Ex 7 Tetra vs Hexa Meshing

2-1

Exercise 7 Tetra versus Hexahedral Meshing

UGS Corp. 2006. All rights reserved.Femap with NX Nastran 1-2

Exercise 7 Tet vs. Hex Meshing

In this example we will explore two mesh representations

of the same model

First build by tetrahedral and second by of hexahedral solid elements

We start with loads and constraints and solution of the tetrahedral model

Next we will delete the Tetrahedral mesh and mesh the part with

Hexahedral elements. Again we will solve the model in Nastran



1. Import the Geometry

a) Select File, Import, Geometry and choose

the file Ch16hexmesh.x_t from Femap

training folder.

b) Accept or enter values in Solid Model Read

Options dialog box by clicking OK.

2. Rotate the view

a) Rotate the model into the position shown

here by clicking the left mouse button, and

holding it while dragging the mouse.



3. Apply the Geometric Load

Select Model, Load, On Surface

Type in title for Load Set, e.g. Vertical load on boss and click OK

Preview Selection icon if needed to display the selected surface before OK

From list select Force and enter value 200 in FZ direction, then OK

Click Cancel to exit



4. Create geometric constraints on

the inside of the bracket

a) Select the Model, Constraint, On

Surface

b) Enter a title for the Constraint Set,

press OK

c) Select three inside surfaces of the

bracket and confirm clicking OK.

d) Select Fixed constraint on the

geometry and press OK to create

the constraint.

e) Click Cancel to exit the command



5. Create the Material

a) Select Model, Material

b) In the Define Material

press Load for the list of

materials in the material

library and elect 2024-

T351 Al Plate .25-.5

and press OK.

c) Femap displays values

of aluminum material

filled in. Press OK then

Cancel



6. Create the Solid Property

a) Select Model, Property

and click on

Elem/Property Type then

select Solid. Press OK.

b) Property dialog box is

changed to Define

Property - Solid Element

Type.

c) Enter the title as Solid

Aluminum 2024-T351.

Set the material to 2024-

T351 Al Plate .25-.5.

d) Press OK and Cancel to

exit



The meshing process generally consists of 2 steps

Establishing mesh sizing and control on the part, and

Meshing the part

7. Mesh the part with tetrahedral

elements

a) Select the Mesh, Mesh Control,

Size on Solid command

b) In Automatic Mesh Sizing box,

change Element Size to 2,

Minimum Elements on Edge to 2,

and Max Angle Tolerance to 15.

c) Uncheck Max Element on Small

Feature option.

d) Press OK to set mesh size.



8. Mesh the part

a) Select the Mesh, Geometry,

Solids

b) In the Automesh Solids box,

set Tet Growth Ratio to 1.

This is recommended when

meshing thin-section parts

with solid elements.

c) Leave the option to create

Midside Nodes enabled (or

checked on).

d) Click OK to mesh the part.



9. Use Analysis Set Manager

to generate an analysis

solution

a) Select Model, Analysis

b) Click on New to create

analysis set.

c) Enter Title, select

31..NEiNastran for the

solver, and 1..Static for

analysis type.

d) Click OK to create the

Analysis Set.



10.Run Analysis

a) Click Analyze button, save Nastran file and press F5 in NEiNastran Editor

11.Close Editor when NEiNastran has finished

12.Display the Von Mises stress contour on the deformed shape

a) Select View, Select (F5)

b) Select Deform for Deformed Style and

Contour for Contoured Style and click

on Deformed and Contour Options.

c) Select Total Translation for deformation

and Solid Von Mises vector for contour

d) Save the model as tetmesh.mod, for

later comparison



13.Prepare the model for remeshing with hex elements

a) Save the model file as hexmesh.mod.

b) Turn off the contour and deformed displays using the Post toolbar

c) Delete the mesh using Delete, Model, Mesh, select All, then press OK

d) Choose NO so that created properties and materials remain in the model

14. Rebuild and compact the model which will also reset the entity counters

a) Select the File, Rebuild command.

b) Select Yes to compact the Femap database.



15.Subdivide the solid into ten

independent solids to aid in meshing

a) Run Geometry, Solid, Embed Face

b) Pick surface A and select Automatic

as the embed option, and press OK.

c) Repeat this for surfaces B and C.

d) For B, select the All Curves button.

A

B

C



16.Continue to subdivide the solid

a) Select the Geometry, Solid, Slice

b) Select solids 5 and 6, then OK

c) Use right-mouse click and select Snap to Point, select Points A, B, and C to define the cutting plane and OK to slice the parts

d) Repeat Geometry, Solid, Slice and select both halves of solids 5 and 7

e) In Plane Locate Specify Plane for click on Methods, and select Global Plane as the cutting method, the YZ plane as the cutting plane, and point B as the base for the plane.

f) Press OK to slice the solid.

ABC



17.Check to see that you sliced your part into the proper sub-solids

In Model Info use the command Transparent Highlight to display and

explore each solid which will highlight to see the separate solids



18. Hex mesh the model

a) Select the menu command

Mesh, Mesh Control, Size on

Solid. Select All solids and

press OK.

b) In the Automatic Mesh Sizing

box select Hex Meshing, set

Min Elements on Edge to 2.

c) Deselect both Max Angle

Tolerance and Max Elem on

Small Feature options.

d) Press OK to set the mesh size

on the parts.



19.The model is ready to mesh

a) Select the command Mesh, Geometry, Hex Mesh Solids.

b) Select All, and press OK.

c) We will use the default hex meshing options, so press OK to hex mesh

the part.



20.Two of sub-solids need to be

mapped meshed

The default free mesher will always use a

combination of triangles and quads on any

surface that is not a simple 4-sided region.

To force Femap to map mesh surfaces that

are not 4-sided, you will use the Mesh,

Mesh Control, Approach on Surface

command.

a) Delete the previous hex mesh using

Delete, Model, Mesh

b) Select All elements

c) Press No when prompted to delete

unused properties and materials.



21. Specify the Mesh Approach

a) Select Mesh, Mesh Control,

Approach on Surface command

b) Pick surface A then, select for

Mesher Mapped, Four Corner

option

c) Since the surface has more than

four corners, Femap will

automatically detect corners to map

between. Press OK.

d) The command will automatically

repeat. Perform the same

procedure for surface B

A

B



22. Continue to specify the Mesh Approach

e) Mesh Approach command continues to automatically repeat

f) Perform the same process for both surfaces C and D

C

D


23. Hex Mesh the Solid

a) Set the mesh size with the Mesh, Mesh Control, Size on Solid command.

Select All of the solids. Set the Size for to Hex Meshing, Element Size to 3,

Min Elements on Edge to 3 and Max Angle Tolerance to 15. Deselect the

Max Elem on Small Feature option.

b) Select the Mesh, Geometry, Hexmesh Solids command, Select All of the

solids, and mesh with the default options.



24. Solve the model and compare Solid Von Mises Stress contour

for hex meshed model with that obtained from the tet meshed

model you analyzed earlier

25. Perform the following below before analyzing the model

a) Because slicing of the solids for hex meshing caused the geometry

ID numbers to change, you cannot use the old load and constraint

sets for this model.

b) Delete the old load and constraint sets

c) Re-create them with the same load values and constraint

configurations



Comparison of the Solid Von Mises

stresses for hex meshed model with

that obtained from the tet meshed

model you analyzed earlier in the

exercise

Hex meshed model statistics

Nodes: 5568

Hex Solid Elements: 3942

Tet meshed model statistics

Nodes: 34307

Tet Solid Elements: 19756


Documents

Ex 7 Tetra vs Hexa Meshing