ansys best practices

7/27/2019 Ansys Best Practices

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2008 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary

FEA Best Practices Seminar


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Introduction

FEA is not a black box; it

is a tool that can be

abused.

It is an approximation

Careful modeling and

interpretation of results

Garbage in, garbage out!


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Example: Sleipner A Oil Platform

Concrete structure with 24 cells,

sank in 1991 during prep for deck

installation

Extensive FEA was performed, butinaccurate modeling of the tricells,

the frame where three cells met, led

to an underconservative design

Shear stresses were

underestimated by 47%


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How can this be prevented?

Thorough planning

Careful modeling

Accurate loading andmodeling of supports

Thorough verification of

results


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Accurate Results Depend on:

Understanding the physics of the problem

Understanding the behavior of the elements

Selecting the correct element, the number of

elements, and their distribution Critically evaluating the results and making

modification in the conceptual model to improve their

accuracy

Understanding the effects of the simplifications andassumptions used

Using FEA wisely requires using best practices: this

seminar presents many of these, and discusses how to

develop best practices for your process.


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Guidelines for Best Practices

Department of Defenses Defense Modeling andSimulation Office

Recommended practices for large-scale simulations

Does not focus on first-principles directly

American Institute of Aeronautics and Astronautics(AIAA) has the Guide for the Veri f icat ion andVal idat ion o f Compu tat ional Fluid Dynam icsSimulat ions

FEA now has the recently formed ASME PTC 60

committee on Verification and Validation Recently published Guide for Verification and

Validation in Computational Mechanics


7/126 2008 ANSYS, Inc. All rights reserved. 7 ANSYS, Inc. Proprietary

V&V 10-2006

Provides structural mechanics community with:

Common language

Conceptual framework

General guidance for implementing V&V NOT a step-by-step guide!

Glossary of terms

Figures illustrating a recommended overall approach

Discussions of factors that should be considered

Written as guidance to developing V & V processes



V&V 10-2006: Elements of V&V

Intended Use

Modeling Activities

-Model Development-Verification

-Predictive Calculations

-Uncertainty assessment

Experimental Activities

-Experimental design-Initial and boundary conditions

-Response measurements

-Uncertainty assessment

Validation

Application



V & V

Model is validated for a specified use

e.g., for a certain range of loads

Modeler should quantify the uncertainties

Due to inherent variability of parameters Lack of knowledge of the parameters

Model form

Combine with calculation verification to get overall

uncertainty estimate Not always possible to test the full range of interest

Should still develop a plan of V&V



Best Practices Approach

Plan your analysis

Materials

Model geometry

Element choice

Meshing

Simplifications

Supports and Loads

Model Calibration

Verification



Plan your analysis

What are the design objectives?

What do you need to know?

Why are you doing FEA?

What is the design criteria?

What engineering criteria will be used to

evaluate the design?



Plan your analysis

What are you trying to find out? How much of the structure needs to be

modeled?

What are the boundary conditions and

loads? Do you need to know stresses,

displacements, frequency, buckling ortemperature?

Get ballpark figures through hand-calculations or test data, so you have anidea of how the structure will behave andwhat numbers are reasonable.



Analysis Decisions

Analysis type

How to idealizematerial properties

Geometry details/simplifications

Element type/options

What are the supports or constraints

What are the loads



Type of analysis

Is it static or dynamic?

Are the loads applied gradually, or quickly?

Vibrations? Seismic?

Linear or nonlinear?

Are there large deflections?

Nonlinear materials?

Contact?



Is it really static?

Static analysis assumes that inertial and

damping effects are negligible

You can use time-dependency of loads as a

way to choose between static and dynamicanalysis.

If the loading is constant over a relatively long

period of time, choose a static analysis.



Is it really static?

In general, if the excitation frequency is less

than 1/3 of the structures lowest natural

frequency, a static analysis may be

acceptable Cyclic loads can be modeled by a harmonic

analysis rather than full transient



Linear vs. Nonlinear

Nonlinear structural behavior is a changing

structural stiffness

Several types of nonlinearities:

Geometric

Material (e.g., plasticity, hyperelasticity)

Changing Status (e.g., contact)



Geometric Nonlinearities

Large deflections

Large rotation

Stress stiffening

CablesMembranes

Membrane underdeformation picks up

bending stiffnessSpinning structures



Material Nonlinearities

Plasticity

Creep/Viscoelasticity

Rate dependence

Viscoplasticity

Time dependent

Hyperelasticity

t

e

e

Yield Point y

Elastic Plastic

Unloading



Changing Status Nonlinearities

Contact



Contact

Bonded vs. nonlinear contact

Welded/glued parts

Gaps in model

Will parts separate from each other?

