NAT'L INST. OF STAND &.T|CH

REFERENCE

AlllDS TbfibM3

NBS

Publications

\

o NBS TECHNICAL NOTE 1075

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

193^
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides; (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities^ — Radiation Research — Chemical Physics —

Analytical Chemistry — Materials Science

THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities;

provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices;

and develops and improves mechanisms to transfer results of its research to the ultimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering^ — Manufacturing

Engineering — Building Technology — Fire Research — Chemical EngineeringTHE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. 'Some divisions within the center are located at Boulder, CO 80303.
OF STAi^Di'J'.D

^ LIBRARY QC

i oo

Electromagnetic-Acoustic-Transducer/ ^^^^

Synthetic-Aperture System for ^^^^ Thick-Weld Inspection

C. M. Fortunko"^

R. E. Schramm

J. C. Moulder

J. D. McColskey

Fracture and Deformation Division

Center for Materials Science

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

^Present address: Aerojet Ordnance, Tustin, California 92680

Funded by:

Ames Laboratory

Energy and Mineral Resources Research Institute Iowa State University

Ames, Iowa 5001

1

/ ¥ %

e

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued May 1984
National Bureau of Standards Technical Note 1075

Natl. Bur. Stand. (U.S.), Tech Note 1075, 130 pages (May 1984)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1984

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402
CONTENTS

Chapter Page

1 INTRODUCTION 1 References 5

2 INSPECTION CONCEPT 7

Ultrasonic Principles 7 Signal Processing 10

Summary 17

References 19

3 SYSTEM CONCEPT - OVERVIEW 21

Reference 27

4 DESIGN OF EFFECTIVE SH-WAVE EMATS 29

Operational Principles 29

Practical Configurations 31

Final Design 35

References 43

5 TRANSDUCER ELECTRONICS 45

Design of Low-Noise Receiver Amplifiers

for EMATs 45

Design of Efficient Power Amplifiers for EMAT

Applications 49

References 55

6 DIGITAL PROCESSING 59

Signal Averaging Method 59

Synthetic Aperture Method 59

Scanner Control and Ultrasonic Data Collection

and Processing 62

References 68

7 MECHANICAL APPARATUS 69

Gimbal -Mounted Transducer Housing 69

Two-Axis Positioner 72

8 MEASUREMENTS 75

Preparation of Calibration Specimens 75

Experimental Procedure 77

Evaluation of Surface Slot Scattering

Measurements 79

References 89

9 SUMMARY AND RECOMMENDATIONS 91

References 96

m
Appendices Page

I SOFTWARE LISTING 97

II SOFTWARE SAMPLE RUN 109

III TECHNICAL PAPERS RESULTING FROM THIS PROGRAM 115

IV ACKNOWLEDGMENTS 117

IV
LIST OF FIGURES

Figure Page

2-1 SH-wave detection of planar flaws in a weld. 8

2-2 Ultrasonic scattering by two-dimensional defects.

(a) Short wavelength, (b) Long wavelength. 10

2-3 Ultrasonic signal paths, (a) Semi-infinite half-space.

(b) Multiple paths in finite thickness plate. 12

2-4 SH-wave reflection signals, (a) EMAT-flaw distance

about 55 mm. Signal interference, (b) EMAT-flaw

distance about 65 mm. Signal separation. 13

2-5 Multipath propagation of ultrasonic pulses in a plate

and the equivalent path length, (a) Even number of

surface reflections, (b) Odd number of surface

reflections. 16

2-6 Synthetic aperture processing of a reflection echo from

a saw cut in the surface of a 25-mm thick plate. Signals

#1 to #10 are raw data taken at incremental increases of

2.1 mm between EMATs and cut. The processed waveform is

at the bottom. 18

3-1 Block-diagram of SH-wave-EMAT weld inspection system. 22

3-2 Photograph of laboratory SH-wave-EMAT weld inspection

system. 23

3-3 Block-diagram of the analog electronics section. 24

3-4 Concept for detecting and characterizing weld flaws.

(a) Longitudinal, (b) Transverse. 26

4-1 Primitive EMAT element. The eddy current J produced by

I in a wire at the surface interacts with the external

mcignetic field Ho to produce traction force T. 30

4-2 The radiation pattern of the simple EMAT in Fig. 4-1

produces a stronger reflection from the plate back

surface than does a flaw. 31

4-3 Schematic of a periodic-permanent-magnet (PPM) EMAT con- taining M pairs of magnets. 32

4-4 Aluminum test block and transducer configuration to

measure the angular distribution of ultrasonic energy. 34

4-5 Simple EMAT (M=l) with maximum sensitivity to SH waves propagating normal to the surface. 34
Figure Page

4-6 Angular radiation pattern of EMAT in Fig. 4-5 (M=l).

Maximum amplitude is normal to the surface. 36

4-7 Angular radiation pattern of EMAT in Fig. 4-3 (M=16).

Maximum amplitude grazes the surface. 36

4-8 Improved EMAT design. The two coils are counter-wound

to minimize pickup of EMI. 37

4-9 Radiation patterns of EMAT in Fig. 4-8. Three cycle tone

burst, (a) 400 kHz. (b) 441 kHz. (c) 500 kHz.

(d) 700 kHz. 39

4-10 Radiation patterns of EMAT in Fig. 4-8. 441 kHz. (a) Four

cycle tone burst, (b) Eight cycle tone burst. 42

4-11 Transverse radiation distribution of M=l EMAT (Fig. 4-5). 42

5-1 Idealized schematic of a receiver-amplifier system. 46

5-2 Schematic of the transmission-line transformers and

J-FET preamplifier used in the current work with a transmission line < one meter long. 48

5-3 Preamplifier schematic. Useful for longer transmission

lines. 48

5-4 (a) Basic capacitive discharge circuit used in most

commercial pulser designs for driving piezoelectric

transducer elements, (b) Voltage waveform seen by

transducer T. 51

5-5 (a) Basic schematic of a power amplifier for a transmitter EMAT. (b) Voltage and current waveforms

produced by this amplifier. 53

5-6 Schematic of amplifier for 300-600 kHz used in this work. 56

5-7 Construction of output power transformers used in the

amplifier of Fig. 5-6. 56

5-8 Oscilloscope traces of the output of the amplifier in

Fig. 5-6. (a) Voltage, (b) Current. 57

6-1 By repeated mathematical processing, the same data set

can be brought to focus at multiple points. 60

6-2 Phase shift of Fourier coefficients in the realimaginary plane. 61

VI
Figure Page

6-3 Flow chart of computer program used to collect, store,

process, and display synthetic aperture data. 63

6-4 Geometrical parameters required for synthetic

aperture processing. 63

6-5 (a) Three sets (out of ten) of raw signal data.

(b) Signal after synthetic aperture processing. 66

6-6 Typical Fourier coefficients for some raw data sets and

after processing. All the information is centered about

the operational frequency of 450 kHz. (a) Real,

(b) Imaginary. 67

6-7 Power distribution of processed signal. 68

7-1 Gimbal -mounted transducer housing for both transmitter

and receiver EMATs. The two axes of rotational freedom

allow the probe surface to comform to the plane of the

plate under inspection. 70

7-2 Transducer housing attached to pneumatic piston and

suspended from the two-axis positioner operated under

computer control

.

71

8-1 Calibration plate with EDM notches. 76

8-2 Detailed scan pattern used for data collection. 78

8-3 Scan pattern used when the slots were on the same side

of the plate as the EMATs. 78

8-4 Peak-to-peak signal voltage from scan of slots on same

side of plate as EMATs. (a) Flaws A-E. (b) Flaws F-J. 81

8-5 Power signal from scan of slots on same side of plate

as EMATs. (a) Flaws A-E. (b) Flaws F-J. 82

8-6 Flaw signal as a function of flaw depth for a constant

length. The results remain the same for flaws up or down, (a) Peak-to-peak voltage, (b) Power. 85

8-7 Flaw signal as a function of flaw length for a constant

depth. The results remain the same for flaws up or down, (a) Peak-to-peak voltage, (b) Power. . 86

8-8 Depth and length measurement capabilities and limitations

of the present EMAT system. The letters A through J - identify the points indicating the dimensions of the

calibration notches listed in Table 8-1. 88

vn
LIST OF TABLES

Table Page

8-1 Dimensions of EDM notches in calibration plate. 76

VI 1 1
ELECTROMAGNETIC-ACOUSTIC-TRANSDUCER/

SYNTHETIC-APERTURE SYSTEM FOR

THICK-WELD INSPECTION

C. M. Fortunko*, R. E. Schramm,

J. C. Moulder, J. D. McColskey

Fracture and Deformation Division

National Bureau of Standards

Boulder, Colorado 80303

ABSTRACT

This report describes a system based on electromagnetic-acoustic transducers

(EMATs) as an approach to automated nondestructive evaluation of thick

weldments (>_25 mm). Good signal-to-noise ratios, often a problem with EMATs, were possible through careful design of the transducers and associated

electronic circuits and the use of signal averaging. At 454 kHz, the

transducers produce shear-horizontal waves of approximately 7-mm wavelength in

steel. The long wavelength permits determination of through-thickness flaw

depth from the amplitudes of scattered ultrasonic waves. A minicomputer

controlled transducer positioning and acquired the digitized ultrasonic

waveforms for synthetic aperture processing. The synthetic aperture technique

further improved signal quality and yielded flaw localization through the weld

thickness. Measurements on artificial flaws demonstrated a detectabil ity

threshold of 0.5 mm (through thickness) and sizing ability up to 2.5 mm, in

agreement with theoretical predictions. Details include the design of the

transducers and electronics, as well as the mechanical positioner, signal

processing algorithms, and complete computer program listing.

Keywords: electromagnetic-acoustic transducer; flaw detection; nondestructive

evaluation; S-H waves; synthetic aperture; ultrasonics; weld inspection

'Present Address: Aerojet Ordnance, Tustin, California 92680

IX

Chapter 1

INTRODUCTION

Rapid advances in automated welding and increased demands for reliable

weld-quality inspection tools have created a need for new ultrasonic

inspection systems. In particular, new systems capable of operation at

elevated temperatures and rapid scan rates are in demand in fully and semiautomated welding applications to complement radiographic and conventional

ultrasonic inspection techniques. In such applications, radiographic

techniques are fundamentally limited because of their inability to detect and

dimension most sharp flaws, and possible exposure of fabrication and

inspection personnel to potential health hazards. On the other hand,

conventional ultrasonic techniques are limited because they tend to be

difficult to automate, require fluid couplants that are incompatible with many

welding environments, and are often operator-dependent.

This report describes progress on a new weld inspection system currently

under development at the National Bureau of Standards for the U.S. Navy. This

system traces its origins to earlier ones developed at the Rockwell

International Science Center (Fortunko, 1979) and a girth-weld inspection

system developed at the National Bureau of Standards (Fortunko and Schramm,

1983). It incorporates many of the features of the earlier systems. For

example, all three systems utilize novel, noncontacting ultrasonic transducers

that can operate on the surfaces of most weldments. In addition, the new

transducers are able to generate and detect special ultrasonic probing

signals, shear waves polarized parallel to the surface of a plate (SH-waves).

These are easier to interpret than the longitudinal waves and shear waves

polarized in the sagittal plane of the plate that are usually generated and

1
detected by conventional piezoelectric transducers. The present inspection

system also incorporates recent improvements in electromagnetic-acoustic

transducer (EMAT) technology and improved digital signal processing routines

that compensate for many hardware-related shortcomings of the earlier systems.

It is also more flexible than the earlier systems since it incorporates

digitally controlled scanning mechanisms that permit the execution of a wide

variety of inspection protocols.

This report contains eight chapters and five appendixes.

Chapter 2 presents an overview of the inspection process concept,

including a perspective on other ultrasonic inspection approaches. This

chapter enumerates the main benefits of using SH-wave EMATs with emphasis on

the special problems of detecting and characterizing weld flaws. Also

included in Chapter 2 is a discussion of the benefits and shortcomings of

operating in the long wavelength region (with respect to the principal flaw

dimensions). Chapter 3 presents an overview of the system with a blockdiagram approach to describe all the main functional elements of the system as

implemented at the National Bureau of Standards for a feasibility

demonstration in laboratory environments. Chapters 2 and 3 familiarize the

reader with the four main themes of this report: 1. use of rapidly scanned,

non-contacting transducers (EMATs); 2. use of special ultrasonic probing

signals (SH-waves); 3. operation in the long-wavelength scattering region;

4. extensive use of digital synthetic aperture and other special signal

processing techniques to improve the overall signal-to-noise performance of

the new inspection system. Details of these special features of the system

are described in detail in the remaining chapters and appendixes forming the

main body of the report.
Chapter 4 includes an extensive description of a new SH-wave EMAT element

specially developed to increase the longitudinal resolution of the inspection

system and reduce the contamination of the ultrasonic signals by unwanted

electromagnetic and ultrasonic interferences. In this context, the radiation

patterns of the new transducer and details of the calibration procedures are

discussed. (The development of a new EMAT was necessary because existing

designs could not be adapted to the inspection of thick plate weldments at in

the 400-800 kHz range which allows flaw sizing in the range of interest.) It

is noted that the new transducer design also incorporates a mechanical faceplate that very effectively protects it from damage by abrasion while causing

only a very small loss in efficiency.

