بسمه تعالی بررسی آزمون های غیر مخرب non-destructive testingndt

تعالی تعالی بسمه بسمهمخرب غیر های آزمون مخرب بررسی غیر های آزمون بررسی

NON-DESTRUCTIVE TESTING

NDTNDT

www.metallurgydata.blogfa.comwww.metallurgydata.blogfa.com

NON-DESTRUCTIVE TESTING

Examination of materials and components in such a way that allows material to be examinated without changing or destroying their usefulness

NDTMost 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 for Internal Testing

NDT

• Which NDT method is the best ?

Depends on many factors and conditions

Basic Principles of Ultrasonic Testing

• To understand and appreciate the capability and limitation of UT

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 1930’s

• Sproule made the first flaw detector in 1942

Ultrasonic Inspection Sub-surface detection

This detection method uses high frequency sound waves, 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 a defect

For ultrasound to enter a material a couplant must be introduced between the probe and specimen

Ultrasonic InspectionUT Set, DigitalPulse echo

signals A scan Display

Compression probe Thickness checking the material

Ultrasonic Inspection

defect

0 10 20 30 40 50

defect echo

Back wall echo

CRT DisplayCompression Probe

Material Thk

initial pulse

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

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

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

Thickness / depth measurement

A

A

B

B

C

C

The THINNER the material the less distance the sound

travel

The closer the reflector to the surface, the signal will be more to the left of

the screen

The thickness is read from the screen

684630


Angle Probe

UT SetA Scan Display


0 10 20 30 40 50

initial pulse defect echo

CRT Display

sound path

Angle Probe

defect

Surface distance

Ultrasonic Inspection AdvantagesRapid resultsSub-surface detectionSafeCan detect planar defectCapable of measuring the

depth of defectsMay be battery poweredPortable

DisadvantagesTrained and skilled operator required

Requires high operator skill

Good surface finish required

Difficulty on detecting volumetric defect

Couplant may contaminate

No permanent record

Ultrasonic Testing

Principles of Sound

What is Sound ?

• A mechanical vibration

• The vibrations create Pressure Waves

• Sound travels faster in more ‘elastic’ materials

• Number of pressure waves per second is the ‘Frequency’

• Speed of travel is the ‘Sound velocity’

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 travels

Distance per unit time– Measured in meter / second (m / sec)

f

V

Velocity

Frequency

Wavelength

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

Particles vibrate about a mean positionParticles vibrate about a mean position

In order to vibrate they require mass and resistance to In order to vibrate they require mass and resistance to changechange

One cycle

Sound WavesSound Waves

Properties of a sound wave• Sound cannot travel

in vacuum• Sound energy to be

transmitted / transferred from one particle to another

SOLID LIQUID GAS

Velocity• The velocity of sound in a particular material is CONSTANT• It is the product of DENSITY and ELASTICITY of the

material• It will NOT change if frequency changes• Only the wavelength changes• Examples:

V Compression in steel : 5960 m/sV Compression in water : 1470 m/sV Compression in air : 330 m/s

STEEL WATER AIR

5 M Hz

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


Ultrasonic• Sound : mechanical vibration

What is Ultrasonic?

Very High Frequency sound – above 20 KHz

20,000 cps

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

Frequency• Frequency : Number of cycles per

second

1 second

1 cycle per 1 second = 1 Hertz



1 second 1 second

THE HIGHER THE FREQUENCY THE SMALLER THE WAVELENGTH

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

DRUM BEAT

Low Frequency Sound

40 Hz

Glass

High Frequency

5 K Hz

ULTRASONIC TESTING

Very High Frequency

5 M Hz

Wavelength and frequency• The higher the frequency the smaller the

wavelength

• 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 ½ (half the wavelength)

High Frequency Sound

f

V

5MHz compression wave probe in steel

mm18.1000,000,5

000,900,5

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 / f

F F

• Which of the following compressional probe has the highest sensitivity?

• 1 MHz

• 2 MHz

• 5 MHz

• 10 MHz

10 MHz

4 times

What is the velocity difference in steel compared with in water?

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

SteelIf the frequency remain constant, in what material does sound has the shortest wavelength, steel, water, or air?

Air

Remember the formula

= v / f

Sound Waveforms

Sound travels in different waveforms in different conditions

•Compression waveCompression wave•Shear waveShear wave•Surface waveSurface wave•Lamb waveLamb wave

Compression / Longitudinal

• Vibration and propagation in the same direction / parallel

• Travel in solids, liquids and gases

Propagation

Particle vibration

Shear / Transverse• Vibration at right angles / perpendicular to

direction of propagation • Travel in solids only• Velocity 1/2 compression (same material)

Propagation

Particle vibration

Compression v ShearFrequency• 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


• Velocity varies according to the material

Compression waves

• Steel 5960m/sec

• Water 1470m/sec

• Air 344m/sec


Shear waves

• Steel 3245m/sec

• Water NA

• Air NA


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

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 and

frequencies

SYMETRIC ASSYMETRIC

The Sound Beam

• Dead Zone

• Near Zone or Fresnel Zone

• Far Zone or Fraunhofer Zone

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 Near zone length as small as possible balanced as possible balanced against acceptable against acceptable minimum detectable defect minimum detectable defect sizesize

The Sound Beam

NZ FZ

Distance

Intensity varies

Exponential Decay

Main Beam

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 high amount of energy

The side lobes has multi minute main beams

Two identical defects may give different amplitudes of signals

Near Zone

V

fD

f

V

D

4Near Zone

4Near Zone

2

2

Near Zone

• What is the near zone length of a 5MHz compression probe with a crystal diameter of 10mm in steel?

mm

V

fD

1.21

000,920,54

000,000,510

4Near Zone

2

2

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

4

4Near Zone

22

1 M Hz 5 M Hz

1 M Hz

5 M Hz

Which of the above probes has the longest Near Zone ?

