ae02

Chapter 1. Introduction to AcousticEmission Testing . . . . . . . . . . . . . . 1

Part 1. Nondestructive Testing . . . . 2Part 2. Management of Acoustic

Emission Testing . . . . . . . . 13Part 3. History of Acoustic Emission

Testing . . . . . . . . . . . . . . . 21Part 4. Measurement Units for

Acoustic EmissionTesting . . . . . . . . . . . . . . . 25

Chapter 2. Fundamentals of AcousticEmission Testing . . . . . . . . . . . . . 31

Part 1. Introduction to AcousticEmission Technology . . . . 32

Part 2. Acoustic Emission Noise . . 41Part 3. Acoustic Emission Signal

Characterization . . . . . . . . 45Part 4. Acoustic Emission

Transducers and TheirCalibration . . . . . . . . . . . . 51

Part 5. Macroscopic Origins ofAcoustic Emission . . . . . . 61

Part 6. Microscopic Origins ofAcoustic Emission . . . . . . 69

Part 7. Wave Propagation . . . . . . . 79

Chapter 3. Modeling of AcousticEmission in Plates . . . . . . . . . . . 109

Part 1. Wave Propagation inPlates . . . . . . . . . . . . . . . . 110

Part 2. Formal AnalyticSolution . . . . . . . . . . . . . 114

Chapter 4. Acoustic Emission SourceLocation . . . . . . . . . . . . . . . . . . 121

Part 1. Fundamentals of AcousticEmission SourceLocation . . . . . . . . . . . . . 122

Part 2. Overdetermined SourceLocation . . . . . . . . . . . . . 131

Part 3. Waveform Based SourceLocation . . . . . . . . . . . . . 135

Chapter 5. Acoustic Emission SignalProcessing . . . . . . . . . . . . . . . . . 147

Part 1. Digital SignalProcessing . . . . . . . . . . . . 148

Part 2. Pattern Recognition andSignal Classification . . . . 157

Part 3. Classification of FailureMechanism Data fromFiberglass Epoxy TensileSpecimens . . . . . . . . . . . . 167

Part 4. Neural Network Predictionof Burst Pressure inGraphite Epoxy PressureVessels . . . . . . . . . . . . . . . 171

Chapter 6. Acoustic Leak Testing . . . 181Part 1. Principles of Sonic and

Ultrasonic LeakTesting . . . . . . . . . . . . . . 182

Part 2. Instrumentation forUltrasound LeakTesting . . . . . . . . . . . . . . 191

Part 3. Ultrasound Leak Testingof Pressurized Industrialand TransportationSystems . . . . . . . . . . . . . . 198

Part 4. Ultrasound Leak Testingof Evacuated Systems . . . 211

Part 5. Ultrasound Leak Testingof Engines, HydraulicSystems, Machinery andVehicles . . . . . . . . . . . . . . 213

Part 6. Electrical Inspection . . . . 215Part 7. Ultrasound Leak Testing

of Pressurized TelephoneCables . . . . . . . . . . . . . . . 218

Part 8. Acoustic EmissionMonitoring of Leakagefrom Vessels, Tanks andPipelines . . . . . . . . . . . . . 220

Acoustic Emission Testing

C O N T E N T S

Chapter 7. Acoustic Emission Testingfor Process and ConditionMonitoring . . . . . . . . . . . . . . . . 227

Part 1. Acoustic Emission Testingin Milling . . . . . . . . . . . . 228

Part 2. Acoustic Emission Testingof Resistance SpotWelding . . . . . . . . . . . . . 232

Part 3. Acoustic Emission WeldMonitoring of AluminumLithium Alloy . . . . . . . . . 235

Part 4. Acoustic Emission Testingfor Machinery ConditionMonitoring . . . . . . . . . . . 243

Part 5. Acoustic Emission Testingduring Grinding . . . . . . . 251

Part 6. Crack Detection duringStraightening of Axlesand Shafts . . . . . . . . . . . . 263

Chapter 8. Acoustic Emission Testingof Pressure Vessels, Pipes andTanks . . . . . . . . . . . . . . . . . . . . . 271

Part 1. Acoustic Emission Testingof Spheres and OtherPressure Vessels . . . . . . . . 272

Part 2. Acoustic EmissionTesting of CompositeOverwrapped PressureVessels . . . . . . . . . . . . . . . 281

Part 3. Acoustic Emission Testingof Pipelines . . . . . . . . . . . 284

Part 4. Acoustic Emission Testingof Delayed CokeDrums . . . . . . . . . . . . . . . 290

Part 5. Acoustic Emission Testingof Tank Floors . . . . . . . . . 296

Chapter 9. Acoustic Emission Testingof Infrastructure . . . . . . . . . . . . 305

Part 1. Acoustic Emission Testingof Bridges . . . . . . . . . . . . 306

Part 2. Acoustic EmissionMonitoring of CrackGrowth in Steel BridgeComponents . . . . . . . . . . 310

Part 3. Evaluation of SlopeStability by AcousticEmission Testing . . . . . . . 315

Chapter 10. Electric PowerApplications of AcousticEmission Testing . . . . . . . . . . . . 331

Part 1. Acoustic Emission Locationof Incipient Faults inPower Transformers . . . . 332

Part 2. Acoustic Emission Testingof High Energy SeamWelded Piping . . . . . . . . 342

Part 3. Acoustic EmissionMonitoring of LooseParts . . . . . . . . . . . . . . . . 347

Part 4. Acoustic Emission Testingof Steam Turbines . . . . . . 354

Chapter 11. Aerospace Applicationsof Acoustic Emission Testing . . 359

Part 1. Acoustic Emission Testingof Aircraft . . . . . . . . . . . . 360

Part 2. Fatigue Crack Monitoringof Aircraft EngineCowling in Flight . . . . . . 367

Part 3. Acoustic EmissionMonitoring of RocketMotor Case duringHydrostatic Testing . . . . 377

Part 4. Acoustic EmissionPrediction of Burst Pressurein Fiberglass EpoxyPressure Vessels . . . . . . . 383

Chapter 12. Special Applications ofAcoustic Emission Testing . . . . 391

Part 1. Acoustic Emission TestingUsing Moment TensorAnalysis . . . . . . . . . . . . . 392

Part 2. Acoustic Emission Testingfor Structural Design ofGrand Prix Cars . . . . . . . 401

Part 3. Acoustic EmissionMonitoring of Sand inPetroleum Wells . . . . . . . 408

Part 4. Active CorrosionDetection Using AcousticEmission . . . . . . . . . . . . . 415

Chapter 13. Acoustic EmissionTesting Glossary . . . . . . . . . . . . 427

Index . . . . . . . . . . . . . . . . . . . . . . . . . 435

Figure Sources . . . . . . . . . . . . . . . . . . . 446

Movie Sources . . . . . . . . . . . . . . . . . . . 447


Chapter 1. Introduction to AcousticEmission Testing

Movie. Discontinuities in steel . . . . 6Movie. Plastic deformation causes

cry of tin . . . . . . . . . . . . . . 21

Chapter 2. Fundamentals of AcousticEmission Testing

Movie. Acoustic emission differsfrom other methods . . . . . 32

Movie. Pencil break source . . . . . . 36Movie. Guard transducers control

noise . . . . . . . . . . . . . . . . . 42

Chapter 5. Acoustic Emission SignalProcessing

Movie. System with onechannel . . . . . . . . . . . . . . 150

Chapter 6. Acoustic Leak TestingMovie. Vibration at ultrasonic

frequencies of gas moleculesescaping from orifice . . . 183

Sound. Audible analog ofultrasonic signal . . . . . . . 194

Movie. Steam system leak test . . 202Movie. Amplitude rise heard

through ultrasounddetector as rough andraspy . . . . . . . . . . . . . . . . 214

Chapter 7. Acoustic Emission Testingfor Process and ConditionMonitoring

Movie. Discontinuities fromwelds . . . . . . . . . . . . . . . . 232

Chapter 13. Acoustic EmissionTesting Glossary

Movie. Pencil break source . . . . . 431


M U L T I M E D I A C O N T E N T S

Chapter 5. Acoustic EmissionSignal ProcessingFigure 1 Vallen-Systeme GmbH, Munich, Germany.

Chapter 7. Acoustic EmissionTesting for Process and ConditionMonitoringFigures 19-26 Holroyd Instruments, Bonsall Near

Matlock, Derbyshire, United Kingdom

Chapter 11. AerospaceApplications for Acoustic EmissionTestingFigures 1-11 ASTM International, West

Conshohocken, PA.

446 Acoustic Emission Testing

Figure Sources

Chapter 1. Introduction toAcoustic Emission TestingMovie. Discontinuities in steel Physical Acoustics

Corporation, Princeton, NJ; for the Federal HighwayAdministration, United States Department ofTransportation, Washington, DC.

Movie. Plastic deformation causes cry of tin PhysicalAcoustics Corporation, Princeton, NJ; for theFederal Highway Administration, United StatesDepartment of Transportation, Washington, DC.

Chapter 2. Fundamentals ofAcoustic Emission TestingMovie. Acoustic emission differs from other methods

Physical Acoustics Corporation, Princeton, NJ; forthe Federal Highway Administration, United StatesDepartment of Transportation, Washington, DC.

