By default, the general contact algorithm uses the finite-sliding,
surface-to-surface contact formulation, which is discussed in
Contact Formulations in Abaqus/Standard.
In addition, it is supplemented by the edge-to-surface, edge-to-edge and
vertex-to-surface formulations, which are also based on the finite-sliding
tracking approach. Optionally, you can specify the small-sliding tracking
approach for portions of the general contact domain and, by extension, the
entire general contact domain.
Main and Secondary Surface Roles of a Contact Formulation
The surface-to-surface contact formulation used by general contact generates individual contact
constraints using a main-secondary approach, as discussed in Contact Formulations in Abaqus/Standard. Abaqus/Standard assigns default pure main-secondary surface roles of a contact formulation for contact
involving disconnected bodies within the general contact domain. Bodies consisting of
connected beam and truss elements are considered disconnected bodies even though these
bodies may share nodes with other faceted bodies. Internal surfaces are generated
automatically using the naming convention
General_Contact_Faces_k,
where k corresponds to an
automatically assigned component number. By default, the lower-numbered component surfaces
act as main surfaces to the higher-numbered component surfaces. An exception is when
component surfaces consisting of beam and truss elements interact with faceted component
surfaces in the edge-to-surface contact formulation. A component surface consisting of beam
and truss elements acts as the main surface in the edge-to-surface formulation if half of
the average element radius is larger than the average smallest facet length of the faceted
component surface.
Self-contact within a body is treated with balanced main-secondary contact by default, with
each surface node acting as a main node in some constraints and as a secondary node in other
constraints.
For example, if the general contact domain spans three disconnected bodies,
the following three internal “component-surfaces” for general contact are
created automatically:
General_Contact_Faces_1
General_Contact_Faces_2
General_Contact_Faces_3
By default, the first surface listed acts as a main surface to the other two, and
General_Contact_Faces_2 acts as a main surface to
General_Contact_Faces_3. If any of these surfaces contain
beam or truss elements interacting with other faceted surfaces in the edge-to-surface
contact formulation, the decision to use these as the main surfaces will depend on the
average element radius and the average smallest facet length of the faceted surfaces. By
default, self-contact within each of these three surfaces is modeled with balanced
main-secondary contact.
All XFEM-based crack surfaces in the general contact domain are
assigned to a separate component and assigned the highest component number. Therefore, crack
surfaces act as secondary surfaces s to other components by default. Contact between
portions of crack surfaces are handled with balanced main-secondary contact since they all
belong to a single component.
Specifying Nondefault Main-Secondary Roles
You can override the default main-secondary roles by specifying pure main-secondary roles or by
specifying that balanced main-secondary contact should be used. The default main-secondary
treatment works well in most cases. Keep the following points in mind when modifying the
main-secondary assignments, in addition to other factors discussed in this section:
Do not use the internally generated component surfaces when assigning alternative
main-secondary roles (instead, use surface names that you define). Named
XFEM-based crack surfaces are not supported for
specifying main-secondary roles.
The main-secondary role assignments are part of the model definition and cannot be modified
from step to step.
The guidelines for assigning pure main-secondary roles for contact pairs discussed in Defining Contact between Two Separate Surfaces are also applicable
for situations in which you reassign pure main-secondary roles for general contact.
The limitations of balanced (symmetric) main-secondary contact pairs discussed in Using Symmetric Main-Secondary Contact Pairs to Improve Contact Modeling are also applicable
for situations in which you reassign balanced main-secondary contact for general
contact. Balanced main-secondary contact can result in reduced robustness due to the
increased number of constraints and the possibility of overconstraints.
Input File Usage
Use the following option to indicate that the first surface should be considered the secondary
surface:
If the first surface name is omitted, a default surface that
encompasses the entire general contact domain is assumed. The second surface
name must be specified.
Use the following option to specify that contact with balanced main and secondary surface
roles should be used between two surfaces:
If the first surface name is omitted, a default surface that
encompasses the entire general contact domain is assumed. If the second surface
name is omitted, contact between the first surface and itself is
assumed.
