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NDT Field Guides

Phased Array TestingBasic Theory for

Industrial Applications


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PhasedArrayTesting:BasicTheoryforIndustrialApplications

Seriescoordinator:MeindertAnderson

Technicalreviewersandadvisers:DanielKass,MichaelMolesPhD,TomNelligan

Layout,graphics,editing,proofreading,andindexing:Olympus NDTsTechnicalCommunicationsService

Publishedby:Olympus NDT,48WoerdAvenue,Waltham,MA 02453,USA

Marketinganddistribution:OlympusNDT

The information contained in this document is subject to change or revisionwithoutnotice.

OlympusNDTpartnumber:DMTA2000301EN,rev. B

2010,2012OlympusNDT,Inc.Allrightsreserved.Nopartofthispublicationmay be reproduced, translated or distributed without the express written

permissionofOlympusNDT,Inc.

PrintedinCanada

Secondedition,February2012

Notice

To thebest of our knowledge, the information in this publication is accurate;however, the Publisher does not assume any responsibility or liability for theaccuracy or completeness of, or consequences arising from, such information.

Thisdocumentisintendedforinformationalpurposesonly.Finaldeterminationofthesuitabilityofanyinformationorproductforusecontemplatedbyanyuser,and the manner of that use, is the sole responsibility of the user. The Publisherrecommendsthatanyoneintendingtorelyonanyrecommendationofmaterialsorproceduresmentionedinthispublicationshouldsatisfyhimselforherselfastosuch suitability, and that he or she can meet all applicable safety and healthstandards.

Allbrands are trademarks or registered trademarks of their respective ownersandthirdpartyentities.


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Olympus TableofContents iii

Table of Contents

Preface .......................................................................................... 1AboutThisGuide .................................................................. 1

AboutOlympus ..................................................................... 2ANoteonTerminology ........................................................ 3

1. Introduction ....................................................................... 51.1 GeneralIntroductiontoPhasedArrayTesting ................. 51.2 WhatIsaPhasedArraySystem?......................................... 71.3 HowDoesUltrasonicPhasingWork?................................. 81.4 AdvantagesofPhasedArrayasComparedwith

ConventionalUT .................................................................. 10

2. PhasedArrayProbes ...................................................... 112.1 UltrasonicBeamCharacteristics ........................................ 112.2 FundamentalPropertiesofSoundWaves ........................ 142.3 PhasedArrayProbeCharacteristics.................................. 212.4 PhasedArrayWedges ......................................................... 242.5 PhasedPulsing ..................................................................... 252.6 BeamShapingandSteering................................................ 272.7 BeamFocusingwithPhasedArrayProbes ...................... 312.8 GratingLobesandSideLobes ........................................... 332.9 PhasedArrayProbeSelectionSummary ......................... 34

3. BasicsofPhasedArrayImaging................................... 373.1 AScanData .......................................................................... 383.2 SingleValueBScans............................................................ 393.3 CrosssectionalBScans....................................................... 403.4 LinearScans .......................................................................... 423.5 CScans .................................................................................. 433.6 SScans ................................................................................... 463.7 CombinedImageFormats .................................................. 483.8 ScanRateandDataAcquisition ........................................ 48

4. PhasedArrayInstrumentation ..................................... 514.1 ImportantSpecifications ..................................................... 51

4.2 CalibrationandNormalizationMethods ......................... 59

5. PhasedArrayTestSetupandDisplayFormat...........63


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iv TableofContents Olympus

5.1 InstrumentSetupConsiderations ..................................... 635.2 NormalBeamLinearScans................................................ 665.3 AngleBeamLinearScans................................................... 695.4 SScanDisplayExamples ................................................... 725.5 InterpretingReflectorPositioning .................................... 76

Appendix A:ConstantsandUsefulFormulaTables.........81

Appendix B:UnitConversion ............................................... 87

Appendix C:SupportandTraining ...................................... 89

Appendix D:TypesofEquipmentAvailable...................... 91D.1 EPOCH 1000SeriesAdvancedUltrasonicFlaw

DetectorswithPhasedArrayImaging............................. 92D.2 OmniScanSeriesModularAdvancedFlawDetectors

withUT,PA,EC,andECATechnologies ........................ 93D.3 TomoScanFOCUSLTPowerful,Flexible,andCompact

UTDataAcquisitionSystem ............................................. 94D.4 TomoViewUTDataAcquisitionandAnalysis

Software ................................................................................ 95

PhasedArrayGlossary............................................................ 97

SelectedReferences ............................................................... 103

Index ......................................................................................... 105


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Olympus Preface 1

Preface

As industrial ultrasonic testing moved into the twentyfirst century,probably the most important development in the field was theincreasing availability and acceptance of portable phased arrayimaging instruments. Phased array testing is grounded in the same

basicwavephysicsthatsupportstheconventionalflawdetectorsandthicknessgagesthathavebeenincommercialuseformorethanfiftyyears. However, the increased capability offered by phased arraytesting frequently requires a higher level of skill and understandingon the part of inspectors using phased array testing. Thus, theintroduction of new phased array instruments calls for thedevelopmentofnewtrainingresourcesaswell.

Olympus is proud to introduce this new PhasedArray Testing fieldguide as a convenient resource for customers and anyone elseinterestedinphasedarraytechnology.Itisdesignedtobeaneasytofollow introduction to ultrasonic phased array testing, both fornewcomersandformoreexperienceduserswhowishtoreviewbasicprinciples.Thisguidebeginsbyexplainingwhatphasedarraytestingis and how it works, then outlines some considerations for selectingprobes and instruments, and concludes with further referenceinformationandaPhasedArrayGlossary.

About This Guide

Thisguideisdividedintothefollowingsections:

Chapter1, Introduction. This chapter provides abrief history ofconventional ultrasonic and phased array testing. It also lists theadvantages of phased array testing as compared with conventionalultrasonics.

Chapter2, Phased Array Probes. This chapter describes howultrasonictransducersandphasedarrayprobesareconstructed,andexplainstheircharacteristics.Inaddition,thereaderlearnsaboutfocallawsequencing,beamshapingandsteering,andtransducerfocusing.

Chapter3,BasicsofPhasedArrayImaging.Thischapterexplainsthe various image formats available for presenting inspection datathrougheasytounderstandillustrationsfrombothconventionalandphasedarrayinstruments,including:Ascans,Bscans,Cscans,linear


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2 Preface Olympus

scansandsectorialscans.

Chapter4,PhasedArrayInstrumentation.Thischapterincludesabriefoverviewofcommerciallyavailableinstrumentcategories.Italsodescribesimportantspecificationsandfeaturestobeconsideredwhenselectingbothconventionalandphasedarrayinstrumentation.

Chapter5, Phased Array Test Setup and Display Format. Thischapterprovidesfurtherhelpwithinterpretingdisplaysandmakingmeasurements.

Appendixes. These sections include a variety of referenceinformation, including useful ultrasonic formulas, material velocityand acoustic impedance information, unit conversion, sources forfurther training and reference, and the types of equipment available

thatutilizethesetechnologies.

PhasedArrayGlossary.Thisfinalsectionpresentsaconvenientlistof definitions for terms used in conventional and phased arrayultrasonictesting.

Wehopethatthisguidewillbehelpfultoyouincarryingoutphasedarrayultrasonicinspections.Commentsandsuggestionsarewelcome,andmaybesentto:[email protected].

About Olympus

Olympus Corporation is an international company operating inindustrial, medical, and consumer markets, specializing in optics,electronics, and precision engineering. Olympus instrumentscontribute to the quality of products and add to the safety ofinfrastructureandfacilities.

Olympus is a worldleading manufacturer of innovativenondestructivetestingandmeasurementinstrumentsthatareusedinindustrial and research applications ranging from aerospace, powergeneration, petrochemical, civil infrastructure, and automotive toconsumer products. Leadingedge testing technologies includeultrasound, ultrasound phased array, eddy current, eddy currentarray, microscopy, optical metrology, and Xray fluorescence. Its

products include flaw detectors, thickness gages, industrial NDTsystems and scanners, videoscopes, borescopes, highspeed videocameras,microscopes,probes,andvariousaccessories.

Olympus NDT isbased in Waltham, Massachusetts, USA, and hassales and service centers in all principal industrial locationsworldwide. Visit www.olympusims.com for applications and salesassistance.


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Olympus Preface 3

A Note on Terminology

Because widespread use of phased array testing is relatively new inultrasonicNDT,someterminologyisstillevolving.Therearecasesinwhich specific industries, such as nuclear power, standards

organizations, such as ASME, and manufacturers of phased arrayequipment use different terms for the same activity. The maindifferencesincludethemanytermsusedforSscan,andtheuseofthetermlinearscan.ThePhasedArrayGlossarypresentedattheendofthisguidecanbereferencedforfurtherexplanation.Theterminologyusedinthisguideisintendedtobeconsistentwiththatincorporatedin Olympus NDT phased array instruments such as the OmniScanandEPOCH 1000.

Thetermlinearscanisusedtodescribethescanformatinwhichthe

activebeamapertureiselectronicallymovedacrossthelengthofalineararrayprobe,eitheratnormalincidenceorafixedangle.Thisformat is alternately known as an Escan in certain ASME andIIWdocuments.

Aprobethathasbeenprogrammedtogeneratealinearscanintheforward direction may also be mechanically moved along thelength of a weld orsimilartestpiece, generating anencoded linear

scan.ThisformatisknownasaonelinescanorCscan. The termSscan is used to describe the scan format in which the

beam angle is electronically swept through a selected range. Thisformat is also known as a sectorial, sector, azimuthal, orswept angle scan. Alternately, in some instruments the termSscan hasbeen applied to any stacked Ascan display, includinglinearscans.

