BMFB 4283 NDT & FAILURE ANALYSIS

BMFB 4283NDT & FAILURE ANALYSIS

Lectures for Week 4

Prof. Qumrul Ahsan, PhD Department of Engineering MaterialsFaculty of Manufacturing Engineering

4.0 Ultrasound 4.1 Introduction4.2 Theory4.3 Equipment4.4 Inspection Principles 4.5 Applications

Issues to address

Ultrasonic Testing

Introduction

• This module presents an introduction to the NDT method of ultrasonic testing.

• Ultrasonic testing uses high frequency sound energy to conduct examinations and make measurements.

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

• A considerable amount of information about the part being examined can be collected, such as the presence of discontinuities, part or coating thickness; and acoustical properties can often be correlated to certain properties of the material.

Ultrasound – An Introduction• Ultrasonic Testing (UT) uses high frequency sound

(ultrasound) energy to conduct examinations and make measurements.

Ultrasonic inspection can be used for flaw detection/ evaluation, dimensional measurements, material characterization, and more.

Basic Principles of Sound•Sound is produced by a vibrating body and travels in the form of a wave.•Sound waves travel through materials by vibrating the particles that make up the material.•The pitch of the soundis determined by the frequency of the wave (vibrations or cycles completed in a certain period of time). •Ultrasound is soundwith a pitch too highto be detected by the human ear.

Basic Principles of Sound (cont.)

• The measurement of sound waves from crest to crest determines its wavelength (λ).

• The time it takes a sound wave to travel a distance of one complete wavelength is the same amount of time it takes the source to execute one complete vibration.

• The sound wavelengthis inversely proportional to its frequency. (λ = 1/f)

• Several wave modes of vibration are used in ultrasonic inspection.The most common arelongitudinal, shear, andRayleigh (surface) waves.

direction ofosscillation

direction of propagation

wave length

Longitudinal wave• a wave formed by individual

particles oscillating in the direction of propagation

• propagated through solids, liquids and gases

• This wave is the most easily generated and detected.

• Almost all of the sound energy used in UT originates as longitudinal sound and then may be converted to the other modes for special applications.

direction ofosscillation

direction of propagation

wave length

Transverse wave• particle motion is

transverse or at an angle to the direction of propagation

• only in solids because distance between molecules is so great in liquid and gases

• This wave is the most easily generated and detected

• speed about half of longitudinal

Surface wave• Rayleigh wave

– elliptical particle motion– penetration approx.

one wavelength– velocity of approx. 90

percent of shear wave– Reflection of Rayleigh

waves from cracks on the surface or from sub-surface discontinuities may be seen on the CRT

Direction of propagationParticle movement

Lamb wave (Plate wave)• generated in a relatively

thin solid substance whose thickness is about one wavelength

• consist of a mixture of zig-zag reflected longitudinal and transverse

• detect surface and sub-surface discontinuities

Summary of WavesThe table below summarizes many, but not all, of the wave modes possible in solids.

Wave Types in Solids Particle Vibrations

Longitudinal Parallel to wave directionTransverse (Shear) Perpendicular to wave directionSurface - Rayleigh Elliptical orbit - symmetrical mode Plate Wave - Lamb Component perpendicular to surface

(extensional wave) Plate Wave - Love Parallel to plane layer, perpendicular

to wave direction

Stoneley (Leaky Rayleigh Waves)

Wave guided along interface

Sezawa Antisymmetric mode

Frequency, Wavelength and Velocity• Frequency is the number of oscillation

in one sec • Period is the time is to make on

oscillation t = 1/f• Wavelength expressed as is given as

the distance between two successive crests in the waveform, this distance varies with frequency and velocity

fλV

One cycle

Time

t * λV • The velocity of sound propagation varies from one material to another.– It depends on the elastic property and

density of the material.

ρEVp

Sound velocity in various materials

Materials Long. Velocity (m/s) Shear Velocity (m/s)AirAluminiumBrass (70-30)Cast IronCopperGoldIronLeadOilPerspexSteel – mildSteel – stainlessWaterTungstenZincZirconium

322640043723500476032405957240014402740596057401480517441704650

-313021002200232512003224790-132032403130-288024802300

Acoustic ImpedanceSound travels through materials under the influence of sound pressure. Because molecules or atoms of a solid are bound elastically to one another, the excess pressure results in a wave propagating through the solid and the impedance restrict movement of velocity.

