BARC/2002/E/034 g

DESIGN AND DEVELOPMENT OF AN ULTRASONIC PULSER-RECEIVER UNIT FOR NON-DESTRUCTIVE TESTING OF MATERIALS

by V. H. Patankar and V. M. Joshi

Electronics Division

Government of India

Bhabha Atomic Research Centre mi Mumbai - 400 085, *rH?r India

2002

BARC/2002/E/034

GOVERNMENT OF INDIA ATOMIC ENERGY COMMISSION

DESIGN AND DEVELOPMENT OF AN ULTRASONIC PULSER-RECEIVER UNIT FOR NON-DESTRUCTIVE

TESTING OF MATERIALS by

V. H. Patankar and V. M. Joshi Electronics Division

BHABHA ATOMIC RESEARCH CENTRE MUMBAI, INDIA

2002

BARC/2002/E/034

B I B L I O G R A P H I C D E S C R I P T I O N S H E E T F O R T E C H N I C A L R E P O R T (as per IS : 9400 - 1980)

01 Security classification : Unclassified

02 Distribution : External

03 Report status : N e w

04 Series : BARC External

05 Report type : Technical Report

06 Report No. : BARC/2002/E7034

07 Part No. or Volume No. :

08 Contract No. :

10 Title and subtitle: Design and development of an ultrasonic pulser-receiver unit for non-destructive testing of materials

11 Collation : 26 p., 3 figs.

13 Project No.:

20 Personal author(s) ; V.H. Patankar; V . M . Joshi

21 Affiliation of author(s) : Electronics Division, Bhabha Atomic Research Centre, Mumbai

22 Corporate aulhor(s) : Bhabha Atomic Research Centre. Mumbai-400 085

23 Originating unit: Electronics Division, BARC, Mumbai

24 Sponsor(s) Name : Department of Atomic Energy

Type : Government

-/- Contd... (ii)

BARC/2002/E/034

30 Date of submission: October 2002

31 Publication/Issue date: November 2002

40 Publisher/Distributor : Head, Library and Information Services Division, Bhabha Atomic Research Centre, Mumbai

42 Form of distribution : Hard copy

50 Language of text : English

51 Language of summary : English

52 No. of references. 17 refs.

53 Gives data on .

60 Abstract: The Pulser/Receiver constitutes the most vital part o f an Ultrasonic Flaw Detector or an Ultrasonic Imaging System used for inspection o f materials. The ultrasonic properties o f the material and resolution requirements govern the choice o f the frequency o f ultrasound that can be optimally used. The Pulser/Receiver in turn decides the efficiency o f excitation o f the transducer and the overall signal to noise ratio of the system for best sensitivity and resolution. A variety of pulsers are used in the ultrasonic instruments employed for materials inspection. This report describes a square wave type of an ultrasonic Pulser-Receiver unit developed at Ultrasonic instrumentation Section, Electronics Division, B A R C . It has been primarily designed for excitation o f the transducer that is used with a multi-channel Ultrasonic Imaging System " U L T I M A 1 0 0 M " targeted for inspection of SS403 billets, which are in turn used as the base material for fabrication o f end Fittings for coolant channels o f Pressurised Heavy Water Nuclear Reactors ( P H W R s ) . The design o f the pulser is based upon very fast M O S F L T s , configured as electronic switches. The pulser is operated with a linear bipolar H.V. supply (+ / - 500V max.) . The receiver provides a 60 dB gain with a -3 d B B W of 40 M H z . This Pulser/Receiver unit has been successfully interfaced with a 4 channel " U L T I M A 100 M 4 " Multichannel Ultrasonic C-Scan Imaging system, also designed and developed by the authors at Ultrasonic Instrumentation Section (Electronics Division, B A R C ) and supplied to Centre for Design and Manufacture - C D M (formerly known as Central Workshops), B A R C . This system is being regularly used in C-Scan imaging mode for volumetric inspection of SS403 billets for end fittings o f 500 M We P H W R s .

70 Keywords/Descriptors : PHWR TYPE REACTORS; ULTRASONIC TESTING; TRANSDUCERS;

ZIRCALOY 2; PRESSURE TUBES; IMAGES; ULTRASONIC WAVES; MULTI-CHANNEL ANALYZERS;

SPECIFICATIONS

71 WIS Subject Category: S21

99 Supplementary elements :

ii-

Abstract

The Pulser/Receiver constitutes the most vital part of an Ultrasonic Flaw

Detector or an Ultrasonic Imaging System used for inspection of materials. The

ultrasonic properties of the material and resolution requirements govern the choice of

the frequency of ultrasound that can be optimally used. The Pulser/Receiver in turn

decides the efficiency of excitation of the transducer and the overall signal to noise

ratio of the system for best sensitivity and resolution.

A variety of pulsers are used in the ultrasonic instruments employed for

materials inspection. This report describes a square wave type of an ultrasonic

Pulser-Receiver unit developed at Ultrasonic Instrumentation Section, Electronics

Division, BARC. It has been primarily designed for excitation of the transducer that is

used with a multi-channel Ultrasonic Imaging System "ULTIMA 100M" targeted for

inspection of SS403 billets, which are in turn used as the base material for

fabrication of end fittings for coolant channels of Pressurised Heavy Water Nuclear

Reactors (PHWRs). The design of the pulser is based upon very fast MOSFETs,

configured as electronic switches. The pulser is operated with a linear bipolar H.V.

supply (+/- 500V max.). The receiver provides a 60 dB gain with a -3 dB BW of 40

MHz.