Movie. Pencil break source Physical AcousticsCorporation, Princeton, NJ.

Movie. Guard transducers control noise PhysicalAcoustics Corporation, Princeton, NJ; for theFederal Highway Administration, United StatesDepartment of Transportation, Washington, DC.

Chapter 5. Acoustic EmissionSignal Processing

Movie. System with one channel Physical AcousticsCorporation, Princeton, NJ; for the Federal HighwayAdministration, United States Department ofTransportation, Washington, DC.

Chapter 6. Acoustic Leak TestingMovie. Ultrasonic vibration of gas molecules escaping

orifice UE Systems, Elmsford, NY.Sound. Audible analog of ultrasonic signal UE Systems,

Elmsford, NY.Movie. Steam system leak test UE Systems, Elmsford,

NY.Movie. Amplitude rise heard through ultrasound detector

as rough and raspy UE Systems, Elmsford, NY.

Chapter 7. Acoustic EmissionTesting for Process andCondition Monitoring

Movie. Discontinuities from welds Physical AcousticsCorporation, Princeton, NJ; for the Federal HighwayAdministration, United States Department ofTransportation, Washington, DC.

Chapter 13. Acoustic EmissionTesting Glossary

Movie. Pencil break source Physical AcousticsCorporation, Princeton, NJ; for the Federal HighwayAdministration, United States Department ofTransportation, Washington, DC.

447Index

Movie Sources

Eric v.K. Hill, New Smyrna Beach, Florida

M. Nabil Bassim, University of Manitoba, Winnipeg,Manitoba, Canada (Part 5)

Franklin R. Breckenridge, Olympia, Washington (Part 4)

Davis M. Egle, Norman, Oklahoma (Part 7)

Donald G. Eitzen, National Institute of Standards andTechnology, Gaithersburg, Maryland (Part 4)

D. Robert Hay, TISEC, Incorporated, Montreal,Quebec, Canada (Part 3)

Adrian A. Pollock, Physical Acoustics Corporation,Princeton, New Jersey (Part 1)

Vasile Mustafa, TISEC, Incorporated, Montreal,Quebec, Canada (Part 3)

Jack C. Spanner, Sr., Richland, Washington (Parts 1 and 2)

Haydn N.G. Wadley, University of Virginia,Charlottesville, Virginia (Part 6)

Fundamentals of AcousticEmission Testing

2C H A P T E R

Phenomenon of AcousticEmissionAcoustic emission is the elastic energyspontaneously released by materials whenthey undergo deformation. In the early1960s, a new nondestructive testtechnology was born when it wasrecognized that growing cracks anddiscontinuities in pressure vessels could bedetected by monitoring their acousticemission signals. Although acousticemission is the most widely used term forthis phenomenon, it has also been calledstress wave emission, stress waves,microseism, microseismic activity and rocknoise.

Formally defined, acoustic emission isthe class of phenomena where transientelastic waves are generated by the rapidrelease of energy from localized sourceswithin a material, or the transient elasticwaves so generated.2 This definitionembraces both the process of wavegeneration and the wave itself.

Source MechanismsSources of acoustic emission includemany different mechanisms ofdeformation and fracture. Earthquakesand rockbursts in mines are the largestnaturally occurring emission sources.Sources that have been identified inmetals include crack growth, movingdislocations, slip, twinning, grainboundary sliding and the fracture anddecohesion of inclusions. In compositematerials, sources include matrix crackingand the disbonding and fracture of fibers.These mechanisms typify the classicalresponse of materials to applied load.

Other mechanisms fall within thedefinition and are detectable withacoustic emission equipment. Theseinclude leaks and cavitation; friction (asin rotating bearings); the realignment orgrowth of magnetic domains (barkhauseneffect); liquefaction and solidification; andsolid-to-solid phase transformations.Sometimes these sources are calledsecondary sources or pseudo sources todistinguish them from the classic acousticemission caused by mechanicaldeformation of stressed materials.

Acoustic EmissionNondestructive TestingAcoustic emission testing is anondestructive method withdemonstrated capabilities for monitoringstructural integrity, for detecting leaks andincipient failures in mechanicalequipment and for characterizing materialbehavior. Applications of acousticemission to engineering structures havebeen documented since the 1960s.

Comparison with OtherTechniquesAcoustic emission testing differs frommost other nondestructive methods intwo significant respects. First, the energythat is detected is released from withinthe test object rather than being suppliedby the test method, as in radiographic orultrasonic testing. Second, the acousticemission method can detect the dynamicprocesses associated with the degradationof structural integrity. Crack growth andplastic deformation are major sources ofacoustic emission. Latent discontinuitiesthat enlarge under load and are activesources of acoustic emission by virtue oftheir size, location or orientation are alsolikely to be significant in terms ofstructural integrity.

Usually, certain areas within astructural system will develop localinstabilities long before the structure fails.These instabilities result in minutedynamic movements such as plasticdeformation, slip or crack initiation andpropagation. Although the stresses in ametal part may be well below the elasticdesign limit, the region near a crack tipmay undergo plastic deformation as aresult of high local stresses. In thissituation, the propagating discontinuityacts as a source of stress waves andbecomes an active acoustic emissionsource.

Acoustic emission testing isnondirectional. Most acoustic emissionsources appear to function as point sourceemitters that radiate energy in sphericalwave fronts. Often, a transducer locatedanywhere in the vicinity of an acousticemission source can detect the resultingacoustic emission. This ability is incontrast to other methods of


PART 1. Introduction to Acoustic EmissionTechnology1

MOVIE. HalDunegan saysacousticemission differsfrom othermethods.

nondestructive testing, which depend onprior knowledge of the probable locationand orientation of a discontinuity todirect a beam of energy on a path thatwill properly intersect the area of interest.

Advantages of Acoustic EmissionTestsThe acoustic emission method offers thefollowing advantages over othernondestructive testing methods:

1. Acoustic emission testing is a dynamictest method in that it provides aresponse to discontinuity growthunder an imposed structural stress;static discontinuities will not generateacoustic emission signals.

2. Acoustic emission testing can detectand evaluate the significance ofdiscontinuities throughout an entirestructure during a single test.

3. Because only limited access isrequired, discontinuities may bedetected that are inaccessible to othermethods.

4. Vessels and other pressure systems canoften be requalified during aninservice test that requires little or nodowntime.

5. The acoustic emission method may beused to prevent catastrophic failure ofsystems with unknown discontinuitiesand to limit the maximum pressureduring containment system tests.

Acoustic emission is a wavephenomenon and acoustic emissiontesting uses the attributes of particularwaves to help characterize the material inwhich the waves are traveling. Frequencyand amplitude are examples of thewaveform parameters that are regularlymonitored in acoustic emission tests.

Table 1 gives an overview of the mannerby which various material properties andtesting conditions influence acousticemission response amplitudes.3 Thefactors are indicative rather than absolute.

Application of AcousticEmission TestsThe functional categories of acousticemission applications are as follows:mechanical property testing andcharacterization, preservice proof testing,requalification testing in service,monitoring on line, in-process weldmonitoring, mechanical signatureanalysis, leak detection, leak location andgeological applications. By definition,online monitoring may be continuous orintermittent and may involve the entirestructure or a limited zone only. Althoughleak detection and acoustic signatureanalysis do not involve acoustic emissionin the strictest sense of the term, acousticemission techniques and equipment areused for these applications.

Structures and MaterialsA wide variety of structures and materials(metals, nonmetals and variouscombinations of these) can be monitoredby acoustic emission techniques duringthe application of an external stress(load). The primary acoustic emissionmechanism varies with different materialsand should be characterized beforeapplying acoustic emission techniques toa new type of material. Once thecharacteristic acoustic emission responsehas been defined, acoustic emission testscan be used to evaluate the structuralintegrity of a component.

33Fundamentals of Acoustic Emission Testing

TABLE 1. Factors that tend to increase or decrease the relative amplitude of acousticemission response.3

Increase Decrease

High strength Low strengthHigh strain rate Low strain rateLow temperature High temperatureAnisotropy IsotropyHeterogeneity HomogeneityThick sections Thin sectionsBrittle failure (cleavage) Ductile failure (shear)Material containing discontinuities Material without discontinuitiesMartensitic phase transformations Diffusion controlled phase transformationsCrack propagation Plastic deformationCast materials Wrought materialsLarge grain size Small grain sizeMechanically induced twinning Thermally induced twinning

Testing of CompositesAcoustic emission monitoring of fiberreinforced composite materials has proveneffective when compared with othernondestructive test methods. However,attenuation of the acoustic emissionsignals in fiber reinforced materialspresents unique problems. Effectiveacoustic emission monitoring of fiberreinforced components requires muchcloser transducer spacings than metalcomponents of similar size andconfiguration. With the proper numberand location of transducers, monitoringof composite structures has proveneffective for detecting and locating areasof fiber breakage, delaminations and othertypes of structural degradation.

Types of StressPressure systems are stressed usinghydrostatic or some other pressure test.The level of stress should normally beheld below the yield stress. Bendingstresses can be introduced to beamedstructures. Torsional stresses can begenerated in rotary shafts. Thermalstresses may be created locally. Tensionand bending stresses should either beuniaxial or cyclic to best simulate serviceinduced stresses.

