, clearance value, coordinates of points a and b on the axis of the bolt/bolt hole

Repeat the second data line as often as necessary.

CONTACT INITIALIZATION DATA, INITIAL CLEARANCE=clrName, NAME=C1

Abaqus/CAE Usage

Interaction module: contact interaction editor: Clearance: Initial clearance: Computed for single-threaded bolt or Specify for single-threaded bolt: clearance value,
Secondary node set for clearance: Edit Region: select region,
Bolt direction vector: Edit: select axis,
Half-thread angle: half-thread angle, Pitch: pitch,
Bolt diameter: Major: major bolt diameter or Mean: mean bolt diameter
Normal adjustment: Uniform

Specifying initial clearance and/or contact directions for contact initialization for general contact is not supported in Abaqus/CAE.

Location-Dependent Association with Top or Bottom Thread Face

Optionally, each secondary node can be associated with the top or bottom thread face based on its initial position with respect to the reference thread geometry, which will influence the sign of the axial and circumferential components of the adjusted contact normal direction for each secondary node. Having some secondary nodes associated with the top thread face and others associated with the bottom thread face restricts motion of the bolt with respect to the hole in both axial directions and supports tension or compression in the bolt. It is unlikely that all secondary nodes will be associated with the same (top or bottom) thread face with this non-default option. For example, consider that a purely circumferential mesh line would be associated with the top thread face over a 180° arc and the bottom thread face over the remaining 180° arc.

With this location-dependent association with top and bottom thread faces, contour plots of contact stress on bolt/hole surfaces often show stripes at intervals corresponding to the threads in the reference thread geometry.

Input File Usage

Use the following option to specify a location-dependent association with the top or bottom thread face for contact pairs:

CLEARANCE, BOLT, NORMAL ADJUSTMENT=LOCATION DEPENDENT

Use the following options to specify a location-dependent association with the top or bottom thread face for general contact:

CLEARANCE, NAME=clrName, BOLT, NORMAL ADJUSTMENT=LOCATION DEPENDENT
CONTACT INITIALIZATION DATA, INITIAL CLEARANCE=clrName, NAME=C1

Abaqus/CAE Usage

Interaction module: contact interaction editor: Clearance: Initial clearance: Computed for single-threaded bolt or Specify for single-threaded bolt: clearance value, Normal adjustment: Location dependent

Specifying initial clearance and/or contact directions for contact initialization for general contact is not supported in Abaqus/CAE.

Right-Handed Versus Left-Handed Reference Thread Geometry

The reference thread geometry corresponds to right-handed threads by default; however, you can specify left-handed reference thread geometry optionally. Left-handed threads have the opposite effect on the circumferential component of the contact normal direction as right-handed threads.

Figure 2 shows that nodal contact force results on a bolt surface are not sensitive to whether thread geometry information is specified for nodes on one surface or the other for a small-sliding general contact interaction. In the case shown on the left, thread geometry information associated with left-handed threads is specified on the bolt (main) surface; in the case shown on the right, thread geometry information associated with left-handed threads is specified on the bolt-hole (secondary) surface. Abaqus/Standard internally assigns the bolt surface to act as the main surface for both of these simulations. For small-sliding contact pairs, you must specify thread geometry information on the secondary surface. General contact automatically assigns main and secondary roles; in this example, the bolt acts as the main surface.

Figure 2. Cut-away views of contact force vectors.

Input File Usage

Use the following option to specify that the reference threads are right-handed for contact pairs:

CLEARANCE, BOLT, HANDEDNESS=RIGHT (default)

Use the following option to specify that the reference threads are left-handed for contact pairs:

CLEARANCE, BOLT, HANDEDNESS=LEFT

Use the following options to specify that the reference threads are right-handed for general contact:

CLEARANCE, NAME=clrName, BOLT, HANDEDNESS=RIGHT (default)
CONTACT INITIALIZATION DATA, INITIAL CLEARANCE=clrName, NAME=C1

Use the following options to specify that the reference threads are left-handed for general contact:

CLEARANCE, NAME=clrName, BOLT, HANDEDNESS=LEFT
CONTACT INITIALIZATION DATA, INITIAL CLEARANCE=clrName, NAME=C1

Abaqus/CAE Usage

Interaction module: contact interaction editor: Clearance: Initial clearance: Computed for single-threaded bolt or Specify for single-threaded bolt: clearance value, Handedness: Right or Left

Specifying initial clearance and/or contact directions for contact initialization for general contact is not supported in Abaqus/CAE.

Visualizing the Precise Initial Clearances or Overclosures

Abaqus/Standard does not adjust the coordinates of the secondary surface when precise initial clearances or overclosures are specified. Therefore, the specified clearances or overclosures cannot be seen in the model in Abaqus/CAE. Thus, depending on the initial geometry of the surfaces and the magnitude of the clearances or overclosures, the surfaces may appear open or closed in Abaqus/CAE when they are actually just in contact. However, the actual clearance can be displayed in Abaqus/CAE by plotting a contour plot of the variable COPEN.