This Pulser/Receiver unit has been successfully interfaced with a 4 channel

"ULTIMA 100 M4" Multichannel Ultrasonic C-Scan Imaging system, also designed

and developed by the authors at Ultrasonic Instrumentation Section (Electronics

Division, BARC) and supplied to Centre for Design and Manufacturer - CDM

(formerly known as Central Workshops), BARC. This system is being regularly used

in C-Scan imaging mode for volumetric inspection of SS403 billets for end fittings of

500 MWe PHWRs.

CONTENTS

Page No.

1. Introduction - 1

2. System Description 8

3. Applications •• - 11

4. Summary and Future work 19

5. Acknowledgement 19

6. References 20

Introduction:

Use of ultrasound for detection of internal flaws of metallic objects is now a well-matured technique. For Non-Destructive Testing (NDT) of metallic objects, ultrasound in the range of 1-15 MHz is generally employed, though for some specialized applications, frequencies upto 100-150 MHz are also required. Use of Piezo-electric transducers is most common in all such applications. Ultrasound is generated when a high voltage, narrow pulse, excites a piezo-electric transducer. A close physical contact with the transducer enables coupling of ultrasonic energy into the desired material/object. Any acoustic discontinuity within the material will result in a change in the acoustic impedance and consequently the incident ultrasonic wave will be partially reflected back from such a location. The receiver transducer will pick up the reflected ultrasound and convert it back into an electrical signal. The intensity of the reflected waves depends upon the geometry and orientation of the discontinuity. Accordingly, the received echo signal displayed on an oscilloscope, can give a fair idea about the presence of defects if any in the object under testing. The typical waveform seen on the oscilloscope is commonly termed as 'A-Scan'.

Ultrasonic Flaw Detector, which is a very widely used instrument for NDT of various types of materials, essentially works on the basic principle of Pulse-echo (PE) operation. In PE mode of excitation, basically the same transducer works as a transmitter of ultrasound and also as the receiver of the reflected ultrasonic waves received from discontinuities within the material under testing. In the Transmit-Receive (T-R) or Tandem technique of excitation, typically used for inspection of metallic plates etc., one transducer operates in transmit mode and the other transducer will have to be placed either on the opposite side of the plate along the line of sight of the transmitter or both transmitting and receiving transducers can be placed in an angular manner. In its simplest form the Flaw Detector consists of electronic circuits for generation and reception of ultrasound and a CRT/LCD with associated electronics for display of A-scan waveforms. Provisions are made so that the operator can set the relevant parameters in an optimum way for obtaining a clear and unambiguous display of echo signals. Such parameters include energy and damping controls, gain/sensitivity, band-pass filters, suppression (of spurious or noise signals), and time delays. User adjustable cursors may be provided for measurement of depths of defects, thickness measurement and measuring the amplitudes of defect echoes.

It must however be emphasized that the basic Pulser/Receiver section constitutes the most vital part of a flaw detector. This report provides information regarding a Square wave type of Pulser/Receiver unit developed recently at Ultrasonic Instrumentation Section, Electronics Division of BARC.

1.1 Historical background:

Within the human range of audible sound frequencies, the wavelength is rather large in most metals. Until 1870, it was not possible to generate and detect sound waves at much higher frequencies needed for detecting smaller discontinuities in metallic objects. During 1870, Lord Rayleigh's work on sound "The Theory of Sound" explained the nature and properties of sound waves in solids, liquids and gases which led to the development of the techniques that are currently in use in ultrasonic NDT. In 1880, Curie brothers and Lippmann found that an

1

electric potential could be generated by applying mechanical pressure to deform plates of certain crystals cut in a particular fashion. This effect is known as the direct Piezo-electric effect. The following year, Lippmann discovered that the inverse piezo­electric effect where in the application of an electric signal to these plates caused a mechanical distortion. In those days, naturally occurring crystals of quartz, tourmaline and Rochelle salt were observed to display the piezo-electric effect. During 1929, S.Y. Sokolov, in Russia, was the first to suggest, usage of ultrasonic waves for detecting discontinuities in metals.

1.1.1 Piezo-electric transducers:

Piezo-electricity is based on a property of the elementary cell of crystalline materials, which have no centre of symmetry. Barium titanate, Lead metaniobate, Lead Zirconate Titanate (PZT), Barium Sodium niobate etc. are some of the popular piezo-electric ceramics used for generating and receiving ultrasound in materials. Piezo-electric monocrystals such as lithium sulphate, lithium niobate, are used for very specific applications. A plastic thin film known as Polyvinylidene Fluoride (PVDF) also shows strong piezo-electric properties and is used for high frequency (upto 100 MHz) ultrasound applications in materials inspection.

