understanding and applying fea - autodesk platinum …ketiv.com/pdf/ama2008/understanding and...

Understanding and Applying FEA

Fred Redell

Course Summary: This session begins with an overview of Finite Element Analysis (FEA) as a method of testing and validating a design under a given set of boundary conditions and forces. You will learn how to apply FEA using Inventor to avoid physical testing of prototypes. We’ll also examine workflows to solving real-world engineering problems. Instructor: Fred Redell President – Redell Engineering, Inc. Fred is a registered PE, and has an MSME from UCSD with specialization in combustion, fluid flow, and numerical analysis. Fred is the owner of Redell Engineering, Inc., a mechanical engineering company focused on unique designs requiring advanced analysis. Using Inventor’s built in ANSYS software, he converges on design solutions that help optimize mass and size for customers.


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FEA Origins ......................................................................................... 3 What is FEA ........................................................................................ 3

The Basics ................................................................................................................... 3 Computerized FEA ...................................................................................................... 4 Verification of Results .................................................................................................. 5

FEA in Inventor ................................................................................... 5 Assumptions ................................................................................................................ 6 Important Terms .......................................................................................................... 6

Loads .................................................................................................................................... 6 Constraints ............................................................................................................................ 6

Typical Workflow .......................................................................................................... 7 Dynamic Simulation ..................................................................................................... 7 Sample Exercises ........................................................................................................ 8

Exercise 1: Shelf Bracket ...................................................................................................... 8 Exercise 2: Bracket Assembly ............................................................................................. 11

Tutorials ..................................................................................................................... 16 Advanced Simulation for Inventor .............................................................................. 17

Resources ......................................................................................... 18


Understanding Finite Element Analysis (FEA) FEA Origins

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The origins of finite element methods traces back to the early 1900s but first published by Courant in 1943 to describe a continua using discrete elements. However significant steps were taken by Boeing in the 1950s for modeling aircraft wings. Developments continued in the 1960s followed by the first release of ANSYS in 1971. What is FEA The Basics Finite Element Analysis, as you might already be aware, is a numerical procedure that can be used to obtain solutions to a large class of engineering problems including stress analysis, heat transfer, electromagnetism, and fluid flow. Most commonly and as described here, the method is employed to determine if the strength of a component or set of components is sufficient when subjected to a given set of applied loads and boundary conditions. In the same way that a system of structural members is solved for a truss to determine the loading on a global scale, FEA can be used to determine the loading in each element and thus characterize the stress in the member. The basic steps of involved in any analysis consist of the following:

1. Create and discretize the part into a system of nodes and elements 2. Assume a shape function to represent the behavior of an element 3. Develop a system of equations by developing an equation for the elements 4. Construct the global stiffness matrix 5. Apply the boundary conditions, initial conditions, and loading. 6. Solve the system of equations using linear algebra 7. Review the results and compare results to ensure


An example of a finite element model to represent a component of varying cross-section will be analyzed during the presentation Computerized FEA Modern FEA consists largely of solid models automatically discretized and solved by sophisticated programs. An example of a structural model similar to the hand solution presented produces a number of elements, dividing the system small parts connected by faces and nodes. Autodesk Inventor Professional utilizes a particular element well suited for structural analysis or solid parts. Several types of elements are available depending upon the analysis in full versions of ANSYS and other software but are not described here and are suited for very complex modeling. In the same manner as described for the hand FEA calculation, a system of equations is developed that can be solved more rapidly by computer. Finite element models can have hundreds to millions of elements. The resulting set of equations is the same though:

K * X = or

In static FEA analyses, the force F is known from loading conditions. However dynamic problems and loading condition require much more care when determining the loading. To aid in this, dynamic simulation can be used to provide static loading conditions.

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Verification of Results With the growth of finite element analysis as a design tool easy to use computer packages become more relied upon to rapidly arrive at a solution reducing the time and cost to bring a part to market. However, all FEA results should be reviewed and the results verified as part of good engineering practice. Various sources of error contribute to incorrect results including:

• Wrong input data such as material properties • Wrong type of element used for the type of analysis (not a large concern with the

integrated tool but an example will be shown)

• Poor element shape and size resulting in inaccurate results near complex shapes. Free meshing v. mapped meshing and how this can be achieved in Inventor.

