UT power point

7/28/2019 UT power point

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NON-DESTRUCTIVE TESTINGNDT

www.metallurgydata.blogfa.com


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NON-DESTRUCTIVE TESTING

Examination of materials and components insuch a way that allows material to be

examinated without changing or destroying

their usefulness


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NDT

Most common NDT methods:Penetrant Testing (PT)

Magnetic Particle Testing (MT)

Eddy Current Testing (ET)

Radiographic Testing (RT)

Ultrasonic Testing (UT)

Mainly used for

surface testing

Mainly used forInternal Testing


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NDT

Which NDT method is

the best ?

Depends on many

factors and conditions


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Basic Principles of Ultrasonic

Testing

To understand and

appreciate the

capability and

limitation of UT


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History of Ultrasonic Testing (UT)

First came sonic testing

The piezo-electric effect discovered in

1880/81

Marine echo sounding developed from 1912

In 1929 Sokolov used vibrations in metals to

find flaws

Cathode ray tubes developed in the 1930s

Sproule made the first flaw detector in 1942


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Ultrasonic Inspection Sub-surface detection

This detection method uses high frequency soundwaves, typically above 2MHz to pass through a material

A probe is used which contains a piezo electric crystal

to transmit and receive ultrasonic pulses and display the

signals on a cathode ray tube or digital display The actual display relates to the time taken for the

ultrasonic pulses to travel the distance to the interface

and back

An interface could be the back of a plate material or adefect

For ultrasound to enter a material a couplant must be

introduced between the probe and specimen


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Ultrasonic InspectionUT Set, DigitalPulse echo

signals

A scan Display

Compression probe Thickness checking the material


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Ultrasonic Inspection

defect

0 10 20 30 40 50

defect

echoBack wall

echo

CRT DisplayCompression Probe

Material Thk

initial pulse


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Basic Principles of Ultrasonic TestingThe distance the sound traveled can be displayed on the Flaw Detector

The screen can be calibrated to give accurate readings of the distance

Bottom / Backwall

Signal from the backwall


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Basic Principles of Ultrasonic TestingThe presence of a Defect in the material shows up on the screen of

the flaw detector 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

to the marker on the screen

0 10 20 30 40 50 60

60 mm


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Thickness / depth measurement

A

A

B

B

C

C

The THINNER the material

the less distance the soundtravel

The closerthe reflector

to the surface, the signalwill be more to the left of

the screen

The thickness is read from the screen

684630


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Angle Probe

UT SetA Scan

Display


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0 10 20 30 40 50

initial pulse defect echo

CRT Display

sound path

Angle Probe

defect

Surface distance


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Advantages

Rapid results

Sub-surface detection

Safe

Can detect planar defectCapable of measuring the

depth of defects

May be battery poweredPortable

Disadvantages

Trained and skilledoperator required

Requires high operator

skillGood surface finish

required

Difficulty on detecting

volumetric defectCouplant may

contaminate

No permanent record


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Ultrasonic Testing

Principles of Sound


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What is Sound ?

A mechanical vibration

The vibrations create Pressure Waves

Sound travels faster in more elasticmaterials

Number of pressure waves per second is

the Frequency Speed of travel is the Sound velocity


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Sound

Wavelength :The distance required to complete a cycle Measured in Meter or mm

Frequency :

The number of cycles per unit time Measured in Hertz (Hz) or Cycles per second (cps)

Velocity :

How quick the sound travelsDistance per unit time

Measured in meter / second (m / sec)


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f

V

Velocity

Frequency

Wavelength


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Sound waves are the vibration of particles in solids liquids or gases

Particles vibrate about a mean position

In order to vibrate they require mass and resistance to change

One cycle

Sound Waves


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Properties of a sound wave

Sound cannot travelin vacuum

Sound energy to be

transmitted /

transferred from one

particle to another

SOLID LIQUID GAS


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Velocity

The velocity of sound in a particular material is CONSTANT

It is the product ofDENSITYand ELASTICITY of thematerial

It will NOT change if frequency changes

Only the wavelength changes

Examples:V Compression in steel : 5960 m/s

V Compression in water : 1470 m/s

V Compression in air : 330 m/s

STEEL WATER AIR

5 M Hz


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Sound travelling through a material

Velocity varies according to the material

Compression waves

Steel 5960m/sec

Water 1470m/sec

Air 344m/sec

Copper 4700m/sec

Shear waves

Steel 3245m/sec

Water NA

Air NA

Copper 2330m/sec


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Ultrasonic

Sound : mechanical vibration

What is Ultrasonic?

