Ut testing section 3 equipments & transducers

Section 3: Equipment & Transducers

Typical sound velocities

Wavelength in mm for Steel

Content: Section 3: Equipment & Transducers

3.1: Piezoelectric Transducers3.2: Characteristics of Piezoelectric Transducers3.3: Radiated Fields of Ultrasonic Transducers3.4: Transducer Beam Spread3.5: Transducer Types3.6: Transducer Testing I3.7: Transducer Modeling3.8: Couplants3.9: Electromagnetic Acoustic Transducers (EMATs)

Continues Next Page

3.10: Pulser-Receivers3.11: Tone Burst Generators In Research3.12: Arbitrary Function Generators3.13: Electrical Impedance Matching and Termination3.14: Transducer Quality Factor “Q”3.15: Data Presentation3.16: Testing Techniques3.17: UT Equipment Circuitry3.18: Further Reading on Sub-Section 3 3.19: Questions & Answers

3.1: Piezoelectric TransducersThe Definitions:

Nominal frequency (F) - nominal operating frequency of the transducer (usually stamped on housing)

Peak frequency (PF) - the highest frequency response measured from the frequency spectrum

Bandwidth center frequency (BCF) - the average of the lowest and highest points at a -6 dB level of the frequency spectrum

Bandwidth (BW) - the difference between the highest and lowest frequencies at the -6 dB level of the frequency spectrum; also % of BCF or of PF

Pulse width (PW) - the time duration of the time domain envelope that is 20 dB above the rising and decaying cycles of a transducer response

Sensitivity is the ability of the search unit to detect reflections or echoes from small defects or flaws.

The acoustic impedance of a transducer is the product of its density and the velocity of sound within it.

Resolution is the resolving power includes the ability to separate reflections from two closely spaced flaws or reflectors.

Front surface pulse (at crystal face), Initial pulse, or “Main Bang” - the first indication on the screen, represents the emission of ultrasonic energy from the crystal face.

Front surface pulse (at interface) - ?

Pulse width (PW) - the time duration of the time domain envelope that is 20 dB above the rising and decaying cycles of a transducer response


Piezoelectric Properties

The conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical energy is the basis for ultrasonic testing. The active element is the heart of the transducer as it converts the electrical energy to acoustic energy, and vice versa. The active element is basically a piece of polarized material (i.e. some parts of the molecule are positively charged, while other parts of the molecule are negatively charged) with electrodes attached to two of its opposite faces. When an electric field is applied across the material, the polarized molecules will align themselves with the electric field, resulting in induced dipoles within the molecular or crystal structure of the material.

The effectiveness of the search unit for a particular application depends onQ factor, bandwidth, frequency, sensitivity, acoustic impedance, and resolving power.

This alignment of molecules will cause the material to change dimensions. This phenomenon is known as electrostriction. In addition, a permanently-polarized material such as quartz (SiO2) or barium titanate (BaTiO3) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. This phenomenon is known as the piezoelectric effect. Additional information on why certain materials produce this effect can be found in the linked presentation material, which was produced by the Valpey Fisher Corporation.

Keyword:

SiO2- QuartzBaTiO3- Barium Titanate

Electric field is applied causing dimensional change: electrostrictionElectric field is generated by dimensional change: piezoelectric effect

Fig. 5.10: Basic design of a single transducer Ultrasound head

Piezoelectric materials have two nice properties:

1. Piezoelectric materials change their shape upon the application of an electric field as the orientation of the dipoles changes.

2. Conversely, if a mechanical forces is applied to the crystal a the electric field is changed producing a small voltage signal.

The piezoelectric crystals thus function as the transmitter as well as the receiver!

Transducer Effectiveness

The effectiveness of the search unit for a particular application depends onQ factor, bandwidth, frequency, sensitivity, acoustic impedance, and resolving power.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/PiezoelectricEffect.ppt

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/PiezoelectricElements.ppt

Piezoelectric crystals

http://www.ndt-kits.com/blog/wp-content/uploads/2013/05/What-is-piezoelectric-transducer.gif

http://www.ndt-kits.com/blog/?cat=7






The active element of most acoustic transducers used today is a piezoelectric ceramic, which can be cut in various ways to produce different wave modes. A large piezoelectric ceramic element can be seen in the image of a sectioned low frequency transducer. Preceding the advent ofpiezoelectric ceramics in the early 1950's, piezoelectric crystals made from quartz crystals and magnetostrictive materials were primarily used. The active element is still sometimes referred to as the crystal by old timers in the NDT field. When piezoelectric ceramics were introduced, they soon became the dominant material for transducers due to their good piezoelectric properties and their ease of manufacture into a variety of shapes and sizes. They also operate at low voltage and are usable up to about 300°C. The first piezoceramic in general use was (1) barium titanate, and that was followed during the 1960's by (2) lead Zirconate Titanate compositions, which are now the most commonly employed ceramic for making transducers. New materials such as piezo-polymers and composites are also being used in some applications.

Keywords:(1) Barium Titanate(2) Lead Zirconate Titanate

The thickness of the active element is determined by the desired frequency of the transducer. A thin wafer element vibrates with a wavelength that is twice its thickness. Therefore, piezoelectric crystals are cut to a thickness that is ½the desired radiated wavelength. The higher the frequency of the transducer, the thinner the active element. The primary reason that high frequency contact transducers are not produced is because the element is very thin and too fragile.

The fundamental frequency of the transducer is determined by its thickness:

From the equation, it can be seen that for high frequency transducer, the thickness is very thin , thus fragile; making its only suitable for immersion techniques only.

At Interface: Reflection & Transmittance

Incoming wave Transmitted wave

Reflected wave

Perspex Steel

1,87

1,00,87


0,13

1,0

-0,87

Perspex Steel

Incoming wave Transmitted wave

Reflected wave



At first glance a sound pressure exceeding 100 % seems paradoxical and one suspects a contradiction of the energy law. However, according to Eq. (1.4) the intensity, i.e. the energy per unit time and unit area, is not calculated from the sound pressure (squared) only but also from the acoustic impedance of the material in which the wave travels. However, since this impedance in steel is very much greater than in water, the calculation shows that the intensity of the transmitted wave is very much smaller there than in water in spite of the higher sound pressure.

Piezoelectric crystals may be X or Y cut depending on which orientation they are sliced. The crystals used in UT testing are X cut, due to the mode of vibration they produced (longitudinal wave). This means that the crystal is sliced with it main axis perpendicular with the X axis.


Q153 A quartz crystal cut so that its major faces are parallel to the X, Y axes and perpendicular to the X axis is called:

a) a Y-cut crystal/ longitudinal waveb) a Y-cut crystal/ shear wavec) a X-cut crystal/ longitudinal waved) a X-cut crystal/ shear wavee) a XY-cut crystal/ longitudinal wave

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html




3.1.1: Type of Piezoelectric Crystal

■ Quartz is a Silicon Oxide (SiO3) ■ Lithium Sulphate LiSO4 Decomposed 130°C■ Barium Titanate (BaTiO3) Curies point 120°C■ Lead Metaniobate (PBNbO6)■ Lead Zirconate Titanate (PBZrO3. PbTiO3)* Curies point 350°C

*Pb[ZrxTi1-x]O3 (0≤x≤1).

■ Quartz is a Silicon Oxide (SiO3) crystal found naturally and X cut across the crustal give compression wave, a Y cut produces shear wave.

Advantages:1. Resistance to wear2. insoluble in water3. resistance to ageing4. easy to cut to give the required frequency

Disadvantage1. It is inefficient, needs a lot of energy to

produce small amount of ultrasound2. Quart crystals are susceptible to

damages (nor robust)3. High voltage to produce low frequency

sound

Quartz

SiO3-Silicon Quartz

■ Lithium Sulphate LiSO4, grows from Lithium Sulphate solution by evaporation.

Advantages:1. Lithium Sulphate is the most efficient receiver of ultrasound2. It has low electric impedance3. Operate well at low voltage4. it does not age5. it has very good resolution6. crystals are easily damp and give a short pulse length

Disadvantage1. It dissolves in water2. It breaks easily 3. It decomposed at temperature above 130°C (what is Curie temperature?)

All of which make it unsuitable for industrial used, except for medical ultrasonic where the temperature restriction is not a concern.

Lithium Sulphate LiSO4 硫酸锂

Followings are Piezoelectric crystals- Polarized crystals made by heating up powders to high temperatures, pressing them into shape and allow them to cool in a very strong electric fields.

Heat applied

Heat applied

Pressed Powders Fused polarized PZT

■ Barium Titanate (BaTiO3) are polarized crystals made by baking Barium Titanate at 1250C and cooling in a 2KV/mm electric field.

AdvantagesIt is efficient ultrasound generatorIt requires low voltageIt has good sensitivity

DisadvantagesIts curies point is about only 120°C, above which it loss it functionalityIt deteriorated over time

BaTiO3

BaTiO3

■ Lead Metaniobate (PBNbO6) crystals are made the similar way as Barium Titanate

AdvantagesIt has high internal dampingIt gives narrow pulse of ultrasound, which gives good resolution

DisadvantageIt has much less sensitivity than Lead Zirconate Titanate PZT

Fig. 3: Comparison between PZT (left) and 1-3 piezocomposite transducer (right) on a prospect wedge

Fig. 4: Comparison between lead Metaniobate (left) and 1-3 piezocomposite transducer (right) for a WSY70-4 probe

http://www.ndt.net/article/splitt/splitt_e.htm

■ Lead Zirconate Titanate (PBZrO3. PbTiO3)* is the best all round crystal for industrial use.

Advantages

■ It has high Curies point 350°C■ It has good resolution■ It does not dissolved in water■ It is tough■ It does not dissolve in water■ It is easily damp.

Other Transducer> Polyvinylchloride probe for high frequency 15MHz, giving high resolution and very high sensitivity.

*Pb[ZrxTi1-x]O3 (0≤x≤1).

■ Lead Zirconate Titanate PZT Curies point 350°C

350°C

350°C is also goof for:



Curie Temperature: In physics and materials science, the Curie temperature (Tc), or Curie point, is the temperature where a material's permanent magnetism changes to induced magnetism. The force of magnetism is determined by magnetic moments. The Curie temperature is the critical point where a material's intrinsic magnetic moments change direction. Magnetic moments are permanent dipole moments within the atom which originate from electrons' angular momentum and spin. Materials have different structures of intrinsic magnetic moments that depend on temperature. At a material's Curie Temperature those intrinsic magnetic moments change direction.

Permanent magnetism is caused by the alignment of magnetic moments and induced magnetism is created when disordered magnetic moments are forced to align in an applied magnetic field. For example, the ordered magnetic moments (ferromagnetic, figure 1) change and become disordered (paramagnetic, figure 2) at the Curie Temperature. Higher temperatures make magnets weaker as spontaneous magnetism only occurs below the Curie Temperature. Magnetic susceptibility only occurs above the CurieTemperature and can be calculated from the Curie-Weiss Law which is derived from Curie's Law.

Lead zirconium Titanate is an intermetallic inorganic compound with the chemical formula Pb[ZrxTi1-x]O3 (0≤x≤1). Also called PZT, it is a ceramic perovskite material that shows a marked piezoelectric effect, which finds practical applications in the area of electroceramics. It is a white solid that is insoluble in all solvents.

Lead zirconium Titanate PZT

http://en.wikipedia.org/wiki/Lead_zirconate_titanate

http://www.ndt.net/article/platte2/platte2.htm

Properties of Piezoelectric Materials

Ceramic Transducer

Q67: Which of the following transducer materials is the most efficient receiver of ultrasonic energy?

(a) Lead metaniobate(b) Quartz(c) Lithium sulphate(d) Barium titanate

Q69: An advantage of using lithium sulphate in search units it that:

(a) It is one of the most efficient generators of ultrasonic energy(b) It is one of the most efficient receivers of ultrasonic energy(c) It is insoluble(d) It can withstand temperatures as high as 700ºC

Q68: Which of the following transducer materials is the most efficient transmitter of ultrasonic energy?

(a) Lead metaniobate(b) Quartz(c) Lithium sulphate(d) Barium titanate

Q17: Which of the following is the least efficient receiver of ultrasonic Energy?

(a) Quartz(b) Lithium sulphate(c) Lead metaniobate(d) Barium titanate

Q21: An advantage of using a ceramic transducer in search units is that:

(a) It is one of the most efficient generators of ultrasonic energy(b) It is one of the most efficient receivers of ultrasonic energy(c) It has a very low mechanical impedance(d) It can withstand temperatures as high as 700oC

Q73: Which of the following is the most durable piezoelectric material?

A. Barium titanateB. QuartzC. Dipotassoium tartrateD. Rochelle salt

Q12: The 1 MHz transducer that should normally have the best time or distance resolution is a:

A. Quartz transducer with air backingB. Quartz transducer with phenolic backingC. Barium titanate transducer with phenolic backingD. Lithium Sulphate transducer with epoxy backing

3.2: Characteristics of Piezoelectric TransducersThe transducer is a very important part of the ultrasonic instrumentation system. As discussed on the previous page, the transducer incorporates a piezoelectric element, which converts electrical signals into mechanical vibrations (transmit mode) and mechanical vibrations into electrical signals (receive mode). Many factors, including material, mechanical and electrical construction, and the external mechanical and electrical load conditions, influence the behavior of a transducer. Mechanical construction includes parameters such as the radiation surface area, mechanical damping, housing, connector type and other variables of physical construction. As of this writing, transducer manufacturers are hard pressed when constructing two transducers that have identical performance characteristics.

