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Shell and Solid Simulation in Autodesk® Inventor® Professional

Shekar SubrahmanyamTech Lead Design Lifecycle & Simulation Autodesk Inc

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Class Summary

Introduction Shell creation tools Shell loads & constraints Shell contacts & mesh Shell results Mixed models Summary

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Learning Objectives

At the end of this class, you will be able to: use tools to generate shells for thin solids add loads, constraints on shells add contact tolerances and contacts analyze results on shells simulate mixed models

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Introduction

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Solid FEA

• Pros: Easy workflow

• Cons: Analyzing shells/beams as solids has the following issues:

• Meshing: small element size leads to large number of elements

• Performance• Accuracy Solid FEA fails for

thin parts

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Shell FEA

Approximation/idealization of a solid into a surface

Thick parts cannot be shelled

Pros: Produces good physical results to simulate mechanical behavior

Cons: Approximation and model preparation

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Mixed model simulation

Solids: Thick or bulky

Shells: Thin sheetmetal, electronic components

Beams*: Frames, Trusses, Springs

Mix of these is mixed model simulation

* Not covered in this presentation

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Shell creation tools

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1. Midsurface Extraction - Interactive

Feature: Midsurface command

Thin solid bodies are candidates for shell

Pick n Bodies n Midsurfaces

MidsurfaceSolid

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Midsurface gaps

Not contiguous

Alternatives Simplify the geometry

Modify the design slightly if possible

Connectors

Offset

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Thickness calculation for Midsurface

Input thickness for Midsurface generation = SM thickness OR (3 * Solid’s Volume ) / Solid’s surface area

Shape Manager returns thickness of each face pair (TOP and BOTTOM faces)

Body thickness = Average of face pair thicknesses

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Solid or Shell?

Thickness threshold is based on input solid body’s volume and area ratio

Length(L) = Overall length of the input solid body Thickness(T) = Thickness of the input solid body

L/T ratio Interpretation Recommendation* for analysisL/T < 100 Body is thick SolidL/T >= 100 && L/T<= 250 Body may be thin Shell or Solid at your own risk. Thin wall modelL/T > 250 && L/T <= 750 Body is thin ShellL/T >750 Body is too thin Certainly Shell

*Specific details on how the L/T ratio is computed and compared are not covered here

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2. Midsurface Extraction - Automated

Find thin bodies command

Detects thin bodies using ratio

Recommendation: Try this first!

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3. Offset

Supplement for midsurface

Contiguous faces as input Offset has no gaps

Enter Thickness. Distance is calculated

Offset direction is always inside

Preview provided

OffsetSolid

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Warnings

Thick solids being treated as thin Thin solid being treated as thick

Good News: Can be over-ridden !!!

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Shell browser

Separate browser node

Edit, Visibility, Delete

Find in Window

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Shell and Solid Visibility

Generation of shell hides the input solid Delete/Hide solid not needed

Shells can be hidden using Visibility command RMB command on Shells better result viewing Shift-select works on multiple shells

Shell visibility does not affect solid’s visibility

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Shell Other

Materials once assigned to solids are transferred to shells

Shells are only relevant in SA environment. Hidden in canvas/browser of native parts, assemblies, drawings and presentation environment

Part: Go below the EOP Assemblies: Suppressed which brings back the solid

Can’t export shells to neutral formats

After all design changes are posted to the models, shells generated in the end

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Shell loads & constraints

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Shell Loads

Loads• Force

Face, Edge, Vertex• Pressure

Face• Moment

Face, Edge• Body Force & Gravity

Face, Body• Remote Force

Face, Edge Bearing Load

Face

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Shell Constraints

Constraints• Fixed constraint

Face, Edge, Vertex• Each node in shell: 6 DOF

Ux, Uy, Uz: Displacement along X,Y,Z axis Rx, Ry, Rz: Rotation along X,Y,Z axis

Pin constraint Cylindrical faces/Circular edges

• Frictionless constraint• Face, Edge

• Note:If you use a pin constraint do not also put a bonded contact

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Loads and Constraints (L&C) transfer

Solid Shell OR Shell Delete Solid L&C go sick

Users can Edit the L&C on solids and reroute it to the shell/solid Delete or suppress

No carryover of L&C from one to another

Note: Solve is not affected on shell while sick loads exist on solid or vice-versa.

