Introduction to Ultrasonics.pdf

7/25/2019 Introduction to Ultrasonics.pdf

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Ultrasonic Testing


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Introduction

This module presents an introduction to the NDTmethod of ultrasonic testing.

Ultrasonic testing uses high frequency soundenergy to conduct examinations and makemeasurements.

Ultrasonic examinations can be conducted on awide variety of material forms including castings,forgings, welds, and composites.

A considerable amount of information about thepart being examined can be collected, such as thepresence of discontinuities, part or coatingthickness; and acoustical properties can often becorrelated to certain properties of the material.


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Outline

Applications

Basic Principles of sound generation Pulse echo and through transmission testing Inspection applications

Equipment

Transducers Instrumentation Reference Standards

Data presentation Advantages and Limitations Glossary of terms


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Basic Principles of Sound

Sound is produced by a vibrating body and travelsin the form of a wave.

Sound waves travel through materials by vibratingthe particles that make up the material.

The pitch of the soundis determined by thefrequency of the wave(vibrations or cyclescompleted in a certainperiod of time).

Ultrasound is soundwith a pitch too highto be detected by thehuman ear.


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Basic Principles of Sound (cont.)

The measurement of sound waves from crest tocrest determines its wavelength ().

The time is takes a sound wave to travel adistance of one complete wavelength is the sameamount of time it takes the source to executeone complete vibration.

The sound wavelengthis inversely proportionalto its frequency. ( = 1/f)

Several wave modes ofvibration are used inultrasonic inspection.The most common arelongitudinal, shear, and

Rayleigh (surface) waves.


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Basic Principles of Sound (cont.)

Ultrasonic waves are very similar tolight waves in that they can be reflected,

refracted, and focused. Reflection and refraction occurs when

sound waves interact with interfaces ofdiffering acoustic properties.

In solid materials, the vibrational energycan be split into different wave modeswhen the wave encounters an interfaceat an angle other than 90 degrees.

Ultrasonic reflections from the presenceof discontinuities or geometric featuresenables detection and location.

The velocity of sound in a given materialis constant and can only be altered by a

change in the mode of energy.


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Ultrasound Generation

The transducer iscapable of bothtransmitting andreceiving soundenergy.

Ultrasound is generated with a transducer.

A piezoelectric elementin the transducerconverts electricalenergy into mechanicalvibrations (sound), and

vice versa.


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Principles of Ultrasonic Inspection

Ultrasonic waves are introduced into a material

where they travel in a straight line and at aconstant speed until they encounter a surface.

At surface interfaces some of the wave energy isreflected and some is transmitted.

The amount of reflected or transmitted energy canbe detected and provides information about the

size of the reflector.

The travel time of the sound can be measured andthis provides information on the distance that the

sound has traveled.


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Ultrasonic testing is a very versatile inspectionmethod, and inspections can be accomplished in a

number of different ways.

Ultrasonic inspection techniques are commonlydivided into three primary classifications.

Pulse-echo and Through Transmission(Relates to whether reflected or transmitted energy is used) Normal Beam and Angle Beam

(Relates to the angle that the sound energy enters the test article)

Contact and Immersion(Relates to the method of coupling the transducer to the testarticle)

Test Techniques

Each of these techniques will be discussed brieflyin the following slides.


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In pulse-echo testing, a transducer sends out a pulse of energyand the same or a second transducer listens for reflected energy

(an echo). Reflections occur due to the presence of discontinuities and the

surfaces of the test article.

The amount of reflected sound energy is displayed versus time,which provides the inspector information about the size and thelocation of features that reflect the sound.

f

Test Techniques - Pulse-Echo

plate

crack

0 2 4 6 8 10

initial

pulse

crack

echo

back surface

echo

UT Instrument Screen


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Test Techniques Pulse-Echo (cont.)

Digital displayshowing signalgenerated fromsound reflectingoff back surface.

Digital displayshowing the presenceof a reflector midwaythrough material, with

lower amplitude backsurface reflector.

The pulse-echo technique allows testing when access to only oneside of the material is possible, and it allows the location of

reflectors to be precisely determined.


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Test Techniques Through-Transmission

0 2 4 6 8 10

2

11

Two transducers located onopposing sides of the test

specimen are used. Onetransducer acts as a transmitter,the other as a receiver.

