can be defined in any connector with available components of relative
motion by specifying the loading and unloading behavior;
can be specified for each available component of relative motion
independently;
can define separate response in the tensile and compressive
directions;
can exhibit nonlinear elastic behavior, damaged elastic behavior, or
elastic-plastic type behavior with permanent deformation upon complete
unloading;
can have an unloading response specified; and
can be specified as dependent on constitutive motions in several local
directions.
The local directions for each connection type (as described in Connection Types) determine the directions in which the forces and moments act and in
which the displacements and rotations are measured.
Specifying Uniaxial Behavior for an Available Component of Relative Motion
Uniaxial behavior can be specified for an available component of relative
motion by defining the loading and unloading response for that component. For
each component, separate loading/unloading response data can be defined for the
response in the tensile and compressive directions. The loading and unloading
response can be classified according to three available behavior types:
nonlinear elastic behavior;
damaged elastic behavior; and
elastic-plastic type behavior with permanent deformation.
To define the loading response, you specify forces or moments as nonlinear
functions of the components of relative motion. These functions can also depend
on temperature, field variables, and constitutive displacements/rotations in
the other component directions. See
Input Syntax Rules
for further information about defining data as functions of temperature and
field variables.
The unloading response can be defined in the following ways:
You can specify several unloading curves that express the forces or
moments as nonlinear functions of the components of relative motion;
Abaqus
interpolates these curves to create an unloading curve that passes through the
point of unloading in an analysis.
You can specify an energy dissipation factor (and a permanent
deformation factor for models with permanent deformation), from which
Abaqus
calculates an exponential/quadratic unloading function.
You can specify the forces or moments as nonlinear functions of the
components of relative motion, as well as a transition slope; the connector
unloads along the specified transition slope until it intersects the specified
unloading function, at which point it unloads according to the function. (This
unloading definition is referred to as combined unloading.)
You can specify the forces or moments as nonlinear functions of the
components of relative motion;
Abaqus
shifts the specified unloading function along the strain axis so that it passes
through the point of unloading in an analysis.
The behavior type that is specified for the loading response dictates the
type of unloading you can define, as summarized in
Table 1.
The different behavior types, as well as the associated loading and unloading
curves, are discussed in more detail in the sections that follow.
Table 1. Available unloading definitions for the uniaxial behavior types.
Material behavior type
Unloading definition
Interpolated
Quadratic
Exponential
Combined
Shifted
Rate-dependent elastic
Damaged elastic
Permanent deformation
Defining the Deformation Direction
The loading/unloading data can be defined separately for tension and
compression by specifying the deformation direction. If the deformation
direction is defined (tension or compression), the tabular values defining
tensile or compressive behavior should be specified with positive values of
forces/moments and displacements/rotations in the specified component of
relative motion and the loading data must start at the origin. If the behavior
is not defined in a loading direction, the force response will be zero in that
direction (the connector has no resistance in that direction).
If the deformation direction is not defined, the data apply to both tension
and compression. However, the behavior is then considered to be nonlinear
elastic and no damage or permanent deformation can be specified. The response
data will be considered to be symmetric about the origin if either tensile or
compressive data are omitted.
Behavior That Depends on Relative Positions or Motions in Multiple Component Directions
By default, the loading and unloading functions depend only on the
displacement or rotation in the direction of the component of relative motion
specified for the connector uniaxial behavior definition (see
Connector Behavior
for details). However, it is also possible to define loading and unloading
functions that depend on the constitutive displacements and rotations in
multiple component directions.
When the loading response is rate independent, the unloading response is
also rate independent and occurs along the same user-specified loading curve as
illustrated in
Figure 1.
An unloading curve does not need to be specified.
