Unknown Author - Tutorials In Finite Element Analysis Using MSC-Patran-Nastran [Unknown].pdf - FEA CFD - annam101

MSC/PATRAN TUTORIAL # 1

MODELING A BAR PROBLEM

I. THE PHYSICAL PROBLEM

In the simple bar problem below, there are three separate sections of the bar.

Each section has different properties. The following properties apply, Al

ã Aluminum, St ã Steel, E for Steel = 200 E9 Pa, E for Al = 70 E9 Pa

All Bars have square cross section and the right and left ends of the bar are

built in. The force "F" = 9000 Newtons

The 2-d model of the problem is shown below.

1 cm

2 cm

5cm

5 cm

10cm

II. THINKING ABOUT THE MECHANICS

The analytic solution for stresses and displacements for this problem is readily available.

Any Mechanics of Materials text will provide equations for the displacements and

stresses throughout the bar. The problem is indeterminant because there are two

reactions (one at each wall) and only one relevant equilibrium equation (

The normal stress due to axial loading is given by :

, where P is the internal force in the axial direction and A is the cross sectional

area of the bar. The displacements are computed from

u =

here L is the bar’s length

and E is the Elastic (Young’s) modulus.

Some basic questions to consider before creating the computational model are:

1. Where will the stresses be tensile and where will they be compressive?

2. What will be the magnitude and direction of the reaction forces?

3. Where will the displacements be greatest?

4. How do the displacements vary along the length (linear, quadratic etc.)?

5. What will the local effect of the concentrated load be on the stresses?

6. Is the model fully constrained from rigid body rotations and displacements?

Answering these questions qualitatively, along with the quantitative analytical solutions

for the stresses and displacements, will provide reinforcement that your computational

model is correctly constructed.

III. GEOMETRIC AND FINITE ELEMENT MODEL

Some general notes on PATRAN:

A general finite element analysis can be broken down into 3 principle tasks;

preprocessing, analysis and post processing. The preprocessing task includes building the

geometric model, building the finite element model, giving these elements the correct

properties, setting the boundary conditions and loading conditions and finally, assembling

these elements into a connected structure for analysis. The analysis stage simply solves

for the unknown degrees of freedom, as well as reactions and stresses. In the

postprocessing stage, the results are evaluated and displayed. The accuracy of these

results is postulated during this postprocessing task.

The Patran and Nastran software together perform all 3 of the principle tasks of a finite

element analysis. The pre and post processors are unique to PATRAN itself. However,

this package allows the user to do the actual solution analysis on a variety of different

packages. At many sites you have the option of using the MSC/Nastran package, which is

probably the most widely used solver in industry. Many of the other packages

commonly used in industrial settings (ABAQUAS, ANSYS, MARC) are also compatible

with PATRAN.

F ).

Therefore, it is necessary to use the Mechanics of materials (stress and or displacement)

equations as well as the force equilibrium equations to solve the problem.

IV. FINITE ELEMENT THEORY

The exact details of the formulation of the rod elements in MSC/Nastran is given in the

MSC/Nastran manuals and is somewhat lengthy. However, the basic formulation of an

isoparametric 2 node rod element is not difficult and will provide us with sufficient

background information to begin to understand the convergence and other accuracy

studies. This basic form can be found in any standard text of finite element analysis. For

Example see Finite Element Modeling for Stress Analysis, by R.D. Cook, John Wiley &

Sons, 1995.

V. STEP BY STEP INSTRUCTIONS FOR MODELING THE BAR PROBLEM

USING MSC/PATRAN

Unless you have used the PATRAN software numerous times in the past, the steps shown

below should be followed exactly. However, in order to prepare you to do independent

finite element work using PATRAN in the future, you are encouraged to go back after

you have completed the assignment and investigate modeling options using different

PATRAN selections. Also, I encourage you to take notes as you go through this exercise

in order to prepare for the time when you will be asked "build a certain geometric

structure" or "apply a certain type of boundary condition" with out being given the

specific steps for carrying out this task.

The MSC/Patran program is menu driven much in the same way that most Windows

programs are driven. Selecting a category from a menu may result in a pull down set of

options or in a subordinate menu. Selections in menus may be in the form of buttons to

turn on or off, or in the form of boxes which require text. Text entered into boxes may be

changed by positioning the cursor at the point of text insertion and either typing the new

text or erasing the incorrect text. A standard finite element analysis normally proceeds

across the top menus starting with Geometry and ending with Results. Selecting one of

these top menus results in a set of menus which allow you to complete that task in the

analysis process. Generally, it is best to attempt to proceed from the top of these menus

toward the bottom, answering questions as you go.

Preliminaries for using MSC Patran and Nastran normally include:

1) Log in to the machine.

2) Change to the directory that you wish to contain your results.

3) To start the program MSC/Patran, click on Start/Programs/MSC(common) and choose

MSC Patran 90.

In the instructions below, the following abbreviations and terms will be used:

TM = Top Menu . This refers to the horizontal menu options residing at the top of the

screen after PATRAN has been initiated.

RM = Right Menu . This refers to the menus that pop up after an option has been chosen

from the top menu. These menus reside on the far right side of the PATRAN desktop.

SM = Subordinate Menu . This referees to the menus that pop up from options selected

in the right menu.

Click = Unless otherwise stated, this indicates a click with the left mouse button.

Boldface will indicate text that occurs in the PATRAN menus.

Italics text will indicate text that you must enter into text boxes in the PATRAN menus or

text that you choose in a menu scroll box.

Our first step is to create a new database:

From the TM choose File

A SM called New Database pops up

Turn off (no check) Modify Preferences

If the new database for has come up showing a directory on a

remote computer (as opposed to a directory on the local machine),

then switch the directory to the local directory c:\MSC

Under New Database Name enter bar.db

Click OK

The geometry of the structure will be determined next:

From the TM choose Geometry

A RM called Geometry will result

Set Action = Create

Object = Point

Method = XYZ

Set the Point ID list to 1

Set Reference Coordinate Frame to Coord 0

Turn off the Auto Execute button

Enter the following into the point coordinates list:

[0,0,0] [.05,0,0] [.10,0,0] [.20,0,0]

(note that PATRAN will accept either commas or blanks as separators

between coordinates)

Click Apply

( At this point 4 points should appear on your "bar.db - default_viewport - default_group

- entity" main viewport)

The next job is to connect these points to form 3 lines:

While still in the Geometry RM,

Set Action = Create

Object = Curve

Turn off the Auto Execute button if it is on

In the resulting pull down menu choose New

Method = Point

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