StrukturlaborFEM With Ansys 2012

www.structures.ethz.ch

CENTRE OF STRUCTURE TECHNOLOGIES

28.02.2012, page 1

Finite Element Modeling with ANSYS

Strukturlabor

Spring Semester 2012

Alberto Sanchez

Sebastian Kollert

Timo Hilsdorf



28.02.2012, page 2

Contents

What is the Finite Element Method?

Modeling with ANSYS

- Element type

- Modelling of composite materials

- Boundary conditions and load introduction

- Analysis



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For many engineering problems analytical solutions are not suitable because of the

complexity of the material properties, the boundary conditions and the structure

itself.

The basis of the finite element method is the representation of a body or a structure

by an assemblage of subdivisions called finite elements.

The Finite Element Method translates partial differential equation problems into a

set of linear algebraic equations.

Reference: Introduction to the finite element method, C. S. Desai and J. F. Abel, Van Nostrand Reinhold

Company, New York, 1972.

FqK Nodal vector force

Stiffness matrix

Nodal displacement vector



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Contents


Modeling with ANSYS

Element type

Modelling of composite materials

Boundary conditions and load introduction



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Modeling with ANSYS

The modeling procedure is the following:

GEOMETRY

ELEMENT TYPE

MATERIAL PROPERTIES

MESH DEFINITION

BOUNDARY CONDITIONS

ANALYSIS

POST PROCESSING

Shell or solid?

Isotropic or anisotropic material?

Static – Buckling analysis

Including the load introduction



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Modeling with ANSYS – Geometry and Element type

The geometry and the element type have to be considered together.

Shell element are typically used for structure where the thickness is negligible

compared to its length and width

Nevertheless, a plate modeled with solid element would provide similar results. The

disadvantage lies in the computation time.

Ansys provides a large choices of elements.

Shell181 Shell281 Solid186



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Modeling with ANSYS – Geometry and Element type

The Ansys command to define the element type is:

Or use menu: (Preprocessor- Element Type- Add/Edit/Delete- Add)

The geometry is defined with key points that are connect together to obtain either an

area or a volume.

Or use menu: (Preprocessor- Modeling- Create)

1:(0,0,0)

2:(1,0,0)

3:(1,0.5,0)

4:(0,0.5,0)



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Modeling with ANSYS – Material properties (1)

Isotropic material like aluminum

Orthotropic material:

1. Determination of the engineering constants (Using C.A.P.)

2. Or implementation of single layers in Ansys

Or use menu: (Preprocessor- Material Props- Material Models- Structural)



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Modeling with ANSYS – Material properties (2)

Sections:

Build laminates in Ansys

(Preprocessor- Sections- Shell- Layup- Add/Edit)

Or:

sect,1,shell,,up

secdata, 0.2,2,45,3

secdata, 0.2,2,-45,3

secdata, 0.2,2,45,3

secoffset,MID

seccontrol,,,, , , ,



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Modeling with ANSYS – Mesh definition (1)

Before meshing, it is necessary

1. To select the geometry to mesh

2. To give a material type

3. To give an element type

4. To select the mesh type

Free or mapped meshing

5. To define the mesh refinement



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Meshing different sections by picking

1. Select an element type

2. Select a coordinate system

3. Select a section

(Preprocessor- Meshing- Mesh Attributes- Picked Areas)



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Coordinate system:

Plot element cs: PlotCtrls- Symbols- ESYS Element coordinate sys (on)

Create local cs:“local,12,0,0,0,0,45,45,45”

give number offset, rotation

Cartesian



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Modeling with ANSYS – Boundary Conditions (1)

Displacements constraints

nsel,s,loc,x,50-1,50+1

nsel,r,loc,y,-1,1

cm,support1,node

nsel,s,loc,x,550-1,550+1

nsel,r,loc,y,-1,1

cm,support2,node

cmsel,s,support1,node

cmsel,a,support2,node

D,all, , , , , ,uy, , , , ,


cmsel,a,support2,node

D,all, , , , , ,uy, , , , ,


nsel,r,loc,z,-1,1

D,all, , , , , ,ux,uy,uz, , ,


nsel,r,loc,z,b-1,b+1

D,all, , , , , ,ux,uy, , ,

Select „support“ nodes

and create components

Select components and

fix vertical displacement

Select other nodes and

fix displacements to get

a statically

determinate structure



Loads introduction:

The load is introduced into the structure by a “rigid” cylinder, that contributes to

distribute the stresses. How can we model this effect?


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Simple line load Modeling the cylinder

and the contact




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Select nodes and create

component „mid“

Create rigid element between

the nodes (Couple DOF)

Apply a force on each node

nsel,s,loc,x,l/2-1,l/2+1

nsel,r,loc,y,h-1,h+1

cm,mid,node

cp,next,all,mid

*get,nodes_num,node,0,count

F,all,Fy,-10000/nodes_num

! Target element type: rigid cylinder

ET,11,Targe170

! Contact element type: deformable elements

ET,12,Conta174

! Setting contact options

KEYOPT,12,4,2

KEYOPT,12,5,1

KEYOPT,12,9,1

KEYOPT,12,10,2

KEYOPT,12,11,1

KEYOPT,12,12,0

KEYOPT,12,2,0

KEYOPT,11,1,0

KEYOPT,11,2,1

KEYOPT,11,3,0

KEYOPT,11,5,0

! Real Constant

radius = 15

R,20,radius,radius,1.0,0.1,0, !Real Constants

Real,20

RMORE,,,1.0E20,0.0,1.0,

RMORE,0.0,0,1.0,,1.0,0.5

RMORE,0,1.0,1.0,0.0,,1.0

! Contact material, friction coefficient

MP,MU,13,0.01

Mat,13

! Mesh deformable contact elements

asel,s,area,,4

nsla

lsel,s,line,,3,7,4

nsll,a

nsel,r,loc,x,l/2-15,l/2+15

Real,20

Type,12

Esurf

! Create rigid target element

N,100000,l/2,h+radius+.1,-b/2

N,100001,l/2,h+radius+.1,1.5*b

N,100002,l/2,h+radius+.1,b/2

Real,20

Type,11

TSHAP,CYLI

E,100000,100001

TSHAP,PILO

E,100002

! Reverse normal vector

ESEL,S,TYPE,,12

ESEL,R,REAL,,20

ESURF,,REVERSE

!Loads

nsel,s,node,,100002

f,all,fy,-10000

Simple line load Modeling the cylinder and the contact




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Simple line load Modeling the cylinder and the contact

CPU Time

8.3 s

„Rigid element“ Displ

-1.01 mm

Max Von Mises stress

150.1 MPa

CPU Time

45.4 s

„Cylinder“ Displ

-1.07 mm

Max Von Mises stress

141.3 MPa

DO NOT USE VON MISES STRESS WITH ORTHOTROPIC MATERIALS!



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Modeling with ANSYS – Analysis

Static Analysis

Buckling analysis



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Modeling with ANSYS

Ansys Help: “help, command”

View command lines: List- Files- LogFile



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Thank you for

your attention.

StrukturlaborFEM With Ansys 2012

Documents

finite element

ansys boundary

ansys mesh

ansys material

boundary conditions

material properties

element type

coordinate