What Is Finite Element Modeling?

What Is Finite Element Modeling?I-DEAS Tutorials: Fundamental Skills

Although typically you’d use many elements, in thistutorial you’ll simulate what happens to a single thin-shellelement in the middle of a machine linkage, under atensile load.

Learn how to:

• create a finite element model• create material properties• create physical properties• create nodes• create an element• apply boundary conditions• solve the model• interpret the results

2

Before you begin...

Prerequisite tutorials:

• Getting Started (I-DEAS Multimedia Training)

–or–

Quick Tips to Using I-DEAS–and–Creating Parts

• Introduction to Simulation

This tutorial assumes you have a background in theconcepts of stress as taught in a typical college coursein strength of materials.

3

Setup 1 of 2

If you didn’t start I-DEAS with a new (empty) model file,open a new one now and give it a unique name.

FileOpen

Open Model File form

Model File name: any unique name

OK

Make sure you’re in the following application and task:

Simulation

Master ModelerSet your units to mm.

OptionsUnits

mm (milli newton)

4

Setup 2 of 2

Save your model file. File

Save

Warning! If you are prompted by I-DEAS to save your model file,respond:

NoSave only when the tutorial instructions tell you to––notwhen I-DEAS prompts for a save.

Why:If you make a mistake at any time between saves andcan’t recover, you can reopen your model file to the lastsave and start over from that point.

Hint To reopen your model file to the previous save, pressControl-Z.

5

Create a finite element model 1 of 2

Before you start an analysis, you should think about thetype of solution you want. Which solution type youchoose depends on loading and other factors. Keyfactors include:

• how loads vary with time, or the duration of theload relative to the period of natural frequency

• whether the expected result is linear

In this example, you’ll use a linear static solution.

You start by creating a finite element (FE) model. An FEmodel is always associated with a part.

In the model file, there may be more than one finiteelement model associated with the same part, withdifferent meshes.

6

Create a finite element model 2 of 2

What: Switch to the Meshing task and create an FEmodel.

How:

Meshing

FE Model Create form

Part or Assembly: (any unique name)

FE Model Name: Tensile Model

OK

Things to notice Ignore the warning message if you get one. When youcreated the FE model above and entered the name of anonexisting part, a null part was created. A null partcontains no surfaces. It is displayed in Simulation as apart coordinate system.

7

Create material properties 1 of 3

What: Create a material for the analysis and define themodulus of elasticity and Poisson’s ratio.

How:

AlCu

Fe

Materials form

Quick Create...

Quick Create form

Material Label & Name: Link Material(enter name only, in second field)

Property: Modulus of Elasticity

2E8

Modify Value

Continued on next page...

8


Property: Poisson’s Ratio

0.3

Modify Value

OK

Do not exit from the Materials form yet.

Things to notice The value for Poisson’s ratio of 0.3 means that if thematerial is stretched one unit in the X-direction, it willshrink 30% in the Y- and Z-directions. Later in thistutorial you’ll check the displacements to see if this holdstrue.

9


What: Examine the new material to make sure youentered the correct values.

How:

Materials form

Link Material

Examine...

Examine form

Review the property values in the table.

LINK MATERIALMODULUS OF ELASTICITY

2.0000E+08POISSONS RATIO

3.L0000E–01

Dismiss

Materials form

OK

Things to notice This material definition is stored in your model file andcan be referenced by any FE model you create in thesame model file.

10

Create physical properties 1 of 3

The thin shell element you’ll create will be defined byfour nodes at its corners. The physical thickness of thinshell elements is defined by a separate physical propertytable.

Things to notice The link part is shown here only to indicate the thicknessof the element. Since you are creating nodes andelements manually, you won’t have a part displayed.

11


What: Create a physical property table that defines ashell thickness of 5mm.

How:

t

Thin Shell

Check I-DEAS Prompt .

physical property name: LinkThickness

No

Directory

Continued on next page...

12


TK THICKNESS [4V]

There are four values of thickness (4V)––one valueat each corner.


