1. Shell and Solid Simulation in Autodesk® Inventor® Professional
Shekar Subrahmanyam
Tech Lead
Design Lifecycle & Simulation Autodesk Inc

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

3. 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

4. Introduction

5. 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

6. 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

7. 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

8. Shell creation tools

9. 1. Midsurface Extraction - Interactive
Feature: Midsurface command
Thin solid bodies are candidates for shell
Pick n Bodies n Midsurfaces
Solid
Midsurface

10. Midsurface gaps Not contiguous Alternatives Simplify the geometry
Modify the design slightly if possible
Connectors
Offset

11. 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

12. 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

13. 2. Midsurface Extraction - Automated
Find thin bodies command
Detects thin bodies
using L T ratio
Recommendation: Try this first!

14. 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

15. Warnings Thick solids being treated as thin
Thin solid being treated as thick
Good News: Can be over-ridden !!!

16. Shell browser Separate browser node Edit, Visibility, Delete
Find in Window

17. 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

18. 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

19. Shell loads & constraints

20. Shell Loads Loads Force Face, Edge, Vertex Pressure Face Moment
Body Force & Gravity
Face, Body
Remote Force
Bearing Load

21. 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

22. 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.

23. Shell contacts & mesh

24. Shell Gaps Closing gap is not mandatory, but desirable
Junction with non-uniform thickness produces gap
Closing gap is not mandatory, but desirable

25. 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

26. Shell Contacts(Shell-Shell)
Contacts between faces and edges
Face-Face contact
Face-Edge contact
Edge-Edge contact

27. 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

28. 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

29. 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

30. Shell results

31. 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.

32. Results
Home View
Home View - flipped

33. 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.

34. Numerical Results Validation
NAFEMS: Testcase LE1 Cylindrical Membrane

35. Numerical Results Validation
Several cases were compared with solid and NAFEMS results

36. 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

37. Mixed Models

38. 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).

39. Mixed Models - Contacts
Add contacts between
Shell-Shell
Shell-Solid
Thickness used to detect contacts
Solid
t/2
Shell
t/2

40. 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

41. Mixed Model examples

42. 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.

43. Demo
Pipeline.iam
SheetMetal_Enclosure.ipt

44. 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,

45. Summary

46. 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

47. 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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