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Shell contacts & mesh

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Shell Gaps

Closing gap is not mandatory, but desirable

Junction with non-uniform thickness produces gap

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Connector – Key Points

System-generated contact

Bridge gaps within a shell

Bonded type

Shell-connector tolerance value is used

Listed under Midsurface in browser

Suppress available, no Edit, Delete etc

Contacts are listed here

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Shell Contacts(Shell-Shell)

Contacts between faces and edges• Face-Face contact

• Face-Edge contact

• Edge-Edge contact

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Contact tolerances

1. Global contact tolerance (length) used for contact types• Solid-Solid• Solid-Shell• Shell-Shell

2. Shell connector tolerance (unit less ratio) • Handle gaps in midsurfaces• Gap tolerance x midsurface shell thickness, is used to detect connectors

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Shell mesh

2D mesh• Users can control the parameters for meshing

New: Average element size in Shells

All other options supported Minimum element size Grading factor Maximum Turn Angle Local mesh control

Note: Fix meshing issues at source

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Shell mesh - Tips

Reducing element size not answerTry coarse size first Fix meshing issues at source

Solid bodies that check fineExclusionsNode count will go down automatically

64-bit machine with 8GB or higher. 3-4 million nodes

Fresh reboot clears memory

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Shell results

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Results

Rotation• Difference from solid analysis• Both displacement and rotation are used to represent the deformation of shell element.

Results on top and bottom surfaces• A shell has top and bottom surfaces. Stress and strain on both surfaces are computed and shown.

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Results

Home View Home View - flipped

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Shell problems

bending approximation through the thickness needs high order elements or multi-layer mesh

potential large deformation and buckling effects in some cases

cannot be used for thick/bulky parts or capture stress concentration due to 3D localized features like fillets/chamfers, etc.

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Numerical Results Validation

NAFEMS: Testcase LE1 Cylindrical Membrane

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Numerical Results Validation

Several cases were compared with solid and NAFEMS results

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Shell Miscellaneous

Supported Reaction Forces and Moments Convergence Parametric Optimization of Inventor parameters allowed.

Tip: Ensure thickness is a parameter. Use Show Thickness command on RMB of shell. Copy Simulation Probes Animate

Not supported DS Load transfer Export to Autodesk Mechanical

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Mixed Models

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Mixed Models

Seamless integrated environment for solids, shells

Body is common denominator Parts: multi-bodies with some bodies being shelled Assemblies: instances where some bodies are shelled

A body can be either midsurfaced/offsetted. In a single simulation, body can be analyzed as solid or shell (midsurface or offset).

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Mixed Models - Contacts

Add contacts between Shell-Shell Shell-Solid

Thickness used to detect contacts

t

tShell

Solid

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Contact- Tips

Easy to forget applying contacts. No contacts results in large deformation

Count how many unique interactions are there between the components and ensure contacts are created

Start simple (large models with all bonded contacts) and ensure everything is solvable and then add contacts in a step-wise fashion

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Mixed Model examples

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Mixed Models

Exclusions are normally applied before shell idealizations

Shells are associative: Since they are part and assembly features

Contacts are automatically generated. Some manual contacts needed.

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Demo

Pipeline.iam

SheetMetal_Enclosure.ipt

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Performance

Model Name Model Number of solid elements

Number of shell elements

Solid Simulation

(secs)

Shell/Mixed model

simulation (secs)

Performance Improvement

1200x1200x1200x1 inch Plate

781724 11858 1860 3 Large

Pipeline.iam 164948 36736 240 30 800%

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Summary

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Conclusions

Better simulation coverage with shells

Shells unify FEA modeling and analysis into a seamless user & computational workflow

Drastic performance gains expected for some classes of models analyzed with shells

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Customer Quotes

The better performance and functionality for FEA on sheet metal (thin-walled) parts, which is the solution we always expected.

Jiandong Guo, Manager of Engineering Department

The shell feature seems to be a specially-tailored function for us.   Technical Department, Manager of Leggett & Platt company

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