Discontinuities in the sound path

will result in a partial or total lossof sound being transmitted andbe indicated by a decrease in thereceived signal amplitude.

Through transmission is useful indetecting discontinuities that arenot good reflectors, and whensignal strength is weak. It doesnot provide depth information.

T R

T R

11

2


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Digital display

showing received

sound through

material

thickness.

Digital display

showing loss of

received signaldue to presence

of a discontinuity

in the sound field.

Test Techniques Through-Transmission


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Test Techniques Normal and Angle Beam

In normal beam testing, the soundbeam is introduced into the test

article at 90 degree to the surface. In angle beam testing, the sound

beam is introduced into the testarticle at some angle other than

90. The choice between normal and

angle beam inspection usuallydepends on two considerations:

- The orientation of the feature ofinterest the sound should bedirected to produce the largestreflection from the feature.

- Obstructions on the surface of the

part that must be worked around.


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0 2 4 6 8 10

FWE

BWEDE

2IP IP = Initial PulseFWE = Front Wall

Echo

DE = Defect Echo

BWE = Back Wall

Echo

0 2 4 6 8 10

FWE

BWE

1IP1 2

Defect

Test Techniques Contact Vs Immersion

To get useful levels of sound energy into a material, the airbetween the transducer and the test article must be removed.

This is referred to as coupling. In contact testing (shown on the previous slides) a couplant

such as water, oil or a gel is applied between the transducerand the part.

In immersion testing, the part and the transducer are place in a

water bath. This arrangement allows better movement of thetransducer while maintaining consistent coupling.

With immersion testing, an echo from the front surface of thepart is seen in the signal but otherwise signal interpretation isthe same for the two techniques.


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Inspection Applications

Some of the applications for which ultrasonic testing

may be employed include: Flaw detection (cracks, inclusions, porosity, etc.)

Erosion & corrosion thickness gauging

Assessment of bond integrity in adhesively

joined and brazed components

Estimation of void content in composites and

plastics

Measurement of case hardening depth in steels Estimation of grain size in metals

On the following slides are examples of somecommon applications of ultrasonic inspection.


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Thickness Gauging

Ultrasonic thicknessgauging is routinely utilized

in the petrochemical andutility industries todetermine various degreesof corrosion/erosion.

Applicationsinclude piping

systems, storageand containmentfacilities, andpressure vessels.


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Flaw Detection - Delaminations

Signal showing multiple back

surface echoes in an unflawed area.

Additional echoes indicate

delaminations in the member.

Contact, pulse-echo inspection for delaminationson 36 rolled beam.


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Flaw Detection in Welds

One of the most widelyused methods ofinspecting weldments isultrasonic inspection.

Full penetration groovewelds lend themselves

readily to angle beamshear wave examination.


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Equipment

Equipment for ultrasonic testing is very

diversified. Proper selection is important toinsure accurate inspection data as desiredfor specific applications.

In general, there are three basic components

that comprise an ultrasonic test system:- Instrumentation

- Transducers

- Calibration Standards


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Transducers

Transducers are manufactured in a variety offorms, shapes and sizes for varying applications.

Transducers are categorized in a number of wayswhich include:

- Contact or immersion

- Single or dual element- Normal or angle beam

In selecting a transducerfor a given application, itis important to choose thedesired frequency,bandwidth, size, and in some cases focusingwhich optimizes the inspection capabilities.


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Contact Transducers

Contact transducers aredesigned to withstandrigorous use, and usuallyhave a wear plate on thebottom surface to protectthe piezoelectric element

from contact with thesurface of the test article.

Many incorporateergonomic designs for

ease of grip whilescanning along thesurface.


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Contact Transducers (cont.) Contact transducers are

available with two piezoelectric

crystals in one housing. Thesetransducers are called dualelement transducers.

One crystal acts as a transmitter,the other as a receiver.

This arrangement improves nearsurface resolution because thesecond transducer does notneed to complete a transmit

function before listening forechoes.

Dual elements are commonlyemployed in thickness gauging

of thin materials.


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Contact Transducers (cont.)

A way to improve near surfaceresolution with a single element

transducer is through the use ofa delay line.