If one considers a plate of such a piezo-electric material, which is deformed by an applied mechanical pressure (direct piezo-electric effect), the system behaves as a receiver of ultrasound. The reciprocal effect forms the basis for an acoustic transmitter in which alternating voltage applied across the two faces of the crystal coated with electrodes, causes the plate to oscillate at the resonance. Corresponding to a sound velocity "c" for longitudinal waves in the plate material, its thickness d is related as:

d = X = _c_ where 2 2f0

X. = wavelength of the ultrasonic wave (mm) c = velocity of propagation (mm/|isec) d = thickness of a piezo-electric plate (mm) f = fundamental frequency of oscillation for the plate (MHz)

Hence the characteristic or natural or fundamental frequency of the plate is: fo = c / 2 d .

It may be noted that, thickness oscillations can be achieved by longitudinal as well as transverse waves.

A freely vibrating piezo-electric plate of thickness d = X/2 excited at its fundamental frequency can be represented electrically by its static capacitance C 0 in parallel with an L S l C s and ohmic resistance R s . Equivalent circuit of a typical piezo­electric crystal is shown in Fig 1.2(A).

2

C o

Rs

Ls

Cs

Fig. 1.2 (A) Equivalent Circuit of Piezo-Electric Crystal/ Ultrasonic Transducer

Modern transmitter circuits usually have a very low o/p impedance. Therefore electrical matching of a transducer is only necessary to the input impedance of the amplifier. If these impedances differ very much (as in case of very long cables) interfering reflections may get introduced in the receiving amplifier. Tuning of the electric circuit to equal the mechanical resonance frequency of the plate will not only improves the sensitivity for the frequency used, but also suppresses interfering frequencies.

1.2 Methods for Generation of Ultrasonic Waves:

Transducers used for ultrasonic testing incorporate a thin plate of a piezo-electric material to convert electrical energy, typically stored in a capacitor, into an ultrasonic signal. In most of the flaw detection and thickness gauging applications, it is advantageous to generate a compact ultrasonic waveform. This is best achieved by exciting the transducer with a short, unipolar voltage pulse whose rise time is shorter than the time required for an ultrasonic pulse to move through the piezo-electric plate. By and large, four different types of ultrasonic transmitters/ generators are widely used for generation of ultrasound: Spike pulser, Square wave pulser, Bipolar tone burst pulser and Step pulser and these are discussed below.

1.3.1 Spike Pulser:

Spike Pulsers are perhaps the most commonly used electronic circuits for exciting piezo-electric transducers. The schematic for spike pulsers is shown in fig. 1.3(A).

3

CHARGING KtSISTOK

+IIV DC POWER SUPPLY Kc

SWITCH i / (SCR/ \ '

TRANSISTOR/ MOSFF.T)

CAPACITOR H — Ct

DAMPING Rd RES1STOJ

OUTPUT o y

-HV f ULTRASONIC TRANSDUCER

FIG. 1.3 (A) SCHEMATIC DIAGRAM OF A SPIKE PULSER

Here, the charging capacitor C c is charged to a high voltage "+HV" (which ranges from 100 to 500 V) through the charging resistor R c and the damping resistor R D . In most applications, the value of Cc ranges from 1 to 4 nanofarads while R c and R D seldom exceed 10 KQ and 200f i respectively. Thus the charging capacitor is charged to the full value (+HV) in a few microseconds. Thus, pulse repetition frequencies upto 5 KHz can be easily obtained.

For the purpose of excitation, the switch S is rapidly closed for a short time interval. The closure of S. causes the voltage of the fully charged capacitor Cc to appear across the terminals of the transducer. This abrupt voltage change causes the piezo-electric material of the transducer to respond by emitting an ultrasonic wave. The exciting voltage then rapidly decays because of the damping resistor Ro, connected in parallel with the transducer. The value of R D typically ranges over 10 to 200ohms. This value can be adjusted by the user to accommodate different transducer impedances. Proper adjustment of R D is important because, it directly determines transducer ringing time and the resulting near surface and axial resolution.

Since the acoustic pressure at the front face of the transducer is directly proportional the time derivative of the applied voltage d (HV) / dt, it is important to minimize the rise time of the applied pulse. The rise time is primarily affected by the speed at which the switch S can be fully closed and by the presence of parasitic inductances in series with the capacitor, switch and transducer.

Ultrasonic transducers are typically connected to the pulse with a length of co-axial cable (either RG174/RG58U) whose capacitance increases at a rate about 100 pF/m. Therefore, the total capacitance of several meters of cable can be easily equal or exceed the capacitance of many transducers. In such cases, a major portion of the pulse energy can be shunted away from the transducer.

The efficiency of the excitation can also be degraded by the series inductances and other parasitic impedances. Series inductances tend to increase rise times and prevent the high frequency portion of the pulse energy from reaching the transducer. These effects may severely affect the ability of a spike pulse to efficiently excite thin film transducers, which are typically 50 j im thick and exhibit capacitances of only a few picofarads.

4

1.3.2 Square Wave Pulser:

The rapid development of new fast switching semiconductor devices has led to the proliferation of square wave pulsers in ultrasonic instruments. The basic disadvantages of square wave pulsers (high component count and appreciable power consumption) are offset by important operational advantages. The use of square wave pulser, increases the ability of the user to control and stabilize important test parameters, including the harmonic content (spectrum) of the transmitted ultrasonic pulses. Also, the use of square wave pulse may result in higher pulse amplitudes and thus provides higher sensitivity needed for detection of smaller discontinuities. Fig.1.3 (B) shows the schematic of a square wave pulser.