Is delamination possible?

Large vs. small sliding

Determines type of element to use

Determines type of contact



Contact

Contact stiffness

Is the contact hard, or is there some

softening?

Is contact pressure an important value?

Does friction need to be modeled?

What value for the coefficient?

May need to run model with different values


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Materials

Material properties used will be approximate!

Is the material homogenous (the same

throughout)?

Is it isotropic, orthotropic or anisotropic?

Is temperature dependence important to the

analysis?

Is there rate or time dependence? Are composites used?


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Material Information

For linear isotropic material, need modulus of

elasticity and Poissons ratio for a static

analysis

Need density for inertial loads For thermal analysis, need thermal

conductivity

Also need Coefficient of Thermal Expansionfor thermal stress

Need test data for nonlinear materials


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Material Data Sources

Testing:

Datapoint Labs: http://www.datapointlabs.com/

Axel Products: http://www.axelproducts.com/

Online:Matweb: http://www.matweb.com

Material Data Network:

http://matdata.net/index.jsp


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Poissons Ratio

Used to calculate shear modulus

Controls expansion/contraction in direction

perpendicular to load direction

If using = 0.5, need to use element withhyperelastic ability

For models that are constrained from

expansion, the value of

is very important!


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Multiple Materials

Model a boundary wherever

material properties change

Make sure the appropriate

material property is

assigned to each part of the

model

Consider interaction

between properties

Affects contact stiffness


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Linear vs. Non-Linear Materials

Will the stresses stay in the elastic region?

Are you using a non-linear material, e.g.

concrete, soil, rubber?

Do you need to consider creep or fatigue?


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Assess by

undertaking

linearanalysis

START

Is material

behaviour

linear?

At operating

conditions,

does material

exhibit rate

dependency?

Is

deformation

fully

recoverable?

Use

viscoelastic

model

Is

deformation

fully

reversible?

Use rate

independent

plasticity

Is there

significant

partial

recovery?

Use

viscoplastic

model

Will loadingvary with

time?

Use a creep

model

Research the

material

Use

hyperelastic

model

Does materialexhibit hydrostatic

stress sensitivity?

Use rate

dependent

yield model

Use von

Mises

plasticity, or

similar

Use Mohr

Coulomb orDrucker-

Prager

Material Non-linearity Flowchart,

by Andrew Crocombe

??

no

no yesyes yes

??

??

??

??

??

no no

no

no

no

yes

yes

yes


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Linear or Nonlinear?

If no stress-strain data is given, the program will assume the

analysis is linear, and will use Youngs Modulus even if the part

yields. This gives erroneous results when the loads cause the

model to exceed yield.

e

Actual stress

Linear stress


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Non-Linear Materials

Enter stress-strain curve using true-stress

and true-strain

true strain = ln(1 + engineering strain)

true stress = eng. stress (1 + eng. strain)

Graph stress-strain curve to check inputs

Still need to enter Youngs modulus

Make sure Youngs modulus matches yieldstress and strain


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Units

Many general purpose FEA codes allow the

user to enter a consistent unit set

Make sure forces, displacements, material

properties have same units these determinethe units of the results.

Use mass = force/area to get proper mass

units


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Consistent Unit Systems

Mass unit kg kg lbf-s2/in slug

Length unit m mm in ft

Time unit s s s s

Gravity const. 9.807 9807 386 32.2

Force unit N mN lbf lbf

Pressure/Modulus of

Elasticity

Pa kPa psi psf

Density Unit kg/m3 kg/mm3 lbf-s2/in4 slug/ft3

Mod. Elasticity Steel 0.2E12 0.2E9 30E6 4.32E9Mod. Elasticity Concrete 30E9 30000 4.5E6 648E6

Density of Steel 7860 7.86E-6 7.5e-4 15.2

Density of Concrete 2380 2.38E-6 2.2e-4 4.61


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How to Check Units

Set loads to zero and run. Check mass and

center of mass.

