Shell and Solid Simulation in Autodesk® Inventor®
Professional
Shekar Subrahmanyam – Autodesk Inc
MA-2691
Thin solid models found in electronic and sheet-metal components are idealized as shell surfaces, which
improves meshing and performance. This class will cover how one can use the automated and
interactive workflows to detect thin components to idealize them to shells such as midsurface or offset
surfaces. Preprocessing steps such as setting up loads, constraints contacts for shells, and connectors
to handle shell gaps will aid in generating meaningful results. We will provide tips and tricks and discuss
result generation for mixed dimensionality models. Shell and solid simulation delivers a unified workflow
and drastic performance gains.
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
About the Speaker
Shekar has worked at Autodesk for the past fifteen years leading key projects. He has seven patents in
CAD and solid modeling. Currently he is the Tech Lead for Inventor Simulation in Design, LifeCycle &
Simulation group at Autodesk. He is one of the authors of the book, Mastering Autodesk Inventor 2009.
He earned his Bachelors, Masters and Doctorate degrees in mechanical engineering. In 2008, he
attended the Advanced Certificate for Executives Program at MIT.
shekar.sub@autodesk.com
Shell and Solid Simulation in Autodesk® Inventor® Professional
Introduction
In CAE analysis, using solid elements for a part or assembly, where the length and width of the
bodies are too large compared to the thickness (thin parts) will lead to performance and accuracy
problems. Example: Sheet-metal and electronic components. Solid meshing becomes much more
challenging for thin bodies. As the mesh element size reduces it leads to large number of mesh elements
and decreased mesh quality. To get correct and faster results, it is always prudent to choose the right
strategy for thin parts. Autodesk Inventor Simulation offers interactive and automatic tools for finding thin
parts and generating shells. Many realistic assemblies have a mix of thin and thick (bulky) parts. While
solids are not good for thin parts, on the same token, shells are not good for thick parts. Ultimately care
has to be taken to identify the thin parts and shell them and ensure the continuity of the deformation and
rotation are enforced at the contact boundaries between the shell and other shells and solid parts.
Shell Creation Tools
Midsurface command
Input Solid
Shell: Output Surface
Midsurface: For a given input solid, an output surface is generated. Loads and constraints are directly
applied to such surfaces.
The Find Thin Bodies command in the Prepare Panel of Stress Analysis environment, automatically finds
thin bodies in parts/assemblies. Thickness is extracted from Sheet Metal (SM) styles. For non-SM parts,
average thickness calculated. This thickness value is stored with the shell to be sent to the solver for
contact generation and analysis.
Offset command
Input Solid
Shell: Output Surface
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Shell and Solid Simulation in Autodesk® Inventor® Professional
Offset: An offset feature is recommended to be used when midsurface fails or produces an unacceptable
(too many gaps) result. This command works very similar to the Thicken/Offset feature in the part
modeling environment. User picks contiguous faces from the input solid body and enters a thickness and
the offset direction always goes inside the input solid body. While midsurface can result in gaps due to
non-uniform thickness at some junctions, offset does not produce gaps. The above tools do not have
additional command inputs like tolerance settings, direction, since they have a specific purpose of
generating shells for CAE analysis.
Thickness threshold is based on the Length to Thickness ratio.

L
T
ratio = Length /Thickness
Length = Overall length of the input solid body
Thickness = 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
Based on the values of these ratios certain recommendations are made to users. The good news is that
these warnings can be always overridden.
For midsurfaces, the average thickness is computed for all the face pairs involved to generate the
midsurface. For offset, the user-entered thickness value is used for contacts and result generation.
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Shell and Solid Simulation in Autodesk® Inventor® Professional
Visibility of shells can be controlled through RMB menu on the shell in the browser. Hiding the shell does
not bring back the solid on screen. Users need to delete the shell to see the input solid.
All shells appear under the Shells node with RMB access for Edit, Visibility and Delete commands. Any
connectors of midsurfaces are shown under Midsurfaces in the browser. Offsets do not need Connectors
since they always produce contiguous shells.
Shell loads & constraints
Loads
Force can be applied to Face, Edge or Vertex. Pressure can be only applied to faces. Userinterface logic ensures a valid pick.
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Shell and Solid Simulation in Autodesk® Inventor® Professional
Constraints
Fixed constraint can be applied to Face, Edge or Vertex. Each node in shell has 6 DOF. The key
difference between constraints on solids is that rotation is exposed in addition to displacement. Ux, Uy,
Uz: Displacement along X,Y,Z axis. Rx, Ry, Rz: Rotation along X,Y,Z axis. Pin constraint can be applied
only to cylindrical faces and circular edges. Frictionless constraints can be applied to face or edge.
When a solid is shelled, the loads and constraints(L&C) on the solid go sick. Users have to Edit
the load/constraint and reroute(pick entities on the shell) it to the shell OR delete them if they wish. When
the shell is deleted the L&C on shells can be rerouted to the solid. Users apply loads and constraints and
local mesh control directly on the shell.
Shell Contacts & Mesh
Shell gaps
F-F
F-E
E-E
Gaps within shells are bridged using system-generated contacts called connectors. Face-Face
(F-F), Face-Edge (F-E or E-F), Edge-Edge (E-E) connectors are generated for shells.They are listed
under the Midsurface in the browser versus under the Contacts node. A connector is a special version of
a contact within the shell body. i.e. system-generated contact which can be suppressed.
The following contact tolerances are of interest
1. Global contact tolerance (length) used for contact types: Solid-Solid, Solid-Shell, Shell-Shell
2. Shell connector tolerance (unit less ratio): This handle gaps in midsurfaces.
Gap tolerance x midsurface shell thickness, is used to detect shell gap connectors.
Users can control the following parameters for shell meshing.
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Shell and Solid Simulation in Autodesk® Inventor® Professional
•
Average element size in Shells: This UI option influences the meshes for shell. There is a
separate UI option called Average element size for solids.
•
The following solid mesh options are also supported for shells
•
Minimum element size
•
Grading factor
•
Maximum Turn Angle
•
Local mesh control
Shell Results
Stress and Strain are calculated on both sides of the surfaces and plotted on top and bottom.
Both rotation and displacement are used to represent the deformation of the shell element. Probes help in
getting actual values on either side of the shell.
Shells have the following limitations:

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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Shell and Solid Simulation in Autodesk® Inventor® Professional
Numerical results validation has been conducted with NAFEMS and other test cases to ensure the results
produced by Autodesk Inventor Professional are comparable to the industry benchmarks. The shell
results are also close to the solid’s results as seen in the chart.
Shells have support for reaction forces and moments, convergence, copy simulation, probes, animate,
etc.
Mixed models
A seamless integrated environment is provided to analyze both solids and shells. The thickness value
(computed or offset thickness) is used to find contacts between shells with solids and shells. In a single
simulation, the input solid body can be analyzed as a solid or shell (midsurface or offset). New
simulations can be created to look at “what-if” scenarios with solid or shell(midsurface or offset).
A Parametric Dimension simulation can be created where Inventor parameters can be varied including
sheet-metal thickness or thickness of the thin solid’s extrusion for example.
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Shell and Solid Simulation in Autodesk® Inventor® Professional
Shell model:Long plate.ipt
Mixed model: Pipeline.iam
Performance numbers on thin parts demonstrates that there are drastic gains to be expected. On a
1200x1200x1200, 1 inch thick plate, it takes 3 sec using shell versus using 31 minutes using solids. For
the Pipeline.iam it takes 30 seconds with shell mixed model as compared to 240 seconds using solids.
Summary

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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