The acoustic impedance (Z) of a material is defined as the product of its density () and acoustic velocity (v).

Z = Driving pressure/velocity of particle = P/v or V

Acoustic impedance is important inthe determination of acoustic transmission and

reflection at the boundary of two materials having different acoustic impedances.

the design of ultrasonic transducers. assessing absorption of sound in a medium.

Features of Ultrasound• Ultrasonic waves are very similar to light

waves in that they can be reflected, refracted, and focused.

• Reflection and refraction occurs when sound waves interact with interfaces of differing acoustic properties.

• In solid materials, the vibrational energy can be split into different wave modes when the wave encounters an interface at an angle other than 90 degrees.

• Ultrasonic reflections from the presence of discontinuities or geometric features enables detection and location.

• The velocity of sound in a given material is constant and can only be altered by a change in the mode of energy.

BEHAVIOR OF SOUND WAVES

• Behavior of sound waves:

–Reflection–Refraction–Diffraction–Mode conversion–Attenuation

• When an ultrasonic wave incidents normal to an interface between two materials and the materials have different acoustic impedance , both reflected and transmitted waves are produced.

• The difference in Z is commonly referred to as the impedance mismatch.

• The greater the impedance mismatch, the greater the percentage of energy that will be reflected at the interface

Reflection at Normal Incidence

Reflection at Normal Incidence• The fraction of the incident wave intensity that is reflected

can be calculated with the equation below and known as the reflection coefficient

• Since the amount of reflected energy plus the transmitted energy must equal the total amount of incident energy, the transmission coefficient is calculated by simply subtracting the reflection coefficient from one.

T = 1 - R = 4Z1Z2 / [Z2+ Z1]2

Reflection at Oblique Incidence

• The reflected energy follows the same laws as light, i.e. the angle of incidence is equal to the angle of reflection

When an ultrasonic wave passes through an interface between two materials at an oblique angle, and the materials have different indices of refraction, both reflected and refracted waves are produced.

• Refraction takes place at an interface due to the different velocities of the acoustic waves within the two materials.

• The refraction can be calculated by Snell’s Law as shown in the following equation.

Refraction at Oblique Incidence

Where:VL1 is the longitudinal wave velocity in material 1.VL2 is the longitudinal wave velocity in material 2.

MODE CONVERSION• Ultrasonic energy

when reflected, may change from one waveform to another (compressional to shear, shear to surface, etc)

• This mode change is accompanied by the appropriate change in velocity.

L

LL

L

L

medium 1medium 2

reflectedwave

refractedwave

incidentwave

Critical Angles

T

L

T

perspex

steel

reflectedwave

refractedtransverse wave

incidentwave

= 36.4°T = 45°

T

L

L

T

Lperspexsteel

reflectedwave

refractedwaves

incidentwave

= 1 = 27.5°° (first critical angle) T = 33.3L = 90°

L

T O

surfacewaveperspex

steel

reflectedwave

incidentwave

= 2 = 57° ( second critical angle) T = 90°

27,5°

57°

33,3°

90°perspex

steel

L

T

Ranges for incident waves

DIFFRACTION• Diffraction is the

apparent bending of sound waves around the tips of a narrow reflector.

• Diffuse reflection or scatter occurs at these positions resulting in a small amount of energy being bent around the defect

ATTENUATION• Attenuation is the loss intensity of

the ultrasonic beam as it passes through a material and is dependent on the physical properties of the material.

• Two factors affect the sound attenuation:• Absorption

– Cause by the interaction of the particles as they vibrate during the passage of sound waves

– The movement of particles cause friction, and dissipated as heat– As the frequency increases, the absorption greater due to raid

movement• Scatter

– Cause by grain boundaries, porosity, non metallic inclusions, etc– Larger grain size greater scattering– Coarse grain will be more attenuative than fine grain material– Rough surface also cause attenuation

Attenuation• Attenuation can be represented as a decaying exponential. The

amplitude change of a decaying plane wave can be expressed as:

• A0 is the unattenuated amplitude of the propagating wave at some location.

• The amplitude A is the reduced amplitude after the wave has traveled a distance z from that initial location.

• The quantity is the attenuation coefficient of the wave traveling in the z-direction. The dimensions of are nepers/length, where a neper is a dimensionless quantity.

