FEM ConstraintDisplacement

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Menu location FEM ConstraintDisplacement Model → Mechanical boundary conditions and loads → Displacement boundary condition FEM None - FEM tutorial

Description

Creates a FEM boundary condition for a prescribed displacement of a selected object for specified degrees of freedom.

Usage

1. Press the button or select the menu Model → Mechanical boundary conditions and loads → Displacement boundary condition.
2. In the 3D view select the object the boundary condition should be applied to, which can be a vertex (corner), edge, or face.
4. Uncheck Unspecified to activate the necessary fields for editing.
5. Set the values or (introduced in version 0.21) specify a formula for the displacements.

Formulas

General

For the solver Elmer it is possible to define the displacement as a formula. In this case the solver sets the displacement according to the given formula variable.

Take for example the case that we want to perform a transient analysis. For every time step the displacement ${\displaystyle d}$ should be increased by 6 mm:

${\displaystyle \quad d(t)=0.006\cdot t}$

enter this in the Formula field:
 Variable "time"; Real MATC "0.006*tx"

This code has the following syntax:

• the prefix Variable specifies that the displacement is not a constant but a variable
• the variable is the current time
• the displacement values are returned as Real (floating point) values
• MATC is a prefix for the Elmer solver indicating that the following code is a formula
• tx is always the name of the variable in MATC formulas, no matter that tx in our case is actually t

Rotations

Elmer only uses the Displacement * fields of the boundary condition. To define rotations, we need a formula.

If for example a face should be rotated according to this condition:

{\displaystyle \quad {\begin{aligned}d_{x}(t)=&\left(\cos(\phi )-1\right)x-\sin(\phi )y\\d_{y}(t)=&\left(\cos(\phi )-1\right)y+\sin(\phi )x\end{aligned}}}

then we need to enter for Displacement x
 Variable "time, Coordinate" Real MATC "(cos(tx(0)*pi)-1.0)*tx(1)-sin(tx(0)*pi)*tx(2)

and for Displacement y
 Variable "time, Coordinate" Real MATC "(cos(tx(0)*pi)-1.0)*tx(2)+sin(tx(0)*pi)*tx(1)

This code has the following syntax:

• we have 4 variables, the time and all possible coordinates (x, y z)
• tx is a vector, tx(0) refers to the first variable, the time, while tx(1) refers to the first coordinate x
• pi denotes ${\displaystyle \pi }$ and was added so that after ${\displaystyle t=1{\rm {\,s}}}$ a rotation of 180° is performed

Notes

For the solver CalculiX: