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Shell and Solid Simulation in Autodesk® Inventor® Professional
Shekar Subrahmanyam Tech 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 fails for thin parts
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 Deteriorates quality of mesh hence leads to inaccurate results 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 Solid Midsurface
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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 L T ratio Length(L) = Overall length of the input solid body Thickness(T) = Thickness of the input solid body L/T ratio Interpretation Recommendation* for analysis L/T < 100 Body is thick Solid L/T >= 100 && L/T<= 250 Body may be thin Shell or Solid at your own risk. Thin wall model L/T > 250 && L/T <= 750 Body is thin Shell L/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 L T 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 the ability for a user to manually nominate solid faces to establish a 2D shell for analysis May generate better result than mid-surface Solid Offset
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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
Body Force & Gravity Face, Body Remote Force Bearing Load
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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 Closing gap is not mandatory, but desirable
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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 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 answer
Try coarse size first Fix meshing issues at source Solid bodies that check fine Exclusions Node 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 Results on top and bottom surfaces
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 Not 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 Solid t/2 Shell t/2
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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 Performance Improvement 1200x1200x1200x1 inch Plate 781724 11858 1860 3 Large Pipeline.iam 164948 36736 240 30 800% Performance data generated on Intel Xeon X5650, 24 CPUs, 12 GB memory, Windows 7,
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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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Autodesk, AutoCAD* [*if/when mentioned in the pertinent material, followed by an alphabetical list of all other trademarks mentioned in the material] are registered trademarks or trademarks of Autodesk, Inc., and/or its subsidiaries and/or affiliates in the USA and/or other countries. All other brand names, product names, or trademarks belong to their respective holders. Autodesk reserves the right to alter product and services offerings, and specifications and pricing at any time without notice, and is not responsible for typographical or graphical errors that may appear in this document. © 2012 Autodesk, Inc. All rights reserved.
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