Shell and Solid Simulation in Autodesk Inventor Professional

Shell and Solid Simulation in Autodesk® Inventor® Professional

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.

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 andother shells and solid parts.

Shell Creation Tools

Midsurface command Input SolidShell: 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

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.

• 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.

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. User-interface logic ensures a valid pick.

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 gapsF-FF-EE-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.

•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 followingsolid 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.

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.

Shell model:Long plate.iptMixed 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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