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GJ, IIT M, Chennai G.Jothinathan Project consultant C N D E. MDS I I T M. Chennai 600 036

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Presentation on theme: "GJ, IIT M, Chennai G.Jothinathan Project consultant C N D E. MDS I I T M. Chennai 600 036"— Presentation transcript:

1 GJ, IIT M, Chennai G.Jothinathan Project consultant C N D E. MDS I I T M. Chennai

2 Ultrasonic Testing Overview of Ultrasonic Testing Methods Contact Testing Through Transmission Pulse Echo Immersion Testing Resonance Technique Normal Probe A Scan Pulse Echo B Scan Angle Probe C Scan

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4 Normal probe – suited for this orientation as reflected waves reach back the crystal Normal probe – unsuitable for oriented defects as the reflected waves do not reach back the crystal Parallel orientation results in virtually no reflection as the length is small

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6 A-Scan – point scanning – CRT gives point by information - two information - echo position & echo amplitude A-Scan: B-Scan and C Scan techniques and images B-Scan –line scanning -CRT gives image for the line scanning two information- probe placement & - depth corresponding to the probe placing C- Scan–Volume Scanning- CRT gives the image for the entire area – plan view similar to radiographic image

7 GJ, IIT M, Chennai Ultrasonic Test (UT)

8 A,B & C Scan images.

9 Applicability of Ultrasonic Testing Materials that allow propagation of US waves can be tested. Most engineering materials can be tested as they propagate US waves. The propagation capability can be ascertained from attenuation coefficient values, which indicate the loss of US wave energy as the waves propagate Table of attenuation values It can be seen low attenuation materials like fine grained materials can be easily tested. Coarse grained metals and metals having second phase particles are difficult to test. For these reasons cast metals and composites are normally difficult to test ultrasonically

10 Applications of Ultrasonic Testing 1.Flaw detection and evaluation (location, size & nature) 2. Thickness measurement -(pipe lines, reactor vessels- one side accessibility) 3. Material characterisation - grain size - proportion of phases - nodularity of cast iron - hydrogen damage - extent of deformation 4. Quality control tool - E and  are related to ultrasonic wave velocitiesCl and Ct 5.Bond integrity testing - Bearings and aircraft structures (adhesive)

11 Ultrasonic Testing (UT) Ultrasonic waves - sound waves of frequency more than 20 KHz - UT frequencies of 0.5 MHz to 15 MHz (25 MHz) choice of frequency depends on sensitivity required and attenuation (loss of US wave energy as it propagates) properties of the material. Higher the frequency – higher the sensitivity Higher the frequency– higher the attenuation of US waves with the result it may not be possible to use high frequency probes with high attenuating materials settling for low sensitivity. As a corrolary wavelength and sensitivity and attenuation Limit of defect detectability = λ/2– Smaller than this cannot be detected - why Generation Piezoelectric materials - presently artificially produced polarised ceramic transducers - BaTiO3, PZT, Pb meta niobate etc- mechanical vibrations to electric pulse electrical pulse to mechanical vibrations Magnetostrictive and electrodynamic- not normally used

12 Following guide-lines may be used for selection of the probes: FREQUENCY APPLICATION 0.5 MHz Very coarse grained materials like C.I., S. G. Iron, austenitic Stainless, Steel, soft plastics, rubber, composites etc. 1.0 MHz For coarse grained materials like steel castings and those with very high thickness. 2.0 MHz For large sized components with fair sensitivity requirement like testing of forgings. 4.0 MHz For optimum sensitivity, resolution and penetration. For inspection of fine grained material and those involving low thickness. 6.0 MHz For very high sensitivity or checking thin walled components used in critical space and nuclear applications MHz For obtaining exceptionally high sensitivity and resolution. For inspection of materials like titanium, managing steel etc.

13 Properties 1. Propagation - most engg. materials allow the propagation of USW - elastic property of the material-they allow the vibration to be transmitted 2. Reflection - Transmission Propagating US waves get reflected/transmitted at interfaces. Large acoustic impedance mismatch between the mediums leads to reflection similar to light reflection by mirror Acoustic impedance = density X wave velocity Reflection energy coefficient R= (Z 2 - Z 1 ) 2 / (Z 2 +Z 1 ) 2 R = 99% for a crack interface: air interface R = % for various inclusions Transmission energy coefficient T = 4 Z1 Z2 (Z 2 +Z 1 ) 2 Probe in direct contact with steel : T (BaTiO3-air-steel) = 0.005% Probe in contact with couplant:T (BaTiO3-any couplant-steel) = 16% (hence use of couplant is must in UT) Other properties