- H V

DC POWKR.SUPPLY

•HIV

DC POWER SUPPLY

Cliatrfinfi Resistor

r * . M O S K K I

T r i e i t r l

Charging Rcf islur

Rrl

P-MOSFET

i i r >--

Resistor

1 >

FIG.1.3 (B) SCHEMATIC DIAGRAM OF A SQUARE-WAVE PULSER

High power MOSFETs are commonly used in square wave pulsers intended to drive transducers between frequencies 0.1 to 50 MHz. Some very fast MOSFETs enable generation of very short pulses of the order of 10-20 nanoseconds with an amplitude of around 1000V. They can handle pulse currents of 30A and higher. Consequently, square wave pulsers are well suited for driving large size, low frequency transducers which frequently exhibit high capacitances.

Fig. 1.3(B) shows...initially, the square wave pulser operates in a manner similar to that of spike pulser. The sharp transition associated with the closing of the switch S1 , causes the generation of an ultrasonic signal by the transducer. Because of high value of C c , the pulse voltage is maintained at -HV while the switch S1 remains in closed position for duration "T". When switch S1 is restored to its original open position, the switch S2 is closed and thus the output pulse voltage returns to +HV. This second abrupt transition also causes the generation of an ultrasonic signal. Here also, the switch S2 is closed for duration T . (Here, +HV can be set to +LV [5-12V] to minimize the effect of second time excitation of transmitter on second abrupt transition due to closing of S2.) After the time interval corresponding to 2T, both the switches remain off, thereby keeping the transducer quiescent and responding to echo signals.

The time duration "T" of the square wave pulse must be carefully adjusted to produce a positive interference between the ultrasonic signals excited by the positive going and negative going transitions of the transducer. If the pulse is too long, then a distorted ultrasonic waveform is observed. If the pulse is too short, then

5

the ultrasonic pulse amplitude is significantly smaller than that achieved with an equivalent spike pulser. In practice, the pulse duration is adjusted precisely, by the operator using front panel controls.

A propedy tuned square wave pulser can generate twice as much echo signal amplitude as a spike pulse charged to the same high voltage. Even larger improvements in signal strength are possible when a suitable impedance-matching device is employed between the transducer and the pulser.

The theoretical and practical advantages of square wave pulsers are now well known. Except for specialized applications, such as thickness gages and very high-resolution discontinuity detectors, the use of square wave pulser leads to a better performance as compared to a spike pulser. To optimize the performance of a superior square wave pulser, the damping resistor R D and pulse duration must be adjusted independently for each transducer. In spike pulser, however, only the value of damping resistor R D is operator adjustable.

1.3.3 Tone Burst Pulser:

Tone burst pulser operation is essentially an extension of a square wave pulser operation and can be achieved by repetitively operating the square wave pulser shown in Fig. 1.3 (B). The resulting voltage waveforms are shown in Fig. 1.3(C).

ov jum SI - ON

SI - OFF

OV

+J1V

OV

UUU1 S2-ON

SI-OFF

TRANSDUCER OUTPUT

•HV

FIG. 1.3 (C) TIMING DIAGRAM FOR TONE - BURST PULSER.

The main advantage of operating the square wave pulser in the tone burst mode is that it allows the operator to maximize the energy of the transmitted signal at a specific frequency.

Tone burst operation can also be achieved when a spike pulser is used to drive an inductively tuned transducers. However, in this case, frequency control can be realized by altering the voltage of the tuning inductor.

Tone burst pulsers are often designed for compatibility with impedance matching networks required to maximize the output of unconventional transducers: Electromagnetic Acoustic transducers, air coupled transducers, dry coupled and roller probes. Pulsers capable of generating 450V tone bursts at frequencies of several MHz are available. Tone burst excitation is often used in special instruments,

6

including acoustic microscopes, where frequencies of several GHz have been demonstrated. Also, tone burst signals are used in many ultrasonic interferometers for material velocity measurements.

1.3.4 Step Pulsers:

The excitation of ultrasonic transducers with step pulses requires circuits that are topologically more complex than those discussed earlier. Fig. 1.3(D) shows a circuit that can imposes a shaped step signal upon a piezo­electric transducer.

DIRECT CURRENT

POWER SUrfLY

TIMING GENERATOR

SWITCH!

- J

• H V

ULTRASONIC

TRANSDUCER

TRANSDUCER EXITATION

J n

SW'I - ON

S W I - OFF

SW2 - ON

SW2 • OFF

Trftniducer

Output

TRANSDUCER EXITATION

Fig.1.3 (D) Schematic Diagram for Step Pulser.

Fig. 1.3 (D) shows the timing diagram for the step pulser. First the switchl is closed to allow the transducer to charge to a high voltage. Next Switch 1 is restored to the open position and switch Switch2 causes the transducer voltage to rapidly decay to zero. This rapid transition causes the generation of the unipolar ultrasonic waveform. In the case of a broadband, thin film, ferroelectric polymer transducer, the transducer output voltage is more compact than the bipolar pulse produced by a spike pulser. In this case, an external damping resistor is not required because a transducer with high internal damping is used.