Attenuation can also measured in terms of decibels (dB)

dB = 20 log (I0/I)

Ultrasound Generation

The transducer is capable of both transmitting and receiving sound energy.

Ultrasound is generated with a transducer. A piezoelectric element

in the transducer converts electrical energy into mechanical vibrations (sound), and vice versa.

Sound beam

• Since the ultrasound originates from a number of points along the transducer face, the ultrasound intensity along the beam is affected by constructive and destructive wave interference . These are sometimes also referred to as diffraction effects.

• This wave interference leads to extensive fluctuations in the sound intensity near the source and is known as the near field. Because of acoustic variations within a near field, it can be extremely difficult to accurately evaluate flaws in materials when they are positioned within this area.

Where: N = Near Field Length or Transition from Near Field to Far Field D = Diameter of the Transducer F = Frequency of the Transducer Wavelength, V Velocity of Sound in the Material

Sound beam• The area beyond the near field where the ultrasonic beam is more

uniform is called the far field. The area just beyond the near field is where the sound wave is well behaved and at its maximum strength. Therefore, optimal detection results will be obtained when flaws occur in this area.

Where: N = Near Field Length or Transition from Near Field to Far Field D = Diameter of the Transducer F = Frequency of the Transducer Wavelength, V Velocity of Sound in the Material

The transition between the near field and the far field occurs at a distance, N, and is sometimes referred to as the "natural focus" of a flat (or unfocused) transducer.

Sound beam

• Far zone is beyond near zone. The beam diverges resulting in decay in sound intensity as the distance from the crystal is increased. In the far zone, large and small reflectors follow different laws:– Large reflectors: follow inverse law- the amplitude is inversely

proportional to the distance– Small reflectors: follow inverse square law- the amplitude is

inversely proportional to square of the distance.

near field

far fieldacoustical axis (central beam)

N = near field length

= angle of divergence

Beam Divergence/Beam Spread• the energy in the beam does not

remain in a cylinder, but instead spreads out as it propagates through the material.

• The phenomenon is usually referred to as beam spread but is sometimes also referred to as beam divergence or ultrasonic diffraction.

• beam spread is twice the beam divergence.

Where: θ = Beam divergence angle from centerline to point where signal is at half strength.

V = Sound velocity in the material. (inch/sec or cm/sec)1

a = Radius of the transducer. (inch or cm)1 F = Frequency of the transducer. (cycles/second)

0 2 4 6 8 10

dead zone

Dead zone

The dead zone is the ringing time of the crystal and is minimized by the damping medium behind the crystal. It is not possible to detect defect in this zone.

Principle of Ultrasonic Inspection• Ultrasonic waves are introduced into a material

where they travel in a straight line and at a constant 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 the size of the reflector.

• The travel time of the sound can be measured and this provides information on the distance that the sound has traveled.

start signal(pulse)

finish signal(echo)

transmitter

transit timemeasurement

probe

work piece

sound transitpath

Stop-watch

Principle of transit time measurement

EquipmentEquipment for ultrasonic testing is very diversified. Proper selection is important to insure accurate inspection data as desired for specific applications.In general, there are three basic components that comprise an ultrasonic test system:

- Instrumentation- Transducers- Calibration Standards

Transducers• Transducers are manufactured in a variety of

forms, shapes and sizes for varying applications. • Transducers are categorized in a number of ways

which include: - Contact or immersion - Single or dual element- Normal or angle beam

• In selecting a transducer for a given application, it is important to choose thedesired frequency, bandwidth, size, and in some cases focusing which optimizes the inspection capabilities.

Chac: of TransducersSome transducers are specially fabricated to be more efficient

transmitters and others to be more efficient receivers.

A transducer that performs well in one application will not always produce the desired results in a different application.

For example, sensitivity to small defects is proportional to the product of the efficiency of the transducer as a transmitter and a receiver.

Resolution, the ability to locate defects near the surface or in close proximity in the material, requires a highly damped transducer.

Highly damped transducers will respond to frequencies above and below the central frequency. The broad frequency range provides a transducer with high resolving power. Less damped transducers will exhibit a narrower frequency range and poorer resolving power, but greater penetration.