Chapter 5 is devoted entirely to EMAT electronics, with special emphasis

on the design and optimization of analog subsystems that determine

sensitivity. Separate sections describe the principles of operation of a very

efficient, high-power amplifier for driving EMAT-like electrical loads and

\jery low-noise preamplifiers needed to establish the low noise-figures,

essential to overcome the inherently low transduction efficiencies of EMATs.

Examples of several important systems, including detailed circuit diagrams are

provided.

The details of the three main signal processing schemes (signal

averaging, digital filtering, and synthetic aperture reconstruction) used to

acquire and process the ultrasonic signals are treated in Chapter 6. This

chapter includes three separate sections, which are devoted to signal

averaging, synthetic aperture reconstruction, and overall software summary.

The synthetic aperture section emphasizes the use of signal processing in the

frequency-domain via the use of digital FFT algorithms. This approach allows

for the full use of the dynamic range of a 16-bit minicomputer to avoid
spatial sidelobe problems associated with time-quantization errors. A

detailed logic diagram illustrates the main steps involved in the gathering

and processing of ultrasonic data. Also included are specific examples of

unprocessed and processed waveforms.

The details of the mechanical apparatus used to move the EMATs on the

surface of the work-piece are given in Chapter 7. This chapter describes a

unique way of mounting the transducer package in a flexible gimbal -derived

enclosure that allows for conformal tracking of the EMATs on the surface.

This feature is particularly important in the inspection of weldments, which

are often deformed in the vicinity of welds by the action of residual

stresses. Also included is a description of the mechanical scanner that

transports the EMATs in a preprogrammed manner while maintaining adequate

phase coherence of the many ultrasonic beams used in synthetic aperture

reconstruction. Although the particular hardware implementation described in

this report is suitable only for laboratory use, the principles are directly

applicable to a scaled-up prototype version that would be suitable for further

evaluation of the system in a technology demonstration phase.

Chapter 8 is a discussion of the experimental results from a laboratory

version of the new SH-wave inspection system used on a calibration specimen

containing well characterized model flaws. Chapter 8 describes the

calibration specimen and the experimental procedures used to collect the

ultrasonic data. Then follows a discussion and analysis of the experimental

results summarized in terms of a flaw length vs. flaw depth diagram that is

compatible with a f itness-for-purpose approach to weld quality validation.

A large number of figures illustrate the principal parametric performance

limits (including transverse and longitudinal resolution) of an SH-wave EMAT

system operating at 454 kHz. Because suitable calibration specimens
containing well characterized interior flaws were unavailable, the discussion

is limited to surface flaws. However, the system's performance limitations

with respect to interior flaws can now be extrapolated from the surface flaw

case on the basis of a wealth of closed-form and finite-element analytical

treatments (Datta, 1983).

The report ends with a short summary. Chapter 9, on the highlights of the

just completed work and an outline of several ideas for future work.

Appendixes provide detailed listings of the software, illustrate a typical

operator-system interactive session, give a list of several technical papers

that have resulted from the work covered in the present report, and

acknowledge those who have helped us along the way.

References

Datta, S. K. , 1983, University of Colorado, private communication.

Fortunko, C. M., 1979, Ultrasonic inspection of weldments with frequency

scanned SH waves, vf}_: "Proceedings of the IEEE Ultrasonics Symposium,:" B. R. McAvoy, ed., IEEE, New York.

Fortunko, C. M., and Schramm, R. E., 1983, Evaluation of pipeline girth welds

using low-frequency horizontally polarized waves, J. NDE, 3:155.

Chapter 2

INSPECTION CONCEPT

Ultrasonic Principles

Nondestructive evaluation techniques are often necessary to verify the

integrity of ferritic butt weldments. They must reliably detect longitudinal

and transverse planar flaws that exhibit a significant projected depth along

the through-thickness direction. As an example, Fig. 2-1 shows two

longitudinal flaws of the planar variety (i.e. flaws whose principal length

dimensions lie along the welding direction). This figure also illustrates the

concept of detecting longitudinal flaws using low-frequency shear waves

polarized along the direction parallel to the surfaces of the butt weldment

(SH-waves). The two planar flaws in Fig. 2-1 are inadequate penetration (IP)

of the weld preparation groove and incomplete (sidewall) fusion (IF) between

the weld deposit and the base metal. A transverse flaw (not shown) would be

largely normal to the welding direction, i.e., the flaw would be in the plane

of Fig. 2-1.

In most cases of practical interest, it is not possible to interrogate

the flaw region ul trasonically along a direction that is normal to the

surfaces of a butt weldment because the weld crowns (shown in Fig. 2-1) are

generally not removed after welding. For this reason, it is usually necessary

to inspect the weld deposit and the fusion zone using a transducer located on

the surface of the base metal. When SH-waves are used for detection of planar

flaws, it is normally advantageous to inspect the region of the weld deposit

and fusion zone at a near-grazing angle with respect to the surfaces of a butt

weldment, as shown in Fig. 2-1, because, in this mode, the flaw presents the

largest obstacle to the illuminating ultrasonic signal.
Weld

Reinforcement

Base Metal

Ultrasonic

Transducer

Ultrasonic Signal

Figure 2-1: SH-wave detection of planar flaws in a weld

Recent theoretical and experimental investigations have shown that planar

flaws and open cracks in butt welds are detectable with low-frequency SH-wave

techniques (Fortunko, King, and Tan, 1982, and Fortunko and Schramm, 1982).

This technique differs from conventional ultrasonic techniques in three

important respects. First, shear waves that are polarized parallel to the

surface of a plate weldment (SH-waves) fill the weld region. Second, the

probing ultrasonic signals are generated and detected using novel

electromagnetic-acoustic transducers (EMATs) that do not require the

application of grease or fluid couplants to the surfaces of the inspected part

and can operate in adverse environments. Third, the ultrasonic frequency of

operation used by the new inspection technique is typically 500 kHz, an order

of magnitude lower than the principal frequencies of conventional systems

(2.25, 5, and 10 MHz).

Setting the frequency of operation at approximately 500 kHz is highly

beneficial when the principal goal of an inspection is the reliable detection

of major flaws. At low frequencies, when the ultrasonic wavelength is comparable to or larger than the principal flaw dimensions, the effect of certain

flaw parameters on the scattered signal amplitude can be substantially

8
reduced. In particular, if the frequency of operation is sufficiently low,

the effects of surface roughness, branching, and departure from planarity are

reduced and the amplitude of the scattered ultrasonic signals can be

unambiguously related to certain principal flaw dimensions (Budiansky and

Rice, 1978; Doyle and Scala, 1978; and Richardson and Elsley, 1979). In

addition, at long wavelengths open planar flaws are more detectable than blunt

voids of comparable dimensions. This is advantageous from the fitness-forpurpose point of view, which emphasizes the importance of principal flaw

dimensions and assumes that sharp flaws are inherently more critical than

blunt flaws (Lundin, 1976; Reed, McHenry, and Kasen, 1979; and British

Standards Institution, 1980). Another advantage of long wavelengths is a

substantial reduction in the need to maintain precise equipment alignments.

Figure 2-2 illustrates the qualitative differences between scattering of

ultrasonic waves in the long (low-frequency) and short (high-frequency)

wavelength regions. (The flaw is assumed planar and relatively long in the

direction of the normal to the plane of the figure.) At high frequencies,

Fig. 2-2a, most of the incident ultrasonic energy reflects specularly from the

front face of the flaw; i.e., the angle of reflection with respect to the face

normal equals the angle of incidence. Also, a significant portion of the

incident signal diffracts from the tips of the flaw, which act like ultrasonic

line sources. If the surface of the flaw exhibits some roughness, a

significant portion of the incident signal can also scatter incoherently (not

shown). Furthermore, depending on the orientation of the flaw with respect to

the incident signal and the type of ultrasonic signal used (i.e., shear vs.

longitudinal), significant mode conversions may occur. As a result, the

scattered ultrasonic field at high frequencies is very complicated and

difficult to interpret.
(a) (b)

Figure 2-2: Ultrasonic scattering by two-dimensional defects

(a) Short wavelength, (b) Long wavelength.

At low ultrasonic frequencies (Fig. 2-2b), in the long wavelength

scattering region, the scattered ultrasonic signals are usually easier to

interpret because the specular reflection component becomes negligible and the

tip-diffracted signals merge. As a consequence, the scattered ultrasonic

signals are much less sensitive to flaw orientation and detail. In addition,

the amplitudes of the scattered ultrasonic fields increase monotonical ly with

flaw dimensions. This useful phenomenon can improve the ability of an

ultrasonic inspection system to discriminate between flaws of different sizes

and types (Datta, 1977, and Richardson and Elsley, 1979).

Signal Processing

In the past, the use of low-frequency SH-wave EMAT inspection systems has

been considered in the context of nondestructive evaluation of pipeline girth

welds in conjunction with acceptance criteria (Reed, Kasen, McHenry, and

Fortunko, 1983). In that case it was convenient to represent the spatial

10
distribution and strength of the illuminating and scattered ultrasonic fields

in terms of modal expansions. However, in the present context it is more

convenient to employ a ray representation that is very closely related to

techniques used in seismology and ocean acoustics (Officer, 1958). The

different approach is justified by the fact that the present SH-wave EMAT

system uses ultrasonic tone-bursts of shorter duration than those in Fortunko

and Schramm (1983), and that the plate weldments are thick relative to the

ultrasonic wavelength.

At low ultrasonic frequencies, flaw detectabil i ty in butt welded plates

may be significantly degraded by multipath propagation effects and ultrasonic

clutter caused by multiple reflections and mode conversion of the incident and

scattered ultrasonic fields by the free surfaces. At high frequencies this

phenomenon is less important because it is much easier to collimate the

ultrasonic signals and remove unwanted signals by time-gating.

Figure 2-3 illustrates the two different physical situations. In both

cases, the probing SH-wave signals are generated and detected by an EMAT pair.

However, Fig. 2-3a illustrates the case when the thickness of the plate under

inspection is sufficiently large compared to the ultrasonic wavelength so that

ultrasonic signals arriving at the receiver EMAT following a reflection from

the bottom surface can be gated-out in the time domain. Effectively, the case

shown in Fig. 2-3a corresponds to that of a flaw embedded near the surface of

a half-space. Figure 2-3b illustrates the case when the signals from the

opposite surface can no longer be obscured by means of signal-gating

techniques because the ultrasonic signals, arriving via different paths,

overlap in the time-domain.

Figure 2-3 does not show the paths of signals generated by mode

conversion effects at the free surfaces of the plate and by the flaw.

11
Flaw

EM AT EM AT

Half Space

(a)

Plate

(b)

Figure 2-3: Ultrasonic signal paths, (a) Semi-infinite half-space,

(b) Multiple paths in finite thickness plate.

Physically, this simplification is justified by the mode-selective properties

of SH-wave EMATs that significantly de-emphasize the detection of signals of

unwanted polarization: Rayleigh waves, shear vertical (SV), and longitudinal

(L) waves.

Figure 2-4 illustrates the effect of the multipath propagation phenomenon

on the appearance of SH-wave signals received at the free surface of a

25-mm-thick ferritic plate containing a planar, surface flaw 6.8-mm deep and

25-mm long. Relatively short, 4-RF-cycle, current bursts at a center

frequency of 500 kHz generated the ultrasonic signals. The top trace shows

the appearance of the received SH-wave signals when the mean distance along

12
50 mV

5 p-s

(b)

Figure 2-4: SH-wave reflection signals, (a) EMAT-flaw distance

about 55 mm. Signal interference, (b) EMAT-flaw

distance about 65 mm. Signal separation.

the surface between the EMATs and the flaw was approximately 50 mm. Here the

first two arrivals are not separated in the time domain and interfere

strongly. These interferences resulted in considerable variations in the

maximum amplitude of this signal due to beating and deconvolution and would be

highly uncertain. The bottom trace shows the same signals when the mean

distance between the EMATs and the flaw was approximately 65 mm and the

several signals present are more clearly separated and measurable. (The

beginning of the frame, showing a remnant from the input drive signal is not

shown.

)

13
An additional major task in the design of ultrasonic inspection systems

that operate at low frequencies involves the elimination of the unwanted

ultrasonic and electromagnetic signals that may mask the presence of a flaw.

