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Ansys Tutorial MEC 209

Academic Year 2001/2002

Dr A. Yoxall


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Dr A Yoxall Department of Mechanical Engineering 2

Introduction

This tutorial is for use with the Ansys overview document. The purpose of the tutorial

is to develop an understanding of Finite Element techniques and relate thosetechniques to the building of structure able to support a load of 116 tonnes.

A block, width 2500 mm, height 3300 mm and depth 1850 mm must be able to pass

underneath your structure.

For your Analysis the maximum deflection is to be 3 mm. The maximum stress

allowed in the model is 200 MPa (localised regions of higher stress maybe allowed).

The Youngs Modulus of steel is to be taken as 0.2e6 MPa and Poissons ratio 0.3

This structure relates to the Make and Break part of MEC 209. However, for the

purposes of this tutorial you will design your structure in steel. This is for ease of

simulation, however there should be correlation between the designs, and you canconsider the wooden Make and Break design a scale model of your final design.


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1 Logging into Unix and starting Ansys

For this tutorial we will be using the Unix version of Ansys, not the one available onthe NT network. You may use the NT version if you like, although you may find

problems with filespace and the number of nodes and elements you can use (1000 of

each).

You should have been given a Unix account with username and password details.

You can access Unix from the start menu as follows:

The following dialogue box should appear.


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Type in your user ID and then click on run. You will be prompted in another dialogue

box for your password then click on OK. This will open an Xterm window similar tothe one shown below.

For further instructions on Unix file handling, see the document on the following web

page:

http://www.shef.ac.uk/~mpe/staff/ay/cae

Note in the xterm window shown above you have a home account and a /scratch

account. Models should not be run in the home account. You wont have enough

space. Instead, save your files in the home account and copy them to the scratch

account for analysis.

To start Ansys just type ansys (in lower case) in the Xterm window.

To open up the menu in Ansys first type:

/show,x11 (note Ansys field are separated by commas)


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And then:

/menu,on


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2 Producing Your Concept

Obviously, you can make the frame any shape or size. However, the object is to be the

stiffest yet the lightest weight. There are really only three types of frame you couldbuild. Either:

A truss structure.

A monocoque structure.

A combination of the above.

The purpose of this tutorial is to demonstrate how you might build concepts of the

kind described above. What the tutorial does not do is produce the optimal design.

That is for you the student to produce to the best of your ability.


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3 A Truss Structure

We should all be familiar with truss structures; electricity pylons are good examples

of this kind a structure.

So how do we go about building a truss structure in Ansys?

To make a truss the structure, the most obvious way is to build a structure using beam

elements.

Such a structure is shown in the figure below.

In 3D the frame structure looks like this:


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3.1 Ansys Beam Elements

The beam element we will use for this analysis is the 3D elastic beam element knownin Ansys parlance as BEAM4. This element is a uniaxial element with tension,

compression and bending capabilities. The element has six degrees of freedom at each

node (three translational and three rotational).

The beam is defined by two or three nodes, the cross sectional area, two area

moments and two thicknesses, an angle of orientation, a torsional moment of inertia

and the material properties.

i

jx

TKY

TKZ

IYY

IZZz

y


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In Ansys these sectional properties are defined by an elements real constant. Real

constants apply properties to elements such as thickness, contact stiffness and so on.

The can also be useful as a method by which you can select a set of elements that may

have the same properties but are in a different location in your model.

3.1.1 Defining Beam Elements

The main Ansys graphical user interface or GUI is shown below:

You can define the beam element from the Ansys pre-processing menu accessed

through the main menu. These menus are shown in their entirety in the following

figures.

Utility menu

Command window

Main Menu

Graphics Area


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By clicking on element type the following sub-menu appears.

We wish to add element types so clicking on the add/edit/delete option brings up thefollowing sub-menu.


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By clicking on add we get to the menu that allows us to define our first element type.

Note that the beam element is given a reference number. This reference number is a

flag that we can use to switch between different element types when meshing.


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The above menu shows the user which element types have been defined. Click on

close to move back to the pre-processor menu. If you are using the NT system this

window may not open properly. To close this window, drag the bottom portion of the

window downwards until the close button appears.

3.1.2 Real Constants

Real constants as explained earlier define Ansys element properties.

