How to Fix solid elements and shell elements

[정민찬]

Hello, I'm M.S. candidate student in korea.

I want to simulate the sled test, we created a shell to act as the sled.

However, there are difficulties in fixing the lower part of the chair (solid element) to the shell.

How can I combine the chair subnodes with the shell element?

Regards

[James Kennedy]

Dear Student,

 

Something to consider.

 

Tied contacts in LS-DYNA, defined using *CONTACT with options TIED or TIEBREAK,

provide a convenient way of treating interaction between disjoint meshes as shown in Figure

below. Tied contacts are one-way contacts in which the slave node is TIED to the closest

master segment. The "closest" segment to a given slave node is based on the nodal projection

distance in which the node that as a segment that yields the smallest projected distance along

its normal is considered "closest". Once a closest node is located, the node is TIED and remains

TIED for the duration of the simulation or until a certain failure condition is reached. Examples

of industrial use of TIED contacts include but not limited to spotwelds to BIW, Trim ribs to

Trim surface, seat cushion to seat pan, structural bumper foams to bumper, etc.

 

Selecting Sides of Contact Interface for TIED_SURFACE_TO_SURFACE. For *CONTACT_

TIED_SURFACE_TO_SURFACE, we generally recommend making SURFA the more finely

meshed part. This recommendation has exceptions. For instance, if the coarsely meshed SURFB

side deforms, then the finer SURFA side will deform with high stress localizations in the folding

lines.

 

Recommendations for TIED contact types:

Using the previous Table as a general, comparative guideline and remembering that TIED_

NODES_TO_SURFACE and TIED_SURFACE_TO_SURFACE are the same contact

algorithm, only with a different form of data input, it would appear that either of these

options would be a good basic choice if the simulation has minimal or no moment due to

offset or conflicts with the constraint based formulation. If rigid bodies come into play, this

would dictate the need for changing to a penalty based formulation. The OFFSET option

would appear to handle this nicely.

 

If the retention of the offset and the moment due to the offset is needed and/or required, the

TIED_SHELL_EDGE_TO_SURFACE_CONSTRAINED_OFFSET would be a good choice

if constraint based formulation is acceptable in the simulation. With rigid bodies (as long as an

accurate mass of the rigid body is made) or other constraint conflicts exist, the penalty based

TIED_SHELL_EDGE_TO_SURFACE_BEAM_OFFSET would be a good choice. An

alternative to this penalty based formulation, once failure feature has been removed, would be

the AUTOMATIC_SURFACE_TO_SURFACE_TIEBREAK (option=-2) option. The rigid

body mass requirement appears to not be of concern with this option.

 

For tying solids-to-solids, that is, for situations where none of the nodes have rotational degrees-

of-freedom, use TIED_NODES_TO_SURFACE and TIED_SURFACE_TO_ SURFACE type

contacts. These contact types may include the OFFSET or CONSTRAINED_OFFSET option.

 

For tying shells-to-shells, beams-to-shells, that is, for situations where all the nodes have rotational

degrees-of-freedom, use TIED_SHELL_EDGE_TO_SURFACE type contacts. This contact type

may include the OFFSET, BEAM_OFFSET, or CONSTRAINED_ OFFSET option.

 

TIED_SHELL_EDGE_TO_SOLID is intended for tying shell edges to solids or beam ends to

solids, that is, situations where only the slave side nodes have rotational degrees-of-freedom.

 

Cantilever models illustrate three good alternatives for constraining shells to solids and one bad one.

The three good approaches are CONSTRAINED_SHELL_TO_SOLID, CONSTRAINED_NODAL_

RIGID_BODY, and CONSTRAINED_INTERPOLATION. The CONSTRAINED_SHELL_TO_

SOLID approach maintains rotational continuity. The bad method employs TIED_SHELL_EDGE_

TO_SURFACE. The main problem here is using solids which do not have rotational DOF and so the

tied contact cannot enforce rotational continuity (in practice works like a hinge). The recently added

option of TIED_SHELL_EDGE_TO_SOLID contact appears to offer a fourth good alternative for

constraining shells to solids in that shell moments are transmittedTIED_SURFACE_TO_SURFACE

are not recommended to be connected to structural nodes, i.e., nodes with rotational degrees-of-

freedom.

 

Sincerely,

James M. Kennedy

KBS2 Inc.

April 23, 2023

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