1st value for thickness: 5

<Return> – for rest of the prompts

Warning! If you enter more than one value, all four values must benon-zero. To enter a uniform thickness at each corner,you need to enter only the first value.

Done

Recovery Point

FileSave

13

Create nodes 1 of 2

In this example, you’ll create four nodes in the finiteelement model.

The displacements of nodes are the variables for whichthe finite element model is solved. Each node has threepotential displacements and three potential rotations, ora total of six degrees of freedom (DOF).

The four nodes in the model contain 24 DOF to besolved.

14

Create nodes 2 of 2

What: Create four nodes in a 10mm square on the XYplane.

How:

Node form

OK


Node 1 location: 0 0




Done

Adjust the display to get a better view of the nodes. Youcould use Zoom All and F2.

Recovery Point

FileSave

15

Create an element 1 of 4

Elements are displayed by lines connecting the nodes.

Why: In this example you’ll use only one element tounderstand how nodes and elements behave. Thecomputed displacements and stresses will be correctonly for simple uniform loads. In most cases, you’ll haveto create many smaller elements to converge on acorrect result.

To create elements you’ll:• specify the type of elements (1D, 2D, 3D)• define material and physical properties• pick the nodes

16


What: Create one thin-shell element to connect the fournodes.

How:

Element form

Make sure the following are selected on the form:

2D

Element Family: Thin Shell

Element Type:

Don’t close the Element form yet.

17


What: Assign material and physical properties to theelements.

How:

Element form

Material Selection...

Materials form

Link Material

OK

Element form

Physical Property:

?

Physical Property form

Link Thickness

OK

Element form

OK

Why: The default is to use the material stored with thepart that’s associated with the finite element model.Since you’re not using a meaningful part here, youmanually selected the material table you created earlier.

18


What: Pick the four nodes with which to create thethin-shell element. In a typical problem, you would usemore automatic methods to create elements.

How:

1

2 shift-pick

3 shift-pick

4 shift-pick

2

34

1

(Done)

If the element looks wrong, you’ve picked the nodesin the wrong order. Press Control-Z and repeat thisstep before saving your model file.

Recovery Point

FileSave

19

Apply boundary conditions 1 of 4

At each degree of freedom (DOF), you can either applyrestraints to the degree of freedom, or you can apply aforce.

You must apply enough restraints to hold the model as arigid body in space. Even though the linkage has equaland opposite forces on the two ends, you will restrainone end, and apply a force on the other.

1

20


What: Restrain all six degrees of freedom of the nodeindicated.

How:

Boundary Conditions

1 pick node

Done

Displacement Restraint on Node form

Clamp

OK

21


What: Restrain the indicated node, and leave only the Ytranslation free.

How:

1

1


Specified

Specify Restraint...

Specified Restraint on Node form

Y Translation: Free

OK


OK

Why: The Y-displacement is free so the element cancontract in the Y-direction as it stretches in theX-direction.

22


What: Apply a force in the X-direction at two nodes.

How:

F 1

2

1

2 shift-pick

Force on Node form

X Force: 100000

OK

Recovery Point

FileSave

Things to notice The total force on the element is the sum of the twonodal forces.

23

Solve the model 1 of 4

The most common type of solution is linear statics, inwhich the software simulates the response of the modelto a steady-state static load.

The software will solve the matrix equation:

{F} = [K] * {d}

At each degree of freedom, either a displacement iscalculated at the given forces, or reaction forces can becalculated where displacements are restrained.

24


What: Create a solution set to define:• the type of solution to perform• the boundary condition set to use• the types of output to calculate and store• other options that affect the solution

How:

Model Solution

Manage Solution Sets form

Create...

Solution Set form

Name: Uniform Static Load

Don’t close the form; continue on the next page.

25


What: Request output types of displacement and stress.List and store the output during the solve.

How:

Solution Set form

Output Selection...

