GJ, IIT M, Chennai

G.Jothinathan

Project consultant

C N D E . MDS

I I T M. Chennai 600 036

Gj@iitm.ac.in

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

                                                                                       

         

                                                                                        

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

                                                                                                               

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

                      

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• Ultrasonic Test (UT)

A,B & C Scan images

 

 

 

 

 

 

 

 

 

 

 

 

 

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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 valuesIt 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

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

deformation4.Quality control tool - E and are related to

ultrasonic wave velocitiesCl and Ct  5.Bond integrity testing - Bearings and aircraft

structures (adhesive)

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

Following guide-lines may be used for selection of the probes:

FREQUENCY APPLICATION0.5 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.10.0 MHz For obtaining exceptionally high sensitivity and resolution. For inspection of materials like titanium, managing steel etc.

Properties1. 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= (Z2 - Z1)2 /

(Z2+Z1) 2

R = 99% for a crack interface: air interface

R = 30 - 60 % for various inclusionsTransmission energy coefficient T = 4 Z1 Z2

(Z2+Z1) 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

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

Ultrasonic testingUltrasonic 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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Pulse Echo Method ( A-Scan)- propagation of US waves in the medium- reflection at interfacesmeasurement 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 Technique1. Same probe acts as Transmitter and Receiver2. 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 settingTime 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

PULSE GENERATOR

REPT. FREQ. AND SWEEP VOLTAGE GENERATOR

AMPLIFIER

BLOCK DIAGRAM OF AN ULTRASONIC PULSE ECHO EQUIPMENT

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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

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Infrasound - 0-20 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 MHzIndustrial Ultrasound –0.5MHz –25 MHz- 100 MHzHigh energy ultrasound generated using magnetostrictive effect is widely used by process industries for mixing up, effectively cleaning using solvents

Wave velocity is a material property dependant on ρand and μ and not thickness, distance or travel or probe frequency.

Definitions1. 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:25MHzTime period and frequency are inversely

related3. Wavelength - displacement for one full oscillation mm, cm, metre4. 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 frequecnyGuided 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

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

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Acoustic impedance (Z)- ratio of acoustic pressure to particle velocity Z = P/V

-from the above one can get an expression Z = ρ Cl or Z = ρ Ct

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.

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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/meter2.

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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

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Coninuous and pulsed waves

Type of waves that could be used in Pulse echo

Through transmission &

Resonance

Modes of vibration (continued)

Topic will be dealt under the following headings1. Definition 2. Example 3. Mediums of wave propagation4. Generation 5. Expression for wave velocity

Longitudinal waves1.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

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Transverse waves

1. Particle movement direction is

perpendicular to the wave

propagation direction

2. Rope pulled from one end3. Propagates only in solid medium Shear forces cannot be sustained by fluids4. No piezoelectric material except Y cut quartz on its own generate transverse waves

5. Expression for wave velocity

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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 l2 — 2 C

t2

2( C l2 — C

t2)

E = ρ C l2 2 C l

2 — 4 C t2

C l2 — C

t2

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Compression waves

• Vibration and propagation in the same direction

• Travel in solids, liquids and gases

Propagation

Particle vibration

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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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Surface waves/((Rayleigh waves) x 1-             (                             

1. The particles move in an elliptical path2. 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. C0 = 0.9 Ct

                                       

            

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Surface Waves• Elliptical vibration• Velocity 8% less than shear• Penetrate one wavelength deep

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1. Useful depth of penetration is limited to one wavelength

2. Reflected by sharp corners3. Propagates along smooth curves4. Damped by oil, grease & dirt5. 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.

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Complicated geometry- turbine blades

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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 = Vl /VP where Vl 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

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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.

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

Advances in NDE II – Newer UT methods Guided waves, Phased array probe, Backscattering techniques and TOFD

Conventional UT & Guided Waves Testing Transducer

Conventional ultrasonic testingRegion 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

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Imperial College NDT Group

Typical Chemical Plant

Imperial College NDT Group

Research Transducers

Imperial College NDT Group

Pipes passing through earth wall II

Guided Ultrasonics Ltd

0.0 5.0 10.0 15.0 20.00.0

2.0

4.0

6.0

8.0

10.0

12.0

Distance (m)

Clean Pipe Generally Corroded Pipe