Successful ApplicationsExamples of proven applications for theacoustic emission method include thefollowing: periodic or continuousmonitoring of pressure vessels and otherpressure containment systems to detect

and locate active discontinuities;detection of incipient fatigue failures inaerospace and other engineeringstructures; monitoring material behaviortests to characterize various failuremechanisms; monitoring fusion orresistance weldments during welding orduring the cooling period; andmonitoring acoustic emission responseduring stress corrosion cracking andhydrogen embrittlement susceptibilitytests.

Acoustic Emission TestEquipmentEquipment for processing acousticemission signals is available in a variety offorms ranging from small portableinstruments to large multichannelsystems. Components common to allsystems are transducers, preamplifiers,filters and amplifiers to make the signalmeasurable. Techniques used formeasurement, display and storage varymore widely according to the demands ofthe application. Figure 1 shows a blockdiagram of a generic four-channel acousticemission system.

Acoustic Emission TransducersWhen an acoustic emission wave frontimpinges on the surface of a test object,very minute movements of the surfacemolecules occur. A transducers function isto detect this mechanical movement andconvert it into a specific, usable electricsignal.


FIGURE 1. Basic four-channel acoustic emission test system.

Transducers

Databuffers

Disk storage

Computer

Screendisplay

Printer andgraphics

copy

Operator keyboard

Preamplifierswith filters

Main amplifierswith filters

Measurementcircuitry

The transducers used for acousticemission testing often resemble anultrasonic search unit in configurationand generally use a piezoelectric sensor asthe electromechanical conversion device.The transducers may be resonant or broadband. The main considerations intransducer selection are (1) operatingfrequency, (2) sensitivity and (3)environmental and physicalcharacteristics. For high temperature tests,waveguides may be used to isolate thetransducer from the environment. This isa convenient alternative to hightemperature transducers. Waveguides havealso been used to precondition theacoustic emission signals as aninterpretation aid.

Issues such as wave type anddirectionality are difficult because thenaturally occurring acoustic emissioncontains a complex mixture of wavemodes.

Preamplifiers and FrequencySelectionThe preamplifier must be located close tothe transducer. Often, it is actuallyincorporated into the transducer housing.The preamplifier provides filtering, gain(most commonly 40 dB) and cable drivecapability. Filtering in the preamplifier(together with transducer selection) is theprimary means of defining themonitoring frequency for the acousticemission test. This may be supplementedby additional filtering at the computer.

Choosing the monitoring frequency isan operator function because the acousticemission source is essentially wide band.Reported frequencies range from audibleclicks and squeaks up to 50 MHz.

Although not always fully appreciatedby operators, the observed frequencyspectrum of acoustic emission signals issignificantly influenced by the resonanceand transmission characteristics of boththe test object (geometry as well asacoustic properties) and the transducer. Inpractice, the lower frequency limit isgoverned by background noise; it isunusual to go below 10 kHz except inmicroseismic work. The upper frequencylimit is governed by wave attenuationthat restricts the useful detection range; itis unusual to go above 1 MHz. The mostcommon frequency range for acousticemission testing is 100 to 300 kHz.

System ComputerThe first elements in the computer are themain amplifiers and thresholds, which areadjusted to determine the test sensitivity.Main amplifier gains in the range of 20 to60 dB are most commonly used.Thereafter, the available processing

depends on the size and cost of thesystem. In a small portable instrument,acoustic emission hits or thresholdcrossings may simply be counted and thecount then converted to an analogvoltage for plotting on a chart recorder. Inmore advanced hardware systems,provisions may be made for energy oramplitude measurement, spatial filtering,time gating and automatic alarms.

Signal Processing and DisplaysNearly all acoustic emission systems usecomputers in various configurations, asdetermined by the system size andperformance requirements. In typicalimplementations, each acoustic emissionsignal is measured by hardware circuitsand the measured parameters are passedthrough the central computer to a diskfile of signal descriptions. The customarysignal description includes the hits, hitrate, amplitude, duration, rise time andoften the energy of the signal, along withits time of occurrence and the values ofslowly changing variables such as loadand background noise level.

During or after data recording, thesystem extracts data for graphic displaysand hard copy reports. Common displaysinclude history plots of acoustic emissionversus time or load, distributionfunctions, cross plots of one signaldescriptor against another and sourcelocation plots. Installed systems of thistype range in size from 4 to 128 channels.

Operator Training and SystemUsesComputer based systems are usuallyversatile, allowing data filtering (toremove noise) and extensive posttestdisplay capability (to analyze andinterpret results). This versatility is anadvantage in new or difficult applications,but it places high demands on theknowledge and technical training of theoperator. Other kinds of equipment havebeen developed for routine industrialapplication in the hands of less highlytrained personnel.

Examples are the systems used forbucket truck testing (providingpreprogrammed data reports inaccordance with recommended practices)and systems for resistance weld processcontrol (which are inserted into thecontrol system and terminate the weldingprocess automatically when expulsion isdetected).

Acoustic emission equipment wasamong the first nondestructive testingequipment to use computers in the late1960s. Performance, in terms ofacquisition speed and real time analysiscapability, has been much aided by


advances in computer technology. Trendsexpected in the future include advancedkinds of waveform analysis, morestandardized data interpretationprocedures and more dedicated industrialproducts.

Acoustic Emission SystemAccessoriesAccessory items often used in acousticemission work include oscilloscopes,transient recorders and spectrumanalyzers, magnetic tape recorders, rootmean square volt meters, specialcalibration instruments and devices forsimulating acoustic emission. A widelyaccepted simulator is a graphite or pencillead break (PLB) source, which uses amodified drafting pencil that provides aremarkably reproducible simulatedacoustic emission signal when thegraphite is broken against the teststructure.

Acoustic Emission TestSensitivityAlthough the acoustic emission method isinherently more sensitive than othernondestructive methods such asultrasonic or radiographic testing, thesensitivity decreases with increasingdistance between the source and thetransducers. The same factors that affectthe propagation of ultrasonic waves alsoaffect the propagation of the stress wavesused in acoustic emission techniques.

Wave mode conversions at the surfacesof the test object and other acousticinterfaces, combined with the fact thatdifferent wave modes propagate atdifferent velocities, are factors thatcomplicate analysis of acoustic emissionresponse signals and produceuncertainties in calculating acousticemission source locations withtriangulation or other source locationtechniques.

Background Noise and MaterialPropertiesIn principle, overall acoustic emissionsystem sensitivity depends on thetransducers as well as the characteristics ofthe specific instrumentation system. Inpractice, however, the sensitivity of theacoustic emission method is oftenprimarily limited by ambient backgroundnoise considerations for engineeringmaterials with good acoustic transmissioncharacteristics.

During the monitoring of structuresmade of materials that exhibit highacoustic attenuation (because of scattering

or absorption), the acoustic properties ofthe material usually limit the ultimate testsensitivity and will certainly dictatemaximum transducer spacings.

Effects of System TransducersTransducer coupling and reproducibilityof response are important factors thatmust be considered when applyingmultiple acoustic emission transducers.Careful calibration checks must beperformed before, after and sometimesduring the acoustic emission monitoringprocess to ensure that all channels of theinstrumentation are operating properly atthe correct sensitivities.

For most engineering structures,transducer selection and placement mustbe carefully chosen based on a detailedknowledge of the acoustic properties ofthe material and the geometric conditionsthat will be encountered. For example, theareas adjacent to attachments, nozzlesand penetrations or areas where thesection thickness changes usually requireadditional transducers to achieveadequate coverage. Furthermore,discontinuities in such locations oftencause high localized stress in areas wheremaximum coverage is needed.

Data InterpretationProper interpretation of the acousticemission response obtained duringmonitoring of pressurized systems andother structures usually requiresconsiderable technical knowledge andexperience with the acoustic emissionmethod. Close coordination is requiredbetween the acoustic emission systemoperators, the data interpretationpersonnel and those controlling theprocess of stressing the structure.

Because most computerized,multichannel acoustic emission systemshandle response data in pseudo batches,an intrinsic dead time occurs during datatransfer. Dead time is usually not aproblem but can occasionally result inanalysis errors when the quantity ofsignals overloads the data handlingcapabilities of the system.

Compensating for BackgroundNoiseWhen acoustic emission monitoring isused during hydrostatic testing of a vesselor other pressure system, the acousticemission system will often provide thefirst indication of leakage. Pump noiseand other vibrations, or leakage in thepressurizing system, can also generatebackground noise that limits the overall


MOVIE. Pencilbreak source.

system sensitivity and hampers accurateinterpretation.

Special precautions and fixturing maybe necessary to reduce such backgroundnoise to tolerable levels. Acoustic emissionmonitoring of production processes in amanufacturing environment involvesspecial problems related to the highambient noise levels, both electrical andacoustical. Preventive measures may benecessary to provide sufficient electrical oracoustical isolation to achieve effectiveacoustic emission monitoring.

Various procedures have been used toreduce the effects of background noisesources. Included among these aremechanical and acoustic isolation;electrical isolation; electronic filteringwithin the acoustic emission system;modifications to the mechanical orhydraulic loading process; specialtransducer configurations to controlelectronic gates for noise blocking; andstatistically based electroniccountermeasures, includingautocorrelation and cross correlation.

