ultrasonics ndt testing

7/29/2019 Ultrasonics NDT Testing

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Nondestructive Testing using

Ultrasonic's

P.V.V.SATYANARAYANA,M.Sc, M.Phil, M.Tech.


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Outline

Introduction to NDT

Overview of Six MostCommon NDT Methods

Selected Applications


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Nondestructive testing isa method of findingdefects in an objectwithout harming the

object.

With NDT, defects may be foundbefore they become dangerous.

i.e. Inspect or measure without doing harm.

Definition of NDT


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Methods of NDT

Visual


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What are Some Usesof NDE Methods?

Flaw Detection and Evaluation

Leak Detection

Location Determination

Dimensional Measurements Structure and Microstructure Characterization

Estimation of Mechanical and Physical Properties

Stress (Strain) and Dynamic ResponseMeasurements

Material Sorting and Chemical Composition

Determination

Fluorescent penetrant indication


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When are NDE Methods Used?

To assist in product development

To screen or sort incoming materials

To monitor, improve or controlmanufacturing processes

To verify proper processing such as heattreating

To verify proper assembly

To inspect for in-service damage

There are NDE application at almost any stagein the production or life cycle of a component.


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Six Most Common NDTMethods

Visual Liquid Penetrant Magnetic Ultrasonic Eddy Current X-ray


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Most basic and common

inspection method.

Tools includefiberscopes,borescopes, magnifying

glasses and mirrors.

Robotic crawlers permitobservation in hazardous ortight areas, such as air

ducts, reactors, pipelines.

Portable video inspectionunit with zoom allows

inspection of large tanksand vessels, railroad tank

cars, sewer lines.

Visual Inspection


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A liquid with high surface wetting characteristicsis applied to the surface of the part and allowedtime to seep into surface breaking defects.

The excess liquid is removed from the surfaceof the part.

A developer (powder) is applied to pull thetrapped penetrant out the defect and spread iton the surface where it can be seen.

Visual inspection is the final step in theprocess. The penetrant used is often loadedwith a fluorescent dye and the inspection isdone under UV light to increase testsensitivity.

Liquid Penetrant Inspection


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Magnetic Particle Inspection

The part is magnetized. Finely milled iron particles coated with adye pigment are then applied to the specimen. These particlesare attracted to magnetic flux leakage fields and will cluster toform an indication directly over the discontinuity. This indicationcan be visually detected under proper lighting conditions.


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Magnetic Particle Crack Indications


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RadiographyThe radiation used in radiographytesting is a higher energy (shorterwavelength) version of theelectromagnetic waves that wesee as visible light. The radiation cancome from an X-ray generator or aradioactive source.

High Electrical Potential

Electrons

-+

X-ray Generatoror RadioactiveSource Creates

Radiation

Exposure Recording Device

RadiationPenetratethe Sample


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Film Radiography

Top view of developed film

X-ray film

The part is placed between theradiation source and a piece of film.The part will stop some of theradiation. Thicker and more dense

area will stop more of the radiation.

= more exposure

= less exposure

The film darkness(density) will vary withthe amount of radiation

reaching the filmthrough the test object.


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Radiographic Images


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Conductivematerial

CoilCoil'smagnetic field

Eddycurrents

Eddy current'smagnetic field

Eddy Current Testing


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Eddy Current TestingEddy current testing is particularly well suited for detecting surfacecracks but can also be used to make electrical conductivity and

coating thickness measurements. Here a small surface probe isscanned over the part surface in an attempt to detect a crack.


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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 certain

period of time). Ultrasound is sound

with 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 same

amount of time it takes the source to execute onecomplete 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) [email protected]


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Ultrasound Generation

The transducer iscapable of bothtransmitting andreceiving soundenergy.

Ultrasound is generated with a transducer.

A piezoelectric element inthe transducer convertselectrical energy intomechanical vibrations

(sound), and vice versa.


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Principles of Ultrasonic

Inspection Ultrasonic waves are introduced into a materialwhere they travel in a straight line and at aconstant speed until they encounter a surface.

At surface interfaces some of the wave energy is

reflected and some is transmitted. The amount of reflected or transmitted energy can

be detected and provides information about thesize of the reflector.

The travel time of the sound can be measured andthis provides information on the distance that thesound has traveled.


