lec 16_tribology in mech eng_erosion_sierros

8/8/2019 Lec 16_Tribology in Mech Eng_Erosion_Sierros

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TribologyinMechanicalEngineeringMAE493N/593T

Dr.Konstantinos

A.

Sierros

WestVirginiaUniversity

Mechanical&AerospaceEngineering

ESBAnnex

263

[email protected]


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Erosionby

solid

particle

impact

Erosivewearbyplasticdeformation

Erosivewearbybrittlefracture

Erosionofengineeringmaterials

Testingmethods

for

erosive

wear

Erosion

https://reader009.{domain}/reader009/html5/0527/5b09

Notesfrom:I.M.Hutchings,Tribology,Ch.6


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Erosionbysolidparticleimpact

Solidparticleerosion:Discretesolidparticlesstrike

asurface

of

interest

Itdiffersfrom3bodyabrasionintheoriginofthe

forcesbetweenparticlesandwearingsurface

Inabrasion,particlesaretrappedandpressed

betweentwo

sliding

surfaces

Inerosion,severalforcesofdifferentoriginsacton

aparticleincontactwithasolidsurface


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In

erosion,

several

forces

of

different

origins

act

on

a

particle

in

contact

with

a

solidsurface

Neighboringparticlesmayexertcontactforcesandaflowingfluidwillcausedrag

Thedominantforceonaparticleisthecontactforceexertedbythesurface

Theextent

of

wear

depends

on

Numberandmassofindividualparticlesstrikingthesurface

Theimpactvelocityofthestrikingparticles


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Behaviour

of

a

single

hard

particle

striking

a

softer

surface

at

normal

incidence Assumptions

Particledoesnotdeform

Problemisquasistatic(ignoredynamiceffects)

Only

force

acting

is

contact

force

exerted

by

the

surface

DeformationofsurfaceisperfectlyplasticwithconstantindentationhardnessH


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Attimet

afterinitialcontactparticlehasindentedsurfacedepthx

Crosssectional

area

of

the

indentation

at

the

surface

is

A(x)

A(x)isdeterminedbytheshapeoftheparticle

Theupwardforcethatdeceleratestheparticleisduetotheplasticflowpressure

actingoverA(x)

)(2

2

xHAdt

xdm = Equationofparticlemotion


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Forsimpleparticleshapestheequationofparticlemotioncanbesolved

analytically

Inthiscaseweneedtoknowthefinalvolumeoftheindentation

whenthe

particlerestsatadepthd

HmUV2

2

=Volumeofindentation Initialvelocityofparticle

Thematerial

displaced

from

the

indentation

Maybedisplaced(byelasticdeformation)awayfromtheindentation

Mayformarimofplasticallydeformedmaterialaroundtheindentation

Mayberemovedasweardebris


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AssumethatsomefractionK

ofthematerialisremovedfromtheindentationas

weardebris

H

mUKremovedmaterialofmass

2

2

=

DensityofmaterialbeingerodedDimensionlessfactor

Summationofaboveequationsuggeststhattotalmassofmaterialremovedis

proportionalto

the

total

mass

of

striking

particles


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Masslostfromasurfacevs.totalmassofstrikingerosiveparticles

For

some

materials

particles

become

embedded

in

the

surface

and

cause

an

initialmassgain(curveb)


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Erosivewearbyplasticdeformation Aftertheinitialweightgaining(incubationperiod)erosionproceedslinearly

For

soft

target

materials

and

high

angles

of

incidence

incubation

period

is

observed

Forductilematerialsanyincubationperiodisnegligibleandmasslossis

proportionaltototalmassofstrikingparticles(linea)


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The

linear

relationship

observed

in

steady

state

erosion

can

lead

to

the

following

definitionoferosionE;

particlesstrikingofmass

removedmaterialofmassE=

dimensionless

H

mUKremovedmaterialofmass

2

2

=

usingE

H

UKE

2

2=


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HUKE

2

2

=

ErosivewearisinverselyproportionaltohardnessH

The

severity

of

wear

is

determined

by

K

(dimensionless) Kcanbethoughtofasameasureoftheefficiencyofmaterialremovalprocess

Kwouldbeequalto1ifallmaterialremovedbytheactionoftheerosiveparticle

Formetals,Kliesintherange5x103101

(verysimilarto2bodyabrasion)


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Erosivewearbyplasticdeformation Geometryofdeformationdependson;

Impact

velocity

Shapeandorientationofparticle

Impactangle

Impactanglesaredefinedwithrespecttotheplaneofthetargetsurface

For

normal

impact:

=90o


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Theerosionofmetals(ductile)dependsstronglyonimpactangle(curvea)showingamaximumat20o30o


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Impactofsingleparticlesonmetalsurfacesat30o

impactanglehas3different

types

of

impact

damage Ploughing

TypeIcutting

TypeIIcutting


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Roundedparticlesdeformthesurfacebyploughing,displacingthematerialto

the

side

and

in

front

of

the

particle

(a)

Furtherimpactsonneighboringareasleadtodetachmentofheavilystrained

materialfromtherimofthecraterorfromtheterminalleap

Deformationcausedbyanangularparticledependsonorientationofparticle

and

whether

particle

rolls

forwards

or

backwards

during

contact TypeIcutting:Particlerollsforwardsandindentsthesurface

(b)