Very High Frequency sound above 20 KHz

20,000 cps


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Acoustic Spectrum

0 10 100 1K 10K 100K 1M 10M 100m

Sonic / Audible

Human

16Hz - 20kHz

Ultrasonic

> 20kHz = 20,000Hz

Ultrasonic Testing

0.5MHz - 50MHz

Ultrasonic : Sound with frequency above 20 KHz


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Frequency

Frequency : Number of cycles per

second

1 second

1 cycle per 1 second =

1 Hertz

18 cycle per 1 second

= 18 Hertz

3 cycle per 1 second =

3 Hertz

1 second 1 second

THE HIGHER THE FREQUENCY THE SMALLER THE

WAVELENGTH

Pg 21


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Frequency

1 Hz = 1 cycle per second

1 Kilohertz = 1 KHz = 1000Hz

1 Megahertz = 1 MHz = 1000 000Hz

20 KHz = 20 000 Hz

5 M Hz = 5 000 000 Hz

Pg 21

ULTRASONIC TESTING


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DRUM BEAT

Low Frequency Sound

40 Hz

Glass

High Frequency

5 K Hz

ULTRASONIC TESTING

Very High Frequency

5 M Hz


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Wavelength and frequency

The higher the frequency the smaller thewavelength

The smaller the wavelength the higher the

sensitivity

Sensitivity : The smallest detectable

flaw by the system or

technique

In UT the smallest detectable flaw is (half the wavelength)


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High Frequency Sound

f

V

5MHz compression

wave probe in steel

mm18.1000,000,5

000,900,5


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Frequency

1 M Hz 5 M Hz 10 M Hz 25 M Hz

Which probe has the smallest wavelength?

SMALLESTLONGEST

Which probe has the longest wavelength?

= v / fF F


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Which of the following compressional

probe has the highest sensitivity?

1 MHz

2 MHz

5 MHz

10 MHz

10MHz


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4 times

What is the velocity difference in steel compared with inwater?

If the frequency remain constant, in what material doessound has the highest velocity, steel, water, or air?

Steel

If the frequency remain constant, in what material does

sound has the shortest wavelength, steel, water, or air?

Air

Remember the formula

= v / f


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Sound Waveforms

Sound travels in different waveforms indifferent conditions

Compression wave

Shear waveSurface wave

Lamb wave


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Compression / Longitudinal

Vibration and propagation in the samedirection / parallel

Travel in solids, liquids and gases

Propagation

Particle vibration


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Shear / Transverse Vibration at right angles / perpendicular to

direction of propagation Travel in solids only

Velocity 1/2 compression (same material)

Propagation

Particle vibration


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Compression v Shear

Frequency 0.5MHz

1 MHz

2MHz 4MHz

6MHZ

Compression 11.8

5.9

2.95 1.48

0.98

Shear 6.5

3.2

1.6 0.8

0.54

The smaller the wavelength the better the

sensitivity


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Sound travelling through a material

Velocity varies according to the material

Compression waves

Steel 5960m/sec

Water 1470m/sec

Air 344m/sec

Copper 4700m/sec

Shear waves

Steel 3245m/sec

Water NA

Air NA

Copper 2330m/sec


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Surface Wave Elliptical vibration

Velocity 8% less than shear

Penetrate one wavelength deep

Easily dampened by heavy grease or wet finger

Follows curves but reflected by sharp corners or

surface cracks


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Lamb / Plate Wave Produced by the manipulation of surface

waves and others Used mainly to test very thin materials /

plates

Velocity varies with plate thickness andfrequencies

SYMETRIC ASSYMETRIC


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The Sound Beam

Dead Zone

Near Zone orFresnel Zone

Far Zone orFraunhoferZone


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Sound Beam

Near Zone

Thickness

measurement

Detection of defects Sizing of large

defects only

Far Zone

Thickness

measurement

Defect detection Sizing of all defects

Near zone length as small aspossible balanced againstacceptable minimumdetectable defect size


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The Sound Beam

NZ FZ

Distance

Intensityvaries

Exponential Decay

Main

Beam


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Main Lobe

Side Lobes

Near

Zone

Main Beam

The main beam or the centre

beam has the highest

intensity of sound energy

Any reflector hit by the main

beam will reflect the highamount of energy

The side lobes has multi

minute main beams

Two identical defects may

give different amplitudes of

signals


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Near Zone

V

fD

f

V

D

4Near Zone

4Near Zone

2

2


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Near Zone

What is the near zone length of a 5MHzcompression probe with a crystal diameter

of 10mm in steel?

mm

VfD

1.21

000,920,54

000,000,510

4Near Zone

2

2

N Z


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Near Zone

The bigger the diameter the bigger the

near zone

The higher the frequency the bigger the

near zone

The lower the velocity the bigger the near

zone

V

fDD

44Near Zone

22


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1 M Hz 5 M Hz

1 M Hz

5 M Hz

Which of the above probes has the longest Near Zone ?


