GuiCommand:Container| {{GuiCommand Name: FEM ConstraintDisplacement MenuLocation: Model , Mechanical boundary conditions and loads , Displacement boundary condition Workbenches: FEM_Workbench Shortcut: SeeAlso: FEM_tutorial }} {{GuiCommandFemInfo Solvers: CalculiX, Elmer }}

FEM ConstraintDisplacement

Description

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

Usage

1. Press the Displacement boundary condition button or select the menu Model → Mechanical boundary conditions and loads → Displacement boundary condition.
2. Press the Add button.
3. In the 3D view select the object the boundary condition should be applied to, which can be a vertex, edge, or face (but all the selected objects have to be of the same type). To remove objects from the list, press the Remove button and click on them.
4. Check the boxes next to the degrees of freedom that you want to use. By default, they are set to zero (fixed) but any value ((v0.21) : or a formula for Elmer) can be specified.

Formulas

(v0.21)

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 $d$ should be increased by 6 mm:

$\quad d(t)=0.006\cdot t$

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:

$\quad \begin{align} d{x}(t)= & \left(\cos(\phi)-1\right)x-\sin(\phi)y\ d{y}(t)= & \left(\cos(\phi)-1\right)y+\sin(\phi)x \end{align}$

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 $\pi$ and was added so that after $t=1\rm\, s$ a rotation of 180° is performed

Notes

For the solver CalculiX:

• This tool uses the *BOUNDARY card.
• Fixing a degree of freedom is explained at http://web.mit.edu/calculix_v2.7/CalculiX/ccx_2.7/doc/ccx/node164.html
• Prescribing a displacement for a degree of freedom is explained at http://web.mit.edu/calculix_v2.7/CalculiX/ccx_2.7/doc/ccx/node165.html

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