Output Selection form

Output Type: Displacements

Store/List: Store and List

Output Type: Stress

Store/List: Store and List

OK

Solution Set form

OK

Manage Solution Sets form

Dismiss

Recovery Point

FileSave

26


What: Define the location where temporary files andoutput files are stored. Then solve the model.

How:

Solve form

Output file: results1.lis

Solve

Things to notice The windows will temporarily disappear when the solverstarts. As the solve progresses, status messages aredisplayed in the I-DEAS List window. After the solve, anyerrors or warnings are listed in the I-DEAS List window.

During the solve, the file results1.lis is created to storedetails of the analysis. You may want to examine this filelater, then delete it.

27

Interpret the results 1 of 8

When reviewing results you must ask:• Are the results from the model correct?• What do the results mean?

Some simple hand calculations can increase yourconfidence that the results are correct.

28


What: Select the deformation results to examine.

How:

Post Processing

Results Selection form

Display Results: Clear

Displacement_1 (select)

Deformation Results

OK

Why: Deformation results will be shown as displacednodes. Display results are shown in color. By clearingthe display results, you can see the displacements moreclearly.

29


What: Display the deformation.

How:

(to select all elements)

It’s good practice to look at displacements firstbecause they are more intuitive than stresses. Makesure the displacements look reasonable before youtry to interpret stresses.

30


What: Perform a hand calculation to check thedisplacements.

How:

The displacement in the X-direction should be:

X = PL/AE

P = 100,000 * 2L = 10A = 10 * 5E = 2E8

X = 0.0002

Things to notice This displacement value is small, verifying that theassumption of using linear static analysis is valid.

Most workstations have a desktop calculator you canuse to try this calculation.

31


What: Generate a report of the displacements to checkthe values.

How:

Report Writer form

Output Similar To: Deformed Display

Generate Report

Check I-DEAS List . Scroll back in the I-DEAS List window.X-displacements at nodes 2 and 4 should matchyour hand calculation. If they don’t match, you mayhave made a mistake in entering boundaryconditions, material properties, or physicalproperties.

32


What: Check Poisson’s ratio.

How:Using the values shown in the I-DEAS List window,divide the Y-translation by the X-translation at node 2:

Y-translation / X-translation = 0.3

Things to notice In the plot of deflection, the element shrinks in theY-direction as it stretches in the X-direction. Under auniaxial load, this value should match the value youentered for Poisson’s ratio (0.3).

33


What: Display the stress results.

How:

Results Selection form

Stress_2 (select)

Display Results

Component: Maximum Principal

OK

Things to notice Due to uniform loading, all values for the element are thesame. The fact that the stresses are less than the yieldstress verifies that using linear static analysis is a goodassumption.

34


What: Perform a hand calculation to check the stress.

How:

Stress = P/A

P = 100,000 * 2A = 10 * 5

Stress = 4000

Things to notice If 4000 is not the value on your plot, you may haveincorrectly entered the nodal coordinates, physicalproperties, or forces.

In this simple case, one element gives the correctanswer since the stress is uniform. In most modelswhere the stress is not uniform, the elements can onlyapproximate the true values, so you’ll require moreelements to obtain a correct answer.

You may want to examine the output.lis file on theoperating system. It will list the same results youdisplayed.

35

Tutorial wrap-up

You have completed the What Is Finite ElementModeling tutorial.

Delete the FE model, then the part. This part is not usedin any other tutorial.

Hint

See also...For additional information on the concepts covered inthis tutorial, see the following:

Help, Manuals, Table of Contents

Simulation: Finite Element Modeling User’s GuideSimulation OverviewUsing Simulation Tools

Simulation: Model Solution/Optimization User’s GuideUsing the Solvers

Using Linear Statics Analysis

What’s next?You know that one element is not enough to model mostproblems, but how many should you use? Here we usedthin-shell elements, but when should you use other typesof elements? These questions and others will bediscussed in other tutorials.

To exit this tutorial, select: File

Exit

Warning! Do not use the menu in the I-DEAS Icons window to exit.Use the menu in the Acrobat Reader window.

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What Is Finite Element Modeling?