Finite Elements - Multi-Point Constraints

Marc > Building A Model > Finite Elements - Multi-Point Constraints

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The Finite Elements application in Patran (or MSC.AFEA) is used to define the basic finite element mesh. Use this application to create Marc nodes, elements, and multi-point constraints.

Nodes

Nodes in Patran (or MSC.AFEA) will generate the Marc COORDINATES keyword in the input file. Create nodes either directly by using the Node object, or indirectly by using the Mesh object. An Marc TRANSFORMATION or CYLINDRICAL keyword and set is generated for each node associated to a local (non-global) analysis coordinate frame.

To modify the analysis coordinate frame of an existing mesh, use the Create|Node|Edit options in this application. When creating a mesh, use the Node Coordinate Frames button when the options are set to Create|Mesh.

Elements

The Finite Elements application in Patran (or MSC.AFEA) assigns element topology, such as Quad4, Hex8, Tri6, etc. The type of Marc elements created however, are not determined until the element properties are assigned. See Element Properties for more information on Marc element types. Either create elements directly, by using the Element object, or indirectly by using the Mesh object. Both elements and nodes can be created simultaneously using the Create|Mesh options in this application or individual elements can be created using the Create|Element options.

The Marc element type or number is entered in the first field of the third card of the CONNECTIVITY option in the Marc input file.


Note:	Actual Marc element types are not assigned until element properties are associated with the elements of the mesh. Care should be taken to make sure the proper element topology is used before assigning properties. For grounded springs/dampers, create point elements.

Multi-Point Constraints

Multi-point constraints (MPCs) are created in the Finite Elements application by setting the Object to MPC. MPCs are special element types which define a rigorous behavior between several specified nodes. The full functionality is described in Create MPC Sliding Surface Form (p. 131) in the Reference Manual - Part III.

Define Terms

In general, for all MPC types except Cyclic Symmetry and Sliding Surface, dependent and independent terms must be specified including any degrees-of-freedom and/or coefficients associated with those terms on the form shown below. The operation is as explained:

A list of MPC types and their expected dependent and independent term information is given in MPC Types below.

Degrees-of-Freedom

When a list of degrees-of-freedom are expected for an MPC term, a listbox containing the valid degrees-of-freedom is displayed on the form. A degree-of-freedom is valid if:

1. It is valid for the current Analysis Code Preference.

2. It is valid for the current Analysis Type (structural/thermal).

3. It is valid for the selected MPC type.

In most cases, all degrees-of-freedom which are valid for the current Analysis Code and Analysis Type are valid for the MPC type.

The following degrees-of-freedom are supported by Marc MPCs for the various analysis types:


Degree-of-freedom	Analysis Type
UX	Structural
UY	Structural
UZ	Structural
RX	Structural
RY	Structural
RZ	Structural
Temperature	Thermal
Top Temperature	Thermal
Middle Temperature	Thermal
Bottom Temperature	Thermal


Note:	No MPC types are defined for Coupled analysis. To use MPCs is a Coupled analysis, set the Analysis Preference to Structural or Thermal to define the MPCs you want, then set the Analysis Preference back to Coupled. Make sure that the degree-of-freedom selected for an MPC actually exists at the nodes. For example, a node that is attached only to solid structural elements will not have any rotational degrees-of-freedom. However, Patran will allow you to select rotational degrees-of-freedom at this node when defining an MPC. This may not be allowed by Marc. Marc axisymmetric have three DOFs, namely Z, R, and Theta which correspond to the X, Y, and RX DOF in the global Patran system (DOFs 1,2 and 4 respectively).

MPC Types

The following table describes the MPC types which are supported for Marc. Either SERVO LINK or TYING keyword options are created in the Marc input file. For TYING keyword options, the dependent node ID is entered in the 2nd field of the 3rd data block, referred to as the tied node. The independent node IDs are entered on the 3a data block, referred to as the retained nodes.


