Element_Selection_CAEA[1].pdf

7/30/2019 Element_Selection_CAEA[1].pdf

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ANSYS Elemente ec on

eLearning

Peter Barrett

October 2012

2012 CAE Associates Inc. and ANSYS Inc. All rights reserved.


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ANSYS Element Selection

What is the best element type(s) for my analysis? Best Answer - It depends!

What is the oal of the anal sis?

Displacements, temperatures, mode shapes, nominal strength, fatigue life,stresses from fluid structure interaction, etc.

What are the shapes of my parts/assembly?

What types of symmetry can I take advantage of? Is it lon and slender where beams mi ht be useful?

Thin walled where Shell or Solid-Shell elements can be used?

Should I model the full assembly in one shot or use submodelingand/orsubstructuring?

What type of computational requirements are needed?

Linear vs. Nonlinear Static vs. D namic Time Domain vs. Fre . Domain?

2

The more complex the analysis, the fewer elements can be effectively solved


3/23

ANSYS V11 186 Elements Available!

Some Beam, Pipe,Spar, Shell, Solid

elements were removed

after this release.

Although undocumented

in the code but requireusers to have access toV11 help or older

3

.


4/23

ANSYS V14 166 Elements Available!

Note:any new eemen s

have been added sinceV11 including newsurface effect and

a vance pp ngelements

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Element Types define the active DOF

The DOFs at a node are a function of the element type connected to thenode.

Be careful about the number of degrees of freedom generated in large

beam/shell models since 6 degrees of freedom are created at each node

3-D Spar (Pin Joints)UX, UY, UZ

3-D BeamUX, UY, UZ,

, ,

- 3-D Quadri lateral ShellUX, UY UX, UY, UZ,

ROTX, ROTY, ROTZ

3-D Thermal SolidTEMP

3-D Structural SolidUX, UY, UZ

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Element Shape Function

. An element shape function is the shape of the results within the

element.

w w :

Linear or lower-order elements: Corner or end nodes only.

.

(Using enhanced strain for quad/brick shapes will adds extra shapes for bending)

Only allows straight sides. Curves are faceted.

Quadratic or hi her-order:

Corner or end nodes and mid-side nodes.

Assume a quadratic variation of DOFs within element.

Allows for modeling of curved boundaries

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Guidelines for Element Selection in Meshing

Element Usage For curved surfaces use higher order

curved elements, especially forcontact problems

Quadrilateral elements are preferredover triangular elements, particularly

- .

For very large strain problems using

distortion

Avoid hi h as ect ratio of the element

1:9

May be1:3

Acceptable

1:1

Ideal

sides.

Need to also consider the gradient

excessive

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across the element to determine the

accuracy of the response


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Define Element Types

Beam Elements: Meshing of Line Bodies or Lines or through direct

node and element generation

Using a third node or orientation key point to orient

View /ESHAPE to check the X-sect orientation

Shell Elements:

Meshing of surface bodies / Areas

Element types controlled by assignment oradvanced mesh settings. Use consistent normals

Solid Element Midside Nodes:

Program controlled (default), dropped, or kept.

Element A Element B

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Kept Dropped


9/23

Why Use Beam Elements?

Advantages:

as o sove, o ow eam eory, ess resu s a a, orces an momen sand linearized stresses directly available

Disadvantages:

anno accoun or oca s ress concen ra ons, ore cu o e ne, m e

to constant or linear tapered cross-sections Common Errors

-

Insufficient constraints when used in conjunction with shell and solid elements

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10/23

Why Use Beam Elements?

Common Modeling Issues

: ape unc ons a ong e eng :

0 -- Linear (default) Recommend either 2 -- Quadratic or 3 -- Cubic (beam theory)

Mechanical defaults to Keyopt (3) = 2

The #of integration points in each cross section can greatly influence solution timeand results file size (Main Menu>Preprocessor>Sections>Beam>Common Sections >secdata)

For linear materials use a coarse cross-section mesh; to capture plasticity a refinedcross section mesh might be needed.

Model connections correctly (Fully fixed vs. pinned vs. partial moment release)

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ANSYS Beam Elements Tips

Lower Order Beam Elements (188)

ec anca e au

Recommended for straight beams and for combined beam/shell models whenconnected to lower order shells and solids ( Shell 181, Solid185 etc)

Higher Order Beams (189) are recommended for curved geometry and forcombined meshes with higher order shells/solids

Mechanical

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ANSYS Beam Elements Tips

spiders to spread the loading between bodies

Use MPC contact, Constraint Equations or spokes to distribute the load Spoke elements with lengths equal to half the member section is an preferred

.