Or, turn on gravity and check reactions.

If using small dimensions (e.g. microns,millimeters) use smaller base unit to reduce

round-off problems


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Create the Model--

What should I model?

Do I model the entire

structure, or only the

part that is of interest? Should I model bolts?

How?

Do I model the welds? What details should be

included?


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How was the part made?

Casting can create variation in material

properties

Forging affects the state of strain

Formed sheet metal can have significantresidual stresses in corners and bends


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Variations and Tolerances

Material variations and tolerances affect

behavior probabilistic analysis tackles this

Consider effects of tolerances on key

locations


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SimplificationBe Careful!

Fillets can prevent singularities

Holes, bosses, etc. in areas of high stress

should be included


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Singularities

FEA uses the theory of elasticity: stress = force/area

If the area=0, then stress=infinite

Theory of FEA: as mesh is refined, the stresses

approach the theoretical stress For a singularity, you would try to converge on infinity

P = P/A

As A

0,


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Singularities

A stress singularity is a location in a finite elementmodel where the stress value is unbounded(infinite). Examples:

A point load, such as an applied force or moment

An isolated constraint point, where the reaction forcebehaves like a point load

A sharp re-entrant corner (with zero fillet radius)

Real structures do not contain stress singularities.They are a fiction created by the simplifying

assumptions of the model. Point loads are best used for line elements


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What do you do with Singularities?

If they are located far away from the regionof interest, you can focus post-processingaway from that part of the model

If they arelocated in the region of interest,you will need to take corrective action


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How do you Correct for Singularities?

Adding a fillet at re-entrant corners and re-

runing the analysis.

Replacing a point force with an equivalent

pressure load.


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How do you Correct for Singularities?

Spreading out displacement constraints

over a set of nodes.

Turning on material plasticity

Using Stress Linearization


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Can I Use Symmetry?

Symmetric structures can be modeled by a smallerrepresentative portion or cross-section

Easier to create, can use a finer mesh

Quicker run times can run multiple load scenarios,

multiple configurations much quicker It can be used when geometry, material behavior and

loading are symmetric about the same plane(s).

Generally can NOT use symmetry in modal analysis,

as the mode shapes are not always symmetric.


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Symmetry

Types of symmetry:

Axisymmetry

Rotational

Planar or reflective

Repetitive or translational


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Symmetry, Interrupted

Sometimes a small detail

interrupts symmetry

Can ignore it, or treat it assymmetric best to do a

small test case if unsure


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Superelement

A superelement is a single element that has the samestiffness as a large portion of a structure

Can model in detail the area of interest, then use oneor more superelements for the rest of the structure

Can use superelements for repeating parts of astructure

Also known as substructuring

Requires more pre/post-processing

The stiffness is exact, damping/mass is not


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Modeling Bolts

Solid model if you need stress in bolt

Beam model if you dont

Modeling threads increases model size may

be too much detail


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Bolt Pretension

Affects stresses anddeflections

Traditionally, imposed astrain on the bolt equal to thepretension strain (requires

multiple solutions) Can instead use pretension

element.

ANSYS can automate boltpretension in either interface


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Check geometry

If importing model, do some checks of the

dimensionsdont assume its right!

Make sure the model is in the required units

system If the model was created in a system different

from the material data and loads, you need to

scale the model by the proper conversion

factor


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Choice of elements

2D vs 3D vs line

2D elements are spatially

3D, but in the model they

are geometrically2D

Element Order: linear,

quadratic, polynomial

Specialized elements?

(composites, concrete,

acoustics, coupled field)

Geometric dimensionality--how the geometry is


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Line Elements

Beamelements have bending and axial

strength. They are used to model bolts,

tubular members, C-sections, angle irons,

etc.

Spar or Linkelements have axial

strength. They are used to modelsprings, bolts, preloaded bolts, and truss

members.

Spr ing or Comb inat ionelements also have axial strength,but instead of specifying a cross-section and material

data, a spring stiffness is entered. They are used tomodel springs, bolts, or long slender parts, or to replacecomplex parts by an equivalent stiffness.


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Which Beam Element?

