Abaqus Shell Edge Load

[Colby DuLin]

Clickthe arrow to the right of the Distribution field, and select the option of your choice from the list that appears:Select Uniform to define a load that is uniform over the shell edge.

By default, the traction components are specified with respect to the global axes. To refer to a local coordinate system for the direction components of the traction:Select CSYS: Picked and click Edit to pick a previously defined local coordinate system.

If you selected CSYS: Picked, you can define an additional rotation about one of the axes. Click the arrow to the right of the Additional rotation about axis field, select the axis about which the other two axes will be rotated, and enter a value for the additional rotation angle.

If desired, click the arrow to the right of the Traction is defined per unit field, and select deformed area to define the shell edge load with respect to the current (deformed) area or undeformed area to define the shell edge load with respect to the reference (original) area.

If you selected the General traction type, you can toggle off Follow rotation to define a non-follower load (i.e., the load always acts in a fixed global direction rather than rotating with the shell edge in a geometrically nonlinear analysis).

may be of follower type, which can rotate during a geometrically nonlinear analysis and result in an additional (often unsymmetric) contribution to the stiffness matrix that is generally referred to as the load stiffness.

Nonuniform distributed loads such as a nonuniform body force in the X-direction can be defined by means of user subroutine DLOAD in Abaqus/Standard or VDLOAD in Abaqus/Explicit. When an amplitude reference is used with a nonuniform load defined in user subroutine VDLOAD, the current value of the amplitude function is passed to the user subroutine at each time increment in the analysis. DLOAD and VDLOAD are not available for surface tractions, edge tractions, or edge moments.

In Abaqus/Standard nonuniform distributed surface tractions, edge tractions, and edge moments can be defined by means of user subroutine UTRACLOAD. User subroutine UTRACLOAD allows you to define a nonuniform magnitude for surface tractions, edge tractions, and edge moments, as well as nonuniform loading directions for general surface tractions, shear tractions, and general edge tractions.

You need not specify an element or an element set as is customary for the specification of other distributed loads. Abaqus/Standard and Abaqus/Explicit automatically collect all elements in the model that have mass contributions (including point mass elements but excluding rigid elements) in an element set called _Whole_Model_Gravity_Elset and apply the gravity loads to the elements in this element set. Abaqus/CFD applies the gravity loading to all user-defined elements.

Rotordynamic loads can be used to study the vibrational response of three-dimensional models of axisymmetric structures, such as a flywheel in a hybrid energy storage system, that are spinning about their axes of symmetry in a fixed reference frame (see Genta, 2005). This is in contrast to the centrifugal loads, Coriolis forces, and rotary acceleration loads discussed above, which are formulated in a rotating frame. Rotordynamic loads are, therefore, not intended to be used in conjunction with these other dynamic load types.

Rotordynamic loads are intended only for three-dimensional models of axisymmetric bodies; you must ensure that this modeling assumption is met. Rotordynamic loads are supported for all three-dimensional continuum and cylindrical elements, shell elements, membrane elements, cylindrical membrane elements, beam elements, and rotary inertia elements. The spinning axis defined as part of the load must be the axis of symmetry for the structure. Therefore, beam elements must be aligned with the symmetry axis. In addition, one of the principal directions of each loaded rotary inertia element must be aligned with the symmetry axis, and the inertia components of the rotary inertia elements must be symmetric about this axis. Multiple spinning structures spinning about different axes can be modeled in the same step. The spinning structures can also be connected to non-axisymmetric, non-rotating structures (such as bearings or support structures).

By definition, the line of action of a follower surface load rotates with the surface in a geometrically nonlinear analysis. This is in contrast to a non-follower load, which always acts in a fixed global direction.

General surface tractions can be specified to be follower or non-follower loads. There is no difference between a follower and a non-follower load in a geometrically linear analysis since the configuration of the body remains fixed. The difference between a follower and non-follower general surface traction is illustrated in the next section through an example.