Such signals may result from electromagnetic and acoustic reverberations of

the transducers, direct electromagnetic leakage between the receiver and

transmitter EMATs, and grain noise, as well as multipath and end effects.

Obviously, the elimination of such unwanted signals is essential to improving

the reliability of the detection process. In the present application we

decided that digital signal processing techniques, including signal averaging,

subtraction, and synthetic aperture, can effectively increase inspection

reliability and diminish the effect of unwanted signals and clutter on the

detectabil ity of flaws (Chapter 6).

Digital signal averaging is important, particularly in an in-process

inspection application, because it can be used to enhance the electronic

sensitivity of the EMATs and, most importantly, reduce the contamination of

the ultrasonic signals by asynchronous electromagnetic and ultrasonic interferences. However, because the time-average values of multipath, end-effect

and grain-noise signals are nonzero, digital subtraction and synthetic

aperture techniques must also be employed. The effectiveness of such

techniques can be greatly increased by proper design of the EMATs and other

analog components of the inspection system that contribute to the generation

of the unwanted electromagnetic and ultrasonic signal.

Figure 2-3b shows the preferred inspection configuration for butt welds,

which takes the full advantage of signal enhancement by digital signal

processing. The transmitter and receiver EMATs are designed to aim the

ultrasonic beam in the direction of the flaw and minimize direct ultrasonic

cross-talk. The ultrasound beams are aimed at the grazing angle, but spread

14
rapidly by diffraction to fill the entire thickness of the plate. As a

consequence, the presence of a flaw causes several signals to arrive at the

receiver EMAT at different times. These signals are shown in Fig. 2-5.

For simplicity, in constructing Fig. 2-5 we assumed that the same

transducer generates and receives the probing and scattered ultrasonic signals

(pulse-echo mode). The flaw is located a distance below the top surface of

the plate. The same surface also supports the transducer, which is at the

observation point 0. Two families of rays are shown. The first family. Fig.

2-5a, is all the rays reflecting first from the top surface of the plate. The

second family, Fig. 2-5b, is all the rays reflecting first from the bottom

surface. The actual experimental situation, as exemplified by the

configuration shown in Fig. 2-3b, is much more complicated in that separate

EMATs are necessary to generate and receive the ultrasonic signals (pitchcatch mode). A consequence of this arrangement is two additional families of

rays. (For a comprehensive discussion of this phenomenon the interested

reader shown consult Brekhovkikh (1980).) It is evident that this complicated

situation causes the receiver-EMAT to detect many signals. Physically, these

additional signals correspond to the multiple rays connecting the two EMATs

with the flaw. Because at long wavelengths it is impractical to generate

highly collimated ultrasonic beams, especially using end-fire transducers such

as the EMAT, an alternative procedure is needed to reduce the received

ultrasonic signal complexity.

A practical way to improve the flaw-signal quality is synthetic aperture

processing. In this technique, the transmitter and receiver EMATs are moved

together along a scan line that lies on the top surface of the plate. As a

result, the principal flaw signal, depicted by a solid line in Fig. 2-3,

arrives slightly later in each successive measurement as the two EMATs are

15
as

C 03 O Q- • •r- I/) -(->-(-> E

03

C

O

cn OJ •-

03 1— +-> Q- 03 U O > QJ

S- T- ,— . Cl 13 4- 00 cr O) c

-C OJ s_ o

03 CU OJ +-> Q..C a u

•r- +-> 03 QJ -M M- I— — -O S- 4- ^ c ::5 OJ S (D 1/5 SLnI

CU

cn

16
moved away from the flaw. The timing of the other flaw signals is shifted

correspondingly. On the other hand, the timing of all electromagnetic and

ultrasonic interferences remains unchanged or undergoes changes in the

opposite direction. This phenomenon can be exploited to significantly enhance

the principal flaw-signal quality with respect to all other signals.

In the synthetic aperture technique, the flaw signals from a given

direction are processed by shifting each received waveform by a time increment

that compensates for the change in arrival time of signals from that direction

and then adding the signals together. This reduces the signals from all other

directions and reinforces the flaw signal.

Figure 2-6 shows the appearance of signals received from a deep surface

flaw in a 25-mm-thick steel plate as the spacing between both EMATs and the

flaw is increased in 2.1-mm steps. The successive waveforms (labeled 1-10)

contain residual electromagnetic interference (A) and ultrasonic cross-talk

signals (B) in addition to the principal flaw signal and those that have

propagated via the alternate paths (C). The reconstructed waveform is shown

at the bottom.

Figure 2-6 shows very substantial improvement in the quality of the flaw

signal by elimination of the electromagnetic and ultrasonic interferences from

the display. In addition, the flaw is represented by a single waveform with a

nearly triangular envelope. This decontaminated signal is much more desirable

for processing by any detection algorithm than any of the 10 unprocessed

waveforms

Summary

The inspection system described in the present report takes advantage of

improvements in SH-wave EMAT design resulting in better collimation of the

sound beams in the flaw direction. The system is operated at relatively low

17
A- rB-r

#1

#2

#3

LU

Q

O

CO Meas. taken at 2.1 mm increments

#8

#9

#10

0.1V

Processed

40 80 120

TIME i/JLs)

Figure 2-6: Synthetic aperture processing of a reflection echo from

a saw cut in the surface of a 25-mm thick plate. Signals

#1 to #10 are raw data taken at incremental increases of

2.1 mm between EMATs and cut. The processed waveform is

at the bottom.

18
ultrasonic frequencies (typically 450 kHz) to reduce sensitivity to flaw

orientation and surface detail. Finally, the quality of the flaw signals is

substantially improved by employing advances in digital signal processing,

including digital background subtraction, synthetic aperture focusing, and

time-averaging. The following sections of this report describe the technical

details of the above features of the SH wave EMAT inspection system.

References

Brekhovkikh, L. M., 1980, "Waves in Layered Media," Academic Press, New York.

British Standards Institution, 1980, Guidance on some methods for the

derivation of acceptance levels for defects in fusion welded joints, PD 6493,

British Standards Institution, London.

Budiansky, B., and Rice, J. R., 1978, On the estimation of a crack fracture

parameter by long wavelength scattering, J. Appl . Mech. , 45:453.

Datta, S.K., 1977, Diffraction of plane elastic waves by ellipsoidal

inclusions, J. Acous. Soc. Amer. , 61:1432.

Doyle, P. A., and Scala, C. M. , 1978, Crack depth measurement by ultrasonics:

a review. Ultrasonics , 16:164.

Fortunko, C. M. , King, R. B,, and Tan, M., 1982, Nondestructive evaluation of

planar defects in plates using low-frequency shear horizontal waves, J . Appl

.

Phys. , 53:3450.

Fortunko, C. M., and Schramm, R. E., 1982, Ultrasonic nondestructive

evaluations of butt welds using electromagnetic-acoustic transducers, Weld.

J_^, 61:39.

Fortunko, C. M., and Schramm, R. E., 1983, Evaluation of pipeline girth welds

using low-frequency horizontally polarized waves, J. NDE , 3:155.

Lundin, C. D., 1976, The significance of weld discontinuities - a review of

current literature, WRC Bulletin 222, Welding Research Council, New York.

Officer, C. B., 1958, "Introduction to the Theory of Sound Transmission."

McGraw-Hill Book Co., Inc., New York.

Reed, R. P., Kasen, M. B., McHenry, H. I., and Fortunko, C. M., 1984,

Fitness-for-service criteria for pipeline girth weld quality, WRC Bulletin (in

press). Welding Research Council, New York.

19
Reed, R. P., McHenry, H. I., and Kasen, M. B., 1979, A fracture mechanics

evaluation of flaws in pipeline girth welds, WRC Bulletin 245, Welding

Research Council, New York.

Richardson, u.M. and Elsley, R.K., 1979, Extraction of low-frequency

properties from scattering measurements, 2_n: "Proceedings of the IEEE

Ultrasonics Symposium," B. R. McAvoy, ed., IEEE, New York.

20
Chapter 3

SYSTEM CONCEPT - OVERVIEW

This chapter presents an overview of a specific implementation of the

inspection concept described in Chapter 2. Specifically, block-diagrams

describe the architecture of a laboratory-scale system, including hardware and

software components. This configuration demonstrates the technology and

establishes the inherent performance limits of our SH-wave EMAT inspection

concept. Additional discussion includes various methods of employing the

system to dectect and characterize different flaw categories in weldments, as

well as factors limiting the overall resolution. Our emphasis here is on an

overview. The functional details of the components, including circuit

diagrams, EMAT-assembly drawings and software listings, are given in the

following chapters and appendixes.

Figures 3-1 and 3-2 show the main functional blocks of the SH-wave

technology demonstration system. The operation of the system is centrally

controlled by a 16-bit minicomputer (block 1 in Fig. 3-1) that uses a highlevel (BASIC language) operating environment. The programs, inspection

parameters, and data are stored by two floppy-disk drives (2). The operator

initiates all commands through a keyboard (3) which is a part of a video

display terminal (VDT) equipped with medium-resolution graphics capability. A

dot-matrix printer (4) can provide a hardcopy of system perameters and plots

of signal waveforms and processed data. The minicomputer directs the

operation of several peripheral devices through a standard interface bus. The

system is capable of fully automated data collection followed by signal

processing. During data collection, the minicomputer controls the. operation

of the mechanical scanners (5 and 6), which move the EMAT search-head (7) on

21
1

stepper Motors on XY

Scanner

6

Mechanical Scan

Controller

5

"\

^ Limit Switches

1

1

1

1

, , IEEE-48a

1 ">

t

16-Bil Minicomputer EM AT Search-Head

1 7

"*"

Analog

Signal Processing

8

Digital Oscilloscope

9

IEEE-488

M

Dot Matrix

Printer ^ ^

Graphical Video

Display Terminal and Keyboard

Duat

Floppy Disk

Drives

Figure 3-1: Block-diagram of SH-wave-EMAT weld inspection system.

the surface of the v/eldment. After analog signal processing (8), a digital

oscilloscope (9) displays the data and transfers it to the computer on the

IEEE-488 bus. Limit switches on the lead-screw mechanisms signal the computer

of any overscanning so it can remove power from the stepper motors and prevent

mechanical damage. (The operation of the mechanical scanners and their

controllers is described in detail in Chapter 7.)

The minicomputer does not directly operate the analog circuits (8), which

generate the high-power electrical signals (typically 1000 watts-peak) driving

the transmitter-EMATs and preamplify the low-level signals detected by the

recei ver-EMATs . This portion of the inspection system operates asynchronously

with respect to the mincomputer clock circuit to reduce electromagnetic

interference levels. The analog receiver signal reaches the computer through

a digitizing oscilloscope (9).

22
• '•'••••Hl«««.,i

Reference

OscJIIatcM'

OigitaJ

Oscilloscope--^^ ,,,, [^'wsa^m ;;

Analog

Electronics

Power

Supplies

Con^Miter

Temmal Main

Computer

Figure 3-2: Photograph of laboratory SH-wave-EMAT weld inspection system.

23
EMAT

S«8rch-H«ad

I 1

Transmitter EMAT

Electrical Impedance

Matching

Network

Free-Running

OscillatOf

(90S l»Hz)

J

TTL

Digital Timer A

Trigger

IEEE-4«fi » To Minicomputer

8 Bit Digital Oscilloscope/Tim«-A»erager

Figure 3-3: Block-diagram of the analog electronics section

Figure 3-3 is a block-diagram for sections 7, 8, and 9 of the inspection

system. To avoid confusion vnth Fig. 3-1, the individual components are

marked by letter codes (A-I), (The construction of the EMATs and other

specially constructed electrical subsystems are described in Chapters 4 and

5.)

The analog section can be functionally subdivided into the "transmitter"

side and the "receiver" side. On the "transmitter" side, the key role is

played by a digital timer circuit (A) that generates a unipolar TTL-compatible

(TTL stands for the transistor-transistor-logic family of digital circuits)

train of pulses whose period is synchronized to the period of a free-running

oscillator (J). The timer, which uses digital counting circuits, also

generates a trigger pulse for an 8-bit digital oscilloscope (I) serving as an

24
A/D converter, and establishes the pulse-repetition-frequency (PRF). The

length-of-burst and PRF parameters are manually selectable using switches

located on the front panel of the timer. Typical PRFs are two thousand times

slower than the frequency of the free-running oscillator (908 kHz). The

TTL-level output of the timer goes directly to the input of a class-D transmitter-amplifier (B) that drives the transmi tter-EMAT (D) through a special

impedance-matching network (C).