Several types of beams are usually available

Different beam theories are used

Bending in BEAM188/189 is linear, unlike that in BEAM4; use

several elements to model a member with 188 or 189

BEAM44 and BEAM188/189 include shear deformation

Some elements have special features:

Tapered beams (BEAM44)

Section offset (BEAM44, BEAM188/189)

Section visualization, including stresses (BEAM44,BEAM188/189)

Initial strain input (BEAM4, BEAM44)

Initial stress input (BEAM188/189)


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Shell Elements

Can use a shell when the maximum unsupported dimension of the

structure is at least 10 times the thickness

Use to model thin panels or tubular structures

Thick shell elements include transverse shear, thin shell elements

ignore this.

Shell elements can be 2D or 3D; 2D shells are drawn as a line, 3D as anarea


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Shell Pitfalls

Modeling tubes with straight-edged shells willresult in a faceted model

The nodes are on the true surface, so thelengths of the elements are smaller than thecircumference inaccurate cross-section foraxial stress

Pressure will produce spurious circumferentialbending moments at the nodes

Use finer mesh, or use elements with midsidenodes


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Shell Pitfalls

When shell elements meet at a T,material is duplicated

Can be easier to simply mesh theouter surface of a model, but theelement does not correspond to thecenter of the actual plate

Local bending is now changed

Can use shell offset toaccommodate this (SHELL91,SHELL99, SHELL181)

Connecting shell and solidelements tricky

Different DOFs


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Solid-Shell Element

3D Solid brick (or prism)element without bendinglocking

Nodes have same DOFs as3D elements can connectthin and thick structureswithout constraint equationsor MPCs

Can model varying thicknessbodies without usingmultiple real constants


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2D Solid Elements

Used to model a cross-section of solid objects.

Must be modeled in the global Cartesian X-Y plane.

All loads are in the X-Y plane, and the response(displacements) are also in the X-Y plane.

Element behavior may be one of the following: plane stress

plane strain

generalized plane strain

axisymmetric axisymmetric harmonic


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Plane Stress

Assumes zero stress in the Z direction.

Valid for components in which the Z dimension is

smaller than the X and Y dimensions.

Z-strain is non-zero.

Optional thickness (Z direction) allowed. Used for structures such as flat plates subjected

to in-plane loading, or thin disks under pressure

or centrifugal loading.

Y

XZ


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Plane Strain

Assumes zero strain in the Z direction.

Valid for components in which the Z

dimension is much larger than the X and

Y dimensions.

Z-stressis non-zero.

Used for long, constant cross-section

structures such as structural beams.

YX

Z

G S


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Generalized Plane Strain

Assumes a finitedeformation domainlength in Z directioninstead of infinite

Gives more practicalresults for when the Zdirection is not longenough

The deformation domain or structure is formedby extruding a plane area along a curve with a

constant curvature.

A i t


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Axisymmetry

Assumes that the 3-D model andits loading can begenerated by revolving a 2-D section 360 about theY axis.

Axis of symmetry must coincide with the global Y

axis. Y direction is axial, X direction is radial, and Z

direction is circumferential (hoop) direction.

Hoop displacement is zero; hoop strains and

stresses are usually very significant. Used for pressure vessels, straight pipes, shafts, etc.

A i t i itf ll i ANSYS


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Axisymmetric pitfalls in ANSYS

Check keyopt(3) setting

otherwise, model will be plane

stress

Pressures are input as force/area

Point loads are the total force for

the circumference

All nodes must have +X

coordinate

Nodes at X=0 need to have UX

constrained

3D S lid El t


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3D Solid Elements

Used for structures which, because of

geometry, materials, loading, or detail of

required results, cannot be modeled with

simpler elements. Also used when the model geometry is

transferred from a 3-D CAD system, and a

large amount of time and effort is required to

convert it to a 2-D or shell form.

El t O d


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Element Order

Element order refers to the polynomial order ofthe elements shape funct ions.

What is a shape function?

It is a mathematical function that gives the shapeof the results within the element. Since FEA solves

for DOF values only at nodes, we need the shapefunction to map the nodal DOF values to pointswithin the element.

The shape function represents assumed behavior

for a given element. How well each assumed element shape function

matches the true behavior directly affects theaccuracy of the solution, as shown on the nextslide.

