NON-DESTRUCTIVE TESTING بسمه تعالی بررسی آزمون های غیر مخرب NON-DESTRUCTIVE TESTING NDT www.metallurgydata.blogfa.com

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

NDT Most common NDT methods: Penetrant Testing (PT) Magnetic Particle Testing (MT) Eddy Current Testing (ET) Mainly used for surface testing Radiographic Testing (RT) Ultrasonic Testing (UT) 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

Thickness checking the material Ultrasonic Inspection Pulse echo signals A scan Display UT Set, Digital Compression probe Thickness checking the material

Ultrasonic Inspection defect echo Back wall echo initial pulse Material Thk defect 10 20 30 40 50 Compression Probe CRT Display

Basic Principles of Ultrasonic Testing The distance the sound traveled can be displayed on the Flaw Detector The screen can be calibrated to give accurate readings of the distance Signal from the backwall Bottom / Backwall

Basic Principles of Ultrasonic Testing The 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

0 10 20 30 40 50 60 60 mm The depth of the defect can be read with reference to the marker on the screen

Thickness / depth measurement The closer the reflector to the surface, the signal will be more to the left of the screen C B A 30 46 68 The thickness is read from the screen The THINNER the material the less distance the sound travel C B A

Ultrasonic Inspection UT Set A Scan Display Angle Probe

Ultrasonic Inspection initial pulse defect echo Surface distance defect sound path 10 20 30 40 50 Angle Probe CRT Display

Ultrasonic Inspection Advantages Rapid results Sub-surface detection Safe Can detect planar defect Capable of measuring the depth of defects May be battery powered Portable Disadvantages Trained 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)

Wavelength Velocity Frequency

Sound Waves 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

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/s V Compression in water : 1470 m/s V Compression in air : 330 m/s 5 M Hz STEEL WATER AIR

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 Sonic / Audible Human Ultrasonic 16Hz - 20kHz > 20kHz = 20,000Hz 0 10 100 1K 10K 100K 1M 10M 100m Ultrasonic Testing 0.5MHz - 50MHz Ultrasonic : Sound with frequency above 20 KHz

THE HIGHER THE FREQUENCY THE SMALLER THE WAVELENGTH Frequency : Number of cycles per second 1 second 1 second 1 second 1 cycle per 1 second = 1 Hertz 3 cycle per 1 second = 3 Hertz 18 cycle per 1 second = 18 Hertz THE HIGHER THE FREQUENCY THE SMALLER THE WAVELENGTH

Frequency 20 KHz = 20 000 Hz 5 M Hz = 5 000 000 Hz Pg 21 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

ULTRASONIC TESTING Very High Frequency 5 M Hz Glass High Frequency 5 K Hz DRUM BEAT Low Frequency Sound 40 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 5MHz compression wave probe in steel

Frequency F  F   = v / f Which probe has the smallest wavelength? 1 M Hz 5 M Hz 10 M Hz 25 M Hz LONGEST SMALLEST  = v / f F  F  Which probe has the smallest wavelength? Which probe has the longest wavelength?

Which of the following compressional probe has the highest sensitivity? 1 MHz 2 MHz 5 MHz 10 MHz 10 MHz

Very high frequency = Very small wavelength Acoustic Spectrum Sonic / Audible Human 16Hz - 20kHz Testing 0.5MHz - 50MHz Ultrasonic > 20kHz = 20,000Hz 0 10 100 1K 10K 100K 1M 10M 100m Ultrasonic : Sound with frequency above 20 KHz Very high frequency = Very small wavelength

What is the velocity difference in steel compared with in water? 4 times If the frequency remain constant, in what material does sound 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

Sound travels in different waveforms in different conditions Sound Waveforms Sound travels in different waveforms in different conditions Compression wave Shear wave Surface wave Lamb wave

Compression / Longitudinal Vibration and propagation in the same direction / parallel Travel in solids, liquids and gases Particle vibration Propagation

Shear / Transverse Vibration at right angles / perpendicular to direction of propagation Travel in solids only Velocity  1/2 compression (same material) Particle vibration Propagation

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

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 D N = 4l 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 as possible balanced against acceptable minimum detectable defect size N = D 4l 2

The Sound Beam FZ NZ Intensity varies Exponential Decay Distance Main Beam Intensity varies Exponential Decay Distance

Near Zone The side lobes has multi minute main beams Two identical defects may give different amplitudes of signals Near Zone Side Lobes 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 Main Lobe Main Beam

Near Zone

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

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

Which of the above probes has the longest Near Zone ? 1 M Hz 5 M Hz

Beam Spread In the far zone sound pulses spread out as they move away from the crystal /2 

Beam Spread Edge,K=1.22 20dB,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?

Which of the above probes has the Largest Beam Spread ? 1 M Hz 5 M Hz

Beam Spread The bigger the diameter the smaller the beam spread The higher the frequency the smaller the beam spread 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

Natural Pulse, No Damping, Pulse Length Natural Pulse, No Damping, Long "Ring Time"

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 Short, Well Damped Pulse Long, Well Damped

5 cycles for weld testing Ideal Pulse Length 5 cycles for weld testing

Resolution RESOLUTION in Pulse Echo Testing is the ability to separate echoes from two or more closely spaced reflectors. RESOLUTION is strongly affected by Pulse Length: Short Pulse Length - GOOD RESOLUTION Long Pulse Length - POOR RESOLUTION RESOLUTION is an extremely important property in WELD TESTING because the ability to separate ROOT GEOMETRY echoes from ROOT CRACK or LACK OF ROOT FUSION echoes largely determines the effectiveness of Pulse Echo UT in the testing of single sided welds.

Resolution 10% 90% > 6dB Good resolution

Resolution 50% 90% < 6dB Poor resolution

Sound travelling through a material Loses intensity due to Beam Spread 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

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

The sound beam spread out and the intensity decreases

Beam spread and Attenuation combined Repeat Back-wall Echoes Beyond The Near Zone 80% 40% 20% 37% 15% ZERO ATTENUATION ATTENUATION 0.02 dB/mm

Sound at an Interface Sound will be either transmitted across or reflected back Reflected How much is reflected and transmitted depends upon the relative acoustic impedance of the 2 materials Interface Transmitted

Acoustic Impedance Definition The Resistance to the passage of sound within a material Formula  = Density , V = Velocity Steel 46.7 x 106 Water 1.48 x 106 Air 0.0041 x 106 Perspex 3.2 x 106 Measured in kg / m2 x sec

% Sound Reflected at an Interface % 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 % 88.09 100 93856 . 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.

Large Acoustic Impedance Ratio Large Acoustic Impedance Ratio Air Steel Air Steel Large Acoustic Impedance Ratio Large Acoustic Impedance Ratio Aluminum Steel Steel Steel No Acoustic Impedance Difference Small Acoustic Impedance Difference

Interface Behaviour Similarly: At an Steel - Air interface 99.96% of the incident sound is reflected At a Steel - Perspex interface 75.99% of the incident sound is reflected

Sound Intensity

2 signals at 20% and 40% FSH. What is the difference between them in dB’s?

2 signals at 10% and 100% FSH. What is the difference between them in dB’s?

Amplitude ratios in decibels 2 : 1 = 6bB 4 : 1 = 12dB 5 : 1 = 14dB 10 : 1 = 20dB 100 : 1 = 40dB