Transducer

Transducer PZT & Matching Layer Thicknesses

3.2.1 Transducer Cut-Out

A cut away of a typical contact transducer is shown above. It was previously learned that the piezoelectric element is cut to ½ the desired wavelength. To get as much energy out of the transducer as possible, an impedance matching is placed between the active element and the face of the transducer. Optimal impedance matching is achieved by sizing the matching layer so that its thickness is ¼ of the desired wavelength. This keeps waves that were reflected within the matching layer in phase when they exit the layer (as illustrated in the image to the top). (HOW?)

For contact transducers, the matching layer is made from a material that has an acoustical impedance “Z” between the active element and steel. Immersion transducers have a matching layer with an acoustical impedance “Z” between the active element and water.

Contact transducers also incorporate a wear plate to protect the matching layer and active element from scratching.

Contact Transducer Types:

socketcrystalDamping

Delay / protecting faceElectrical matchingCable

Straight beam probe Angle beam probeTR-probe

Transducer

Transducer: Straight Beam

Transducer: Angle Beam

Transducer Cut-Out

3.2.2 The Active Element (Crystal)

The active element, which is piezo or ferroelectric material, convertselectrical energy such as an excitation pulse from a flaw detector intoultrasonic energy. The most commonly used materials are polarizedceramics which can be cut in a variety of manners to produce different wavemodes. New materials such as piezo polymers and composites are alsobeing employed for applications where they provide benefit to transducerand system performance.

3.2.3 Design of Matching Layer

The matching layer consists of a layer of material with acoustic impedance that of intermediate between the top & bottom mediums. The thickness its thickness is ¼ of the desired wavelength , determined from the center operating frequency of the transducer and the speed of sound of the matching layer.

Matching Layer: Immersion & Delay Transducers

As wear plate

λ /2

λ /4

Active Element

Matching Layer

Backing

3.2.4 Backing (Damping)

The backing is usually a highly attenuative, high density material that is usedto control the vibration of the transducer by absorbing the energy radiating from the back face of the active element. When the acoustic impedanceof the backing matches the acoustic impedance of the active element,the result will be a heavily damped transducer that displays good range resolution but may be lower in signal amplitude. If there is a mismatch in acoustic impedance between the element and the backing, more sound energy will be reflected forward into the test material. The end result is a transducer that is lower in resolution due to a longer waveform duration, butmay be higher in signal amplitude or greater in sensitivity.

Note on Backing:

The backing material supporting the crystal has a great influence on the damping characteristics of a transducer.

Using a backing material with an impedance similar to that of the active element will produce the most effective damping. Such a transducer will have a wider bandwidth resulting in higher sensitivity.

As the mismatch in impedance between the active element and the backing material increases, material penetration increases but transducer sensitivity is reduced.

Keywords:Backing impedance mismatch small: Higher sensitivity Backing impedance mismatch high: Higher penetration.

3.2.5 Wear Plate

The basic purpose of the transducer wear plate is to protect the transducer element from the testing environment. In the case of contact transducers, the wear plate must be a durable and corrosion resistant material in order to withstand the wear caused by use on materials such as steel.

Matching Layer (Wear Plate)

For immersion, angle beam, and delay line transducers the wear plate hasthe additional purpose of serving as an acoustic transformer between thehigh acoustic impedance of the active element and the water, the wedgeor the delay line all of which are of lower acoustic impedance.

This is accomplished by selecting a matching layer that is ¼ λwavelength thick and of the desired acoustic impedance (the active element is nominally ½ λ wavelength). The choice of the wear surface thickness is based upon the idea of superposition that allows waves generated by the active element to be in phase with the wave reverberating in the matching layer as shown in Figure (4).

When signals are in phase, their amplitudes are additive, thus a greateramplitude wave enters the test piece. Figure (12) shows the active elementand the wear plate, and when they are in phase. If a transducer is not tightlycontrolled or designed with care and the proper materials, and the soundwaves are not in phase, it causes a disruption in the wave front.

Transducers

Transducers

http://www.ndt-kits.com/Angle-Beam-Ultrasonic-Transducer-UT0013-s-381-428.html

3.2.6 Transducer Efficiency, Bandwidth and Frequency

3.2.6.1 Resolution

Some transducers are specially fabricated to be more efficient transmitters and others to be more efficient receivers. A transducer that performs well in one application will not always produce the desired results in a different application. For example, sensitivity to small defects is proportional to the product of the efficiency of the transducer as a transmitter and a receiver.

Resolution, the ability to locate defects near the surface or in close proximity in the material, requires a highly damped transducer.

Resolution: BS4331 Pt 3. the recommended resolution should be able to distinguished two discrete echoes less than two wavelength apart. By discrete echoes mean they are split by more than 6dB.

(Vertical spatial resolution)

50% Amplitude or 6dB line.

50% Amplitude or 6dB line.

2 λ

2 λ

In the early days of ultrasonic testing we used the 100, 91 and 85mm steps, at the radius end of the V1 block to test resolving power. However, today this is regarded as too crude a test and BS 4331 Part 3 (now obsolete) recommended that we should be able to recognise two discrete echoes less than two wavelengths apart. By discrete echoes they mean split by more than 6dB, or to more than half the total height of the signals.

3.2.6.2 Transducer Damping

It is also important to understand the concept of bandwidth, or range of frequencies, associated with a transducer. The frequency noted on a transducer is the central or center frequency and depends primarily on the backing material.

Highly damped transducers will respond to frequencies above and below the central frequency. The broad frequency range provides a transducer with high resolving power. Less damped transducers will exhibit a narrower frequency range and poorer resolving power, but greater penetration.

The central frequency will also define the capabilities of a transducer. Lower frequencies (0.5MHz-2.25MHz) provide greater energy and penetration in a material, while high frequency crystals (15.0MHz-25.0MHz) provide reduced penetration but greater sensitivity to small discontinuities. High frequency transducers, when used with the proper instrumentation, can improve flaw resolution and thickness measurement capabilities dramatically. Broadband transducers with frequencies up to 150 MHz are commercially available.

Transducer Damping (illustration with X-axis frequency domain)

Highly damped transducers will respond to frequencies above and below the central frequency. The broad frequency range provides a transducer with high resolving power.

Less damped transducers will exhibit a narrower frequency range and poorer resolving power, but greater penetration.

Transducer (Backing) Damping:

• Highly damped transducers will respond to frequencies above and below the central frequency. The broad frequency range provides a transducer with high resolving power.

• Less damped transducers will exhibit a narrower frequency range and poorer resolving power, but greater penetration.

Transducer Damping

X-axis time domain

X-axis time domain

Narrow bandwidth

Wide bandwidth

Transducer Damping

Transducer Damping

Transducer Damping at -20dB

Transducer Damping at -14dB

Transducer Damping

Transducer Damping- Pulse Length

Wave form Duration at -10dB

Transducer Damping- Low Damping (X-axis time domain)

Transducer Damping- High Damping (X-axis time domain)

48. A more highly damped transducer crystal results in:

(a) Better resolution(b) Better sensitivity (mistake)(c) Lower sensitivity(d) Poorer resolution

3.2.6.3 Bandwidth:

It is also important to understand the concept of bandwidth, or range of frequencies, associated with a ultrasonic transducer. The frequency noted on a transducer is the central or center frequency and depends primarily on the backing material.

Highly damped ultrasonic transducers will respond to frequencies above and below the central frequency. The broad frequency range provides a transducer with high resolving power.

Less damped transducers will exhibit a narrower frequency range and poorer resolving power, but greater penetration.

The central frequency will also define the capabilities of a transducer. Lower frequencies (0.5MHz-2.25MHz) provide greater energy and penetration in material, while high frequency crystals (15.0MHz-25.0MHz) provide reduced penetration but greater sensitivity to small discontinuities. High frequency transducers, when used with the proper instrumentation, can improve flaw resolution and thickness measurement capabilities dramatically. Broadband transducers with frequencies up to 150 MHz are commercially available.

Bandwidth:

The unit for bandwidth is MHz The unit for pulse length is mm or time

The central frequency will also define the capabilities of a transducer.

1. Lower frequencies (0.5MHz-2.25MHz) provide greater energy and penetration in a material,

2. while high frequency crystals (15.0MHz-25.0MHz) provide reduced penetration but greater sensitivity to small discontinuities. High frequency transducers, when used with the proper instrumentation, can improve flaw resolution and thickness measurement capabilities dramatically.



The relation between MHz bandwidth and waveform duration is shownin Figure below. The scatter is wider at -40 dB because the 1% trailing end of the waveform contains very little energy and so has very little effect on the analysis of bandwidth. Because of the scatter it is most appropriate to specify waveforms in the time domain (microseconds) and spectra in the frequency domain.

Transducers are constructed to withstand some abuse, but they should be handled carefully. Misuse, such as dropping, can cause cracking of the wear plate, element, or the backing material. Damage to a transducer is often noted on the A-scan presentation as an enlargement of the initial pulse.

The approximate relations shown in Figure (6) above, can be used to assist intransducer selection. For example, if a -14 dB waveform duration of onemicrosecond is needed, what frequency transducer should be selected?From the graph, a bandwidth of approximately 1 to 1.2 MHz correspondsto approximately 1 microsecond -14 dB waveform duration. Assuming anominal 50% fractional bandwidth transducer, this calculates to a nominalcenter frequency of 2 to 2.4 MHz. Therefore, a transducer of 2.25 MHz or3.5 MHz may be applicable.

http://olympus-ims.com/data/File/panametrics/UT-technotes.en.pdf

Instrumentation Filtered Band Width:

1. Broad band instrument means a wide array of frequencies could beprocessed by the instrument. The frequencies shown will be a close representation of the actual electrical signal measured by the receiver transducer. The S/N may not be very good, the shape of the amplitude tend to be the actual representation.

2. Narrow band instrument, suppressed a portion of frequencies above and below the center frequency. With the high frequencies noise suppressed, gain could be increase, leading to improved sensitivity. However the shape and relative amplitude of pulse frequency components often altered

Instrumentation Band Width:

Q8: Receiver noise must often be filtered out of a test system. Receiver amplifier noise increases proportionally to:

A. the square root of the amplifier bandwidthB. the inverse square of the amplifier bandwidthC. attenuationD. temperature

Q164: The resolving power of a transducer is directly proportional to its:

A. DiameterB. BandwidthC. Pulse repetition rateD. None of the above

Bandwidth is the frequency range of the pulse, it is not the pulse length

Q48: The approximate bandwidth of the transducer with the frequency response shown in figure 1 (-3dB) is:

A. 4 MHz (standard answer)B. 8 MHzC. 10 MHzD. 12 MHz

≈6.5MHz

3.3: Radiated Fields of Ultrasonic TransducersThe sound that emanates from a piezoelectric transducer does not originate from a point, but instead originates from most of the surface of the piezoelectric element. Round transducers are often referred to as piston source transducers because the sound field resembles a cylindrical mass in front of the transducer. The sound field from a typical piezoelectric transducer is shown below. The intensity of the sound is indicated by color, with lighter colors indicating higher intensity.

Ɵ

Since the ultrasound originates from a number of points along the transducer face, the ultrasound intensity along the beam is affected by constructive and destructive wave interference as discussed in a previous page on wave interference. These are sometimes also referred to as diffraction effects. This wave interference leads to extensive fluctuations in the sound intensity near the source and is known as the near field. Because of acoustic variations within a near field, it can be extremely difficult to accurately evaluate flaws in materials when they are positioned within this area.

The pressure waves combine to form a relatively uniform front at the end of the near field. The area beyond the near field where the ultrasonic beam is more uniform is called the far field. In the far field, the beam spreads out in a pattern originating from the center of the transducer. The transition between the near field and the far field occurs at a distance, N, and is sometimes referred to as the "natural focus" of a flat (or unfocused) transducer. The near/far field distance, N, is significant because amplitude variations that characterize the near field change to a smoothly declining amplitude at this point. The area just beyond the near field is where the sound wave is well behaved and at its maximum strength. Therefore, optimal detection results will be obtained when flaws occur in this area.

Near Field

Angular characteristics for large distances from the oscillator.

a: Values of the sound pressure in a linear plot; b: the same plotted in dB

Angular characteristics: Lines of equal sound pressure, plotted in dB. Also the distance from the radiator is plotted in a logarithmic measure

Angular characteristics: Spatial distribution of the sound pressure plotted in linear values on a half plane through the radiator

Angular characteristics: Sound-pressure mountain measured in a plane parallel to the oscillator

Angular characteristics: Sound pressure on the axis of a piston oscillator

For a piston source transducer of radius (a), frequency (f), and velocity (V) in a liquid or solid medium, the applet below allows the calculation of the near/far field transition point. In the Java applet below, the radius (a) and the near field/far field distance can be in metric or English units (e.g. mm or inch), the frequency (f) is in MHz and the sound velocity (V) is in metric or English length units per second (e.g. mm/sec or inch/sec). Just make sure the length units used are consistent in the calculation.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/applet_3_3/applet_3_3.htm

Spherical or cylindrical focusing changes the structure of a transducer field by "pulling" the N point nearer the transducer. It is also important to note that the driving excitation normally used in NDT applications are either spike or rectangular pulsars, not a single frequency. This can significantly alter the performance of a transducer. Nonetheless, the supporting analysis is widely used because it represents a reasonable approximation and a good starting point.

Beam Spreads

http://www.eclipsescientific.com/Software/ESBeamToolAScan/index.html

Probe Dimension & Spread angle探子小,近场杂波短,声扩张度较大.

Probe Dimension & Spread angle探子大,近场杂波长,声扩张度较小.

Probe dimension & Zf, , Ɵ探子小,近场杂波短,声扩张度较大.

Probe dimension & Zf, , Ɵ探子小,近场杂波短,声扩张度较大.

