CONSTRUCT LMASK [lmask] SURFACE [INVERSE] Umin [Umax [Vmin [Vmax]]]

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CONSTRUCT LMASK [lmask] SURFACE [INVERSE] Umin [Umax [Vmin [Vmax]]]

This command is used to define a load mask for use with a load applied to a surface. When attached to a load using the command PROPERTY ATTACH the load mask will restrict the action of the load to those nodes or elements which lie between Umin and Umax, Vmin and Vmax. The INVERSE option may be used to restrict the action of the load to those nodes or elements which lie outside Umin, Umax, Vmin and Vmax where:

lmask	=	name of the load mask
INVERSE	=	load acts on area outside mask
Umin	=	lower limit as a fraction of line length in first surface direction (0 to 1)
Umax	=	upper limit as a fraction of line length in first surface direction (0 to 1)
Vmin	=	lower limit as a fraction of line length in second surface direction (0 to 1)
Vmax	=	upper limit as a fraction of line length in second surface direction (0 to 1)

**Figure 2.11:** Definition of a surface load mask
$\begin{figure} \centerline{ \psfig {figure=diagrams/lmsurf1.ps,width=5.0in} }\end{figure}$

**Figure 2.12:** Definition of an INVERSE surface load mask
$\begin{figure} \centerline{ \psfig {figure=diagrams/lmsurf2.ps,width=5.0in} }\end{figure}$

Notes:

1.: Default names
If no name is entered a default name is created, this is LMn where n is a count of defined load masks. The default name can be changed with the CONSTRUCT NAME command.
2.: Masking criteria
The load mask bases its masking action upon either the coordinates of a node or the centroid of an element or element face. Thus for element loads the mask will simply include or exclude whole elements.
3.: Interaction with space curves
The surface load mask will compress a surface space curve so that all of the curve fits within the load mask limits. Where the INVERSE option has been used the space curve will be repeated once from 0 to Umin and once from Umax to 1, once from 0 to Vmin and once for Vmax to 1.
4.: Viewing load masks
The limits of a surface load mask can only be viewed by attaching it to a load and using the command LABEL MESH LOAD.
5.: Permitted surface topologies
The surface load mask can only be applied to four-sided surfaces with the topologies indicated in the following figures. The sides of the surfaces can be combined lines.

Examples:

1.: CONSTRUCT LMASK LM1 SURFACE .2 .8 .2 .8
This creates a load mask which when applied to a surface load will restrict the loaded nodes or elements to those in the central section from .2 to .8 of both side lengths.
2.: CONSTRUCT LMASK LM1 SURFACE INVERSE .2 .8 .2 .8
This creates a load mask which when applied to a surface load will restrict the loaded nodes or elements to those which lie outside the central section from .2 to .8 of both side lengths.

**Figure 2.13:** Flat surface with four straight sides
$\begin{figure} \centerline{ \psfig {figure=diagrams/lmsurfa.ps,width=2.8in} }\end{figure}$

**Figure 2.14:** Flat surface with two straight sides and two constant radius arc sides
$\begin{figure} \centerline{ \psfig {figure=diagrams/lmsurfb.ps,width=2.8in} }\end{figure}$

**Figure 2.15:** Cylindrical surface with two straight sides and two constant radius arc sides
$\begin{figure} \centerline{ \psfig {figure=diagrams/lmsurfc.ps,width=2.8in} }\end{figure}$

**Figure 2.16:** Flat surface with one constant radius arc and one degenerate side
$\begin{figure} \centerline{ \psfig {figure=diagrams/lmsurfd.ps,width=2.8in} }\end{figure}$

See also the following commands

'CONSTRUCT NAME'
'CONSTRUCT SCURVE'
'CONSTRUCT TCURVE'
'PROPERTY ATTACH'
'PROPERTY LOAD'
'UTILITY DELETE'
'UTILITY TABULATE'

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Femsys Limited
1st October 1999