About general contact in Abaqus Explicit PDF

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12/10/2019

About general contact in Abaqus/Explicit

About general contact in Abaqus/Explicit Abaqus/Explicit provides two algorithms for modeling contact and interaction problems: the general contact algorithm and the contact pair algorithm. See About contact interactions 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. The general contact algorithm in Abaqus/Explicit: is specified as part of the model or history definition of the model; allows very simple definitions of contact with very few restrictions on the types of surfaces involved; uses sophisticated tracking algorithms to ensure that proper contact conditions are enforced efficiently; 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);

Related Topics About contact interactions In Other Guides *CONTACT *CONTACT INCLUSIONS *CONTACT EXCLUSIONS Defining general contact

can be used only with three-dimensional surfaces; can be used only in mechanical finite-sliding contact analyses; and does not support kinematic constraint enforcement (contact constraints are enforced with the penalty method). The following topics are discussed: Defining a general contact interaction Surfaces used for general contact Including general contact in an analysis Defining the general contact domain Output

Products:

Abaqus/Explicit Abaqus/CAE

Defining a general contact interaction The definition of a general contact interaction consists of specifying: the general contact algorithm and defining the contact domain (i.e., the surfaces that interact with one another), as described in this section; the contact surface properties (Assigning surface properties for general contact in Abaqus/Explicit); the mechanical contact property models (Assigning contact properties for general contact in Abaqus/Explicit); the contact formulation (Contact formulation for general contact in Abaqus/Explicit); the initial clearance between contact surfaces (Controlling initial contact status for general contact in Abaqus/Explicit); and the algorithmic contact controls (Contact controls for general contact in Abaqus/Explicit).

Surfaces used for general contact The general contact algorithm allows for very general characteristics in the surfaces that it uses, as discussed in About contact interactions. For detailed information on defining surfaces in Abaqus/Explicit for use with the general contact algorithm, see Element-based surface definition, Node-based surface definition, Analytical rigid surface definition, Eulerian surface definition, and Operating on surfaces. Two-dimensional surfaces cannot be used with the general contact algorithm. 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. 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. 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.

Considerations for edge-to-edge contact 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 1) usually is detected only as edge-to-edge contact.

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Figure 1. General contact domain, including 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 crosssection, 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 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. 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). 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. 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. Geometric feature edges and 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.

Eulerian-Lagrangian contact 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). 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 for more information.

Contact involving DEM or SPH particles The general contact algorithm enforces the following types of contact involving DEM or SPH particles: contact between DEM or SPH particles and other Lagrangian surfaces; and contact among DEM particles. See Discrete element method and Smoothed particle hydrodynamics for more information regarding contact involving DEM and SPH particles, respectively.

Including general contact in an analysis 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. 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. Input File Usage:

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About general contact in Abaqus/Explicit Use the following option to indicate the beginning of a general contact definition: *CONTACT This option can appear only once per step.

Abaqus/CAE Usage: Interaction module: Create Interaction: General contact (Explicit)

Removing general contact definitions You can remove the previously specified general contact definition and specify a new one. Input File Usage: *CONTACT, OP=NEW Abaqus/CAE Usage: Interaction module: interaction manager: select interaction, Deactivate

Modifying general contact definitions 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. 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. Input File Usage: *CONTACT, OP=MOD Abaqus/CAE Usage: Interaction module: interaction manager: select interaction, Edit Example 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): *CONTACT *CONTACT INCLUSIONS surf_1, *CONTACT EXCLUSIONS surf_a, surf_b This contact definition is then modified in Step 2 with the following input: *CONTACT, OP=MOD *CONTACT INCLUSIONS surf_2, surf_3 *CONTACT EXCLUSIONS surf_a, surf_c An equivalent contact definition for Step 2 could be specified as follows: *CONTACT, OP=NEW *CONTACT INCLUSIONS surf_1, surf_2, surf_3 *CONTACT EXCLUSIONS surf_a, surf_b surf_a, surf_c

Defining the general contact domain 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. 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. 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.