3.4: Transducer Beam SpreadAs discussed on the previous page, round transducers are often referred to as piston source transducers because the sound field resembles a cylindrical mass in front of the transducer. However, the energy in the beam does not remain in a cylinder, but instead spreads out as it propagates through the material. The phenomenon is usually referred to as beam spread but is sometimes also referred to as beam divergence or ultrasonic diffraction. It should be noted that there is actually a difference between beam spread and beam divergence. Beam spread is a measure of the whole angle from side to side of the main lobe of the sound beam in the far field. Beam divergence is a measure of the angle from one side of the sound beam to the central axis of the beam in the far field. Therefore, beam spread is twice the beam divergence.

Far field, or Fraunhofer zone

Although beam spread must be considered when performing an ultrasonic inspection, it is important to note that in the far field, or Fraunhofer zone, the maximum sound pressure is always found along the acoustic axis (centerline) of the transducer. Therefore, the strongest reflections are likely to come from the area directly in front of the transducer.

Beam spread occurs because the vibrating particle of the material (through which the wave is traveling) do not always transfer all of their energy in the direction of wave propagation. Recall that waves propagate through the transfer of energy from one particle to another in the medium. If the particles are not directly aligned in the direction of wave propagation, some of the energy will get transferred off at an angle. (Picture what happens when one ball hits another ball slightly off center). In the near field, constructive and destructive wave interference fill the sound field with fluctuation. At the start of the far field, however, the beam strength is always greatest at the center of the beam and diminishes as it spreads outward.

As shown in the applet below, beam spread is largely determined by the frequency and diameter of the transducer. Beam spread is greater when using a low frequency transducer than when using a high frequency transducer. As the diameter of the transducer increases, the beam spread will be reduced.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/toplinks-rev2.swf

Near/ Far Fields

Near field, constructive and destructive wave interference fill the sound field with fluctuation- reverberence

Far field, however, the beam strength is always greatest at the center of the beam and diminishes as it spreads outward.

Beam angle is an important consideration in transducer selection for a couple of reasons. First, beam spread lowers the amplitude of reflections since sound fields are less concentrated and, thereby weaker. Second, beam spread may result in more difficulty in interpreting signals due to reflections from the lateral sides of the test object or other features outside of the inspection area. Characterization of the sound field generated by a transducer is a prerequisite to understanding observed signals.

Numerous codes exist that can be used to standardize the method used for the characterization of beam spread. American Society for Testing and Materials ASTM E-1065, addresses methods for ascertaining beam shapes in Section A6, Measurement of Sound Field Parameters. However, these measurements are limited to immersion probes. In fact, the methods described in E-1065 are primarily concerned with the measurement of beam characteristics in water, and as such are limited to measurements of the compression mode only. Techniques described in E-1065 include pulse-echo using a ball target and hydrophone receiver, which allows the sound field of the probe to be assessed for the entire volume in front of the probe.

For a flat piston source transducer, an approximation of the beam spread may be calculated as a function of the transducer diameter (D), frequency (F), and the sound velocity (V) in the liquid or solid medium. The applet below allows the beam divergence angle (1/2 the beam spread angle) to be calculated. This angle represents a measure from the center of the acoustic axis to the point where the sound pressure has decreased by one half (-6 dB) to the side of the acoustic axis in the far field.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/applet_3_4/applet_3_4.htm

3.5: Transducer TypesUltrasonic transducers are manufactured for a variety of applications and can be custom fabricated when necessary. Careful attention must be paid to selecting the proper transducer for the application. A previous section on Acoustic Wavelength and Defect Detection gave a brief overview of factors that affect defect detectability. From this material, we know that it is important to choose transducers that have the desired;

■ frequency, (thickness of piezoelectric material)■ bandwidth, (Back damping) ■ Focusing (curvature probe)

to optimize inspection capability. Most often the transducer is chosen either to enhance the sensitivity or resolution of the system. Transducers are classified into groups according to the application.

3.5.1 Contact transducers

are used for direct contact inspections, and are generally hand manipulated. They have elements protected in a rugged casing to withstand sliding contact with a variety of materials. These transducers have an ergonomic design so that they are easy to grip and move along a surface. They often have replaceable wear plates to lengthen their useful life. Coupling materials of water, grease, oils, or commercial materials are used to remove the air gap between the transducer and the component being inspected.

Contact Transducers

Contact probe

Contact Transducer

http://static2.olympus-ims.com/data/Flash/dual.swf?rev=6C5C

http://www.olympus-ims.com/en/ultrasonic-transducers/dualelement/

Practice Makes Perfect

43. Which of the following is a disadvantage of contact testing?

(a) Ability to maintain uniform coupling on rough surface(b) Ease of field use(c) Greater penetrating power than immersion testing(d) Less penetrating power than immersion testing

3.5.2 Immersion transducers

In immersion testing, the transducer do not contact the component. These transducers are designed to operate in a liquid environment and all connections are watertight. Immersion transducers usually have an impedance matching layer that helps to get more sound energy into the water and, in turn, into the component being inspected. Immersion transducers can be purchased with a (1) planer, (2) cylindrically focused or (3) spherically focused lens. A focused transducer can improve the sensitivity and axial resolution by concentrating the sound energy to a smaller area. Immersion transducers are typically used inside a water tank or as part of a squirter or bubbler system in scanning applications.

Unfocused & Focused

Focusing Ration in water/steel (F=4)

http://www.olympus-ims.com/en/ndt-tutorials/flaw-detection/beam-characteristics/

Focused Transducer (Olympus)

ZB = Beginning of the Focal ZoneFZ = Focal ZoneZE = End of the Focal ZoneD = Element Diameter

Focal Length Equation:The focal length F is determined by following equation;

Where:

F = Focal Length in waterR = Curvature of the focusing lensn = Ration of L-velocity of epoxy to L-velocity of water

F

Focal Length Variations

Focal Length Variations due to Acoustic Velocity and Geometry of the Test Part. The measured focal length of a transducer is dependent on the material in which it is being measured. This is due to the fact that different materials have different sound velocities. When specifying a transducer’s focal length it is typically specified for water. Since most materials have a higher velocitythan water, the focal length is effectively shortened. This effect is caused byrefraction (according to Snell’s Law) and is illustrated in Figure (18).

Focal Length Variations

This change in the focal length can be predicted by Equation (13).For example, given a particular focal length and material path, this equationcan be used to determine the appropriate water path to compensate for thefocusing effect in the test material.

Eqn. 13

WP = F – MP.(Ctm/Cw)

WP = Water PathMP = Material DepthF = Focal Length in WaterCtm = Sound Velocity in the Test MaterialCw = Sound Velocity in the water

In addition, the curvature of surface of the test piece can affect focusing.Depending on whether the entry surface is concave or convex, the soundbeam may converge more rapidly than it would in a flat sample or it mayspread and actually defocus.

Cylindrical & Spherical Focused

Cylindrical & Spherical Focused

Q79: What type of search unit allows the greatest resolving power with standard ultrasonic testing equipment?

a) Delay tipb) Focusedc) Highly dampedd) High Q

Q165: Acoustic lens elements with which of the following permit focusing the sound energy to enter cylindrical surface normally or along a line of focus.

a) Cylindrical curvatureb) Spherical lens curvaturesc) Convex shapesd) Concave shapes

Q18: Which of the following is an advantage of a focused transducer?

(a) Extended useful range(b) Reduced sensitivity in localised area(c) Improved signal to noise ratio over an extended range(d) Higher resolution over a limited range

Q67: A divergent sound beam is produced by:

(a) Concave mirror(b) Convex mirror(c) Convex lens(d) None of the above

Q78: Which of the following is not an advantage of a focused transducer?

(a) High sensitivity to small flaws(b) Deep penetration(c) High resolving power(d) Not much affected by surface roughness

Q79: What type of search unit allows the greatest resolving power with standard ultrasonic testing equipment?

(a) Delay tip(b) Focused(c) Highly damped(d) High Q

3.5.3 Dual element transducers

contain two independently operated elements in a single housing. One of the elements transmits and the other receives the ultrasonic signal. Active elements can be chosen for their sending and receiving capabilities to provide a transducer with a cleaner signal, and transducers for special applications, such as the inspection of course grained material. Dual element transducers are especially well suited for making measurements in applications where reflectors are very near the transducer since this design eliminates the ring down effect that single-element transducers experience (when single-element transducers are operating in pulse echo mode, the element cannot start receiving reflected signals until the element has stopped ringing from its transmit function). Dual element transducers are very useful when making thickness measurements of thin materials and when inspecting for near surface defects. The two elements are angled towards each other to create a crossed-beam sound path in the test material.

Keywords: For near surface effects■ Fresnel zone (near zone)■ Ring down effect

For a single crystal probe the length of the initial pulse is the dead zone andany signal from a reflector at a shorter distance than this will be concealedin the initial pulse. We deliberately delay the initial pulse beyond the left ofthe time base, by mounting the transducers of a twin (or double) crystalprobe onto plastic wedges. This and the focusing of the crystals reduces thedead zone considerably and it is only where the transmission and receptivebeams do not overlap that we cannot assess flaws.

A twin or double crystal probe is designed to minimise the problem of deadzone. A twin crystal probe has two crystals mounted on Perspex shoesangled inwards slightly to focus at a set distance in the test material. Werethe crystals not angled, the pulse would be reflected straight back into thetransmitting crystal.

The Perspex shoes hold the crystals away from the test surface so that theinitial pulse does not appear on the CRT screen. The dead zone is greatlyreduced to the region adjoining the test surface, where the transmission andreception beams do not overlap.

More on Dead Zone BS EN 12668-Part1 Section: 3.5

Dead time after transmitter pulsetime interval following the start of the transmitter pulse during which the amplifier is unable to respond to incoming signals, when using the pulse echo method, because of saturation by the transmitter pulse

There are other advantages

1. Double crystal probes can be focused2. Can measure thin plate3. Can detect near surface flaws4. Has good near surface resolution

Disadvantages

1. Good contact is difficult with curved surfaces2. Difficult to size small defects accurately as the width of a double crystal3. probe is usually greater than that of a single crystal probe4. The amplitude of a signal decreases the further a reflector is situated5. from the focal distance - a response curve can be made out.

Therefore single and twin crystal probes are complementary.

Other Reading (Olympus): Dual element transducers utilize separate transmitting and receiving elements, mounted on delay lines that are usually cut at an angle (see diagram on page 8). This configuration improves near surface resolution by eliminating main bang recovery problems. In addition, the crossed beam design provides a pseudo focus that makes duals more sensitive to echoes from irregular reflectors such as corrosion and pitting.

One consequence of the dual element design is a sharply defined distance/amplitude curve. In general, a decrease in the roof angle or an increase inthe transducer element size will result in a longer pseudo-focal distance andan increase in useful range, as shown in Figure (13).

Advantages:Improves near surface resolution (sensitivity?)Provide a pseudo focus (improve sensitivity in the Far Zone?)Less affected by surface roughness due to the pseudo focus effect

Disadvantage(?)The pseudo focus by tilting the active elements (roof angle?) reduces the useful range of transducer?

Figure (13).

Duo Elements Transducer

Roof Angle

Transmitting Crystal

Receiving Crystal

Acoustic Barrier

Casing

Cross Beam Sound path

Duo Elements Transducer

3.5.4 Delay line transducers

provide versatility with a variety of replaceable options. Removable delay line, surface conforming membrane, and protective wear cap options can make a single transducer effective for a wide range of applications. As the name implies, the primary function of a delay line transducer is to introduce a time delay between the generation of the sound wave and the arrival of any reflected waves. This allows the transducer to complete its "sending" function before it starts its "listening" function so that near surface resolution is improved. They are designed for use in applications such as high precision thickness gauging of thin materials and delamination checks in composite materials. They are also useful in high-temperature measurement applications since the delay line provides some insulation to the piezoelectric element from the heat.

Delay Lined Transducer:

Advantages:

1. Heavily damped transducer combined with the use of a delay line provides excellent near surface resolution

2. Higher transducer frequency improves resolution 3. Improves the ability to measure thin materials or find small flaws while

using the direct contact method 4. Contouring available to fit curved parts

Applications:

1. Precision thickness gauging 2. Straight beam flaw detection 3. Inspection of parts with limited contact areas 4. Replaceable Delay Line Transducers 5. Each transducer comes with a standard delay line and retaining ring 6. High temperature and dry couple delay lines are available 7. Requires couplant between transducer and delay line tip

Other Reading (Olympus): Delay Line Transducers

Delay line transducers are single element longitudinal wave transducersused in conjunction with a replaceable delay line. One of the reasons for

choosing a delay line transducer is that near surface resolution can be improved.

The delay allows the element to stop vibrating before a return signal from the reflector can be received. When using a delay line transducer, there will be multiple echoes from end of the delay line and it is important to take these into account. Another use of delay line transducers is in applications in which the test material is at an elevated temperature. The high temperature delay

line options listed in this catalog (page 16, 17, 19) are not intended for continuous contact, they are meant for intermittent contact only.

Advantages:

■ Improve near surface resolution■ High temperature contact testing

Delay Lined Transducer

Delay lined Transducer

TR-Probe / Dual Crystal Probe- Transmitting Receiving Probe

http://www.weldr.net/simple/skill/html/content_10802.htm

Probe Delay with TR-Probe

Cross Talk at High Gain

Probe Delay

Probe Delay

Delay Line UT 1 Lab 8

www.youtube.com/embed/lelVZ9OGli8

3.5.5 Angle beam transducers

Angle beam transducer and wedges are typically used to introduce a refracted shear wave into the test material. Transducers can be purchased in a variety of (1) fixed angles or in (2) adjustable versions where the user determines the angles of incidence and refraction.

In the fixed angle versions, the angle of refraction that is marked on the transducer is only accurate for a particular material, which is usually steel. The angled sound path allows the sound beam to be reflected from the backwall to improve detectability of flaws in and around welded areas. They are also used to generate surface waves for use in detecting defects on the surface of a component.

