Free Free Plate Modeling

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Free Plate Natural Frequencies and ModeShapes using ANSYS Modal analysis

Mohammad A Gharaibeh11/12/2014


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Goal

This Lecture aims to obtain natural frequencies and mode shapesof a free isotropic plate using ANSYS Software

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Problem Description

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Problem:

Obtain natural frequencies and mode shapes offree squared isotropic plate using ANSYS Modalanalysis

Material Parameter SI units English units

Youngs modulus 73.1 GPa 10.6 MSI

Density 2780 Kg/m3 2.6*10-4 lb.sec2/in4

Poissons ratio 0.3 0.3

Be careful of this!!

Material: Aluminum

Geometry Parameter SI units (m) English units (inch)

Length (a) 0.3 12

Width (b) 0.3 12Thickness (h) 6.35*10-3 0.25

Plate Size:

We will useEnglish units


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What is ANSYS?

ANSYS is a finite element analysis (FEA) software that canbe used to solve various engineering problems.

Every analysis involves three main steps:

1. Preprocessing Create model geometry

Define material

Element type

Mesh2. Solution

Apply loads and define analysis type

Solve

3. Postprocessing

Review results

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Start ANSYS

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Start >> All Programs >> ANSYS15.0 >> Mechanical APDL 15.0

In windows 7


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ANSYS Environment

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Utility Menu: File controls, plotting, selecting parameters, etc.Input line:Allows you to type in commands directlyToolbar: Used to open and save ANSYS jobs.Main Menu:The place you will use a lot. Preprocessing, Solution and Postprocessing

Graphics window:

Where the model,Postprocessing andresults will appear.


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Start modeling - Preprocessing

In this step we will learn how:

Define Element Type.

Specify Material Parameters

Build the Model

Mesh

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Element type defines the way how do you want to mesh your model. In FEA, it dependson the problem type and it is very necessary to be selected carefully.


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Define Element Type

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Main Menu:

Preprocessor >> Element Type >> Add/Edit/Delete


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Define Element Type (contd)

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Element type window: Add

Library of Element type: Solid >> Brick 8 Node 185This is SOLID185 elementsuitable for various 3-D modeling of solid structures

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Define Element Type - Keyoption

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We need to specify little more details for this element to make match our desired problem

using element KEYOPTIONElement types window: OptionsSOLID185 element type options: Element technology (K2) >> Simple enhanced Strain

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3

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You can testother optionsavailable andcompare withtheoreticalresults


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Specify Material Properties - 1

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Now we need to input material properties

Main Menu: Preprocessor >> Material Props >> Material Models


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From Window appears

Material Model Number 1 >> Structural >> Linear >> Elastic >> IsotropicThen Type in your values for Youngs Modulus and Poissons ratio >> OK

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10.6e6equals to10.6 MSI

Take care ofyour units!


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Specify density

Material Model Number 1 >> Structural >> DensityThen Type in your value for Density>> OKClose window (X)

Take care ofyour units!

2.6e-4 equalsto 2.6*10-41

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5

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Build Geometry - 1

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Main Menu: Preprocessor >> Modeling >> Create >>Volumes >> Block >> By dimensions

Type in your plate dimensions in x,yand zcoordinates and hit OK.Other Modeling procedure ??Try keypoints, lines, areas and end up with volume


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Build Geometry - 2

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Meshing

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Meshing is the most essential step in FEA modeling. It means that use the element type you

specified to fill in (mesh) your solid model with nodes and elements.

ImportantYou need nodes and elements for the finite element solution NOT just the

solid model weve just created. The solid model does NOT participate in the FEA solution.

It requires three steps:

Attribute element type and material properties to the solid model

Mesh Controlspecify number of divisions. Meshing the solid model (create elements and nodes)

Solid Model FEA Model

Meshing


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Main Menu: Preprocessor >> Meshing >> Mesh Attributes >>Picked volumes

Click on the Solid model

Meshing: 1- Attribution

Click on thesolid model

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When you click on the solid model its color will turn into Purple

Now, hit OK


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Volume attributes window: OK

Here we specify which material and/or element type to use. In our case, it should look like:


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In order to specify number of divisions, we need first to observe the LINES

Utility Menu: Plot >> Lines

Meshing: 2- Mesh Control

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In order to specify number of divisions, we need first to observe the LINES

Utility Menu: Plot >> Lines



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In order to specify number of divisions, we need first to observe the LINES and lines NUMBERS

Plot Number Controls window: LINE >> ON

Hit OK


Turn this ON

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Meshing: 2- Mesh ControlTo changes the angle of view use Dynamic Model Mode

Or simple using keyboard and mousePress and hold Ctrl + Mouse right clickFor zoomMouse scroll

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Meshing: 2- Mesh ControlAfter plotting lines and numbers we need now to divide them

Main Menu: Preprocessor >> Meshing >> Size Cntrls >> Picked LinesThen, click on the desired lines. In our case just select the in-plane 8 lines (top and bottom) of plate.Hit OK

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Meshing: 2- Mesh ControlAfter plotting lines and numbers we need now to divide them

Element size on Picked Lines window: NDIV type in 30 (or any other number of divisions)OK

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Meshing: 2- Mesh ControlWe can specify number of divisions of out-of-plane lines but ANSYS will have three divisions

from them by default, which is more than enough.

