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250 lines
18 KiB
Markdown
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# 36.3.10 CONTACT MODELING IF ASYMMETRIC-AXISYMMETRIC ELEMENTS ARE PRESENT
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Product: Abaqus/Standard
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# References
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• “Slide line contact elements,” Section 40.4.1
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• “Rigid surface contact elements,” Section 40.5.1
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• \*ASYMMETRIC-AXISYMMETRIC
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# Overview
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Modeling contact in asymmetric-axisymmetric problems:
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• requires the use of contact elements (ISL or IRS);
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• requires independent contact elements on each circumferential plane; and
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• can be done only on certain circumferential planes.
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# Modeling contact in asymmetric-axisymmetric problems
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CAXA or SAXA elements (see “Axisymmetric solid elements with nonlinear, asymmetric deformation,” Section 28.1.7, and “Axisymmetric shell elements with nonlinear, asymmetric deformation,” Section 29.6.10) are used to model problems where initially axisymmetric structures may undergo asymmetric deformations. These asymmetric deformations may include asymmetric contact conditions. The surface-based contact capability cannot be used to model such problems; contact elements (ISL or IRS) must be used.
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Independent sets of two-dimensional contact elements must be created for each circumferential plane in the CAXA or SAXA elements. You must specify the angle, , of the circumferential plane with which each set of contact elements is associated and the number of Fourier modes, n, used with the underlying CAXA or SAXA elements.
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Input File Usage: Use both of the following options:
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```txt
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*INTERFACE, ELSET=element_set_name
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*ASYMMETRIC-AXISYMMETRIC, MODE=n, ANGLE=θ
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```
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where the ELSET parameter refers to a set of ISL- or IRS-type contact elements.
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# Limitations on contact in asymmetric-axisymmetric problems
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If the circumferential planes in an asymmetric-axisymmetric problem rotate more than a few degrees, Abaqus/Standard can model contact conditions correctly only on the =0 and 180 circumferential planes. The asymmetric-axisymmetric elements have internal degrees of freedom for the rotation and out-ofplane motion of the circumferential planes, but these degrees of freedom are not accounted for in the
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contact elements. Ignoring these degrees of freedom means that Abaqus/Standard keeps the contact directions fixed in initial circumferential planes and the position of the nodes is projected back onto these initial planes for contact calculations. If the rotation and motion of the nodes from these initial planes are small, the errors caused by this approach are minimal. If they are large, the errors will become very large, making the results unrealistic.
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# 36.4 Defining general contact in Abaqus/Explicit
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• “Defining general contact interactions in Abaqus/Explicit,” Section 36.4.1
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• “Assigning surface properties for general contact in Abaqus/Explicit,” Section 36.4.2
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• “Assigning contact properties for general contact in Abaqus/Explicit,” Section 36.4.3
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• “Controlling initial contact status for general contact in Abaqus/Explicit,” Section 36.4.4
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• “Contact controls for general contact in Abaqus/Explicit,” Section 36.4.5
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<!-- source-page: 534 -->
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<!-- source-page: 535 -->
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# 36.4.1 DEFINING GENERAL CONTACT INTERACTIONS IN Abaqus/Explicit
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Products: Abaqus/Explicit Abaqus/CAE
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# References
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• “Contact interaction analysis: overview,” Section 36.1.1
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• \*CONTACT
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• \*CONTACT INCLUSIONS
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• \*CONTACT EXCLUSIONS
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• “Defining general contact,” Section 15.13.1 of the Abaqus/CAE User’s Guide, in the HTML version of this guide
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# Overview
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Abaqus/Explicit provides two algorithms for modeling contact and interaction problems: the general contact algorithm and the contact pair algorithm. See “Contact interaction analysis: overview,” Section 36.1.1, for a comparison of the two algorithms. This section describes how to include general contact in an Abaqus/Explicit analysis, how to specify the regions of the model that may be involved in general contact interactions, and how to obtain output from a general contact analysis.