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Why 2-d or Axisymmetric Solid Elements? Advantages:

, ,

Disadvantages:

Creating the 2-d surface and correct orientation

-

Common Errors Not modeling @ z=0 with positive X for Radius

e ectng ncorrect ormuaton ane tress vs. . tran vs. xsymmetrc

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Generalized Plane Strain

Generalized Plane Strain

A finite deformation domain length in the z direction (Z direction strain is notrequired to be zero)

Simulate 3-D deformations using 2-D options. Example Section of a cylinderor ong o y un er un orm emp. e omman s ,

GSGDATA, GSSOL,GSLIST)

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15/23

Why Use Shell Elements?

Advantages: Faster to Solve, No length to thickness requirements, Less data created,

Forces and Moments easily obtained

Disadvantages:

Difficult to define contact, normal's and connections

Common Errors

Definin normal's inconsistentl

Postprocessing the wrong surface

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16/23

Why Use Shell Elements?

Common Modeling Issues SHELL188: KEYOPT(3) Integration option:

0 -- Reduced integration with hourglass control (default)

2 -- Recommend Full integration with incompatible modes

Mechanical defaults to Shell 181 - Keyopt(3) = 2

Symmetry Boundary Conditions Make sure to also constrain the in-plane rotations for symmetric surfaces

urn arge e ec on n

For Pressure loads large deflection is often required since membrane stiffness is notaccounted for with small deflections

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17/23

Shell Elements Tips

Lower Order Shell Elements (181) Mechanical Default

ecommen e or a o es an or com ne eams e mo e s w enconnected to lower order beams and solids ( Beam 188, Solid185 etc)

combined meshes with higher order beams/solids Use Mesh Details > Advanced Settings to Force Higher Order Shells (281) in

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Solid Shell Elements

Mechanical.

Set the mesh method to Sweep.

.

Element Option: Solid Shell.

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Why Use Solid Shell Elements?

Advantages:

Faster to Solve, No length to thickness requirements, Thickness defined bygeometry, Auto Contact generation available, Easy transition to standard 3-delements, Automatically creates tapered elements

:

Sweepable Geometry and correct Element Orientation required Common Errors

Defining normal's inconsistently. Thin Section must be in element Z direction


Lower Order Elements require refined mesh to modal small radii geometry Meshing of complex geometry can be difficult

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20/23

Why Use Solid Elements?

Advantages:

Easy to Model, Provides 3-d geometry with results, Can include stressconcentrations

Disadvantages:

Computationally expensive, For thin and slender bodies may require excessive

element count

Common Errors

Using Singular results stresses at sharp corners, fixed supports, point loads


SOLID185 (Lower Order Brick): KEYOPT(3) Element technolo :

0 -- Full integration with B-BAR method (default)

1 -- Uniform reduced integration with hourglass control

2 -- Enhanced strain formulation Recommended for Bending Problems

3 -- Simplified enhanced strain formulation Mechanical defaults to Keyopt (3) = 0 when midside nodes are dropped

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ANSYS Mechanical Mixed Order Meshing

Solid186Solid185

Solid186Brick w/dropped

midnodes Solid186

Solid186Brick w/dropped

midnodes

Pyramid

Solid187

rc Solid186Pyramid

Solid187

Tet e

Edges ofMeshedVolumes

Nodes

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Elements (Side view of 1 row of elements from model above)


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Contact Elements

Contact Element Types

2D Surface-to-Surface

Plane Stress, Strain,Axisymmetric Elements

.

3D Surface-to-Surface Shell, Solid,-shell, SolidElements

Rigid or Flex. Bodies

Point-to-Surface

Beams to shells or solids Line-to-Line

Beam-to-Beam

Bending or solid-to-

Line-to-Surface

Beam or Shell edge toSolid

22

Node-to-Node

Specialty elements


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Specialty Elements

Lumped Mass - Mass21

Keyoptdefines DOF (6 by default)

Set Keyopt3=2 to reduce DOF ifrotatry interia is not required

Make sure to s ecif Mass in all threedirections

Multi-Point Constraint - MPC184 Link, Slider, Revolute, Universal, Slot,

- - , , , ,Spherical, Screw, etc.

Large deflection response

Automated Generation in WB

Bolt Pretension PRETS179

Small deflection Only

Use MPC184 or contact surface offset

Spring - (Dashpot) Elements

Combin14, 37, 39, 40, 214

Original Spring Elements

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Combin214

Unsymmetrical Stiffness/Damping

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