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Beam Spread In the far zone sound pulses spread out

as they move away from the crystal

DfKV

DKSine or

2

/2


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Beam Spread

DfKV

DKSine or

2

Edge,K=1.22

20dB,K=1.08

6dB,K=0.56

Beam axis

or Main Beam


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Beam Spread What is the beam spread of a 10mm,5MHz

compression wave probe in steel?

o

Df

KV

Sine

35.71278.0

105000

592008.1

2

Whi h f th b b h th L t B


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1 M Hz 5 M Hz

1 M Hz

5 M Hz

Which of the above probes has the Largest Beam

Spread ?


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Beam Spread

The bigger the diameter the smaller thebeam spread

The higher the frequency the smaller

the beam spread

DfKV

DKSine or

2

Which has the larger beam spread, a compression

or a shear wave probe?

Ultrasonic Pulse


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Ultrasonic Pulse A short pulse of electricity is applied to a

piezo-electric crystal The crystal begins to vibration increases

to maximum amplitude and then decays

Maximum

10% of

Maximum

Pulse length


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Pulse Length

P l L th


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Pulse Length The longer the pulse, the more

penetrating the sound

The shorter the pulse the better thesensitivity and resolution

Short pulse, 1 or 2 cycles Long pulse 12 cycles


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Pulse Length


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Ideal Pulse Length

5 cycles for weld testing

Resolution


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ResolutionRESOLUTION in Pulse Echo Testing is the ability to

separate echoes from two or more closely spacedreflectors.

RESOLUTION is strongly affected by Pulse Length:

Short Pulse Length - GOOD RESOLUTIONLong Pulse Length - POOR RESOLUTION

RESOLUTION is an extremely important property in

WELD TESTING because the ability to separate ROOTGEOMETRY echoes from ROOT CRACK or LACK OFROOT FUSION echoes largely determines theeffectiveness of Pulse Echo UT in the testing of singlesided welds.


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Resolution

Good resolution


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Resolution

Poorresolution


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Loses intensitydue to

Soundtravellingthroughamaterial

AttenuationSound beam comparable

to a torch beam

Reduction differs for small

and large reflectors

Energy losses due to

material

Made up of absorption

and scatter

BeamSpread

S


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Scatter

The bigger the grain

size the worse the

problem

The higher the

frequency of theprobe the worse the

problem

1 MHz 5 MHz

B S d


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Beam Spread

The sound beam

spread out and the

intensity decreases

Beam spread and Attenuation


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Beam spread and Attenuation

combinedRepeat Back-wall Echoes Beyond The Near Zone

ZERO ATTENUATION ATTENUATION0.02 dB/mm


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Sound at an Interface

Sound will be either transmitted acrossor reflected back

Reflected

Transmitted

Interface

How much is reflected and

transmitted depends upon the

relative acoustic impedance ofthe 2 materials


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Acoustic Impedance

Definition

The Resistance to the

passage of sound

within a material

Formula

VZ

Measured in

kg / m2 x sec

Steel 46.7 x 106

Water 1.48 x 106

Air 0.0041 x 106

Perspex 3.2 x 106

= Density , V = Velocity

% Sound Reflected at an


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% Sound Reflected at an

Interface

reflectedZZ

ZZ%100

2

21

21

% Sound Reflected + % Sound Transmitted = 100%

Therefore

% Sound Transmitted = 100% - % Sound Reflected

How much sound is reflected at a steel to water


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How much sound is reflected at a steel to water

interface?

Z1 (Steel) = 46.7 x 10

6

Z2 (Water) =1.48 x 10

6

reflected%10048.17.46

48.17.462

reflected%100

18.48

22.452

reflected%88.0910093856.02


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How much sound transmitted?

100 % - the reflected sound

Example : Steel to water

100 % - 88 % ( REFLECTED) = 12 % TRANSMITTED

The BIGGER the Acoustic Impedance Ratio

orDifference between the two materials:

More sound REFLECTED than transmitted.


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Steel

AirSteel

Air

Steel

Steel Aluminum

Steel

Large Acoustic Impedance

Ratio

Large Acoustic Impedance

Ratio

No Acoustic Impedance

Difference

Small Acoustic Impedance

Difference


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InterfaceBehaviour

Similarly:

At an Steel - Air interface 99.96% of the

incident sound is reflected

At a Steel - Perspex interface 75.99% ofthe incident sound is reflected


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Sound Intensity

2 signals at 20% and 40% FSH


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1

0

10..20

H

HLogdB

2 signals at 20% and 40% FSH.

What is the difference between them in dBs?

2..2020

4020

1010..LogLogdB

3010.020dB

dBdB 6

2 signals at 10% and 100% FSH


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1

0

10..20

H

HLogdB

2 signals at 10% and 100% FSH.

What is the difference between them in dBs?

10..2010

10020

1010..LogLogdB

120dB

dBdB 20


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Amplitude ratios in decibels

2 : 1 = 6bB

4 : 1 = 12dB

5 : 1 = 14dB

10 : 1 = 20dB

100 : 1 = 40dB

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