On the "receiver" side, the output of the recei ver-EMAT (E) is first

preamplified by a specially designed, low-noise amplifier (G), passed through

a band-pass filter (H) and, then, digitized by the oscilloscope (I). The

digital oscilloscope also computes a time-average of up to 256 individual

waveforms. Normally, 16 time-averages are used. This capability is needed to

compensate for the low transduction efficiencies of SH-wave EMATs and to

reduce the influence of implusive and other electromagnetic interferences. An

impedance-matching network (F) is used to minimize the noise figure of the

receiver-preamplifier. The digital oscilloscope transfers the time-averaged

output waveform to the minicoputer over the IEEE-488 data bus.

As configured in Fig. 3-2, with both EMATs located on the same side of

the weld and aligned along a plane normal to the welding direction, the

SH-wave EMAT system is mainly intended to detect longitudinal flaws in butt

weldments. This category includes elongated slag, inadequate penetration

(IP), incomplete fusion (IF), and certain cracks. By definition, longitudinal

flaws are aligned with the welding direction, as shown in Fig. 3-4a. Because

the wavelength of the ultrasonic signal is large (7.4mm) in relation to

conventional ultrasonic systems, small concentrations of weld porosity cannot

normally be detected. In the noramal inspection mode, the weld region is

interrogated by moving the EMAT search unit in a pattern on the surface of the

25
Figure 3-4: Concept for detecting and characterizing weld flaws

(a) Longitudinal, (b) Transverse.

weldment such as shown in Fig. 8-2. To obtain better transverse and longitudinal resolution, extensive use is made of digital signal processing,

including synthetic aperture reconstruction. (The details of the digital

signal processing procedures are treated separately in Chapter 6.)

The inspection configuration of Fig. 3-4a is not suitable for detecting

the important category of transverse weld flaws. Figure 3-4b illustrates this

category of weld flaw along with a possible inspection approach. In contrast

to Fig. 3-4a, the two EMATs in Fig. 3-4b are positioned on opposite sides of

the weld and aligned symmetrically with respect to the welding direction to

exploit the phenomenon of specular reflection. Implementation of the

inspection concept shown in Fig. 3-4b would require a substantially different

26
and more complicated mechanical arrangement than that shown in Fig. 3-2. For

this reason, detection of transverse weld flaws is not included in the present

report.

Although the laboratory-scale demonstration system shown in Fig. 3-2 does

not allow very long scan distances, it is believed that the SH-wave inspection

concept is inherently suitable for the in-process inspection role in many

shipyard applications with very long machine-welded joints. Of course, a

different arrangement for moving the EMAT search-heads on the surface would be

necessary. Perhaps, already existing mechanisms can be adapted to fill this

role (de Sterke, 1981). Although no provisions were made to ruggedize and

shield the laboraory set-up for field use, industrial or military-quality

versions of all components, except for the specially made EMATs and portions

of the analog electronics section, are available commercially.

Reference

de Sterke, A., 1981, Automated Ultrasonic Inspection of Circumferential Welds

in Pipelines, Met. Constr. , 12:42.

27

Chapter 4

DESIGN OF EFFECTIVE SH-WAVE EMATS

In the inspection of plate butt weldments it is desirable to maximize the

collimation of the ultrasonic beam in the direction of the flaw. There are

two ways to accomplish this: 1) using synthetic aperture processing, and

2) designing an SH-wave electromagnetic-acoustic transducer (EMAT) with the

desired beam shape. In practice, to satisfy system design requirements, a

combination of the above two techniques yields the desired beam collimation.

This section emphasizes the details of SH-wave EMAT design necessary to obtain

the desired beam shape characteristics and, simultaneously, reduce the effect

of unwanted ultrasonic and electromagnetic interference.

Operational Principles

Figure 4-1 shows a primitive EMAT element composed of a wire conductor

carrying a dynamic current I and a source of strong magnetic bias field H .

The current I induces dynamic eddy currents J in the metal conductor

CO

-^ -^ 00 surface. The strong magnetic bias field, H , causes the deflection of the

moving electrons in a direction defined by the cross-product of the direction

vectors associated with J and H . The resultant Lorentz body forces, T,

00

J' ' 5 generate ultrasonic signals that propagate radially into the bulk of the metal

conductor, away from the wire. The signal polarization depends on the

direction of the static bias field H with respect ^ to the free surface (this

report does not discuss the subject of generating ultrasonic signals using

magnetostrictive transduction mechanisms). Waves with particle displacements

parallel to the free surface are called SH-waves.

The primitive transducer element in Fig. 4-1 is not very useful in a

practical application. First, it is not very efficient, because of the

29
Figure 4-1 Primitive EMAT element. The eddy current J produced by

I in a wire at the surface interacts with the external

magnetic field Ho to produce traction force T.

practical difficulties of efficiently matching isolated wire radiators to

transmitter-amplifiers and receiver-preamplifiers in the frequency band used

in flaw detection (450 kHz typically). Second, the radiation pattern exhibits

cylindrical symmetry when generating SH-waves. Cylindrical radiation patterns

are not useful for inspecting butt welds in plates, because as much energy is

aimed at the opposite surface of the plate as at the flaw (Fig. 4-2).

Because the opposite surface presents a greater obstacle to the ultrasonic

signal than the flaw, detection may be difficult. Clearly, a practical EMAT

must be able to generate most of the signal in the general direction of the

flaw, i.e. along the surface, and discriminate against signals that can

propagate along directions near the normal to the plate.

30
FlawPhV;

Source of magnetic field

^Wire carrying dynamic current

-incident Wave^

- Reflected Waves

Figure 4-2: The radiation pattern of the simple EMAT in Fig. 4-1

produces a stronger reflection from the plate back

surface than does a flaw.

Practical Configurations

In the past, a number of different EMAT configurations have been

proposed. The majority of applications involving the use of SH-waves made

extensive use of the periodic-permanent-magnet (PPM) EMATs of the general type

depicted in Fig. 4-3. It is easily seen that the EMAT shown in Fig. 4-3 can

be constructed by superposing a number of the primitive EMATs shown in Fig.

4-1. However, in Fig. 4-3 the sources of the static magnetic field range

symetrically along the conductor carrying the dynamic current I . To confine

the eddy current distribution to the region directly below the periodicpermanent-magnet array, there are two oppositely polarized rows of magnets and

the conductor is in the form of an elongated spiral coil.

Detailed factors governing the beam-forming characteristics (e.g.,

directivity) of periodic-permanent-magnet EMATs are in Fortunko and Schramm

31
2D -«

-

1 2 3 M

N S N S N S N S N S N S

Permanent

Magnet Assembly

-<RF Coil

t

1 r 1 i t —

/"

1 w

V

v.

1 N

\ --- ------ --- --|--

r^ '

RF Coil

Figure 4-3: Schematic of a periodic-pernianent-magnet (PPM) EMAT con- taining M pairs of magnets.

(1983), while Thompson (1973, 1977, 1978) discusses the principles of the

transduction mechanisms. In the sagittal plane (i.e., the plane normal to the

surface of the plate and parallel to the principal propagation direction), the

directivity pattern of a PPM EMAT can be determined approximately from the

relation (Vasile and Thompson, 1979, and Fortunko, King and Tan, 1982):

32
s i n ( ^ s i n

' ' ' —r- Sin |9

sin

sin 2 -'^' ^-D

where the angle is defined by the ray from the PPM EMAT center to an

observation point and the normal to the plate surface. As shown in Fig. 4-3,

the PPM EMAT is a periodic array of M permanent magnets (usually made from

high-strength samarium-cobalt) with a period L=2D and half-width W, and a

spiral-wire coil placed between the magnet assembly and the plate surface. In

practice, a spacer of thickness 6 (not shown in Fig. 4-3) is inserted between

the magnets to reduce eddy current losses. Then, L becomes 2(D+6). In Eq.

4-1, X is the bulk ultrasonic wavelength (x = v /f).

The PPM EMAT of Fig. 4-3 is M essentially independent cells. The cells

are connected in-parallel acoustically and in-series electrically. As a

consequence, the PPM EMAT can be viewed as an ultrasonic equivalent of an

electromagnetic end-fire antenna. This fact is reflected in Eq. 4-1, which

can be interpreted as a product of element-structure and array-form factors.

For clarity, braces isolate the element and array factors. Narrow beam widths

can be realized only when the EMAT is driven electrically by a tone-burst at

least M cycles long. Then, the principal beam direction is:

= arcsin [x/2(D+6)] Eq. 4-2

The end-fire properties of the PPM EMAT in Fig. 4-3 can be verified

experimentally using the calibration configuration in Fig. 4-4, a 152-mm

radius, 203-mm wide aluminum test-block with a semi-cylindrical cross-section.

The angles e and define the coordinates at the observation point. The PPM

EMAT under test (in this case, the transmitter EMAT) is placed symmetrically

33
Transmitter

EMATx

Receiver

EMAT

ALUMINUM

Figure 4-4: Aluminum test block and transducer configuration to

measure the angular distribution of ultrasonic energy.

Sm-Co Magnet

Magnet Shunt

Magnet Keeper

Glass/Epoxy Spacer

(0.5 mm)

Magnet Shunt

Glass/Epoxy Spacer

(0.5 mm)

24 Turns #32 AWG

Figure 4-5: Simple EMAT (M=l) with maximum sensitivity to SH waves propagating normal to the surface.

34
at the intersection of the two symmetry planes. A second EMAT serves as a

probe. The simple EMAT of Fig. 4-5 exhibits maximum sensitivity to shear wave

signals that propagate along the normal to the surface. (Figure 4-6 shows its

theoretical and experimental radiation patterns.) As a result, it is very

useful in diagnosing the performance characteristics of the more complicated

PPM EMATs.

The experimental and theoretical radiation patterns of the EMAT in Fig.

4-3 are compared in Fig. 4-7. In this case the measurements were taken in the

sagittal plane (9=0). The EMAT characteristics were: M=16, W=12.5 mm,

D=3.2 mm and 6=0.5 mm. The frequency was 410 kHz.

A comparison of the experimental and theoretical results shown in Fig.

4-7 verifies the end-fire characteristics of the EMAT shown in Fig. 4-3 over

most of the range of 0. In particular, the maximum amplitude of the radiated

signal occurs at the grazing angle, 0=0°. A discrepancy between the measured

and theoretical values exists for 0<6O°. This is attributed to

second-harmonic frequency components and transducer end-effects. It is

important to note that the amplitude of the signal transmitted along the

surface normal is negligible.

Final Design

Although the SH-wave EMAT shown in Fig. 4-3 exhibits the desired

directivity characteristics, it suffers from excessive sensitivity to electromagnetic interference (EMI). A better design in Fig. 4-8 uses two separate

RF coils to induce eddy currents at the surface of the metal plate. The coils

are symmetrical with respect to the sagittal plane (not shown) and connected

in-series electrically to increase the input electrical impedance. However,

one of the coils is wound clockwise while the other is wound counterclockwise.

This winding arrangement reduces the EMAT sensitivity to EMI and minimizes

35
9 (degrees)

-90

SIGNAL (mV)

Aluminum, I 1 52 mm

• Experimental

•^Theoretical

M=1

= 0° D=3.2mm

428 kHz

Figure 4-6: Angular radiation pattern of EMAT in Fig. 4-5 (M=l)

Maximum amplitude is normal to the surface.

i> (degrees)

-90

40

\ "^

\

1

*..,.><X\'\.

•Ay ^

1

Vo

30 20 10 EMAT 10 20 30 40

90

SIGNAL (mV)

e= 0°

Aluminum, I 152 mm

• Experimental

•^Theoretical

M= 16 D = 3.2 mm

410 kHz

Figure 4-7: Angular radiation pattern of EMAT in Fig. 4-3 (M=16)

Maximum amplitude grazes the surface.

36
Sm-Co Magnet

Glass/Epoxy Spacer

(1.3 mm)

Magnet Shunt

Magnet Keeper

Glass/Epoxy Spacer

(0.5 mm)

Magnet Shunt

Glass/Epoxy Spacer

(0.5 mm)

24 Turns #32 AWG

Figure 4-8: Improved EMAT design. The two coils are counter-wound

to minimize pickup of EMI.

direct electromagnetic cross-talk between adjacent EMATs. To maintain phase

coherence, there are four rows of permanent magnets. The four magnet

structures are polarized to maintain mirror symmetry with respect to the

sagittal plane.

Figure 4-8 shows all the essential EMAT components, except for a thin (80

ym), 304-type stainless steel face plate protecting the EMAT coils from

damage. Fiberglass-epoxy spacers, 0.5 mm thick, separate the individual

37
magnets to reduce eddy current flow in the samiarium-cobal t (Sm-Co) magnets.

A central 1.3-mm-thick spacer of the same material separates the magnet

assembly into double rows associated with each spiral coil. In addition,

3.7-mm thick soft-iron plates serve as magnet-keepers and shunts.