Delay line transducers have aplastic piece that is a sound paththat provides a time delay

between the sound generationand reception of reflectedenergy.

Interchangeable pieces make itpossible to configure the

transducer with insulating wearcaps or flexible membranes thatconform to rough surfaces.

Common applications includethickness gauging and high

temperature measurements.


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Transducers (cont.)

Angle beam transducersincorporate wedges to

introduce a refracted shearwave into a material.

The incident wedge angle isused with the material

velocity to determine thedesired refracted shearwave according to SnellsLaw)

Transducers can use fixedor variable wedge angles. Common application is in

weld examination.


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Transducers (cont.)

Immersion transducers aredesigned to transmit soundwhereby the transducer andtest specimen are immersedin a liquid coupling medium(usually water).

Immersion transducersare manufactured withplanar, cylindrical or

spherical acousticlenses (focusing lens).


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Instrumentation

Ultrasonic equipment is usually purchased tosatisfy specific inspection needs, some users

may purchase general purpose equipment tofulfill a number of inspection applications.

Test equipment can be classified in a number ofdifferent ways, this may include portable orstationary, contact or immersion, manual orautomated.

Further classification of instruments commonlydivides them into four general categories: D-meters, Flaw detectors, Industrial and specialapplication.


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Instrumentation (cont.)

D-meters or digitalthickness gauge

instruments provide theuser with a digital(numeric) readout.

They are designedprimarily forcorrosion/erosioninspection applications.

Some instruments provide the user with both a

digital readout and a display of the signal. Adistinct advantage of these units is that they allowthe user to evaluate the signal to ensure that thedigital measurements are of the desired features.


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Flaw detectors areinstruments designed

primarily for the inspectionof components for defects.

However, the signal can beevaluated to obtain other

information such asmaterial thickness values.

Both analog and digitaldisplay.

Offer the user options ofgating horizontal sweepand amplitude threshold.


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Industrial flaw detectioninstruments, provideusers with more optionsthan standard flawdetectors.

May be modulated unitsallowing users to tailorthe instrument for theirspecific needs.

Generally not as portableas standard flawdetectors.


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Instrumentation (cont.) Immersion ultrasonic scanning

systems are used forautomated data acquisitionand imaging.

They integrate an immersiontank, ultrasonicinstrumentation, a scanningbridge, and computer controls.

The signal strength and/or thetime-of-flight of the signal ismeasured for every point in thescan plan.

The value of the data is plottedusing colors or shades of grayto produce detailed images ofthe surface or internal featuresof a component.


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Images of a Quarter Produced With an

Ultrasonic Immersion Scanning System

Gray scale image produced using

the sound reflected from the front

surface of the coin

Gray scale image produced using the

sound reflected from the back surface

of the coin (inspected from heads side)


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Calibration Standards

Calibration is a operation of configuring theultrasonic test equipment to known values. Thisprovides the inspector with a means of comparingtest signals to known measurements.

Calibration standards come in a wide variety of

material types, and configurations due to thediversity of inspection applications.

Calibration standards are typically manufacturedfrom materials of the same acoustic properties as

those of the test articles.

The following slides provide examples of specifictypes of standards.


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Calibration Standards (cont.)

Thickness calibrationstandards may be flat orcurved for pipe and tubingapplications, consisting ofsimple variations inmaterial thickness.

Distance/Area Amplitudestandards utilize flat bottomholes or side drilled holes to

establish known reflectorsize with changes in soundpath form the entry surface.

ASTM Distance/Area Amplitude

NAVSHIPS


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Calibration Standards (cont.)

There are also calibrationstandards for use in anglebeam inspections whenflaws are not parallel toentry surface.

These standards utilizedside drilled holes, notches,and geometricconfiguration to establish

time distance andamplitude relationships.

IIW

DSC DC Rhompas

SC

ASME Pipe Sec. XI


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Qualification Standards

Qualificationstandards differ fromcalibration standardsin that their use is forpurposes of varyingproper equipmentoperation andqualification ofequipment use forspecific codes andstandards.

AWS Resolution

IOW Beam Profile

DC-dB Accuracy


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Data Presentation

Information from ultrasonic testing can bepresented in a number of differing formats.

Three of the more common formats include: A-scan

B-scan C-scan

These three formats will be discussed in the nextfew slides.