• Applying the wrong boundary conditions and loads. This is the most common

source of error and requires good engineering judgment and a practical knowledge of how the component will function.

Even with the most careful application of FEA tools it is important to find a way to check your results. For static models the most common method is by reviewing the forces and reactions; since it is a static model the sum of the forces and reactions must equal zero.

FEA in Inventor In Inventor the stress analysis environment uses FEA to simulate the behavior of a component under externally calculated loads and frequencies. It’s important to note that Inventor performs the analysis at the part level.

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Assumptions If analysis is to be done on an assembly or if one of the assumptions below is not valid then you should consider using another analysis package such ANSYS or Advanced Simulation for Inventor (preview technology available on Autodesk Labs)

• Deflection and stress are linearly proportional to the load. If you double the load, the deflection an stress double

• Material Properties are linear. The stress strain curve is a straight line, with the

stress remaining proportional to the strain. There is no yielding of the material.

• The load in is static and is applied slowly. Dynamic loading effects such as sudden load application or impact are not considered.

• Temperature has no effect on the part geometry or material properties.

• The deformation of the part is small when compared to the dimensions of the

part. Large deflection requires a nonlinear analysis to account for changing part and load geometry and is not considered in linear analysis.

• Other nonlinear effects such as buckling are not considered

Important Terms Loads A load is an external force that is exerted on a component directly or indirectly. Loads can occur in various locations on the component, and their magnitude and types can vary. Constraints Constraints are used to define how components would be fixed in a real world assembly. They can be controlled by defining the directions in the X, Y, Z axes or radial, axial, and tangential directions for cylindrical components.


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Typical Workflow

• Select material for the part. • Click Application pull down > Stress Analysis

• Click Stress Analysis Settings, and then adjust the mesh to the appropriate

fineness or coarseness. • Select the analysis style from the Analysis Type pull-down list (either Stress

Analysis or Modal Analysis. • Click appropriate load(s) on the stress analysis panel bar, select the faces,

edges, or vertices, and then enter the magnitude to apply the load. • Click appropriate constraint(s), and then select faces, edges, or vertices to apply

a constraint. This step is not necessary for Modal analysis. • Click Stress Analysis Update to start the analysis.

• Review Results in the browser. • Change the part by either adding features or suppressing problematic ones using

Suppress for Stress Analysis. Note - Using Suppress for Stress Analysis is not supported for features of Sheet Metal iParts.

• Use result convergence and resolve to ensure that the results are as accurate as

possible. Result convergence is blocked for multiple-time step support and prestressed modal results.

Dynamic Simulation Dynamic Simulation removes the dependence on educated guesses for applied forces in part FEA analysis by introducing Newton’s second law of motion into the mix. By animating an assembly, we can then extract motion loads directly into a part and run our analysis with more accurate forces applied.


Sample Exercises Before starting the exercises;

• Browse to the CD and copy the 3B Understanding and Applying FEA to your C: drive.

• Launch Inventor and go to the File pull down > Projects. • In the Project Editor click Browse and go to C:\Understanding FEA folder and

select Understanding FEA.ipj, click Open. • Click Done

Exercise 1: Shelf Bracket

1. In Inventor, Open AMA_Shelf_Bracket.ipt 2. Go to the File pull down >

iProperties > Physical Tab set the material to Steel, Mild.

3. Go to the Applications pull down > Stress Analysis.

4. Click Stress Analysis Settings and select

Stress Analysis from the Analysis Type pull-down list

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5. Apply constraints.

a. From the Stress Analysis panel bar choose and apply to face on the bracket as shown.

b. From the Stress Analysis panel bar choose and apply to face on the bracket as shown. Make sure to click the more button , enter a check next Use Vector Components, and edit the Y component to read -.25 (in).

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6. Click Stress Analysis Update to start the analysis.

