Area Command

The Area command defines a 2D area or a 1D interface which will automatically be divided into elements to define all or part of an FEA mesh.

Scripted Area Block

In scripted files, the Area command is a block that begins with Area command and ends with an EndArea command. Between the Area and its matching EndArea, the area is defined by a collection of paths that enclose the area. To define a solid 2D area, the area block must include four paths:

Area #1,#2,<#3>
  Paths "Bottom"
    Keypoint "BotLeft",0,0
    Keypoints "BotRight"
  EndPath
  Paths "Right", "Top" "Left"
EndArea

To define a 1D interface, the area block must includes two separate, but identical paths:

Area #1,#2,<#3>
  Path "Left",6
   Keypoint "LeftBot",0,0
   Keypoints "LeftTop"
  EndPath
  Paths "Right"
EndArea

The Area command starts with three arguments

• #1 is the material ID for a previously defined material. All elements in the area will be set to this material. When the mesh will be defined using images and shapes, set the material ID for any region that will use the images and shapes to be _NONE_; the images and shapes will fill in the materials later.
• #2 is the thickness of all the elements in mm for 2D plane stress or plain strain analyses. It is ignored for axisymmetric analyses, but is still required.
• #3 is an optional material angle for all the elements. It may be needed for certain anisotropic materials. It can be entered as a number or as a function. A function is evaluated at the midpoint of an element. Triangular elements are created in pairs and the function is evaluated at the midpoint of each pair.

Within the area block the Path and Paths commands a series of paths that must circumnavigate the area in the counter clockwise direction. The paths must be connected and define a closed area. Each path can be explicitly defined (e.g., the paths with subordinate [[Keypoint Command|Keypoins commands</a>) or can refer to a previously defined path (e.g., the Paths commands with one or more path names). See the Path and Keypoint commands for details on defining paths and keypoints.

XML Area Block

In XML input files, the Area block must be within the single Mesh block and have the form

  <Area mat="1" thick="10" type="4" flip="0" angle="0">
    <path id="bottom"/>
    <path id="right"/>
    <path id="top"/>
    <path id="left"/>
  </Area>

An Area block links a series of paths into a region to be meshed. Subordinate to each Area block are a series of path commands that list the paths that define the area. Currently NairnFEA only supports areas with four paths, to mesh quadrilateral areas, or areas with two paths, to mesh interfaces. The attributes for an Area block are:

• mat: Material number in the list of defined materials when referring to material "By Number". When the mesh will be defined using images and shapes, the mat should be 0 for elements that will be set later by images and shapes.
• matname: Alternatively, materials can be specified "By Name".
• angle: Optional material angle relevant to anisotropic materials. It can be entered as a number or as a function. A function is evaluated at the midpoint of an element. Triangular elements are created in pairs and the function is evaluated at the midpoint of each pair.
• thick: Thickness in mm (for plane stress or plane strain 2D calculations); not needed for axisymmetric analysis.
• type: The type of element to create in the meshed area.

The first <Area> command must define an element type. Subsequent <Area> commands can omit the element type unless the element type is changed to have an analysis with mixed elements. When mixed elements are used, they must be compatible elements (i.e., you can mix linear elements (types 1, 2, and 5) with each other or quadratic elements (types 3, 4, 6, and 8) with each other, but cannot mix linear elements with quadratic elements). Whenever a type attribute is used, it will apply to that area and all subsequent areas until it is changed. Finally, quadrilateral areas can only use solid elements (types 1, 2, 3, 4, and 8) and interface areas can use only interface elements (type 5 and 6).
• flip: For triangular elements, the initially created quadrilaterials are split into two elements by drawing a diagonal across the quadrilaterial. The flip option can change which diagonal is used. It should be 0 or 1. Once it is set, it will apply to that area and all subsequent areas until it is changed.

Notes

1. For quadrilateral areas, the end of the last path must connect to the start of the first path. The direction of each path does not matter as they will be rearranged if needed to define the area; but the paths must circumnavigate the area in the counter-clockwise direction.
2. Currently all solid areas must be defined by four paths. Furthermore, if n₁, n₂, n₃, and n₄ are the number of intervals along the four paths, they must satisfy either n₁+n₂=n₃+n₄ or n₁+n₄=n₂+n₃.
3. For interface meshing, the two paths must be different, but indentical paths. In other words, they must connect different keypoints, but the coordinates of the keypoints, the number of intervals along the path, and the path ratios must be identical. The order and orientation of the two paths in the area command does not matter. Finally, interfaces only function when they connect two areas. To insure this connection, the paths for an interface must have each been previously used in exactly one quadrilateral area.
4. Solid (or quadrilateral) areas must use normal materials while interface areas must use interface materials.
5. An FEA analysis can have any number of areas. To connect areas into a solid object, the areas must share common paths.
6. The area will be meshed into elements of the type defined in the most recent <a href="element.html">Element command</a>. Solid areas must use solid elements. Interfaces must use <a href="imperfect.html">interface elements</a>.
7. The Area command is only allowed for <a href="analysis.html">FEA analyses.</a>