Kaiser EffectJosef Kaiser is credited as the founder ofmodern acoustic emission technology andit was his pioneering work in Germany inthe 1950s that triggered a connected,continuous flow of subsequentdevelopment. He made two majordiscoveries. The first was the nearuniversality of the acoustic emissionphenomenon. He observed emission in allthe materials he studied. The second wasthe effect that bears his name.

In translation of his own words: Testson various materials (metals, woods ormineral materials) have shown that lowlevel emissions begin even at the loweststress levels (less than 1 MPa or100 lbfin.2). They are detectable all theway through to the failure load, but onlyif the material has experienced noprevious loading. This phenomenon lendsa special significance to acoustic emissioninvestigations, because by themeasurement of emission during loadinga clear conclusion can be drawn about themagnitude of the maximum loadingexperienced before the test by thematerial under investigation. In this, themagnitude and duration of the earlierloading and the time between the earlierloading and the test loading are of noimportance.4

This effect has attracted the attentionof acoustic emission workers ever since. Infact, all the years of acoustic emissionresearch have yielded no othergeneralization of comparable power. Astime went by, both practical applications

and controversial exceptions to the ruleswere identified.

Acoustic emission testing is a passivemethod that monitors the dynamicredistribution of stresses within a materialor component. Therefore, acousticemission monitoring is only effectivewhile the material or structure is subjectedto an applied stress. Examples of thesestresses include pressure testing of vesselsor piping and tension loading or bendloading of structural components.

IrreversibilityAn important feature affecting acousticemission applications is the generallyirreversible response from most metals. Inpractice, it is often found that once agiven load has been applied and theacoustic emission from accommodatingthat stress has ceased, additional acousticemission will not occur until that stresslevel is exceeded, even if the load iscompletely removed and then reapplied.This often useful (and sometimestroublesome) behavior has been namedthe kaiser effect.

The degree to which the kaiser effect ispresent varies between metals and mayeven disappear completely after severalhours or days for alloys that exhibitappreciable room temperature annealing(recovery) characteristics. Some alloys andmaterials may not exhibit measurablekaiser effect at all.

Because of the kaiser effect, eachacoustic signal may only occur once, sotests have a now or never quality. In thisrespect, acoustic emission is at adisadvantage when compared totechniques that can be repeated bydifferent operators or with differentinstruments, without affecting thestructure or the discontinuity.

The outcome in practical terms is thatacoustic emission testing must be used atcarefully planned times: during prooftests, before plant shutdowns or duringcritical moments of continuous operation.This seeming restriction sometimesbecomes the biggest advantage of theacoustic emission method. By usingacoustic emission during service,production can continue uninterrupted.Expensive and time consuming processessuch as the erection of scaffolding andextensive surface preparation can becompletely avoided.

Dunegan CorollaryAn early major application of the kaisereffect was for diagnosing damage inpressure vessels and other engineeringstructures.5 The strategy included aclarification of the behavior expected of apressure vessel subjected to a series of


loads (to a proof pressure withintervening periods at a lower workingpressure).

Should the vessel suffer no damageduring a particular working period, thekaiser effect dictates that no emission willbe observed during the subsequent proofload. In the event of discontinuity growthduring a working period, subsequentproof loading would subject the materialat the discontinuity to higher stressesthan before and would make thediscontinuity emit. Emission during theproof loading is therefore a measure ofdamage experienced during the precedingworking period.

This so called dunegan corollary becamea diagnostic approach in practical fieldtesting. Field operators learned to payparticular attention to emission betweenthe working pressure and the proofpressure and thereby made many effectivediagnoses. A superficial review of thekaiser effect might lead to the conclusionthat practical application of acousticemission techniques requires a series ofever increasing loads. However, effectiveengineering diagnoses can be made byrepeated applications of the same proofpressure.

Felicity EffectThe second major application of thekaiser effect arose from the study of caseswhere it did not occur. Specifically in fiberreinforced plastic components, emission isoften observed at loads lower than theprevious maximum, especially when thematerial is in poor condition or close tofailure. This breakdown of the kaiser effectwas successfully used to predict failureloads in composite pressure vessels6 andbucket truck booms.7

The term felicity effect was introducedto describe the breakdown of the kaisereffect and the term felicity ratio names theassociated quantitative measure. Thefelicity ratio has proven to be a valuablediagnostic tool in one of the mostsuccessful of all acoustic emissionapplications, the testing of fiberglassvessels and storage tanks.8 In fact, thekaiser effect may be regarded as a specialcase of the felicity effect (a felicity ratioof 1).

The discovery of cases where the kaisereffect breaks down was at first confusingand controversial but eventually somefurther insights emerged. The kaiser effectfails most noticeably in situations wheretime dependent mechanisms control thedeformation. The rheological flow orrelaxation of the matrix in highly stressedcomposites is a prime example. Flow ofthe matrix at loads below the previousmaximum can transfer stress to the fibers,causing them to break and emit. Other

cases where the kaiser effect will fail arecorrosion processes and hydrogenembrittlement, which are also timedependent.

Kaiser PrincipleFurther insight can be gained byconsidering load on a structure versusstress in the material. In practicalsituations, test objects or engineeringstructures experience loading and mostdiscussions of the kaiser effect come fromthat context. But actually, the kaiser effectapplies more fundamentally to stress in amaterial. The principle is that materialsemit only under unprecedented stress.Evaluated point-by-point through thethree-dimensional stress field within thestructure, this principle has wider truththan the statement that structures emitonly under unprecedented load. Providedthat the microstructure has not beenaltered between loads, the kaiser principlemay even have the universal validity thatthe kaiser effect evidently lacks, at leastfor active deformation and discontinuitygrowth.

In composite materials, an importantacoustic emission mechanism is frictionbetween free surfaces in damaged regions.Frictional acoustic emission is alsoobserved from fatigue cracks in metals.Such source mechanisms contravene boththe kaiser effect and the kaiser principle,but they can be important for practicaldetection of damage and discontinuities.

System Selection

Financial ConsiderationsAs with any purchase of technicalapparatus, choosing an acoustic emissionsystem is a process that includescompromises between cost andperformance. The total cost of a system isdetermined by several factors: (1) thesophistication of the systems software,(2) the value and reliability of themanufactured components, (3) theresources needed to support thenondestructive test, (4) the amount ofdata required and (5) the kind ofinformation needed and how it ispresented.

Performance OptionsBecause acoustic emission systems areused in a number of ways, manufacturersprovide instrumentation with a variety ofoptions, ranging from simple displayoptions (analog, digital or a combination)to options in size and cost that extendover several orders of magnitude.


Below is a list of generalized questionswhose answers will help define the bestsystem for a particular application.

1. Is the test primarily to determineincipient failure?

2. Is a potential discontinuity in the testobject detectable during operatingstresses but undetectable otherwise?

3. Is there a metallurgical problem thatmust be addressed (such as weld orfastener integrity)?

4. Is there damage caused by themanufacturing technique that couldbe detected during the machining orforming processes?

5. Can the instrumentation be used tomaterially reduce waste inmanufacturing?

6. Is the instrumentation expected tooperate in harsh industrial areas orspecial environments (explosive orhigh temperature)?

7. Will the instrumentation be used formedical purposes?

8. Will ancillary sources of sound(background noise) obscure the desiredacoustic emission sources?

9. Is data analysis necessary forinterpretation of test results?

System ExpansionAcoustic emission instrumentation in itsmost basic form consists of a detectordevice (transducer) and an oscilloscope.The transducer is acoustically coupled tothe test object and electrically coupled tothe oscilloscope. The operator watches foran identifiable set of signals emanatingfrom the test object while under aninduced stress. These stresses may bethermal, residual, corrosive or mechanical.Occasionally, the test object is subjectedto dangerously high stresses.

Signal losses in the cable connectingthe transducer with the oscilloscope maybecome excessive, requiring the additionof a preamplifier close to or integral withthe transducer. Fixture noise and otherspurious electrical or acoustic signals maythen require specific filtering techniques.

Attenuation or additional amplificationof the signals may be required, dependingon the character of the acoustic emissionsource. Additionally, care must beexercised when selecting the continuousrange of signal amplitudes that will beaccepted or rejected. Acoustic emissionfrom metal, wood, plastic and biologicalsources generates signals ranging from1 V to 1 kV. The dynamic range ofacoustic emission signal voltageamplitudes from a test object may be120 dB, or a ratio of one million to one. Acompromise must be made to choosethose amplitudes that fit the linear rangeof the instrumentation.

Placement and acoustic coupling oftransducers is probably the most exactingchallenge in an acoustic emission test andthey are aspects of the technique thataffect many levels of the test technique,including instrumentation. Transducersmust be placed on test object areas thatwill not be acoustically shadowed fromthe source. Equal care must be given tothe number of interfaces (acousticimpedances) permitted between thesource and transducer. The material undertest is intrinsically part of theinstrumentation because the material actsas a mechanical filter and a waveguide.

It is important that new test data berelated to the recent history of similarsignals. Recording devices are needed toensure that interpretation does not relyon human memory. After conditioning,signals are processed for electronicstorage.

Source LocationBefore an acoustic emission test canbegin, signal calibration is required toestablish the sensitivity and selectivity ofthe acoustic emission instruments and toensure the repeatability of source location.To narrow an area of acoustic emissionactivity to a specific location, additionalacoustic emission channels may be usedon the test object.