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The general contact algorithm in Abaqus/Explicit:
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• is specified as part of the model or history definition of the model;
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• allows very simple definitions of contact with very few restrictions on the types of surfaces involved;
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• uses sophisticated tracking algorithms to ensure that proper contact conditions are enforced efficiently;
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• can be used simultaneously with the contact pair algorithm (i.e., some interactions can be modeled with the general contact algorithm, while others are modeled with the contact pair algorithm);
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• can be used only with three-dimensional surfaces;
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• can be used only in mechanical finite-sliding contact analyses; and
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• does not support kinematic constraint enforcement (contact constraints are enforced with the penalty method).
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# Defining a general contact interaction
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The definition of a general contact interaction consists of specifying:
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• the general contact algorithm and defining the contact domain (i.e., the surfaces that interact with one another), as described in this section;
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• the contact surface properties (“Assigning surface properties for general contact in Abaqus/Explicit,” Section 36.4.2);
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• the mechanical contact property models (“Assigning contact properties for general contact in Abaqus/Explicit,” Section 36.4.3);
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• the contact formulation (“Contact formulation for general contact in Abaqus/Explicit,” Section 38.2.1);
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• the initial clearance between contact surfaces (“Controlling initial contact status for general contact in Abaqus/Explicit,” Section 36.4.4); and
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• the algorithmic contact controls (“Contact controls for general contact in Abaqus/Explicit,” Section 36.4.5).
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# Surfaces used for general contact
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The general contact algorithm allows for very general characteristics in the surfaces that it uses, as discussed in “Contact interaction analysis: overview,” Section 36.1.1. For detailed information on defining surfaces in Abaqus/Explicit for use with the general contact algorithm, see “Element-based surface definition,” Section 2.3.2; “Node-based surface definition,” Section 2.3.3; “Analytical rigid surface definition,” Section 2.3.4; “Eulerian surface definition,” Section 2.3.5; and “Operating on surfaces,” Section 2.3.6. Two-dimensional surfaces cannot be used with the general contact algorithm.
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A convenient method of specifying the contact domain is using cropped surfaces. Such surfaces can be used to perform “contact in a box” by using a contact domain that is enclosed in a specified rectangular box in the original configuration. For more information, see “Operating on surfaces,” Section 2.3.6.
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In addition, Abaqus/Explicit automatically defines an all-inclusive surface that is convenient for prescribing the contact domain, as discussed later in this section. The all-inclusive automatically defined surface includes all element-based surface facets as well as all analytical rigid surfaces and surfaces on all Eulerian materials.
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The general contact algorithm generates contact forces to resist node-into-face, node-into-analytical rigid surface, and edge-into-edge contact penetrations. The primary mechanism for enforcing contact is node-to-face contact (the only mechanism used in the contact pair algorithm). If analytical rigid surfaces are present in the contact domain, the general contact algorithm also enforces node-to-analytical rigid surface contact.
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# Considerations for edge-to-edge contact
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The general contact algorithm also considers edge-to-edge contact, which is very effective in enforcing contact that cannot be detected as penetrations of nodes into faces. For example, contact between beam segments and shell perimeter edges (see Figure 36.4.1–1) usually is detected only as edge-to-edge contact. The terminology “contact edges” refers to feature edges of surface facets (on both shells and solids) as well as to segments representing beam and truss elements. The contact edges representing beam and truss elements have a circular cross-section, regardless of the actual cross-section of the beam or truss element. The radius of a contact edge representing a truss element is derived from the cross-sectional area specified on the truss section definition (it is equal to the radius of a solid circular section with an equivalent cross-sectional area). For beams with circular cross-sections, the radius of the contact edge is equivalent to the section radius. For beams with non-circular cross-sections, the radius of the contact edge is equal to the radius of a circumscribed circle around the section. If
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<!-- source-page: 537 -->
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<details>
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<summary>text_image</summary>
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Thin solid lines indicate geometric feature edges, which can optionally be included in the contact domain.
