Element Activation and Deactivation

Element activation and deactivation is a feature for structural static analysis that represents construction sequences, damage progression, and staged structural changes on an element-group basis. This page explains the differences among fixed mode, time-table mode, and stress/strain threshold mode, and how to choose between them.

Overview

Element activation and deactivation is defined for each element group with !ELEMENT_ACTIVATION, and the corresponding group is enabled on the step side. In structural static analysis, elements targeted by an element-activation setting that is not enabled in the step are treated as active. For the syntax for specifying enabled groups using a !STEP data line (ELEMACT, id), see !STEP. For the overall concept of step control, see Step Control.

Elements deactivated in structural static analysis do not contribute as normal elements and have a dummy stiffness coefficient for numerical stabilization. For the syntax and default value of the coefficient, see !ELEMENT_ACTIVATION.

Mode	How the state is determined	Suitable applications
Fixed mode	Directly specifies an element group as active or inactive	Construction sequence; staged addition or removal of members
Time-table mode	Switches between active and inactive according to values in a time table defined by !AMPLITUDE	Analyses in which elements appear or disappear according to a known time history, such as welding
Stress/strain threshold mode	Deactivates an element when the stress or strain criterion value falls outside the specified range	Damage progression; approximate models involving fracture or removal

Fixed mode applies the state as specified when it is enabled in a step. Time-table mode and stress/strain threshold mode update the element state at each substep.

Because different element groups can be enabled for each step, fixed mode can be used to represent an initial construction sequence and stress/strain threshold mode can be enabled in a different step. However, if the same element is included in multiple element-activation settings, its state may be overwritten by a later setting; therefore, as a rule, the target element groups should not overlap.

Fixed Mode

In fixed mode, the target element group is directly specified as active or inactive. Its state does not change automatically within a step according to stress, strain, or time.

To represent a construction sequence, prepare a control group that deactivates members not yet constructed and specify that group in ELEMACT for steps before construction. From the step in which the member is added onward, remove the corresponding deactivation group from ELEMACT so that the target elements are treated as active.

Fixed mode is the simplest mode and allows the analyst to control the element state completely. On the other hand, because it does not automatically switch according to load history or damage criteria, it is suitable for problems in which the time of the state change is known.

Time-Table Mode

In time-table mode, the element state is switched according to the value of a time table defined by !AMPLITUDE. A table value greater than 0.5 is treated as active, and a value of 0.5 or less as inactive.

This mode is suitable for problems in which the time history of the element state is known in advance. For example, it can represent cases in which elements appear and disappear according to a known time history, such as successively introducing weld-metal elements as the heat source advances in a welding analysis. A table can also express controls such as keeping elements inactive from time 0 to 1 and active from time 1 onward, or temporarily deactivating them during the analysis.

For how to define a time table and use it with other boundary conditions and loads, see the time-table section on the boundary-conditions and loads page. For the input syntax of !AMPLITUDE, see !AMPLITUDE.

Stress/Strain Threshold Mode

In stress/strain threshold mode, stress or strain is selected as the criterion quantity, and an element is deactivated when its Mises equivalent value falls outside the specified range. The criterion range is given by lower and upper limits, and elements within the range remain active.

This mode is used when material damage or fracture is to be represented in a simplified manner. The criterion is evaluated at each substep, and an element that falls outside the threshold range transitions from active to inactive. While the same stress/strain threshold mode remains enabled, an element that has once been deactivated is not reactivated, so this mode is not suitable for representing reversible opening/closing or recovery. If damage is to remain irreversible in subsequent steps, keep the same element-activation setting enabled in those subsequent steps.

When stress is used as the basis, the criterion value is calculated from the stress state; when strain is used, it is calculated from the strain state. Set the physical units and magnitude of the threshold according to the selected criterion quantity and material model. For the input syntax, see !ELEMENT_ACTIVATION.

Related Topics

• Step Control — ELEMACT specification for each step and its relationship with automatic incrementing and cutback.
• !STEP — Input syntax for step data lines, including ELEMACT.
• Boundary conditions and loads — Overview of time-table control using !AMPLITUDE.
• !ELEMENT_ACTIVATION — Input syntax and parameters for element activation.
• !AMPLITUDE — Input syntax for time tables.