TimeVaried Gain (TVG) is also known as TimeCorrected Gain

(TCG).

Inthisguide,wewilluseSscanforsweptanglescan,linearscanforswept aperture scan, andC

scan orone

line

scan for any encoded

scan.

Activity Nuclear ASME

Mechanicalscanalongweld(encoded)

Onelinescan Linearscan

Electronicscanatfixedangle

Linearscan Escan

Sscan Sectorscan,sectorialscan,orSscan

Sscanalsosectorialscan,sectorscan,orsweptanglescan

Cscan Onelinescan,ormultiplelinescans

Cscan


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Olympus Introduction 5

1. Introduction

1.1 General Introduction to Phased Array

TestingManypeoplearefamiliarwiththemedicalapplicationsofultrasonicimaging, in which highfrequency sound waves are used to createhighly detailed crosssectional pictures of internal organs. Medicalsonograms are commonly made with specialized multielement

probes1 known as phased arrays and their accompanying hardwareand software. But the applications of ultrasonic phased array

technology are not limited to medical diagnosis. In recent years,phased array systems have been increasing in use in industrialsettings to provide new levels of information and visualization incommon ultrasonic tests that include weld inspection,bond testing,thicknessprofiling,andinservicecrackdetection.

During their first couple of decades, commercial ultrasonicinstruments relied entirely on single element transducers that usedone piezoelectric crystal to generate and receive sound waves, dualelement transducers that had separate transmitting and receivingcrystals, and pitchandcatch or throughtransmission systems thatused a pair of single element transducers in tandem. Theseapproaches are still used by the majority of current commercialultrasonic instruments designed for industrial flaw detection andthickness gaging; however, instruments using phased arrays aresteadilybecoming more important in the ultrasonic nondestructivetesting(NDT)field.

TheprincipleofconstructiveanddestructiveinteractionofwaveswasdemonstratedbyEnglishscientistThomasYoungin1801inanotableexperiment that utilized two point sources of light to createinterference patterns. Waves that combine in phase reinforce eachother, while waves that combine outofphase cancel each other (seeFigure11).

1. Asaglobalcompany,Olympus NDThaschosentousetheISOtermsforequipment;forexample,anarrayisspecificallycalledaprobeinthisguide,notatransducer.


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6 Chapter1 Olympus

Figure 1-1 Two-point source interference pattern

Phase shifting, or phasing, is in turn a way of controlling theseinteractionsby timeshifting wavefronts that originate from two ormore sources. It canbe used tobend, steer, or focus the energy of awavefront. In the 1960s, researchers began developing ultrasonicphasedarraysystemsthatutilizedmultiplepointsourcetransducersthat were pulsed so as to direct soundbeamsby means of these

controlled interference patterns. In the early 1970s, commercialphasedarraysystemsformedicaldiagnosticuse,firstappearedusingsteeredbeamstocreatecrosssectionalimagesofthehumanbody(seeFigure12).

Figure 1-2 Phased arrays used for medical diagnoses

Initially, the use of ultrasonic phased array systems was largelyconfined to the medical field, aidedby the fact that the predictablecomposition and structure of the human body make instrument

designandimageinterpretationrelativelystraightforward.Industrialapplications, on the other hand, represent a much greater challengebecause of the widely varying acoustic properties of metals,composites,ceramics,plastics,andfiberglass,aswellastheenormous

=Maximum pressure

=Minimum pressure


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varietyofthicknessesandgeometriesencounteredacrossthescopeofindustrial testing. The first industrial phased array systems,introduced in the 1980s, were extremely large, and required datatransfer to a computer in order to do the processing and imagepresentation. These systems were most typically used for inservicepower generation inspections. In large part, this technology was

pushed heavily in the nuclear market, where critical assessmentgreatly allows the use of cutting edge technology for improvingprobability of detection. Other early applications involved largeforgedshaftsandlowpressureturbinecomponents.

Portable,batterypoweredphasedarrayinstrumentsforindustrialuseappearedintheearly2000s.Analogdesignshadrequiredpowerandspace to create the multichannel configurations necessary forbeamsteering.However,thetransitionintothedigitalworldandtherapiddevelopment of inexpensive embedded microprocessors enabledmore rapid development of the next generation phased arrayequipment. In addition, the availability of lowpower electroniccomponents, better powersaving architectures, and industrywideuse of surfacemountboard designs led to miniaturization of thisadvanced technology. This resulted in phased array tools, whichallowed electronic setup, data processing, display, and analysis allwithin a portable device, and so the doors were opened to more

widespread use across the industrial sector. This in turn gave theability to specify standard phased array probes for commonapplications.

1.2 What Is a Phased Array System?

Conventional ultrasonic transducers for NDT commonly consist of

either a single active element thatboth generates and receives highfrequencysoundwaves,ortwopairedelements,onefortransmittingand one for receiving. Phased array probes, on the other hand,typicallyconsistofatransducerassemblywith16toasmanyas256small individual elements that can eachbe pulsed separately (seeFigure 13 and Figure 14). These canbe arranged in a strip (lineararray), 2D matrix, a ring (annular array), a circular matrix (circulararray), or a more complex shape. As is the case with conventionaltransducers, phased array probes canbe designed for direct contactuse, as part of an angle beam assembly with a wedge, or forimmersionusewithsoundcouplingthroughawaterpath.Transducerfrequencies are most commonly in the 2 MHz to 10 MHz range. Aphased array system also includes a sophisticated computerbasedinstrument that is capable of driving the multielement probe,receiving anddigitizing the returningechoes,andplotting thatechoinformation in various standard formats. Unlike conventional flawdetectors, phased array systems can sweep a soundbeam through a

rangeofrefractedanglesoralongalinearpath,ordynamicallyfocusat a number of different depths, thus increasingboth flexibility andcapabilityininspectionsetups.


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8 Chapter1 Olympus

Figure 1-3 Typical phased array probe assemblies

Figure 1-4 Typical multielement construction

1.3 How Does Ultrasonic Phasing Work?

In the mostbasic sense, a phased array system utilizes the wavephysics principle of phasing. It varies the timebetween a series ofoutgoing ultrasonic pulses in such a way that the individualwavefrontsgeneratedbyeachelementinthearraycombinewitheachother. This action adds or cancels energy in predictable ways thateffectivelysteerandshapethesoundbeam.Thisisaccomplishedbypulsingtheindividualprobeelementsatslightlydifferenttimes.

Frequently, the elements are pulsed in groups of 4 to 32 in order toimprove effective sensitivity by increasing aperture, which thenreduces unwanted beam spreading and enables sharper focusing.Softwareknownasafocallawcalculatorestablishesspecificdelaytimesfor firing each group of elements in order to generate the desired

beam shape, taking into account probe and wedge characteristics as

wellasthegeometryandacousticalpropertiesofthetestmaterial.Theprogrammedpulsingsequenceselectedbytheinstrumentsoperatingsoftwarethenlaunchesanumberofindividualwavefrontsinthetestmaterial. These wavefronts, in turn, combine constructively anddestructivelyintoasingleprimarywavefrontthattravelsthroughthetest material and reflects off cracks, discontinuities,back walls, andother materialboundaries like a conventional ultrasonic wave. The

beam can be dynamically steered through various angles, focaldistances, and focal spot sizes in such a way that a single probeassembly is capable of examining the test material across a range ofdifferent perspectives. Thisbeam steering happens very quickly sothatascanfrommultipleanglesorwithmultiplefocaldepthscanbe

Piezocomposite

Individual elements


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performedinafractionofasecond.

Thereturningechoesarereceivedbythevariouselementsorgroupsofelementsandtimeshiftedasnecessarytocompensate forvaryingwedge delays, and then summed. Unlike a conventional singleelement transducer, which effectively merges the effects of allbeam

componentsthatstrikeitsarea,aphasedarrayprobecanspatiallysortthereturningwavefrontaccordingtothearrivaltimeandamplitudeat each element. When processed by instrument software, eachreturnedfocallawrepresentsthereflectionfromaparticularangularcomponentofthebeam,aparticularpointalongalinearpath,and/ora reflection from a particular focal depth (see Figure 15 and Figure16). The echo information can thenbe displayed in any of severalformats.

Figure 1-5 Example of an angle beam generated by a flat probe by means of thevariable delay

Figure 1-6 Example of focused angle beam linear scan

Angle steering

Incident wavefront

PA probe

Delay (ns)

Scanning direction

Active group

128116


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10 Chapter1 Olympus

1.4 Advantages of Phased Array asCompared with Conventional UT

Ultrasonic phased array systems can potentially be employed in

almost any test where conventional ultrasonic flaw detectors havetraditionallybeen used. Weld inspection and crack detection are themost important applications, and these tests are done across a widerange of industries including aerospace, power generation,petrochemical, metal billet and tubular goods suppliers, pipelineconstruction and maintenance, structural metals, and generalmanufacturing. Phased arrays can alsobe effectively used to profileremainingwallthicknessincorrosionsurveyapplications.

Thebenefitsofphasedarraytechnologyover conventionalUTcomefrom its ability to use multiple elements to steer, focus, and scan

beams with a single probe assembly. Beam steering, commonlyreferredtoasSscanning(sectorialscanning),canbeusedformappingcomponents at appropriate angles. This can greatly simplify theinspectionofcomponentswithcomplexgeometry.Thesmallfootprintof the probe and the ability to sweep thebeam without moving theprobealsoaidstheinspectionofsuchcomponentsinsituationswhere

there is limited access for mechanical scanning. Sectorial scanning isalsotypicallyusedforweldinspection.Theabilitytotestweldswithmultiple anglesfrom asingle probe greatlyincreases the probabilityof detection of anomalies. Electronic focusing optimizes thebeamshape and size at the expected defect location, as well as furtheroptimizing probability of detection. The ability to focus at multipledepths also improves the ability for sizing critical defects forvolumetricinspections.Focusingcansignificantlyimprovesignaltonoiseratioinchallengingapplications,andelectronicscanningacross

manygroupsofelementsallowsrapidproductionofCscanimages.The ability to simultaneously test across multiple angles and/or toscanalargerareaofthetestpiecethroughLinearscanningincreasesinspection speed. Phased array inspection speeds canbe as much as10 times faster as compared to conventional UT thus providing amajoradvantage.

