is meant to model situations, such as deployment maneuvers, where a
motor attached to the body loads the connection with an internal force or
moment history or a hydraulic system imposes a known motion;
can be used to fix available components of relative motion; and
consists of driving an available component of relative motion by a
prescribed displacement (rotation) or by a specified force (moment).
The prescribed relative motions and loads are in the local directions
associated with the available components of relative motion for the connector.
Prescribing displacements/rotations for available components of relative
motion that also include connector stop or connector lock behaviors may lead to
overconstraints.
Abaqus
will issue a warning message if an overconstraint occurs.
A common practice is to fix available components of motion. Such fixed
motion conditions can be used to customize connection types for specific
applications. As an example, the REVOLUTE connection type uses the local 1-direction as the shared
revolute axis and, hence, the available component of relative motion. If, for
convenience, a revolute connection about the local 3-direction were needed, you
could fix the relative rotations about the local 1- and 2-directions in a CARDAN connection type. In doing so, a connection type identical to
the REVOLUTE connection type would be created; however, the shared axis
would be the local 3-direction instead of the local 1-direction.
An example is provided later in this section in which the pin part of a
pin-in-slot connection is modeled with a CARDAN connection type with fixed rotations.
Input File Usage
Use the following
option in the model portion of the input file to fix available connector
components of relative motion:
You can specify a relative displacement, velocity, or acceleration between
two parts in the connector's local directions in a manner similar to defining a
boundary condition (see
Boundary Conditions).
You specify the connector element set name or connector element number; the
component number identifying the available component of relative motion being
actuated; and the value of the relative displacement, velocity, or
acceleration.
The penalty used for enforcing connector motion may lead to a noisy
solution, particularly in single precision for some models. Use of double
precision is, therefore, preferable in such situations. If performance is a
concern for double precision, you can run the constraint packaging and
constraint solver in double precision (see
Abaqus/Standard and Abaqus/Explicit Execution).
You cannot specify the motion of connectors in a subspace dynamic analysis.
You cannot prescribe relative motions and loads on a connector element with
the complex connection type SLIPRING in
Abaqus/Standard.
Input File Usage
Use the following
option in the history portion of the input file to specify a relative
displacement for a connector:
Figure 1
illustrates a pin-in-slot connection oriented at 45° from the global 1-axis
modeled with element type CONN3D2.
Figure 1. A pin-in-slot connection modeled with SLOT and CARDAN connection types.
The figure on the left is a schematic representation of the connection to be
modeled, while the figure on the right is the finite element mesh.
Displacements in the slot are allowed only along the line of the slot, and
connection type SLOT is appropriate for enforcing these kinematics. Assume the pin
and slot are constructed in such a way that the only rotation of the pin
relative to the slot is along the local 3-direction. This is a revolute
constraint; however, basic rotation connection type REVOLUTE uses the local 1-direction as the revolute axis. In this case
connection type CARDAN combined with a specified constraint can be used to define a
revolute-type connection with the appropriate revolute axis.
For illustrative purposes assume the connection is actuated by a rotational
velocity of
radians per second around the pin's axis. Using input parametrization for
convenience, the following lines are used:
You can specify concentrated loads applied to the available components of
relative motion in a manner similar to defining concentrated loads for other
elements in
Abaqus
(see
Concentrated Loads).
However, connector loads are always follower loads that rotate with the
rotation of the available components of relative motion as the connector
element moves. You specify the connector element set name or connector element
number, the component number identifying the available component of relative
motion being loaded, and the value of the actuation force or moment.
Input File Usage
Use the following
option in the history portion of the input file to specify a concentrated load
for a connector:
Load module: Create Load: Mechanical: Connector force or Connector moment
Example
Returning to the example in
Figure 1,
assume that the pin is pushed along the slot by a constant force of 1000.0
units (for example, through a hydraulic system). The following lines should be
added to the input file:
Connector Actuation in Linear Perturbation Procedures
Nonzero magnitude connector motions are allowed only in the eigenvalue
buckling, direct-solution steady-state dynamic, and linear static perturbation
procedures. Any nonzero magnitude specified during an eigenfrequency extraction
procedure is ignored, and the specified available component of relative motion
is held fixed. Connector motions cannot be used in any modal-based procedure.
In direct-solution steady-state dynamic analyses the real and imaginary
parts of any available connector component of relative motion are either
restrained or unrestrained simultaneously; it is physically impossible to have
one part restrained and the other part unrestrained.
Abaqus/Standard
will automatically restrain both the real and the imaginary parts of a
component of relative motion even when only one part is prescribed
specifically. The unspecified part will be assumed to have a perturbation
magnitude of zero.
A nonzero prescribed connector motion in an eigenvalue buckling step will
contribute to the incremental stress and, thus, will contribute to the
differential initial stress stiffness. When prescribing nonzero connector
motions, you must interpret the resulting eigenproblem carefully. See the
discussion for boundary conditions in
Eigenvalue Buckling Prediction
for more details.