The SH-wave EMAT in Fig. 4-8 was originally designed to operate in the

440 to 500-kHz region. The magnets are 5-mm deep along the polarization

direction, 3.3-mm long along the propagation direction and 8-mm wide along the

direction that is normal to the propagation direction. Each elongated spiral

coil contains 24 turns of #32 AWG magnet wire. At 500 kHz the input

electrical impedence when placed on ferritic steel is (19 + 15. 3j) ohms

(j=/-l). This is a relatively high impedence level that is intermediate

between the low impedance levels required by the transmitter amplifier (less

than 1 ohm) and the high impedance levels required by the receiver

preamplifier (typically 500-1000 ohms). It is convenient to raise or lower

this impedance to maximize the electrical efficiency of the system. (Chapter

5 covers this topic extensively.)

It is interesting to examine the radiation patterns of the EMAT design in

Fig. 4-8 as a function of frequency and length-of-signal parameters. Figures

4-9 (a) to (d) show measurements at several selected frequencies. The

measurements were made in the sagittal plane using the EMAT in Fig. 4-5 as a

probe and the semicyl indrical test-block in Fig. 4-4 as an experimental range.

The radiation patterns are for center frequencies of 400, 441, 500 and 700 kHz

with a three-cycle RF tone burst. In addition. Figs. 4-10 (a) and (b) show

the radiation patterns at 441 kHz with, respectively, four and eight RF-cycle

drive signals. In some of the figures, the actual received waveforms are also

shown to better illustrate certain peculiarities of the PPM SH-wave EMATs.

38
>E «

T3

".So

03

OJ

JZo

+->

O) r^o

>>u • N

o) rc

O) -i^: S- x: o 1— oLn . .-—. 00 o 1 ''*^—

=dcn

N

1- DC U :^ ^-* C t—

«i^ "U •r- ^

(0o

rns

of

EMAT

kHz.

(b) 4 h F >. O) o E F o -M O

(M <vj

m

03 * en Q---

—

N

II II II ^ 03 nz F

-> o oo o

-1 in •1-

O c 4-> • e<

gure

4-9:

Radia burst (d) 7

39
Even a cursory examination of the ultrasonic radiation patterns and

shapes reveals several interesting features. For example, in Figs. 4-9 (a)

and (b), a three-RF-cycle tone-burst drives the EMAT. Comparing the two

figures shows that the major lobes of the two radiation patterns lie in the

range 90-45 degrees with respect to the surface normal. However, in addition

to the two major lobes, which are distributed symmetrically with respect to

the surface normal (PPM EMATs are bidirectional), two additional lobes exist,

aimed at approximately 15 degrees with respect to the surface normal. A

closer examination of the actual received waveform reveals that the center

frequency of the signal transmitted through the two subsidiary lobes is mainly

second-harmonic components. (The presence of such signals is not detrimental

to the inspection process, because they can be efficiently filtered out by

zero-padding the Fourier spectra during synthetic aperature processing.) The

major lobes aimed along the surface are the most important aspects of the two

radiation patterns in Figs. 4-9 (a) and (b). (At 90 degrees the amplitude of

the signal appears to decrease by approximately two-thirds. However, this is

due to the M=l receiver EMAT near the corner formed by the cylindrical an-d

flat surfaces of the test-block.) These radiation patterns are very desirable

for inspecting butt weldments, because they insure that the flaw remains in

the field of view of the transducer over maximum scan distances. This feature

aids in realizing long synthetic apertures to help discriminate against

unwanted ultrasonic signals.

The radiation patterns in Figs. 4-9 (c) and (d) exhibit major lobes aimed

into the material at 47 and 30 degrees with respect to the surface normal. We

have included the two figures for completeness, to illustrate the fact that

the direction of the SH-wave signals can be smoothly varied by simple changing

the center frequencies. These radiation patterns are useful in angle-beam

40
inspections and as a means of localizing the flaw with respect to the plate

through-thickness direction by triangulation. (At grazing incidence the

transverse resolution of an ultrasonic inspection system is very poor

(Fortunko and Schramm, 1983).

Finally, it is interesting to compare the radiation pattern in Fig.

4-9(b) with those Figs. 4-10 (a) and (b). The three patterns were generated

by the same EMAT when driven by three, four and eight RF-cycles, respectively.

The pattern for four RF-cycles appears to be smoother than the patterns for

three and eight RF-cycles. However, the quantitative differences in the three

radiation patterns are very minor. For this reason, to increase the longitudinal resolution of the SH-wave inspectin system, we have used the shortest

pulse, three cycles.

For completeness, we also show typical transverse distributions of an

ultrasonic beam generated by a SH-wave EMAT. In particular we show the

transverse distributions at 520 kHz in Fig. 4-11 of the M=l EMAT shown in Fig.

4-5. The measurements used an identical EMAT as a probe. (The results were

corrected to account for the fact that the signal passes through two identical

transducers.) An examination of the radiation patterns at zero and 45 degrees

with respect to the surface normal shows that the two patterns are nearly

identical and can be accurately predicted by simple theory. The close

adherence of the two patterns also demonstrates the great repeatability of the

measurements, a very desirable feature for synthetic aperture systems. (We

have also shown that the EMATs preserve not only the amplitude, but also the

phase of the received signals (King and Fortunko, 1983).) The above results

demonstrate that the radiation properties of the PPM SH-wave EMATs are very

well understood and can be custom-tailored to specific applications.

41
O

N

rc.^ • +-> I—

I/) "vt- S- ^ 13 jn

, cu CO £Zo

"^ +-'

, QJ en (

•I— O

Ll_ >iu

+->

+-> CO

03

S_

OJ

S->

fD

SCO

4- -Q

a O — o

fc E >- to E E u c •

tNI (N

•* S- +J

in

«T n

I

OJ CO

II II II -:^ 4-> =5 V ? n

^ ro JD

F » CL

3

<r QJ c c

e o o 3 •.- +-> < +-> 03 OJo

u

I

O)

s_13

CD

sC7)

-

1 1

o •

•E

-

- o*

o* E

eg E

-

• o

0»

o •

II

e'CM

e

eg«

a W=

12.5

m

520

kHz

p 3<

•

• o -

o • o= sn O t

a»

.o •

11 II « -e-

• o o

o

•

•

• o

I 1 1

m

_ o

(A

0)wo

00d

egd

(sjjun aAijeiej) 3anindi/\iv aaiimsNvai

42
References

Fortunko, CM., King, R.B. and Tan, M. , 1982, Nondestructive evaluation of

planar defects using low-frequency shear horizontal waves, J. Appl . Phys. , 53: 3450.

Fortunko, CM. and Schramm, R.E., 1983, An analysis of electromagnetic

acoustic transducer arrays for nondestructive evaluation of thick metal

sections and weldments, jiji: "Review of Progress in Quantitative Nondestructive

Evaluation, Vol. 2," D.O. Thompson and D.E. Chimenti, eds.. Plenum Press,

New York.

King, R.B. and Fortunko, CM., 1983, Determination of in-plane residual stress

states in plates using horizontally polarized shear waves, J. Appl . Phys .

,

54: 3027.

Thompson, R.B., 1973, A model for the electromagnetic generation of Raleigh

and Lamb waves, IEEE Trans. Sonics-Ul trasonics , 20: 340.

Thompson, R.B., 1977, Mechanism of electromagnetic generation and detection of

ultrasonic Lamb waves in iron-nickel alloy polycrystal s , J. Appl . Phys. , 48: 4942.

Thompson, R.B., 1978, A model for the electromagnetic generation of guided waves in ferromagnetic metal polycrystal, IEEE Trans. Sonics-Ul traonsics

25: 7.

Vasile, CF., and Thompson, R.B., 1979, Excitation of horizontally polarized

elastic waves by electromagnetic transducers with periodic magnets,

J. Appl. Phys. , 50: 2583.

43

Chapter 5

TRANSDUCER ELECTRONICS

Design of Low-Noise Receiver Amplifiers for EMATs

The principal challenge facing the designer of an EMAT-based ultrasonic

inspection system is the need to design an optimal receiver-EMAT to

preamplifier interface. In particular, extreme care is necessary to ensure

use of an efficient electrical impedance-matching network to minimize the

preamplifier noise-figure. (The preamplifier converts the low-level output of

an EMAT to a high level signal and drives a length of transmission line

feeding the rest of the signal-processing chain.) Although this subject is

treated in great detail in a number of specialized texts (Ott, 1976; Krauss,

Bostian, and Raab, 1980; Clarke and Hess, 1971), it is useful to outline the

impedance matching techniques that are particularly useful for matching

SH-wave EMATs in the 400 to 1500 kHz region. It has been the authors'

experience that these techniques are simple to implement and verify

experimentally without sophisticated instrumentation.

Thermal noise generated by the random motions of electrons within the

EMAT ultimately limits the sensitivity of the EMAT-based ultrasonic inspection

system. However, in most cases of practical importance this ultimate

sensitivity is not attainable because of other sources of electrical noise and

ultrasonic and external electromagnetic interferences. Chapter 4 addresses

limiting the influences of ultrasonic and electromagnetic interferences

through proper SH-wave EMAT design. This chapter specifically addresses the

problems associated with minimizing the amount of excess electrical noise that

can exist due to improper design or implementation of the impedance-matching

network connecting the receiver-EMAT to the receiver-preamplifier. In the

45
present context, the term excess noise refers to all active and passive

sources of electrical noise that contribute to a 1 arger-than-one noise-figure

in the receiver-preamplifier chain.

Figure 5-1 illustrates a very useful way of characterizing the noise

performance of a receiver-EMAT/receiver-ampl if ier combination. A resistive

sensor R^ is connected to a noisy amplifier represented by a noise-free

amplifier and two equivalent noise sources l^ and \l^. The quantities I^, and

V|^ represent the current and voltage excess noise generated within the

amplifier. An equivalent voltage source, V^,, represents the Johnson noise

associated with R^-. Using the circuit shown in Fig. 5-1, it is

straightforward to show that the noise-figure is minimized when the value of

the resistor R^- is set equal to the ratio Vj^/I^,. This ratio is in the range

of 50 ohms for bipolar transistors and 500-1000 ohms for junction field-effect

transistors (J-FETs), which are typically used as the front-ends of low-noise

receiver-preamplifiers. Both types of transistors can be used in the 400 to

1500 kHz region. In our designs we have used J-FETs because of their biasing

simplicity. Since the electrical impedance levels of our SH-wave EMATs are in

AAAr

Sensor-^

IN o Noise-free

amplifier

Preamplifier

Figure 5-1: Idealized schematic of a receiver-amplifier system.

46
the range 5-15 ohm, an appropriate impedance transformer is necessary to

obtain a 500-1000 ohm impedance level at the input to the

receiver-preamplifier. Fortunately, this large transformation ratio is

possible with properly designed transmission-line transformers (Krauss,

Bostian, and Raab, 1980). This technique is preferred to resonant matching

because it offers superior efficiencies and bandwidths compatible with the

acoustic bandwidths of the SH-wave EMATs described in Chapter 4.

An example of a transmission-line transformer/J-FET receiver-amplifier

chain is shown in Fig. 5-2. In this particular design, a chain of four

transmission-line transformers, with 1:4 impedance-transformation ratios,

matches the SH-wave EMAT to the input of the receiver-amplifier. The

transformers are approximately 18 turns of #30 AWG bifilar wire on two 13-mm

diameter toroidal ferrite cores with an initial permeability of 1000 (Krauss,

Bostian and Raab, 1980). (It is interesting to note that the efficiency of

the fourth transformer in the chain is degraded substantially because its

magnetizing reactance is comparable to the optimum value of the source

resistance Re-.)

In Fig. 5-2, the receiver-preamplifier is a two-stage design. The J-FET

device in the first stage conditions the noise figure of the preamplifier

while the second device acts as an active impedance transformer to the lowimpedance (200 ohm) twisted-wire-pair transmission line that connects to the

remaining signal processing chain. The overall voltage gain of this amplifier

is approximately 12. Two sets of back-to-back diodes limit high-voltage

signals that can exist when the transmitter-amplifier is active or when

impulsive noise interferences are present. This is useful primarily when the

receiver-amplifier can be located in close physical proximity to the receiverEMAT and the output transmission line is very short (less than 1 meter).

47
10/iF:

IN4004 M

1 * EMAT » f—^*>1J r^ |r^ I j^ I r^ '.'. i;

-»> Amplifier

All resistors metal film, 1%

All resistances in il

All capacitances in pF unless otherwise noted

Ti .Tj. 13,14 : 15 turns #30 AWG bifilar wire on two 13mm dia., 3D3 cores CD Ferrite Beads

EMAT PREAMPLIFIER AND LINE DRIVER

Figure 5-2: Schematic of the transmission-line transformers and

J-FET preamplifier used in the current work with a transmission line < one meter long.