Thepotentialdisadvantagesofphasedarraysystemsareasomewhathigher cost and a requirement for operator training. However, thesecostsarefrequentlyoffsetbytheirgreaterflexibilityandareductioninthetimeneededtoperformagiveninspection.


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Olympus PhasedArrayProbes 11

2. Phased Array Probes

2.1 Ultrasonic Beam Characteristics

Conventional longitudinalwave ultrasonic transducers work as apiston source of highfrequency mechanical vibrations, or soundwaves. As voltage is applied, the piezoelectric transducer element(often called a crystal) deforms by compressing in the directionperpendiculartoitsface.Whenthevoltageisremoved,typicallylessthan a microsecond later, the element springsback, generating thepulse of mechanical energy that comprises an ultrasonic wave (seeFigure21).Similarly,iftheelementiscompressedbythepressureofan arriving ultrasonic wave, it generates a voltage across its faces.Thusasinglepiezoelectricelementcanactasbothatransmitterandreceiverofultrasonicpulses.

Figure 2-1 Principle of the piezoelectric transducer element

AlltransducersofthekindmostcommonlyusedforultrasonicNDT

++++++++++++++++++++++++++

+

+

+

Rest state

Voltage applied

Voltage removed

Return to rest state


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havethefollowingfundamentalfunctionalproperties:

Type. The transducer is identified according to function as a contact,delay line, angle beam, or immersion type. Inspected materialcharacteristics (such as surface roughness, temperature, accessibilityas well as the position of a defect within the material, and the

inspectionspeed)allinfluencetheselectionoftransducertype.

Size. The diameter or length and width of the active transducerelement,whichisnormallyhousedinasomewhatlargercase.

Frequency. The number of wave cycles completed in one second,normally expressed in kilohertz (kHz) or megahertz (MHz). Mostindustrial ultrasonic testing is done in the 500 kHz to 20 MHzfrequencyrange,somosttransducersfallwithinthatrange,although

commercial transducers are available frombelow 50 kHz to greaterthan 200 MHz. Penetration increases with a lower frequency, whileresolutionandfocalsharpnessincreasewithahigherfrequency.

Bandwidth. The portion of the frequency response that falls withinspecified amplitude limits. In this context, it shouldbe noted thattypicalNDTtransducersdonotgeneratesoundwavesatasinglepurefrequency, but rather over a range of frequencies centered at thenominal frequency designation. The industry standard is to specifythisbandwidthatthe6 dB(orhalfamplitude)point.

Waveform duration. The number of wave cycles generated by thetransducereachtimeitispulsed.Anarrowbandwidthtransducerhasmorecyclesthanabroaderbandwidthtransducer.Elementdiameter,

backingmaterial,electricaltuning,andtransducerexcitationmethodallimpactwaveformduration.

Sensitivity. The relationshipbetween the amplitude of the excitationpulseandthatoftheechoreceivedfromadesignatedtarget.

Beamprofile. As a working approximation, thebeam from a typicalunfocuseddisktransducerisoftenthoughtofasacolumnofenergyoriginatingfromtheactiveelementareathatexpandsindiameterandeventuallydissipates(seeFigure22).

Figure 2-2 Beam profile

Infact,theactualbeamprofileiscomplex,withpressuregradientsinboth the transverse and axial directions. In the beam profileillustrationbelow(Figure23),redrepresentsareasofhighestenergy,whilegreenandbluerepresentlowerenergy.


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Figure 2-3 Areas of energy in the beam profile

The sound field of a transducer is divided into two zones: the nearfieldandthefarfield(seeFigure24).Thenearfieldistheregioncloseto the transducerwhere thesound pressure goes through aseries ofmaximumsandminimums,anditendsatthelastonaxismaximumat distance N from the face. Near field distance N represents thenaturalfocusofthetransducer.

Figure 2-4 The sound field of a transducer

The far field is the region beyond N where the sound pressuregraduallydropstozeroasthebeamdiameterexpandsanditsenergydissipates. The near field distance is a function of the transducersfrequency and element size, and the sound velocity in the testmedium, and it can be calculated for the square or rectangularelementscommonlyfoundinphasedarraytestingasfollows:

or

where:

N =nearfieldlength

k =aspectratioconstant(seebelow)

L =lengthofelementoraperture

f =frequency

c =soundvelocityintestmaterial

=wavelength=

N kL2f

4c-----------= N kL

2

4---------=

c

f--


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The aspect ratio constant is as shown in Table 21. It isbased on theratio between the short and long dimensions of the element oraperture.

Inthecaseofcircularelements,kisnotusedandthediameteroftheelement(D)isusedinsteadofthelengthterm:

or

Becauseofthesoundpressurevariationswithinthenearfield,itcanbe difficult to accurately evaluate flaws using amplitude basedtechniques (although thickness gaging within the near field is not aproblem).Additionally,Nrepresentsthegreatestdistanceatwhicha

transducerbeamcanbefocusedbymeansofeitheranacousticlensorphasing techniques. Focusing is discussed further in section2.7, onpage31.

2.2 Fundamental Properties of SoundWaves

Wavefrontformation.Whileasingleelementtransducercanbethought

of as a piston source, a single disk, or plate pushing forward on thetestmedium,thewaveitgeneratescanbemathematicallymodeledasthesumofthewavesfromaverylargenumberofpointsources.ThisderivesfromHuygensprinciple,firstproposedbyseventeenthcenturyDutchphysicistChristiaanHuygens,whichstatesthateachpointonan advancing wavefront maybe thought of as a point source thatlaunches a new spherical wave, and that the resulting unified

wavefrontisthesumofalloftheseindividualsphericalwaves.

Beam spreading. In principle, the sound wave generated by atransducer travels in a straight line until it encounters a material

Table 2-1 Aspect ratio constant

Ratioshort/long k1.0 1.37(squareelement)

0.9 1.25

0.8 1.15

0.7 1.09

0.6 1.04

0.5 1.01

0.4 1.00

0.3andbelow 0.99

N D2f

4c---------= N

D2

4-------=


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boundary. What happens then is discussedbelow. But if the soundpath length is longer than the nearfield distance, the beam alsoincreases in diameter, diverging like thebeam of a spotlight (seeFigure25).

Figure 2-5 Beam spread

Thebeam spread angle of an unfocused circular transducerbeyondthenearfieldcanbecalculatedasfollows:

D =elementdiameteroraperture

f =frequencyc =soundvelocityintestmedium

=wavelength=

6 dBhalfbeamspreadangle()ofanunfocusedtransducer:

From this equation it is seen thatbeam spread angle increases withlowerfrequenciesandsmallerdiameters.Alargebeamspreadanglecancausesound energyperunitareatoquickly dropwithdistance.This effectively decreases sensitivity to small reflectors in someapplicationsinvolvinglongsoundpaths.Insuchcases,echoresponsecanbe improvedby using higher frequency and/or larger diametertransducers.

In the case of rectangular elements, the beam spreading isasymmetrical, with a largerbeam spread angle across the smallerdimension of thebeam. The angle for each axis canbe calculatedusingtheformulagivenbelow,usingtheappropriatelengthorwidthforterm L:

or

0 N 2N 3N 4N

N

BEAM AXISD

Near field length D2f

4c---------

D2

4-------= =

c

f--

sin 1 0.514c

fD

----------------

=

sin 1 0.44c

fL

-------------

= sin 1 0.44

L

--------------

=


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16 Chapter2 Olympus

The following graphics show some generalized changes in beamspreadingwithchangesintransducerdiameterandfrequency.Ifthefrequency is constant, thenbeam spreading decreases as transducerdiameterincreases(seeFigure26andFigure27).

Figure 2-6 Beam spreading with a 3mm element

Figure 2-7 Beam spreading with a 13mm element

Ifthetransducerdiameterisconstant,thenbeamspreadingdecreasesasfrequencyincreases(seeFigure28andFigure29).

Figure 2-8 Beam spreading with a 2.25 MHz element

Velocity: 5850m/s (0.230 in./s) Diameter: 3mm (0.125 in.)Frequency: 5.0 MHz


Velocity: 5850m/s (0.230 in./s) Diameter: 13mm (0.5 in.)Frequency: 2.25MHz


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Figure 2-9 Beam spreading with a 10MHz element

Attenuation.Asittravelsthroughamedium,theorganizedwavefrontgeneratedbyanultrasonictransducerbeginstobreakdownduetoanimperfect transmission of energy through the microstructure of anymaterial. Organized mechanical vibrations (sound waves) turn intorandommechanicalvibrations(heat)untilthewavefrontisnolongerdetectable.Thisprocessisknownassound

attenuation.