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Thick solid lines indicate shell perimeter edges and "contact edges" corresponding to beams.
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Beam
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Solid
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Shells
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Dashed lines indicate element boundaries for which edge-to-edge contact is not modeled.
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</details>
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Figure 36.4.1–1 General contact domain, including edge-to-edge contact.
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connected edges have different radii, a nodal radius is first computed as the minimum radius of the adjacent contact edges, and the radius of the edge cross-section is interpolated linearly over the length of the contact edge from the nodal values. Shell element edges reflect the shell thickness in the normal direction and do not extend past the perimeter (similar to shell nodes and facets). Some numerical rounding of features occurs for both node-to-facet and edge-to-edge contact.
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To model contact between edges that are not cylindrical in shape, surface elements can be attached to the edge nodes using surface-based tie constraints and node-to-face contact can be defined between the surface elements (see “Surface elements,” Section 32.7.1). This technique is useful for modeling geometric details important to the contact definition that are not modeled with the underlying element geometry. Surface elements can also be defined around shell elements in which Abaqus has reduced the contact thickness (i.e., if the thickness exceeds the surface facet edge lengths or diagonal lengths) so that the true surface thickness can be modeled. However, using surface elements with general contact requires a physically reasonable mass to be associated with the surface element nodes, and care must be taken not to alter the bulk mass properties when transferring mass to the surface elements from the underlying elements.
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By default, when a surface is used in a general contact interaction, all applicable facets, analytical rigid surfaces, nodes, perimeter edges, and beam and truss segments are included in the contact definition. You can control which feature edges are considered for edge-to-edge contact, as discussed in “Assigning surface properties for general contact in Abaqus/Explicit,” Section 36.4.2. Geometric feature edges and
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<!-- source-page: 538 -->
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perimeter edges do not have to be included explicitly in a surface definition (by using edge identifiers) for them to be considered for edge-to-edge contact.
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# Eulerian-Lagrangian contact
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The general contact algorithm also enforces contact between Eulerian materials and Lagrangian surfaces. This algorithm automatically compensates for mesh size discrepancies to prevent penetration of Eulerian material through the Lagrangian surface. The all-inclusive surface that is defined by Abaqus/Explicit can be used to enforce contact between all Eulerian materials and all Lagrangian bodies in a model; you can also specify individual Eulerian surfaces in the contact domain (see “Eulerian surface definition,” Section 2.3.5). Eulerian-Lagrangian contact is enforced only for Lagrangian surfaces defined on solid and shell elements. Other surface types, such as beam edges and analytical rigid surfaces, are ignored. Contact interactions between Eulerian materials and interactions due to Eulerian material self-contact are handled naturally by the Eulerian formulation; these interactions do not require a general contact definition. See “Interactions” in “Eulerian analysis,” Section 14.1.1, for more information.
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# Contact involving DEM or SPH particles
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The general contact algorithm enforces the following types of contact involving DEM or SPH particles:
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• contact between DEM or SPH particles and other Lagrangian surfaces; and
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• contact among DEM particles.
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See “Discrete element method,” Section 15.1.1, and “Smoothed particle hydrodynamics,” Section 15.2.1, for more information regarding contact involving DEM and SPH particles, respectively.
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# Including general contact in an analysis
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If a general contact definition does not appear in a step, any general contact definition active in the previous step will be propagated to the current step.
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For convenience, general contact can be defined as model data. A general contact definition specified as model data is considered to be defined in the initial step, or “Step 0,” of the analysis; it can be modified or removed in Step 1 or later steps.
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Input File Usage: Use the following option to indicate the beginning of a general contact definition:
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\*CONTACT
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This option can appear only once per step.
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Abaqus/CAE Usage: Interaction module: Create Interaction: General contact (Explicit)
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# Removing general contact definitions
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You can remove the previously specified general contact definition and specify a new one.