+ 15Vdc »- IT

-QID— 7812 :io^F 2700=!= rt-:iO/iF

IK'

2700=^

3D3

20T *30AWG IN4004 ln» ^-rrvY-, vw 1 - ^ 1

-"T

'—T—T—T—T—I n'—T—

T—T—T

Agi ^ ,,'i-L_L§i \i'sk -- „^A . 5 o? ii '-=t-i i-s-.i 1 o 5 <»> " '-4-1 T' ;t

r^ ilfjl [

i|^|i==

jM'^

CM =

28K'

Ol co> i^ jl:^—I— -I— (V. ICP-I -T «

I i i 1 . I 1

1

Gain Ad). 50K I 10T 1 22* n

=^1^F

k—

I

J100K*

S?K-r i *2-T=r°^-^'' 1^°

All capacitances in pF unless marked otherwise

All resistances in il

• 1% metal film

Ferrite bead

i^F

IN4004 *

J 390

1/iF

' — »Out i^fI 1

0.1/iF

.6K? 10 J g'TT M

500 kHz EMAT PREAMPLIFIER

Figure 5-3: Preamplifier schematic. Useful for longer transmission

1 ines.

48
A higher-gain receiver-preamplifier design useful for driving very long

50-ohm transmission lines is shown in Fig. 5-3. In this case only one input

transformer is shown in the Figure, because the remaining three transformers

are usually integrated directly within the EMAT housing. The individual

mid-stages are tuned to a center frequency of approximately 500 kHz. Typical

voltage gains are in the range of 500-1000.

Both amplifiers include an on-board voltage-regulated power supply and

ferrite beads that help to minimize electromagnetic interferences. Metal-film

resistors at the input stages in both designs reduce shot noise. In addition,

good soldering practices eliminate contact noises. It is believed that both

amplifier designs offer noise-figures in the range 3-4 dB when transformer

losses are taken into account.

It should be noted that although the receiver-preamplifier designs shown

in Figs. 5-2 and 5-3 utilize overload protection features, they are not well

suited for operation in the pulse-echo mode in which the same EMAT acts as a

transmitter and receiver of elastic waves. The transformers impose the

principal limitation because they store a very substantial portion of the

transmitter output energy. In spite of repeated efforts, we have not been

able to devise a satisfactory scheme for designing a functional transmitreceive (T/R) switch for EMAT systems operating below 1 MHz. This remains one

of the main obstacles to designing more compact, low-frequency, SH-wave-EMAT

systems.

Design of Efficient Power Amplifiers for EMAT Applications

To maximize the conversion efficiencies for generating acoustic waves,

both EMATs and piezoelectric transducers must be appropriately matched to

their power amplifiers to ensure optimum utilization of the available

transmitter power output. However, because piezoelectric transducers are

49
generally much more efficient than EMATs and produce higher open-circuit

voltages for a given surface displacement, the design philosophy of pulsers

for driving piezoelectric transducers has been traditionally guided by

spectral considerations and the voltage breakdown characteristics of

semiconductor components used in capacitance-discharge circuits. (The

possibility of depoling the piezoelectric drive elements traditionally sets

the high-voltage limits.) As a consequence, resonant matching of

piezoelectric transducers has not been widely used to maximize generation

efficiencies.

The electrical characteristics of most EMATs, as viewed from the

electrical input port, are significantly/ different from those of piezoelectric

transducers designed primarily for NDE applications. Electromagnetic

transducers generally behave as inductive loads, while the bulk capacitance of

the piezoelectric crystal or ceramic drive element dominates the electrical

input impedances of most piezoelectric transducers. (Reactances caused by

mechanical motion of the elements themselves are generally smaller than the

bulk capacitive reactance.) As a consequence, to maximize transduction

efficiencies and optimize temporal responses, different electrical circuit

configurations are necessary for EMATs and for piezoelectric transducers.

The conceptual differences between electrical circuits used for driving

EMATs and those used for driving most piezoelectric transducer elements can be

best illustrated by reference to Fig. 5-4, which shows the basic capacitive

discharge circuit used in most commercial pulsers for driving piezoelectric

transducer elements. Only six circuit elements comprise the circuit in Fig.

5-4a: 1) dc voltage source, V ; 2) current limiting resistor, R ; 3) ideal

switch, S; 4) charging capacitor, C; 5) damping resistor, Rr.; and

6) piezoelectric transducer, T. In operation, the capacitor C is first

50
'Rp 3m: T

(a)

O)

COo

>

time

(b)

Figure 5-4: (a) Basic capacitive discharge circuit used in most

commercial pulser designs for driving piezoelectric

transducer elements, (b) Voltage waveform seen by

transducer T.

charged through the resistors R and Rr.. The maximum voltage is V . On

closing the switch S, the capacitor discharges through the parallel

combination of Rp, and T. (It is assumed that R >>Rn.) The resulting voltage

D c '^D' waveform V(t) at the electrical terminals of the transducer T is shown in Fig.

5-4b. V(t) first drops rapidly to a value that is approximately equal to -V .

In practice, the non-ideal behavior of S and series inductances in the circuit

determine the initial risetime. Then V(t) increases exponentially to the

ground potential, as the charge initially stored by capacitor C discharges

through the damping resistor R^.

51
The major limitation of the circuit in Fig. 5-4a is that it is useful

mainly for producing unipolar signals. As a consequence, the damping resistor

Rp, and switch S unproductively absorb most of the energy initially stored in

the capactior C. Clearly, because of this inherent inefficiency, this circuit

is not suitable for EMAT applications.

In many NDE applications using EMATs it is necessary to use gated bursts

of RF to drive the transducer. For example, 4 cycles of RF are required to

maximize the output of the EMAT in Fig. 4-8 when it is operated in an end-fire

mode. (Here, it is assumed that the electrical of the EMAT is smaller than

the acoustical Q.) Such applications require very efficient power amplifiers.

Figure 5-5a shows an example of a circuit configuration that is inherently

efficient and easily lends itself to EMAT applications.

Technically, the circuit configuration in Fig. 5-5a is a complementary,

voltage-switching, Class-D power amplifier. However, this circuit includes

extra components needed in a practical implementation. Those needed to assure

Class-D operation are the two constant voltage sources Vp, the two ideal

switches S, and $2, and the series tank circuit composed of the EMAT and

external capacitor C„. The waveforms associated with the operation of this

basic Class-D amplifier are shown in Fig. 5-5b.

The circuit shown in Fig. 5-5a operates as follows. The switches S, and

Sp alternately close and open at a fundamental frequency f . Ideally each

switch operates at a 50% duty cycle. As a result of this commutating action,

a square-wave voltage waveform V(t) (Fig. 5-5b) appears at the output of the

amplifier, 0. The series tank circuit, composed of the EMAT and series

capacitor C,, acts as a filter that presents the minimum impedance to the

52
(a)

V(t)

time

Figure 5-5: (a) Basic schematic of a power amplifier for a transmitter

EMAT. (b) Voltage and current waveforms produced by this

ampl if ier.

fundamental -frequency component of the applied voltage waveform. Ideally, the

symmetry of the network in Fig. 5-5a ensures that the zeroth (dc) and

even-harmonic terms of the Fourier series associated with the driving voltage

waveform are not present. The resultant current waveform I(t) is shown in

Fig. 5-5b and is nearly sinusoidal and free of even order harmonic components.

53
This circuit configuration is very efficient since there is no damping

resistor and the switches dissipate little power because they drop very little

voltage while passing maximum currents. Compared with a classical Class-B

amplifier used in many RF transmitter designs, the Class-D can be 27% more

efficient at similar maximum current/maximum voltage ratings (Raab, 1974).

When implemented with practical switching devices, the performance of the

Class-D circuit is, of course, subject to the effects of saturation,

parasitics, finite switching times, and device breakdown considerations.

Because of practical considerations associated with the breakdown

characteristics of semiconductor switching elements and the input impedance

characteristics of typical EMATs, the circuit in Fig. 5-5a includes additional

components: current limiting resistors R, , and Rp, storage capacitors C, and

Cp, protection diodes CR, , and CRp, impedance matching transformers T, and T^,

and a parallel tank circuit L^-C^.

The capacitor-resistor networks R-i-C-, and Rp-Cp provide sufficient energy

for short RF bursts at the fundamental frequency f . In the event of a short,

the resistors R-, , and Ro limit the output current. The diodes CR, and CR^

provide a path for inductively fed currents when both S, and S^ are in the off

position. (Cross-over distortion is used in many designs to reduce the

potential of damaging the semiconductor devices used to implement the switches

S, and Sp.) Finally, the circuit of Fig. 5-5a includes two high-power

impedence matching transformers T, and Tp and a parallel tank circuit L-i-Cn

tuned to the fundamental frequency f . The two transformers reduce the input

impedance of the EMAT (tuned to frequency f by C.) to match the low output

impedance levels of the Class-D output stage while the tank circuit L,-Co

provides a bypass to ground for higher-than-fundamental harmonics.

54
Figure 5-6 shows a circuit diagram of a practical power amplifier

designed to operate in the 300-600 kHz range. This particular amplifier can

produce approximately 1500 W when used in conjunction with the SH-wave EMAT in

Fig. 4-8. Figure 5-7 shows the physical construction of the two output power

transformers T, and T^,. Both transformers are wound using 8 turns of small

diameter, flexible coaxial cable (RG-174) on 9.3-mm outside diameter, Q-l-type

ferrite cores. The large number of cores (24) prevents saturation. Figures

5-8 a and b are oscilloscope traces of the voltage and current waveforms at

the output of the amplifier. The amplifier in Fig. 5-7 consumes approximately

2 W when operated at a 0.4% duty cycle. Figure 5-8b shows that at f =450 kHz

the current waveform is sinusoidal, as expected, and the peak-to-peak current

is 80 amperes. However, the voltage waveform is not square. This results

from the finite switching and storage times of the high-power semiconductor

devices used to implement the switches S, and S^. (At 250 kHz, the output

voltage waveform becomes nearly square.) The amplifier in Fig. 5-6 uses

high-power, bipolar switching transistors, because faster devices with

comparable breakdown characteristics were not available. Currently, the same

amplifier could be implemented using high-power MOS-FET devices.

References

Clarke, K. E. and Hess. D.T., 1971, "Communication Circuits: Analysis and

Design," Addison-Wesley , Reading, PA.

Krauss, H. L., Bostian, C.W., and Raab, F.H., 1980, "Solid State Radio

Engineering," Wiley, New York.

Ott, H.W., 1976, "Noise Reduction Techniques in Electronic Systems,"

Wiley-Interscience, New York.

Raab, F. H,, 1974, High efficiency RF power amplifiers. Ham Radio , 7:8.

55
t^I2;XT2I2z—^^° '"*

24*01' cores. 9.5 mm dra., 8 turns RG-174 coax

All capacitances in ^F

Ail resistances in H EMAT POWER AMPLIFIER

Figure 5-6: Schematic of amplifier for 300-600 kHz used in this work.

1 : 16 Transformer

Q-1 cores

(24)

Input

IlliE^llI

^i^

28 Awg plastic jacketed

minature coaxial cable

(8 turns)

Input

Output Lo Hi EMAT

Transducer Amp

EMAT Transducer — 1:16 -^ EMAT Transducer

"^" Amp Xfrmr

Output

Figure 5-7: Construction of output power transformers used in the

amplifier of Fig. 5-6.

56
Output of EMAT Power Amplifier

1 1 f \ A A i \ 50 V

Voltage

(a)

A \ A t ^^

/ / / \y

1^

V, \L If 5 / V

Current

(b)

^

\

\ %hA /I /Vj 1 ^r\i V V/

^ / 20 A

,

2 /i-s

Figure 5-8: Oscilloscope traces of the output of the amplifier in

Fig. 5-6. (a) Voltage, (b) Current.

57

Chapter 6

DIGITAL PROCESSING

Signal Averaging Method

The receiver EMAT signal is displayed on a digital oscilloscope that

internally performs the initial time averaging. Since the background noise is

mainly electronic and random, an average of 8 to 32 scope traces improves the

S/N ratio and reduces the effects of impulsive interference. An IEEE-488 bus

transfers the digital data to a minicomputer for storage and analysis.

Synthetic Aperture Method

The synthetic aperture focusing method and its advantages were discussed

in Fortunko and Schramm (1983). It requires obtaining data from several

locations along a scan line that is approximately normal to the long flaw

dimension. From the geometric parameters and sound velocity information, it

is possible to time-shift the individual waveforms so they combine coherently.

In effect, the sound energy focuses at a particular resolution cell to be

examined (Kino, et al . , 1980). Subsequently, the same data can be reprocessed

to change the location of the focal point (Fig. 6-1). Repetition of this

process results in the interrogation of the whole weld in the direction normal

to the surface. Only one set of ultrasonic data is necessary.