Chac: of Transducers The central frequency will also define the capabilities of a transducer. Lower frequencies (0.5MHz-2.25MHz) provide greater energy and penetration in a material, while high frequency crystals (15.0MHz-25.0MHz) provide reduced penetration but greater sensitivity to small discontinuities.

High frequency transducers, when used with the proper instrumentation, can improve flaw resolution and thickness measurement capabilities dramatically.

Broadband transducers with frequencies up to 150 MHz are commercially available.

Transducers are constructed to withstand some abuse, but they should be handled carefully. Misuse, such as dropping, can cause cracking of the wear plate, element, or the backing material. Damage to a transducer is often noted on the A-scan presentation as an enlargement of the initial pulse.

Contact TransducersContact transducers are designed to withstand rigorous use, and usually have a wear plate on the bottom surface to protect the piezoelectric element from contact with the surface of the test article. Many incorporate ergonomic designs for ease of grip while scanning along the surface.

Probe construction - Contact Transducers

• The cork separator and corrugations in the Perspex reduce the cross-talk/chatter between crystals

• The probe are used mainly for testing thin sections (thickness measurement) and to detect near surface defect

Contact Transducers (cont.)• A way to improve near surface

resolution with a single element transducer is through the use of a delay line.

• Delay line transducers have a plastic piece that is a sound path that provides a time delay between the sound generation and reception of reflected energy.

• Interchangeable pieces make it possible to configure the transducer with insulating wear caps or flexible membranes that conform to rough surfaces.

• Common applications include thickness gauging and high temperature measurements.

socket

matching-element

damping-block

crystalprotecting face(probe delay)

housing

workpiece Sound pulse

Straight beam probe

Probe construction - Contact Transducers

• Contact transducers are available with two piezoelectric crystals in one housing. These transducers are called dual element transducers.

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

• This arrangement improves near surface resolution because the second transducer does not need to complete a transmit function before listening for echoes.

• Dual elements are commonly employed in thickness gauging of thin materials.

receiversocket

transmittersocket

damping blocks

crystal

delay

acousticalbarrier

TR-probe / dual crystal probe

0 2 4 6 8 10

IPBE

work piece

TR-probe

Probe delay with

TR-probes

0 2 4 6 8 10

IPBE

flaw

cross talkecho

flawecho

TR-probe

Cross talk at high gain

Transducers (cont.)• Angle beam transducers

incorporate wedges to introduce a refracted shear wave into a material.

• The incident wedge angle is used with the material velocity to determine the desired refracted shear wave according to Snell’s Law)

• Transducers can use fixed or variable wedge angles.

• Common application is in weld examination.

crystalperspex wedge(probe delay)

damping blocks

socket

housing

workpiece Sound pulse

Angle beam probe

Transducers (cont.)• Immersion transducers

are designed to transmit sound whereby the transducer and test specimen are immersed in a liquid coupling medium (usually water).

• Immersion transducersare manufactured withplanar, cylindrical or spherical acoustic lenses (focusing lens).

Selection of Probe FrequencyLow frequency probe High frequency probe

Long wavelength Short wavelength

More beam spread Less beam spread

Shorter near zone Longer near zone

Better penetration Shorter penetration

Less attenuation More attenuation

Longer dead zone Shorter dead zone

Less sensitivity High sensitivity

Instrumentation• Test equipment can be classified in a

number of different ways, this may include portable or stationary, contact or immersion, manual or automated.

• Further classification of instruments commonly divides them into four general categories: D-meters, Flaw detectors, Industrial and special application.

Pulser- Receivers• Ultrasonic pulser-receivers

can be used for flaw detection and thickness gauging in a wide variety of metals, plastics, ceramics, and composites.

• The pulser section of the instrument generates short, large amplitude electric pulses of controlled energy, which are converted into short ultrasonic pulses when applied to an ultrasonic transducer.

• Control functions associated with the pulser circuit include:Pulse length or damping (The amount of time the pulse is applied to the transducer.) Pulse energy (Typical pulser circuits will apply from 100 volts to 800 volts to a transducer.)

Pulser- Receivers In the receiver section the voltage

signals produced by the transducer, which represent the received ultrasonic pulses, are amplified.

The amplified radio frequency (RF) signal is available as an output for display or capture for signal processing.