The real constant menu can be accessed from the pre-processor menu.


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3.1.3 Defining Material Properties

Real constants define element properties. Material properties such as Youngs

Modulus and Poissons ratio are defined using the material properties sub-menu. This

menu is also accessed from the pre-processor menu.


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We will be using isotropic material properties.

Note, just like the real constant menu, Ansys prompts use for a material reference

number. Again like the real constant menu, this is a switch used to define elements

before meshing.


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Note that you only have to define the Youngs Modulus, the Poissons ratio and the

density for the model to run successfully.

It is the material properties menu that sets the dimensions of your model. If you

choose a Youngs Modulus in N and mms then the distances in your model will be in

mm. If you choose N and metres then the distances in your model will be in metres.

The units that you choose for Youngs modulus will also affect your deformation and

stress output units.


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3.2 Defining Geometry

One of the most useful parts of the pre-processing menu is the modelling section. In

this part of the menu users can create, copy, reflect and operate on geometry.The easiest way of creating beam elements is to mesh them onto a set of lines created

using Ansys solid geometry (lines, keypoints etc) commands.

Remember that there is a hierarchy of objects in Ansys solid geometry. The basic

geometry point is called a keypoint, lines connect keypoints and surfaces (called areas

in Ansys) are a connection of lines. From a set of areas, 3D shapes called volumes can

be created. User cannot delete lines unless the corresponding area is deleted and

keypoints cannot be deleted unless lines are deleted and so on.

This sort of geometry creation is called solid geometry in Ansys and can also be

created using CAD software.

From the pre-processing menu select the create option.

The following sub-menu appears that allows the creation of lines, arc, keypoints areas

and so on. As with any software there are many different ways in which the model

you require can be created. This example demonstrates only one way. You may in

time find better more efficient methods or one that particularly suits the way in which

you want to model. There is no right way.


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From this menu the sub-menu asking us how we want to create the keypoint. Click

on in Active CS. This means the keypoint geometry will be placed in the active co-

ordinate system. The default system is a Cartesian system with the origin and axis

based on the X-Y-Z marker shown in the graphics screen. This marker is called the

triad.


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The next sub-menu asks for the keypoint number and the position in the Cartesian

system you want to create that keypoint. Leaving the boxes blank inputs a zero. If thekeypoint number field is left blank the keypoint number will increment automatically.

The following figure shows keypoint number one placed at the position x=4400 in the

global Cartesian (active) system.

Using this menu we can place keypoints at positions we would like to enable us to

create our geometry. Remember that the menu will remain open if the user click the

apply option. Clicking on the OK option will close the window and finish thecommand.


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At this point its is also useful to remember that you dont have to use the window

system to create the keypoints you can type commands in the command editor

window. Note that all Ansys commands are generic, k creates keypoints, l lines and a

areas and so on.

In the above window the typed command is l,p. This means create lines using

graphical picking. Note that during any window the typed equivalent command isshown in brackets. When you become more familiar with Ansys, you will find typing

far quicker and easier than using the menu options.

Hence we can start to create some initial geometry. You can plot any geometry item

using they plot command from the utility men. An initial keypoint plot is shown

below.


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Of course you dont have to create all the keypoints you need and then attempt to

create lines between them. You can use other modelling commands, such as reflect or

copy.

Copying geometry is one of the most useful Ansys modelling commands. It can be

accessed from the Ansys pre-processing menu. As with most Ansys sub-menus you

are initially asked what do you want to copy?

If you click on keypoints an Ansys pop-up menu appears. These menus are used for

selecting objects within Ansys. Note that you can select the items you want to copy,

unselect them, pick them singularly or using a box etc. The number of items selectedis also displayed. If you are having difficulty selecting an item using the pop-up menu

then you can use the select menu from the utility menu to grab your objects and then

use the pop-up menu to perform the required operation on those selected objects. This

versatility within Ansys is one of the reasons why the pre-processor is so powerful.


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If you do select your items first using the select menu then you can use the pick all

function on the pop-up menu. Remember that clicking on apply will keep the menu

open. So we can create the base of our geometry, select the keypoints and start to

copy them to build up our geometry.