Acoustic emission hit timing becomescritically important when source locationis required. Time of arrival, duration ofacoustic emission signals and acousticemission signal amplitude are importantdata for interactive process controlsystems that depend on gating orwindowing.

Computers can calculate and respondvery quickly. These characteristics producehigh capture rates, some greater than100 000 acoustic emission hits per second,with the entire waveform digitallyrecorded. Depending on signal durationsand the number of online graphics andcomputational iterations, some computersmay be able to capture less than 50 hitsper second.

Other Computer UsesComputer algorithms have been writtenfor a number of acoustic emissionapplications, including (1) one-, two- andthree-dimensional acoustic emissionsource location schemes, (2) fast fouriertransform calculations and(3) measurement of hit amplitude,duration, rise time, signal energy andpower. Computer systems also provide forrecording of test parameters such aspressure, strain and temperature.

Few acoustic emission systems canrecord all the test data. An interrupted or


nearly continuous flow of acousticemission will inhibit some computersystems. Therefore, analog instrumentsare still useful for the capture of nearlycontinuous acoustic emission such asleakage noise. The most common analoginstruments consist of true root meansquare volt meters, frequency countersand hit counters.

System CalibrationWith all of the features provided bymodern instrumentation, calibration ofsignal input and instrument calibrationscan still be a formidable problem. Severalstandards have been published governingacoustic emission acquisition and datareduction. Among these are documentsfrom ASTM International, including onethat specifically treats the calibration ofacoustic emission instrumentation.9

However, with such a wide selection ofinstrumentation, no single document cancover all possible contingencies. Acousticemission instrument manufacturers striveto produce systems with better quality,higher efficiency and faster speeds yetmore needs to be done to improve theassurance that instruments in the field areproperly calibrated. Sufficiently detailedinformation on the acoustic emissioninstrument calibration is often difficult toobtain. The end user has the ultimateresponsibility for maintaining propercalibration of the system.

Greater complexity of instrumentationprovides the chance for a greater numberof malfunctions. Acoustic emission testpersonnel need a process to certify theoperation of computer hardware andsoftware combinations. Such processesshould be designed for field use.


Hydraulic and MechanicalNoiseAcoustic emission monitoring mustalways recognize the presence of noise(extraneous or interfering acoustic signalscarrying no relevant data). Noise signalsmay be continuous or intermittent andthe source may be either internal orexternal to the test object. Sources shouldbe examined to discriminate betweennoise and relevant acoustic emissionsignals. The most effective remedy fornoise interference is to identify the noisesources and then eliminate or inhibitthem.

Most acoustic emission monitoring isdone using a frequency threshold of100 kHz or above. Fortunately, mostacoustic noise diminishes in amplitude atthese higher frequencies. Less noise isdetected as the frequency pass bandbecomes narrow and higher, but thesefrequencies can also inhibit acousticemission detection. An unacceptableremedy for noise interference is to reducethe instrument gain or raise the detectionthreshold because this will also excludevalid acoustic emission signals.

As a rule, an indication of the level ofinterference from electrical or mechanicalnoise can be obtained by comparing thesystem noise on the test structure withthe noise in an ambient environment. Itis important to test for noise in anoperating plant, including machinery thatwill be active during the acoustic emissiontest. Noise measured in terms of peak orpeak-to-peak amplitude relates mostdirectly to acoustic emission detection.Other measurement parameters such asroot mean square voltages will generallynot recognize noise spikes or very shortduration intermittent signals.

Hydraulic NoiseAn example of hydraulic noise may occurduring acoustic emission monitoring of ahydrostatic pressure vessel test. If thepressure system develops a leak, the leakwill result in continuous high amplitudenoise that can completely obscureacoustic emission from a discontinuity. Inaddition to the noise from a leak, thereare other noise sources such as boiling of

a liquid, cavitation and turbulent fluidflow.

One modern servo hydraulic testingmachine can present a serious noiseproblem during acoustic emission tests.10If the valves controlling the hydraulicloading are integral with the yoke of themachine, hydraulic noise will betransmitted to the acoustic emissiontransducer through the load train. Thesource of the noise in this machine is thecavitating hydraulic fluid in narrowchannels of the load piston and cylinder.The high frequency content is oftencoupled to the test object and this can becontrolled by separating the servocontroller from the test frame with highpressure hoses.

Leaking air lines in the test vicinity canbe a source of continuous noise. Turbulentflow or cavitation can also be a seriousproblem during continuous monitoring ofpressurized systems. One cure is tooperate the acoustic emission monitoringsystem in a frequency range above thenoise frequency.11-14

Mechanical NoiseAny movement of mechanical parts incontact with the structure under test is apotential source of noise. Rollerassemblies with spalled bearings or racesare examples of noisy parts. Mechanicalnoise can be beneficial when acousticemission techniques are applied to detectincipient mechanical failure.15

Fretting. Rubbing or fretting is aparticularly difficult noise source toovercome because it often has a verybroad frequency content. A pressure vesselunder hydrostatic test will expandelastically in response to the imposedstress and may rub against its supports.Guard transducers can sometimes helpidentify this noise source. Riveted, pinnedor bolted structures are often noisy.Initially, bearing points on a pinnedstructure are limited. As these points areloaded to yield stress, the joint will shiftto pick up the load and simultaneouslyproduce bursts of noise.Cyclic Noise. Repetitive noise such as thatfrom reciprocating or rotating machinerycan sometimes be rejected by blankingthe acoustic emission instrument duringthe noise bursts. Blanking is especiallyeffective if the repetition rate is low and


PART 2. Acoustic Emission Noise1

uniform relative to acoustic emission.Causes of cyclic noise include positioningof spot welding electrodes, clamping ofsheets before shearing and fatigue testingmachines.

Serrated wedge grips used on manyphysical testing machines are nearlyalways a source of noise. As the test objectis loaded, two sources of noise can occur:(1) the serrations of the grips bite into thetest object and (2) the grips slip in theyokes of the testing machine. Pinned loadconnections will usually alleviate thisproblem.

Noise Signals and Rise TimeMechanical noise has characteristics thathelp distinguish it from the acousticemission signals of cracks. Noise signalsare of relatively low frequency with lowrise time. The acoustic emission burstsfrom cracks generally have rise times(from threshold to peak) of less than25 s if the transducer is near the source.Mechanical noise rarely has such a fastrise time.

The rise time of both noise andacoustic emission signals increases withsource-to-transducer distance because ofthe attenuation of the high frequencycomponents. In some cases, a rise timediscriminator can be effective in isolatingacoustic emission from mechanical noise.Frequency discrimination is usually morereliable.

Control of Noise SourcesWhen multichannel systems are used forsource location, noise signals from outsidethe zone of coverage can be controlledusing signal arrival times (t) at thevarious transducers in the array. Tomeasure arrival times, the signal densitymust be relatively low (less than 10 persecond). At higher signal densities, thenoise and acoustic emission signals mayinteract to produce erroneous t values.

Another technique for noise rejectionis called the master slave technique. Mastertransducers are mounted near the area ofinterest and are surrounded by slave orguard transducers relatively remote fromthe master. If the guard transducers detecta signal before the master transducers,then the hit occurred outside the area ofinterest and is rejected by the instrumentcircuitry.

Noise during Welding

Effect of Grounding on WeldingNoiseThe terminals of most welding machinesare not grounded to facilitate reversing

the polarity. Thus, a structure beingwelded is said to be floating with respectto ground and this serves to aggravate theacoustic noise problem. One simplesolution for this is to remove the acousticemission instruments ground from themain voltage supply and to then connectit along with a blocking capacitor to thestructure under test. This remedy bringsthe welding machine and the structure tothe same noise potential and greatlyreduces the detected noise. This proceduremay not be legal in some locationsbecause of electricity regulations in force.

Another solution is to use a batterypowered acoustic emission systemgrounded to the structure. An isolationtransformer between the instrument andthe power supply is also effective.Similarly, care should be taken to ensurethat contact between different parts of thesystem (transducer, preamplifier, mainmodule) and different parts of thestructure do not result in different groundpotentials or ground loops. Ground loopsmake acoustic emission test systemshighly vulnerable to electromagneticinterference.

A ground loop occurs when the metalcase of a single-ended transducer is inelectrical contact with a metallic testobject. This contact creates the possibilitythat a current can flow in a loop from thetest structure to the transducer case, to acoaxial cable screen, to the instrumentground, to the earth itself and so back tothe test object. The conducting loop actslike a giant antenna. Externalelectromagnetic radiation induces currentin this antenna. The current produces anelectric potential between the two ends ofthe cable screen in which it is flowing.This potential manifests as noise. Groundpotential differences less than 1 mV cancause noise.

Noise from Welding ProceduresMetal arc welding gives off spatter thatstrikes the weldment and may causeappreciable noise. The noise may beenough to make acoustic emissiondetection impossible with the arc on. Ifthe welding process produces slag on topof the bead, the slag should be removedto enhance acoustic emission detection.Slag fracturing is a true acoustic emissionsource having no effect on the quality ofthe weld but it can obscure acousticemission from the weld.

Welding requires heat, which causesthermal expansion, followed bycontraction and warpage with cooling.These movements, especially over a grittysurface, cause random noise bursts untilthe weldment reaches ambienttemperature. Good welding practice is towipe the weld piece, parts and work tableclean before assembly.


