ate-shells-14218.pdf

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© 2011 ANSYS, Inc. July 3, 20141

Using Shell Elements in

ANSYS Mechanical

ANSYS Technical Support


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© 2011 ANSYS, Inc. July 3, 20142

Agenda

• Background

• Element Technology

– Solids

– Shells

– Solid-Shell• Geometry Modification Options

– Mid Surface Extraction

– Surface Extension

– Misc. (Virtual Topology, Repair, …)

• Shell Connections Methods

• Demo


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© 2011 ANSYS, Inc. July 3, 20143

Background

The analysis of thin and slender structures can

benefit from computationally efficient shell

modeling.

Thin structures:


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© 2011 ANSYS, Inc. July 3, 20144

• Element topologies: shells, beams,

solid shells, connections (MPC

contact, springs, dampers, joints,

spotwelds…).

• Mid-surfacing Technology enables

3D solids to be modified so that

they can be meshed with shellelements.

Thin Structures:

Background


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© 2011 ANSYS, Inc. July 3, 20145

Conventional 3D Solid 185/6/7 elements

Element Technology

• Geometrically & spatially 3D

• Three displacement degrees of

freedom at each node

• Supported by Enhanced StrainTechnology

• Suitable for modeling 3-D bulk solid

structures

• Not typically suitable for thin

structures in bending


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© 2011 ANSYS, Inc. July 3, 20146

Conventional 3D Shell181/281 elements

• Geometrically 2D, but spatially 3D

• Six degrees of freedom (three in

translation, three in rotation) at each

node.

• Suitable for modeling 3-D thin to

moderately-thick shell structures.

Element Technology


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Thin Shells (Love/Kirchhoff)

– The cross-sectional plane initially

normal to the midsurface of the shell is assumed to

remain straight and normal to the neutral axis

during loading. This assumption excludes shear

deformations.

Moderately-Thick Shells (Mindlin/Reissner)

– The cross-sectional plane initially

normal to the midsurface of the shell is assumed to

remain straight but not remain normal to theneutral axis during loading. The shear strain, as a

result, is constant across the section.

Element Technology


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3D Solid-Shell Element (SOLSH190)

Element Technology

• Geometrically and spatially 3D

• Three translational DOF at each node.

• Has 7 internal DOF, similar to Enhanced Strain but

decoupled in bending direction.

─ condensed out at the element level

• Performs well in simulating plate structures with a wide range of

thicknesses (from extremely thin to moderate thick).


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Solid185 with enhanced strain and Solid186 show transverse shear locking, when

the structure is really thin, e.g L/t =1000

L/t = 1000, transverse shear stress

Umax=0.035, but should be 1

Solid185 Solid186

Umax=0.83, but should be 1

L/t = 1000, transverse shear stress

Element Technology


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Thin wall structure – with Solids

Remedy:

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Number of Elements Per Edge

N o r m a l i z e d M a x .

D e f l e c t i o n

Solid185 (enhanced strain)

1. Increase the mesh density (very expensive)

2. Use midside node Solid elements (expensive)

Umax=1

Umax=1

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D e f l e c t i o n

Element Technology


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Thin wall structure – with Shells

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D e f l e c t i o n

3. Use Shell elements - needs the mid-surface

transverse shear stress Umax=1

4. Use Solid-Shell elements - needs a swept mesh

transverse shear stress Umax=1 00.1

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D e f l e c t i o n

Element Technology


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Shell vs Solid vs Solid-Shell