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Once we have created the keypoints (or at least some of them) we can start connecting

them together with lines. We can either use the create menu again and this time select

lines (which brings up the following menu) or use the l,p command.

Notice that if you use the l,p command that the cursor will start to rubber-band

between keypoints. This feature is intended to aid you in deciding how to create your

line.


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Eventually we can create a fairly simple shape as shown in the figure below.

As I said previously you dont have to do it this way. You could create all your

keypoints and connect them up. However, if you create your simple geometry as I

have done you can then break the geometry into smaller parts letting Ansys do thehard work for you. So in this tutorial I created a simple outline of the frame and then

broke the lines into smaller lines, each one generating a new keypoint. Hence Ansys

has created keypoints for me that I can then connect with lines. The Ansys command

for dividing lines is the ldiv command. Alternatively the command is available from

the operate menu.

Using the divide option the following sub-menu appears.


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Clicking on the Line into N Lns option allows us split any line into N number of

lines. The following pop-up menu will appear.

Eventually you should be able to create the geometry you want. Note that so far weve

only worked in 2D (the x-y plane). To make the geometry 3D we can use the copy

command an this time copy lines rather than keypoints (the command will copy the

keypoints automatically for you) into the z plane. We can then connect the parts of the

frame together and produce our finished truss structure.


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4. Monocoque

In this type of structure (railway vehicle body structures, car bodies etc) thin panels

are used to take the stresses (particularly in shear) rather than using the triangulationmethod of truss structures. Hopefully your structure should be lighter.

So this time we have to use a combination of shell and beam elements.

4.1 Shell Elements

Shell elements are used to represent surfaces that are thin relative to their length (I

dont want to confuse you, but for completeness there are actually thick shell

formulations). Examples where you might use thin shells are for car body panels.

Unlike beams, shells are meshed onto surfaces or areas. Four nodes and four

thicknesses define shell elements. The elements have two surfaces and four edges.

Shell elements have generally only one real constant to define, that of the element

thickness. Note that if your surface is of uniform thickness you only have to define the

thickness at one of the nodes.

Hence for our monocoque structure we can define areas onto which we can map our

shell elements.

4.1.1 Defining shell elements

To define shell elements we use the same menu paths as we did for defining beam

elements. Note that in this instance if we have already defined some beam elements

then the shell elements will be given an element type number one higher than thatused for the beam elements. This element type number is used as a flag for setting

i

k

l

3

1

2

6

4

5


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which elements you are going to use when meshing. For example you cant mesh

areas with beams, nor can you mesh lines with shells so Ansys needs some switch to

enable the different geometries to be meshed.

We can define areas in two ways. Either, by picking on a set of keypoints or selectingthe lines that make up the area in question.

4.2 Defining Geometry

You can define areas using the create option from the pre-processor menu. After

clicking on create areas you will be prompted with the following sub-menu.


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Note that just like when you define lines through keypoints a rubber band will appear

to help you define your areas. This is shown in the figure below.


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You can define you areas using any number of lines for example but be careful! How

you define your areas affects Ansys ability to map the elements onto the surface. Ifpossible four sided surfaces are the best.


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The monocoque structure minus the shear panel (i.e. beams only) is shown below.


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5 Combination

You could if you so wanted use a combination of panels and trusses. Building the

geometry for this type of structure is as described in the preceding sections.

So weve created our geometry. Were not finished yet however. Geometry creation is

often the easy part. Getting a properly converged solution that gives you decent

results is another thing entirely. A good FE result can often depend on the quality of

the mesh you choose and how the model is loaded and constrained.


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6 Meshing

In Ansys there are many ways in which you can decide on the how many elements

you are going to have in your model. Because of the way FEA works you will wantmore elements in the regions of high stress gradient. You will also want to use enough

elements so that you get an adequately converged solution; the term used for this is

mesh density.

6.1 Global Element Size

Mesh control items are found from the middle part of the pre-processor menu.

By clicking on the Size Cntrls option the following menu appears.


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This sub-menu allows access to the different methods in Ansys that you can choose to

set your element density. In Ansys you can set the element density using areas,

keypoints, lines, global element density and smart sizing. Forget smartsizing for now

however, it is dealt with later.

