lab#1 introduction to ansys finite element analysiscdaley/6002/6002_ansys_1_19.pdf · introduction...

6002 ANSYS Lab#1, page 1

Engineering 6002 - Ship Structures I

Lab#1

Introduction to ANSYS Finite Element Analysis By C. Daley

Overview The first lab is an introduction to ANSYS,

including Workbench, Space Claim and ANsys

“Mechanical”

The lab will familiarize you to the following;

Into to Workbench

- Project window

- Geometry modelling (SpaceClaim + )

- Finite Element Analysis

We will create a simple cantilever model and

- Apply boundary conditions

- Apply a load

- Solve

- Plot Stress

- Plot deflection

In the exercises, you will

- See what happens when you refine the mesh

- Make same model with shell elements (vs brick elements)


ANSYS Model #1 – simple cantilever

Step 1: describe and sketch the problem:

In this first example we will model a simple steel cantilever, to see how the simple structure

responds to load. The problem is sketched below.

The problem description is as follows:

Geometry: 1200 x 180 x 10 mm

Load: 18000 N applied at the end of the cantilever in the string direction

Supports: the base is fixed in all degrees of freedom, all other boundaries are free.

Material: Steel, with E = 200GPa (2e11 N/m2)

Units: N, m, Pa

Step 2: estimate expected results (analytically):

The bar has the following properties:

Moment of inertia : I = 1/12 t h3 = 10 x 1803 /12 = 4.860e06 mm4

Section Modulus: Z = I/(h/2) = 54000 mm3

Base Bending Moment: M = 18000 x 1200 N-mm

Maximum stress (at base): sig = M/Z = 400 N/mm2 or MPa

Maximum deflection: d=FL3/(3 EI) = 10 mm

It is likely that the ANSYS results will be close to these, but not exactly the same. The % error

will depend on the assumptions, but differences of say +- 10% would not be unusual. ANSYS

considers effects that are not in the analytical calculation, such as shear deformation, and includes

various numerical approximations. It is an essential part of engineering analysis and design to cross

check results and compare assumptions.


Step 3: open ANSYS Workbench 19.0 and create a project

1) First, save the (empty) project as Cantilever1.wbpj 2) The left hand window shows a set of analysis type options. Select Static Structural and drag

the icon to the right, placing it in the Project Schematic window.

The Workbench user interface, with a Static Structural analysis set selected.

Step4: open Geometry and create the CAD model

1) By Clicking on Geometry in the Project window, ANSYS will open a CAD modeling

program called SpaceClaim. 1a) IF you had right-clicked on Geometry, you would have seen;

giving you alternatives for modelling your problem.


2) You now see this window:

The main window (slightly shaded and titled Graphics) is where the CAD model will be displayed.

The left side (Tree Outline on white background) lists the components in the model (initially just

3 drawing planes and no bodies or parts). Close the welcome window.

3) We want to do some sketching on a vertical (x,y) plane rather than on the default horizontal

plnae (x,z). At the bottom of the main screen , there are 4 buttons. Click the New Sketch Plane

button.

Then click on the screen above the grid and you should see a new vertical grid.


Before you start to draw, create a base plane.

You should see:

The sketching window lets the user create and edit a variety of 2D geometric objects. This is part

of creating a ‘sketch’ from which 3D objects can be made.


4) Select the rectangle tool and sketch a rectangle, taller than wide. After you click on a

starting point, you will see the dimensions of the rectangle and one will be highlighted in blue.

You can type ‘10’ and width will snap to 10mm. Then you can type 180 to snap the height to

180mm.

When you finish you should see;

If you want to, you can just draw any rectangle, and then re-size it to 10mm x 180mm. when you

select any edge of a rectangle, you will see the distance to the other edge highlighted. You can

just type the desired dimension.


5) To create the bean, we will pull the rectangle 1200mm. Select the pull tool and hover

over the rectangle. It will highlight;

You can click and hold on the rectangle and pull the face. While pulling you will see the highlighted length. Type 1200 while

pulling and the bar will snap to a length of 1200mm. To check your drawing you can add dimensions using the dimension

tool under Details.


6) This is all we need to do in the SpaceClaim for now.

Step4: open Model and create the Finite Element model

1) Return to the ANSYS Project window, and click on the Model feature. You don’t to close

SpaceClaim, but you can if you wish.