7

2.1 System Description:

Ultrasonic Instrumentation Section, Electronics Division, B.A.R.C. has designed and developed an Ultrasonic, a square wave type Pulser-receiver for NDT of metallic parts. The operation of such an ultrasonic pulser has been discussed in section 1.3.

This Pulser-Receiver has been successfully interfaced with a 4-channel Ultrasonic C-Scan Imaging System "ULTIMA 100M4" designed and developed by the authors at Ultrasonic Instrumentation Section [ref. 15, 16]. Recently "ULTIMA 100M4" system has been supplied to CDM, B.A.R.C for inspection of SS403 -stainless steel forging billets (190 mm 4>and 2500 mm in length). Here, for these billets, a minimum size of the defect to be detected (i.e. the defect standard) is 1.2 mm Flat Bottom Hole (FBH) of 10% of the wall thickness deep (i.e. 19 mm deep). With the PC Add on "PR1000" Spike type Pulser Receiver board [ref. 10] developed earlier, picking up of an echo of this minimum defect standard was found to be very difficult because of insufficient output power and larger noise level. Hence to meet this specific NDT requirement, a square wave type, multichannel ultrasonic Pulser and Receiver has been designed, evaluated and also incorporated in the ULTIMA 100M4 system.

The design approach adopted here has been fairly a straight forward. The square wave type Pulser is configured in a stand-alone unit (19" rack mountable type, 2U in height approximately) with a linear, low ripple, regulated power supply for both low and high voltage sections. The unit is named as "SQ-PR". The SQ-PR has been equipped with a maximum 2 channels (can be expanded to 4 channels) of ultrasonic Pulsers and respective receiver sections. With such a modular approach, the SQ-PR services as a rack mountable 2-channel, low noise Ultrasonic Pulser Receiver Unit. Two such SQ-PR units have been fabricated and supplied to CDM, BARC for the volumetric inspection of SS403 billets. Ultrasonically inspected, defect free billets are then machined for making the end fitting bodies for 500 MWe PHWRs.

2.2 Pulser-Receiver Details:

The SQ-PR unit is a stand-alone unit and consists of following circuits:

1. Timing Generation logic 2. Ultrasonic Pulser 3. Limiter and Wideband, High Gain Receiver Section 4. Linear, regulated DC power supply (+/-100 to 500V).

The SQ-PR unit has facilities for tuning all the parameters of pulser and receiver at the factory. Parameters such as pulse repetition frequency (for Internal Trigger mode), pulse width and receiver gains are preset such that the SQ-PR unit is tailored for specific applications. The block diagram of the SQ-PR is shown in fig. 2.2. (A).

8

I N T - T R C I X T T K C

J TIMING i GENERATOi

( T O

LEVEL —f j n u M A T n i —f—— -f j r m T o u v i — f

T i - o i n r i HIGH SQURE

MOSTET , DRIVERS :

WAVE T i - O V R TVW I

ULTRASONIC TRANSDUCER

CHI CHI

T X - O U T l > -

TX.oun>-

•urn£> A M P ^ >

n.irrej> A M P > 1

BANDPASS FILTER

BANDPASS FILTER

LINE

LIVER RX-OUTI

r i LINE DRIVER

1 Fig. 2.2(A) Schematic Diagram For 2-Channel Square Wave Pulser - Receiver

2.2.1. Timing Generation:

N - MOSFET ON 1R.

P - MOSFET ON

PRF - I I -

- I I -

Trigjjerl

Trigger?

Tx - OUT

- H V ~ l l

+HV

Fig. 2.2(B) Timing Diagram For Square Wave Pulser

The entire timing generation circuit is implemented using fast 74 F family devices. Each pulser channel can work either with external or (on board) internal trigger signal. The phase-shifted signals needed for the operation of the pulser are generated using mono-stable multivibrators and then fed through 50 ohms line drivers to the level translator circuit. Corresponding block diagram is shown in fig. 2.2(A).

2.2.2. Level Translators:

The fast MOSFET switches used at the final stage of the step pulser being enhancement type, the gate driving pulses are required to have a peak 12 V excursions. Fast MOSFET drivers are used to maintain sharp rise and fall times (few nanoseconds) for the pulses shown in fig. 2.2(B). The output MOSFET devices operate in Push-Pull manner.

9

2.2.3. Complementary output Power MOSFET stage:

The output stage consists of a Push Pull configuration power MOSFETs as shown in Fig. 2.2 (C) The gate driving pulses received from the high speed MOSFET drivers are fed to output devices. The closing of n-channel MOSFET switch (SW1), energises the piezo-electric crystal with the sharp rising edge making it resonate. The SW1-switch, remains ON for duration T V As soon as the SW1 switch is turned OFF, the complementary switch (SW2) would be closed for a similar time interval (T) thereby applying an additional pulse of reverse polarity. Hence, the piezo-electric crystal will resonate again. The pulse width "T" for closing of switches SW1 & SW2 is carefully so adjusted that the final ultrasound wave generated due to turning ON of SW1 followed by SW2 will have a constructive effect, also thus generates maximum energy. The TX-OUT signal for each channel has been brought on to the respective BNC connector, provided on the front panel. Each Pulser when suitably tuned, is capable of exciting ultrasonic transducers over 1-30 MHz frequency range.