Control functions associated with the receiver circuit include :Signal rectification (The RF signal can be viewed as positive half wave, negative half wave or full wave.) Filtering to shape and smooth return signals Gain, or signal amplification Reject control

0 2 4 6 8 10

work pieceprobe

sound wave starts at crystal

light point

transmittertransmission

pulse

Instrumentation (cont.)• D-meters or digital

thickness gauge instruments provide the user with a digital (numeric) readout.

• They are designed primarily for corrosion/erosion inspection applications.

• Some instruments provide the user with both a digital readout and a display of the signal. A distinct advantage of these units is that they allow the user to evaluate the signal to ensure that the digital measurements are of the desired features.

Instrumentation (cont.)• Flaw detectors are

instruments designed primarily for the inspection of components for defects.

• However, the signal can be evaluated to obtain other information such as material thickness values.

• Both analog and digital display.

• Offer the user options of gating horizontal sweep and amplitude threshold.

Instrumentation (cont.)• Industrial flaw

detection instruments, provide users with more options than standard flaw detectors.

• May be modulated units allowing users to tailor the instrument for their specific needs.

• Generally not as portable as standard flaw detectors.

Instrumentation (cont.)• Immersion ultrasonic scanning

systems are used for automated data acquisition and imaging.

• They integrate an immersion tank, ultrasonic instrumentation, a scanning bridge, and computer controls.

• The signal strength and/or the time-of-flight of the signal is measured for every point in the scan plan.

• The value of the data is plotted using colors or shades of gray to produce detailed images of the surface or internal features of a component.

Data Presentation• Information from ultrasonic testing can be

presented 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 next few slides.

Data Presentation - A-scan• A-scan presentation

displays the amount of received ultrasonic energy as a function of time.

• Relative discontinuity size can be estimated by comparing the signal amplitude to that from a known reflector.

• Reflector depth can be determined by the position of the signal on the horizontal sweep.

Time

Sign

al A

mpl

itude

Time

Data Presentation - B-scan• B-scan presentations display a profile

view (cross-sectional) of a test specimen.

• Only the reflector depth in the cross-section and the linear dimensions can be determined.

• A limitation to this display technique is that reflectors may be masked by larger reflectors near the surface.

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 an automated data acquisition system, such as in immersion scanning.

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

• The C-scan presentation provides an image of the features that reflect and scatter the sound within and on the surfaces of the test piece.

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)

COUPLING

• Water, glycerine, oils, petroleum grease, silicon grease, wall paper paste, etc.

• Water with wetting agent suitable for smooth surface

• Grease/heavy oil for hot, vertical or rough surface

• Ultrasonic testing is a very versatile inspection method, and inspections can be accomplished in a number of different ways.

• Ultrasonic inspection techniques are commonly divided 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 test article)

Test Techniques

Test Techniques – Pulse-Echo (cont.)

Digital display showing signal generated from sound reflecting off back surface.

Digital display showing the presence of a reflector midway through material, with lower amplitude back surface reflector.The pulse-echo technique allows testing when access to only one side of the material is possible, and it allows the location of reflectors to be precisely determined.

Digital display showing received sound through material thickness.

Digital display showing loss of received signal due to presence of a discontinuity in the sound field.

Test Techniques – Through-Transmission

Test Techniques – Normal and Angle Beam

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

• In angle beam testing, the sound beam is introduced into the test article at some angle other than 90.

• The choice between normal and angle beam inspection usually depends on two considerations: - The orientation of the feature of

interest – the sound should be directed to produce the largest reflection from the feature.

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

NORMAL BEAM TECHNIQUE• Thickness Measurement

– Single Crystal Probe• normally for wall thickness more than 30mm• single backwall• multiple backwall - eliminate paint thickness

– Twin crystal (TR) Probe• normally for thickness less than 30mm• be careful with cross talk echo

– Velocity correctionSpecimen thickness = indicated thick X Velocity in specimen

Velocity in cal. block

ANGLE BEAM TECHNIQUE• Angle Beam Transducers

and wedges are typically used to introduce a refracted shear wave into the test material.

• An angled sound path allows the sound beam to come in from the side, thereby improving detect-ability of flaws in and around welded areas.

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 air

between 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 transducer and the part.

• In immersion testing, the part and the transducer are place in a water bath. This arrangement allows better movement of the transducer while maintaining consistent coupling.

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

Calibration Standards• Calibration is a operation of configuring the

ultrasonic test equipment to known values. This provides the inspector with a means of comparing test signals to known measurements.

