structure shell create command

Syntax

structure shell create keyword

Primary keywords:

by-block-face    by-nodeids    by-quadrilateral    by-triangle    by-zone-face

Create a shell consisting of a collection of shell finite elements. The five main keywords articulate five creation techniques: (1) create a collection of elements from a set of block faces (by-block-face); (2) create a single element from three existing nodes (by-nodeids); (3) create a collection of elements from a quadrilateral shape (by-quadrilateral); (4) create a collection of elements from a triangular shape (by-triangle); and (5) create a collection of elements from a set of zone faces (by-zone-face). For techniques 1, 3, 4 and 5, newly created nodes will attach to the first valid object (e.g., zones in FLAC3D and 3DEC, particles in PFC3D) found at their location, with attachment conditions appropriate to a shell (see this table in the topic Structural Element Links). For technique 2, newly created nodes will remain unattached. If you wish to attach them to a target object, position them with the structure node initialize position command. A shell can also be created by the structure shell import command.

by-block-face [shellcreateblock] <range>

Create new elements that are attached to the set of 3-sided 3DEC zone faces in the range. Elements can only be created on block surfaces. New nodes associated with each element will also be created. The nodes of each element will be ordered counter-clockwise with respect to the outside of the zone faces, thereby making each element \(z\) axis point outward. The orientation of the \(z\) axis can be reversed with the reverse keyword.

by-nodeids i1 i2 i3 [shellcreateblock]

Create a single element from three existing nodes.

by-quadrilateral v1 v2 v3 v4 [shellcreateblock]

Create elements based on four points forming a quadrilateral, in order. The element \(z\)-axis will point in the direction determined by applying the right-hand rule to the quadrilateral.

by-triangle v1 v2 v3 [shellcreateblock]

Create elements based on three points forming a triangle. The element \(z\)-axis will point in the direction determined by applying the right-hand rule to the triangle.

by-zone-face [shellcreateblock] <range>

Create new elements that are attached to the set of three- or four-sided zone-faces in the range. By default, only surface faces will be considered, and the internal keyword can be used to select internal faces. New nodes associated with each element will also be created. The nodes of each element will be ordered counter-clockwise with respect to the outside of the zone faces, thereby making each element \(z\) axis point outward. The orientation of the \(z\) axis can be reversed with the reverse keyword. Note that after creating the elements with this command, the zones may be deleted and the elements may be positioned by moving their nodes with the structure node initialize position command.

structure shell create Keyword Block

The following keywords may be optionally provided on element creation performed using any of the major keywords: by-block-face, by-nodeids, by-quadrilateral, by-triangle and by-zone-face.

cross-diagonal <b >

If specified, a cross-diagonal mesh of four triangles will be created with a node lying at the centroid of all quadrilaterals; otherwise, a cross-hatch mesh of two triangles will be created dividing the quadrilateral. If this keyword is not specified, then a cross-hatch mesh will be created.[1]

distinct

Eliminate sharing of nodes among created elements. Every element will create a new set of nodes, even if they are at the same location in space and elements share the same ID number. This option is most commonly used with a later structure node join command to create links connecting nodes at the same location in order to apply custom inter-element behavior.

element-type keyword

Specify the type of finite element to be used by each structural element. A description of the available shell finite elements is provided here. If no element type is specified, then the element type will be DKT-CST.[2] Note that the type of finite element is associated with each structural element when it is created and cannot subsequently be altered. Plastic material behavior is supported by the DKT, CST and DKT-CST finite elements. The keyword must be one of the following entries:

s

Attempt to match the string s to one of the available keywords below, and treat as that option.

cst

CST (6 degrees-of-freedom) finite element. This is a membrane element; it resists membrane, but not bending, loading.

csth

CST Hybrid (9 degrees-of-freedom) finite element. This is a membrane element; it resists membrane, but not bending, loading.

dkt

DKT (9 degrees-of-freedom) finite element. This is a plate-bending element; it resists bending, but not membrane, loading.

dkt-cst

DKT-CST (15 degrees-of-freedom) finite element. This is a shell element that combines the DKT and CST elements to resist both bending and membrane loading.

dkt-csth

DKT-(CST Hybrid) (18 degrees-of-freedom) finite element. This is a shell element that combines the DKT and CST Hybrid elements to resist both bending and membrane loading.

group s1 <slot s2 >

Assign the created elements to group s1. The group is assigned to the slot named s2 if supplied; it is assigned to the slot named Default if not. Use of the group logic is described in Groups.

id i

Specify the ID (this is the collection ID) that will be assigned to all new elements created by this command. If this value is not given, then the next available ID will be used. The ID controls potential joining of the element nodes to an existing element, as follows. At each potential new node location, a new node is created unless all of the following conditions are fulfilled: 1) the ID is given; 2) an existing node is found “near” the potential new node location; and 3) the found node is part of an element of the same type with ID equal to i.

internal

This keyword only applies if the by-zone-face option was used. Specifies that all internal and surface faces in the range will have elements created on them. Normally only surface faces are considered.

maximum-length f

Break the element into segments no greater than length f. The segments setting, if provided, is overridden as needed by this setting.

reverse

This keyword only applies if the by-zone-faces option was used. Specifies that the orientation of the element will be reversed from the default with respect to the face, so that the element \(z\) axis points in the other direction. Also, if the cross-diagonal keyword was not used, then the diagonal will appear in the other direction with respect to quadrilateral faces.

segments i

Break the element into i equal segments, creating new nodes as needed. The default value is 1, indicating that only one element will be created.

size i1 i2

This keyword only applies if the by-quadrilateral option was used. Increases the number of elements created by subdividing the quadrilateral into i1 segments along the first edge and i2 segments along the second. By default, only one quad is created.

Endnotes