17.6.4.7. Element Birth and Death

The Element Birth and Death object enables you to activate and/or deactivate the element status for a specific load step in your analysis.

When you deactivate an element or elements (Death), the solver does not remove elements. Instead, the solver deactivates the elements by multiplying their stiffness (or conductivity, or other analogous quantity) by a reduction factor so severe that the application renders the elements inactive.

Any load associated with deactivated elements:

  • Zero-out of the loading vector. This includes mass, damping, specific heat, and other such effects.

  • Does not include the mass and energy of deactivated elements in the summations over the model.

The application also sets strains to zero as soon as that element is deactivated.

Likewise, for activated elements (Alive), the application does not add the elements to the model. They are simply reactivated. You need to first deactivate an element in order to reactivate the element for the desired load step.

When you reactivate elements, stiffness, mass, element loads, etc., return to their original values. The application reactivates elements with no record of strain history (heat storage, etc.); the application considers reactivated elements to be generally strain-free. However, thermal strains are computed for newly-activated elements based on the current step temperature and the reference temperature. Therefore, newborn elements with thermal loads may not be stress-free as intended.

This feature is useful for analyzing excavation (as in mining and tunneling), staged construction (as in shored bridge erection), sequential assembly (as in fabrication of layered computer chips), and many other applications in which you can easily identify activated or deactivated elements by their known locations.

Element Birth and Death Requirements

Review the following guidelines when applying the Element Birth and Death object:

  • You cannot apply Constraint Equations to inactive DOFs. Inactive DOFs occur when a node has no active (Alive) elements attached to it.

  • You can model stress-relieving operations (such as annealing) by deactivating and then reactivating elements.

  • During nonlinear analyses, do not deactivate or reactivate elements in such a way as to create singularities (such as sharp re-entrant corners in a structural analysis) or sudden large changes in stiffness. Convergence difficulties may result.

  • The Program Controlled setting for the Newton-Raphson Option property (Analysis Settings> Nonlinear Controls ) automatically specifies the Full setting if a Dead element status is detected.

  • The Nonlinear Solution read-only property (Analysis Settings > Analysis Data Management) is automatically set to the Yes setting.

  • In order to obtain a good result, Ansys recommends that you set the Large Deflection property (Analysis Settings> Solver Controls) to On.

  • If you deactivate a Contact Region using the Contact Step Control feature or if you deactivate the underlying elements of a Contact Region using the Element Birth and Death feature, the application changes the status of the contact pair to far field contact (open and not near contact). This includes bonded contact. To reestablish the pre-death contact status when reactivating the Contact Region, you may need to deactivate both the Contact Region and their underlying elements.

  • If an element in a Static Structural analysis is killed during the first load step and then activated in the subsequent load step, any downstream Linear Perturbation analyses restarted from the activated load step will activate the element during the first phase of the analysis. However, if an element in a Static Structural analysis is active during the first load step and then deactivated in the subsequent load step, any downstream Linear Perturbation analyses restarted from the deactivated load step will deactivate the element during the first phase of the analysis. For additional information, see the First Phase of the Linear Perturbation Analysis in the Mechanical APDL Structural Analysis Guide.

For additional information, see the Element Birth and Death topic in the Mechanical APDL Advanced Analysis Guide.

This page includes the following sections:

Analysis Types

Element Birth and Death is available for the following analysis types:
Dimensional Types

The supported dimensional types for the Element Birth and Death boundary condition include:

  • 3D Simulation

  • 2D Simulation

Geometry Types

The supported geometry types for the Element Birth and Death boundary condition include:

  • Solid

  • Surface/Shell

  • Wire Body/Line Body/Beam

Topology Selection Options

The supported topology selection options for Element Birth and Death include:

  • Body

  • Elements

Applying an Element Birth and Death Boundary Condition

To apply an Element Birth and Death condition:

  1. On the Environment Context tab, click Conditions>Element Birth and Death. Alternatively, right-click the Environment tree object or within the Geometry window and select Insert>Element Birth and Death.

  2. Define your Scoping Method and select geometry.

  3. Define the desired number of steps in the Step Controls category of the Analysis Settings.

  4. Select a desired step in the Current Step property of the Element Birth and Death object.

  5. Specify the Status of each load step as desired, Alive or Dead. The default setting for each step is Alive. The Graph and Tabular Data windows each provide the right-click option Swap Status. This option also changes the Status of a load step.

Details Pane Properties

The selections available in the Details pane are described below.

CategoryProperties/Options/Descriptions
Scope

Scoping Method: Options include:

  • Geometry Selection: Default setting, indicating that the condition is applied to a geometry or geometries, which are chosen using a graphical selection tool.

    • Geometry: Visible when the Scoping Method is set to Geometry Selection. Displays the type of geometry (Body or element only) and the number of geometric entities (for example: 1 Body, 2 Bodies) to which the condition has been applied using the selection tools.

  • Named Selection: Indicates that the geometry selection is defined by a Named Selection.

    • Named Selection: Visible when the Scoping Method is set to Named Selection. This field provides a drop-down list of available body- or element-based Named Selections.

Definition

Type: Read-only field that describes the object - Element Birth and Death.

Suppressed: Include (No - default) or exclude (Yes) the condition.

Step Controls

Current Step: This property displays the currently selected Step. This step is also highlighted in the Graph window.

Status. The options for this property include:

  • Alive: Scoped bodies/elements for the current step are active.

  • Dead: Scoped bodies/elements for the current step are deactivated.

Mechanical APDL References and Notes

The following Mechanical APDL commands and considerations are applicable for this condition.

  • In order to create the Element Birth and Death component, the application uses the CM command for body-based scoping and the CMBLOCK command for element-based scoping.

  • When the application completes the creation of the Element Birth and Death components, it issues the ALLSEL ,ALL command.

  • The application issues the EKILL command when the Status is changed from Alive to Dead or Dead at the first step.

  • The application issues the EALIVE command when the Status is changed from Dead to Alive.

API Reference

For specific scripting information, see the Element Birth and Death section of the ACT API Reference Guide.