C i El t O d


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Comparing Element Order

Quadratic distribution of

DOF values

Actual quadratic

curve

Linear approximation(Poor Results)

Linear approximation

with multiple elements

(Better Results)

Quadratic approximation

(Best Results)

Li Q d ti


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Linear vs. Quadratic

Linear elements

Can support only a linear

variation of displacement and

therefore (mostly) only a constant

state of stress within a single

element.

Highly sensitive to elementdistortion.

Acceptable if you are only

interested in nominal stress

results.

Need to use a large number of

elements to resolve high stress

gradients.

Quadratic elements

Can support a quadratic variation

of displacement and therefore a

linear variation of stress within a

single element.

Can represent curved edges and

surfaces more accurately thanlinear elements. Not as sensitive

to element distortion.

Recommended if you are

interested in highly accurate

stresses.

Give better results than linear

elements, in many cases with

fewer number of elements and

total DOF.

S l ti El t O d


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Selecting Element Order

When you choose an element type, you areimplicitly choosing and accepting the elementshape function assumed for that elementtype. Therefore, check the shape function

information before you choose an elementtype.

Typically, a linear element has only cornernodes, whereas a quadratic element also has

midside nodes.



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For shell models, the difference between linear andquadratic elements is not as dramatic as for solidmodels. Linear shells are therefore usually preferred.

Besides linear and quadratic elements, a third kind is

available, known as p-elements. P-elements cansupport anywhere from a quadratic to an 8th-ordervariation of displacement within a single element andinclude automatic solution convergence controls.

Mesh Considerations


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Mesh Considerations

For simple comparison,coarse mesh is OK

For accurate stresses, finer

mesh is needed

Need finer mesh for fatigue

Invest elements at locations of

interest

Avoid connecting quadratic

and linear elements

Mesh Distortion


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Mesh Distortion

Elements distorted from their basic shapecan be less accurate

Greater the distortion, the greater the error

Four types of distortion:Aspect ratio (elongation)

Angular distortion (skew and taper)

Volumetric distortion

Mid node position distortion (higher orderelements)

Mesh Distortion


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Mesh Distortion

Most FEA packages have distortion

checks, and will warn you if elementsexceed those limits

These limits are subjective, and a badelement might not give erroneous results,

and a good element might not giveaccurate results!


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Connecting Different Element Types

Meshing your entire structure is not alwaysfeasibleits nice to model some parts withsimpler elements

Can embed shells in solid elements to

connect them, but be careful of doubling thestiffness better to use MPC connection orSolid-Shell element

Use constraint equation or MPC to connectshell to solid, beam to solid or beam to shell.

Connecting Different Element T pes


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Connecting Different Element Types

Constraint equation and MPCs allows singlenode to drive a set of nodes so that

moments are transferred properly

Check Real Constants


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Check Real Constants

Beams often have a strong and weak axisimportant to have proper stiffness resisting

loads

In ANSYS, use /eshape,on to turn on beamand shell cross-sections

Check shell normals


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Check shell normals

Shells have a bottom, middle andtop

Bending through thickness meansstress on top and bottom will differ

Want all tops in the same direction, sothat the stresses make sense

Positive pressure is oriented oppositeto the element normal (i.e., into theelement)

/PSYM,ADIR to view normals

Element connectivity


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Element connectivity

Make sure there are no cracksin the model

Turn on edge plotting

Apply a dummy load and solve,

then view the displacements Can also use a shrink plot to

check connectivity

Can add density and do a modalanalysis-- mode shapes will showcracks

Mesh Convergence


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Mesh Convergence

FEA Theory: as mesh gets finer, it gets closer to realanswer

Mesh once, solve, mesh finer, solve again; if results

change within a certain percentage, the mesh is

converged, otherwise, repeat Perform a mesh convergence on a problem with a

known answer to get a better understanding

Displacement results converge faster than stress

results

Boundary conditions


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Boundary conditions

Boundary conditions provide the knownvalues for the matrix solution

You always have to include them even for a

balanced pressure situation Can use symmetry to support a balanced load

situation

Can use weak springs to support a balanced

structure

Boundary conditions


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Boundary conditions

Do the boundary conditionsadequately reflect real life?