A practical problem limiting the method is the result of time

quantization. Each of the 512 data points in an oscilloscope trace is the

signal voltage at a discrete point in time. The time scale on the

oscilloscope fixes the amount of time between these points. As a result, an

integral number of sampling time intervals will not, in general, give the

necessary time-shift. This may cause the appearance of unwanted spatial

59
Synthetic

Aperture

Weld

Reinforcement

v-rry free

Surface

Defect below surface

Focal

Point 2

Figure 6-1: By repeated mathematical processing, the same data set

can be brought to focus at multiple points.

sidelobes which would have to be eliminated by time-consuming interpolation

techniques. Time-shifting in the frequency domain using a Fast Fourier

Transform (FFT) overcomes the detrimental effects of time quantization. This

takes full advantage of the 16 bit dynamic range of the computer. The

transform yields a series of coefficients, A, composed of real (Re) and

imaginary (Im) parts. Multiplying the coefficients, A, by a complex phase

factor gives the effect of time-shifting (indicated by primes). Note

Fig. 6-2.

A' = A exp (ja3At) = (Re + jIm) (coswAt + jsinooAt)

So that Re' = Re coscoAt - Im sin toAt

Im' = Im cosuAt + Re sinooAt

where j = /-I, At = change in path length/sound velocity, u) = n(2TTf ),

n = harmonic number of Fourier coefficient, and f^ - 1/measurement period (the

oscilloscope display sweep time).

60
Phase Shift for

Synthetic Aperture

R' R

Figure 6-2: Phase shift of Fourier coefficients in the real-imaginary plane,

For convenience, the transducer postion nearest the focal spot usually

serves as the reference point (forward shift). Conversely, for a backward

shift, the reference becomes the position farthest from the focus. Simple

trigonometry determines the change in ultrasound path length at other

transducer positons. The oscilloscope produces 512 eight-bit samples for each

trace (the values of n range from -256 to 255). Thus, for a typical time

scale, the total measurement period is 51.2 ys and the frequency step in the

FFT (f ) is about 19.5 kHz. Since the input data are entirely real, there are

FFT symmetry relations that reduce the mathematical manipulations by half

(Bergland, 1969).

Examination of the FFT coefficients can reveal electromagnetic noise at

other than the operating frequency. The application of suitable windows

61
(Oppenheim and Schafter, 1975) and other techniques (Taylor, 1983) can

minimize this problem. In particular, zero padding the base band can

eliminate the effect of dc ground loops.

After time-shifting, the coefficients from each data set are added, and

the result is normalized and reverse transformed to produce a reconstructed

time signal .

Scanner Control and Ultrasonic Data Collection and Processing

Figure 6-3 is a flowchart of the BASIC computer program used to

synchronize the operation. Detailed listings are in Appendix I and an example

of typical operator-computer interactions is in Apperrdix II.

One of the main functions of the computer program is control of the

mechanical scanning mechanisms (Chapter 7) that determine the EMAT positions

on the surface of the part under inspection. To accomplish this, the computer

program transfers appropriate control codes from the main minicomputer to a

special, microprocessor controlled, subsystem that directly controls the power

supplies of the stepper motors. The master minicomputer transmits the digital

codes to the slave microcomputer via an industry-standard IEEE-488 bus.

Another primary function of the computer program involves collecting and

processing ultrasonic signals. Because of very high electrical noise levels,

the data collection and processing is possible only while the stepper motor

power supplies are inactive. When the EMATs are stationary, the computer

program executes a data collection and processing sequence.

The first decision block in Fig. 6-3, determines whether to collect and

process new ultrasonic data or to recall previously recorded data. The next

block allows the console operator to input signal processing and scanner

control parameters. Figure 6-4 illustrates the meaning of various geometrical

parameters.

62
5 <Analyze'?>

6

FFT

C VP^ )

Figure 6-3: Flow chart of computer program used to collect, store,

process, and display synthetic aperture data.

Test plate

EMAT Transducer

Transmitter Reciever

J

(n-1)d

Focal Spot

Figure 6-4:

n = number of transducer positions

d = distance between transducer positions

S = separation between transmitter and reciever

T = plate thickness

H = horizontal distance between initial transmitter

position and focal spot

V = vertical depth of focal spot

Geometrical parameters required for synthetic

aperture processing.

63
The input block (3) in Fig. 6-3 involves reading the output of the

digital oscilloscope and transferring it to magnetic memory (standard 8-inch

diskette). Since the oscilloscope settings are not externally adjustable via

the IEEE-488 bus, manual adjustment of control knobs on the front panel is

necessary. Parameters in this category include the trigger time delay, sweep

rate, vertical amplifier sensitivity, and the number of time-averages. After

storing the data, the program instructs the scanner controls via the IEEE-488

bus to move the EMAT search head to the next location.

The next query (6) allows the operator the option of subtracting a

coherent background from the ultrasonic data. Elsley and Fortunko (1981)

discuss the rationale for this step in detail. This allows the operator to

remove from the new data certain common features such as transducer

reverberations and reflections from common geometrical artifacts. It is

particularly useful in the inspection of simple weldments whose profile does

not vary along the transverse scan direction. The option is implemented by

first digitally averaging many sets of data from different EMAT positions and,

then, subtracting the average from each of the individual data sets.

If the option in step 6 is not selected, then the program moves directly

to the digital computation of the discrete Fourier transform (DFT) of each of

the individual data sets in step 8. The DFT is carried out using a

library-function fast Fourier transform (FFT) algorithm. At step 9,

zero-padding removes the central (dc) coefficient (assuming a two-sided

transform). This step helps to remove ground loop and dc offset effects that

exist in the analog electronics. The results are stored magnetically along

with the original ultrasonic data.

The next query in the program involves a window option in the frequency

domain. Several different window are available, including Bartlett and

64
Manning (Oppenheim and Schafter, 1975). This option supplements the analog

band-pass filters in the receiver EMAT preamplifier chain. However, it is

seldom used, and can be bypassed.

Steps 12-14 involve the actual implementation of the synthetic aperture

algorithm that is the heart of the computer program. First, the FFTs

corresponding to different data sets are phase shifted as determined a-priori

from a knowledge of the EMAT positions and details of the scan parameters

(e.g., plate thickness, scan length). The phase-shifted FFT's are then added

together and renormalized in step 13. The resultant FFT thus represents an

ultrasonic data set as obtained by a single array composed of N EMATs that are

connected in-parallel acoustically. Because the output waveform from such an

array must be real the resultant FFT is symmetrized in step 14 such that the

complex conjungate of the negative frequency spectrum is identical to the

positive frequency spectrum (Bergland, 1969). After reverse transformation

the results are stored magnetically and then are available through the display

routines in step 16.

Several forms of output are available, many recorded only to allow a

check of the process at this still-experimental stage. Graphical displays

include:

1. Original raw data (and averaged data if this option was exercised),

as well as the processed result (Fig. 6-5).

2. Fourier coefficients, as transformed, shifted, or averaged (Fig.

6-6).

3. Signal power as a function of frequency (power =

(Real2+Imaginary2)^), (Fig. 6-7).

All of these graphs can be transferred from a VDT to a printer.

65
15 30 45

Microseconds

.3

.15

-.15

-.3

T

Final Data

^\/\/yy\/vv\A/—

15 30 45

Microseconds

(b)

Figure 6-5: (a) Three sets (out of ten) of raw signal data,

(b) Signal after synthetic aperture processing,

66
N

I

>•o

z

111 Z)O

LU

CC

LlI I

I siN3ioidd300 damnod AdVNiovHi

N

I

o

z

LU3

o

111 CC

Li.

(0

C O

+-> TD

OJ OJ 00 S- • O) r—

n3 +-> fO +J C QJ -o o

2 I/) re TSc:

O) o

E -r- O -t-J CO roE

o o

^- Mc:

00 -t-

+->

CU -E

•r- +J

(J

•r— ^-~

<+- r—

I+- <C

O)O

u

n3

O

M-O

>:> U

C

CD

13CT

O)

SOJ

z.o

en

oooo

O)U

O

s-OI- O) Q.-l-> — ro

vo

ro • o s-

•I- fO

+-> c

03 -c- S_ CD

OJ ro Q. E O 1—'

I I siN3ioidd30o aaianod iv3d

67
100

1-25 2.5 3.75 5.0 Frequency, MHz

Figure 6-7: Power distribution of processed signal

Numerical outputs include:

1. Signal amplitude of the raw or processed data.

2. Total relative power represented by any data set.

See Appendix II for a typical run.

References

Bergland, G.D., 1969, A guided tour of the Fourier transform, IEEE Spectrum

6(7):41.

Elsley, R.K., and Fortunko, CM., 1981, Improvements in flaw detection in

austenitic stainless steel weldments, ui_: "Proceedings of the IEEE Ultrasonics

Symposium," B.R. McAvoy, ed., IEEE, New York.

Fortunko, C. M. and Schramm, R. E., 1983, An analysis of electromagnetic

acoustic transducer arrays for nondestructive evaluation of thick metal

sections and weldments, ij}_: "Reveiw of Progress in Quantitative

Nondestructive Evaluation, Vol. 2," D.O. Thompson and D. E. Chimenti, eds.,

Plenum Press, New York.

Kino, G.S., Corl , D., Bennett, S., and Peterson, K., 1980, Real time synthetic

aperture imaging system, vn_: "Proceedings of the IEEE Ultrasonics Symposium"

B. R. McAvoy, ed., IEEE, New York.

Oppenheim, A.V., and Schafter, R.W., 1975, "Digital Signal Processing,"

Prentice Hall, Englewood Cliffs, NJ.

Taylor, J.S.W., 1983, Digital techniques for enhancing and processing dynamic

stress-analysis data, Exp. Tech 7(6):31.

68
Chapter 7

MECHANICAL APPARATUS

In order to make routine use of the synthetic aperture technique

discussed in Chapter 6, it is necessary to have some means of positioning the

EMATs accurately and reproducibly, under computer control. Additionally, a

rugged transducer housing is required to maintain proper alignment of the

transmitter and receiver EMATs on the surface of the weldment and to provide

electromagnetic shielding. In this chapter we describe the gimbaled

transducer housing and the computer-controlled, two-axis positioner that were

designed for the new weld inspection system.

Gimbal-Mounted Transducer Housing

At low frequencies it is difficult to use the same EMAT for both

generating and detecting ultrasonic waves, owing to the problems encountered

in designing a satisfactory transmit/receive switch (see Chapter 5). It is

therefore necessary to use separate EMATs for the transmitter and receiver.

The preferred configuration is to position the two transducers symmetrically

with respect to a common sagittal plane as in Fig. 6-5. If the transducers

are to be mounted in the same housing, some freedom of motion must be provided

in the mounting arrangement to allow them both to conform to the surface of

the weldment. Often the surface of a weldment is deformed in the vicinity of

the weld by the action of residual stresses and the transducer housing must be

able to accommodate such deformation without one of the transducers lifting

off the surface.

A gimbal mount, shown in Fig. 7-1, provides the freedom of motion

necessary to ensure that the EMAT coils and magnet assemblies conform to the

surface of the plate under inspection. The gimbals' two axes of rotation are

69
Transducer Case

Fig. 7-»l: Gimbal -mounted transducer

housing for both transmitter

and reciever EMATs. The two axes of rotational freedom

allow the probe surface to

conform to the plane of the

plate under inspection.

mutually orthogonal and parallel to the free surface of the plate. A third

degree of freedom perpendicular to the plate's surface is provided by

attaching the gimbaled transducer case to a pneumatically driven piston that

can raise or lower the entire assembly under computer control.

Raising and lowering of the transducers is accomplished by reversing the

air pressure supplied to the two sides of the piston with a solenoid-operated

valve. Power is supplied to the solenoid valve through a relay controlled by

the computer. As the transducer housing is lowered onto the plate, free

rotation about the gimbals' axes allows the EMATs to contact the surface

uniformly. The pneumatic piston and the transducer housing are shown in Fig.

7-2 mounted on the two-axis positioner. The shaft connecting the piston and

the transducer housing passes through a bearing sleeve to ensure that raising

and lowering the transducers does not degrade the positional accuracy of the

assembly.

70
71
The cylindrical transducer case was machined from mild steel and divided

into two compartments by welding a steel plate across the diameter of the

cylinder. Rectangular openings were milled into the bottom of each compartment for the EMAT coil and magnet assemblies (Fig. 4-8), which were carefully

aligned with each other and then epoxied into place. Dividing the case into

two compartments provides a certain degree of electromagnetic shielding of the

receiver EMAT from the transmitter EMAT. The bottom of the transducer case is

covered with stainless steel tape approximately 0.1 mm-thick. The tape

provides excellent protection from abrasion of the EMAT coils when sliding the

transducers along the surface of the plate, yet has a minimal effect on the

signal strength.