7. Right click on Fixed Constraint 2 in the Stress Analysis browser and choose Reaction Forces to reveal how much force would be required displace the face down .25 in.

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Exercise 2: Bracket Assembly

8. In Inventor, Open AMA_Bracket_Assy.iam 9. Double click on Bracket:1 in the browser to activate it.

10. Go to the File pull down > iProperties > Physical Tab set the material to Steel,

High Strength Low Alloy. 11. Go to the Applications pull down > Stress

Analysis.

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12. Click Stress Analysis Settings and select Stress Analysis from the Analysis Type pull-down list

13. Apply loads.

a. From the Stress Analysis panel bar choose .

b. Rotate the model and select the blue faces as shown.

c. Click the Force direction arrow and choose the edge as shown, flip the

direction as necessary to point down and match the image.

d. Enter a magnitude of 100 (N), click OK.

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e. From the Stress Analysis panel bar choose .

f. Select the blue face as shown

g. Click the Moment direction arrow and choose the edge shown in red, flip

the direction as necessary to match the image .

h. Enter a magnitude of 100 (N mm), click OK Notice that bracket.ipt has a split face feature. This method can be used if a load should be applied to a localized area of a face

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14. Apply constraints. From the Stress Analysis panel bar choose

and apply to the red edges on the bracket as shown.

15. .Click Stress Analysis Update to start the analysis.

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16. Review the results by double clicking on each result type in the browser.


Tutorials

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If you have Inventor Professional or Inventor Simulation Suite you have access to tutorials that guide you through a few stress analysis scenarios, including some that employ Dynamic Simulation. To access these tutorials,

• Launch Inventor • Go to the Help pull-down > Learning Tools > Tutorials • Go to the Contents tab > Autodesk Inventor Analysis and Simulation > Using

Stress Analysis


Advanced Simulation for Inventor

• Simulate Inventor parts and assemblies - Users can directly read in Inventor parts and assemblies and perform stress and modal analysis on them

• Parametric simulation of Inventor models - The Advanced Simulation Technology

Preview allows users to change parameters, simulate various permutations, and compare results

• Multi-criteria optimization - Users can determine the optimal values of design

parameters to meet their objectives using optimization

• Automated tools to set up simulation model - Automated mesh refinement and contact detection allow users to quickly set up their simulation models and get usable results

Fast facts about the Advanced Simulation Technology Preview for Inventor 2009:

• Inventor Simulation Suite or Professional for Advanced Simulation Technology Preview - The technology preview requires a licensed version of Inventor Simulation Suite 2009 or Inventor Professional 2009 to be installed on the machine

• English only but installs with all languages of Inventor Simulation Suite and

Professional - The technology preview is available in English only but can be installed alongside any language version of Inventor Simulation Suite 2009 or Professional 2009

The expiration date of the licensing for the technology preview is June 1, 2009.

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Resources For more information see the following:

• Introduction to FEA 1 http://www.chalice-engineering.com/Analysis_Basics/Introduction_to_FEA1.htm

• Introduction to FEA 2 http://www.chalice-engineering.com/Analysis_Basics/Introduction_to_FEA2.htm

• Advanced Simulation Technology Preview for Autodesk Inventor http://labs.autodesk.com/technologies/advanced_simulation/

• Materials www.matweb.com.

©2008 KETIV Technologies of California, Inc. All rights reserved.

About KETIV Technologies

KETIV Technologies is a leading Autodesk solutions provider with 25 years’ experience delivering CAD software and services. KETIV’s team of industry experts increase the profitability of engineering services companies by helping them simplify the process of conceptualization, design and production. KETIV serves the manufacturing, plant and civil engineering industries. For more information, visit www.ketivtech.com or call 866.465.3848.

http://www.chalice-engineering.com/Analysis_Basics/Introduction_to_FEA1.htm

http://www.chalice-engineering.com/Analysis_Basics/Introduction_to_FEA2.htm

http://labs.autodesk.com/technologies/advanced_simulation/

http://www.matweb.com/

http://www.ketivtech.com/

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