There are no single-point or linesupports; these areapproximations we use. Reallife has some small area

Be wary of singularities

If deformation of support isn'tnegligible, model support with

coarse elements and usebonded contact to tie support tomodel

Be Careful about Fixed Supports


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Be Careful about Fixed Supports

The Poisson effect: whensomething is pulled in onedirection, it shrinks in theperpendicular directions

A fixed support will prevent thisshrinking, which leads to asingularity

Is there a potential of lift-off atthe support? Might need to model

support and contact

Symmetric and Antisymmetric Constraints


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Symmetric and Antisymmetric Constraints

Symmetry BC: Out-of-planedisplacements and in-plane rotationsare fixed.

Antisymmetry BC: In-planedisplacements and out-of-planerotations are fixed.

Many packages have a singlecommand to apply these constraints

Symmetric Constraints


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Symmetric Constraints

plane of symmetry

deflected shape

Antisymmetric Constraints


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Antisymmetric Constraints

deflected shape

Prevent rigid body motion


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Prevent rigid body motion

For structural static, need to constrain fortranslation and rotation.

Even though the beam below is in

equilibrium, you need a constraint in thehorizontal direction, or you will get pivot

errors

Pivot Errors?


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Pivot Errors?

The program doesnt have any known valuesfor displacements in that direction, so it cant

solve the matrix

Prevent Rigid Body Motion


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Prevent Rigid Body Motion

Set loads to zero and runif model doesnt converge,or results look odd, you have rigid body motion.

Or, add density and do modal analysis

Loading


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Loading

Do the loads match real life?

There are no point loads in real life, just really

small areas with pressures on them

Investigate all possible combinations of loads Make sure you have density entered if turning

on gravity or doing modal analysis

Might need to model load application

device/structure

Loading


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Loading

Consider range of load values

Parametric analysis of different values

Consider long-term vs. short-term loads

Long-term analyzed for creep and fatigueShort-term analyzed for yielding

Think about the load path through the

structure

St. Venants Principle


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St. Venant s Principle

Two sy stems of loads w hich are stat ical lyequivalent w i l l prod uce approx imately the same

effect at locat ions remo te from the loads.

This can be used to simplify loads orstructures

Check Reactions


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C c c s

For statics, sum of forces = sum of reactions

Can identify misplaced loads, incorrect units,

geometry mistakes, typos

Check reactions at contact pairs

Check for buckling


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g

Occurs when part of structure is slender, andunder compressive stress (will not happen

under tension).

Linear buckling (eigenvalue) isUNCONSERVATIVE!

Nonlinear buckling is more accurate, but also

more difficult (need to use full Newton-

Raphson method)

Watch Your Errors and Warnings


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g

Warnings about loads or constraints notapplied because the node or element are

nonexistent

Undefined material properties

Large differences in stiffness


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g

Possible source of problem when the ratio ofthe maximum and minimum element stiffness

coefficients should be less than 1e8

Otherwise, the stiffer part of the model willcrash through the less stiff part

Can indicate problem with inputs

Evaluating Results


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g

Stress criteria

Factor of safety

Is stress greater than yield?

Dont assume the results are correct! Are the displacements in the expected range?

Compare to tests or theory, when possible

Does the displaced shape make sense? Check out stress hotspots

Evaluating Results


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g

Use linearization, if needed

Check the whole modeldont focus so much

on one spot, you miss a problem elsewhere

Check reactions against applied loads Check contact pairs for penetration

Check element error to assess mesh

Plot unaveraged stresses

Results Verification


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Use deformed animation to check loads andlook for cracks in model

Combined load behavior is sometimes

difficult to predict consider separating eachload into its own load case to check

Do Test Cases


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Do a simplified case to learn about behavior

Do 2D instead of 3D, beam instead of 3D

Bonded contact instead of frictional

Elastic instead of plastic or hyperelasticHand calcs!