• Calibration standards come in a wide variety of material types, and configurations due to the diversity of inspection applications.

• Calibration standards are typically manufactured from materials of the same acoustic properties as those of the test articles.

• To produce reliable and reproducible test results

Calibration Standards (cont.)Thickness calibration standards may be flat or curved for pipe and tubing applications, consisting of simple variations in material thickness.

Distance/Area Amplitude standards utilize flat bottom holes or side drilled holes to establish known reflector size with changes in sound path form the entry surface.

ASTM Distance/Area Amplitude

NAVSHIPS

Calibration Standards (cont.)There are also calibration standards for use in angle beam inspections when flaws are not parallel to entry surface. These standards utilized side drilled holes, notches, and geometric configuration to establish time distance and amplitude relationships.

IIW

DSC DC Rhompas

SC

ASME Pipe Sec. XI

Qualification StandardsQualification standards differ from calibration standards in that their use is for purposes of varying proper equipment operation and qualification of equipment use for specific codes and standards.

AWS Resolution

IOW Beam Profile

DC-dB Accuracy

Standard Test Blocks

• To perform– Equipment characteristic verification– Range/Time base calibration– Reference level or sensitivity setting

• Calibration block?• Reference block?

EQUIPMENT CHARACTERISTIC

• Horizontal linearity• Screen height linearity• Amplitude control linearity• Resolution• Maximum penetrating power• Pulse length

CALIBRATION-Normal probe

• Determine test range• Min. TR at least = block thickness• Calibration echoes – 1st and 2nd BWE

w

w

flaw

probe

beam path distance

CALIBRATION-Angle probe

• Use 100mm curvature of V1 or 25mm/50mm curvature of V2 block

• Range – min. 100mm for V1• Range – 100mm/125mm for V2

Beam profile

• Use IOW or ASME block

CALIBRATION

• Probe index

• Probe angle

0 2 4 6 8 10

0 100 mm50steel, L

div.

Range calibration

5 10 15

70°

45°

100 m

m

60°

Calibration block 1 with angle beam probes

5 10 15

0 2 4 6 8 10

100 mm

1st echo from circular section

5 10 15

0 2 4 6 8 10

100 mm 200 mm 300 mm

Echo sequence from 100 mm radius

s = 25 mms = 100 mms = 175 mmetc.

1

2

3

this wave will be absorbed !

25 mm radius of calibration block 2

s = 50 mms = 125 mms = 200 mmetc.

1

2

3

50 mm radius of calibration block 2

5 10 15

0 2 4 6 8 100 100 mm

steel

100 mm range calibration on V2

SENSITIVITY SETTING• Distance Amplitude Correction (DAC)

– Acoustic signals from the same reflecting surface will have different amplitudes at different distances from the transducer.

– Distance amplitude correction (DAC) provides a means of establishing a graphic ‘reference level sensitivity’ as a function of sweep distance on the A-scan display.

• The use of DAC allows signals reflected from similar discontinuities to be evaluated where signal attenuation as a function of depth has been correlated.

• Most often DAC will allow for loss in amplitude over material depth (time), graphically on the A-scan display

Inspection ApplicationsSome 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 some common applications of ultrasonic inspection.

Thickness Gauging• Ultrasonic thickness gauging

is routinely utilized in the petrochemical and utility industries to determine various degrees of corrosion/erosion.

• Applications include piping systems, storage and containment facilities, and pressure vessels.

0 2 4 6 8 10

work piece

probe

sound wave

transmitter

Principle, sound wave in the workpiece

0 2 4 6 8 10

work piece

probe

transmitter

Principle, sound pulse at the back wall

0 2 4 6 8 10

work piece

probe

transmitter

Principle, sound pulse at the coupling surface

0 2 4 6 8 10

work piece

probeback wall

echo

transmitter

Principle, echo display and 2nd run

10 155 10 155T R

a1

Tandem technique (top)

10 155 10 155T R

a2

Tandem technique (middle)

10 155 10 155T R

a 3

Tandem technique (bottom)

2 4 6 8 10

protecting face

crystal

probe

electricalzero

(initial pulse)

mechanicalzero

(surface)