The easiest method of meshing anything is to set a global element size. This means

that Ansys will try and make all the elements the same size to suit your geometry. The

global element size panel is shown in the figure below. Note that you have two

options, the element edge length (how physically big the element size is) or the

number of divisions. What this means in practice is that if are meshing a line 120

units long and set an element edge length of 10 units you will get 12 elements (each

10 units long). If however you set the number of divisions to 10 then you will get 10elements 12 units long.


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6.2 Area, Line and keypoint sizing

You can in Ansys set the number of element divisions you require on areas, lines and

keypoints. Ansys then maps the requested mesh density onto the solid geometry. The

sub-menus to do this are very similar to our esize sub-menu shown earlier.

As an alternative to having the same element size all over your model, you can set

different element sizes in different parts. This is really useful since you dont wantlots of elements in low stress areas since the computational solving time and model

size are proportional to the number of elements you use.

One of the handiest ways of having a variable element density around your model is

setting the mesh density using lines. The line sizing sub-menu is shown below.


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With the line sizing option you can either set the size on all selected lines or select the

lines yourself using the line size command (lesize) and the standard pop-up menu

shown below.

A line that has been line sized will appear dashed. Each dash represents the element

edge length. An example of this is shown below.


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6.3 Smartsizing and the meshtool

All the commands described in the earlier section are available using the meshtool,

again available from the pre-processor menu.

The mesh tool also includes smart sizing and element shape options.

Smartsizing is an Ansys auto-meshing option that applies greater element densities atareas likely to have high stress gradients. However, it can be a little fiddly to use and


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can create large meshes. Its a personal thing but for most mesh situations it is best

avoided.

Note that the mesh tool also allows us to set our element shapes. If possible we should

always try to use rectangular (quad) elements. Sometimes this just isnt possible and a

mixture of triangles and quads has to be used.

6.4 Connectivity

Except in very special software (or some element types) the basic premise of FEA is

of nodal connectivity. Elements must be connected together so that the stiffness of the

object you are meshing is accurately represented and stress and strains can be

calculated from one element to the next. Whatever mesh you choose, it must have this

nodal connectivity. An example of what is meant by nodal connectivity is shown in

the following figure. Elements that are not connected properly are shown on the left;those with good nodal connectivity are shown on the right.

In Ansys there are several ways in which you can ensure you have nodal connectivity:

Make sure connecting areas share the same lines

Make sure connecting lines share the same keypoints.

Merge keypoints and nodes so that duplicate objects are removed.

Merging keypoints and nodes can be done using the numbering ctrls option from

the pre-processor menu.


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Then select the merge items option from the following sub-menu.

This brings up the following sub-menu. The tolerance option is the range that

Ansys uses to look for coincident objects. Be very careful with this command! If

too large a tolerance is used than you can end up distorting your model.


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In Ansys there are several ways in which you can check you have nodal connectivity.

Visual inspection

Run your model. If bits flap about that are not supposed to then are

probably poorly connected

6.5 Mesh Attributes


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6.6 Clearing Meshes and deleting


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It is always very useful to be able to clear your mesh if you have made a mistake the

clear option is available from the pre-processor sub-menu.

From this sub-menu we are given the option of clearing lines, keypoints etc. To

actually clear the mesh the standard pop-up menu appears.


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7 Loading and Constraining

Idealising the geometry and meshing the model are sadly only two parts to the battle

to win the war that is Finite Element Analysis. Loading and constraining models aremassively important in ensuring as accurate solution to the problem as possible.

You can load and constrain a model in Ansys either from the pre-processing menu or

the solution menu. To enter the solution menu click on solution on the Ansys main

menu.

We wish to apply load and constraints so click on apply and the following sub-menu

will appear.


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From this sub-menu we can either apply loads or constraints. Constraints are

necessary in a model since the load must be reacted in some way as to prevent rigid

body motion; that is the movement of the body uniformly through space (the model

will continue to move in space until the programme crashes).


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We can apply constraints on nodes or keypoints. The advantage of applying the

constraints on keypoints is that you can modify the mesh but keep the same

constraints. However it is often more reliable to apply the constraints on nodes. If we

select this option the following pop-up menu appears.

This is our standard pop-up menu. We can select the nodes in the usual way.