• Simple example of buckling of arch

shown on right

– Calculation of load required to inducebuckling

– Comparison of SHELL181, SOLID185and SOLSH190

• For thin structures, SOLSH190

matches SHELL181

– SOLID185 requires additional elements

along edge

• For thick structures, SOLSH190matches SOLID185

3rd mode thick

elem/thick elem/edge 1.00E-03 1.00E-02 1.00E-01 1.00E+00 2.00E+00

SHELL181 1 10 3.7496 3750 3.74E+06 3.09E+09 1.64E+1020 3.4509 3451 3.44E+06 2.89E+09 1.57E+10

50 3.3743 3374 3.37E+06 2.84E+09 1.55E+10

SOLID185 1 10 3533.8000 39403 4.31E+06 3.55E+09 2.23E+10

20 50.9320 4096 3.48E+06 3.23E+09 2.07E+10

50 3.7035 3386 3.38E+06 3.14E+09 2.04E+10

3 10 3534.0000 39403 4.31E+06 3.49E+09 2.13E+10

20 50.8300 4096 3.48E+06 3.18E+09 1.99E+10

50 3.6230 3386 3.38E+06 3.10E+09 1.96E+10

5 10 3533.8000 39403 4.31E+06 3.45E+09 2.04E+10

20 50.9040 4096 3.48E+06 3.14E+09 1.91E+10

50 3.6708 3386 3.37E+06 3.07E+09 1.88E+10

SOLSH190 1 10 3.7232 3722 3.72E+06 3.40E+09 2.23E+10

20 3.4530 3445 3.44E+06 3.17E+09 2.07E+10

50 3.3751 3373 3.37E+06 3.11E+09 2.04E+10

3 10 3.6055 3722 3.72E+06 3.37E+09 2.13E+10

20 3.4384 3445 3.44E+06 3.15E+09 1.99E+10

50 3.3764 3373 3.37E+06 3.09E+09 1.96E+10

5 10 3.4980 3722 3.72E+06 3.33E+09 2.04E+10

20 3.4201 3445 3.44E+06 3.12E+09 1.91E+10

50 3.2714 3373 3.37E+06 3.06E+09 1.88E+10

Element Technology


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Thin wall structure – options

3. Solid-Shell:

- needs to be sweep meshed

2. Surface Body mesh (Shells):

- might need mid-surface tool

and shell mesher

1. Solid Body mesh:

- needs a robust solver

for very large models

Element Technology


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© 2011 ANSYS, Inc. July 3, 201414

How do we analyze thin structures effectively?

Assuming the structure is too

large and complex to bemeshed effectively with

conventional solid elements ,

there are two options

available

• Shells with mid-plane

extraction (right)

• Solid-Shells (Left)


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© 2011 ANSYS, Inc. July 3, 201415

Only Surface Bodies can be meshed with SHELL elements

Surface Bodies can be generated by multiple methods:

• From Edges

• From Sketches

• From Faces

Defining Surface Bodies

Planar

surface

Non-planarsurface

Existing solid body edges

are selected for new

surface boundary.

New, frozen, surface body generated(note, solid body is hidden).


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© 2011 ANSYS, Inc. July 3, 201417

Mid-Surface ToolManual Method

– Select pair of faces, one pair at a time, in FacePairs input

– The order of selection determines the surface

normal directionDetails View of Mid-Surface

Color of faces after selectionSolid Model Color of faces during selection

Normal

Direction

1st Pick

2nd Pick


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© 2011 ANSYS, Inc. July 3, 201418

Mid-Surface Tool

Automatic Method (many control options)

• Bodies to search: Limits search to VisibleBodies, Selected Bodies, or All Bodies

• Minimum and maximum threshold sets search

range (thickness) for face pairs

•

Selection Tolerance: Allowable deviation fromperfect set

• Thickness Tolerance: Allows grouping ofmultiple pairs into one set within this value.

• Sewing Tolerance: max allowable gap between

surfaces

• Extra Trimming provides options for removing unwanted face pairs

• Preserve Bodies? allows you to save the solid bodies from which

surfaces are created (default is No)


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© 2011 ANSYS, Inc. July 3, 201419

Gap

Mid-Surface ToolTolerances

Sewing Tolerance:

• If gaps exist in adjacent face pairs, they will be sewn

together within the Sewing Tolerance

• If Gap < Sewing Tolerance, Surfaces are grouped and

connected (Conformal Mesh between them)

Selection Tolerance:

• Tolerance is used to detect face pairs in case of imperfect

offsets

Without Selection

Tolerance Selection tolerance value is

suggested to user

All the pairs detected

successfully with

Selection tolerance


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© 2011 ANSYS, Inc. July 3, 201420

Joint ToolJoint

Joins surface bodies together such that their contact

regions share common edge. Prerequisite for conformalmeshes.

