INTRODUCTION TO FINITE ELEMENT ANALYSIS …home.agh.edu.pl/~pawelp/?download=FEA.pdfINTRODUCTION TO FINITE ELEMENT ANALYSIS USING MSC.PATRAN/NASTRAN Objective The goal of this exercise

INTRODUCTION TO FINITE ELEMENT ANALYSIS USING

MSC.PATRAN/NASTRAN

Objective

The goal of this exercise is to prepare a finite element model using surface representation of an object

from CAD system. Then a linear static analysis of the model is performed. Boundary conditions are

applied according to the loading for the real component of the structure.

Typical workflow

A typical workflow for an analysis using the finite element method and MSC.Patran consists of the

following steps:

importing the three dimensional model of an object into preprocessing tool (MSC.Patran) - once

the 3D model of the part is prepared in CAD system it is imported to FEA preprocessor. It is

worth noting that the model is typically imported as a boundary representation of the object (B-

REP).

surface mesh generation - various meshing tools are used to prepare the surface mesh based on B-

REP. The mesh has to be checked for consistency (gaps) and the orientation.

volume mesh based on the surface mesh - if a proper surface mesh has been prepared it may be

used to create the volume mesh.

All three stages must be completed before proceeding to the analysis. The above tasks must be

accomplished by using the tools described below.

Requirements

The following requirements will be checked during the exercise:

1. CAD model import (File - Import)

2. Control of the surface mesh (Mesh seed tool, element size)

3. Surface mesh definition (Elements menu):

- closed mesh (no gaps in the model; tools: Nodes equivalence, move, translate etc.)

- normal vectors orientation (consistent orientation for the whole surface mesh; tools: Verify

element normals, reverse direction)

4. Volume mesh definition (tools: Create Mesh Solid)

5. Boundary conditions (BCs menu)

6. Analysis definition (Analysis menu)

Exercise outline

This section presents some important points of the modelling process. Generally, the following

substages may be distinguished.

Model setup

Model definition

Geometry

Mesh

Boundary conditions (BCs)

Material properties

Loadcase

Analysis

Results evaluation

Model setup

Warning! Due to specific work organisation of the program it is particularly important to follow

exactly the steps outlined below. Otherwise problems with results evaluation may occur!

Create new working folder.

Create new file, change its name and extension to *.db

Double click to open the file in default program - Patran. In Patran choose new file

and select previously created file

confirm creating new file message

Next, choose model preferences

Model definition

Model geometry

Open geometry modeller

Import the CAD model file by using File - Import.

You may change display mode by using various options from the bar at the top of the screen

Lighting can be managed by through Light sources menu

Select all light sources and apply

Model rotation, panning and zooming may be carried out by selecting the mode (F2-F6 buttons),

pressing the middle mouse button and moving the mouse.

Mesh definition

Once the geometry has been imported, proceed to mesh definition

Use the tools described in the tools section of this document to apply mesh seed and create the surface

mesh. Then, use the surface mesh to create volume mesh.

Boundary conditions

Open Loads/BCs menu

Define new boundary condition - fix nodes at one side of your model. Specify BC's name, ...

... constraints (DOFs), ...

... and application region

apply changes

Fixed nodes should look like in the figure below

Prescribe a force at selected node

Change BC type to force

Specify desired force magnitude and apply

Select application region, apply

The force boundary condition should look like this

Material properties

Open material definition menu

Specify material's name

and its properties

Note that you must specify all parameters in consistent unit system. Do you know why?

Save changes by clicking apply

Open properties menu from the bar at the top of the screen, specify name

Select "Input properties", and choose already defined material by clicking material button

choose defined material

Specify application region

select all the elements and apply

Loadcase definition

Apply specified loads to create a loadcase. Open loadcase menu, select Action: Modify

Select default Loadcase

check if both previously defined boundary conditions are selected

Confirm by clicking apply

Analysis

proceed to analysis menu

switch analysis method to "analysis deck", this will create solver input file, but will not submit it to the

solver

Check it the solution type is set to static analysis and apply

Find the BDF file in your working folder

Open file in notepad, try to recognize entries in BFD file.

Run Nastran

Browse BDF model file

Run the analysis

After Nastran finishes, check if *.xdb result file has been created

Result evaluation

In Analysis menu, find "Access Results" option

Select results file and apply

Go to Results menu

Check results you want to display

You can change display settings, e.g. turn off undeformed model configuration

and geometry in Plot/Erase menu

Tools

The following tools might be useful when defining a finite element mesh based on the surface

representation of an object:

Menu: Elements

Create - Mesh seed: allows to define a set of points on the surface' edges that will be subsequently

used for the mesh definition. The nodes of the final mesh will be created in those points, thus it is

possible to control the mesh resolution at the surface outline. This option should be used to adjust the

mesh around corners, holes etc.

Create - Mesh - Surface (Elem shape: Tria): this option is used for surface mesh definition. Element

size may be input here. Surface mesh is defined on a surface, preferably with mesh seed defined in the

previous step. Make sure that the triangular element shape is selected.

Create - Mesh - Solid: enables the volume mesh definition. Solid mesh requires consistent surface

mesh with correctly oriented normal vectors. Make sure that the mesh does not contain any gaps and

forms a closed volume. This option uses a set of element as an input.

Verify - Element - Boundaries: allows to check whether the surface mesh is correctly defined. If the

surface mesh does not contain gaps only the outline and sharp edges should be visible in the preview

window. To return to model view use Reset Graphics button.

Verify - Element - Normals: the normal direction for surface elements may be checked. The surface

mesh is defined correctly if the surface normals are pointing inwards or outwards of the component

(direction is not important). The guiding element may be chosen to change the normals according to

its normal vector. In order to change the normal vector orientation choose the button in the Test

Control box within this tab. Next, choose the guiding element and apply changes.

Equivalence - List: this option enables to merge nodes that are close to each other. In this way a gaps

in the surface mesh may be closed. If there is a gap between two neighbouring elements (as checked

with Verify - Element - Boundaries) the nodes of these elements may be selected and equivalenced.

The effect of the equivalencing may be checked by using the Verify - Element - Boundaries tool.

Other tools may be also used. Especially those from Transform action of the Elements menu.