MOVIE. Guardtransducerscontrol noise.

Considerable welding is done on hotrolled steel with the mill scale notremoved. Such scale (iron oxide) has adifferent coefficient of thermal expansionthan the steel and being brittle tends tofracture with heating and cooling.

Figure 2 shows the acoustic emissiondetected after heating the edge of a25 mm (1 in.) plate to 150 C (300 F). Inthis case, the acoustic emissioninstruments gain was set at 60 dB.Acoustic emission count rate peaked at7500 counts per second when heating wasstopped. The count droppedexponentially from this high point to zeroin 4 min. The best way to eliminate thenoise was found to be descaling beforewelding. However, when descaling is notpossible, signal pattern recognition mayhelp identify the valid acoustic emissionsignals in the presence of scale noise.

When welding alloy and high carbonsteels, a transformation to martensite ispossible. Martensitic transformation is atrue and energetic source of acousticemission and is caused by rapid cooling.Figure 3 is a plot of acoustic emissionfrom spot welding of high carbon steel.The square wave in the upper left portionof the illustration indicates weld powerduration. The associated acoustic emissionis from nugget formation. When the weldinterval ended, the nugget cooled rapidly,

causing (1) martensitic transformationand (2) the large envelope of acousticemission during the immediate postweldinterval.16

Miscellaneous NoiseMetals such as aluminum give offconsiderable acoustic emission when incontact with acids during etching orpickling. Some solution annealedaluminum alloys will emit acousticemission when heated to 190 C (375 F)because of precipitation hardening (seeFig. 4).


FIGURE 2. Acoustic emission signals ( 60 dB) from fracturingof mill scale.

Aco

ustic

em

issi

on (

103

coun

ts p

er s

econ

d)

7

6

5

4

3

2

1

Time (60 s intervals)

0 4 8 12 16 20

FIGURE 3. Postweld acoustic emission signalsfrom spot weld martensitic transformation.

Postweldacoustic emissionWeld acoustic

emission

FIGURE 4. Acoustic emission signals from precipitationhardening of aluminum heated to 190 C (375 F).

Aco

ustic

em

issi

on (

103

coun

ts p

er s

econ

d)

10

9

8

7

6

5

4

3

2

1

0

Time (80 s intervals)

1 2 3 4 5 6 7 8

Noise from Solid State BondingWhen clean metallic surfaces are broughtinto intimate contact, some degree ofsolid state bonding is possible. This effectincreases with temperature and pressure.Such incidental bonding usually does nothave much strength and may rupture atthe first change of stress, creating a newfree surface and a stress wave that is truebut undesired acoustic emission.

Fretting is an example of thisphenomenon. An underwelded spot weldcalled a sticker causes a similar condition.Even a properly welded spot may besurrounded by solid state bonding andwill result in acoustic emission when thestructure is first loaded. Such acousticemission is termed secondary emission17and may be an indicator of discontinuitygrowth.

Electromagnetic InterferenceElectromagnetic interference consists ofnoise signals coupled to the acousticemission instrumentation by electricalconduction or radiation. Sources ofelectromagnetic interference includefluorescent lamps, electric motor circuits,welding machines and turning electricalpower on and off by relays with inductiveloads. Large spikes of electrical noise arethus created and may contain the highestamplitude and frequencies of any signalsdetected.

The extent to which electromagneticinterference is encountered is a functionof the environment, shielding and thetransducer design. Hostile environmentsoften require better shielding anddifferential transducers. Optimumstandard shielding can be provided byenclosing the transducers, preamplifiersand main amplifiers in high conductivitymetal cases, by grounding the cases at acommon point or by using special leadsscreened with alternate layers of metaland copper.18

Commercial acoustic emissioninstruments usually include features toreject some electrical noise on the supplyline and noise radiated as electromagneticinterference. An isolation transformer onthe power supply can also reduceelectromagnetic interference. A majorproblem can arise if the test structure hasa ground system different from theacoustic emission instrumentation.

The electrical potential differencebetween the structure and the acousticemission system can be substantial andmay produce high frequency noise spikesmuch greater than the valid signalsdetected by the transducers. Noinstrument can reject this magnitude ofnoise. Even a transducer insulated fromthe structure can pick up capacitivelycoupled noise. A differential transducer

will provide considerable relief from suchnoise.

ConclusionsThere is a wide range of potential noisesources associated with acoustic emissionmonitoring. Before performing acousticemission tests, it is essential to check forbackground noise, for spurious acousticemission and for their effects on testresults. This check should include allelectrical equipment and machineryoperating during the acoustic emissiontest. Noise should then be eliminated,either by mechanically removing theidentified noise source from the test setupor by selectively analyzing the test data.


Purpose of SignalCharacterizationThe objective of an acoustic emission testis to detect emission sources and toprovide as much information as possibleabout them. The technology for detectingand locating sources is well established.Acoustic emission signals can providemuch information about the source of theemission and the material and structureunder test.

The purpose of source characterizationis to use the transducer output waveformto identify the sources and to evaluatetheir significance. There is thus aqualitative (source identification) and aquantitative (source intensity or severity)aspect to characterization.

Data InterpretationThe signal waveform is affected by(1) characteristics of the source, (2) thepath taken from the source to thetransducer, (3) the transducerscharacteristics, and (4) the measuringsystem. Generally, the waveforms arecomplex and using them to characterizethe source can be difficult. Information isextracted by techniques ranging fromsimple waveform parametermeasurements to artificial intelligence(pattern recognition) approaches. Theformer often suffice for simple preserviceand inservice tests. The latter may berequired for online monitoring ofcomplex systems.

In addition to characteristics of thewaveforms themselves, there isinformation available from thecumulative characteristics of the signals(including the amplitude distribution andthe total number of signals detected) andfrom rate statistics (such as the rate ofsignal arrival or energy at the transducer).

There are thus several options availablefor acoustic emission sourcecharacterization. An appropriate approachmust be determined for each application.

Characteristics of DiscreteAcoustic EmissionDiscrete or burst type acoustic emissioncan be described by relatively simple

parameters. The signal amplitude is muchhigher than the background and is ofshort duration (a few microseconds to afew milliseconds). Occurrences ofindividual signals are well separated intime. Although the signals are rarelysimple waveforms, they usually riserapidly to maximum amplitude and decaynearly exponentially to the level ofbackground noise. The damped sinusoidin Fig. 5 is often used to represent a burstof acoustic emission.

Acoustic emission monitoring isusually carried out in the presence ofcontinuous background noise. A thresholddetection level is set somewhat above thebackground level (Fig. 6) and serves as areference for several of the simplewaveform properties.

Using this model, the waveformparameters in Fig. 7 can be identified:(1) acoustic emission hit, (2) acousticemission count (ringdown count),(3) acoustic emission hit energy,(4) signal amplitude, (5) signalduration and (6) signal rise time.


PART 3. Acoustic Emission SignalCharacterization1

FIGURE 5. Idealized representation of an acoustic emissionburst signal.

Am

plit

ude

(rel

ativ

e sc

ale)

Time(relative scale)

FIGURE 6. Threshold setting to avoid triggering bycontinuous background noise.

Trigger levelNoise

Am

plit

ude

(rel

ativ

e sc

ale)

Time(relative scale)

Cumulative representations of theseparameters can be defined as a function oftime or test parameter (such as pressure ortemperature), including: (1) total hits,(2) amplitude distribution and(3) accumulated energy. Once a specificparameter is selected, rate functions maybe defined as a function of time or testparameter: hit rate, count rate or energyrate.

Based on Fig. 7, several techniques forcharacterizing acoustic emission aredescribed below.

Waveform Parameters

Emission Hits and CountAcoustic emission hits are individualsignal bursts produced by local materialchanges. The hit count is the number oftimes a signal crosses a preset threshold.High amplitude hits of long duration tendto have many threshold crossings. Thenumber of threshold crossings per unittime depends on (1) the transducerfrequency, (2) the damping characteristicsof the transducer, (3) the dampingcharacteristics of the structure and (4) thethreshold level.

The idealized signal in Fig. 5 can berepresented by Eq. 1:19

(1)

where B is the decay constant (greaterthan 0), e is the root of naturallogarithms, t is time (second), V is theoutput potential (volt) of the transducer,V0 is the initial signal amplitude and isangular frequency (radian per second).

A common way to measure acousticemission activity is by ringdown counts,the number of times the transducer signalexceeds a counter threshold. When thetime t required for the signal to decay tothe threshold voltage Vt is long compared

to the period of oscillation, then thenumber of counts N from a given hit canbe given:

(2)

where Vt is the threshold potential (volt)of the counter.

Correlation of Count Rate andMaterial PropertiesAs shown in Fig. 7, a single acousticemission hit can produce several counts.A larger hit requires more cycles to ringdown to the trigger level and will producemore counts than a smaller hit. Thisprovides a measure of the intensity of theacoustic emission hit.

Correlations have been establishedbetween total counts, count rate andvarious fracture mechanics parameters(such as stress intensity factor)20-25 asexpressed in Eq. 3 or fatigue crackpropagation rate22-25 as expressed in Eq. 4:

(3)

where K is the stress intensity factor, N isthe total number of counts and n is aconstant with a value between 2 and 10.

