تعالی تعالی بسمه بسمهمخرب غیر های آزمون مخرب بررسی غیر های آزمون بررسی
NON-DESTRUCTIVE TESTING
NDTNDT
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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
Ultrasonic Inspection
Angle Probe
UT SetA Scan Display
Ultrasonic Inspection
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
• Copper 2330m/sec
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
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
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
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
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
Sound travelling through a material
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