Kinematic vs. distributing coupling 1

[PamCrash]

Hi

I've read the manual many times, but it's still not 100% clear to me what the difference is between *kinematic coupling and *distibuting coupling. Does anybody can give a good explanation. Thanks in advance.

Pam

 

[acafilip]

This is from My Abaqus (

http://www.abaqus.com)

pagego and search using "kinematic coupling")
Both types of coupling constraint have the common purpose of coupling the motion of a collection of nodes on a surface (the coupling nodes) to the motion of a reference node. Some differences between the two methods are outlined below.

Nature of the constraint enforcement

Kinematic coupling is enforced in a strict master-slave approach. Degrees of freedom (DOFs) at the coupling nodes are eliminated, and the coupling nodes will be constrained to move with the rigid body motion of the reference node.

Distributing coupling is enforced in an average sense. Degrees of freedom at the coupling nodes are not eliminated. Rather, the constraint is enforced by distributing loads such that:

the resultants of the forces at the coupling nodes are equivalent to the forces and moments at the reference node, and
force and moment equilibrium of the distributed loads about the reference point is maintained.
Usage notes

A kinematic coupling constraint does not allow relative motion among the constrained DOFs. It does allow relative motion among the unconstrained DOFs.

A distributing coupling allows relative motion among the constrained and unconstrained DOFs. The relative motion of the coupling nodes will be such that the equilibrium condition on the distributed loads is maintained.

As an example, consider the cantilever beam shown in Figure 1. It is meshed with second order brick elements and is built-in at the right end. A coupling constraint is defined at the free end. Degrees of freedom 1 through 6 of the end surface nodes are included in the constraint. At the reference node, a displacement is applied in the vertical (2) direction, while all other displacement and rotation components are held to zero.


Figure 1: Cantilever beam, undeformed Figure 2: Cantilever beam, deformed

The model is analyzed using the kinematic and distributing constraint methods. Viewed globally, the deformed shape of the beam for both constraint types is similar, and is shown in Figure 2.


Figure 3: Axial displacement, distributing coupling Figure 4: Axial displacement, kinematic coupling

A closer inspection of the displacements at the coupled end of the beam reveals the difference between the results of the two constraint methods. Figures 3 and 4 show contours of the axial (3-direction) displacement at the free end of the beam for the distributing and kinematic methods, respectively. In both plots, the legend scale has been adjusted to make the displacement gradient more visible. The distributing coupling allows the nodes at the end of the beam to experience relative deformation. The kinematic coupling, because it constrains the motion of the coupling nodes to the rigid body motion of the reference node, does not. The displacement in the axial (as well as lateral) direction is identically zero because of the boundary condition on the reference node.

Continuing with the above example, consider the case when only degrees of freedom 1-3 are coupled; the contours of axial displacement are shown in Figures 5 and 6 for the distributing and kinematic methods, respectively.


Figure 5: Distributing coupling, DOFs 1-3 coupled Figure 6: Kinematic coupling, DOFs 1-3 coupled

Figure 5 shows that with the distributing coupling, the end of the beam is free to rotate. Figure 6 shows that with the kinematic coupling, rotation of the beam end is constrained. The reason is that rotational degrees of freedom at the reference node of a distributing coupling are only active when at least one slave rotational DOF is coupled. In contrast, all degrees of freedom are active at the reference node of a kinematic coupling constraint, independent of the slave DOFs participating in the constraint. Proper constraints must be placed on the unconstrained DOFs of the reference node to avoid numerical singularities.

A kinematic coupling constraint is advantageous when a particular kinematic mode in a structure must be suppressed. An application is the simulation of pure bending in thin-walled pipes, in which the cross section must ovalize but remain plane. The details of this example are outlined in ABAQUS Answer 1139.

A distributing coupling allows more control of the distribution of load from the reference node to the coupling nodes. In addition to a uniform distribution, distributions can be made to decrease linearly, quadratically, or cubically with distance from the reference node. No control is provided with the kinematic method.

The distributing coupling must always constrain all available translational degrees of freedom at the coupling nodes. This is not necessary with the kinematic coupling constraint.

Once any combination of degrees of freedom at the coupling nodes is specified in a kinematic coupling constraint, none of the remaining degrees of freedom are available for further constraint. This is not the case for the distributing coupling. Additional details are provided in ABAQUS Answer 1125.

For either type of constraint, concentrated loads or displacements can be applied at the reference node.

A large number of coupling nodes in a distributing coupling definition can cause excessive memory usage and long run times; this is a result of the large wavefront that is produced when forming the constraint.

Defining constraints in ABAQUS/CAE

Coupling constraints are defined in the Interaction module of ABAQUS/CAE. First define the surface to be coupled. Then, if the reference node is not part of the existing geometry, a separate reference node must be created (Tools ® Reference Point). Then select:

Constraint ® Create... ® Coupling ® select the reference node ® select the coupling surface

The Edit Constraint dialog box will appear, from which the constraint type and coupled degrees of freedom can be selected.