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Input File Usage: \*CONTACT, OP=NEW
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Abaqus/CAE Usage: Interaction module: interaction manager: select interaction, Deactivate
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<!-- source-page: 539 -->
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# Modifying general contact definitions
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Alternatively, you can make changes to an existing general contact definition. In this case the existing general contact definition remains active and any additional information specified is appended to the general contact definition.
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Contact state information (such as the proper contact normal orientation for double-sided surfaces) is transferred across step boundaries even if the contact domain is modified.
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Input File Usage: \*CONTACT, OP=MOD
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Abaqus/CAE Usage: Interaction module: interaction manager:
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select interaction, Edit
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# Example
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Each part of a general contact definition is considered independently when it is modified. For example, the following contact definition is specified in Step 1 (the individual options are discussed later in this section):
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```txt
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*CONTACT
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*CONTACT INCLUSIONS
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surf_1,
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*CONTACT EXCLUSIONS
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surf_a, surf_b
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```
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This contact definition is then modified in Step 2 with the following input:
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```c
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*CONTACT, OP=MOD
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*CONTACT INCLUSIONS
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surf_2, surf_3
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*CONTACT EXCLUSIONS
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surf_a, surf_c
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```
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An equivalent contact definition for Step 2 could be specified as follows:
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```c
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*CONTACT, OP=NEW
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*CONTACT INCLUSIONS
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surf_1,
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surf_2, surf_3
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*CONTACT EXCLUSIONS
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surf_a, surf_b
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surf_a, surf_c
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```
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# Defining the general contact domain
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You specify the regions of the model that can potentially come into contact with each other by defining general contact inclusions and exclusions. Only one contact inclusions definition and one contact exclusions definition are allowed per step.
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<!-- source-page: 540 -->
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All contact inclusions in an analysis are applied first, then all contact exclusions are applied, regardless of the order in which they are specified. The contact exclusions take precedence over the contact inclusions. The general contact algorithm will consider only those interactions specified by the contact inclusions definition and not specified by the contact exclusions definition.
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General contact interactions typically are defined by specifying self-contact for the default automatically generated surface provided by Abaqus/Explicit. All surfaces used in the general contact algorithm can span multiple unattached bodies, so self-contact in this algorithm is not limited to contact of a single body with itself. For example, self-contact of a surface that spans two bodies implies contact between the bodies as well as contact of each body with itself.
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# Specifying contact inclusions
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Define contact inclusions to specify the regions of the model that should be considered for contact purposes.
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# Specifying “automatic” contact for the entire model
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You can specify self-contact for a default unnamed, all-inclusive surface defined automatically by Abaqus/Explicit. This default surface contains, with the exceptions noted below, all exterior element faces, all analytical rigid surfaces and all edges based on beam and truss elements in the model, as well as the nodes attached to these faces and edges; in addition, feature edges are included according to the user-specified criteria (see “Assigning surface properties for general contact in Abaqus/Explicit,” Section 36.4.2). This is the simplest way to define the contact domain. With this approach contact is modeled for all node-to-facet, node-to-analytical rigid surface, and edge-to-edge interactions of the nodes, facets, analytical rigid surfaces, and contact edges of the default surface. This default surface does not include the following:
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• Nodes that cannot be part of an element-based surface; for example, nodes attached only to point masses or connectors.
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• Faces, edges, and nodes that belong only to cohesive elements. In fact, this default surface is generated as if cohesive elements were not present. See “Modeling with cohesive elements,” Section 32.5.3, for further discussion of contact modeling issues related to cohesive elements.
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Input File Usage: Use both of the following options to specify “automatic” contact for the entire model:
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\*CONTACT \*CONTACT INCLUSIONS, ALL EXTERIOR
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The \*CONTACT INCLUSIONS option should have no data lines when the ALL EXTERIOR parameter is used.
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Abaqus/CAE Usage: Interaction module: Create Interaction: General contact (Explicit): Included surface pairs: All\* with self
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