14 Pulse Echo Technique Almost entire UT is carried out with this technique The principle is similar to echo hearing by bats to locate obstacles or prey In this method, the elapsed time between the sending of the waves at the front surface and receiving of reflected waves is measured. The time information is converted into thickness information through the wave velocity in the material. The interfaces are identified- how Wave velocity in steel is 5900 m/sec. From this it is evident that that the time of travel Ultrasonic waves in 100 mm of steel is of the order of microseconds To measure time of this order a CRT is used

15 Ultrasonic testing Ultrasonic waves are sent and Reflected ultrasonic waves are received and elapsed time is measured. Defect detected and located Bats can accurately size their prey

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17 Pulse Echo Method ( A-Scan) - propagation of US waves in the medium - reflection at interfaces measurement of elapsed time between sending and the receiving of waves- time information converted to depth information thro the velocity of USW in the medium-material.constant- from depth, location is identified ie whether backwall interface or flaw interface - -Essentially time measurement and CRT is used-- Features of Pulse Echo A-scan Technique 1. Same probe acts as Transmitter and Receiver 2. Simultaneous application of pulse to Xal and X-electrodes of CRT Waves generated,get transmitted and propagating Bright spot appears on screen at the left corner and travels left to right- the speed of travel determined by time base setting Time elapses 3. reflected waves reach the Xal- gets converted to an electric pulse amplified and fed to the Y-electrodes which causes a deflection of the moving bright spot vertically - the point deflection in the X-axis indicates elapsed time (depth) 4. Next pulse is applied (why next pulse need be applied 50 –1200 pulses.sec

18 PULSE GENERATOR REPT. FREQ. AND SWEEP VOLTAGE GENERATOR AMPLIFIER BLOCK DIAGRAM OF AN ULTRASONIC PULSE ECHO EQUIPMENT

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20 GJ, IIT M, Chennai Ultrasonic wave propagation Propagation of vibrations or oscillations – ( to and fro motion)unlike electromagentic radiation needs a medium for propagation Wave - disturbance that travels through a medium, transmitting energy from one location to another location. Medium - the material through which the disturbance is moving – medium is permanently displaced

21 GJ, IIT M, Chennai Infrasound Hz- used by elephants Audible sound - 20 Hz to 20,000 Hz Ultrasound - >20,000 Hz (or 20 KHz) - used by bats to locate obstacles as well as to find prey- dolphins and cats and dogs Medical ultrasound- 2.5 MHz to 15 MHz Industrial Ultrasound –0.5MHz –25 MHz- 100 MHz High energy ultrasound generated using magnetostrictive effect is widely used by process industries for mixing up, effectively cleaning using solvents

22 Wave velocity is a material property dependant on ρand and μ and not thickness, distance or travel or probe frequency. Definitions 1. Time period – time for one full oscillation- secs, microsecs, nanosecs 2. Frequency–no. oscillations/unit time-cycles/sec Hz, KHz, MHz US waves above 20 KHz. 0.5 MHz – 15 MHz:25MHz Time period and frequency are inversely related 3. Wavelength - displacement for one full oscillation mm, cm, metre 4. Wave velocity– phase velocity – different from particle velocity velocity with which energy transferred or the velocity with disturbance travels - C = f/λ C being a material constant,‘λ’ is inversely proportional to ‘f’the frequecny Guided waves are dispersive in nature- the velocity is dependant not only on material but also on thickness of the material and frequency of the probe

23 Wave velocity (contd) Wave velocity is a material property determined by density, Youngs Modulud and Poisssons’ ratio Wavelength contd The ultrasonic wave interaction with obstacles or interfaces is determined by the relative sizes of the obstacle and wavelength Defect detecability in UT = wavelength / 2 Anything below this cannot be detected - why

24 GJ, IIT M, Chennai Acoustic impedance (Z) - ratio of acoustic pressure to particle velocity Z = P/V -from the above one can get an expression Z = ρ C l or Z = ρ C t Z is an important property of ultrasonic waves as the entire property of reflection /transmission is determined the acoustic impedances of the two mediums Unit of Z 1.the design of ultrasonic transducers. 2.assessing absorption of sound in a medium. The acoustic impedance (Z) of a component is the ratio of the acoustic (or AC) pressure p across it to the flow of fluid U through it. Like electrical impedance, acoustic impedance is complicated by the fact that the current and pressure are not necessarily in phase -- the maximum voltage may be ahead of the maximum current, or vice versa. As in electricity, we use complex numbers to handle this, where the real part represents the in-phase component and the imaginary part the out-of-phase component.

25 GJ, IIT M, Chennai Pressure, Energy and Intensity: (indicative of amount of X-rays ) Sound pressure: pressure or stress oscillation in a medium with wave propagation ie  x for longitudinal and  xy for transverse waves. Energy density: Intensity : They are proportional to square of sound pressure. The above three terms denote the quantity of sound waves in a medium. I or E  P 2 The sound pressure is the most important in UT since echo height at the screen is proportional to the sound pressure. Intensity = Energy /unit area/unit time since the energy/time ratio is equivalent to the quantity power, intensity is simply the power/area. Typical units for expressing the intensity of a sound wave are Watts/meter 2.