The mathematical theory of attenuation and scattering is complex.Thelossofamplitudeduetoattenuationacrossagivensoundpathisthe sum of absorption effects and scattering effects. Absorptionincreases linearly with frequency, while scattering varies throughthree zones depending on the ratio of wavelength to grain size

boundariesorotherscatterers.Inallcases,scatteringeffectsincrease

withfrequency.Foragivenmaterialatagiventemperature,testedatagivenfrequency,thereisaspecificattenuationcoefficient,commonlyexpressed in Nepers per centimeter (Np/cm). Once this attenuationcoefficient is known, losses across a given sound path can becalculatedaccordingtotheequation:

where:p =soundpressureatendofpath

p0 =soundpressureatbeginningofpath

e =baseofnaturallogarithm

a =attenuationcoefficient

d =soundpathlength

As a practical matter, in ultrasonic NDT applications, attenuationcoefficients are normally measured rather than calculated. Higherfrequenciesareattenuatedmorerapidlythanlowerfrequenciesinanymedium, so low test frequencies are usually employed in materialswith high attenuation coefficients such as lowdensity plastics andrubber.

Reflection and transmission at aperpendicularplane boundary. When a

soundwavetravelingthroughamediumencountersaboundarywitha dissimilar medium that lies perpendicular to the direction of thewave, a portion of the wave energy is reflected straightback and a


p p0e ad=


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18 Chapter2 Olympus

portioncontinuesstraightahead.Thepercentageofreflectionversustransmissionisrelatedtotherelativeacousticimpedancesofthetwomaterials,withacousticimpedanceinturnbeingdefinedasmaterialdensity multipliedby speed of sound. The reflection coefficient at aplanarboundary (the percentage of sound energy that is reflected

backtothesource)canbecalculatedasfollows:

where:

R =reflectioncoefficientinpercent

Z1 =acousticimpedanceoffirstmedium

Z2 =acousticimpedanceofsecondmedium

From thisequation itcanbe seen thatas the acousticimpedances ofthe two materials become more similar, the reflection coefficientdecreases, and as the acoustic impedancesbecome less similar, thereflection coefficient increases. In theory the reflection from the

boundarybetween two materials of the same acoustic impedance is

zero, while in the case of materials with very dissimilar acousticimpedances, as in aboundarybetween steel and air, the reflectioncoefficientapproaches100 %.

Refractionandmodeconversionatnonperpendicularboundaries.When asoundwavetravelingthroughamaterialencountersaboundarywithadifferentmaterialatanangleother thanzerodegrees,aportion ofthewaveenergyisreflectedforwardatanangleequaltotheangleofincidence. At the same time, the portion of the wave energy that istransmitted into the second material is refracted in accordance withSnells Law, which was independently derived by at least twoseventeenthcentury mathematicians. Snells law relates the sines oftheincidentandrefractedangletothewavevelocityineachmaterialasdiagramedbelow.

RZ2 Z1

Z2 Z1+-------------------=


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Figure 2-10 Sound wave refraction and mode conversion

where:

i =incidentangleofthewedge

rl =angleoftherefractedlongitudinalwave

rs =angleoftherefractedshearwave

ci =velocityoftheincidentmaterial(longitudinal)

crl =materialsoundvelocity(longitudinal)

crs

=velocityofthetestmaterial(shear)

Figure 2-11 Relative amplitude of wave modes

Ifsoundvelocityinthesecondmediumishigherthanthatinthefirst,

isin

ci

------------rlsin

crl

--------------rssin

crs

---------------= =

Surface

Shear

S SL

Longitudinal

1st Criticalangle

2nd Criticalangle

Incident angle

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

R


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then above certain angles this bending is accompanied by modeconversion, most commonly from a longitudinal wave mode to ashear wave mode. This is the basis of widely used angle beaminspection techniques. As the incident angle in the first (slower)medium (such as a wedge or water) increases, the angle of therefracted longitudinal wave in the second (faster) material such as

metalincreases.Astherefractedlongitudinalwaveangleapproaches90 degrees, a progressively greater portion of the wave energy isconvertedtoalowervelocityshearwavethatisrefractedattheanglepredictedby Snells Law. At incident angles higher than that whichwould create a 90 degree refracted longitudinal wave, the refractedwave exists entirely in shear mode. A still higher incident angleresultsinasituationwheretheshearwaveistheoreticallyrefractedat90 degrees,atwhichpointasurfacewaveisgeneratedinthesecond

material. The diagrams in Figure 212, Figure 213, and Figure 214showthiseffectforatypicalanglebeamassemblycoupledintosteel.

Figure 2-12 Incident angle: 10. Strong longitudinal wave and weak shear wave.

Figure 2-13 Incident angle: 30. Beyond the first critical angle, the longitudinal waveno longer exists, and all refracted energy is contained in the shear wave.


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Figure 2-14 Incident angle: 65. Beyond the second critical angle, the shear wave nolonger exists, and all refracted energy is contained in a surface wave.

2.3 Phased Array Probe Characteristics

Figure 2-15 Phased array probes

Anarrayisanorganizedarrangementoflargequantitiesofanobject.ThesimplestformofanultrasonicarrayforNDTwouldbeaseriesofseveral single element transducers arranged in such a way as toincrease inspection coverage and/or the speed of a particularinspection.Examplesofthisinclude:

Tube inspection, where multiple probes are often used for crackdetection, finding laminar flaws, and overall thicknessmeasurement.

Forgedmetalpartsinspection,whichoftenrequiremultipleprobesfocusedatdifferentdepthstoenablethedetectionofsmalldefectsinazonalmanner.

Composite and metal inspection, where a linear arrangement ofprobesalongasurfaceisrequiredtoincreasedetectionoflaminarflawsincompositesorcorrosioninmetals.

These inspections require highspeed, multichannel ultrasonicequipment with proper pulsers, receivers, and gate logic to processeach channel as well as careful fixturing of each transducer toproperlysetuptheinspectionzones.


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Initssimplestform,onecanthinkofaphasedarrayprobeasaseriesof individual elements in one package (see Figure 216). While theelements in reality are much smaller than conventional transducers,theseelementscanbepulsedasagroupsoastogeneratedirectionallycontrollable wavefronts. This electronic beam forming allowsmultiple inspection zones tobe programmed and analyzed at very

high speeds without probe movement. This is discussed in greaterdetailinlaterpages.

Figure 2-16 Phased array probe

While phased array probes come in a wide range of sizes, shapes,frequencies,andnumberofelements,whattheyallhaveincommonisa piezoelectric element that has been divided into a number ofsegments.

Contemporary phased array probes for industrial NDT applicationsaretypicallyconstructedaroundpiezocompositematerials,whicharemadeupofmanytiny,thinrodsofpiezoelectricceramicembeddedina polymer matrix. While they can be more challenging tomanufacture, composite probes typically offer a 10 dB to 30 dBsensitivity advantage over piezoceramic probes of otherwise similardesign.Segmentedmetalplatingisusedtodividethecompositestripinto a number of electrically separate elements that canbe pulsedindividually. This segmented element is then incorporated into aprobeassemblythat includesaprotectivematching layer,abacking,cableconnections,andahousing(seeFigure217).

Figure 2-17 Phased array probe cross-section

Phased array probes are functionally categorized according to the

Multiconductor

coaxial cable

Inner

sleeve

Matching

layer

External

housing

Backing

Piezocomposite

element

Metallic

plating


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followingbasicparameters:

Type.Mostphasedarrayprobesareoftheanglebeamtype,designedfor use with either a plastic wedge or a straight plastic shoe (zerodegreewedge),ordelayline.Directcontactandimmersionprobesarealsoavailable.

Frequency.Mostultrasonicflawdetectionisdonebetween2 MHzand10 MHz, so most phased array probes fall within that range. Lowerandhigherfrequencyprobesarealsoavailable.Aswithconventionaltransducers, penetration increases with lower frequency, whileresolutionandfocalsharpnessincreasewithhigherfrequency.

Numberofelements. Phased array probes most commonly have 16 to128elements,withsomehavingasmanyas256.Alargernumberof

elements increases focusing and steering capability, which alsoincreases area coverage,butboth probe and instrumentation costsincrease as well. Each of these elements is individually pulsed tocreate the wavefront of interest. Hence the dimension across theseelementsisoftenreferredtoastheactiveorsteeringdirection.

Size of elements. As the element width gets smaller,beam steeringcapabilityincreases,butlargeareacoveragerequiresmoreelementsatahighercost.

Thedimensionalparametersofaphasedarrayprobearecustomarilydefinedasfollows:

Figure 2-18 Dimensional parameters of a phased array probe

A = totalapertureinsteeringofactivedirection

H = elementheightorelevation.Sincethisdimensionisfixed,itisoftenreferredtoasthepassiveplane.

p = pitch,orcentertocenterdistancebetweentwosuccessiveelements

e = widthofanindividualelement

g = spacingbetweenactiveelements

This information is used by instrument software to generate thedesired beam shape. If it is not entered automatically by proberecognitionsoftware,thenitmustbeenteredbytheuserduringsetup.


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2.4 Phased Array Wedges

Phased array probe assemblies usually include a plastic wedge.Wedges are used in both shear wave and longitudinal waveapplications, including straight beam linear scans. These wedges

performbasically the same function in phased array systems as inconventional single element flaw detection, coupling sound energyfrom the probe to thetest piece in such a waythat it mode convertsand/orrefractsatadesiredangleinaccordancewithSnellslaw.Whilephased array systems do utilizebeam steering to createbeams atmultipleanglesfromasinglewedge,thisrefractioneffectisalsopartofthebeamgenerationprocess.Shearwavewedgeslookverysimilarto those used with conventional transducers, and like conventional

wedgestheycomeinmanysizesandstyles.Someofthemincorporatecouplant feed holes for scanning applications. Some typical phasedarrayprobewedgesareseeninFigure219.

Figure 2-19 Phased array probe wedges

Zerodegreewedgesarebasicallyflatplasticblocksthatareusedforcoupling sound energy and for protecting the probe face fromscratches or abrasion in straight linear scans and lowanglelongitudinalwaveangledscans(seeFigure220).

