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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
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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.
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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.
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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.
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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).
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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