Angle Beam Transducers- Angle beam transducers are typically used to locate and/or size flaws which are oriented non-parallel to the test surface.










ϴ2L ϴ2S

ϴ1L

Angle Beam Transducers

ϴ2L ϴ2S

ϴ1L

Angle Beam Transducers

Angle Beam Transducers- Mode ConversionFigure (15) below shows the relationship between the incident angle and the relative amplitudes of the refracted or mode converted longitudinal, shear, and surface waves that can be produced from a plastic wedge into steel.

Angle Beam Transducers- Common Terms

ϴ = Refracted angle T= Thickness LEG1=LEG2= T/Cos ϴ

V PATH= 2x LEG= 2T/Cos ϴ SKIP= 2.T Tan ϴ

ϴ

Angle Beam Transducers- Common Terms

ϴ = Refracted angle T= Thickness Surface Distance= S.Sin ϴ

Depth= S.Cos ϴ

ϴ

Angle Beam Transducers- Longitudinal / Shear Wave Inspection

Many AWS inspections are performed using refracted shear waves.However, grainy materials such as austenitic stainless steel may requirerefracted longitudinal waves or other angle beam techniques for successfulinspections.

Angle Beam Transducer

http://static4.olympus-ims.com/data/Flash/wedge_weld.swf?rev=EF60

http://www.olympus-ims.com/en/ultrasonic-transducers/dualelement/

3.5.6 Normal incidence shear wave transducers

Normal Incidence Shear Wave transducers incorporate a shear wave crystalin a contact transducer case. These transducers are unique because they allow the introduction of shear waves directly into a test piece without the use of an angle beam wedge. Rather than using the principles of refraction,as with the angle beam transducers, to produce shear waves in a material,the crystal itself produces the shear wave (Y-cut). Careful design has enabled manufacturing of transducers with minimal longitudinal wave contamination. The ratio of the longitudinal to shear wave components is generally below -30dB.

Because shear waves do not propagate in liquids, it is necessary to use avery viscous couplant when making measurements with these. When usingthis type of transducer in a through transmission mode application, it isimportant that direction of polarity of each of the transducers is in line withthe other. If the polarities are 90° off, the receiver may not receive the signalfrom the transmitter.

Application of Normal incidence shear wave transducers

Typically these transducers are used to make shear velocity measurements of materials. This measurement, along with a longitudinal velocity measurement can be used in the calculation of Poisson’s Ratio, Young’s Modulus, and Shear Modulus. These formulas are listed below for reference.

Keys:

S = Poisson’s RatioVL = Longitudinal VelocityVT = Shear Velocityr = Material DensityE = Young’s ModulusG = Shear Modulus

Normal incidence shear wave transducers

http://static3.olympus-ims.com/data/Flash/shear_wave.swf?rev=3970

Normal incidence shear wave transducers

Advantages:

1. Generate shear waves which propagate perpendicular to the test surface 2. For ease of alignment, the direction of the polarization of shear wave is

nominally in line with the right angle connector 3. The ratio of the longitudinal to shear wave components is generally below

-30 dB

Applications:

1. Shear wave velocity measurements 2. Calculation of Young's Modulus of elasticity and shear modulus (see

Technical Notes, page 46) 3. Characterization of material grain structure

http://www.olympus-ims.com/en/ultrasonic-transducers/shear-wave/

3.5.7 Paint brush transducers

Paint brush transducers are used to scan wide areas. These long and narrow transducers are made up of an array of small crystals that are carefully matched to minimize variations in performance and maintain uniform sensitivity over the entire area of the transducer. Paint brush transducers make it possible to scan a larger area more rapidly for discontinuities. Smaller and more sensitive transducers are often then required to further define the details of a discontinuity.

Q: To evaluate and accurately locate discontinuities after scanning a part with paintbrush transducer, it is generally necessary to uae a:

A. Transducer with a smaller crystalB. ScrubberC. Grid mapD. Crystal collimator

3.5.8 Wheel Transducer

Wheel Transducer Probe Features:

The main driving advantage of this dry coupled solid contact wheel probe is that it works to overcome problems with couplant contamination (application & removal) as well as eliminating the practicalities of immersion systems.The "tyre" or delay material is constructed of hydrophilic polymers which have acoustic properties that lend themselves ideally to the implementation of ultrasonics. Applications include thickness measurement, composite inspection, delamination detection and general flaw detection.

Q: A special scanning device with the transducer mounted in a tire-like container filled with couplant is commonly called:

A. A rotating scannerB. An axial scannerC. A wheel transducerD. A circular scanner

Q: A wheel transducer scanning method is consider as:

A. Contact methodB. Immersion methodC. Wheel methodD. Not allowed

UT Technician At works- Salute!

3.6: Transducer TestingSome transducer manufacturers have lead in the development of transducer characterization techniques and have participated in developing the AIUM Standard Methods for Testing Single-Element Pulse-Echo Ultrasonic Transducers as well as ASTM-E 1065 Standard Guide for Evaluating Characteristics of Ultrasonic Search Units.

Additionally, some manufacturers perform characterizations according to AWS, ESI, and many other industrial and military standards. Often, equipment in test labs is maintained in compliance with MIL-C-45662A Calibration System Requirements. As part of the documentation process, an extensive database containing records of the waveform and spectrum of each transducer is maintained and can be accessed for comparative or statistical studies of transducer characteristics.

Manufacturers often provide time and frequency domain plots for each transducer. The signals below were generated by a spiked pulser. The waveform image on the left shows the test response signal in the time domain (amplitude versus time). The spectrum image on the right shows the same signal in the frequency domain (amplitude versus frequency). The signal path is usually a reflection from the back wall (fused silica) with the reflection in the far field of the transducer.

TRANSDUCER EXCITATION

As a general rule, all of our ultrasonic transducers are designed for negative spike excitation. The maximum spike excitation voltages should be limited to approximately 50 volts per mil of piezoelectric transducer thickness. Low frequency elements are thick, and high frequency elements are thin.

A negative-going 600 volt fast rise time, short duration, spike excitation can be used across the terminals on transducers 5.0 MHz and lower in frequency. For 10 MHz transducers, the voltage used across the terminals should be halved to about 300 volts as measured across the terminals.

Although negative spike excitation is recommended, continuous wave or tone burst excitations may be used. However there are limitations to consider when using these types of excitation. First, the average power dissipation to the transducer should not exceed 125 mW to avoid overheating the transducer and depoling the crystal.

http://www.olympus-ims.com/en/5072pr/

Excitation: Spiked Pulser (negative spike excitation)

http://www.olympus-ims.com/en/5072pr/

Time

ΔT

Pulse Width @50%

0V10%

90%

Square Wave Spiked Pulser: (negative spike excitation)

Square wave has controlled rise and fall times with directly adjustable voltage and pulse width. Precautions on the average power dissipation to the transducer should not exceed 125 mW to avoid overheating the transducer and depoling the crystal.

Adjustable Pulse width

Adjustable Voltage

Time →

0V

Pulse energy: Broad band versus Narrow band.

0.1 1.0 5.0 10 20

0

5

10

1

5

20

25

30

Frequency MHz

Ener

gy (d

B)

Narrow band

Broad band

UT Flaw Detector – Olympus EPOCH 600

Other tests may include the following:

Electrical Impedance Plots provide important information about the design and construction of a transducer and can allow users to obtain electrically similar transducers from multiple sources.

Beam Alignment Measurements provide data on the degree of alignment between the sound beam axis and the transducer housing. This information is particularly useful in applications that require a high degree of certainty regarding beam positioning with respect to a mechanical reference surface.

Beam Profiles provide valuable information about transducer sound field characteristics. Transverse beam profiles are created by scanning the transducer across a target (usually either a steel ball or rod) at a given distance from the transducer face and are used to determine focal spot size and beam symmetry. Axial beam profiles are created by recording the pulse-echo amplitude of the sound field as a function of distance from the transducer face and provide data on depth of field and focal length.

Effects of Probe Frequencies:

1. Higher frequencies give better resolution2. Higher frequencies give better sensitivity3. Lower frequencies give better penetration4. Lower frequencies less attenuation5. Lower frequencies probe wider beam spread with more coverage to detect

reflectors and reflectors with unfavorable orientation.

6. Higher frequencies the beams are more focused and the sensitivity and resolution are better.

Effects of Probe Sizes:

1. The larger the probe produce more energy thus more penetration2. Small probe small near zone3. The larger the probe the poorer the contacts on a curve substrate.

Single or Double Crustal Probe Selection:

1. Single crystal probe should be used for material thickness 15mm and above, according to the probe the near zone

2. Single crystal probe should be used for thickness above 30mm3. Double crystal should be used for thin material

As noted in the ASTM E1065 Standard Guide for Evaluating Characteristics of Ultrasonic Transducers, the acoustic and electrical characteristics which can be described from the data, are obtained from specific procedures that are listed below:

Frequency Response--The frequency response may be obtained from one of two procedures: shock excitation and sinusoidal burst.

Sinusoidal excitation.

Shock excitation

Relative Pulse-Echo Sensitivity--The relative pulse-echo sensitivity may be obtained from the frequency response data by using a sinusoidal burst procedure. The value is obtained from the relationship of the amplitude of the voltage applied to the transducer and the amplitude of the pulse-echo signal received from a specified target.

Time Response--The time response provides a means for describing the radio frequency (RF) response of the waveform. A shock excitation, pulse-echo procedure is used to obtain the response. The time or waveform responses are recorded from specific targets that are chosen for the type of transducer under evaluation, for example, immersion, contact straight beam, or contact angle beam.

Frequency Response--The frequency response of the above transducer has a peak at 5 MHz and operates over a broad range of frequencies. Its bandwidth (4.1 to 6.15 MHz) is measured at the -6 dB points, or 70% of the peak frequency. The useable bandwidth of broadband transducers, especially in frequency analysis measurements, is often quoted at the -20 dB points. Transducer sensitivity and bandwidth (more of one means less of the other) are chosen based on inspection needs.

Complex Electrical Impedance--The complex electrical impedance may be obtained with commercial impedance measuring instrumentation, and these measurements may provide the magnitude and phase of the impedance of the search unit over the operating frequency range of the unit. These measurements are generally made under laboratory conditions with minimum cable lengths or external accessories and in accordance with specifications given by the instrument manufacturer. The value of the magnitude of the complex electrical impedance may also be obtained using values recorded from the sinusoidal burst.

Sound Field Measurements--The objective of these measurements is to establish parameters such as the on-axis and transverse sound beam profiles for immersion, and flat and curved transducers. These measurements are often achieved by scanning the sound field with a hydrophone transducer to map the sound field in three dimensional space. An alternative approach to sound field measurements is a measure of the transducer's radiating surface motion using laser interferometry.

3.7: Transducer ModelingIn high-technology manufacturing, part design and simulation of part inspection is done in the virtual world of the computer. Transducer modeling is necessary to make accurate predictions of how a part or component might be inspected, prior to the actual building of that part. Computer modeling is also used to design ultrasonic transducers.

As noted in the previous section, an ultrasonic transducer may be characterized by detailed measurements of its electrical and sound radiation properties. Such measurements can completely determine the response of any one individual transducer.

There is ongoing research to develop general models that relate electrical inputs (voltage, current) to mechanical outputs (force, velocity) and vice-versa. These models can be very robust in giving accurate prediction of transducer response, but suffer from a lack of accurate modeling of physical variables inherent in transducer manufacturing. These electrical-mechanical response models must take into account the physical and electrical components in the figure below.

The Thompson-Gray Measurement Model, which makes very accurate predictions of ultrasonic scattering measurements made through liquid-solid interfaces, does not attempt to model transducer electrical-mechanical response. The Thompson-Gray Measurement Model approach makes use of reference data taken with the same transducer(s) to deconvolve electro-physical characteristics specific to individual transducers. See Section 5.4 Thompson-Gray Measurement Model.

The long term goal in ultrasonic modeling is to incorporate accurate models of the transducers themselves as well as accurate models of pulser-receivers, cables, and other components that completely describe any given inspection setup and allow the accurate prediction of inspection signals.

3.8: CouplantsA couplant is a material (usually liquid) that facilitates the transmission of ultrasonic energy from the transducer into the test specimen. Couplant is generally necessary because the acoustic impedance mismatch between air and solids (i.e. such as the test specimen) is large. Therefore, nearly all of the energy is reflected and very little is transmitted into the test material. The couplant displaces the air and makes it possible to get more sound energy into the test specimen so that a usable ultrasonic signal can be obtained. In contact ultrasonic testing a thin film of oil, glycerin or water is generally used between the transducer and the test surface.

Couplant

Immersion Method - Water as a couplantWhen scanning over the part or making precise measurements, an immersion technique is often used. In immersion ultrasonic testing both the transducer and the part are immersed in the couplant, which is typically water. This method of coupling makes it easier to maintain consistent coupling while moving and manipulating the transducer and/or the part.

Squirter Column (bubbler)- Water as a couplant

Squirter Column (bubbler)- Water as a couplant

https://www.youtube.com/user/UltrasonicSciences

Couplant

Couplant

3.9: Electromagnetic Acoustic Transducers (EMATs)As discussed on the previous page, one of the essential features of ultrasonic measurements is mechanical coupling between the transducer and the solid whose properties or structure are to be studied. This coupling is generally achieved in one of two ways. In immersion measurements, energy is coupled between the transducer and sample by placing both objects in a tank filled with a fluid, generally water. In contact measurements, the transducer is pressed directly against the sample, and coupling is achieved by the presence of a thin fluid layer inserted between the two. When shear waves are to be transmitted, the fluid is generally selected to have a significant viscosity.