This will start the ANSYS ‘Mechanical’ program, to setup the actual finite element model.

2) The Mechanical window looks like this;

On the right is the model geometry, but with no mesh or loads yet. On the left is a list of the

model features that have to be set. By default, the material to be used will be structural steel. We

can skip the Coordinate Systems and Mesh for now. The program will use defaults. We do have

to set the loads and supports (if we would hit solve now, the program would fail and give us an

error)


3) First we will set the support conditions at the base of the cantilever.

You will need to bring the back of the bar into view. You can use these tools.

Rotate, pan, zoom smooth, zoom select and zoom all:

With the face that you want to fix in view, you need to insert a fixed support. To do this right-

click on the Static Structural component in the left hand Outline window. This will open a sub

menu. Move the mouse over Insert and a 2nd submenu opens. Select Fixed Support (see below).


Make sure the face select option is on (in the menu bar at the top of the screen):

Point, line, face and body selector:

Now when you move the mouse over model in the main window, various faces will be

temporarily selected. Select the slender end face of the bar. The face will turn green. You are not

done yet. You need to click the Apply button on the lower left to confirm that you want fixity

applied to the selected face.

Now the Fixed Support is added to the outline tree, with a check mark to indicate that all is ok

and up to date. When it is selected, the support is shown in the main window and in the details

window. It can be later deleted or edited (moved) by selecting it in the outline tree.


4) Next we add the 18kN force to the free end of the bar. Again right click on Static Structural in

the Outline tree, select Insert, Force.

Select the line that defines the top-end corner of the bar, and click Apply. Then type 18000 into

the Magnitude cell (shown in yellow until given a value) in the lower left. By default, the

program picks a direction for the force and draws an arrow. You may need to select the Click to Change box under the Magnitude box. Define the direction of the force by selecting another line

or face to show direction. Keep selecting until the arrow points where you want it to. Then hit

Apply.


5) There should be no question marks left in the Outline Tree, with some lightning bolts (see

below). You can solve the model now, but first we will specify what information we want to plot

(this could also be done after solution).

6) To specify output, right click on Solution in the tree, and select Insert, then Stress, then

Equivalent Stress. Do the same to select Total Deformation.


7) Hit the button in the menu at the top of the screen.

8) When you select the Equivalent Stress under Solution in the tree, the von-Mises equivalent

stresses will be plotted on the deformed shape. The max stress is 3.8788e8, which is in Pa. This

is 388 MPa, reasonably close to our simple estimate of 400MPa. If you click on Total Deformation, it shows a max value of .0109, or 10.9 mm, compared with our estimate of 10 mm.

These values are reasonably close to the simple analytical estimates. Which value do you think is

more correct?

9) Examine the equivalent stress plot (next page). There are very localized stresses at the tip

under the load. Are these correct? The pattern of stresses near the base of the cantilever look

slightly odd. What looks odd? Why?


Ansys Lab #1 Exercises: Student:______________

For each of these exercises, modify the model that you have developed above to explore the

model behavior and answer the questions given. Show the instructor your results and make sure

that it is recorded that you have completed the exercises.

Exercise #1 – Refine the mesh. The default mesh results in only 3 bricks across the 180mm

web. So the elements are about 60x60x10mm. Set the mesh size to 20mm and compare the

results. Do this by inserting a sizing control in the mesh part of the project.

Deflection at end 60mm mesh 20mm mesh

deflection at end [mm]:

Eqv. Stress at base [MPa]

Eqv.Stress at end [MPa]

Comment:

(how does this illustrate

St.Venant’s Principle ?)


Exercise #2 – Redo the analysis using plate elements. Start by returning to SpaceClaim and

modifying the CAD. Beside your current brick model, draw a line 180mm high and use the pull

tool to pull it 1200mm. You will now have a plane that will be meshed as a plane, as the web of

the beam. There are several things that you need to do to make this work, but in the end you can

create two beams, practically identical, but one made from brick elements, while the other is

made from plate elements, as shown below. For the new shell model you will need to fix one end

and add the load to the tip of the opposite end

Deflection at end 20mm mesh brick 20mm mesh shell

deflection at end [mm]:

Eqv. Stress at base [MPa]

Eqv.Stress at end [MPa]

Comment:

brick

shell

lab#1 introduction to ansys finite element analysiscdaley/6002/6002_ansys_1_19.pdf · introduction...

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