Figure 2-20 A zero-degree wedge


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2.5 Phased Pulsing

Wheneverwavesoriginatingfromtwoormoresourcesinteractwitheachother,therearephasingeffectsleadingtoanincreaseordecreaseinwaveenergyatthepointofcombination.Whenelasticwavesofthe

same frequency meet in such a way that their displacements areprecisely synchronized (in phase, or zerodegree phase angle), thewave energies add together to create a larger amplitude wave (seeFigure221a).Iftheymeetinsuchawaythattheirdisplacementsareexactly opposite (180 degrees out of phase), then the wave energiescancel each other (see Figure 221c). At phase angles between0 degrees and 180 degrees, there is a range of intermediate stages

between full addition and full cancellation (see Figure 221b). By

varyingthetimingofthewavesfromalargenumberofsources,itispossible touse these effects toboth steerand focus the resultingcombined wavefront. This is an essential principlebehind phasedarraytesting.

Waves in phase Reinforcement

Intermediate condition

Waves out of phase Cancellation

Figure 2-21 Phasing effects: combination and cancellation

a

b

c


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Inconventionaltransducers,constructiveanddestructiveinterferenceeffects create the nearfield and farfield zones and the variouspressure gradients therein. Additionally, a conventional anglebeamtransducerusesasingleelementtolaunchawaveinawedge.Pointsonthiswavefrontexperiencedifferentdelayintervalsduetotheshapeof the wedge. These are mechanical delays, as opposed to the

electronic delays employed in phased array testing. When thewavefront hits the bottom surface it can be visualized throughHuygens principle as a series of point sources. The theoreticallyspherical waves from each of these points interact to form a singlewaveatanangledeterminedbySnellslaw.

In phased array testing, the predictable reinforcement andcancellationeffectscausedbyphasingareusedtoshapeandsteertheultrasonicbeam. Pulsing individual elements or groups of elementswith different delays creates a series of point source waves thatcombine into a single wavefront that travels at a selected angle (seeFigure 222). This electronic effect is similar to the mechanical delaygeneratedby a conventional wedge,but it canbe further steeredbychangingthepatternofdelays.Throughconstructiveinterference,theamplitudeofthiscombinedwavecanbeconsiderablygreaterthantheamplitude of any one of the individual waves that produce it.Similarly, variable delays are applied to the echoes receivedby each

element of the array. The echoes are summed to represent a singleangular and/or focal component of the totalbeam. In addition toaltering the direction of the primary wavefront, this combination ofindividualbeamcomponentsallowsbeamfocusingatanypointinthenearfield.

Figure 2-22 Angled waveform

Elementsareusuallypulsedingroupsof4to32inordertoimproveeffectivesensitivitybyincreasingaperture,whichreducesunwanted

beamspreadingandenablessharperfocusing.

Thereturningechoesarereceivedbythevariouselementsorgroups

Resulting wavefront


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ofelementsandtimeshiftedasnecessarytocompensate forvaryingwedge delays and then summed. Unlike a conventional singleelement transducer, which effectively merges the effects of allbeamcomponentsthatstrikeitsarea,aphasedarrayprobecanspatiallysortthereturning wavefrontaccordingtothearrivaltimeandamplitudeat each element. When processed by instrument software, each

returnedfocallawrepresentsthereflectionfromaparticularangularcomponentofthebeam,aparticularpointalongalinearpath,and/ora reflection from a particular focal depth. The echo information canthenbedisplayedinanyofseveralstandardformats.

Asnotedpreviously,phasedarraybeamsaregeneratedbypulsingtheindividual probe elements or groups of elements in a particularpattern. Phased array instruments generate these patternsbased oninformationthathasbeenenteredbytheuser.

Softwareknownasafocallawcalculatorestablishesspecificdelaytimesfor firing each group of elements in order to generate the desired

beamshapethroughwaveinteraction,takingintoaccountprobeandwedge characteristics as well as the geometry and acousticalproperties of the test material. The programmed pulsing sequenceselected by the instruments operating software, then launches anumber of individual wavefronts in the test material. These

wavefronts in turn combine constructively and destructively into asingle primary wavefront that travels through the test material andreflects off cracks, discontinuities, back walls, and other material

boundariesas withanyconventionalultrasonicwave.Thebeamcanbe dynamically steered through various angles, focal distances, andfocalspotsizesinsuchawaythatasingleprobeassemblyiscapableofexaminingthetestmaterialacrossarangeofdifferentperspectives.Thisbeamsteeringhappensveryquickly,sothatascanfrommultipleanglesorwithmultiplefocaldepthscanbeperformedinafractionofasecond.

2.6 Beam Shaping and Steering

Theresponseofanyultrasonictestsystemdependsonacombinationof factors: the transducer used, the type of instrument used and itssettings,andtheacousticpropertiesofthetestmaterial.Theresponses

produced by phased array probes, like those from any otherultrasonictransducersforNDT,arerelatedbothtotransducerdesignparameters(suchasfrequency,size,andmechanicaldamping),andtotheparametersoftheexcitationpulsethatisusedtodrivetheprobe.

Four important probe parameters have a number of interrelatedeffectsonperformance.

Frequency. As noted in the previous section, the test frequency has a

significanteffectonnearfieldlengthandbeamspreading.Inpractice,higherfrequenciescanprovidebettersignaltonoiseratiothanlowerfrequencies,becausetheyofferpotentiallysharperfocusingandthus


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Figure 2-23 Focal law sequences

As previously explained, ultrasonicbeam characteristics are definedby many factors. In addition to element dimension, frequency, and

damping that govern conventional single element performance,phased array probebehavior is affectedby how smaller individualelements are positioned, sized, and grouped to create an effectiveapertureequivalenttoitsconventionalcounterpart.

ForphasedarrayprobesNelementsaregroupedtogethertoformtheeffective aperture for whichbeam spread canbe approximatedbyconventionaltransducermodels(seeFigure224).

Figure 2-24 Effective aperture

Conventionaltransducer

Single pulse

16-element arrayAll elements pulsing

16-element array4 elements pulsing


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Forphasedarrayprobes,themaximumsteeringangle(at6 dB)inagivencaseisderivedfromthebeamspreadequation.Itcanbeeasilyseenthatsmallelementshavemorebeamspreadingandhencehigherangularenergycontent,whichcanbecombinedtomaximizesteering.Aselementsizedecreases,moreelementsmustbepulsedtogethertomaintainsensitivity.

where:

sinst =sineofthemaximumsteeringangle

=wavelengthintestmaterial

e =elementwidth

Figure 2-25 Beam steering limits: When the element number is constant, 16 asshown, the maximum beam steering angle increases as the aperture size decreases.

Recalling that the practical limit for phased array probemanufacturing restricts the smallest individual element width to0.2 mm, the active aperture for a 16element probe with 0.2 mmelements wouldbe 3.2 mm. Creating an aperture of 6.4 mm would

require 32 elements. While these probes would no doubt maximizesteering, the small apertures would limit static coverage area,sensitivity,penetration,andfocusingability.

Thesteeringrangecanbefurthermodifiedbyusinganangledwedgeto change the incident angle of the soundbeam independently ofelectronicsteering.

Fromthebeamspreadangle,thebeamdiameteratanydistancefrom

the probe canbe calculated. In the case of a square or rectangularphasedarrayprobe,beamspreadinginthepassiveplaneissimilartothat of an unfocused transducer. In the steered or active plane, the

beam canbe electronically focused to converge acoustic energy at a

stsin 0.514e---=

64 mm aperture

32 mm aperture

16 mm aperture

9

18

36


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desireddepth.Withafocusedprobe,thebeamprofilecantypicallyberepresentedby a tapering cone (or wedge in the case of singleaxisfocusing)thatconvergestoafocalpointandthendivergesatanequalanglebeyondthefocalpoint,asdescribedasfollows:

The nearfield length and hence the natural divergence of an

ultrasonic beam are determined by aperture (equal to elementdiameter in the case of conventional monolithic transducers) andwavelength (wave velocity dividedby frequency). For an unfocusedcircular probe, the nearfield length,beam spread angle, andbeamdiametercanbecalculatedasfollows:

where:


f =frequency

c =soundvelocityintestmedium

=wavelength=

Fortheformulaforsquareorrectangularelements,seepages1314.

2.7 Beam Focusing with Phased ArrayProbes

Soundbeams canbe focused like light rays, creating an hourglass

shapedbeamthattaperstoaminimumdiameteratafocalpointandthenexpandsoncepastthatfocalpoint(seeFigure226).

Figure 2-26 Focused sound beam

Near-field length D2f

4c---------

D2

4-------= =

c

f--


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The depth at which thebeam from a phased array focuses canbevariedbychangingthepulsedelays.Thenearfieldlengthinagivenmaterialdefines the maximum depth atwhichasoundbeamcanbefocused.Abeamcannotbefocusedbeyondtheendofthenearfieldinthetestmaterial.

A focused probes effective sensitivity is affected by the beamdiameterat thepointofinterest.Thesmallerthebeamdiameter,thegreater is the amount of energy that is reflectedby a small flaw.Additionally,thesmallbeamdiameteratthefocuscanimprovelateralresolution. The 6 dBbeam diameter or width of a focused probe atthefocalpointcanbecalculatedasfollows:

where:

F =focallengthintestmedium

c =soundvelocityintestmedium


For rectangular elements, this is calculated separately for the active

andpassivedirections.

Fromtheseformulasitcanbeseenthatastheelementsizeand/orthefrequencyincrease,thebeamspreadangledecreases.Asmallerbeamspreadangleinturncanresultinhighereffectivesensitivityinthefarfieldzoneduetothebeamenergydissipatingmoreslowly.Withinitsnear field, a probe canbe focused to create abeam that convergesrather than diverges. Narrowing thebeam diameter or width to afocalpointincreasessoundenergyperunitareawithinthefocalzone

and thus increases sensitivity to small reflectors. Conventionaltransducers usually do this with a refractive acoustic lens, whilephasedarraysdoitelectronicallybymeansofphasedpulsingandtheresultingbeamshapingeffects.