Probe delay

electicalzero

(initial pulse)

mechanicalzero

(surface)sound wave

work piece

delay(wedge)

probe 0 2 4 6 8 10

Probe delay

0 2 4 6 8 10

work piece

probeback wall

echo

flaw

flawecho

Flaw location and echo display

0 2 4 6 8 10

work piece

probe

back wallecho

flaw

flawecho


0 2 4 6 8 10

flaw echocoveredby initialpulse

work piece

probe

back wallecho

flaw


0 2 4 6 8 10

back wallecho:

without

with flaw

work piece

probe

flaw


0 2 4 6 8 10

flaw echosequence

work piece

probe

flaw


0 2 4 6 8 10

10 20 30 40

LT

possible flawlocations

Angle beam probe with both wave types

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 delaminations on 36” rolled beam.

Flaw Detection in Welds• One of the most widely

used methods of inspecting weldments is ultrasonic inspection.

• Full penetration groove welds lend themselves readily to angle beam shear wave examination.

Flaw detection

10 20 30 40

Flaw detection

Bad flaw orientation

crack

Improper flaw location

10 20 30 40

Improper flaw orientation

10 20 30 40

Perfect flaw orientation

sound beam

flat defect

15105

reflected sound waves

Flaw detectability with improper flaw orientation

s

Flaw distance

10 20

Near surface detectability with angle beam pobes

10 20 30 40

crack

Angle reflection

Angle reflection

10 20 30 40

Vertical, near surface flaw

51015

work piece reflector

s

0 2 4 6 8 10

s = k•Rs = sound pathk = scale factorR = screen reading

Flaw location

flaw location

a

s d

Sound entry point projectionpoint

a = s•sin ßd = s•cos ß

ß

ßß = probe angles = sound patha = surface distanced = depth

"flaw triangle"

Flaw location with angle beam probes

51015

work piece reflector

aa'

s d

x

index point -front edge of probe

a = surface distancea' = reduced surface distancex = x-value = distance:

Flaw location with angle beam probes

51015

apparent flaw location

a

sd = apparent depth

T

Flaw location with an angle beam probe on a plate

51015

apparent flaw location

real flaw location

a

s dd'

d' = apparent depthd = real depthT = work piece thickness

T

a = s • sind‘ = s • cosd = 2T - d

Flaw location with an angle beam probe on a plate

Large defects parallel to the scanning surface

Scanning the edge of the defect

Flaw echo drops to 50% of its maximum value

"half value" positions

delaminationprobe positionwith echo amplitudereduced to 50 %

Determination of the defect area

0 2 4 6 8 100 2 4 6 8 10 0 2 4 6 8 10

IP BER IP BER IP BER

Flaw sizes and echo amplitudes

0 2 4 6 8 10

0 2 4 6 8 10

0 2 4 6 8 10

IP BER

IP BER

IP BER

Flaw distances and

echo amplitudes

0 500100 200 300 400

B 4 S6 mm

4 mm3 mm

BE

F

Distance amplitude curves on the CRT screen

0 2 4 6 8 10

IP BEF

F

instrument gain: G = 34 dB

80 %

Defect evaluation by comparison - 1

0 2 4 6 8 10

IP BER

instrument gain: 34 dB


0 2 4 6 8 10

IP BERE

+ 8 dB

instrument gain: 42 dB


0 2 4 6 8 10

10 20 30 40 10 20 30 40 10 20 30 40 10 20 30 40

1

1

2

2

3

3

4

4

Echo

Position

Distance amplitude curve (DAC)

0 2 4 6 8 10 0 2 4 6 8 10

time corrected gainDAC

DAC and TCG

Echodynamicpress ESC to quit

press ESC to quit Echodynamic

Advantages of Ultrasonic Testing• Sensitive to both surface and subsurface discontinuities.• Depth of penetration for flaw detection or measurement is

superior to other methods.• Only single-sided access is needed when pulse-echo

technique is used.• High accuracy in determining reflector position and

estimating 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, in

addition to flaw detection.

Limitations of Ultrasonic Testing• Surface must be accessible to transmit ultrasound.• Skill and training is more extensive than with some other

methods.• 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 to inspect.

• Cast iron and other coarse grained materials are difficult to 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 equipment calibration, and characterization of flaws.

BMFB 4283 NDT & FAILURE ANALYSIS

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Transcript of BMFB 4283 NDT & FAILURE ANALYSIS