You can assign the name of a connector behavior to particular connector elements.

*CONNECTOR SECTION, ELSET=name, BEHAVIOR=behavior name
*CONNECTOR BEHAVIOR, NAME=behavior name

Connector behaviors allow for modeling of the following types of effects:

A conceptual model illustrating how connector behaviors interact with each other is shown in Figure 25.2.1–1.

Multiple definitions for the same behavior type are permitted. For example, if connector elasticity (or damping) is defined several times in an uncoupled fashion for the same available component of relative motion, in a coupled fashion, or in both fashions, the spring-like (or dashpot-like) responses are added together. Multiple definitions of friction, plasticity, and damage behaviors are permitted as long as the rules outlined in the corresponding behavior sections are followed. Multiple uncoupled stop and lock definitions for the same component are permitted, but only one will be enforced at a time.

Tabular data are often used to define connector behaviors, such as nonlinear elasticity, isotropic hardening, etc. As shown in Figure 25.2.1–2, the data points make up a nonlinear curve in the constitutive space. The curve is flat outside of the range defined by the two endpoints and will cause zero stiffness in the connector elements.

ABAQUS/Explicit regularizes the data into tables that are defined in terms of even intervals of the independent variables. In some cases where the dependent quantities are defined at uneven intervals of the independent variables and the range of the independent variable is large compared to the smallest interval, ABAQUS/Explicit may fail to obtain an accurate regularization of your data in a reasonable number of intervals. In this case the program stops after all data are processed and issues an error message that you must redefine the behavior data. See “Material data definition,” Section 16.1.2, for a more detailed discussion of data regularization.

In many cases connector behavior is specified in an uncoupled manner in individual available components of relative motion. Coupled behavior can be defined for all or some of the available components of relative motion in a connector.

For coupled plasticity, damage, and, in certain situations, friction behavior, additional functions describing the nature of the coupling effects must be defined (see “Connector functions for coupled behavior,” Section 25.2.4). These functions do not define a behavior by themselves but are used as tools for building a desired behavior. For example, these functions may be used to define:

*CONNECTOR BEHAVIOR OPTION, COMPONENT=n

*CONNECTOR BEHAVIOR OPTION

In all nonlinear uncoupled connector kinetic behaviors the independent variable is the connector available component in the direction for which the response is defined. When modeling the following connector behaviors, the properties can also depend on relative positions or constitutive displacements/rotations in several component directions:

*CONNECTOR BEHAVIOR OPTION, 
INDEPENDENT COMPONENTS=POSITION (default)

*CONNECTOR BEHAVIOR OPTION, 
INDEPENDENT COMPONENTS=CONSTITUTIVE MOTION

In many connector behavior definitions, material-like behavior has a reference position where the force or moment is zero, which is different from the initial position. This is the case, for example, in a spring that has nonzero force or moment in the initial configuration. In these situations the most convenient way to define the connector behavior is relative to the nominal or reference geometry where the forces or moments vanish.

You can define the translational or angular positions at which constitutive forces and moments are zero by specifying up to six reference values (one per component of relative motion): three lengths and three angles (in degrees). The reference lengths and angles affect only spring-like elastic connector behavior and, if the friction-generating contact force (moment) is a function of the relative displacement (rotation), connector friction behavior. By default, the reference lengths and angles are the length and angle values determined from the initial geometry. See “Connection-type library,” Section 25.1.5, for the meaning of the reference lengths and angles for each connection type.

*CONNECTOR CONSTITUTIVE REFERENCE
length 1, length 2, length 3, angle 1, angle 2, angle 3

Defining precompressed or preextended linear elastic behavior

In many cases connectors are precompressed or preextended when installed in assemblies. In such cases the connector force is nonzero in the initial configuration. While nonlinear elasticity could be used to define nonzero force in the initial configuration, it is often more convenient to specify a (linear) spring stiffness plus a reference length or angle at which the force or moment is zero. For example, linear uncoupled elastic behavior defined with the connection type AXIAL would have force given by the equation

where . l is the current length of the AXIAL connection, and is the user-defined constitutive reference length. The connector constitutive displacement quantities, , are defined for different connection types as described in “Connection-type library,” Section 25.1.5.

*CONNECTOR BEHAVIOR, NAME=sbehavior
...
*CONNECTOR CONSTITUTIVE REFERENCE
 , , , 22.5

In ABAQUS/Explicit kinematic constraints, stops, locks, and actuated motion in connector elements are treated with implicit time integration. By default, connector constitutive behavior (for example, elasticity, damping, and friction) is also integrated implicitly. The advantage of implicit time integration is that elements with these behaviors do not affect the stability or time incrementation of the analysis in any way.

In certain problems, such as single precision analyses with “soft” springs together with “stiff” springs (i.e., several orders of magnitude difference in spring stiffness) or soft springs together with kinematic constraints, implicit time integration of the constitutive response may lead to poor solution resolution. In this case you can specify a more traditional explicit time integration for the constitutive response. This explicit time integration may lead to a small improvement in computational performance. However, explicit integration of relatively stiff springs will reduce the global time increment size, since such connector elements are included in the stable time increment size calculation.

*CONNECTOR BEHAVIOR, INTEGRATION=IMPLICIT

*CONNECTOR BEHAVIOR, INTEGRATION=EXPLICIT

In linear perturbation procedures (see “General and linear perturbation procedures,” Section 6.1.2) the connector element kinematics are linearized about the base state. Hence, linearized versions of kinematic constraints are applied, and the connector behavior is linearized about the state at the end of the previous general analysis step.

Connector element behaviors allow for proper modeling of most physical connection behaviors within a single connector element. However, in rare circumstances more complex connection behaviors may require multiple connector elements to be used in parallel or in series. You place connector elements in parallel by defining two or more connector elements between the same nodes. You place connectors in series by specifying additional nodes (most often in the same location as the nodes of interest) and then stringing connector elements between these nodes.

For example, assume that you would like to define a connector stop that exhibits elastic-plastic behavior upon contact. Since this is not permitted within the context of one connector behavior definition, you can circumvent the limitation by using two connector elements in series. This concept is illustrated in Figure 25.2.1–4.

Connectors in parallel can be used as well to model complex kinetic behavior. For example, assume that you need to define an elastic-viscous connector with spring-like and dashpot-like behaviors in parallel (for example, the strut in an automotive suspension). Assume that damage can occur only in the dashpot once it is stretched/compressed beyond specified limits. Since this is not permitted within the context of one connector behavior definition, you can circumvent the limitation by using two connector elements in parallel. This concept is illustrated in Figure 25.2.1–5.