TypeIIcutting:Particlerollsbackwardsandatruemachiningactionoccurs


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Classificationofthe3typesofabrasion

wearisnotsimplewhen;

Randomly

oriented

particles

strike

surface

Irregularshapeparticlesimpingeona

previouslyerodedsurface


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Fig(a)showssteelsurfaceerodedbySiC

particles

Plasticruptureanddetachmentofmetaldisplacedfromtheimpactsitesintoraisedcraterrimsandlips

Weardebriswillnotbeproduceduntil

severalcycles

of

plastic

deformation


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Intheextremecaseofspherical

particlesatnormalincidence,materialis

removedonly

after

neighboring

impacts

haveimposedmanycyclesofplastic

deformation(b)

Surfacelooksverydifferentas

comparedto

an

eroded

surface

by

angularparticles(a)


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Theoreticalmodelsconsiderimpactofasingleparticleonaplanesurface

Idealized2D

model

is

presented

below;

Rigidparticlecutsintoplanesurface

Volumeofmaterialisthatsweptoutbythemotionoftheparticletip(asdistinctfromthetotalvolumedisplacedbyindentation) Therefore,predictederosioniszerofornormalincidence


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Erosivewearbyplasticdeformation Modelinwhichtheforcesontheparticleareassumedtoactat

itsextremetip

)(2

fHUKE=

ThevalueofKdependsonparticlegeometry

Depends

on

the

fraction

of

particles

actually

cutting

(in

an

ideal

manner) f()predictedbytheoryissimilartoexperimentallyobservedcurve

Theory Experiment


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Asimilarbutmorerealisticmodelassumesthatthepointofactionoftheforces

onthe

particle

is

allowed

to

move

during

impact

)(11

fH

UKE

n

=

2


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Extremecaseoferosionatnormalincidencebysphericalparticleswherecutting

playsno

role

can

be

modeled

in

two

ways;

Byassumingthatsurfacematerialbecomesdetachedwhenaccumulatedplastic

strainreachesacriticalvalue

Bytreatingtheproblemasoneoflowcyclefatiguecausedbycyclicplastic

deformationcaused

by

successive

particle

impacts

Bothmodelsleadtosimilarconclusions

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32/12

H

UKEc

=

isdensityofsphericalerosiveparticlesc

iscriticalstrainatwhichdetachmentofweardebrisoccurs

b l d f


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Inpractice,erosionofmetalsshowsstrongdependenceonparticleimpact

velocity

n

UE

Valuesofnbetween2.3and3.0areexperimentallyobserved

Velocityexponentnisalwaysgreaterthan2.0andoftenliesaround2.4for

ductilemetals

(predicted

by

all

models)

Alltheoreticalmodelspredictthaterosionrates(involvingplasticdeformation)

areinverselyproportionaltomaterialhardnessH

Erosionalso

depends

on

ductility

of

material

Erosionisproportionaltomaterialdensity

E i b b i l f


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Whenimpactoferosiveparticlecausesbrittlefracture,materialisremovedfrom

surfaceby

formation

and

intersection

of

cracks

Thereisstillaplasticflowcomponentaroundthepointofcontact

Generalmorphologyisshownbelow;

E i b b ittl f t


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Erosivewearbybrittlefracture Extentofcrackingismoreseverewhenimpactdirectionisnormaltosurface

Erosionis

most

rapid

in

this

case

Erosionfallsmonotonicallywithangleawayfrom90o (curveb)

Modelsforbrittleerosionhavebeendevelopedforimpactatnormalincidence

Mostofthesemodelsassumecrackingandleadtoexpressionsas

follows;

nmUrE risparticleradius

E i b b ittl f t


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ValuesofmandnarerelatedtoWeibullconstants(brittlebehaviorstatistical

distribution) mvaluesaround1

nvaluesintherangefrom2.63.0

nmUrE

Erosive wear by brittle fracture


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Forangularshapedparticlesmodelsforerosionbyelasticplasticfractureare

more

applicable


Materialisremovedbytheintersectionoflateralcrackswitheachotherandthe

surface

Quasistatic

model

assumes

that

hardness

H

is

the

pressure

resisting

penetration

Dynamicwavepropagationmodeldeterminestheimpactpressure



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Quasistaticmodel

3.1

1.02.04.27.0

cK

HUr

E

Dynamicwavepropagationmodel

25.03.1

6.02.37.0

HKUr

E

c

Bothmodelspredictaweakdependenceoferosiononparticledensity

The

most

important

material

property

for

erosion

resistance

is

fracture

toughnessKc HardnessHismuchlesssignificant

Erosiondependsonparticlesizer



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Experimentalstudiesonerosionofbrittlematerialsshowthatthemassloss

fromthesurfacedependslinearlyonthetotalmassoferodentparticles



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Considerablevariationoferosion

betweendifferentmaterialsis

found

Experimentalresultsdonot

conformwellwiththeoretical

predictions


Erosion of engineering materials


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Twotypesofbehavior:

Ductile

Brittle

Angulardependenceoferosiondependsonmaterialandtheconditionsof

erosion

Example:Erosionofmetalsurfacebyangularparticlesisductile

Erosionofmetalsurfacebysphericalparticlesisbrittle



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Testing methods for erosive wear


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Testingmethodsforerosivewear

Fourdifferenttests

Testing methods for erosive wear


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Summary


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y






lec 16_tribology in mech eng_erosion_sierros

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