AAE 320 - Introduction to PATRAN And

8/14/2019 AAE 320 - Introduction to PATRAN And

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AAE 320 - Introduction to PATRAN and ABAQUS

Lab. 2 : A simple 2D plane stress problem with PATRAN

1. Introduction and objectives

During this lab, we will use PATRAN to analyze a simple 2D plane stress problem. This lab will demonstratesome of the graphic capabilities of PATRAN. Some of the concepts that will be introduced are:

-creating a 2D geometry and finite element mesh using PATRAN-applying a pressure type boundary condition-introducing 2D 4-node quad elements-controlling the size of the elements-obtaining 2D contour plots of the results

y

x

2 cm

4 cm

4 cm

1000 N/cm

Figure 1. Problem geometry.

As illustrated in Figure 1, we will investigate the problem of a horseshoe-shaped plate subjected to asymmetric distributed loading. Taking advantage of the symmetry of the problem, we just need to discretizethe upper half of the plate. What boundary condition do we need to apply along y=0?

The plate is made of Aluminum (E=70*109 Pa and =0.3) and has a thickness of 0.2 cm.

2. PATRAN session

Before you can run PATRAN, you must first modify your .cshrc file as described in the general manualpages describing the various commercial codes available on the EWS (enter man software for more info)

set path=($path /patran3/bin)setenv P3_ENABLE_NFS_DB_ACCESS yes

Since the files that PATRAN generates are always very large, it is highly recommended to first move to thescratch space.

Start PATRAN by enteringp3 & (use the & to run PATRAN in the background while keeping control on your window)


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The main window will open. It is divided in four parts arranged horizontally. The top section corresponds to executive, or housekeeping functions (such as opening a newdatabase or changing the display). These selections will be referred to in bold characters on thefollowing pages. The selections below the horizontal line and indicated with circular buttons correspond to separatefunctional areas of the program (creation of the geometry, creation of the mesh, application of theboundary conditions, definition of the material properties, definition of the element properties, ...).Only one can be selected at a time. Once selected, these will open additional windows on the right

side of the display. These windows may have functions that will open sub-windows. These functionbuttons are denoted with a diamond u in the following pages. The third section is the Quick Pick section which controls the display. By using the Quick Pick, itis easy to change the viewpoint, zoom in and out, hide node numbers, etc. Finally, the last section provides details on the commands executed by PATRAN and gives errormessages.

Note that PATRAN has a very large number of options. In the following text, only some of the mostimportant options and those which need to be modified will be mentioned. You should feel free toexperiment with all aspects of PATRAN in order to familiarize yourself with its abilities.

Once PATRAN has started, the analysis can begin. All analyses performed with PATRAN include thefollowing series of steps:

1) creation of a new database and selection of model preferences2) creation of the model geometry3) meshing the created geometry4) application of the various loading conditions5) entering the material properties6) defining the element properties7) generating a load case8) submitting the model for analysis9) reading the results of the analysis

You will be led through each of these steps in the following pages.

1) Creation of a new database and selection of model preferences.

First, we need to create a new database (which we will call lab2). This means that most of the files thatPATRAN creates will have the prefix lab2. The most important one will be lab2.db, which contains thewhole model. We will also create a session file (with suffix .ses) which will contain the list of all thecommands that you are about to enter. This file is very useful when you have to create a complicated modeland want to modify only some par ts of the model. You can edit the session file and then re-run itautomatically. This file is also considerably smaller than the .db file, and it may be useful to save only thisfile in order to conserve disk space.

The default name for the session file is patran.ses.01. Before starting the analysis, we will change this

name to lab2.ses. Select

FileSession Record Recording file=lab2.ses Apply

Next, we create the database and select the model preferences (i.e., the code we will use to perform theanalysis after we create the finite element model),


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FileNew... New Database Name=lab2 OK

This step will take a few moments as PATRAN creates a very large file (about 6 MB). Once again, if youhave not moved to the scratch space before starting the analys is, you will most probably get a quotaexceeded error message at this point. If so, quit PATRAN, move to the scratch space and restart the code.You may also get an error message regarding some protection problem. Disregard the message and clickYES to continue. The viewport window (on which all the graphics will be displayed) will be created. Also,a window entitled New Model Preferences) will appear, giving you the choice of various codes to perform

the analysis (remember, PATRAN is just a pre- and post-processor). In this series of labs, we will chooseABAQUS which is available on the EWS. You must also enter the type of analysis you plan to do (structural,thermal, ...). You can review and modify these preferences at all times by selecting

PreferencesAnalysis...

2) Creation of the model geometry.