26 GJ, IIT M, Chennai Types and modes of vibration Types of ultrasonic waves : continuous and pulsed Modes of vibration–the relationship between particle movement direction and wave propagation direction Modes of vibration are 1.Longitudinal – compressional 2.Transverse - shear 3. Surface - Rayleigh 5. Plate waves - Lamb 6. Rod waves - Love waves Guided waves dispersive

27 GJ, IIT M, Chennai Coninuous and pulsed waves Type of waves that could be used in Pulse echo Through transmission & Resonance

28 Modes of vibration (continued) Topic will be dealt under the following headings 1. Definition 2. Example 3. Mediums of wave propagation 4. Generation 5. Expression for wave velocity Longitudinal waves 1.Particle movement direction is parallel to wave propagation direction 2. Sound in air 3. Longitudinal waves propagate in all mediums gas, liquid and solid 4.All piezoelectric materials generate longitudinal waves. Exception is Y cut quartz 5. Expression for wave velocity

29 GJ, IIT M, Chennai Transverse waves 1. Particle movement direction is perpendicular to the wave propagation direction 2. Rope pulled from one end 3. Propagates only in solid medium Shear forces cannot be sustained by fluids 4. No piezoelectric material except Y cut quartz on its own generate transverse waves 5. Expression for wave velocity

30 GJ, IIT M, Chennai Longitudinal and transverse waves Substitute μ for steel Cl/Ct = 91/50 Long. 91mm in steel is equivalent to 50 mm of shear Meaning of the above For same frequency of probe in steel, which mode is sensitive – long or trans Cl and Ct equations can be solved to get E & μ Applications : 1. good for quality control tool 2. material chracterization μ = C l 2 — 2 C t 2 2( C l 2 — C t 2) E = ρ C l 2 2 C l 2 — 4 C t 2 C l 2 — C t 2

31 GJ, IIT M, Chennai Compression waves Vibration and propagation in the same direction Travel in solids, liquids and gases Propagation Particle vibration

32 GJ, IIT M, Chennai Shear waves Vibration at right angles to direction of propagation Travel in solids only Velocity  1/2 compression (same material) Propagation Particle vibration

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34 Surface waves/((Rayleigh waves) x 1- ( 1. The particles move in an elliptical path 2. Example- Earth quake 3. Only in solids- contains transverse wave component 4. Oblique incidence of longitudinal wave: the angle corresponding to second critical angle 5. C 0 = 0.9 C t

35 GJ, IIT M, Chennai Surface Waves Elliptical vibration Velocity 8% less than shear Penetrate one wavelength deep

36 GJ, IIT M, Chennai 1.Useful depth of penetration is limited to one wavelength 2.Reflected by sharp corners 3.Propagates along smooth curves 4.Damped by oil, grease & dirt 5.Very good candidates for complicated shapes for surface defects turbine blades curved and holes below. Rayleigh waves are useful because they are very sensitive to surface defects and since they will follow the surface around, curves can also be used to inspect areas that other waves might have difficulty reaching.

37 GJ, IIT M, Chennai Complicated geometry- turbine blades

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39 Plate,Rod waves -Lamb waves & Love wavesGuided waves) Plate thickness or dia. of rod Is equal to the wavelength pure L,T and S cannot exist. In these cases Plate waves and Rod waves are generated. 1. Complicated motion of particles : symmetrical and assymetrical 2.They are dispersive: wave velocity not only depends on , E &  but also on frequency and thickness of the material. 3. Sin  = V l /V P where V l is desired velocity Frequency & thickness and velocity relationship-dispersion curves As these waves involve the entire thickness for the propagation, the frequency need be so chosen that the wavelength correspond to the thickness of the plate. The velocity can be found using dispersion curves

40 GJ, IIT M, Chennai Lamb and Love waves (continued) Lamb waves are similar to longitudinal waves, with compression and rarefaction, but they are bounded by the sheet or plate surface causing a wave-guide effect.rarefaction As the entire thickness is involved, normally these waves are generated in thin plates and rod. Velociy need be found out for frequency-thickness combination and graphs (dispersion curves)are available

41 Advances in NDE II – Newer UT methods Guided waves, Phased array probe, Backscattering techniques and TOFD Conventional UT & Guided Waves Testing Transducer Conventional ultrasonic testing Region of inspection Transducer Guided wave inspection Global inspection Length of coverage limited to the probe size Length of coverage high upto 100 mtrs Buried pipelines and insulation coatings pose problems Buried structures with insulation coatings can be tested

42 GJ, IIT M, Chennai Distance (m) Clean Pipe Generally Corroded Pipe


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