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Basic Principles of UltrasonicTesting

Sound is transmitted in the material to be tested

The sound reflected back to the probe isdisplayed on

the Flaw Detector


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Basic Principles of Ultrasonic TestingThe distance the sound traveled can be displayed on the Flaw DetectorThe screen can be calibrated to give accurate readings of the distance

Bottom / Backwall

Signal from the backwall


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Basic Principles of UltrasonicTestings

The presence of a Defect in the material shows up on the screen of the flawdetector with a less distance than the bottom of the material

The BWE signal

Defect signal

Defect


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The depth of the defect can be read with reference tothe marker on the screen

0 10 20 30 40 50 60

60 mm

Thickness / depth


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Thickness / depthmeasurement

A

A

B

B

C

C

The THINNER the material the

less distance the sound travel

The closer the reflector tothe surface, the signal will

be more to the left of thescreen

The thickness is read from the screen

684630


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

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

Test Techniques

Each of these techniques will be discussed briefly in 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 thesurfaces 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

initialpulse

crackecho

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 midway

through material, withlower 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 testspecimen are used. Onetransducer acts as a transmitter,the other as a receiver.

Discontinuities in the sound pathwill 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

[email protected]


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Digital displayshowingreceivedsound through

materialthickness.

Digital displayshowing loss ofreceived signaldue to presenceof a discontinuityin the sound field.

Test Techniques Through-Transmission


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Test Techniques Normal and Angle Beam

In normal beam testing, the sound beamis introduced into the test article at 90degree to the surface.

In angle beam testing, the sound beam isintroduced into the test article at someangle other than 90.

The choice between normal and anglebeam inspection usually depends on twoconsiderations:

- The orientation of the feature ofinterest the sound should be

directed to produce the largestreflection from the feature.

- Obstructions on the surface of thepart that must be worked around.


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0 2 4 6 8 10

FWE

BWEDE

2IP IP = Initial PulseFWE = Front Wall

EchoDE = Defect EchoBWE = 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 couplantsuch as water, oil or a gel is applied between the transducerand the part.

In immersion testing, the part and the transducer are place in awater 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.

[email protected]


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Transducers

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

Transducers are categorized in a number of ways whichinclude:

- Contact or immersion

- Single or dual element

- Normal or angle beam

In selecting a transducerfor a given application, it

is 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 protect

the piezoelectric elementfrom contact with thesurface of the test article.

Many incorporateergonomic designs forease of grip whilescanning along thesurface.


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Contact Transducers (cont.)

Contact transducers areavailable with two piezoelectriccrystals 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 not

need to complete a transmitfunction before listening forechoes.

Dual elements are commonlyemployed in thickness gaugingof thin materials.

[email protected]


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Contact Transducers (cont.)

A way to improve near surfaceresolution with a single elementtransducer is through the use ofa delay line.

Delay line transducers have a

plastic piece that is a sound paththat provides a time delaybetween the sound generationand reception of reflectedenergy.

Interchangeable pieces make it

possible to configure thetransducer 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 tointroduce a refracted shearwave into a material.

The incident wedge angle isused with the materialvelocity to determine thedesired refracted shearwave according to SnellsLaw)

Transducers can use fixedor variable wedge angles.

Common application is inweld examination.

[email protected]


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Transducers (cont.)

Immersion transducers aredesigned to transmit soundwhereby the transducer andtest specimen are immersed

in a liquid coupling medium(usually water).

Immersion transducersare manufactured withplanar, cylindrical or

spherical acousticlenses (focusing lens).

[email protected]


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Inspection Applications

Some of the applications for which ultrasonic testingmay 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 andplastics Measurement of case hardening depth in

steels

Estimation of grain size in metals

Ult i I i


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Ultrasonic Imaging

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)

High resolution images can be produced by plottingsignal strength or time-of-flight using a computer-

controlled scanning system.


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Images of a Quarter Produced With anUltrasonic Immersion Scanning System

Gray scale image producedusing the sound reflected fromthe front surface of the coin

Gray scale image produced usingthe sound reflected from the backsurface of the coin (inspected fromheads side)


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Calibration Standards (cont.)

Thickness calibrationstandards may be flat orcurved for pipe and tubingapplications, consisting of

simple variations inmaterial thickness.

Distance/Area Amplitudestandards utilize flat bottom

holes or side drilled holes toestablish known reflectorsize with changes in soundpath form the entry surface.

ASTM Distance/AreaAmplitude

NAVSHIPS


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Calibration Standards (cont.)

There are also calibrationstandards for use in anglebeam inspections whenflaws are not parallel to

entry surface.These standards utilizedside drilled holes, notches,and geometric

configuration to establishtime distance andamplitude relationships.