In the case of the most commonly used linear phased arrays withrectangular elements, thebeam is focused in the steering directionand unfocused in the passive direction. Increasing the aperture sizeincreases the sharpness of the focusedbeam, as canbe seen in these

beam profiles (see Figure 227). Red areas correspond to the highestsoundpressure,andblueareastolowersoundpressure.

6 dB beam diameter or width 1.02 Fc

fD------------------=


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Figure 2-27 Beam focusing with different aperture sizes

2.8 Grating Lobes and Side Lobes

Another phenomenon associated with phased array probes is thegeneration of unwanted grating lobes and side lobes. These twocloselyrelatedphenomenaare causedbysoundenergy thatspreads

outfromtheprobeatanglesotherthantheprimarysoundpath.Sidelobesare not limited to phased array systemssidelobes also occurwithconventionaltransducersaselementsizeincreases.Gratinglobesonly occur in phased array probes as a result of ray componentsassociated with the regular, periodic spacing of the small individualelements.Theseunwantedraypathscanreflectoffsurfacesinthetestpieceandcausespuriousindicationson animage. Theamplitude ofgrating lobes is significantly affectedby pitch size, the number of

elements, frequency, andbandwidth. Thebeam profiles shown inFigure 228 compare two situations where the probe aperture isapproximatelythesame,butthebeamontheleftisgeneratedbysixelementsat0.4 mmpitch,andthebeamontherightbythreeelementsat 1 mm pitch. Thebeam on the left is somewhat shaped as a cone,whilethebeamontherighthastwospuriouslobesatanapproximate30 degreeangletothecenteraxisofthebeam.


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Figure 2-28 Beam profiles with different number of elements

Grating lobes occur whenever the size of individual elements in anarrayisequaltoorgreaterthanthewavelength.Therearenogratinglobes when the element size is smaller than half a wavelength. (Forelementsizesbetweenonehalfandonewavelength,thegeneratingofgratinglobesdependsonthesteeringangle.)Thusthesimplestwaytominimizegratinglobesinagivenapplication,istouseaprobewithasmall pitch. A specialized probe design incorporating subdicing

(cuttingelementsintosmallerelements)andvaryingelementspacing,alsoreducesunwantedlobes.

2.9 Phased Array Probe Selection Summary

Designing phased array probes is always a compromise betweenselectingtheproperpitch,elementwidth,andaperture.Usingahigh

numberofsmallelementstoincreasesteering,reducessidelobesandprovides focusing,but canbe limitedby cost of manufacturing andinstrumentcomplexity.Moststandardinstrumentssupportaperturesof up to 16 elements. Separating elements at greater distances canseem tobe the easy way of gaining aperture size,but this createsunwantedgratinglobes.

Itisimportanttonotethatvendorsofphasedarrayprobesoftenofferstandardprobesthathavebeendesignedwiththesecompromisesin

mind, resulting in optimized performance for the intended use.Actual probe selection is ultimately drivenby the end applicationneeds.Insomecases,multianglesteeringisrequiredoversmallmetal


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pathssolargeaperturesizesarenotneededordesired.Inothercases,theapplication,whichmaybetocoverlargeareasforlaminardefects,requireslargeaperturesandlinearscanformatwithmultiplegroupedelementswheresteeringisnotrequiredatall.Ingeneral,theusercanapply thebest practice from their conventional UT knowledge forfrequencyandapertureselection.

The Olympus phased array probe catalog can be viewed at thefollowingaddress:

www.olympusims.com/en/probes/pa/

Consult it to view the full selection of probes and wedges that isavailable.


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3. Basics of Phased Array Imaging

Both conventional and phased array ultrasonic instruments utilizehighfrequency sound waves to check the internal structure of a testpieceormeasureitsthickness.Theybothrelyonthesamebasiclawsofphysicsthatgovernsoundwavepropagation.Similarconceptsareemployedinbothultrasonictechnologiestopresentultrasonicdata.

Conventional ultrasonic instruments for NDT commonly consist ofeither a single active element thatboth generates and receives highfrequencysoundwaves,ortwopairedelements,onefortransmittingand one for receiving. A typical instrument consists of a singlechannelpulserandreceiverthatgeneratesandreceivesanultrasonicsignal with an integrated digital acquisition system, which iscoordinated with an onboard display and measurement module. Inmore advanced units, multiple pulser receiver channels canbe usedwith a group of transducers to increase zone of coverage forevaluating different depths or flaw orientations, and can furtherprovide alarm outputs. In more advanced systems, conventionalultrasonics canbe integrated with positional encoders, controllers,andsoftwareaspartofanimagingsystem.

Phased array instruments, on the other hand, are naturallymultichanneled as they need to provide excitation patterns (focal

laws) to probes with 16 to as many as 256 elements. Unlikeconventionalflawdetectors,phasedarraysystemscansweepasoundbeam from one probe through a range of refracted angles, along alinearpath,ordynamicallyfocusatanumberofdifferentdepths,thus


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increasingboth flexibility and capability in inspection setups. Thisadded ability to generate multiple sound paths within one probe,addsapowerfuladvantageindetectionandnaturallyaddstheabilityto visualize an inspectionby creating an image of the inspectionzone.Phasedarray imaging providestheuserwiththeability toseerelative pointtopoint changes and multiangular defect responses,

which can assist in flaw discrimination and sizing. While this canseem inherently complex, it can actually simplify expandinginspection coverage with increased detection by eliminating thecomplex fixtures and multiple transducers that are often requiredwithconventionalUTinspectionmethods.

The following sections further explain the basic formats forconventionalandphasedarraydatapresentation.

3.1 A-Scan Data

All ultrasonic instruments typically record two fundamentalparametersofanecho:howlargeitis(amplitude)andwhereitoccursintimewithrespecttoazeropoint(pulsetransittime).Transittime,inturn,isusuallycorrelatedtoreflectordepthordistance,basedonthesound velocity of the test material and the following simple

relationship:

Distance=VelocityTime

The mostbasic presentation of ultrasonic waveform data is in theformofanAscan,orwaveformdisplay,inwhichechoamplitudeandtransit time are plotted on a simple grid with the vertical axisrepresentingamplitudeandthehorizontalaxisrepresentingtime.Theexample in Figure 31 shows a version with a rectified waveform;

unrectified RF displays are also used. The redbar on the screen is agatethatselectsaportionofthewavetrainforanalysis,typicallythemeasurementofechoamplitudeand/ordepth.


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Figure 3-1 A-scan data

3.2 Single Value B-Scans

AnotherwayofpresentingtheAscandataisasasinglevalue

B

scan.

Thisformatiscommonlyusedwithconventionalflawdetectorsandcorrosionthicknessgagestoplotthedepthofreflectorswithrespecttotheirlinearposition.Thethicknessisplottedasafunctionoftimeorposition,whilethetransducerisscannedalongtheparttoprovideitsdepth profile. Correlating ultrasonic data with the actual transducerposition allows a proportional view tobe plotted and allows theability to correlate and track data to specific areas of the partbeing

inspected.Thispositiontrackingistypicallydonethroughtheuseofelectromechanicaldevicesknownasencoders.Theseencodersareusedeither in fixtures, which are manually scanned, or in automatedsystems that move the transducer by a programmable motor


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controlledscanner.Ineithercase,theencoderrecordsthelocationofeach data acquisition with respect to a desired userdefined scanpatternandindexresolution.

InthecaseshowninFigure32,theBscanshowstwodeepreflectorsand one shallower reflector, corresponding to the positions of the

sidedrilledholesinthetestblock.

Figure 3-2 B-scan data

3.3 Cross-sectional B-Scans

A crosssectionalBscan provides a detailed end view of a test piecealong a single axis. This provides more information than the singlevalue Bscan presented earlier. Instead of plotting just a singlemeasured value from within a gated region, the whole Ascanwaveformisdigitizedateachtransducerlocation.SuccessiveAscansare plotted over the elapsed time or the actual encoded transducer


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positionssoastodrawcrosssectionsofthescannedline.Thisallowsthe user to visualizeboth the near and farsurface reflectors withinthesample.Withthistechnique,thefullwaveformdataisoftenstoredat each location, and maybe recalled from the image for furtherevaluationorverification.

Toaccomplishthis,eachdigitizedpointofthewaveformisplottedsothatcolorrepresentingsignalamplitudeappearsattheproperdepth.

Successive Ascans are digitized, related to color, and stacked atuserdefinedintervals(elapsedtimeorposition)toformatruecrosssectionalimage(seeFigure33).

Figure 3-3 Cross-sectional B-scan


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3.4 Linear Scans

Aphasedarraysystemuseselectronicscanningalongthelengthofalinear array probe to create a crosssectional profile without movingthe probe. As each focal law is sequenced, the associated Ascan is

digitized and plotted. Successive apertures are stacked creating alivecrosssectionalview.Inpractice,thiselectronicsweepingisdonein real time so a live cross section canbe continually viewed as theprobe is physically moved. Figure 34 is an image made with a 64element linear phased array probe. In this example, the userprogrammedthefocallawtouse16elementstoformanapertureandsequencedthestartingelementincrementsbyone.Thisresultedin49individualwaveformsthatwerestackedtocreatetherealtimecross

sectionalviewacrosstheprobes1.5 in.length.

