© 2012 autodesk shell and solid simulation in autodesk® inventor® professional shekar...
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© 2012 Autodesk
Shell and Solid Simulation in Autodesk® Inventor® Professional
Shekar SubrahmanyamTech Lead Design Lifecycle & Simulation Autodesk Inc
© 2012 Autodesk
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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