The traction load acts in the fixed direction in a geometrically linear analysis or if a non-follower load is specified in a geometrically nonlinear analysis (which includes a perturbation step about a geometrically nonlinear base state).

If a follower load is specified in a geometrically nonlinear analysis, the traction load rotates rigidly with the surface using the following algorithm. The reference configuration traction vector, , is decomposed by Abaqus into two components: a normal component,

The shear traction load acts in the fixed direction in a geometrically linear analysis. In a geometrically nonlinear analysis (which includes a perturbation step about a geometrically nonlinear base state), the shear traction vector will rotate rigidly; i.e., , where is the standard rotation tensor obtained from the polar decomposition of the local two-dimensional surface deformation gradient .

It is sometimes convenient to give shear and general traction directions with respect to a local coordinate system. The following two examples illustrate the specification of the direction of a shear traction on a cylinder using global coordinates in one case and a local cylindrical coordinate system in the other case. The axis of symmetry of the cylinder coincides with the global z-axis. A surface named SURFA has been defined on the outside of the cylinder.

If you choose not to have a constant resultant, the traction vector is integrated over the surface in the current configuration, a surface that in general deforms in a geometrically nonlinear analysis. By default, all surface tractions are integrated over the surface in the current configuration.

Distributed pressure loads can be specified on any two-dimensional, three-dimensional, or axisymmetric elements. Hydrostatic pressure loads can be specified in Abaqus/Standard on two-dimensional, three-dimensional, and axisymmetric elements. Viscous and stagnation pressure loads can be specified in Abaqus/Explicit on any elements.

Distributed pressure loads can be specified on any elements. For beam elements, a positive applied pressure results in a force vector acting along the particular local direction of the section or a global direction, whichever is specified. For conventional shell elements, the force vector points along the element SPOS normal. For continuum solid or a continuum shell elements with the distributed load on an explicitly identified facet, the force vector acts against the outward normal of that facet. Distributed pressure loads are not supported for pipe and elbow elements.

Viscous pressure loading is most commonly applied in structural problems when you want to damp out dynamic effects and, thus, reach static equilibrium in a minimal number of increments. A common example is the determination of springback in a sheet metal product after forming, in which case a viscous pressure would be applied to the faces of shell elements defining the sheet metal. An appropriate choice for the value of is important for using this technique effectively.

For typical structural problems it is not desirable to absorb all of the energy (as is the case in the infinite elements). Typically is set equal to a small percentage (perhaps 1 or 2 percent) of as an effective way of minimizing ongoing dynamic effects. The coefficient should have a positive value.

You can specify external pressure, internal pressure, external hydrostatic pressure, or internal hydrostatic pressure on pipe or elbow elements. When pressure loads are applied, the effective outer or inner diameter must be specified in the element-based distributed load definition.

The loads resulting from the pressure on the ends of the element are included: Abaqus assumes a closed-end condition. Closed-end conditions correctly model the loading at pipe intersections, tight bends, corners, and cross-section changes; in straight sections and smooth bends the end loads of adjacent elements cancel each other precisely. If an open-end condition is to be modeled, a compensating point load should be added at the open end. A case where such an end load must be applied occurs if a pressurized pipe is modeled with a mixture of pipe and beam elements. In that case closed-end conditions generate a physically non-existing force at the transition between pipe and beam elements. Such mixed modeling of a pipe is not recommended.

Plane stress theory assumes that the volume of a plane stress element remains constant in a large-strain analysis. When a distributed surface load is applied to an edge of plane stress elements, the current length and orientation of the edge are considered in the load distribution, but the current thickness is not; the original thickness is used.

Distributed edge tractions (general, shear, normal, or transverse) and edge moments can be applied to shell elements in Abaqus as element-based or surface-based distributed loads. The units of an edge traction are force per unit length. The units of an edge moment are torque per unit length. References to local coordinate systems are ignored for all edge tractions and moments except general edge tractions.

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