Figure 3-4 Normal beam linear scan


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Itisalsopossibletoscanatafixedangleacrosselements(seeFigure35). As discussed in section5.3, on page69, this is very useful forautomatedweldinspections.Usinga64elementlinearphasedarrayprobe with wedge, shear waves canbe generated at a userdefinedangle(often45,60,or70 degrees).Withaperturesequencingthroughthe length of the probe, full volumetric weld data canbe collected

without physically increasing the distance to weld center line whilescanning. This provides for singlepass inspection along the weldlength.

Figure 3-5 Angle beam linear scan

3.5 C-Scans

Another presentation option is a Cscan. A Cscan is a two

dimensionalpresentationofdatadisplayedasatoporplanarviewofatestpiece.It issimilarinitsgraphicperspectivetoanxrayimage,where color represents the gated signal amplitude or depth at eachpoint in the test piece mapped to its position. Planar images canbe


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generated on flat partsby tracking data to the XY position, or oncylindrical parts by tracking axial and angular positions. Forconventionalultrasound,amechanicalscannerwithencodersisusedtotrackthetransducerscoordinatestothedesiredindexresolution.

A Cscan from a phased array system is very similar to one from a

conventionalprobe.Withphasedarraysystems,however,theprobeistypically moved physically along one axis while the beamelectronically scans along the other, according to the focal lawsequence.SignalamplitudeordepthdataiscollectedwithinthegatedregionofinterestjustasinconventionalCscans.Inthecaseofphasedarrays, data is plotted with each focal law progression, using theprogrammedbeamaperture.

Figure36isaCscanofatestblockusinga5 MHz,64elementlinear

array probe with a zerodegree wedge. Each focal law uses 16elements to form the aperture, and at each pulsing the startingelementincrementsbyone.Thisresultsinfortyninedatapointsthatareplotted(horizontallyintheimageofFigure36)acrosstheprobes37 mm (1.5 in.) length. As the probe is moved forward in a straightline, a planar Cscan view emerges. Encoders are normally usedwheneveraprecisegeometricalcorrespondenceofthescanimagetothepartmustbemaintained,althoughnonencodedmanualscanscan

alsoprovideusefulinformationinmanycases.

Figure 3-6 C-scan data using 64-element linear phased array probe

While the graphic resolution might not be fully equivalent to aconventional Cscanbecause of the larger effectivebeam size, thereare other considerations. The phased array system is field portable,whichtheconventionalsystemisnot,anditcostsaboutonethirdtheprice.Additionally,aphasedarrayimagecanoftenbemadeinafew

Generalized beam profile anddirection of motion

Phased array C-scan imageshowing hole position


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seconds,whileaconventionalimmersionscantypicallytakesseveralminutes.

Linear phased array probes are also commonly used for performingrefractedshearwaveinspectionsalongthelengthofwelds.Figure37shows a 2.25 MHz 64element phased array probe mounted on an

angledwedgetocreateshearwavesatauserdefinedangle,typically45,60,or70 degrees.Withtheprobepositionedperpendiculartotheweld,theaperturecanbesequencedoverthelengthoftheprobe.Thiseffectivelyallowstherefractedshearwavetomovethroughtheweldvolume without mechanical movement of the probe from the weldscenterline.Fullvolumetricdatacanbepresentedbyslidingtheprobeparallel to the weld line. Using an encoder, data canbe plotted in aCscan like format where amplitude of the reflector is plotted as afunctionofapertureposition(Yaxis)anddistancetraveledalongtheweld(Xaxis).Thisscanningformatisoftenreferredtoasaonelinescan. For producing repeatable results, a mechanical scanner issuggested.InFigure37,areflectionfromtheungroundweldbottomis plotted along the whole weld length at the top of the image. TheAscanandcursorsmarkalargeindicationfromanareaoftheweldwithlackofsidewallfusion.

Figure 3-7 One-line scan for weld inspection using an encoded 2.25MHz 64-element probe steered at 60degrees


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3.6 S-Scans

Ofallimagingmodesdiscussedsofar,theSscanisuniquetophasedarrayequipment.Inalinearscan,allfocallawsemployafixedanglewith sequencing apertures. Sscans, on the other hand, use fixed

aperturesandsteerthroughasequenceofangles.

Twomainformsaretypicallyused.Themostfamiliar,verycommonin medical imaging, uses a zerodegree interface wedge to steerlongitudinal waves, creating a pieshaped image showing laminarandslightlyangleddefects(seeFigure38).

Figure 3-8 30 to +30 S-scan

The second format employs a plastic wedge to increase the incidentbeamangletogenerateshearwaves,mostcommonlyintherefracted


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angle range of 30 to 70 degrees. This technique is similar to aconventional anglebeam inspection, except that thebeam sweepsthrougharangeofanglesratherthanasinglefixedangledetermined

byawedge.Aswiththelinearsectorialscan,theimagepresentationisa crosssectional picture of the inspected area of the test piece (seeFigure39).

Figure 3-9 +35 to +70 S-scan

The actual image generation works on the same stacked Ascanprinciplethatwasdiscussedinthecontextoflinearscansintroducedintheprevioussection.Theuserdefinestheanglestart,end,andstepresolution to generate the Sscan image. Notice that the apertureremains constant, each defined angle generating a corresponding

beam with characteristics definedby aperture, frequency, damping,and the like. The waveform response from each angle (focal law) isdigitized, colorcoded, and plotted at the appropriate correspondingangle,buildingacrosssectionalimage.

In actuality the Sscan is produced in real time so as to continuallyofferdynamicimagingwithprobemovement.Thisisveryusefulfordefect visualizationandincreasesprobabilityofdetection,especially


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acquisition with respect to a desired userdefined scan pattern andindexresolution.

Inordertoavoidgapsindataacquisition,itisimportanttoconsiderthespeedatwhichtheprobeismovingandthedistanceresolutionofthe encoder. In short, the instruments data acquisition rate mustbe

greater than the scanning speed, dividedby the encoder resolution.The acquisition rate is determinedby instrument design and setup,mostimportantlybythepulserepetitionfrequency(PRF),andbythenumber of focal lawsbeing generated for each acquisition,both ofwhich are setup variables. The PRF dividedby the number of focallawsrepresentsthefastestpossibleacquisitionrateforaphasedarraysystem.However,thatnumbercanbefurtheradjustedbyfactorssuchasaveraging,digitalsamplingrate,andprocessingtime.Consulttheinstrumentmanufacturerfordetails.

Once the acquisition rate hasbeen established, the maximum scanspeed canbe calculatedbased on the desired encoder resolution, orvice versa. The effect of an excessive scanning speed for a givenencoderresolutioncanbeseeninthescanimagesinFigure311.

Figure 3-11 Example of the scanning speed influence on acquisition rate

IMPORTANT

1.

2. IfthesamePRFissetforallAscans,then:

Acquisition rate Scanning speed

Scan axis resolution------------------------------------------------

Acquisition rate RecurrenceNumber of focal laws----------------------------------------------------

20 mm/s 10 mm/s

scanning speed

encoder resolutionsAcquisition rate >


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Olympus PhasedArrayInstrumentation 51

4. Phased Array Instrumentation

There is a wide variety of phased array probes commerciallyavailable.Whilethelineararrayprobeiscertainlythemostcommonlyusedconfiguration,customizedprobeswithhighelementcountsandvarying element placements, are also available. They are oftendesigned to meet demanding application needs that require highspeed, full volumetric coverage, and/or complexbeam steering. Tomeet these needs, there are varying levels of phased arrayinstrumentation now commercially available in three generalclassifications: field portable manual, field portable automated, andrackinstrumentsforinlineinspection.

4.1 Important Specifications

Whenevaluatingconventionalflawdetectors,anumberoffunctionalcharacteristics are often specified. These characteristics are generallyshared with phased array instruments. Not all of the items listed

belowareavailableinallinstruments.

Pulser and receiver

Parametersthatlargelydefinetheoperatingrangeoftransducersthatcanbeusedwiththeinstrument

Measurement and display

Parameters defining the general measurement and display modes ofaninstrument:

Numberofalarm/measurementgates

Pulser Receiver

Availablespikepulser Overallbandwidth

Availablesquarewavepulser Availablenarrowbandfilters

Pulserrepetitionfrequency Timevariedgain

Overalldynamicrange


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52 Chapter4 Olympus

Ascan display modes: Rectification (RF, Full Wave, Half Wave),Maximum, Composite, Averaged, Hollow, Filled, and PeakMemory

Range

Measurementresolution

Measurementtypes(thatis,soundpath,depth,distancefromfrontofprobe,dB,dBtocurve,etc.)

SinglevalueBscanmode(notavailableonmostflawdetectors)

Sizing options

Avarietyofflawdetectionstandardsandcodeshavebeendevelopedand are in practice for sizing a variety of defects using conventional

ultrasonics. These apply to the inspection of welds as well as to avariety of metallic and composite structures. Certain inspectionsrequirethataspecificcodebefollowed.Asaresult,awidevarietyoftools are now available in conventional digital flaw detectors toautomatedataacquisitionandrecordtestresultsasrequiredbycodes.

Inputs and outputs

Inputsandoutputsgenerallydefinehowtheinstrumentcanbeusedwithexternaldevicesand/orsoftware:

Numberandtypeofalarmoutputs

USBforprinting,saving,ordatatransfer

Availabilityofencoderinputsforlinkingdatatoposition

Trigger input for external control of pulser firing and acquisitioncycle

Additional phased array speci fications

Becauseofthemultielementnatureofphasedarrayinstruments,thereareadditional keyspecifications that needfurtherconsideration andreview.

Numberofpulsers. Defines the maximum number of elements thatcanbegroupedtoformanactiveapertureorvirtualprobeaperture.

Numberofchannels.Definesthetotalnumberofchannelsthatcanbeused for sequencing apertures that leads to the potential increase incoveragefromasingleprobefootprint.