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Meshing: 3- Mesh CreationTo create mesh (elements and nodes)

Main Menu: Preprocessor >> Meshing >> Mesh >> Volumes >> Mapped >> 4 to 6 sidedClick on the model (as done previously) >> Hit OK

To plot volumes:

Utility Menu: Plot>> Volumes

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Meshing: 3- Mesh CreationThe FEA Model

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Solution step

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As discussed previously ANSYS has three main analysis step:

Preprocessing

Solution

Postprocessing

DONE!

We are here!

In solution step: Define Analysis TypeModal analysis. Loads and Boundary Conditions (BCs)

Solve the FEA model

In our case we

dont have loads

and/or BCs

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Solution: 1- Define analysis typeTo define the analysis type (which is here model analysis)Main Menu: Solution >> Analysis Type >> New AnalysisSelect Modal and OK

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Solution: 1- Define analysis typeMore analysis optionsMain Menu: Solution >> Analysis Type >> Analysis OptionsSolver: Block LanczosNumber of Modes: 20 (or else)Hit OK

Make sure to havesame number

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Solution: 1- Define analysis typeMore analysis optionsMain Menu: Solution >> Analysis Type >> Analysis OptionsSpecify frequency range that youd like to look in (make sure to be large enough)

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Solution: 2- Solve the FE modelSolutionMain Menu: Solution >> Solve >> Current LS (Load Step)OK

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Solution: 2- Solve the FE modelSolutionFor this warning hit YES - this is because we did not constrain the model

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Solution: 2- Solve the FE modelSolution is DONE!Life is Good!Hit OK and close /Status window.

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Postprocessing step

As discussed previously ANSYS has three main analysis step:

Preprocessing

Solution

Postprocessing

DONE!

In PostProcessing step: List Natural Frequencies Select mode shape you want to see Plot it!

We are here!

DONE!

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Postprocessing: 1- List Natural FrequenciesGeneral Postprocessor: Results summaryA list of natural frequencies will appear

First 6 natural frequencies will be zero oralmost zero rigid body modes (free BCs)

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Postprocessing: 1- List Natural FrequenciesIf you want to save natural frequencies as an ASCII fileFile >> Save

First 6 natural frequencies will be zero oralmost zero rigid body modes (free BCs)

Units here are in Hz!

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Postprocessing: 2- Select mode shape neededTo select which mode shape to observeMain Menu: General Postproc. >> Read results >> By load stepLSTEP = 1, SBSTEP = 7remember we have six zero modes!OK

First Mode shape

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Postprocessing: 3- Plot mode shapeTo plot mode shapeMain Menu: General Postproc. >> Plot results >> Contour Plot >> Nodal SolutionFrom tree >> DOF Solution >> Displacement vector SUMOK

First Mode shape

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Postprocessing: 3- Plot mode shapeFirst Mode shape

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Postprocessing: Other Mode Shape? Same procedureTo select which mode shape to observeMain Menu: General Postproc. >> Read results >> By load stepLSTEP = 1, SBSTEP = 8OK

Second Mode shape

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Postprocessing: Other Mode Shape? Same procedure

Second Mode shape

To plot mode shapeMain Menu: General Postproc. >> Plot results >> Contour Plot >> Nodal Solution

From tree >> DOF Solution >> Displacement vector SUMOK

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Postprocessing: Other Mode Shape? Same procedureHere is 2nd mode shapeSame procedure can be used to observe all other modes

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Postprocessing: Save a photo of a mode shapeUtility Menu: PlotCtrls >> Hard Copy >> To FileSpecify Name, file type >> OK

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Other Places to Learn ANSYS

University of Alberta: They have a very nice and user friendly tutorialhttp://www.mece.ualberta.ca/tutorials/ansys

ANSYS.com

ANSYS Verification Manual.

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http://www.mece.ualberta.ca/tutorials/ansyshttp://www.mece.ualberta.ca/tutorials/ansyshttp://www.mece.ualberta.ca/tutorials/ansys


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Questions??

Mohammad Gharaibeh

Email: [email protected]

If you have questions, I will be happy to assist:
mailto:[email protected]:[email protected]

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