+HV.-LV

TRIGGI-:R2^

SATE: 1 SOURCE!

SW2 MOSFF.T P

DRAIN:

+HV.VLV

TRIGC iERl\^_ OATH I

DRAIN I

SWI

MOSFET N

SOURCE I

-HV

TX-OUT

ULTRASONIC TRANSDUCER

V V

-I IV

Fig. 2.2 (C) Complementary Type (Power MOSFET) Output Stage

2.2.4. Receiver Section: As the pulser excites the ultrasonic transducer, ultrasonic waves get

generated. This energy is coupled to the material under testing when it maintains a firm contact with the transducer. Whenever there is a change in the acoustic impedance, within the material, a partial reflection of ultrasound occurs. The

10

ultrasonic waves thus reflected back are picked up by the piezo-electric crystal, which now operates as a receiver. Thus a single transducer, operates as a transmitter and receiver (Pulse-Echo mode). Through the limiter circuit followed by a buffer, the echo signals are fed to a low noise, wide band amplifier. To wideband, high gain amplifier stages are cascaded to provide a maximum gain of 66 dB. The gain is optimised in accordance to the transducer's sensitivity so that the echo from defect is easily detected. The -3dB bandwidth of the receiver stage is 40 MHz. With the help of suitable band-pass filters, the SNR is enhanced. The RX-OUT signal for each channel (max. 2 channels) is brought onto the BNC connector on the front panel and can be routed to the oscilloscope for display of A-scan trace or to the Ultrasonic Imaging System for Digitization, Storage and Display of A-Scan waveform or B/C-Scan imaging purpose.

3.1 Applications:

The SQ-PR Ultrasonic Pulser Receiver Unit has been designed and developed for Non-destructive Testing/Evaluation (NDT/NDE) of fabricated mechanical components/parts. This unit is stand alone in nature and can be very easily interfaced to any Ultrasonic Imaging System or high-speed data acquisition system for NDT of materials due to features such as INT-Trg or EXT-Trg. In the following sub-sections, two practical applications for the SQ-PR unit are described.

3.1.1 Ultrasonic Inspection / C-Scan Imaging of SS403 billet forgings for End-fittings of 500 MWe PHWRs:

For a 500 MWe, Indian, PHWR, more than 600 End fittings are required and they are located at both the ends of every coolant channel. End fitting is < a very critical mechanical component for the PHWR. Before the actual machining of the End fitting, the billet (in solid form has dimensions of 190 mm <D and 2500 mm of length) is volumetrically inspected using C-Scan Imaging Technique in order to detect defects if any.

At Ultrasonic Instrumentation Section, Electronics Division, B.A.R.C. the authors have designed and developed an "ULTIMA 100M4" a 4-Channel ultrasonic C-scan imaging system tailored for inspection of End-fitting billets as per specific requirement of Centre for Design and Manufacture (CDM), (formerly known as CWS), BARC.

For ultrasonic volumetric inspection of billets, contact type transducer of 4 MHz frequency has been used for each channel. Each channel of SQ-PR unit has been tuned and optimized to individual transducer, in order to get the best sensitivity for easy detection of standard defect. (The defect standard for the End fitting billet is, 1.2 mm. Flat Bottom Hole (FBH), 19 mm deep i.e. 10% of the 190 mm billet dia). The stipulated inspection requirement for this defect standard is such that, the echo from this defect should be at least 80% of the full screen height (FSH). Results achieved with the help of a SQ-PR unit have been found to be extremely satisfactory. The pictorial view of the Square wave Pulser-Receiver unit; ULTIMA 100M4 system; SS Billets stacking arrangement and 4-Axis mechanical scanner assembly for C-Scan imaging of the billets (all located at CDM, BARC) are shown in Fig. 3.1.1 (A).

11

Standard B-Scan and C-Scan type cross sectional images of a defect standard and C-scan projection image of the defect standard for SS billets are shown in fig. 3.1.1 (B), 3.1.1 (C) respectively.

3.1.2 Ultrasonic Inspection of Zircaloy-2 Pressure Tubes (PT) for220MWe PHWRs:

The SQ-PR unit has been successfully interfaced to "Winspect" Ultrasonic C-Scan Imaging System (UTEX, Canada make) at Atomic Fuels Division (AFD), BARC. Since the SQ-PR unit can operate with EXT-Trg, the excitation of the transducer has been synchronized with the 1GSPS. 8 bits, STR81G, Sonix, USA make Digitizer board employed in the "Winspect" system. Some trials on the Zr-2.pressure Tubes of 220 MWe PHWR with standard defects have been carried out at AFD, BARC.