IIW

DSC DCRhompas

SC

ASME Pipe Sec. XI


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Qualification Standards

Qualificationstandards differ fromcalibration standardsin that their use is for

purposes of varyingproper equipmentoperation andqualification ofequipment use for

specific 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 presentation displaysthe amount of receivedultrasonic energy as afunction of time.

Relative discontinuity sizecan be estimated bycomparing the signalamplitude to that from aknown reflector.

Reflector depth can bedetermined by the position ofthe signal on the horizontalsweep.

TimeS

ignalAmplitude

SignalAmp

litude

Time


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Data Presentation - B-scan

B-scan presentations display aprofile view (cross-sectional)of a test specimen.

Only the reflector depth in thecross-section and the lineardimensions can bedetermined.

A limitation to this displaytechnique is that reflectorsmay be masked by largerreflectors near the surface.


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Data Presentation - C-scan

The C-scan presentation displays a plan type viewof 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 UltrasonicTesting

Sensitive to small discontinuities both surface andsubsurface.

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 automatedsystems.

Has other uses such as thickness measurements, in

addition to flaw detection.


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Limitations of UltrasonicTesting

Surface must be accessible to transmit ultrasound.

Skill and training is more extensive than with some othermethods.

Normally requires a coupling medium to promote transfer

of 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 difficult

to inspect due to low sound transmission and high signalnoise.

Linear defects oriented parallel to the sound beam maygo undetected.

Reference standards are required for both equipment

calibration, and characterization of flaws.


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Common Application of NDT

Inspection of Raw Products

Inspection Following SecondaryProcessing

In-Services Damage Inspection


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Inspection of Raw Products

Forgings, Castings, Extrusions,

etc.


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Machining Welding

Grinding Heat treating Plating etc.

Inspection FollowingSecondary Processing


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Cracking

Corrosion

Erosion/Wear Heat Damage

etc.

Inspection ForIn-Service Damage


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Power Plant Inspection

Probe

Signals producedby variousamounts ofcorrosionthinning.

Periodically, power plants are

shutdown for inspection.Inspectors feed eddy currentprobes into heat exchangertubes to check for corrosiondamage.

Pipe with damage

Wire Rope Inspection


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Wire Rope InspectionElectromagnetic devicesand visual inspections are

used to find broken wiresand other damage to thewire rope that is used inchairlifts, cranes and otherlifting devices.

Storage Tank Inspection


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Storage Tank Inspection

Robotic crawlersuse ultrasound toinspect the walls oflarge above groundtanks for signs ofthinning due tocorrosion.

Cameras onlongarticulatingarms are usedto inspectundergroundstorage tanksfor damage.

Aircraft Inspection


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Aircraft Inspection Nondestructive testing is used

extensively during the

manufacturing of aircraft. NDT is also used to find cracks

and corrosion damage duringoperation of the aircraft.

A fatigue crack that started atthe site of a lightning strike is

shown below.

J E i I i


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Jet Engine Inspection Aircraft engines are overhauled

after being in service for a period

of time. They are completely disassembled,

cleaned, inspected and thenreassembled.

Fluorescent penetrant inspection

is used to check many of the partsfor cracking.

Crash of United Flight 232


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Sioux City, Iowa, July 19, 1989A defect that went

undetected in anengine disk wasresponsible forthe crash ofUnited Flight 232.

Crash of United Flight 232

Pressure Vessel Inspection


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Pressure Vessel InspectionThe failure of a pressure vesselcan result in the rapid release of

a large amount of energy. Toprotect against this dangerousevent, the tanks are inspectedusing radiography andultrasonic testing.

Rail Inspection


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Rail Inspection

Special cars are used to

inspect thousands of milesof rail to find cracks thatcould lead to a derailment.

B id I i


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Bridge Inspection

The India has 2,78,000

highway bridges. Corrosion, cracking and

other damage can allaffect a bridgesperformance.

The collapse of the SilverBridge in 1967 resulted inloss of 47 lives.

Bridges get a visualinspection about every 2years.

Some bridges are fittedwith acoustic emissionsensors that listen forsounds of cracks growing.

Pipeline Inspection


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NDT is used to inspect pipelinesto prevent leaks that could

damage the environment. Visualinspection, radiography andelectromagnetic testing are someof the NDT methods used.

Remote visual inspection usinga robotic crawler.

Radiography of weld joints.

Magnetic flux leakage inspection.This device, known as a pig, isplaced in the pipeline and collectsdata on the condition of the pipe as itis pushed along by whatever is beingtransported.

Pipeline Inspection


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Questions?Thank YouAll the Best For your Exams

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