Electromagnetic-acoustic transducers (EMAT) acts through totally different physical principles and do not need couplant. When a wire is placed near the surface of an electrically conducting object and is driven by a current at the desired ultrasonic frequency, eddy currents will be induced in a near surface region of the object. If a static magnetic field is also present, these eddy currents will experience Lorentz forces of the form

F = I x B

F the Lorentz force is the body force per unit volume, I is the induced dynamic current density, and B is the static magnetic induction.

The most important application of EMATs has been in nondestructive evaluation (NDE) applications such as (1) flaw detection or (2) material property characterization. Couplant free transduction allows operation without contact at elevated temperatures and in remote locations. The coil and magnet structure can also be designed to excite complex wave patterns and polarizations that would be difficult to realize with fluid coupled piezoelectric probes. In the inference of material properties from precise velocity or attenuation measurements, using EMATs can eliminate errors associated with couplant variation, particularly in contact measurements.

F is the body force per unit volume, I is the induced dynamic current density, and B is the static magnetic induction.

EMAT

A number of practical EMAT configurations are shown below. In each, the biasing magnet structure, the coil, and the forces on the surface of the solid are shown in an exploded view. The first three configurations will excite beams propagating normal to the surface of the half-space and produce beams with radial, longitudinal, and transverse polarizations, respectively. The final two use spatially varying stresses to excite beams propagating at oblique angles or along the surface of a component. Although a great number of variations on these configurations have been conceived and used in practice, consideration of these three geometries should suffice to introduce the fundamentals.

http://www.mie.utoronto.ca/labs/undel/index.php?menu_path=menu_pages/projects_menu.html&content_path=content_pages/fac2_2.html&main_menu=projects&side_menu=page1&sub_side_menu=s2

Electromagnetic acoustic transducerhttp://en.wikipedia.org/wiki/Electromagnetic_acoustic_transducer

Electromagnetic Acoustic Transducer (EMAT) is a transducer for non-contact sound generation and reception using electromagnetic mechanisms. EMAT is an ultrasonic nondestructive testing (NDT) method which does not require contact or couplant, because the sound is directly generated within the material adjacent to the transducer. Due to this couplant-free feature, EMAT is particularly useful for automated inspection, and hot, cold, clean, or dry environments. EMAT is an ideal transducer to generate Shear Horizontal (SH) bulk wave mode, Surface Wave, Lamb waves and all sorts of other guided-wave modes in metallic and/or ferromagnetic materials. As an emerging ultrasonic testing (UT) technique, EMAT can be used for thickness measurement, flaw detection, and material property characterization. After decades of research and development, EMAT has found its applications in many industries such as primary metal manufacturing and processing, automotive, railroad, pipeline, boiler and pressure vessel industries.

Comparison between EMAT and Piezoelectric Transducers

As an Ultrasonic Testing (UT) method, EMAT has all the advantages of UT compared to other NDT methods. Just like piezoelectric UT probes, EMAT probes can be used in pulse echo, pitch-catch, and through-transmission configurations. EMAT probes can also be assembled into phased array probes, delivering focusing and beam steering capabilities.

AdvantagesCompared to piezoelectric transducers, EMAT probes have the following

advantages:1. No couplant is needed. Based on the transduction mechanism of EMAT,

couplant is not required. This makes EMAT ideal for inspections at temperatures below the freezing point and above the evaporation point of liquid couplants. It also makes it convenient for situations where couplant handling would be impractical.

2. EMAT is a non-contact method. Although proximity is preferred, a physical contact between the transducer and the specimen under test is not required.

3. Dry Inspection. Since no couplant is needed, the EMAT inspection can be performed in a dry environment.

4. Less sensitive to surface condition. With contact-based piezoelectric transducers, the test surface has to be machined smoothly to ensure coupling. Using EMAT, the requirements to surface smoothness are less stringent; the only requirement is to remove loose scale and the like.

5. Easier for sensor deployment. Using piezoelectric transducer, the wave propagation angle in the test part is affected by Snell’s law. As a result, a small variation in sensor deployment may cause a significant change in the refracted angle.

6. Easier to generate SH-type waves. Using piezoelectric transducers, SH wave is difficult to couple to the test part. EMAT provide a convenient means of generating SH bulk wave and SH guided waves.

Challenges and Disadvantages

The disadvantages of EMAT compared to piezoelectric UT can be summarized as follows:

1. Low transduction efficiency. EMAT transducers typically produce raw signal of lower power than piezoelectric transducers. As a result, more sophisticated signal processing techniques are needed to isolate signal from noise.

2. Limited to metallic or magnetic products. NDT of plastic and ceramic material is not suitable or at least not convenient using EMAT.

3. Size constraints. Although there are EMAT transducers as small as a penny, commonly used transducers are large in size. Low-profile EMAT problems are still under research and development. Due to the size constraints, EMAT phased array is also difficult to be made from very small elements.

4. Caution must be taken when handling magnets around steel products.

Applications of EMATsEMAT has been used in a broad range of applications and has potential to be

used in many other applications. A brief and incomplete list is as follows.

1. Thickness measurement for various applications2. Flaw detection in steel products3. Plate lamination defect inspection4. Bonded structure lamination detection5. Laser weld inspection for automotive components6. Various weld inspection for coil join, tubes and pipes.7. Pipeline in-service inspection.8. Railroad and wheel inspection9. Austenitic weld inspection for power industry10. Material characterization

http://mdienergy.com/emat.html

Cross-sectional view of a spiral coil EMAT exciting radially polarized shear waves propagating normal to the surface.

http://www-ndc.me.es.osaka-u.ac.jp/pmwiki_e/pmwiki.php?n=Research.EMATs

EMAT Transducer

Cross-sectional view of a tangential field EMAT for exciting polarizedlongitudinal waves propagating normal to the surface.

Cross-sectional view of a normal field EMAT for exciting plane polarized shear waves propagating normal to the surface.

EMATS

The bulk-shear-wave EMAT consists of a pair of permanent magnets and a spiral-elongated coil. Driving currents in the coil generate the electromagnet forces (Lorentz force and magnetostriction force) parallel to the surface to generate the shear waves propagating normal to the surface

Cross-sectional view of a meander coil EMAT for exciting obliquely propagating L or SV waves, Rayleigh waves, or guided modes (such as Lamb waves) in plates.

Cross-sectional view of a periodic permanent magnet EMAT for exciting grazing or obliquely propagating horizontally polarized (SH) waves or guided SH modes in plates.

Practical EMAT designs are relatively narrowband and require strong magnetic fields and large currents to produce ultrasound that is often weaker than that produced by piezoelectric transducers. Rare-earth materials such as Samarium-Cobalt and Neodymium-Iron-Boron are often used to produce sufficiently strong magnetic fields, which may also be generated by pulsed electromagnets.

The EMAT offers many advantages based on its couplant-free operation. These advantages include the abilities to operate in remote environments at elevated speeds and temperatures, to excite polarizations not easily excited by fluid coupled piezoelectrics, and to produce highly consistent measurements.

These advantages are tempered by low efficiencies, and careful electronic design is essential to applications.

3.10: Pulser-ReceiversUltrasonic pulser-receivers are well suited to general purpose ultrasonic testing. Along with appropriate transducers and an oscilloscope, they can be used for flaw detection and thickness gauging in a wide variety of metals, plastics, ceramics, and composites. Ultrasonic pulser-receivers provide a unique, low-cost ultrasonic measurement capability

The pulser section of the instrument generates short, large amplitude electric pulses of controlled energy, which are converted into short ultrasonic pulses when applied to an ultrasonic transducer. Most pulser sections have very low impedance outputs to better drive transducers. Control functions associated with the pulser circuit include:

1. Pulse length or damping (The amount of time the pulse is applied to the transducer.)

2. Pulse energy (The voltage applied to the transducer. Typical pulser circuits will apply from 100 volts to 800 volts to a transducer.)

100 volts to 800 volts (1KV~2KV could be used)

Transducer Cut-out

Pulse characteristics

Pulse lengthN= Pulse Rate

Pulse energy

Pulse Length: BS4331 Pt2.

Pulse length

N= Pulse Rate

Pulse energy

Pulse Length: BS EN 12668- Part 1 Instrumentation

3.22pulse durationtime interval during which the modulus of the amplitude of a pulse is 10 % or more of its peak amplitude.

Pulse Length: A long pulse length may be 15 wavelength λ, a short pulse length may be only 2 λ and a normal pulse length usually about 5 λ.

The longer the pulse length the more energy, thus more penetrating, however the resolution and sensitivity deteriorated.

Pulse Length

Pulse Length

Pulse Length

Pulse Length

Pulse Length and Wave form

Pulse-Length and Wave form Quality FactorTwo different pulses with the same frequency, but different duration (pulse length), i.e. Number of oscillations. The shortest pulse has a wider dispersion of frequencies, i.e. a greater bandwidth.

Wave form Quality Factor

Pulse Length- x axis time domainQuality factor- x axis frequency domain

Q Factor = fo/(f1-f2)

Frequency

Pulse-Echo mode of operation, narrow band excitation (tone burst). Conventional air-coupled transducer with passive matching layers

http://www.mdpi.com/1424-8220/13/5/5996/htm

Two types of excitation: Sinusoidal/Shock.

Pulse-echo mode of operation, wideband excitation (spike). 1. (Red) Air-coupled transducer with active matching layer. 2. (Blue) Conventional air-coupled transducer with passive matching layers.

λ /4 impedance matching layers

Modulus of the electrical impedance of the piezocomposite disk vs frequency. Circles: experimental measurements, solid red line: theoretical calculation.

Z= pV

Sensitivity in pulse-echo mode of operation wideband excitation (spike). 1. (Red) Air-coupled transducer with active matching layer. 2. (Blue) Conventional air-coupled transducer with passive matching layers

Transducers

Damping:

Shock wave transducer and low damped transducer : Shock wave transducers should always be used for wall thickness measurement. For smaller wall thicknesses this is as important for the pulse separation as is the frequency itself. For large wall thickness the shock wave is required also for a perfect start and stop trigger of the time measurement. Low damped transducers are not recommended.

http://www.ndt.net/article/rohrext/us_pk/us_pk_e.htm

In the receiver section the voltage signals produced by the transducer, which represent the received ultrasonic pulses, are amplified. The amplified radio frequency (RF) signal is available as an output for display or capture for signal processing. Control functions associated with the receiver circuit include:

1. Signal rectification (The RF signal can be viewed as positive half wave, negative half wave or full wave.)

2. Filtering to shape and smooth return signals3. Gain, or signal amplification4. Reject control

The pulser-receiver is also used in material characterization work involving sound velocity or attenuation measurements, which can be correlated to material properties such as elastic modulus. In conjunction with a stepless gate and a spectrum analyzer, pulser-receivers are also used to study frequency dependent material properties or to characterize the performance of ultrasonic transducers.

Pulse/Beam Characteristics

High frequency, short duration pulse exhibit better depth resolution but allow less penetration. A short time duration pulse only a few cycle is known as broad band pulse, because its frequency domain equivalent is large. Such pulse exhibit good depth resolution.

http://www.olympus-ims.com/en/ndt-tutorials/thickness_gage/transducers/beam_characteristics/

Transducers of the kind most commonly used for ultrasonic gauging will have these fundamental functional properties, which in turn affect the properties of the sound beam that they will generate in a given material:

Type - The transducer will be identified according to its design and function as a contact, delay line, or immersion type. Physical characteristics of the test material such as surface roughness, temperature, and accessibility, as well as its sound transmission properties and the range of thickness to be measured, will all influence the selection of transducer type.

Diameter - The diameter of the active transducer element, which is normally housed in a somewhat larger case. Smaller diameter transducers are often most easily coupled to the test material, while larger diameters may couple more efficiently into rough surfaces due to an averaging effect. Larger diameters are also required for design reasons as transducer frequency decreases.

Frequency - The number of wave cycles completed in one second, normally expressed in Kilohertz (KHz) or Megahertz (MHz). Most ultrasonic gauging is done in the frequency range from 500 KHz to 20 MHz, so most transducers fall within that range, although commercial transducers are available from below 50 KHz to greater than 200 MHz. Penetration increases with lower frequency, while resolution and focal sharpness increase with higher frequency.

Waveform duration - The number of wave cycles generated by the transducer each time it is pulsed. A narrow bandwidth transducer has more cycles than a broader bandwidth transducer. Element diameter, backing material, electrical tuning and transducer excitation method all impact waveform duration. A short wave duration (broadband response) is desirable in most thickness gauging applications.

Bandwidth - Typical transducers for thickness gauging do not generate sound waves at a single pure frequency, but rather over a range of frequencies centered at the nominal frequency designation. Bandwidth is the portion of the frequency response that falls within specified amplitude limits. Broad bandwidth is usually desirable in thickness gauging applications involving contact, delay line, and immersion transducers.

Sensitivity - The relationship between the amplitude of the excitation pulse and that of the echo received from a designated target. This is a function of the energy output of the transducer.

Beam profile - As a working approximation, the beam from a typical unfocused disk transducer is often thought of as a column of energy originating from the active element area that travels as a straight column for a while and then expands in diameter and eventually dissipates, like the beam from a spotlight.

In fact, the actual beam profile is complex, with pressure gradients in both the transverse and axial directions. In the beam profile illustration below, red represents areas of highest energy, while green and blue represent lower energy.

The exact shape of the beam in a given case is determined by transducer frequency, transducer diameter, and material sound velocity. The area of maximum energy a short distance beyond the face of the transducer marks the transition between beam components known as the near field and the far field, each of which is characterized by specific types of pressure gradients. Near field length is an important factor in ultrasonic flaw detection, since it affects the amplitude of echoes from small flaws like cracks, but it is usually not a significant factor in thickness gauging applications.