MPC Type	Analysis Type	Description
• Explicit	Structural Thermal Coupled	Creates a SERVO LINK explicit MPC between a dependent degree-of-freedom and one or more independent degrees-of-freedom. The dependent term consists of a node ID and a degree-of-freedom, while an independent term consists of a coefficient, a node ID, and a degree-of-freedom. An unlimited number of independent terms can be specified, while only one dependent term can be specified.
• Rigid (Fixed)	Structural Coupled	Creates TYING Type 100 MPCs which constrains all degrees-of-freedom at one or more dependent nodes to the corresponding degrees-of-freedom at one independent node. An unlimited number of dependent terms can be specified, while only one independent term can be specified. Each term consists of a single node.
• Linear Surf-Surf	Structural Coupled	Creates a TYING Type 31 MPC which constrains a dependent node on one linear 2D element to two independent nodes on another linear 2D element to model a continuum. One dependent term is specified, while two independent terms are specified. Each term consists of a single node.
• Linear Surf-Surf	Thermal	Creates a TYING Type 87 MPC which constrains one dependent node to one independent node, which ties temperatures between shell elements. One dependent and one independent term are specified. A second independent term must be supplied but is ignored (it can be the same node). Each term consists of a single node.
• Linear Surf-Vol	Thermal	Creates a TYING Type 85 MPC which constrains a dependent node on one linear 2D element to two independent nodes on another linear 2D element to tie temperatures. One dependent term is specified, while two independent terms are specified. Each term consists of a single node.
• Linear Vol-Vol	Structural Thermal Coupled	Creates a TYING Type 33 MPC which constrains a dependent node on one linear 3D solid element to four independent nodes on another linear 3D solid element to model a continuum. One dependent term is specified, while four (three for degenerate face) independent terms must be specified. Each term consists of a single node.
• Quad Surf-Surf(quadratic)	Structural Coupled	Creates a TYING Type 32 MPC which constrains a dependent node on one quadratic 2D element to three independent nodes on another quadratic 2D element to model a continuum. One dependent term is specified, while three independent terms are specified. Each term consists of a single node.
• Quad Surf-Surf	Thermal	Identical to Linear Surf-Surf for Thermal analysis except a third independent term must be supplied but is also ignored.
• Quad. Surf-Vol	Thermal	Creates a TYING Type 86 MPC which constrains a dependent node on one quadratic 2D element to three independent nodes on another quadratic 2D element to tie temperatures. One dependent term is specified, while three independent terms are specified. Each term consists of a single node.
• Quad Vol-Vol	Structural Thermal Coupled	Creates a TYING Type 34 MPC which constrains a dependent node on one quadratic 3D solid to eight independent nodes on another quadratic 3D solid element to model a continuum. One dependent term is specified, while eight (six for degenerate face) independent terms are specified. Each term consists of a single node.
• Tie DOFs	Structural Thermal Coupled	Creates a TYING Types 1-6 or 102-506 MPC which constrains two nodes at a selected degree-of-freedom or at a range of degrees-of-freedom. One dependent term is specified which consists of a single node. One independent term is specified which consists of a single node and either one or two selected degrees-of-freedom. The Marc type number will be determined by the selected degrees-of-freedom. If one degree-of-freedom is specified, a Type 1-6 MPC is created. If two degrees-of-freedom are selected, a Type 102-506 MPC is created.
• Axi Shell-Solid	Structural Coupled	Creates a TYING Type 26 MPC which connects an axisymmetric shell element to a solid element. One dependent term is specified which consists of a single node. One independent term is specified which also consists of a single node.
• Tri Plate-Plate	Structural Coupled	Creates a TYING Type 49 MPC which connects triangular flat plate elements. One dependent term is specified which consists of a single node. One independent term is specified which also consists of a single node.
• Quad Plate-Plate	Structural Coupled	Creates a TYING Type 50 MPC which connects rectangular flat plate elements. One dependent term is specified which consists of a single node. One independent term is specified which also consists of a single node.
• Pinned Joint	Structural Coupled	Creates a TYING Type 52 MPC which creates a pinned joint between beam elements. One dependent term is specified which consists of a single node. One independent term is specified which also consists of a single node.