Fig.3.1.2 (A) shows A-scan data collected from the OD side of Zr-2 PT using 10MHz transducer in contact mode. Here, waveform (a) shows the A-Scan data acquired using a square wave pulser (channel-1 of SQ-PR unit) and (b) shows the A-Scan data acquired using the spike pulser (channel-2 of SQ-PR unit). Figure 3.1.2 (B) shows the spectrum response of the backwall echo signal. Fig. 3.1.2 (C) depicts the B-Scan image collected in the same manner as for A-Scan. This image shows a B-Scan of the PT collected with a square wave Pulser. The image shown in Fig. 3.1.2 (D) shows a B-Scan image acquired at the same location on the PT but with the help of spike Pulser. It can be clearly seen for fig. 3.1.2 (A), 3.1.2 (C) and 3.1.2 (D) that, echo signals received with the spike pulser are weak compared to the one received with square wave pulser. (Here, the first channel of the SQ-PR unit was configured to operate as square wave pulser mode whereas the second channel was configured for spike-pulser. Also the +HV supply, for both the channels of SQ-PR unit has been set at 125V and gain for receivers of each channel has been set at 26dB.) The advantage of square wave pulser over its spike counterpart is very clearly seen.

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(a) "SQ-PR" - Square Wave Pulser-Receiver Unit (c) Machined Endfittings stacked at CDM, BARC

Fig 3.1.1(B): B-Scan Image of a Defect Standard For SS Billet, Image acquired using "SQ-PR" -Square wave Pulser and ULTIMA 100M2 system

j i

Fig 3.1.1(C): C-Scan Image of a Defect Standard For SS Billet, Image acquired using "SQ-PR" -Square wave Pulser and ULTIMA 100M2 system

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(a) A-Scan data acquired on OD of ZR-2 Pressure Tube of 200 MWe PHWR, Data Acquired using Winspect system and 10 MHz Contact type Ultrasonic Transducer and Square Wave Pulser (Channel-1 of SQ-PR Unit)

(b) A-Scan data acquired on OD of ZR-2 Pressure Tube of 200 MWe PHWR, Data Acquired using Winspect system and 10 MHz Contact type Ultrasonic Transducer and spike type Pulser (Channel-2 of SQ-PR Unit)

Fig. 3.1.2(A)

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Fig. 3.1.2(B): Spectrum Response of an A-Scan data acquired on OD of ZR-2 Pressure Tube of 200 MWe PHWR, Data Acquired using Winspect system and 10 MHz Contact type Ultrasonic Transducer

and Square Wave Pulser (Channel-1 of SQ-PR Unit)

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usee 4.0 4.5 5.0 5.5 6.0

OD ID

Fig. 3.1.2(C): B-Scan Image of a Zircaloy-2 Pressure Tube of 220 MWe PHWR, Image Acquired Using Winspect system and 10 MHz Contact type Ultrasonic Transducer and Square Wave Pulser (Channel-1 of SQ-PR Unit)

usee

OD ID

Fig. 3.1.2(D): B-Scan Image of a Zircaloy-2 Pressure Tube of 220 MWe PHWR, Image Acquired Using Winspect system and 10 MHz Contact type Ultrasonic Transducer and Spike Pulser (Channel-2 of SQ-PR Unit)

4.1 Summary and Future Work:

The "SQ-PR" - Multichannel Square Wave type Ultrasonic Pulser-Receiver Unit has been developed successfully at Ultrasonic Instrumentation Section, Electronics Division, BARC for NDT of mechanical components.

It has been successfully interfaced with Multichannel Ultrasonic Imaging System "ULTIMA 100M4" designed and developed by the authors for C-Scan Imaging of tubular mechanical components. Along with the "ULTIMA 100M4" system, two units of square wave Pulser-receiver "SQ-PR" have been supplied to CDM, BARC for C-Scan Imaging of End fitting SS forgings in the form of billets, for 500 MWe PHWR.

SQ-PR has been interfaced with "Winspect", Ultrasonic Imaging System of UTEX, Canada make. With the help of "Winspect" System, the SQ-PR unit has been evaluated with Zircaloy-2 Pressure Tubes of 220 MWe PHWR. At AFD, BARC, these trials have been carried out with 10 MHz, unfocused, contact type Ultrasonic transducer.

As a consequence of these successful development and evaluation, a HMC (Hybrid Micro-elecronic Circuit) of the pulser section is planned to be developed for achieving better efficiency and usage with high frequency (upto 50MHz) Ultrasonic transducers. Also, it has been planned to incorporate micro­controller controls for providing user selectable parameters such as Pulse width, Pulse energy, PRF, Band-pass filter, Receiver gain, RF or Envelope detected rectified output of receiver, Int / Ext Trigger etc.

5.1 Acknowledgements:

The authors are grateful to Dr. S.K. Kataria (Head, Electronics Division) for his continuous encouragement and support for this development. The active participation of Shri S.R. More (Ex-Scientific Officer, ED, BARC), in the design stage, deserves to be respectfully acknowledged.

The untiring and dedicated efforts contributed by Mrs. PJyothi (Scientific Assistant) and Miss. Shobha Chavan (Tradesman) of Electronics Division towards assembly, testing/calibration and evaluation-installation of the SQ-PR units at CDM and AFD, BARC are gratefully acknowledged by the authors.

The authors are also thankful to their colleagues Shri. A. Agashe and Shri. P.D.Motiwala for their co-operation, support.