In thi s step, we genera te the geomet ry to be later discretized with finite elements. This is done usinggeometric entities of increasing dimension (i.e., 0D entities:points, 1D entities:curves, 2D entities:curves,

and finally 3D entities:solids). Note that 3D entities will not be used in the present lab and will be introducedduring a later lab session.

In this case, we will subdivide the top half of the horseshoe plate into two separate sub-surfaces as shown inFigure 2. Then, we will define the mesh on the created surfaces. We will use a meshing pattern thatgenerates a finer mesh on the inside of the curved portion where the gradients are expected to be the highest.

Lets begin by defining the points. The only reference points that need to be defined are :Grid point A located at (0,0) (Note that we are using cm as the unit of length.)Grid point B located at (2,0)Grid point C located at (8,2)Grid point D located at (8,4)

To enter these points, use

uGeometryAction=Create Object=Point Method=XYZPoint Coordinate List=[0 0 0] Apply

Note that the first point will appear on your screen. The syntax for entering coordinates is either [0 3 -6.2]or [0,3,-6.2] or [0/3/-6.2]. You may also use operations to define a point. For example, the point

(3/64, 2 ,0) may be entered as [`-3/64`,`sqrt(2.0)`,0] (use back quotes). Continue to enter the remainingpoints as follows.

Point Coordinate List=[2 0 0]Point Coordinate List=[8 2 0]

Point Coordinate List=[8 4 0]

Note that, if you enter a wrong coordinate, you may always use either the UNDO button in the top rightcorner of the main window, or the Action=Delete option. If nothing appears on your graphic window, useDisplay/Geometry to increase the size of the points, to show the labels of the geometrical objects,

After defining the 0D objects (points), we move on to the 1D objects (lines and curves). There are manyways to create a wide range of different curves in PATRAN, starting from the simplest ones (lines) to morecomplex ones (arcs, splines, ...). Spend a few moments reviewing some of the methods available.


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Let us start by creating the arcs AE and BF (Figure 2).

Action=Create Object=Curve Method=Revolve

The Axis box defines the direction of rotation of the curve using the right hand rule. The first coordinate isthe bottom of the arrow, and the second coordinate is the tip of the arrow. To create AE and BF, we willrevolve points A and B about an axis centered at (4,0) and parallel to the z-axis. So in the Axis box enter:

{[4 0 0] [4 0 -1]}

Make sure that the rotation angle is set to 90 degrees in the box marked Total Angle and use the mouse toselect point A and then point B. Curves AE and BF are then automatically created.

Next we create the straight line curves AB, CD, EF, CF, and DE.Action=Create Object=Curve Method=Point (2 Point option)Starting Point List=Ending Point List=

Since it is in Auto Execute mode, the curve AB will be automatically created. Repeat this procedure forthe remaining lines CD, EF, CF and DE.

Now that all of the 1D objects (curves) have been defined, we can create the 2D elements (subsurfaces).Once again, there are numerous ways to create surfaces in PATRAN. Spend a few seconds reviewing them.One of the simplest way is to define a surface between two curves

Action=Create Object=Surface Method=Curve (2 Curve option)Starting Curve List=Ending Curve List=

Sub-surface #1 should appear in green. Repeat to create sub-surface #2. You should get a geometry similar tothat shown in Figure 3 (without the grid).

3) Create the finite element mesh.

This is the most important step of the pre-processing. The quality of your finite element mesh will stronglyinfluence the precision of your results. It is always advisable to use smaller elements in regions where thingshappen (i.e., where you expect the stresses to be the highest, or the deformation gradients to be thestrongest). The meshing process is performed in three steps:

a) Preparation of the meshing by defining mesh seedsb) Actual meshingc) Cleaning and optimization

However, before we proceed, it is usually a good idea to create a new group for the finite element model,

thereby separating it from the geometry. This allows us to create (and save) many finite element modelsassociated with the same geometry. It also makes it easier to keep track of things and make selections,because the two models can be displayed separately. So far, all we have created has been put in thedefault_group. Lets create a fem_model group containing the whole mesh.

GroupCreate

New Group Name=fem_modelnMake Current (all new entities will belong to that group)Apply


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3 a) In the first step, the user tells PATRAN how to distribute the elements to be created on the geometricalmodel. In our case, we want 10 equally spaced elements along curves AE, ED, BF, and FC. To do this:

uFinite ElementsAction=Create Object=Mesh Seed Method=UniformNumber=10Curve List=Apply

The mesh seeds will appear as yellow circles on the model.

Curve List=Apply

Repeat this procedure for curves BF and FC.