• Full Moment Joint	Structural Coupled	Creates a TYING Type 53 MPC which is a full moment joint between beam elements. One dependent term is specified which consists of a single node. One independent term is specified which also consists of a single node.
• Rigid Link	Structural Coupled	Creates a TYING Type 80 MPC which creates a pinned rigid link between two nodes. One dependent term is specified, while two independent terms are specified. The dependent term and the first independent term are the nodes at the ends of the link, while the second independent term is an unattached node that provides the rotational information about the link.
• Cyclic Symmetry	Structural Coupled	Creates a TYING Type 100 MPC which ties all degrees-of-freedom between matched nodes on opposite sides of the cyclic sector. Unlimited nodes may be entered in the dependent and independent regions; however, the same number of unique nodes must be specified in both regions.
• Sliding Surface	Structural Coupled	Creates a SERVO LINK explicit MPC which ties the normal to the surface degrees-of-freedom between matched nodes on opposite sides of the interface. Unlimited nodes may be entered in the dependent and independent regions; however, the same number of unique nodes must be specified in both regions.
• RBE2	Structural	Creates an MD Nastran style RBE2 element, which defines a rigid body between an arbitrary number of nodes. Although the user can only specify one dependent term, an arbitrary number of nodes can be associated to this term. The user is also prompted to associate a list of degrees of freedom to this term. A single independent term can be specified, which consists of a single node. There is no constant term for this MPC type. The RBE parameter is also written.
• RBE3	Structural	Creates an MD Nastran style RBE3 element, which defines the motion of a reference node as the weighted average of the motions of a set of nodes. A finite number of dependent terms can be specified, each term consisting of a single node and a list of degrees of freedom. The first dependent (tied) term is used to define the reference node. Any (optional) dependent terms define additional nodes/degrees of freedom (dofs) that are added to the m-set. These additional dependent (tied) nodes/dofs MUST be a subset of the independent (retained) nodes/dofs as defined next. An arbitrary number of independent (retained) terms must also be specified. Each independent term consists of a constant coefficient (weighting factor), a node, and a list of degrees of freedom. All nodes with the same weighting factor and dof list should be grouped together. There is no constant term for this MPC type and at the present time, the Thermal Expansion coefficient is ignored. The RBE parameter is also written.
• Overclosure	Structural Thermal Coupled	Creates a TYING Type 69 MPC which is used for creating gaps or overlaps between two parts of a model either by prescribing the total force on the nodes on either side of the gap/overlap or by prescribing the size of the gap/overlap. This is typically used for pretensioning of bolts or rivets. Dependent terms contain one node each and independent terms contain two nodes each. Each dependent (tied) term consists of a node on one side of the gap/overlap. The first node of the independent (retained) term consist of the corresponding node on the other side of the gap/overlap. The second node of the independent term is a control node to which LBCs may be applied. Each independent term must have the same control node otherwise an error is issued. There must be the same number of independent vs dependent terms also, otherwise an error is issued. The control node should not be associated to any elements. In non-mechanical passes, this MPC reduces to a Type 100 between the dependent and first independent term internally to MSC.Marc.

Cyclic Symmetry

This form appears when Cyclic Symmetry is the selected Type. Use this form to create the TYING Type 100 keyword option. The dependent (or tied) node IDs are entered in the 2nd field of the 3rd data block, and the independent (or retained) node IDs are placed on the 3a datablock.

Cyclic symmetry in Marc is generally performed with the CYCLIC SYMMETRY option rather than through MPC definitions. See Cyclic Symmetry.

Sliding Surface

This form appears when Sliding Surface is the selected Type. Use this form to create the SERVO LINK keyword option. This MPC ties the normal to the surface degrees-of-freedom between matched nodes on opposite sides of the interface. The dependent and independent node IDs are entered on the second card of the SERVO LINK option.