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Engineering 5003 - Ship Structures I
Lab#1
Introduction to ANSYS Finite Element AnalysisBy C. Daley
Overview
ANSYSis a general purpose program, capable of numerical simulation of a variety of physicalproblems. The types of problems include solid mechanics, thermal, electromagnetic and fluid
dynamics. The focus in this introduction will be on solid mechanics and structural behavior. We
will be demonstrating with the release 13.0.
ANSYS has existed as a program for many years (decades). However, it has been updated
significantly over the years, and is now very much more advanced in capability. The user
interface is a modern GUI that looks similar to many CAD packages. ANSYS includes a masterprogram called Workbench, that lets the user set up a project and keep all aspects of the
simulation together and connected. From Workbench the user opens various pre- and post-
processors that allow the user to describe the problem, specify the type and aspects of the
simulation and review the results. ANSYS is like a physical laboratory, where experiments canbe constructed, tested and measured.
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ANSYS Model #1simple 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. ANSYSconsiders 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.
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Step 3: open ANSYS Workbench 13.0 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 Structuraland
drag the icon to the right, placing it in the Project Schematicwindow.
The Workbench user interface, with a Static Structuralanalysis set selected.
Step4: open Geometry and create the CAD model1) By Clicking Geometry in the Project window, ANSYS will open a CAD modeling
program called Design Modeler.
2) Select Millimeterlength unit for the model.
3) You now see this window:
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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). This window has another tab at
the bottom called sketching. Click it now.
4) The Sketching window lets you do 2D sketching on a selected plane:
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The sketching window lets the user create and edit a variety of 2D geometric objects. This is partof creating a sketch from which 3D objects can be made.
5)
Select the rectangle tool and sketch a rectangle, taller than wide (you mayclick the LOOK ATicon );
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6)
Click the Dimensiontab (below Modify), and using the Generaldimension tool, selectthe right side of the rectangle and drag a dimension a bit to the right. Do the same for the
top. You will now see something like (a) below. In this case H5 is the width of the
rectangle and V1 is the height. Notice that the actual dimensions are shown in the Details
Viewat the lower left. You can directly type the dimensions you want and update the
height and width of the rectangle. So type in a height of 180, and a thickness of 10mm. as
in (b) below.
(a) (b)
7) To create a solid bar, hit the extrude icon
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8) Then hit Generate, and change view to isometric. You will see the solid bar in grey. This
is all we need to do in the DesignModelerfor now. (if the extrude distance is not correct ,
you can select the Extrudecomponent in the upper left window and edit its value in the
lower left. Then hit the button.
Step4: open Model and create the Finite Element model
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1) Return to the ANSYS window, and click on the Modelfeature in the Projectwindow.
This will start the ANSYS Mechanical program, to setup the actual finite elementmodel.
2) The Mechanicalwindow 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 Systemsand Meshfor 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.
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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 Outlinewindow. This will
open a sub menu. Move the mouse over Insertand 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 Applybutton on the lower left to confirm that you
want fixity applied to the selected face.
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Now the Fixed Supportis 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 inthe 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
Structuralin the Outlinetree, select Insert, Force.
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Select the line that defines the top-end corner of the bar, and click Apply. Then type
18000 into the Magnitudecell (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 Changebox under the Magnitudebox. 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 OutlineTree, 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).
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6) To specify output, right click on Solutionin the tree, and select Insert, then Stress, then
EquivalentStress. Do the same to select TotalDeformation.
7) Hit the button in the menu at the top of the screen.
8) When you select the Equivalent Stressunder Solutionin 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?
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Self Study 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 #1Refine 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 sizingcontrol in the meshpart 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 illustrateSt.Venants Principle ?)
Ex#1Initials of Instructor
_________
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Exercise #2Redo the analysis using plate elements.Start by returning to Design
Modeler and modifying the CAD. Create a new sketch beside the rectangle. Draw a line
180mm high and extrude it 1200mm. You will now have a plane that will be meshed as aplane, 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.
Deflection at end 20mm mesh brick 20mm mesh shell
deflection at end [mm]:
Eqv. Stress at base [MPa]
Eqv.Stress at end [MPa]
Comment:
Ex#2
Initials of Instructor
_________
brick
shell