Focusing - Immersion transducers can be focused with acoustic lenses to create an hourglass-shaped beam that narrows to a small focal zone and then expands. Certain types of delay line transducers can be focused as well. Beam focusing is very useful when measuring small diameter tubing or other test pieces with sharp radiuses, since it concentrates sound energy in a small area and improves echo response.

Attenuation - As it travels through a medium, the organized wave front generated by an ultrasonic transducer will begin to break down due to imperfect transmission of energy through the microstructure of any material. Organized mechanical vibrations (sound waves) turn into random mechanical vibrations (heat) until the wave front is no longer detectable. This process is known as sound attenuation. Attenuation varies with material, and increases proportionally to frequency. As a general rule, hard materials like metals are less attenuating than softer materials like plastics. Attenuation ultimately limits the maximum material thickness that can be measured with a given gage setup and transducer, since it determines the point at which an echo will be too small to detect.

http://www.olympus-ims.com/en/ndt-tutorials/thickness_gage/transducers/beam_characteristics/

Q15: A significant limitation of a lower frequency, single element transducer is:

a) Scatter of sound beam due to microstructure of test objectb) Increased grain noise or ‘hash’c) (Less beam spreadd) Impaired ability to display discontinuities just below the entry surface

How & Why ?Reasoning: Pulse/Beam CharacteristicsHigh frequency, short duration pulse exhibit better depth resolution but allowless penetration. Lower frequency, longer duration pulse.

3.11: Tone Burst Generators In ResearchTone burst generators are often used in high power ultrasonic applications. They take low-voltage signals and convert them into high-power pulse trains for the most power-demanding applications. Their purpose is to transmit bursts of acoustic energy into a test piece, receive the resulting signals, and then manipulate and analyze the received signals in various ways. High power radio frequency (RF) burst capability allows researchers to work with difficult, highly attenuative materials or inefficient transducers such as EMATs. A computer interface makes it possible for systems to make high speed complex measurements, such as those involving multiple frequencies.

Tone burst generators

Tone burst generators

http://www.seekic.com/circuit_diagram/Signal_Processing/SINGLE_TONE_BURST_GENERATOR.html

3.12: Arbitrary Function GeneratorsArbitrary waveform generators permit the user to design and generate virtually any waveform in addition to the standard function generator signals (i.e. sine wave, square wave, etc.). Waveforms are generated digitally from a computer's memory, and most instruments allow the downloading of digital waveform files from computers.

Ultrasonic generation pulses must be varied to accommodate different types of ultrasonic transducers. General-purpose highly damped contact transducers are usually excited by a wideband, spike-like pulse provided by many common pulser/receiver units. The lightly damped transducers used in high power generation, for example, require a narrowband tone-burst excitation from a separate generator unit. Sometimes the same transducer will be excited differently, such as in the study of the dispersion of a material's ultrasonic attenuation or to characterize ultrasonic transducers.

Section of biphase modulated spread spectrum ultrasonic waveform

http://www.mpi-ultrasonics.com/content/mmm-signal-processing-examples

In spread spectrum ultrasonics (see spread spectrum page), encoded sound is generated by an arbitrary waveform generator continuously transmitting coded sound into the part or structure being tested. Instead of receiving echoes, spread spectrum ultrasonics generates an acoustic correlation signature having a one-to-one correspondence with the acoustic state of the part or structure (in its environment) at the instant of measurement. In its simplest embodiment, the acoustic correlation signature is generated by cross correlating an encoding sequence (with suitable cross and auto correlation properties) transmitted into a part (structure) with received signals returning from the part (structure).

3.13: Electrical Impedance Matching and TerminationWhen computer systems were first introduced decades ago, they were large, slow-working devices that were incompatible with each other. Today, national and international networking standards have established electronic control protocols that enable different systems to "talk" to each other. The Electronics Industries Associations (EIA) and the Institute of Electrical and Electronics Engineers (IEEE) developed standards that established common terminology and interface requirements, such as EIA RS-232 and IEEE 802.3. If a system designer builds equipment to comply with these standards, the equipment will interface with other systems. But what about analog signals that are used in ultrasonics?

Data Signals: Input versus Output

Consider the signal going to and from ultrasonic transducers. When you transmit data through a cable, the requirement usually simplifies into comparing what goes in one end with what comes out the other. High frequency pulses degrade or deteriorate when they are passed through any cable. Both the height of the pulse (magnitude) and the shape of the pulse (wave form) change dramatically, and the amount of change depends on the data rate, transmission distance and the cable's electrical characteristics. Sometimes a marginal electrical cable may perform adequately if used in only short lengths, but the same cable with the same data in long lengths will fail. This is why system designers and industry standards specify precise cable criteria.

1. Recommendation: Observe manufacturer's recommended practices forcable impedance, cable length, impedance matching, and any requirements for termination in characteristic impedance.

2. Recommendation: If possible, use the same cables and cable dressing for all inspections.

Cable Electrical Characteristics

The most important characteristics in an electronic cable are impedance, attenuation, shielding, and capacitance. In this page, we can only review these characteristics very generally, however, we will discuss capacitance in more detail.

Impedance (Ohms) represents the total resistance that the cable presents to the electrical current passing through it. At low frequencies the impedance is largely a function of the conductor size, but at high frequencies conductor size, insulation material, and insulation thickness all affect the cable's impedance. Matching impedance is very important. If the system is designed to be 100 Ohms, then the cable should match that impedance, otherwise error-producing reflections are created.

Attenuation is measured in decibels per unit length (dB/m), and provides anindication of the signal loss as it travels through the cable. Attenuation is very dependent on signal frequency. A cable that works very well with low frequency data may do very poorly at higher data rates. Cables with lower attenuation are better.

Shielding is normally specified as a cable construction detail. For example, the cable may be unshielded, contain shielded pairs, have an overall aluminum/mylar tape and drain wire, or have a double shield. Cable shields usually have two functions: to act as a barrier to keep external signals from getting in and internal signals from getting out, and to be a part of the electrical circuit. Shielding effectiveness is very complex to measure and depends on the data frequency within the cable and the precise shield design. A shield may be very effective in one frequency range, but a different frequency may require a completely different design. System designers often test complete cable assemblies or connected systems for shielding effectiveness.

Capacitance in a cable is usually measured as picofarads per foot (pf/m). It indicates how much charge the cable can store within itself. If a voltage signal is being transmitted by a twisted pair, the insulation of the individual wires becomes charged by the voltage within the circuit. Since it takes a certain amount of time for the cable to reach its charged level, this slows down and interferes with the signal being transmitted. Digital data pulses are a string of voltage variations that are represented by square waves. A cable with a high capacitance slows down these signals so that they come out of the cable looking more like "saw-teeth," rather than square waves. The lower the capacitance of the cable, the better it performs with high speed data.

3.14 Transducer Quality Factor “Q”The quality factor “Q” of tuned circuit, search units or individual transducer element is a performance measurement of their frequency selectivity. It is thru ration of search unit fundamental (resonance ) frequency fo to the band width (f2-f1) at 3dB down point at both sides.

Quality Factor “Q”

Quality Factor “Q”

High quality Q-factor has a narrow frequency range (bandwidth) (i.e. little damping) and a correspond long spatial pulse length, where as a Low quality Q-factor transducer has a wide frequency range (bandwidth) and a shorter spatial pulse length.

As discussed previously highly damped transducer, gives a wider frequency range provide better spatial resolution. Thus a Low quality Q-factor does not mean poor choice of transducer.

Continuous-wave ultrasound testing usually employed High qiality Q-factor transducer.

http://www.slideshare.net/vsrbhupal/echo-meet-final?related=2&utm_campaign=related&utm_medium=1&utm_source=6

3.15: Data PresentationUltrasonic data can be collected and displayed in a number of different formats. The three most common formats are know in the NDT world as:

A-scan, B-scanC-scan presentations D-scan presentations. Shadow Methods (modified A-Scan ?)

Each presentation mode provides a different way of looking at and evaluating the region of material being inspected. Modern computerized ultrasonic scanning systems can display data in all three presentation forms simultaneously.

Data Presentation: A, B and C-scan recording and principle of scanning

Data Presentation:

3.15.1 A-Scan Presentation

The A-scan presentation displays the amount of received ultrasonic energy as a function of time. The relative amount of received energy is plotted along the vertical axis and the elapsed time (which may be related to the sound energy travel time within the material) is displayed along the horizontal axis. Most instruments with an A-scan display allow the signal to be displayed in its:natural radio frequency form (RF), as a fully rectified RF signal, or as either the positive or negative half of the RF signal.

In the A-scan presentation, relative discontinuity size can be estimated by comparing the signal amplitude obtained from an unknown reflector to that from a known reflector. Reflector depth can be determined by the position of the signal on the horizontal sweep.

In the A-scan presentation, relative discontinuity size can be estimated by comparing the signal amplitude obtained from an unknown reflector to that from a known reflector. Reflector depth can be determined by the position of the signal on the horizontal sweep.

Reflector depthRelative discontinuity size

A-Scan

A-Scan

http://static3.olympus-ims.com/data/Flash/HTML5/a_scan/A-scan.html?rev=F2E2

In the illustration of the A-scan presentation to the right, the initial pulse generated by the transducer is represented by the signal IP, which is near time zero, the transducer is scanned along the surface of the part, four other signals are likely to appear at different times on the screen. When the transducer is in its far left position, only the IP signal and signal A, the sound energy reflecting from surface A, will be seen on the trace. As the transducer is scanned to the right, a signal from the backwall BW will appear later in time, showing that the sound has traveled farther to reach this surface. When the transducer is over flaw B, signal B will appear at a point on the time scale that is approximately halfway between the IP signal and the BW signal. Since the IP signal corresponds to the front surface of the material, this indicates that flaw B is about halfway between the front and back surfaces of the sample. When the transducer is moved over flaw C, signal C will appear earlier in time since the sound travel path is shorter and signal B will disappear since sound will no longer be reflecting from it.

3.15.2 B-Scan

http://static2.olympus-ims.com/data/Flash/HTML5/B_Scan/B-scan.html?rev=5E4D

B-Scan

B-Scan


B-Scan Presentation

The B-scan presentations is a profile (cross-sectional) view of the test specimen. In the B-scan, the time-of-flight (travel time) of the sound energy is displayed along the vertical axis and the linear position of the transducer is displayed along the horizontal axis. From the B-scan, the depth of the reflector and its approximate linear dimensions in the scan direction can be determined. The B-scan is typically produced by establishing a trigger gate on the A-scan. Whenever the signal intensity is great enough to trigger the gate, a point is produced on the B-scan. The gate is triggered by the sound reflecting from the backwall of the specimen and by smaller reflectors within the material. In the B-scan image above, line A is produced as the transducer is scanned over the reduced thickness portion of the specimen. When the transducer moves to the right of this section, the backwall line BW is produced. When the transducer is over flaws B and C, lines that are similar to the length of the flaws and at similar depths within the material are drawn on the B-scan. It should be noted that a limitation to this display technique is that reflectors may be masked by larger reflectors near the surface.

It should be noted that a limitation to this display technique is that reflectors may be masked by larger reflectors near the surface.

Masked by “C” above

B-Scan

Q: In a B-scan display, the length of a screen indication from a discontinuity is related to:

A. A discontinuity’s thickness as measured parallel to the ultrasonic beamB. The discontinuity’s length in the direction of the transducer travelC. Both A and BD. None of the above

3.15.3 C-Scan Presentation

The C-scan presentation provides a plan-type view of the location and size of test specimen features. The plane of the image is parallel to the scan pattern of the transducer. C-scan presentations are produced with an automated data acquisition system, such as a computer controlled immersion scanning system. Typically, a data collection gate is established on the A-scan and the amplitude or the time-of-flight of the signal is recorded at regular intervals as the transducer is scanned over the test piece. The relative signal amplitude or the time-of-flight is displayed as a shade of gray or a color for each of the positions where data was recorded. The C-scan presentation provides an image of the features that reflect and scatter the sound within and on the surfaces of the test piece.

C-Scan

http://www.ndt.net/article/pohl/pohl_e.htm

The (1) relative signal amplitude or (2) the time-of-flight is displayed as a shade of gray or a color for each of the positions where data was recorded.

C-Scan

C-Scan / A-Scan

High resolution scans can produce very detailed images. Below are two ultrasonic C-scan images of a US quarter. Both images were produced using a pulse-echo technique with the transducer scanned over the head side in an immersion scanning system. For the C-scan image on the left, the gate was setup to capture the amplitude of the sound reflecting from the front surface of the quarter. Light areas in the image indicate areas that reflected a greater amount of energy back to the transducer. In the C-scan image on the right, the gate was moved to record the intensity of the sound reflecting from the back surface of the coin. The details on the back surface are clearly visible but front surface features are also still visible since the sound energy is affected by these features as it travels through the front surface of the coin.

C-Scan

C-Scan Recording

C-Scan Recording

3.15.4 The D scan- The D scan gives a side view of the defect seen from a viewpoint normal tothe B scan. It is usually automated, and shows the length, depth andthrough thickness of a defect. The D scan should not be confused with thedelta technique.

The D scan- The D scan gives a side view of the defect seen from a viewpoint normal tothe B scan. It is usually automated, and shows the length, depth andthrough thickness of a defect. The D scan should not be confused with thedelta technique.

AUT Displays

3.15.5 The Through Transmission Shadow Method

This method is also called the intensity-measurement or through-transmission method and is explained in Fig. 12.1. The shadow of an in-homogeneity, which is illuminated by an ultrasonic wave, reduces under certain conditions the intensity of the wave received by a second probe. The name through-transmission method arises obviously from the fact that two probes are often positioned face to face on opposite sides of the specimen but that may not always be the case. Figure 12.2 shows an alternative arrangement of the shadow method where the beam is reflected before being influenced by the defect, and equally is could also be reflected afterwards.