Defining Rate-Dependent Behavior
The rate-dependent models require the specification of force-displacement
curves at different rates of deformation to describe both loading and unloading
behavior. If unloading behavior is not specified, the unloading occurs along
the loading curve with the smallest rate of deformation. As the rate of
deformation changes, the response is obtained by interpolation of the specified
loading/unloading data. Unphysical jumps in the forces due to sudden changes in
the rate of deformation are prevented using a technique based on viscoplastic
regularization. This technique also helps model relaxation effects in a very
simplistic manner, with the relaxation time given as
where ,
,
and
are material parameters and
is the stretch.
is a linear viscosity parameter that controls the relaxation time when
.
Small values of this parameter should be used.
is a nonlinear viscosity parameter that controls the relaxation time at higher
values of .
The smaller this value, the shorter the relaxation time.
controls the sensitivity of the relaxation speed to the stretch in the
component of relative motion. Suggested values of these parameters are
,
,
and .
Figure 2
illustrates the loading/unloading behavior as the connector is loaded at a rate
and then unloaded at a rate .
Figure 3
shows the loading/unloading response of a connector element for two different
relaxation times
and
with .
The larger the relaxation time, the longer it takes to achieve the specified
loading/unloading response for the applied deformation rate.
Defining Models with Damage
The damage models dissipate energy upon unloading, and there is no permanent
deformation upon complete unloading. The unloading behavior controls the amount
of energy dissipated by damage mechanisms and can be specified in one of the
following ways:
an analytical unloading curve (exponential/quadratic);
an unloading curve interpolated from multiple user-specified unloading
curves; or
unloading along a transition unloading curve (constant slope specified
by user) to the user-specified unloading curve (combined unloading).
You can specify the onset of damage by defining the displacement below which
unloading occurs along the loading curve.
Specifying Exponential/Quadratic Unloading
The damage model in
Figure 4
is based on an analytical unloading curve that is derived from an energy
dissipation factor,
(fraction of energy that is dissipated at any displacement level). As the
connector is loaded, the force follows the path given by the loading curve. If
the connector is unloaded (for example, at point B), the
force follows the unloading curve .
Reloading after unloading follows the unloading curve
until the loading is such that the displacement becomes greater than
,
after which the loading path follows the loading curve. The arrows shown in
Figure 4
illustrate the loading/unloading paths of this model.
The unloading response follows the loading curve when the calculated
unloading curve lies above the loading curve to prevent energy generation and
follows a zero force response when the unloading curve yields a negative
response. In such cases the dissipated energy will be less than the value
specified by the energy dissipation factor.
Specifying Interpolated Curve Unloading
The damage model in
Figure 5
illustrates an interpolated unloading response based on multiple unloading
curves that intersect the primary loading curve at increasing values of
forces/displacements. You can specify as many unloading curves as are necessary
to define the unloading response. Each unloading curve always starts at point
O, the point of zero force and zero displacements, since
the damage models do not allow any permanent deformation. The unloading curves
are stored in normalized form so that they intersect the loading curve at a
unit force for a unit displacement, and the interpolation occurs between these
normalized curves. If unloading occurs from a maximum displacement for which an
unloading curve is not specified, the unloading is interpolated from
neighboring unloading curves. As the connector is loaded, the force follows the
path given by the loading curve. If the connector is unloaded (for example, at
point B), the force follows the unloading curve
.
Reloading after unloading follows the unloading path
until the loading is such that the displacement becomes greater than
,
after which the loading path follows the loading curve.
If the loading curve depends on the constitutive displacements/rotations in
several component directions, the unloading curves also depend on the same
component directions. The unloading curves also have the same temperature and
field variable dependencies as the loading curve.
Specifying Combined Unloading
As illustrated in
Figure 6,
you can specify an unloading curve
in addition to the loading curve
as well as a constant transition slope that connects the loading curve to the
unloading curve. As the connector is loaded, the force follows the path given
by the loading curve. If the connector is unloaded (for example, at point
B), the force follows the unloading curve
.
The path
is defined by the constant transition slope, and
lies on the specified unloading curve. Reloading after unloading follows the
unloading path
until the loading is such that the displacement becomes greater than
,
after which the loading path follows the loading curve.