(4)

where a is the crack size, c is the numberof cycles and N is the total number ofcounts.

The counts are a complex function ofthe frequency response of thetransducer and the structure, the dampingcharacteristics of the transducer, the hitand the propagation medium B, signalamplitude, coupling efficiency, transducersensitivity, amplifier gain and thethreshold voltage Vt. Maintaining stabilityof these parameters throughout a test orfrom test to test is difficult but essentialfor consistency of interpretation.Nevertheless, counts are widely used as apractical measure of acoustic emissionactivity.

Acoustic Emission Event EnergyBecause acoustic emission activity isattributed to the rapid release of energy ina material, the energy content of theacoustic emission signal can be related tothis energy release26,27 and can bemeasured in several ways. The true energyis directly proportional to the area underthe acoustic emission waveform. Theelectrical energy U present in a transienthit can be defined as:

dNdc

dadc

N Kn

NB

VV

=

2ln 0

t

V V e Bt t= ( )0 sin


FIGURE 7. Definition of simple waveform parameters.

Maximumamplitude

(relative scale)

Duration(relative scale)

Risetime

Decay time

Counts

Threshold

(5)

where R is the electrical resistance (ohm)of the measuring circuit. Direct energyanalysis can be performed by digitizingand integrating the waveform signal or byspecial devices performing the integrationelectronically.

The advantage of energy measurementover ringdown counting is that energymeasurements can be directly related toimportant physical parameters (such asmechanical energy in the emission hit,strain rate or deformation mechanisms)without having to model the acousticemission signal. Energy measurementsalso improve the acoustic emissionmeasurement when emission signalamplitudes are low, as in the case ofcontinuous emission.

Squaring the signal for energymeasurement produces a simple pulsefrom a burst signal and leads to asimplification of hit counting. In the caseof continuous emission, if the signal is ofconstant amplitude and frequency, theenergy rate is the root mean squarevoltage. The root mean square voltagemeasurement is simple and withoutelectronic complications. However, rootmean square meter response is generallyslow in comparison with the duration ofmost acoustic emission signals. Therefore,root mean square measurements areindicative of average acoustic emissionenergy rather than the instantaneousenergy measurement of the directapproach.

Regardless of the type of energymeasurement used, none is an absoluteenergy quantity. They are relativequantities proportional to the true energy.

Acoustic Emission SignalAmplitudeThe peak signal amplitude can be relatedto the intensity of the source in thematerial producing an acoustic emission.The measured amplitude of the acousticemission waveform is affected by thesame test parameters as the hit counts.Peak amplitude measurements aregenerally performed using a log amplifierto provide accurate measurement of bothlarge and small signals.

Amplitude distributions have beencorrelated with deformation mechanismsin specific materials.28,29 For practicalpurposes, a simple equation can be usedto relate signal amplitudes, hits andcounts:

(6)

where b is the amplitude distributionslope parameter, f is the resonantfrequency (hertz) of the transducer, Nrepresents the cumulative counts, Prepresents the cumulative hits and isthe decay time (second) of the hit.

Limitations of the SimpleWaveform ParametersIn general, measurements of hits, countsand energy provide an indication ofsource intensity or severity. Thisinformation is useful for determiningwhether the test object is accumulatingdamage and, in turn, for decidingwhether the test should continue or if thestructure in question should remain inservice.

In many cases, the high sensitivity ofacoustic emission testing also detectsunwanted background noises that cannotbe removed by signal conditioning. Foracoustic emission to be used effectively inthese applications, it is necessary toidentify the source of each signal as it isreceived.

In spite of many attempts to establishfundamental relationships between thesimple parameters, correlations betweensignal and source by analytical techniquesremain elusive. This dilemma is a result ofthe complexity of modeling the source,the material, the structure, the transducerand the measurement system. Advanceddigital computing techniques provide analternative, permitting successfulcharacterization and source identification.

Advanced CharacterizationTechniquesThe objective of source characterization ina specific application is to classify eachsignal as it arrives at the transducer. Inacoustic emission testing, characterizationof both noise and discontinuities isdesirable for the rejection of noise and theclassification of signals by discontinuitytype. This characterization permitsqualitative interpretations based oninformation contained in the signal ratherthan by inference based on filtering,thresholding and interpretation of thesimple parameters. It also extends therange of applications for acoustic emissiontesting to areas where noise interferencepreviously made the method impracticalor ineffective.

The area of artificial intelligenceknown as pattern recognition can be used torelate signal characteristics to acousticemission sources.30-33 There are severalclassification techniques34-36 that havebeen used in nondestructive testing.

NPfb

=

UR

V t dt= ( )1 20


Knowledge RepresentationTechniqueSignals emitted from an acoustic emissionsource are highly unreproducible in boththe time and frequency domains.Advanced techniques are needed toidentify common discriminatory features.This can be achieved with knowledgerepresentation (also called adaptivelearning), where information in theacoustic emission pulse is represented inthe most complete and effective way.

Statistical techniques are used toextract data from a group of signalsoriginating from the same source. Itwould be virtually impossible to obtainthe same information from one signalalone. Pattern recognition techniquesusing statistical features are particularlysuitable for analyzing nonlinear, timevarying acoustic emission signals.

The first step in knowledgerepresentation is to extract features fromthe signal waveform that fully representthe information contained in the signal.In addition to general time domain pulseproperties and shape factors, the acousticemission signal time series is transformedinto other domains. Table 2 lists featuresused in a typical acoustic emission testapplication.

Time domain pulse information iscombined with partial power distributionin different frequency bands together withfrequency shifts of the cumulative powerspectrum. A total of thirty features areextracted in this example. In some cases,more than a hundred features in severaldomains may be extracted to providesource characterization.

Feature Analysis and SelectionThe transformations carried out togenerate new features for sourcecharacterization do not produce newinformation. They represent the existingwaveform information in new ways thatfacilitate source characterization. In fact,only a few well chosen features amongthe available domains are normallyrequired for characterization.

Selection of the best combination offeatures often requires special signalclassifier development tools to searchamong the features and select an optimalset. Acoustic emission is sensitive not onlyto the source but also to transducer,material and structural factors, so thatseparate signal classifiers must bedeveloped for each application. Acombination of computer techniques andexpert insight is required to select the bestfeature set.

In a manual approach, the analyst isusually guided by experience whensearching for the optimal feature set for a

particular application. When severalclasses of signal are involved,development systems use statisticaltechniques such as interclass andintraclass distances and a biserialcorrelation estimate of each individualfeature as a measure of its discriminatingindex among the signal classes ofinterest.37 Separability arises fromdifferences between class mean featurevalues.

Pattern Recognition Training andTestingGenerally, a pattern recognition systemgoes through a learning or training stage,in which a set of decision rules isdeveloped. The system is supplied with aset of representative signals from eachsource to be classified. The classifier is saidto be trained when it can use the decisionrules to identify an input signal.


TABLE 2. Waveform parameters extracted from acousticemission signals using the knowledge representationtechnique.

Standard deviation of acoustic emission signalSkewness coefficient (third moment) of acoustic emission signalKurtosis coefficient (fourth moment) of acoustic emission signalCoefficient of variation of acoustic emission signalRise time of largest pulse in time domainFall time of largest pulse in time domainPulse duration of largest pulse in time domainPulse width of largest pulse in time domainRise time of second largest pulse in time domainFall time of second largest pulse in time domainPulse duration of second largest pulse in time domainPulse width of second largest pulse in time domainPulse ratio of second largest pulsesDistance between two largest pulsesPartial power in frequency band 0 to 0.25 MHzPartial power in frequency band 0.25 to 0.5 MHzPartial power in frequency band 0.5 to 0.75 MHzPartial power in frequency band 0.75 to 1.0 MHzPartial power in frequency band 1.0 to 1.25 MHzPartial power in frequency band 1.25 to 1.5 MHzPartial power in frequency band 1.5 to 1.75 MHzPartial power in frequency band 1.75 to 2.0 MHzRatio of two largest partial powersRatio of smallest and largest partial powerFrequency of largest peak in power spectrumAmplitude of largest peak in power spectrumFrequency at which 25 percent of accumulated power was

observedFrequency at which 50 percent of accumulated power was

observedFrequency at which 75 percent of accumulated power was

observedNumber of peaks exceeding a preset threshold

To test the performance of theclassifier, sample signals not used duringthe training process are presented to thesystem, which will identify the class towhich each unknown signal belongs.Successful identification of input signals isa good indication of a properly trainedsystem.

Signal ClassificationThe data used to train a classifiercorrespond to a group of points in featurespace. If two features are used for a systemwith three source classes, then the feature

space might appear as shown in Fig. 8.The classification mechanism mustdistinguish among the classes.

One simple way to distinguish theclasses is to place planes between theclasses (hyperplanes in amultidimensional space with manyfeatures). The classifier is a linearcombination of feature elements thatdefines a hyperplane to separate one classof signals from another in the featurespace. This is called a linear discriminantfunction classifier.

When the classes are distributed in amore complex arrangement, moresophisticated classifiers are required. Anexample would be the K nearest neighborclassifier that considers every member inthe training set as a representation point.It determines the distance of an unknownsignal from every pattern in the trainingset and it thus finds the K nearest patternsto the unknown signal. The unknown isthen assigned to the class in which themajority of the K nearest neighborsbelongs. In other cases, the patternrecognition process is modeledstatistically and statistical discriminantfunctions are derived.