The transmission method, which may include either reflection or through transmission, involves only the measurement of signal attenuation. This method is also used in flaw detection.


In the pulse-echo method, it is necessary that an internal flaw reflect at least part of the sound energy onto a receiving transducer. However, echoes from flaws are not essential to their detection. Merely the fact that the amplitude of the back reflection from a test piece is lower than that from an identical workpiece known to be free of flaws implies that the test piece contains one or more flaws. The technique of detecting the presence of flaws by sound attenuation is used in transmission methods as well as in the pulse-echo method. The main disadvantage of attenuation methods is that flaw depth cannot be measured.

Fig. 12.1 Principle of the shadow method

Fig. 12.2 Shadow method with reflectionFig. 12.3 Shadow method with guidance of the sound

3.15.6 Other Presentations

3.16 Testing Techniques3.16.1 Pulse Echo Method1. The advantages of pulse echo method is that the deflector could be locate

and assess accurately from one side of specimen. 2. The disadvantage ids that the sound path has to travel twice the distance,

thus more attenuations. 3. The presentation is an A-Scan Dispaly

3.16.2 Through Transmission Techniques

Two probes are used, positioned on opposite sides. The present of reflector is indicated by reduction or loss of receiving signal amplitude.

1. The advantages is that the sound has to travel a single path, thus material with higher attenuation could be checked, thicker material could be checked and higher frequency with improved sensitivity and resolution could be realized.

2. The disadvantages is that there is no indication of depth, access to both sides of specimen is required and change in coupling condition may be mistaken as defect. More elaborate set-up

3. The presentation is a Shadow Method

Through Transmission Techniques

The Through Transmission Shadow Method


3.16.3 The Tandem Techniques

The tandem method employed 2 probe on the same side , with each other spaced at a predetermined length. One transmitting signal the other set to received signal if reflected from a defect,\. The distance between the probe depends on the probe angle, material thickness and the depth of expected defects. The techniques are used to find for defects at predetermined depth such as in the root of double V weld. The presentation could be a A-Scan display.

The Tandem Techniques

Illustration showing the inspection of one zone. Phased array technology allows the simultaneous inspection of all zones with the same probe. Phased array offers complete coverage of the weld with one probe on either side of the weld.

Illustration showing the inspection of one zone. With conventional UT technology several probes are needed to cover all zones.

Phased array: Complete coverage with two probes

Conventional UT: Complete coverage with > 24 probes

http://www.olympus-ims.com/en/pipewizard/

3.16.4 Immersion Methods

In immersion method, compressional probe is mounted on a bridge immersed in water. The probe could be normal to the test piece as compressional probe or the bridge could be tilted to generate shear wave of various shear angle. Probe frequency of 25MHz is not uncommon for immersion method unlike the contact methods where the thin crustal may be too fragile to handle. The display could be a A, B, C Scans or through transmission shadow display.

During the set-up of immersion methods, the water path between the probe and the material surface is delay off the screen, so that the Zero starting point at the screen represent the front surface of the test material.

It is important to note that the longitudinal velocity in steel is 4 times of that of water, so the testing of steel the water gap should be greater than one quarter ( ¼ ) of steel thickness

Gap water > ¼ Steel Thickness, <(e.g. for 100mm steel the water gap shall be >25mm)

T

¼ T

3.17 UT Equipment Circuitry & ControlsAs with computers, the technology concerning ultrasonic equipment and systems is becoming somewhat transitory. Ultrasonic systems are either battery operated portable units, multi-component laboratory ultrasonic systems, or something in between. Whether they are based on modern digital technology or the fast disappearing analog original, systems (often defined as instrument plus transducer and cable) basically comprise the following components circuitry and controls.

3.17.1 Instrument Circuitry:

Although the electronic equipment used for ultrasonic inspection can vary greatly in detail among equipment manufacturers, all general-purpose units consist of a power supply, a pulser circuit, a search unit, a receiver-amplifier circuit, an oscilloscope, and an electronic clock. Many systems also include electronic equipment for signal conditioning, gating, automatic interpretation, and integration with a mechanical or electronic scanning system. Moreover, advances in microprocessor technology have extended the data acquisition and signal-processing capabilities of ultrasonic inspection systems.

Instrument Circuitry: Time base

The function of the time base, also called "sweep generator" in analog-display instruments, is to establish a display of sound travel time on the horizontal scale of the display. The horizontal scale can then be used for distancereadout. The range (coarse range, test range) control adjusts the scale for the range of distance to be displayed.

Instrument Circuitry: Screen picture of a specimen with back echo R and a group of defect indications F, with normal sweep at I-m range (a) and with scale expansion to 250 mm (b)

Instrument Circuitry: Power Supply.

Circuits that supply current for all functions of the instrument constitute the power supply, which is usually energized by conventional 115-V or 230-V alternating current. There are, however, many types and sizes of portable instruments for which the power is supplied by batteries contained in the unit.

Instrument Circuitry: Pulser Circuit.

When electronically triggered, the pulser circuit generates a burst of alternating voltage. The principal frequency of this burst, its duration, the profile of the envelope of the burst, and the burst repetition rate may be either fixed or adjustable, depending on the flexibility of the unit.

Instrument Circuitry: Receiver-amplifier circuits

electronically amplify return signals from the receiving transducer and often demodulate or otherwise modify the signals into a form suitable for display. The output from the receiver-amplifier circuit is a signal directly related to the intensity of the ultrasonic wave impinging on the receiving transducer. This output is fed into an oscilloscope or other display device.

Instrument Circuitry: Oscilloscope.

Data received are usually displayed on an oscilloscope in either video mode or radio frequency mode. In video mode display, only peak intensities are visible on the trace; in the RF mode, it is possible to observe the waveform of signal voltages. Some instruments have a selector switch so that the operator can choose the display mode, but others are designed for single-mode operation only

Instrument Circuitry: Signal-conditioning and gating

circuits are included in many commercial ultrasonic instruments. One common example of a signal-conditioning feature is a circuit that electronically compensates for the signal-amplitude loss caused by attenuation of the ultrasonic pulse in the test piece. Electronic gates, which monitor returning signals for pulses of selected amplitudes that occur within selected time-delay ranges, provide automatic interpretation. The set point ofa gate corresponds to a flaw of a certain size that is located within a prescribed depth range. Gates are often used to trigger alarms or to operate automatic systems that sort test pieces or identify rejectable pieces.

Instrument Circuitry: Image- and Data-Processing Equipment.

As a result of the development of microprocessors and modernelectronics, many ultrasonic inspection systems possess substantially improved capabilities in terms of signal processing and data acquisition. This development allows better flaw detection and evaluation (especially in composites) by improving the acquisition of transient ultrasonic waveforms and by enhancing the display and analysis of ultrasonic data. The development of microprocessor technology has also been useful in portable C-scan systems with hand-held transducers (see the section "Scanning Equipment" in this article).

Instrument Circuitry: Clock

The clock circuit initiates a chain of events that results in one complete cycle of a UT examination. The clock sends a trigger signal, at a regular interval, to both the (1) time base and to the (2) pulser. As the name “clock”' implies, thistrigger signal is repeated at a given frequency, called the pulse repetition rate(PRR). On some instruments pulse repetition rate is adjustable by the examiner; other instruments do it automatically. The electronic clock, or timer, serves as a source of logic pulses, reference voltage, and reference waveform. The clock coordinates operation of the entire electronic system.

Instrument Circuitry: Pulse Repetition Rate PRR

The pulse repetition rate establishes the number of times per second that a complete test cycle will occur. In instruments with adjustable pulse repetition rate, adjustment is made by a pulse repetition rate control, sometimes labeledREP RATE.

Instrument Circuitry: Pulser-Receiver

The pulser emits the electrical signal that activates the transducer. This signal, known as the initial pulse, is quite brief, usually lasting only several nanoseconds (10-9, billionths of a second). The output of the initial pulse is in the order of hundreds of volts; the brief duration provides a fast rise time to the full voltage. The pulser is connected via output connectors on theinstrument front panel to the transducer cable. The pulser is also connected, internally, through the receiver circuit, to the display, thus making available (depending upon the delay setting) a displayed initial pulse signal. This signal is, of course, present whether or not a transducer is connected to the instrument. When a transducer is connected, it is in the signal path between the pulser and the receiver and its output is displayed.

3.17.2 Instrument Control:

Even though the nomenclature used by different instrument manufacturers may vary, certain controls are required for the basic functions of any ultrasonic instrument. These functions include power supply, clock, pulser, receiver-amplifier, and display. In most cases, the entire electronic assembly, including the controls, is contained in one instrument.

Instrument Control: REJECT Control

It is intended for preventing the display of undesired low amplitude signals, called grass or hash, caused by metal noise such as echoes from materialgrain boundaries or inherent fine porosity. There are two types of REJECTcontrols installed on UT instruments: nonlinear REJECT and the more recently linear REJECT controls. Linear REJECT controls offer the advantage in that they do not affect vertical linearity of the display.

Instrument Control: DELAY and RANGE Controls

The controls are used to adjust the instruments time base for proper display of distances. The delay control shifts the horizontal signals to the left and rightwithout altering the spacing between them. The RANGE control expands or contracts the spacing between horizontal signals, corresponding to the Range of the sound travel to be displayed.

Instrument Control: GAIN Control

The sound amplitudes of individual reflectors returning to the transducer determine the relative heights of the corresponding vertical signals on the CRT. By adjusting the Gain Control, vertical display sensitivity and therefore determines the actual amplitude at which signals are displayed.

Gain controls for the receiver-amplifier circuit usually consist of fine- and coarse-sensitivity selectors or one control marked "sensitivity." For a clean video display, with low-level electronic noise eliminated, a reject control can be provided.

Instrument Control: Display Control

The display (oscilloscope) controls are usually screwdriver-adjusted, with the exception of the scale illumination and power on/off. After initial setup and calibration, the screwdriver-adjusted controls seldom require additional adjustment. The controls and their functions for the display unit usually consist of the following:

Controls for vertical position of the display on the oscilloscope screen. Controls for horizontal position of display on the oscilloscope screen. Controls for brightness of display. Control for adjusting focus of trace on the oscilloscope screen. Controls to correct for distortion or astigmatism that may be introduced as

the electron beam sweeps across the oscilloscope screen.

An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances.

A control that varies the level of illumination for a measuring grid usually incorporated in the transparent faceplate covering the oscilloscope screen.

Timing controls, which usually consist of sweep-delay and sweep-rate controls, to provide coarse and fine adjustments to suit the material and thickness of the test piece. The sweep-delay control is also used to position the sound entry point on the left side of the display screen, with a back reflection or multiples of back reflections visible on the right side of the screen.

On/off switch.

Instrument Controls:

A marker circuit, which provides regularly spaced secondary indications (often in the form of a square wave) on or below the sweep line to serve the same purpose as scribe marks on a ruler. This circuit is activated or left out of the display by a marker switch for on/off selection. Usually there will also be a marker-calibration or marker-adjustment control to permit selection of marker-circuit frequency. The higher the frequency, the closer the spacing of square waves, and the more accurate the measurements. Marker circuits are controlled by timing signals triggered by the electronic clock. Most modern ultrasonic instruments do not have marker circuits

A Gain circuit to electronically compensate for a drop in the amplitude of signals reflected from flaws located deep in the test piece. This circuit may be known as distance-amplitude correction, sensitivity-time control, time-corrected gain, or time-varied gain

Damping controls that can be used to shorten the pulse duration and thus adjust the length of the wave packet emanating from the transducer. Resolution is improved by higher values of damping

High-voltage or low-voltage driving current, which is selected for the transducer with a transducer voltage switch.

Gated alarm units, which enable the use of automatic alarms when flaws are detected. This is accomplished by setting up controllable time spans on the display that correspond to specific zones within the test piece. Signals appearing within the gates may automatically operate visual or audible alarms. These signals may also be passed on to display devices or strip-chart recorders or to external control devices. Gated alarm units usually have three controls: the gate-start or delay control, which adjusts the location of the leading edge of the gate on the oscilloscope trace; the gate-length control, which adjusts the length of the gate or the location of the gate trailing edge; and the alarm-level or sensitivity control, which establishes the minimum echo height necessary to activate an alarm circuit. A positive/negative logic switch determines whether the alarm is triggered above or below the threshold level.

Instrument Control: Gates

Most UT equipment is equipped with “gates” that can be superimposed on the time base so that a rapid response from a particular reflector can be obtained when they reach a certain predetermined amplitude. This can be adapted asa “go/no-go” monitoring device for some examinations. Gates can be set for an alarm to be triggered at a pre-determined amplitude (positive) with an increasing signal or (negative) with a decreasing signal amplitude. Gates areessential for some types of recording systems where they also serve to provide information to the recording devices or storage systems.

3.17.3 Pulse-Echo Instrumentation (A-Scan)

The UT system includes: the instrument, transducers, calibration standards, and the object being examined. These elements function together to form a chain of events during a typical UT that can be summarized as follows:

1. The instrument’s pulser electrically activates the transducer, causing it to send sound pulses into the test object.

2. The activation signal, called the initial pulse, is displayed as a vertical signal on the CRT.

3. As sound travels through the test object, it reflects from boundaries as well as from discontinuities within the material.

4. The instrument's time base initiates readout of time/distance information on the horizontal scale of the display.

5. A reflection from the surface opposite the entry surface is called a back reflection. These reflections reach the transducer, which converts them into electrical signals that are displayed on the CRT.

Figure above Block diagram circuitries are:

1. Transducer2. Pulser (clock)3. Receiver/amplifier4. Display (screen)

To understand how a typical ultrasonic system operates, it is necessary to view one cycle of events, or one pulse. The sequence is as follows.