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The above menu prompts us for what type of constraints we wish to apply. Note that

in Ansys a constraint is a displacement of zero at a node. Constraints are displayed in

Ansys as blue arrows for translational constraints and brown arrows for rotational

constraints. These are shown in the following figure.


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From the sub-menu that allows us to apply displacements to a model (they dont have

to be zero) you can also select to apply forces. Again, forces can be applied on eithernodes or keypoints. The sub-menus for applying forces are similar to those for

applying displacements. The sub-menu for applying the force magnitude is shown

below.

This menu shows a force of -550000 N acting in the global Y direction. An appliedforce is shown as a red arrow in Ansys.


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Obviously you can apply other kinds of loads in Ansys such as pressures, body forces

and temperatures.


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8 Solving

This is known as the solution phase within Ansys. There are obviously different types

of analyses that you may wish to undertake, such s thermal, modal, magnetic andstructural.

Ansys, unlike older FEA packages (it used to be Ansys main selling point) has its

pre-processor, solution processor and post-processor all in the same GUI.

To solve you analysis simply go to the solution processor and click on solve current ls

(loadstep).


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9 Post-processing

Post-processing is the means by which we can analyse the results from our solution.

Typically we would want to look at deformation and stress.

Firstly enter the post-processor from the main menu. The post-processing sub-menu is

shown below.

Click on plot results. The following sub-menu appears.


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Click on nodal solution and the sub-menu shown below appears.


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From this menu we can plot items such as stress, strain and displacement. In the

example above we are plotting total displacement (usum). For a beam structure

example the results might look like that shown in the following figure.


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The key on the right shows displacement graded from zero to the maximum in nine

contour bands.The displacement in this example is higher than we would like (the structure would

fail) although the mass is low at 15 kg.

By iterating the size of our beams elements (changing the values in the real constant

table) we can arrive at a structure that has a lower displacement (and corresponding

stress). However, the mass of the structure is significantly increased.


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However this structure now weighs 221 kg. The displacement has however been

significantly reduced.

The equivalent monocoque structure is shown in the following figure. This model also

has a mass of approximately 221 kg. Note that the displacement of the monocoque

structure (for the same weight) is under half that of the truss structure.


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We can reduce the weight of the monocoque structure by placing holes in the shear

panels.


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The mass of the structure can be obtained from the xterm window during the analysis.

Basically click on the window so it is brought to the front during the analysis and note

the mass. This is shown in the following figure.

Stress results can be plotted for beam elements using the following etable commands.

(Obtaining stress output from Ansys is fiddly for beams and the detail could take upanother entire document).

etab,smax,nmisc,1

for the i node

and

etab,smax,nmisc,3

for the j node

pletab,smax

Plots the stress in the beam elements.

Stresses in shell elements can be obtained from the post-processor menu in the same

way as deflection shown above.

When plotting stresses it is worth selecting the elements you want to analyse first.


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10 Saving Your Model

Saving your work is obviously very important. Ansys does not have a proper undo

function as such. You can however resume your work from the last time you saved.

The save sub-menu is obtained under the file option from the utility menu. Note thatyou are offered two choices: either save as jobname.db or save as.

The default filename for an Ansys model (or database) is file.db. Note the .db

extension name. Backup database files are given the default extension .dbb. A .dbb

file is created after you have saved your file more than once and as such is a copy of

when you saved the model prior to your last save.

If you use the save as option, you can save your model with any name and any

extension you like. Take care however; Unix does not like spaces in filenames (use

underscores, e.g. alaster_yoxall.db instead). Also when you want to resume a model

(say you are logging into Ansys for a second time) the default filter for looking for

your models is *.db. If you save your model as just alaster_yoxall, the filter will notfind it; you will have to alter the extensions it is looking for.

The resume menu is shown below.


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It also very important to note which directory you are looking for your results in.

Again, you can use the filter command to find the correct directory.

I recommend all users to start Ansys in the same directory as the model they want to

work in is stored.


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11 Logging out

Logging out of Ansys is also performed from the utility menu file sub-menu (or the

Ansys toolbar). If you click on file and then exit the following sub-menu appears.

Remember the default save option is file.db.

If you exit Ansys after solving a model you will need to resume the geometry and the

results. You do this by using the resume menu for the geometry and clicking on thelast set option on the post processing menu.

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