The users of the ULTIMA 100M - Multichannel Ultrasonic Imaging Systems have given a great support and encouragement during the field evaluation of SQ-PR. Efforts of Shri. S.P. Srivastava (Scientific Officer, CDM, BARC) and his colleagues from NDT group are gratefully acknowledged for the evaluation of SQ-PR unit in conjunction with ULTIMA 100M4 system for C-Scan imaging of SS billets (for Endfittings of 500 MWe PHWR) in contact scanning mode using a 4-axis mechanical scanner. Authors are also thankful to Shri A. Manjunatha (Head, CDM) for his support and encouragement for indigenous developments of such systems.

Co-operation and support from Shri. P.P. Nanekar, Shri. M. Bandopadhyay and Shri. M.D. Mangasulikar (all Scientific Officers of AFD, BARC) is gratefully acknowledged for providing their expertise in evaluating the SQ-PR unit with the Winspect, Canadian ultrasonic imaging system, at AFD, BARC.

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6.1 References:

1. Josef krautkramer and Herbert krautkramer, "Ultrasonic Testing of Materials" 4 t h Fully

Revised Edition 1993/Narosa Publishing House, New Delhi, ISBN 81-85198-71-3.

2. Non destructive Testing Hand Book, second edition, Volume 7 - Ultrasonic Testing,

American Society for Non Destructive Testing, 1991, ISBN-0-931403-04-09.

3. Handbooks of Non Destructive Evaluation - Charles J. Hellier, McGraw-Hill, USA, 2001,

ISBN-0-07-028121-1.

4. Posakony G.J., "Influence of the Pulser Parameters on the Ultrasonic Spectrum", Materials

Evaluation, Vol.43, No.4, 1985 page413-419.

5. Hayward G., T h e influence of Pulser Parameters on the Transmission Response of Piezo-

Etectric Transducers", Ultrasonics, Vol.23, No.3,1985, Page 103-112.

6. J.Buchler, M.PIatte, H.Schmidt, "Electronic Circuit For High Frequency and Broad-Band

Ultrasonic Pulse Echo Operation", Ultrasonics, Vol.25, March 1987, Page 112-114.

7. R. Martin, "Variable Puise Width Piezo-electric Ultrasonic Transducer - Driver", NDT

International, August 1984, page 209.

8. Patankar V.H., More S.R., Joshi V.M., Bairi B.R." Ultrasonic C-Scan Imaging System For

Inspection of Fabricated Mechanical Parts", Proceedings of National Symposium on

Computer Applications in Power Plants, B.A.R.C. Mumbai, December 8-10, 1992, pp 425-

428.

9. Patankar V.H. "PC Based Ultrasonic Imaging System", 8 1 s t Session of Indian

Science Congress Association, Jaipur, January 3-8, 1994.

10. More S.R., Patankar V.H., Joshi V.M., Gopaiakrishnan K.R. "Advanced Ultrasonic Imaging

System for NDT/NDE", Proceedings of 14 t h World Conference on NDT (Vol. 4), New Delhi,

December 8-13,1996, pp 2071-2074, ISBN 81-204-1126-9.

11 . Patankar V.H., More S.R., Joshi V.M." Development of High Frequency Pulser-Receiver for

Ultrasonic Inspection of Materials", Proceedings of Symposium on Advances in Nuclear and

Allied Instrumentation (SANAI-97), B.A.R.C. Mumbai, February 5-7,1997, pp 259-263, ISBN

0-07-463147-0.

12. P.P. Nanekar, Mangsulikar M.D., Bandyopadhyay M., Bandyopadhyay A.K., Patankar V.H.,

Kulkarnl P.G." Detection of Zirconium Hydride Blisters in Pressure Tubes of Pressurised

Heavy Water Reactors", Insight - British Journal of NDT, U.K., October 1998, pp 722-724.

13. Patankar V.H., More S.R., Joshi V.M." Ultrasonic Imaging System - ULTIMA 100+ For NDT

of Materials", Proceedings of the 7* International Conference on NDT, CHSNDT-Shantou,

China, October 19-23,1999.

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14. Patankar V.H., More S.R., Joshi V.M. " Remote Pulser-Receiver Module for NDT of Thin

metallic Specimens", Proceedings (Vol. 1) of Intematbnal Conference and Exhibition on

Ultrasonics (ICEU-99), NPL, New Delhi, December 2-4, 1999, pp 55-58.

15. Joshi V.M., More S.R., Patankar V.H., P. Jyothi "ULTIMA 100+: An Ultrasonic Imaging

System For Non-Destructive Testing Of Materials ", BARC News Letter No. 204, January

2001.

16. Patankar V.H., Agashe A.A., P. Jyothi, Joshi V.M. "ULTIMA 100M8 - Multichannel

Ultrasonic Imaging System for NDE of Tubes/Pipes ", X t h National Conference On

Ultrasonics, Osmania University, Hyderabad, March 15-16, 2001.

17. Patankar V.H., Joshi V.M., Kataria S.K. "Multichannel - Ultrasonic C-Scan Imaging System

For NDE of Metallic Tubular Objects", National Seminar and Exhibition NDE 2001, Lonavala.

December 7-9,2001.

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Published by : Dr. Vijai Kumar, Head Library & Information Services Division Bhabha Atomic Research Centre, Mumbai - 400 085, India.