XX:YY.Namingconventionused,whereXX =numberofpulsers,andYY = total number of available channels. The number of channels isalwaysgreaterorequaltonumberofpulsers.Instrumentsfrom16:16to32:128areavailableinfieldportablepackaging.Higherpulserand

receiver combinations are available for inline inspections and/orsystemsthatuselargerelementcountprobes.

Focallaws.Thenumberoffocallawsthatcanbecombinedtoforman


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imageisoftenspecified.Ingeneral,higherXX:YYconfigurationscansupport more focal laws as they support greater element aperturesand/or more aperture steps in linear scanning. Note that more focallaws does not always mean more functionality. Take the example

below:a64elementprobeperforminga40to70 degreessectorialscanof three sidedrilled holes, comparing steering with 1 degree (31

laws), 2 degree (16 laws), and 4 degree (8 laws) steps over a 2 in.(50 mm)metalpath(seeFigure41,Figure42,andFigure43).Whilethe image is slightly better defined with finer angle increments,detection at a coarser resolution is adequate. Unless the beamdiameter is drastically reduced with focusing, sizing from imagesdoesnotdramaticallychangeeither.

Figure 4-1 40 to 70degrees S-scan: steering with 1 degree (31 laws) steps


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Figure 4-2 40 to 70degrees S-scan: steering with 2degree (16 laws) steps

Figure 4-3 40 to 70degrees S-scan: steering with 4degree (8 laws) steps


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Table 41 shows examples for the number of focal laws required toperform linear scans with varying combinations of virtual probeaperturesandtotalelementcounts.

Itcanbeseenthatastheaperturebecomessmaller,orthenumberofelementsbecomeslarger,thenumberoffocallawsrequiredperscanincreases.Thiswillhaveaneffectondisplayupdaterateascalculated

below.

PRF/Display update rate. Instruments can vary greatly in displayupdateinvariousimagemodes.Forphasedarrayimagingmodes:

An example of a reduced fourfocallaw linear scan sequence with a60 Hz image display update, is shown in Figure 44 forconceptualization.

Table 4-1 Number of elements and focal laws required for linear scans

Linearscan

Aperture Totalelements Elementstep Numberoflaws

4 16 1 13

8 16 1 9

4 32 1 29

8 32 1 25

16 32 1 17

4 64 1 61

8 64 1 57

16 64 1 49

8 128 1 12116 128 1 113

8 256 1 249

16 256 1 241

Maximum image display rate PRF

Number of focal laws----------------------------------------------------=


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56 Chapter4 Olympus

Figure 4-4 Example of a reduced four-focal-law linear scan sequence

Theactualimagedisplayratecanbeaffectedbyotherparameters.TheAscanrefreshrateofasinglefocallawvariesbetweeninstruments.Insome instruments, the Ascan PRF rate is limitedby the maximum

image display update, whether it is shown with the phased arrayimage or even when maximized to a full Ascan. For this reason, insome applications it mightbe important to verify the Ascan PRFwhen derived from a focal law sequence in various image displaymodes.

Probe recognition. The ability to recognize phased array probesreduces operator setup time and potential errorsby automaticallyconfiguringaninstrumentsetupwiththepropernumberofelementsandprobegeometry.

Image types. Sectorial and linear scans are typically available inphased arrayinstruments. Theability to stacktheseimagemodestocreate amplitude and depth Cscans, allows planar images to beformedandprovidesexpandedmeansforsizingdefects.

Waveformstorage.TheabilitytostorerawRFwaveformsallowsdata

tobe reviewed offline. This is particularly useful when collectingdataoveralargearea.

Multigroup support. More capable phased array instruments allowmultiplefocallawgroupstobesequencedononeormoreconnectedprobes. This is especially useful in cases where it is important tocollectvolumetricdatawhichistobeanalyzedoffline.Forexample,a5 MHz, 64element probe canbe programmed to use elements 116for a 40 to 70 degree Sscan, while a second group canbe used to

perform a 60 degree linear scan with an aperture of 16 elements,steppingbyoneelementovertheentire64elementlength.

4.15 ms 8.30 ms 12.45 ms 16.6 ms

Image display update=60 Hz

Focal law 1

150 ns

Focal law 2

150 ns

Focal law 3

150 ns

Focal law 4

150 ns

Focal law 1

150 ns

PRF=240 Hz PRF=240 Hz PRF=240 Hz PRF=240 Hz


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Encoding. There are two classes of instruments generally available:manualandencoded.

A manual phased array instrument works much like a conventionalflawdetectorasitprovidesrealtimedata.AlongwithanAscan,theinstrument also shows realtime Sscan or linearscan images, which

canaidindetectionanddiscontinuityanalysis.Theabilitytouseandvisualizemorethanoneangleorpositionatatimeinatestisthemainreasonforusingthistypeofinstrument.Insomecases,suchascracksizing,theimagecanbeusedasatooltohelpsizecrackdepth.

A phased array instrument with an encoder interface merges probepositional data, probe geometry, and programmed focal lawsequencestoallowtop,end,andsideviewimagesoftestspecimen.In instruments that also store full waveform data, images can be

reconstructedtoprovidecrosssectionalviewsalongthelengthofthescan or regenerate planar Cscans at various levels. These encodedimagesallowplanarsizingofdefects.

Reference cursors. Instruments provide various cursors that canbeused on an image as aids for interpretation, sizing, and depthmeasurement.InanSscan,itispossibletousecursorsformeasuringcrackheight.Anapproximatedefectsizecanbemeasuredwhenusing

encoded data sets. The images that follow show some examples ofavailablecursors.

Inthesimplestdisplaybelow(Figure45),thebluecursorshowstheangularcomponentoftheSscanthatisrepresentedbytheAscan,thehorizontalredlinesmarkthebeginningandendofthedatagateusedformeasurement,andtheverticalgreenlinemarksthepositionontheimage that corresponds to the front of the wedge. The latter iscommonlyusedasareferencepointforcalculatingreflectorlocation,

notingthatnearsurfacereflectorsmightbelocatedunderthewedge,sincetheexactbeamindexpoint(BIP)foraphasedarrayprobevarieswithangleand/oraperturegroup.


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Figure 4-5 Angular cursor

The Sscan image in Figure 46 includes horizontal cursorsrepresentingtheendofthefirstandsecondlegsoundpathsinthetest

material. It also shows the angular cursors marking the three mostcommontestanglesof45,60,and70 degrees.Inaddition,theAscanis marked with a vertical cursor at the 80 % amplitude point that iscommonlyusedasareferencelevel.

Figure 4-6 Angular and horizontal cursors

Advanced interpretive software further enhances visualization andanalysis. The display in Figure 47 shows a singleangle Ascan, aSscan, a raytracing diagram with a weld overlay that shows thepositionofreflectorswithinaweld,andasummarychartshowingthe


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calculatedpositionandmeasuredamplitudeofeachindication.

Figure 4-7 Multiple display formats

4.2 Calibration and Normalization Methods

Zero calibration. Because wedge delay varies with the angle in aphased array system, it is necessary to vary the probe zero offsetacross the angles. Typically, default zero profilesbased on wedgegeometry are programmed in instrumentsoftware,butthese defaultprofiles canbe adjusted for higher accuracy through a calibration

procedurebysweepingthebeamacrossareferencereflectoratafixeddepthordistance.

Gain normalization. Because beam formation relies on varyingelementdelaysandgroups,itisimportanttonormalizetheamplituderesponse from each focal law to compensate forboth elementtoelement sensitivity variations in the array probe and for varyingwedgeattenuationandenergytransferefficiencyatdifferentrefractedangles. Calibration of wedge delay and sensitivity over the entire

inspectionsequencenotonlyprovidesclearerimagevisualization,butalso allows measurement and sizing from any focal law. Olympusnondestructive testing instruments offer full calibration, whereasmany other instruments in the industry can only calibrate one focallaw at any one time. The Olympus instruments provide full AngleCorrected Gain (ACG) and TimeCorrected Gain (TCG), as required

byASMESection V.

In the Figure 48 example, prior to gain normalization, the response

from a reference reflector at 65 degrees, is significantly lower thanfromthesamereflectorat45 degrees.


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Figure 4-9 Response following gain normalization

TVG/DAC for phased array. For sizing defects, Ascan amplitudetechniques using DAC curves or timecorrected gain, are common.

These methods account for material attenuation effects andbeamspreading by compensating gain levels (TVG/TCG) or drawing areference DAC curve based on same size reflector response as afunction of distance. As in conventional UT sensitivity calibrations,


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Olympus PhasedArrayTestSetupandDisplayFormat 65

not only allows accurate measurements of amplitude and depthacross the entire programmed focal sequence, but also providesaccurateandenhancedvisualizationthroughtheimagesthatphasedarrayinstrumentsproduce.

Oneofthemajordifferencesbetweenconventionalandphasedarray

inspections,occursinanglebeaminspections.WithconventionalUT,input of an improper wedge angle or material velocity will causeerrors in locating the defect,butbasic wave propagation (and hencetheresultantAscan)isnotinfluenced,asitreliessolelyonmechanicalrefraction. For phased array, however, proper material and wedgevelocities, along with probe and wedge parameter inputs, arerequiredtoarriveattheproperfocallawstoelectronicallysteeracrossthe desired refracted angles and to create sensible images. In morecapableinstruments,proberecognitionutilitiesautomaticallytransfercriticalphasedarrayprobeinformationandusewellorganizedsetuplibrariestomanagetheuserselectionofthecorrectwedgeparameters.

Thefollowingvaluesmustnormallybeenteredinordertoprogramaphasedarrayscan:

Probe parameters

Frequency Bandwidth

Size

Numberofelement

Documents

Olympus Phased Array[1]