If the loading curve depends on the constitutive displacements/rotations in
several component directions, the unloading curve also depends on the same
component directions. The unloading curve also has the same temperature and
field variable dependencies as the loading curve.
Defining Models with Permanent Deformation
These models dissipate energy upon unloading and exhibit permanent
deformation upon complete unloading. The unloading behavior controls the amount
of energy dissipated as well as the amount of permanent deformation. The
unloading behavior can be specified in one of the following ways:
an analytical unloading curve (exponential/quadratic);
an unloading curve interpolated from multiple user-specified unloading
curves; or
an unloading curve obtained by shifting the user-specified unloading
curve to the point of unloading.
By default, the onset of yield will be obtained as soon as the slope of the
loading curve decreases by 10% from the maximum slope recorded up to that point
while traversing along the loading curve. To override the default method of
determining the onset of yield, you can specify either a value for the decrease
in slope of the loading curve other than the default value of 10% (slope drop =
0.1) or by defining the displacement below which unloading occurs along the
loading curve. If a slope drop is specified, the onset of yield will be
obtained as soon as the slope of the loading curve decreases by the specified
factor from the maximum slope recorded up to that point.
Specifying Exponential/Quadratic Unloading
The model in
Figure 7
illustrates an analytical unloading curve that is derived based on an energy
dissipation factor,
(fraction of energy that is dissipated at any displacement level) and a
permanent deformation factor, .
As the connector is loaded, the force follows the path given by the loading
curve. If the connector is unloaded (for example, at point
B), the force follows the unloading curve
.
The point D corresponds to the permanent deformation,
.
Reloading after unloading follows the unloading curve
until the loading is such that the displacement becomes greater than
,
after which the loading path follows the loading curve. The arrows shown in
Figure 7
illustrate the loading/unloading paths of this model.
The unloading response follows the loading curve when the calculated
unloading curve lies above the loading curve to prevent energy generation and
follows a zero force response when the unloading curve yields a negative
response. In such cases the dissipated energy will be less than the value
specified by the energy dissipation factor.
Specifying Interpolated Curve Unloading
The model in
Figure 8
illustrates an interpolated unloading response based on multiple unloading
curves that intersect the primary loading curve at increasing values of
forces/displacements. You can specify as many unloading curves as are necessary
to define the unloading response. The first point of each unloading curve
defines the permanent deformation if the connector is completely unloaded. The
unloading curves are stored in normalized form so that they intersect the
loading curve at a unit force for a unit displacement, and the interpolation
occurs between these normalized curves. If unloading occurs from a maximum
displacement for which an unloading curve is not specified, the unloading curve
is interpolated from neighboring unloading curves. As the connector is loaded,
the force follows the path given by the loading curve. If the connector is
unloaded (for example, at point B), the force follows the
unloading curve .
Reloading after unloading follows the unloading path
until the loading is such that the displacement becomes greater than
,
after which the loading path follows the loading curve.
If the loading curve depends on the constitutive displacements/rotations in
several component directions, the unloading curves also depends on the same
component directions. The unloading curve also has the same temperature and
field variable dependencies as the loading curve.
Specifying Shifted Curve Unloading
You can specify an unloading curve passing through the origin in addition to
the loading curve. The actual unloading curve is obtained by horizontally
shifting the user-specified unloading curve to pass through the point of
unloading as shown in
Figure 9.
The permanent deformation upon complete unloading is the horizontal shift
applied to the unloading curve.
If the loading curve depends on the constitutive displacements/rotations in
several component directions, the unloading curve also depends on the same
component directions. The unloading curve also has the same temperature and
field variable dependencies as the loading curve.
Using Different Uniaxial Models in Tension and Compression
When appropriate, different uniaxial behavior models can be used in tension
and compression. For example, a model with permanent deformation and
exponential unloading in tension can be combined with a nonlinear elastic model
in compression (see
Figure 10).