Automatically Generated Contact Exclusions
Abaqus/Standard automatically generates contact exclusions for the main-secondary roles opposite to
specified pure main-secondary roles; therefore, self-contact is excluded for any regions
of the two surfaces that overlap. For example, specifying that the general contact
interaction between surf_A and
surf_B should use pure main-secondary contact with
surf_A considered to be the secondary surface would result
in exclusions being generated internally for main faces of
surf_A contacting secondary faces of
surf_B; self-contact would be excluded for the region of
overlap between surf_A and surf_B.
An error message is issued if the second surface name is omitted or is the same as the
first surface name since this input would result in the exclusion of self-contact for
the surface.
Specifying Small Sliding within General Contact
You can specify the small-sliding tracking approach for interactions involving portions of the
entire general contact domain and, by extension, the entire contact domain. The
small-sliding approach avoids repeated contact tracking and the need to re-establish the
nodes involved in the constraint connectivity, which makes it more efficient than the
default finite-sliding approach for general contact. However, you should be aware of the
approximations involved in small sliding to decide whether they are appropriate (The Small-Sliding Tracking Approach). The
small-sliding approach involves only the surface-to-surface contact over opposing surfaces;
edges and vertices are deactivated. The surface pairs you specify to utilize small sliding
are excluded automatically for the default finite-sliding approach. Subsequently, none of
the types of contact based on finite-sliding assumptions (such as surface-to-surface contact
or edge and vertex contact) are active over the pairings with the small-sliding
specification.
You should not use the internally generated component surfaces when
assigning the small-sliding approach; instead, you should define the surface
names. Named XFEM-based crack surfaces are not
allowed for specifying the small-sliding approach.
Input File Usage
Use the following
option to specify the surface pairings to consider for the small-sliding
approach:
Use the following option to specify the small-sliding
approach everywhere (first data line) except contact involving
surf_1 with surf_2 (where
the default finite-sliding approach is active):
CONTACT FORMULATION, TYPE=SLIDING FORMULATION
, , SMALL SLIDINGsurf_1, surf_2,
A blank entry in the first and second data entries of the
data lines refers to the default surface that encompasses the entire general
contact domain.
Smoothness of Contact Force Redistribution upon Sliding
You can control the smoothness of nodal contact force redistribution upon sliding. The default
setting, which is generally appropriate, results in the smoothness of the nodal force
redistribution being of the same order as the elements underlying the secondary surface;
that is, linear redistribution smoothness for linear elements, and quadratic redistribution
smoothness for second-order elements. Quadratic redistribution smoothness usually tends to
improve convergence behavior and improve resolution of contact stresses within regions of
rapidly varying contact stresses. However, quadratic redistribution smoothness tends to
increase the number of nodes involved in each constraint, which can increase the
computational cost of the equation solver. Linear redistribution smoothness tends to provide
better resolution of contact stresses near edges of active contact regions and, therefore,
occasionally results in better convergence behavior.
Input File Usage
Use the following option to indicate that the smoothness of the contact force redistribution
upon sliding should be of the same order as the elements underlying the secondary
surface:
CONTACT FORMULATION, TYPE=SLIDING TRANSITIONsurf_1, surf_2, ELEMENT ORDER SMOOTHING
If the first surface name is omitted, a default surface that
encompasses the entire general contact domain is assumed. If the second surface
name is omitted, contact between the first surface and itself is
assumed.
Use the following option to indicate linear smoothness of
the contact force redistribution upon sliding:
CONTACT FORMULATION, TYPE=SLIDING TRANSITIONsurf_1, surf_2, LINEAR SMOOTHING
If the first surface name is omitted, a default surface that
encompasses the entire general contact domain is assumed. If the second surface
name is omitted, contact between the first surface and itself is
assumed.
Use the following option to indicate quadratic smoothness
of the contact force redistribution upon sliding:
If the first surface name is omitted, a default surface that
encompasses the entire general contact domain is assumed. If the second surface
name is omitted, contact between the first surface and itself is
assumed.
Additional Global Numerical Controls for General Contact
Some additional numerical contact controls can be modified globally from
step-to-step for general contact; you cannot specify contact controls for
individual surface pairings within the general contact domain. You can apply
contact stabilization to address rigid body modes that occur prior to the
establishment of contact in the model, and you can adjust the tolerances used
by
Abaqus/Standard
to determine contact penetrations and separations; both techniques are
discussed in
Adjusting Contact Controls in Abaqus/Standard.