Find pitfalls BEFORE running a model for

several days

Sensitivity Analysis


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y y

Process of discovering the effects of modelinput parameters on response

Can provide insight into model

characteristics Can assist in design of experiments

Should be subject to same scrutiny as all

V&V

Sensitivity Analysis


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A probabilistic analysis using yourmodel and statistical data of input

parameters to see how much

variation there is in output

Generally requires several analysesvery time consuming

Positive sensitivity indicates that

increasing the value of the

uncertainty variable increases thevalue of the result parameter

Global Sensitivity

Local Sensitivity

Calibration


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Also known as model updating, modeltuning, parameter calibration

Adjust for

Compliance in jointsDamping

Unmeasured excitations

Uncertain boundary conditions

Material variations

Model Calibration


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Determines models fitting ability, NOTpredictive capability

Use tests or hand calcs to tweak the model

Tests used for calibration CANT be used forvalidation

Mirror the test behavior as close as possible

when calibrating

Calibration


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Calibrate against

Test data

Field data

Engineering experienceHand calculations

Simplified analyses

Validation


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Making sure the FE model will be accurate fora specified range of loads

Use experimental data (different from

calibration data) Engineering experience

Hand calculations

Dont just assume the model is correct!!

Document Everything!


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Detail all decisions made

Explain simplifications

Detail loads and supports

Document material data Document test data

Document as much results data as possible

List reaction forcesStresses

Displacements

Build a Best Practices Library


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Create documentationfor common analysis

problems

Detail geometry, mesh

requirements, loading,boundary conditions,

materials, results

analysis

Library Example 1:

Ball Joint


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Ball Joint

ANALYSIS STANDARD FOR LBF, INC.SERIES 5a BALL JOINTS

Geometry

All models shall be analyzed as symmetry for efficiency of solution.

Mesh

The maximum element size shall be nogreater than the base pipe thicknessdivided by five (t/5). Either quadratictetrahedral elements (10-noded),quadratic brick elements (20-noded)or linear brick elements (8-noded)shall be used.

Library Example 1 (contd):


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Material Properties

Structural Steel material properties shall be usedas below:

Youngs Modulus 2.9 E7 psi

Poissons Ratio 0.3

Mass Density 0.284 lbm/in^3

Tensile Yield 36,000 psi

Compressive Yield 36,000 psi

Tensile Ultimate 66,000 psi

Boundary Conditions

The model shall have a symmetric boundarycondition on the faces along the two cut planes.The bottom surface of the base pipe shall have avertical-only support, and the bottom face of theflange on the top pipe shall have a vertical-onlysupport.



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Pressure LoadsApply the internal pressure on the

inside faces shown below.

Results

At minimum, the von Mises stress, vonMises strain and total deformationneed to be determined. The factor ofsafety should be determined, andshall not be less than 2.5.

Library Example #2:

Bolted Flange


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g

ACE, Inc., Best Practices Library:

Bolted Flange Problem

Geometry

All models shall be analyzed as 1/2symmetry for efficiency of solution.

Mesh

The maximum element size shall be no

greater than the pipe thickness dividedby three (t/3). Either quadratictetrahedral elements (10-noded),quadratic brick elements (20-noded) orlinear brick elements (8-noded) shall beused.

The contact between the bolts and nutsshould be set to Bonded. Contactbetween the gasket and hub should be

set to Frictionless, and all other contactshould be set to Rough.

Library Example #2 (contd):


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Material Properties

Structural Steel material properties shall be used as below for the hub, ring, nuts andbolts:


Poissons Ratio 0.3



Compressive Yield 36,000 psi

Tensile Ultimate 66,000 psi

Rubber material properties shall be used for the gasket. Hyperelastic modeling is notrequired, and you may treat the gasket material as a simple linear-elastic material asbelow:


Poissons Ratio 0.3



Compressive Yield 15,000 psiTensile Ultimate 20,000 psi



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Boundary Conditions

The model shall have a symmetric boundary

condition on the faces along the two cutplanes. The bottom surface of the base pipeshall have a fixed support. The top flangesurface should be fixed for the first(pretension) load step, then removed for thesecond load step.

Loads

Apply bolt pretension to all bolts in the first step.

Apply axial and bending moment to the topface of the upper flange in the second step.The moment should be in the cut-plane.

Results

At minimum, the von Mises stress, von Misesstrain and total deformation need to bedetermined. The factor of safety should bedetermined, and shall not be less than 2.5.