Beam Spread• In the far zone sound pulses spread out

as they move away from the crystal

Df

KV

D

KSine or

2

/2

Beam Spread

Df

KV

D

KSine or

2

Edge,K=1.2220dB,K=1.08

6dB,K=0.56

Beam axis or Main Beam

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

compression wave probe in steel?

o

Df

KVSine

35.7 1278.0

105000

592008.1

2

1 M Hz 5 M Hz

1 M Hz

5 M Hz

Which of the above probes has the Largest Beam Spread ?

Beam Spread

• The bigger the diameter the smaller the beam spread

• The higher the frequency the smaller the beam spread

Df

KV

D

KSine or

2

Which has the larger beam spread, a compression or a shear wave probe?

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

• Pulse Length

Pulse Length• The longer the pulse, the more

penetrating the sound

• The shorter the pulse the better the sensitivity and resolution

Short pulse, 1 or 2 cycles Long pulse 12 cycles

• Pulse Length

Ideal Pulse Length

5 cycles for weld testing5 cycles for weld testing

ResolutionRESOLUTIONRESOLUTION in Pulse Echo Testing is the ability in Pulse Echo Testing is the ability to separate echoes from two or more closely to separate echoes from two or more closely spaced reflectors.spaced reflectors.

RESOLUTION is strongly affected by Pulse RESOLUTION is strongly affected by Pulse Length:Length:

Short Pulse Length - GOOD RESOLUTIONShort Pulse Length - GOOD RESOLUTIONLong Pulse Length - POOR RESOLUTIONLong Pulse Length - POOR RESOLUTION

RESOLUTION is an extremely important property RESOLUTION is an extremely important property in WELD TESTING because the ability to in WELD TESTING because the ability to separate ROOT GEOMETRY echoes from ROOT separate ROOT GEOMETRY echoes from ROOT CRACK or LACK OF ROOT FUSION echoes largely CRACK or LACK OF ROOT FUSION echoes largely determines the effectiveness of Pulse Echo UT determines the effectiveness of Pulse Echo UT in the testing of single sided welds.in the testing of single sided welds.

Resolution

Good resolutionGood resolution

Resolution

PoorPoor resolutionresolution

Loses intensity

due to


Attenuation

• Sound beam comparable to a torch beam

•Reduction differs for small and large reflectors

• Energy losses due to material

•Made up of absorption and scatter

Beam Spread

Scatter• The bigger the grain

size the worse the problem

• The higher the frequency of the probe the worse the problem

1 MHz 5 MHz

Beam Spread

The sound beam spread out and the intensity decreases

Beam spread and Attenuation combined

Repeat Back-wall Echoes Beyond The Near Repeat Back-wall Echoes Beyond The Near ZoneZone

ZERO ATTENUATIONZERO ATTENUATION ATTENUATIONATTENUATION 0.02 dB/mm0.02 dB/mm

Sound at an Interface

• Sound will be either transmitted across or reflected back

Reflected

Transmitted

Interface How much is reflected and transmitted depends upon the relative acoustic impedance of the 2 materials

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 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 interface?

• Z1 (Steel) = 46.7 x 106

• Z2 (Water) =1.48 x 106

reflected%10048.17.46

48.17.462

reflected%10018.48

22.452

reflected%88.0910093856.0 2

How much sound transmitted?

100 % - the reflected sound

Example : Steel to water

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

The BIGGER the Acoustic Impedance Ratio or Difference between the two materials:

More sound REFLECTED than transmitted.

Steel

AirSteel

Air

Steel

Steel Aluminum

Steel

Large Acoustic Impedance Ratio

Large Acoustic Impedance Ratio

No Acoustic Impedance Difference

Small Acoustic Impedance Difference

Interface Behaviour

Similarly:Similarly:

At an Steel - Air interface 99.96% of At an Steel - Air interface 99.96% of the incident sound is reflected the incident sound is reflected

At a Steel - Perspex interface 75.99% At a Steel - Perspex interface 75.99% of the incident sound is reflectedof the incident sound is reflected

Sound Intensity

1

010..20H

HLogdB

2 signals at 20% and 40% FSH.

What is the difference between them in dB’s?

2..2020

4020 1010.. LogLogdB

3010.020dB

dBdB 6

1

010..20H

HLogdB

2 signals at 10% and 100% FSH.

What is the difference between them in dB’s?

10..2010

10020 1010.. LogLogdB

120dB

dBdB 20

Amplitude ratios in decibels

• 2 : 1 = 6bB

• 4 : 1 = 12dB

• 5 : 1 = 14dB

• 10 : 1 = 20dB

• 100 : 1 = 40dB

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