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Data Presentation - A-scan

A-scan presentationdisplays the amount of

received ultrasonicenergy as a function oftime.

Relative discontinuitysize can be estimated bycomparing the signalamplitude to that from aknown reflector.

Reflector depth can bedetermined by theposition of the signal onthe horizontal sweep.

Time

SignalAm

plitude

SignalAmplitude

Time


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Data Presentation - B-scan

B-scan presentationsdisplay a profile view(cross-sectional) of a testspecimen.

Only the reflector depth inthe cross-section and thelinear dimensions can bedetermined.

A limitation to this displaytechnique is thatreflectors may be maskedby larger reflectors nearthe surface.


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Data Presentation - C-scan The C-scan presentation displays a plan type view

of the test specimen and discontinuities.

C-scan presentations are produced with anautomated data acquisition system, such as inimmersion scanning.

Use of A-scan in conjunction with C-scan isnecessary when depth determination is desired.

Photo of a CompositeComponent

C-Scan Image ofInternal Features


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Advantage of Ultrasonic Testing

Sensitive to both surface and subsurfacediscontinuities.

Depth of penetration for flaw detection or measurementis superior to other methods.

Only single-sided access is needed when pulse-echotechnique is used.

High accuracy in determining reflector position andestimating size and shape.

Minimal part preparation required.

Electronic equipment provides instantaneous results. Detailed images can be produced with automated

systems.

Has other uses such as thickness measurements, inaddition to flaw detection.


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Limitations of Ultrasonic Testing

Surface must be accessible to transmit ultrasound.

Skill and training is more extensive than with some othermethods.

Normally requires a coupling medium to promote transferof sound energy into test specimen.

Materials that are rough, irregular in shape, very small,exceptionally thin or not homogeneous are difficult toinspect.

Cast iron and other coarse grained materials are difficultto inspect due to low sound transmission and high signal

noise. Linear defects oriented parallel to the sound beam may

go undetected.

Reference standards are required for both equipmentcalibration, and characterization of flaws.


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Glossary of Terms

Acoustical properties: ultrasonic material characteristicssuch as velocity, impedance, and attenuation.

ASTM: acronym for American Society for Testing andMaterials. This society is extensively involved inestablishing standards for materials and the testing ofmaterials.

Back reflection: a display signal that corresponds to thefar surface of a test specimen, side opposite to transducerwhen testing with longitudinal waves.

Band width: a range of frequencies either transmitted orreceived, may be narrow or broad range.

B-scan: presentation technique displaying data in a cross-sectional view.


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Glossary of Terms

Calibration: a sequence of instrument controladjustments/instrument responses using known values to

verify instrument operating characteristics. Allowsdetermination of unknown quantities from test materials.

CRT: acronym for Cathode Ray Tube. Vacuum tube thatutilizes one or more electron guns for generating an image.

C-scan: presentation technique that displays specimendata in a plan type view.DAC (Distance Amplitude Correction-curves): a

graphical method of allowing for material attenuation.Percentage of DAC is often used as a means of acceptance

criteria.

Discontinuity: an interruption in the physical structure of amaterial, examples include fissures, cracks, and porosity.


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Glossary of Terms IIW: calibration standard meeting the specification of the

International Institute of Welding.

Longitudinal (Compression) waves: ultrasonic mode ofpropagation in which the particle vibration is parallel to thedirection of propagation.

Near Surface Resolution: the ability of an ultrasonic systemto display reflectors located close to the entry surface.

Pulse-echo: ultrasonic test method that utilizes reflectedsound as a means of collecting test data.

Rayleigh (Surface) waves: ultrasonic mode of propagation

where the sound travels along the surface, particle vibration iselliptical.


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Glossary of Terms Reflection: the changing in direction of sound waves as they

strike a surface.

Snells Law: an equation of ratios used to determine incidentor refracted angle of sound, denotes angle/velocityrelationship.

Sweep display: horizontal line on the lower portion of thedisplay, often called the time base line. Through transmission: test technique in which ultrasound istransmitted from one transducer and received by a separatetransducer on the opposite side of the test specimen.

Wavelength: the distance that a sound wave travels as itcompletes one cycle, normally measured in inches ormillimeters.

Documents

Introduction to Ultrasonics.pdf