Using the WB Wizard for Best Practices


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Edit an existing wizard withthe editor

Add required steps (to avoid

forgotten loads)

Add pictures to show where

loads and supports should beapplied

Add alerts to flag when

design criteria are not met

Example: Ball Joint Best Practice in Wizard


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Checklist for Success


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Gravity pushes downwardSpinning objects move radially outward

Heated objects grow

No real object has 1,000,000 psi stress

Axisymmetric objects rarely have 0 hoopstress

A bending load causes compressive stress

on one side, tensile stress on the other

Checking the Mesh


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Can use error estimationPlot unaveraged stress and compare to

averaged stress to check mesh

Or, do a mesh convergence study

Sanity Check: Peer Review


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Having a fellow engineer review your analysis can help you catchproblems in the model.

Can be informal, one-on-one, or a formal review, with a teamlooking over the analysis.

Either way, it's better to be embarrassed in front of yourcolleagues, than in front of your customer!

"The Cardinal Sins of FEA"by Jim Wood, University of Strathclyde, Glasgow


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1. Forgetting to ratio the load(s) correctly when usingsymmetry

2. Not using a consistent set of units, e.g. Tonne/mm3density

3. Incorrectly mixing units in a model, e.g., inputtingplate thickness in mm and building model inmeters.

4. Trying to constrain or load degrees of freedom thatnodes don't have

5. Quoting FE stresses at re-entrant corners andpoint loads

"The Cardinal Sins of FEA", cont'd


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6. Trying to analyze a 'flying structure', including a lack ofunderstanding as to why a constraint is needed in a directionfor which there is no out-of-balance force

7. Not considering convergence issues or verification checks

8. Getting the axis wrong in axisymmetric models

9. Not applying a degree of common sense to the results: doesit look sensible, are the "field stresses" correct, is the masscorrect

10. Relying wholly on graphics and not double-checking inputdata that cannot always be shown in graphical form(materials, beam properties)



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11. Not archiving model files12. Carrying out analyses when you have little or no

understanding of the theoretical underpinnings of theanalysis type

13. Carrying out analyses when you have little or nounderstanding of the enginering significance of the resultsproduced

14. Underestimating the resources and timescale required forjobs

15. Deliberately selecting views on fringe plots that avoid hot-spots caused by approximations in the model, but have no

relevance to the areas of interest.



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16. Actually 'painting out' hotspots with a paint program whereyou can't avoid the view.

17. Not proof-reading your reports properly because of timepressures.

18. Not listing the assumptions made in the analysis.

19. Not making it clear where your responsibilities lie and whatyou are accepting liability for. E.G., not accepting designresponsibility for products simply being analyzed; notaccepting responsibility for ensuring that what is built waswhat you analyzed.

20. Problems in ascertaining the boundary conditions at the

interface between structures or components being suppliedby different contractors.



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21. Overstating your capabilities and perhaps thesoftware's to potential clients.

22. Quoting results to an accuracy which is notwarranted.

23. Accepting bad meshes because of timescales,sheer recklessness or ignorance.

24. Including too much detail and/or too manyelements in a model, without question.

25. Using averaged or continuous tone fringe plots

when assessing results.



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26. Thoughtlessly solving the idealizedproblem and not the real one, including not

considering the scatter inherent in

materials properties, loads, geometry, etc.

27. Simplifying material constitutive laws,

without consideration of the implications.

28. Making the display color of your finite

elements black with a black background.

Code Guidance on using FEA


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ASME VIII Div2 2007: The Committee recognizes that tools and techniques

used for design and analysis change as technologyprogresses and expects engineers to use goodjudgement in the application of these tools

The Code neither requires nor prohibits the use ofcomputers. However, designers and engineers usingcomputer programs for design or analysis arecautioned that they are responsible for all technical

assumptions inherent in the programs they use andthey are responsible for the application of theseprograms to their design.

References


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K.A. Honkala, Adequate Mesh Refinement forAccurate Stresses, NAFEMS Benchmark, p. 4-9,January 2000

D. Baguley & D. R. Hose, How to Model & InterpretResults Part 3: Validation & Interpretation

Considerations, NAFEMS Benchmark, p. 4-8,January 2001

G. Roth, Pitfalls in Determining Analysis Accuracy,ANSYS Solu tion s, Vol. 2, No. 3, p. 30-31, Summer

2000

References


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D. Baguley & D. R. Hose, How to Model w i thFini te Elements, NAFEMS, 1997

ansys best practices

Documents