Signal Treatment andClassification SystemA classification system is shown in Fig. 9.Acoustic emission source characterizationand recognition begins with signaltreatment. The acoustic emission signalsfrom different sources are first fed into atreatment system that computes thenecessary waveform features of eachsignal. The identity of the signal is taggedto each of the feature vectors. Thesevectors are then divided into two datasets, one for training the classifier and theother to test its performance.

Generally, a normally distributedrandom variable is used for assigning thesignal to the two data sets and thisensures that each set has the same a prioriprobability. Once the classifier is trained,it is tested by supplying a signal featurevector with a known identity and then


FIGURE 8. Feature space for two featuresused for a system with three source classes:(a) linearly separable; (b) nonlinearlyseparable; (c) piecewise linear separation oftwo regions.

(a)

(b)

(c)

+

+

FIGURE 9. A classification system for treatment of acoustic emission signals.

Batchcontrol file

Batch featureextraction

or

Preprocessor

Extractedfeature data

Trainingsample

Control

Testingsample

Featureselection and

decisionlogic

Trainedinformation

Classifier

Trainedinformation

Decision library

Classification output

Signaldata

comparing the known information to theclassification output. The traininginformation and the trained classifiersetup can be stored in a decision librarythat can be used to characterize andclassify the acoustic emission sources.

Table 3 shows the output of a classifierdevelopment system trained to classifyacoustic emission from plasticdeformation, stress corrosion, fracture anduniversal testing machine noise in aUnified Numbering System A97075,temper 651, wrought aluminum alloy.This means of developing acharacterization and classification systemshows clearly where the source of error is.In this case, the error lies primarily inidentifying the fracture signals.

Guidelines for SourceCharacterizationThe simple waveform parameters and theadvanced characterization techniquescomprise the tools available for acousticemission source characterization. Simpletechniques are required to measure thelevel of acoustic emission activity. Moreadvanced techniques are generallyrequired to identify the nature of theemission source.

Of the simple parameters, ringdowncounts are sensitive to threshold and gainsettings and, for reproducibility from testto test, hit measurements are less sensitiveto instrument settings.

Several powerful classifiers are availablein development tools. However, the threetechniques outlined above provide anindication of the decisions that have to bemade when selecting an advancedcharacterization technique. Factors thatmust be considered include the following:(1) available computing power, (2) thesample size available for training, (3) theperformance required of the classifiersystem and (4) the type of data available

for training. It is often difficult to obtain alarge enough number of data samplesfrom a particular source.

The linear discriminant function is thesimplest technique and involves the leastcomputation time. It is therefore readilyimplemented and rapidly computed.However, the classes of signals must belinearly separable. The K nearest neighborand empirical bayesian classifiers are morepowerful and address more difficultproblems.

The nearest neighbor classifier usesevery individual sample in the training setto determine the identity of the unknownsignal and each sample bears equal weightin the process of classification. When thetraining set from a particular source maycontain sample signals from other sources,the incorrect sample in the training setcarries as much weight as a good sample.

Comparison of ClassifierTechniquesBy comparison, the empirical bayesianclassifier (using statistical informationbased on the training set) will producerather stable recognition performance, solong as the numbers of impure datasamples do not dominate the training set.The K nearest neighbor and the lineardiscriminant function classifiers arenonparametric classifiers and the selectionof features in these two techniquesdepends on the sample size.

Compared to K nearest neighbor andthe linear discriminant functionclassifiers, the empirical bayesian classifiertechnique is more sensitive to the samplesize because the number of samples affectsthe estimates of the probabilitydistribution. The K nearest neighbor andthe linear discriminant function classifiersdo not generally require as large a data setso long as it is representative.


TABLE 3. Performance of a linear discriminant classifier.

RecognitionClassification Decisions Rate_______________________________

Source Size Class 1 2 3 4 (percent)

Performance on trainingStress corrosion 19 1 18 0 1 0 95Plastic deformation 30 2 0 29 1 0 97Fracture 27 3 1 1 25 0 93Machine noise 16 4 0 0 0 16 100Overall recognition rate 96

Performance on testingStress corrosion 19 1 14 0 4 1 74Plastic deformation 32 2 0 32 0 0 100Fracture 27 3 0 1 26 0 96Machine noise 16 4 0 0 0 16 100Overall recognition rate 94

DefinitionAcoustic emission transducers are used ona test objects surface to detect dynamicmotion resulting from acoustic emissionhits and to convert the detected motioninto a voltage-versus-time signal.

This voltage-versus-time signal is usedfor all subsequent steps in the acousticemission method. The electrical signal isstrongly influenced by characteristics ofthe transducer. Because the test resultsobtained from signal processing dependso strongly on the electrical signal, thetransducers characteristics are importantto the success and repeatability of acousticemission testing.

Transducer TypesBasic transduction mechanisms can beused to achieve a transducers functions:the detection of surface motion and thesubsequent generation of an electricalsignal. Capacitive transducers have beensuccessfully used as acoustic emissiontransducers for special laboratory tests.Such transducers can have good fidelity,so that the electrical signal very closelyfollows the actual dynamic surfacedisplacement.

However, the typical minimumdisplacement measured by a capacitivetransducer is on the order of 1010 m(4 109 in.). Such sensitivity is notenough for actual acoustic emissiontesting.

Laser interferometers have also beenused as acoustic emission transducers forlaboratory experiments.39,40 However, ifthis technique is used with a reasonablebandwidth, the technique lacks sufficientsensitivity for acoustic emission testing.

Piezoelectric TransducersAcoustic emission testing is nearly alwaysperformed with transducers that usepiezoelectric elements for transduction.The element is usually a special ceramicsuch as lead zirconate titanate and isacoustically coupled to the surface of thetest item so that the dynamic surfacemotion propagates into the piezoelectricelement. The dynamic strain in theelement produces a voltage-versus-timesignal as the transducer output. Because

virtually all acoustic emission testing isperformed with a piezoelectric transducer,the term acoustic emission transducer ishere taken to mean a sensor with apiezoelectric transduction element.

Direction of Sensitivity to MotionSurface motion of a point on a test objectmay be the result of acoustic emission.Such motion contains a componentnormal to the surface and two orthogonalcomponents tangential to the surface.Acoustic emission transducers can bedesigned to respond principally to anycomponent of motion but virtually allcommercial acoustic emission transducersare designed to respond to the componentnormal to the surface of the structure.Because waves traveling at thelongitudinal, shear and rayleigh wavespeeds all typically have a component ofmotion normal to the surface, acousticemission transducers can often detect thevarious wave arrivals.

Frequency RangeThe majority of acoustic emission testingis based on the processing of signals withfrequency content in the range from30 kHz to about 1 MHz. In specialapplications, detection of acousticemission at frequencies below 20 kHz ornear audio frequencies can improvetesting and conventional microphones oraccelerometers are sometimes used.

Attenuation of the wave motionincreases rapidly with frequency and, formaterials with higher attenuation (such asfiber reinforced plastic composites), it isnecessary to sense lower frequencies todetect acoustic emission hits. At higherfrequencies, the background noise islower; for materials with low attenuation,acoustic emission hits tend to be easier todetect at higher frequencies.

Acoustic emission transducers can bedesigned to sense a portion of the wholefrequency range of interest by choosingthe appropriate dimensions of thepiezoelectric element. This, along with itshigh sensitivity, accounts for thepopularity of this transductionmechanism.

In fact, by proper design of thepiezoelectric transducer, motion in thefrequency range from 30 kHz to 1 MHz(and more) can be transduced by a single


PART 4. Acoustic Emission Transducers and TheirCalibration38

transducer.41-43 This special type oftransducer has applications (1) inlaboratory experiments, (2) in acousticemission transducer calibration and (3) inany tests where the actual displacement isto be measured with precision andaccuracy.

Transducer DesignFigure 10 is a schematic diagram of atypical acoustic emission transducermounted on a test object. The transduceris attached to the surface of the test objectand a thin intervening layer of couplant isusually used. The couplant facilitates thetransmission of acoustic waves from thetest object to the transducer. Thetransducer may also be attached with anadhesive bond designed to act as anacoustic couplant.

An acoustic emission transducernormally consists of several parts. Theactive element is a piezoelectric ceramicwith electrodes on each face. Oneelectrode is connected to electrical groundand the other is connected to a signallead. A wear plate protects the activeelement.

Behind the active element is usually abacking material, often made by curingepoxy containing high density tungstenparticles. The backing is usually designedso that acoustic waves easily propagateinto it with little reflection back to theactive element. In the backing, much ofthe waves energy is attenuated byscattering and absorption. The backingalso serves to load the active element sothat it is less resonant or more broad band(note that in some applications, aresonant transducer is desirable). Lessresonance helps the transducer respondmore evenly over a somewhat wider rangeof frequencies.

The typical acoustic emissiontransducer also has a case with aconnector for signal cable attachment.The case provides an integratedmechanical package for the transducercomponents and may also serve as ashield to minimize electromagneticinterference.

There are many variations of thistypical transducer design, including(1) designs for high temperatureapplications, (2) transducers with built-inpreamplifiers or line drive transformers,(3) transducers with more than one activeelement and (4) transducers with activeelements whose geometry or pola

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