Specifying contact inclusions Define contact inclusions to specify the regions of the model that should be considered for contact purposes. Specifying “automatic” contact for the entire model 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,

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feature edges are included according to the user-specified criteria (see Assigning surface properties for general contact in Abaqus/Explicit). This is the simplest way to define the contact domain. With this approach contact is modeled for all nodeto-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: Nodes that cannot be part of an element-based surface; for example, nodes attached only to point masses or connectors. 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 for further discussion of contact modeling issues related to cohesive elements. Input File Usage: Use both of the following options to specify “automatic” contact for the entire model: *CONTACT *CONTACT INCLUSIONS, ALL EXTERIOR The *CONTACT INCLUSIONS option should have no data lines when the ALL EXTERIOR parameter is used. Abaqus/CAE Usage: Interaction module: Create Interaction: General contact (Explicit): Included surface pairs: All* with self Specifying individual contact interactions Alternatively, you can define the general contact domain directly by specifying the individual contact surface pairings. Selfcontact will be modeled only if the two surfaces specified in a pair overlap (or are identical) and will be modeled only in the overlapping region. Multiple surface pairings can be included in the contact domain. At least one surface in each pair must be either an element-based surface or an analytical rigid surface. Input File Usage: Use both of the following options to specify individual contact interactions: *CONTACT *CONTACT INCLUSIONS surface_1, surface_2 At least one data line must be specified when the ALL EXTERIOR parameter is omitted. Either or both of the data line entries can be left blank, but each data line must contain at least a comma; an error message will be issued for empty data lines. If the first surface name is omitted, the default unnamed, all-inclusive, automatically generated surface is assumed. If the second surface name is omitted or is the same as the first surface name, contact between the first surface and itself is assumed. Leaving both data line entries blank is equivalent to using the ALL EXTERIOR parameter. Abaqus/CAE Usage: Interaction module: Create Interaction: General contact (Explicit): Included surface pairs: Selected surface pairs: Edit, select the surfaces in the columns on the left, and click the arrows in the middle to transfer them to the list of included pairs Examples The following input specifies that contact should be enforced between the default all-inclusive, automatically generated surface and surface_2, including self-contact in any overlap regions: *CONTACT *CONTACT INCLUSIONS , surface_2 Either of the following methods can be used to define self-contact for surface_1: *CONTACT *CONTACT INCLUSIONS surface_1, or *CONTACT *CONTACT INCLUSIONS surface_1, surface_1 The following input can be used to introduce a node-based surface containing point masses to the contact domain as well as specify self-contact for the default all-inclusive, automatically generated surface: *CONTACT *CONTACT INCLUSIONS , , node_based_surf

Specifying contact exclusions You can refine the contact domain definition by specifying the regions of the model to exclude from contact. The primary motivation for specifying contact exclusions is to avoid physically unreasonable contact interactions. For example, a finite element model may contain multiple forming tools, but not all of the tools participate in the forming process simultaneously; you can specify contact exclusions to prevent certain tools from participating in the contact model in certain steps.

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You do not need to be concerned with specifying contact exclusions for parts of the model that are not likely to interact, since these exclusions typically will have minimal effect on computational performance. Contact will be ignored for all the surface pairings specified, even if these interactions are specified directly or indirectly in the contact inclusions definition. Multiple surface pairings can be excluded from the contact domain. At least one surface in each pair must be either an element-based surface or an analytical rigid surface. Keep in mind that surfaces can be defined to span multiple unattached bodies, so self-contact exclusions are not limited to exclusions of single-body contact. You cannot exclude only one side of shell-like surfaces. If a side label (SPOS or SNEG) is used in defining an element-based shell-like surface and that surface is excluded from contact, Abaqus/Explicit will exclude all faces associated with these elements. Input File Usage: Use both of the following options to specify contact exclusions: *CONTACT *CONTACT EXCLUSIONS surface_1, surface_2 Either or both of the data line entries can be left blank. If the first surface name is omitted, the default unnamed, all-inclusive, automatically generated surface is assumed. If the second surface name is omitted or is the same as the first surface name, contact between the first surface and itself is excluded from the contact domain. Abaqus/CAE Usage: Interaction module: Create Interaction: General contact (Explicit): Excluded surface pairs: Edit, select the surfaces in the columns on the left, and click the arrows in the middle to transfer them to the list of excluded pairs Automatically generated contact exclusions Abaqus/Explicit automatically generates contact exclusions for general contact in some situations. Contact exclusions are generated automatically for interactions that are defined with the contact pair algorithm or surface-based tie constraints to avoid redundant (and possibly inconsistent) enforcement of these interaction constraints. For example, if a contact pair is defined for surface_1 and surface_2 and “automatic” general contact is defined for the entire model, Abaqus/Explicit would generate a contact exclusion for general contact between surface_1 and surface_2, so that interactions between these surfaces would be modeled only with the contact pair algorithm. These automatically generated contact exclusions are in effect only during the steps in which the contact pair algorithm or surface-based tie constraint interactions are active. Abaqus/Explicit automatically generates contact exclusions for self-contact of each rigid body in the model, because it is not possible for a rigid body to contact itself. When you specify pure master-slave contact surface weighting for a particular general contact surface pair, contact exclusions are generated automatically for the master-slave orientation opposite to that specified (see Contact formulation for general contact in Abaqus/Explicit for more information on this type of contact ...