Along the remaining curves, we want to vary the size of the elements in order to capture the stressconcentration that will occur along the inside of the curve. To do this we will place mesh seeds that arebiased toward the inside curve.

Action=create Object=Mesh Seed Method=One Way BiasNumber=5L1/L2=2.0 or 0.5 (depending on the direction of the blue arrow appearing along thecurve AB on your screen)Curve List=Apply

Repeat this procedure for curves EF and CD

3 b) Next we perform the actual meshing. We will use 4-node quads on both sub-surfaces. Note that manyother types of elements are available (Quad5, Quad8, Quad9, Quad12, Quad16, Tria3, Tria6, ...)

Action=Create Object=Mesh Method=SurfaceElement Topology=Quad4Mesher=IsoMeshSurface List=Apply

Surface List=Apply

The created elements will appear on the screen and your mesh should look like that presented in Figure 4.

3 c) Since the two surfaces share a common side, there are redundant nodes along this side (curve EF). Wewill use the Equivalence action to collapse these nodes, and then we will optimize the new mesh numberingwith the Optimize option.

Action=Equivalence Object=All Method=Tolerance CubeApply

Note that the redundant nodes (i.e., those to be collapsed) appear in purple on your screen.

Action=Optimize Object=Nodes Method=Both Minimization Criterion=BandwidthApply

A table will appear showing the evolution of the bandwidth during the optimization process. Note the drasticreduction achieved... You can view the node and/or element numbers with Display/Finite Elements Tomake sure that no element is missing, you can shrink all the elements with the FEA Shrink Ruler.


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4) Application of the boundary conditions.

In this step we apply the symmetry boundary conditions and the applied uniform load. To do this, we willfix the displacement along the bottom edge of sub-surface #1, and apply a uniform pressure along the topedge of sub-surfaces #1 and #2.

Lets begin with the displacement boundary condition on AB. To indicate that AB is a line of symmetry, wewant to impose zero v-displacement (i.e., no vertical displacement) along the whole line AB.

u Load/BCsAction=Create Object=Displacement Method=NodalNew Set Name= symm_bottom (the name is arbitrary)Input Data...

Translations=< , 0, >OK

Select Application Region...Select Geometric Entities=

(Note: PATRAN may not select the desired entity. Make sure that the Curve icon is selected at the bottomof the screen. Note also that you can apply the boundary conditions directly on the finite element modelinstead of applying them on the geometry)

Click Add

Click OKClick ApplySymbols indicating constrained degrees of freedom will appear on the screen.

But this is not enough : the whole model is still able to translate freely in the x-direction. We must thereforefix completely one of the nodes along AB (say, point A).

Action=Create Object=Displacement Method=NodalNew Set Name= fixed_A (or whatever you might choose)Input Data...

Translations=< 0 , 0 , >OK

Select Application Region...

Select Geometric Entities=Click AddClick OK

Click Apply

Now we apply the uniform load along the top edge of our domain. From the current Load/BCs window:

Action=Create Object=Pressure Type=Element UniformNew Set Name= press_topTarget Element=2DInput Data ...

Edge Pressure=-1000. (Note that we apply a negative outward pressure)

OKSelect Application Region

Select Surfaces=

(Note: Patran may try to select the entire surface. If this is the case, select the edge icon in the little windowthat opened on your screen).

Click AddClick OK

Click Apply


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5) Entering the material properties

This step is self-explanatory

uMaterialsAction=Create Object=Isotropic Method=Manual InputMaterial Name=AluminumInput Properties...

Constitutive Model=ElasticElastic Modulus=70e5 (in N/cm2, remember, all our dimensions are in cm)Poissons Ratio=0.3ApplyCancel (Note : there is a major and obvious difference between Cancel and Clear)

6) Defining the element properties

Here, we decide the type of element to be used and associate a material property with each element. In thiscase, we want to use plane stress elements made of Aluminum.

u Properties

Action=Create Dimension=2D Type=2D SolidProperty Set Name=set1 (or whatever name you wish to give it)Option(s)=Plane StressInput Properties

Material Name=Thickness=0.2OK

Application RegionSelect members=Add

Apply

7) Defining a load case

In this step, we define a load case to be analyzed using ABAQUS. This load case can be any combination ofthe Load/BCs defined earlier.

uLoad CasesAction=CreateLoad case name=load1Assigned Loads/BCs Sets=OKApply

8) Submitting the model for analysis

uAnalysisAction=Analyze Object=Entire Model Method=Full RunStep Creation

Job Step Name=my_stepSolution Type: Linear StaticSelect Load Case=

OKApply


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AAE 320 - Introduction to PATRAN And