1. The clock signals the pulser to provide a short, high-voltage pulse to the transducer while simultaneously supplying a voltage to the time-base trigger module.

2. The time-base trigger starts the spot in the CRT on its journey across the screen.

3. The voltage pulse reaches the transducer and is converted into mechanical vibrations (see piezoelectricity ), which enter the test piece. These vibrations (energy) now travel along their sound path through the test piece. All this time, the spot is moving horizontally across the CRT.

4. The energy in the test piece now reflects off the interface (back wall) back toward the transducer, where it is reconverted into a voltage. (The reconverted voltage is a fraction of its original value.)

5. This voltage is now received and amplified by the receiver/amplifier

Pulse Repetition Rate

- Gain - Frequency- Reject

Sweep & Range

Typical block diagram of an analog A-scan setup, including video-mode display, for basic pulse-echo ultrasonic inspection

A basic instrument contains several circuits:

power supply, clock (also called synchronizer or timer), time base (called sweep generator), pulser (also called transmitter), receiver (also called receiver-amplifier), and display.

3.17.4 B Scan Block diagram:

B-scan display is a plot of time versus distance, in which

one orthogonal axis on the display corresponds to elapsed time (depth), while the other axis represents the position of the transducer along a line

on the surface of the test piece relative to the position of the transducer at the start of the inspection.

Echo intensity is not measured directly as it is in A-scan inspection, but is often indicated semi quantitatively by the relative brightness of echo indications on an oscilloscope screen. A B-scan display can be likened to an imaginary cross section through the test piece where both front and back surfaces are shown in profile. Indications from reflecting interfaces within the test piece are also shown in profile, and the position, orientation, and depth of such interfaces along the imaginary cutting plane are revealed.

Applications.

The chief value of B-scan presentations is their ability to reveal the distribution of flaws in a part on a cross section of that part. Although B-scan techniques have been more widely used in medical applications than in industrial applications, B-scans can be used for the rapid screening of parts and for the selection of certain parts, or portions of certain parts, for more thorough inspection with A-scan techniques. Optimum results from B-scan techniques are generally obtained with small transducers and high frequencies.

Typical B-scan setup, including video-mode display, for basic pulse-echo ultrasonic inspection

First, the display is generated on an oscilloscope screen that is composed of a long-persistence phosphor, that is, a phosphor that continues to fluoresce long after the means of excitation ceases to fall on the fluorescing area of the screen. This characteristic of the oscilloscope in a B-scan system allows the imaginary cross section to be viewed as a whole without having to resort to permanent imaging methods, such as photographs. (Photographic equipment, facsimile recorders, or x-y plotters can be used to record B-scan data, especially when a permanent record is desired for later reference.)

Second, the oscilloscope input for one axis of the display is provided by an electromechanical device that generates an electrical voltage or digital signals proportional to the position of the transducer relative to a reference point on the surface of the test piece. Most B-scans are generated by scanning the search unit in a straight line across the surface of the test piece at a uniform rate. One axis of the display, usually the horizontal axis, represents the distance traveled along this line.

Third, echoes are indicated by bright spots on the screen rather than by deflections of the time trace. The position of a bright spot along the axis orthogonal to the search-unit position axis, usually measured top to bottom on the screen, indicates the depth of the echo within the test piece. Finally, to ensure that echoes are recorded as bright spots, the echo-intensity signal from the receiver-amplifier is connected to the trace-brightness control on the oscilloscope. In some systems, the brightness corresponding to different values of echo intensity may exhibit enough contrast to enable semi quantitative appraisal of echo intensity, which is related to flaw size and shape.

Signal Display.

The oscilloscope screen in Fig. 11 above illustrates the type of video-mode display that is generated by B-Scan equipment. On this screen, the internal flaw in the test piece shown at left in Fig. 11 above is shown only as a profile view of its top reflecting surface. Portions of the test piece that are behind this large reflecting surface are in shadow. The flaw length in the direction of search-unit travel is recorded, but the width (in a direction mutually perpendicular to the sound beam and the direction of search-unit travel) is not recorded except as it affects echo intensity and therefore echo-image brightness. Because the sound beam is slightly conical rather than truly cylindrical, flaws near the back surface of the test piece appear longer than those near the front surface.

3.17.5 C-scan display

C-scan display records echoes from the internal portions of test pieces as a function of the position of each reflecting interface within an area. Flaws are shown on a readout, superimposed on a plan view of the test piece, and both flaw size (flaw area) and position within the plan view are recorded. Flaw depth normally is not recorded, although it can be measured semiquantitatively by restricting the range of depths within the test piece that is covered in a given scan. With an increasing number of C-scan systems designed with on-board computers, other options in image processing and enhancement have become widely used in the presentation of flaw depth and the characterization of flaws. An example of a computer-processed C-scan image is shown in Fig. 11, in which a graphite-epoxy sample with impact damage was examined using time-of-flight data. The depth of damage is displayed with a color scale in the original photograph.

Typical C-scan setup, including display, for basic pulse-echo ultrasonic immersion inspection

System Setup.

In a basic C-scan system, shown schematically in Fig. 12 above, the search unit is moved over the surface of the test piece in a search pattern. The search pattern may take many forms; for example, a series of closely spaced parallel lines, a fine raster pattern, or a spiral pattern (polar scan). Mechanical linkage connects the search unit to x-axis and y-axis position indicators, which in turn feed position data to the x-y plotter or facsimile device. Echo recording systems vary; some produce a shaded-line scan with echo intensity recorded as a variation in line shading, while others indicate flaws by an absence of shading so that each flaw shows up as a blank space on the display (Fig. 12) above.

Gating. (Depth Gate)An electronic depth gate is another essential element in C-scan systems. A depth gate is an electronic circuit that allows only those echo signals that are received within a limited range of delay times following the initial pulse or interface echo to be admitted to the receiver-amplifier circuit. Usually, the depth gate is set so that front reflections and back reflections are just barely excluded from the display. Thus, only echoes from within the test piece are recorded, except for echoes from thin layers adjacent to both surfaces of the test piece. Depth gates are adjustable. By setting a depth gate for a narrow range of delay times, echo signals from a thin slice of the test piece parallel to the scanned surface can be recorded, with signals from other portions being excluded from the display.

Some C-scan systems, particularly automatic units, incorporate additional electronic gating circuits for marking, alarming, or charting. These gates can record or indicate information such as flaw depth or loss of back reflection, while the main display records an overall picture of flaw distribution.

Q79: In the pulse echo instrument, the synchronizer, clock, or timer circuit determine the:

a) Pulse lengthb) Gainc) Pulse repetition rate d) Sweep range

Q1: In an ultrasonic test system where signal amplitudes are displayed, an advantage of a frequency independent attenuator over a continuously variable gain control is that:

A. The pulse shape is less distortedB. The signal amplitude measured using the attenuator is independent

of frequencyC. The dynamic range of the system id decreasedD. The effect of amplification threshold is avoided.

Definition: Switch that controls the output power of the HV generator isthe attenuator.

Q1: The rate generator in B-scan equipment will invariably be directly connected to the:

A. The display intensity circuitB. The pulser circuitC. The RF amplifier circuitD. The horizontal sweep circuit

Q30: The time from the start of the ultrasonic pulse to the reverberations complete decay limit the maximum usable:

A. Pulse time-flaw rateB. Pulse/receiver rateC. Pulse repetition rateD. Modified pulse-time rate

Hint: A/B/D could not be the correct answers as they were not even the standard terms used.

Q129: An A-scan display, which shows a signal both above and below the sweep line is called:

A. A video displayB. A RF displayC. An audio displayD. Frequency modulated display

Q166: In a basic pulse echo instrument, the sunchronizer, clock or timer circuit determines the:

A. Pulse lengthB. GainC. Pulse repetition rateD. Sweep length

Q32: On many ultrasonic testing instruments, an operator conducting animmersion test can remove that portion of the screen presentation thatrepresents water distance by adjusting a:

A. Pulse length control.B. Reject control.C. Sweep delay control.D. Sweep length control.

121. In an ultrasonic instrument, the number of pulses produced by an instrument in a given period of time is known as the:

A. Pulse length of the instrumentB. Pulse recovery timeC. FrequencyD. Pulse repetition rate

122. In a basic pulse echo ultrasonic instrument, the component that coordinates the action and timing of other components is called a:

A. Display unitB. ReceiverC. Marker circuit or range marker circuitD. Synchronizer, clock, or timer

123. In a basic pulse echo ultrasonic instrument, the component that produces the voltage that activates the transducer is called:

A. An amplifierB. A receiverC. A pulserD. A synchronizer

124. In basic pulse echo ultrasonic instrument, the component that produces the time base line is called a:

A. Sweep circuitB. ReceiverC. PulserD. Synchronizer

125. In a basic pulse echo ultrasonic instrument, the component that produces visible signals on the CRT which are used to measure distance is called a:

A. Sweep circuitB. Marker circuitC. Receiver circuitD. Synchronizer

126. Most basic pulse echo ultrasonic instruments use:

A. Automatic read-out equipmentB. An A-scan presentationC. A B-scan presentationD. A C-scan presentation

3.18 Further Reading on Sub-Section 33.18.1 What is reflection, refraction, diffraction, and interference?

What exactly is reflection, refraction, diffraction, and interference?Reflection occurs when a wave hits something and then bounces it off it.Refraction is the bending of a wave caused by a change in its speed as it moves from one medium to another.

Diffraction occurs when an object causes a wave to change direction and bend around it. Interference is when two or more waves overlap and combine to make a new wave of lesser or more amplitude.

This picture shows how reflection of light works and the names of the beams in a reflection.

http://light-and-sound-project.wikispaces.com/3.+What+is+reflection,+refraction,+diffraction,+and+interference%3F

3.18.2 Reflection西塘

In this picture there is two different beams, and those beams create angles. The beams are referred to as the reflected beam and the incident beam. The dotted line is the line that is perpendicular to the mirror, and it splits the large angle into the two different angles. The first angle is the angle of reflection, and it is formed by the reflected beam and the perpendicular line. The other angle is the angle of incidence which is formed by the incident beam and the perpendicular line. These two angles are always the same measure, although it sometimes might be a larger or smaller angle.

How do reflection, refraction, and diffraction relate to light?Reflection happens when a light is turned on, and it is in an enclosed area. If someone is in a enclosed area, and a light is turned on they are going to be able to see it. Then the light will continue, hit a wall, and it would reflect back to the human eye.

This picture shows how water waves will diffract around an island. This picture also shows constructive and destructive interference.

The diffraction happens in this picture when the water waves pass between the two rocks. When the waves get onto the other side of the two rocks the waves are shaped as an arc (a U shape). The constructive and destructive interference happens by the rock in the middle of the picture to the left. The waves that are passing between the two rocks meet up with the waves passing around the one rock to the left, and the waves combine. Some waves will cancel each other out, and some will add to each other and make a bigger amplitude.

3.18.3 Refraction happens when light is shown through another material, and it changes the way it is being shown. An example is when you fill a cup with water, and then you place a pencil in the water. When you look at the pencil from the side it looks as though the pencil is broken where the pencil enters the water. This is due to refraction, and the bending of the waves before it enters your eyes. This picture shows the broken pencil experiment.

3.18.4 Diffraction

Diffraction

Diffraction

Diffraction

Diffraction

Diffraction

Diffraction

Diffraction happen when light tries to go through an opening. If you are in a dark hallway, and a room has a light on, you will be able to see he light, but it will only light up a section of the hallway, and you won't be in the light until you are almost directly in front of the room.

This diagram shows an interference. In this diagram it happens to be constructive interference, but this is not the only type of interference.

3.18.5 Interference

Interference

Interference

3.19 Questions & Answers

Q11: When maximum sensitivity is required from a transducer:

A. Straight beam transducer should be usedB. Large diameter crystals are requiredC. The piezoelectric element should be driven at its fundamental

frequencyD. The bandwidth of the transducer should be as large as possible.

Q12: The 1 MHz transducer that should normally have the best time of distance resolution is a:

A. Quartz crystal with air backingB. Quartz crystal with phenolic backingC. Barium titanate transducer with phenolic backingD. Lithium Sulphate transducer with epoxy backing

Hint: 1 MHz as Lithium Sulphate is not easily cut to very thin thickness, best distance resolution due to the fact the Lithium Sulphate is the best receiver of ultrasound energy.

Q3: The ultrasonic instrument used for examination of welding shall becapable of generating frequencies:

A. more than 5 MHzB. more than 10 MHzC. less than 1 MHzD. 1 MHz to 5 MHz

Q4. Calibration of ultrasonic equipment shall be doneA. at beginning of examinationB. both at beginning and end of the examinationC. both at beginning and also at every two hours intervalD. at beginning end, every two hours interval and whenever a change

operator

Q4. Calibration of ultrasonic equipment shall be done• at beginning of examination• both at beginning and end of the examination• both at beginning and also at every two hours interval• at beginning end, every two hours interval and whenever a change

operator

Q15: Entry surface resolution is a characteristic of an ultrasonic testing system which defines its ability to:

A. Detect discontinuities oriented in a direction parallel to the ultrasonic beam.B. Detect discontinuities located in the center of a forging containing a fine

metallurgic structure.C. Detect minute surface scratches.D. Detect discontinuities located just beneath the entry surface in the

part being tested.

Discussion Topic: Factors affecting the Entry Surface Resolution

Q15: Entry surface resolution is a characteristic of an ultrasonic testing system which defines its ability to:

A. Detect discontinuities oriented in a direction parallel to the ultrasonic beam.B. Detect discontinuities located in the center of a forging containing a fine metallurgic structure.C. Detect minute surface scratches.D. Detect discontinuities located just beneath the entry surface in the part being tested.

List of factors:

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