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AAE 320 - Intr oduction to PATRAN and ABAQUS

Lab . 4: Moda l an alysis of a plate

1. Int r oduction and objectives

The objective of the lab is to obtain the first five natural vibration modes of a 2D cantilever plate withABAQUS and PATRAN. We will use PATRAN only as the pre-processor. We will use the *FREQUENCYfeature of ABAQUS to get the mode shapes and will use ABAQUS/Post to visualize them.

The new concepts introduced in this lab are:- basic commands for a simple modal analysis- some new commands with ABAQUS/Post

The plate geometry is described in Figure 1.

1 m

0.5 m

Figure 1

Our objective is to obtain the frequency and the deformed shape associated with the first five naturalvibration modes of the plate which has the following material properties:

Youngs modulusE = 206.8 GPaPoissons ratio = 0.3Density = 7827 kg/m3

Thickness t = 1 mm

2. Pr e-pr ocessing with PATRAN

The mesh generation step is very simple. Use 8-node quadrilateral plate (shell) elements. Use a simpleregular 10*5 mesh (Figure 2). The only load conditions you need to apply are the fixed displacements androtations on the right-hand side of the domain. When you enter the material properties, dont forget thedensity. Since there is only one surface, you dont need to perform an equivalence operation. However,you must optimize your mesh numbering. When you define your element properties, dont forget to enter theappropriate thickness (1 mm).

Finally, since we are going to run ABAQUS manually, select Ana lysis Deck(instead ofFull Run) in theuAnalysis step. At this point, leaves the Solution Type as Linear Static. We will modify the ABAQUS

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input file directly later.

Figure 2: Finite element mesh for plate vibration problem.

When you are done, exit PATRAN.

3. Solving with ABAQUS

Before running ABAQUS, we must make a few modifications to the input file (prefix.inp)

- to simplify the output file, enter, just after the title (i.e., on the third line of the .inp file)*PREPRINT,MODEL=NO,HISTORY=NO

- replace *STATIC by the following two lines*FREQUENCY5 (since we want to compute the first 5 natural frequencies)

- remove lines starting with *CLOAD and *DLOAD

- the end of the file, which contains all the information relative to the requested output, should look like this*NODE PRINT, FREQ=0 (this turns off the nodal output)*EL PRINT, FREQ=0 (this turns off the element output)*MODAL FILE (to write the eigenvalues in the output file)*RESTART, WRITE (to create the file needed for ABAQUS/Post)*END STEP

Run ABAQUS with the usual commandaba qus job=

After you get the solution (wait to get the Job completed message from ABAQUS), check the result file

(.dat) to get the values of the first five natural frequencies. Note that it also provides the location of thecentroid of the structure and the values of the moments and products of inertia.

4. Post P r ocessing with ABAQUS

Start ABAQUS/Post withaba qus post r estart =

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First, we want to change the view to better visualize the deformed shapesview, rot = (0,0,225) (rotation of 225 degrees about the z-axis)view, rot = (30) (rotation about x)view, rot = (0,-30) (rotation about y)

Then, enterset, eigen = 1 (or 2 or 3 or 4 or 5) to specify the eigenmode.draw, displaced

and the eigenmode will appear on the screen. Note the value of the corresponding eigenvalue.

To obtain an animation of, say, the second eigenmode, enter the following commandsset,eig=2drawset,capture=mode2.flcsequence,mode,frames=20 (this might take a little while...)set,capture=closeanimate, file=mode2.flc

Exercise

As an exercise, we will recreate the mesh of the reinforced plate of Lab 3 and find the natural frequencies.To recreate the mesh we will play the session file as follows:

File... Session... PlaySelect the appropriate session file. (make sure to move the session file to the scratch directory)

The session file is a list of the commands used during that particular PATRAN session. As such, it containsall of the steps performed (including the incorrect ones). A major advantage associated with the session filesis that they are very small compared to the PATRAN database file. However, if the session file is large, itmay take a while to run through the entire file.

You will notice that as the file plays several errors are encountered when the Analysis and Results steps areperformed. This happens because PATRAN is trying to read and display results before a results file ispresent. When these errors occur, simply click OK since we are not interested in the static displacements.

After the session file is finished running, you will probably be left without a viewport, or you will be dumpedout of PATRAN altogether. Since PATRAN plays all commands in the session file, it played the File... Quitor the File... Close command last. If you want to avoid that, just edit the end of the session file.

Now that we have the database file, we need to perform the following modifications in order to perform themodal analysis.

In Load/BCs delete the pressure loadIn Load Cases delete all but the Default load caseIn Fields delete all fieldsIn Materials, add the density to the list of material propertiesRedefine the element properties with the modified material properties.

Make sure that ABAQUS is selected as the analysis code in Preferences/Analysis. From here the analysis isthe same as in the example above.