Takes two or more surface bodies as input

Imprints edges on all bodies where they make contact

Surface bodies after Joint

Surface bodies to be Joined

Details View of Joint


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© 2011 ANSYS, Inc. July 3, 201422

Repair holes

Repair Holes Tool


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© 2011 ANSYS, Inc. July 3, 201423

• When to use?

– To merge together a number ofsmall (connected) faces/edges

– To simplify small features in themodel

– To simplify load abstraction for

mechanical analysis – To create edge splits for better

control of the surface mesh

control

• Virtual cells modify topology

– Original model geometry remainsunchanged

– New faceted geometry is createdwith virtual topology

Virtual TopologyWithout VT With VT


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© 2011 ANSYS, Inc. July 3, 201425

• The “ideal” connection method for shells is to have

shared topology so the mesh is fully connected.

• In some instances, this is not possible/desirable, so

need to use:

• Contact

• Mesh Connections

• Spot Welds

Shell Connection Methods


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• SHELL-to-SHELL Linear Connection (Edge/Edge)

MultiBody Part:



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© 2011 ANSYS, Inc. July 3, 201427

• Types of Contact Detection available between solid and surface bodies:

– Face/Face: contact between faces of different bodies

– Face/Edge: contact between faces and edges of different bodies

– Edge/Edge: contact between edges of different bodies

• Face/Edge and Edge/Edge contact only applies to solid and surface bodies. – Contact relationships involving line bodies are not supported.



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• By default only Face/Face contact is considered

Global control of all connections Local control of grouped connections

Face/Edge and/or Edge/Edge contactoptions should be turned to Yes



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• SHELL Connectivity

• SHELL-to-SHELL Linear Connection (Edge/Edge)

Edge-to-Edge Contact :

Every formulation available



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• SHELL Connectivity

•SHELL-to-SHELL Linear Connection (Edge/Edge)

The default, “Target Normal, Couple U to ROT” , creates too many constraints, causing

an artificial stiffness at the connection and resulting in a discontinuity of stress

and strain distribution that should not be there



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© 2011 ANSYS, Inc. July 3, 201431

“Target Normal, Uncouple U to ROT” creates CEs that separate the rotational and

displacement DOFs into separate equations….

SHELL Connectivity

SHELL-to-SHELL Linear Connection (Edge/Edge)



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- “Target Normal, Uncouple U to ROT” produces expected results

… to remove artificial stiffness at the connection,

thus improving results for special applications.

SHELL Connectivity

SHELL-to-SHELL Linear Connection (Edge/Edge)



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© 2011 ANSYS, Inc. July 3, 201433

Mesh connection : Allows you to join the meshes of topologically disconnected

surface bodies:

– Previously connections such as this required a geometry application to repairgaps (e.g. DesignModeler or CAD).

– Mesh connections are made at the mesh level using either edge to edge or edgeto face configurations.

– Unlike geometry solutions, a multibody part is not required.

Mesh Connection

Example



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• Mesh connections use the concept of master and slave geometry to control

how the connection is made: – Master: indicates the geometry/topology onto which other geometry is projected.

– Slave: indicates the geometry that will be projected onto the master geometry.

– Master geometry can be faces or edges whereas slave geometry can only be edges.

Slave

MasterProjection



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When to Use Mesh Connections:

• When there are disconnects in a surface model, the mesh connection feature

allows the model to be joined via a conformal mesh.

• Mid surfacing often results in situations where gaps exist in a surface model.

Mesh connections are especially useful in these situations.



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Mesh connections work at

part level:

• As a post mesh operation

• Base part mesh is stored to

allow for quick changes in

connections



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• For further information on ANSYS software please contactyour local representative

(www.ansys.com/About+ANSYS/Contacts+and+Locations)

• or email [email protected]

• Forthcoming webinars - www1.ansys.com/customer/webinars

•

Thank you for your interest today

Thank you!
http://www.ansys.com/About+ANSYS/Contacts+and+Locationsmailto:[email protected]:[email protected]://www.ansys.com/About+ANSYS/Contacts+and+Locations

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