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Acoustic
Emission
Howdoesitwork?
Whenaloadisappliedtoasolidstructure(e.g.byinternalpressureorbyexternalmechanical
means),itbeginstodeformelastically.Associatedwiththiselasticdeformationarechangesinthe
structure'sstressdistributionandastorageofelasticstrainenergy.Astheloadincreasesfurther,
somepermanent
microscopic
deformation
may
occur,
which
is
accompanied
by
arelease
of
storedenergy,partlyintheformofpropagatingelasticwavestermed'AcousticEmission'(AE).If
theseemissionsareaboveacertainthresholdleveltheycanbedetectedandconvertedtovoltage
signalsbysensitivepiezoelectrictransducersmountedonthestructure'ssurface.
AtypicalAEsystemconsistsofsignaldetection,amplification,dataacquisition,processingand
analysis.VariousparametersareusedinAEtoidentifythenatureofthesource,including:count,
duration,amplitude,risetime,energy,frequencyandRMS(RootMeanSquare).
AnimportantaspectofAEtestingissignalprocessing.Thereisaneedtoseparategenuinestress
waveemissions,originatingfromwithinthematerial,fromexternalsignals,suchasenvironmental
noise(rain,windwithsandparticles),mechanicalnoise(movementofthecomponentduring
testing),electricnoise,etc.Muchofthisisachievedbycarefulelectronicfilteringofthereceived
AEdatabutbestpracticeisstilltoidentifyandremoveasmanysourcesofextraneousnoiseas
possiblepriortotesting.
Thefrequencyofthestresswavesemittedisnormallyintherange30kHzto1MHz.Triangulation
andothertechniquescangivepositionalinformationandlocalisethesourcesoftheemissions.
Acousticemission
refers
to
the
generation
of
transient
waves
during
the
rapid
release
of
energy
fromlocalizedsourceswithinamaterial.Thesourceoftheseemissionsiscloselyassociatedwith
thedislocationaccompanyingplasticdeformationandtheinitiationandextensionoffatigue
cracksinmaterialunderstress.Othersourcesofacousticemissionare:melting,phase
transformation,thermalstress,cooldowncrackingandthefailureofbondsandfibresin
compositematerials.
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AcousticEmissionhowdoesitwork
Herloadisappliedatboththedirection,onthesimilarconceptandprincipleA.E.doeswork
Energyrealizedfromthebrakeageofbondsinamaterial.
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Acoustic
Emission
System
What
will
it
find?
Sourcesofacousticemissionare:
Plasticdeformation,dislocationmotion,ruptureoftheinclusion,phasetransformation,
twin/slipdeformation;
Differentstagesofcrackpropagation(static,fatigue,stresscorrosion).AEissensitive
enoughtodetectnewlyformedcracksurfacedowntoafewhundredsquaremicrometers
andless.
Thewelddefects:lackofpenetrationandfusion,cracks,inclusionandporosity;
Corrosion:localisedcorrosion/pittingcorrosion.Detectingandmonitoringofactive
corrosion,hydrogenembrittlement,corrosionfatigue,andintergranularstresscorrosion
cracking.Hydrogen
embrittlement;
dissolution
of
metal;
hydrogen
gas
evolution;
the
breakdownofthicksurfaceoxidefilms;
Friction;
Mechanicalimpact;
Leaks(liquidorgas);
Externalnoise(mechanical,electrical,andenvironmental).
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WhatkindofMaterialscanbemonitoredbyAE?
Acousticemissioncanbeusedinnondestructivemonitoringofdifferentkindsofmaterialssuch
as:
Metals:steels,stainlesssteel,carbonsteel,alloy,ferriticsteel,aluminium,aluminium
alloys,magnesiumalloys,andothers(e.g.,copperanditsalloys,uraniumalloys,titanium,
andzirconium
alloys);
Compositematerialsandpolymer:sandwichcomposite,glassreinforcedplastic(GRP)and
carbonfibre;
Concrete,reinforcedconcrete;
Rocks;
Woods.
Whereisitused?
Pressureequipment:Fundamentalresearchanddevelopmenteffortsinthecontrolofthe
damagein
materials
by
acoustic
emission
have
grown
in
the
last
twenty
years.
This
techniquehasbecomeareliableandstandardmethodofnondestructivetestingfor
pressurevessels.AEisusedtomonitorflaws,corrosion,andleakageinpressurevessels,
LPG,tanks,pipingsystems,steamgenerators;
ForCreepmonitoring
ForStrainMeasurements
Fatiguedetection
ConditionMonitoringinInServiceInspectionofPressureVessels.
Aircraftandaerospace:aerospacestructures,wings,bulkhead,fueltanks,Rocketengine,
realtimemonitoring;
Petrochemicaland
chemical:
storage
tanks,
reactor
vessels,
offshore
and
onshore
platforms,drillpipe,pipeline;
Marine:corrosion,compositeshell,engineandpowerplant;
Civilengineering:bridges,dams,suspensioncablebridges,concretestructurereinforced
bycomposite;
Researchanddevelopment:acousticemissionisagoodtechniquetomonitorandstudy
thedamageinmaterialsandtheirmechanicalproperties(newmaterials,smartmaterials,
Shapememoryalloys(SMA)).
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Conclusion
AEisadevelopingtechnology,afieldinwhichthereisongoingresearch.Itisavaluabletool
foranalysisthatwillgrowinacceptanceasitsadoptionincreases.
Becauseofproblemsassociatedwithtests,negativeopinionsdodevelop.Wheneverpossible,
AEtechniciansshouldworkcloselywithmanufacturerstoeliminatethoseconditionsthatcan
leadtotestfailures.Successfullydoingsowillhelptoeliminatetheexpenseandinconvenience
ofrepeated
testing,
increase
understanding,
and
improve
relationships.
The
positive
long
term
benefitofthisactioncanbeenjoyedbyall.
SUNILB.GAVADE
MumbaiMobileNo:+091919768700717
Email:[email protected]