(At the time of this writing, the control logic to operate the pneumatic

valve has not been written into the software system in Appendix I. For the

work reported here, the transducers have been simply pulled along the plate

surface between data collection points. The stainless steel tape over the

EMAT coils has proven to be effective wear protection.)

There is sufficient room in the transducer case to permit the impedancematching components to be mounted in close proximity to the EMATs. The

transmitter EMAT is series-tuned with a capacitor so that is presents a purely

resistive load to the power amplifier at the frequency of operation. The

receiver EMAT has a gang of four 4:1 transformers mounted inside the

transducer case to match the impedance of the EMAT to the input impedance of

the preamplifier, as discussed in Chapter 5.

Two-Axis Positioner

The two-axis positioner that is used to scan the plate under inspection

is shown in Fig. 7-2. It consists of two commercially available precision

lead screw translators driven by dc stepping motors. The two lead screws are

72
mounted orthogonally, with the transducer housing and pneumatic piston fixed

to the translation stage of one of the lead screws. The total range of travel

on either axis is approximately 230 mm, which is adequate for a laboratory

system such as this. Adjustable limit switches located at each end of the

lead screws provide a means to prevent jamming the drive mechanism. When one

of these limit switches is engaged, a dc voltage is applied to a Schmidttrigger on a module in the minicomputer, activating an interrupt routine (not

yet written into the software of Appendix I) that cuts off power to the

stepper motor.

The pitch of the lead screws is 1.58 threads/mm (40 threads/in) and the

stepping motors provide a resolution of 400 steps per revolution when operated

in the "half-step" mode. This results in an overall resolution of 1.6

micrometers. Naturally, the accuracy with which the transducers can be

positioned is degraded by hysteresis in the mechanical linkages, but

independent measurements indicate that the positioning accuracy is better than

±0.1 mm. Since the wavelength of the ultrasonic waves is '^^7 mm, the precision

afforded by the positioner is sufficient for the purpose of synthetic aperture

processing.

Electronic controls for the two-axis positioner were also obtained

commercially. Necessary components include: dc power supplies for each motor

and the control electronics, printed circuit driver cards for each motor,

indexer cards to provide properly sequenced phase control signals for the

driver cards and an IEEE-488 interface card that links the computer to the

controller. Actual command sequences used to position the transducers may be

found in the software listings in Appendix I.

73

Chapter 8

MEASUREMENTS

Preparation of Calibration Specimens

To evaluate the intrinsic and operator-dependent sensitivity limits and

other performance parameters of the 450-kHz SH-wave EMAT system, we have

prepared a special 25.4-mm thick flat-plate calibration specimen, Fig. 8-1, of

HY-80 steel supplied by the U.S. Navy. It contains a family of ten 0.25-mmwide electric-discharge-machined (EDM) s^lots, all located on the same surface.

The plate was used as-received (with protective paint on the surface) without

any additional surface preparation. This is very significant since, in a

practical application, the SH-wave EMAT system would be expected to operate on

similarly unprepared surfaces.

The calibration specimen's width (330 mm) and length (660 mm) eliminate

the end-reflection effect on the ultrasonic data. Table 8-1 summarizes the

individual calibration-slot dimensions as determined by plastic replica

techniques. The letter codes (A through J) denote the slot locations in Fig.

8-1. The slot dimensions span the range of weld flaw sizes that would be of

interest from a practical viewpoint (i.e., sharp flaw dimensions typically

found at weld root and toe region). These dimensions also correspond to the

initial assumptions made in the system design parameters (i.e., EMAT

dimensions and operating frequencies). Thus, to separately evaluate the

system sensitivity limitations with respect to flaw depth and surface length,

the artificial flaws in Table 8-1 form three sets. The first set

(A,B,C,D,E,I ,and J) has a surface length of approximately 25 mm and depths

ranging from 0.48 mm to 6.68 mm. The second set is flaws (C,D,G,H) with a

depth of approximately 1.75 mm, but a surface length ranging from 12.8 to

75
Surface Flaw Standard

EDM Notches in HY80 Steel Plate

(all notches on same surface)

660 >-

L

|d = 0.5

|d = 1.8

id = 2.3 1=7.2

|d=3.9

30 |d = 0.9

|d=2.1

d=1.8

^ = 52.3

|d=6.7

'

|d=1.7 ,d = 1.8 '?=12.8

25

All distances in mm

d = flaw depth

J = flaw length, all flaws are 25 mm, except as noted

Figure 8-1: Calibration plate with EDM notches

Table 8-1: Dimensions of EDM notches in calibration plate,

EDM Slot Label

Slot Surface

Length, mm

Slot Depth,

mm

A 25.7 0.48

B 25.4 0.91

C 25.1 1.68

D 25.7 1.76

E 24.9 2.14

F 7.2 2.29

G 52.3 1.78

H 12.8 1.78

1 26.2 3.94

J 25.2 6.68

76
52.3 mm. The Third set contains only one flaw (F), which is 2.29 mm-deep and

only 7.2-mm long along the surface.

In the design of the calibration specimen shown in Fig. 8-1 considerable

attention went into properly choosing the individual flaw positions. For

example, note that the slots stagger through the width of the plate to avoid

data contamination by end-effects and mutual interferences. In addition, the

distances between the slots along the specimen length are sufficiently large

to permit SH-wave EMAT-system operation in the synthetic aperture mode, needed

to remove the effects of unwanted ultrasonic reverberations and certain

electromagnetic phenomena (e.g., direct and impulsive noise) from the

processed ultrasonic scattering data.

The surface calibration slots in Table 8-1 are vertically oriented with

respect to the plane of the plate. Consequently, they present the maximum

cross-section to the ultrasonic beam, aimed parallel to the plane of the

plate. Canted slots were not possible because of physical limitations of the

EDM equipment. At project inception, provisions were also made to fabricate a

representative family of interior flaws by in-welding procedures similar to

those described in Fortunko and Schramm (1983). However, such interior flaws

were not available because of scheduling difficulties at an independent

research laboratory collaborating in the project. Hopefully, the additional

tests will be possible in the future using the present SH-wave EMAT system.

Experimental Procedure

The experimental procedure, used to obtain the ultrasonic signals from

the calibration plate in Fig. 8-1, was specially designed to take full

advantage of the synthetic aperture capabilities of the inspection system. To

maximize the number of independent ultrasonic measurements, each slot was

illuminated ul trasonical ly while located on the top and bottom surfaces of the

77
T- C\J

CO

ffl c

0)o CD c

(D CC

(D n.

0) M- cr o

>^ Cl o ,o1— co

-*-• COoo

COc

CD CD DC crCD DC

0) DC

<l "

c^l

U-i

CQ

O

LU -3 o

-, ^ X , o .< ^ O ^ o , o ,

CUT3 • ^ -r- l/l +J C/5 I

<:

oj E lj-I S c>^ OJ -O CD -M

<D -E

uo +-> uo

=3 fti

£ZO OJ i- +-> aj CD 'O

4-) s--—

+-> OJ Q03 S

Q. CU 00 -E C +-> +-> 03 O

CJ^ 4-

t/^ 00

C

nI (XI

O)SZ5CD

-0

<u

I/) 3C

S- •

cj; e

+J

-1^ •1— fC +j

Q.

CD c 1

(t3 1

<_> m

-0 03

O) -M 1

03 • I— -0

03

+-> SCD Q M- , , CvJ 00

QJS13cr

78
plate (with respect to the EMAT location). In addition, the slots located in

the central portion of the calibration specimen were scanned ul trasonical ly

from the right and the left with respect to long slot dimensions. It was

possible to scan the slots A,B,C,I, and F from only one side due to an

inadequate separation from the specimen ends. Figure 8-2 shows the scan

pattern for moving the EMAT-pair ten to fifteen 2-mm steps along the direction

normal to the long slot dimensions, and 10-mm steps along the orthogonal

direction. The ultrasonic data from each scan-cycle was stored on magnetic

media, but not processed. (At each scan location the digital oscilloscope

calculated 16 to 32 time-averages before transmitting the data.)

Figure 8-3 shows the detailed scan-pattern used to interrogate slots

located on the same surface as the EMAT scanning head. Also shown are the

reference lines used in the synthetic aperture reconstruction process as well

as the starting points and initial directions of individual scans. (Digital

processing details used are given in Chapter 6.) It should be noted that in

all cases the reference lines are 40 mm away from the slots, a distance

corresponding to the recovery time of the EMAT receiver-amplifier following

the transmitter-amplifier pulse. With the plate turned over and the EMATs on

the side opposite the slots, the scan pattern and procedures were identical.

Evaluation of Surface Slot Scattering Measurements

Data reduction included the synthetic aperture process and filtering to

remove unwanted artifacts caused by electromagnetic interferences (Chapter 6).

After this we ploted the peak-to-peak signal voltages and integrated power

(computed in the frequency-domain by summing the squares of the Fourier

coefficients in the carrier-frequency neighborhood) from the reconstructed

signals as a function of position along the scan lines. The results in Figs.

8-4 and 8-5 correspond to the slot-up reference lines in Fig. 8-3. The

79
vertical dotted lines indicate the flaw extremes. It is interesting first to

examine the data on a figure-by-figure basis. This is helpful in identifying

certain common features and systematic trends in the data that will help later

to develop a chart defining the performance limits of the 450-kHz SH-wave

inspection system with respect to detection and dimensioning of surface flaws.

Of course, similar procedures are equally applicable to defining the system's

performance limits with respect to other types of flaws. While examining the

data in the figures, it is helpful to refer to Table 8-1, which summarizes the

dimensions of the calibration slots. The results for the case of the EMATs

and calibration slots located on opposite sides of the plate are very similar

and not shown.

Before evaluating the individual data sets in detail, it is important to

note the common ultrasonic noise floor in all of the figures. This is

probably due to ultrasonic reverberations in the calibration plate and not

electronic noise. The fact that the mean level of the noise floor is the same

in all the figures is important since it reinforces our claim of high

measurement reproducibility for the SH-wave inspection system. The noise

floor in Figs. 8-4 and 8-5 determines the minimum detectable flaw sizes. A

caution here, however, is that these measurements are on a plate with no

surface irregularities. With a weld reinforcement, weld root, etc, present,

the noise floor may be higher. The scattering data obtained along reference

line 1 (Fig. 8-3) are in the top half of Figs. 8-4a and 8-5a. The three flaws

along this line (A, B, and C) are of approximately equal surface length

(25 mm), but have depths ranging from 0.48 to 1.68 mm. The voltages and

powers of the reflected signals exhibit a monotonic increase with flaw depth.

This is an obviously desirable effect, and is a basis for flaw dimensioning in

80
o

(0

0)c 1

oc o n f O I oo 5 o

CO o

u. o n O o o 5

(0 o o vU- o o

U. o o 5 o n o

J.1 o t oo E s/

—i

CO

QC , EEo

<M o o o \ o o o oo \ oo o o t5

CO u.

A SOO

aovnoA nvNOis >iv3d oi >iv3d

QC

u.i o o o o t u

GO oo 5 o

CO u.1 o oo 1 o < o s o

CO o

u. ooooo

Ya

0)c

-" CM

DC li

CM

A 200 EEzo

\- O) '"D co E 1 O I/) Q. LO 1— c 5 < o ro 2

-M

LU o.

CO r— JD

OC l/l^ LU>

^-o • CO UJ z C 1 < fO <X. cc o

1- C/l CO

/-» 5

J3 E ro ^^ O 1—

S- Li_ 4-

*.- ^ OJ fO

fO

+-> r— • O i/l > 1—< .— 2:

fO LU c

en t/1 •I- fO

(/) O)

J:^ +-> fO 03

OJ r- E Q. Q. E H- z

•t-J

O ^ O) 1- fO "O

QJ -r- co Q_ CO o

Q.

1- ^d- <2 OO

LU

O) LU s- CO 13 OC ai

LU

•1

> u_

COz<

OC

1- ^CO

aovinoA nvNOis >iv3d Oi >iv3d

81
o

(0

% o

-^

CO cc

li.

\ o o

f o

o

o

o

o

o

k O o 5

CO o

v"- o o

o

LI. o 5 o

CO o

u. o

i o

o

1 o

y

C

DC

5

Ll.

EII

CM

simn AjBJiiqjB 'a3M0d IVNOIS

^

0) .c

-I CM •5*

ir

1 o

o

o o

t o

CO oo S o

iS

u.i o o

o

o

t o

o < o g o

iS o

u.1 o

o

o

t o

E m

Eo 5 CM

to u.

i —

o

o

o

A o

oo

o

o

o

1 o

o

o

t

o O

g

CO u.

g

(0O

Q.

I-<

LUHICO

CC>

COz

<

cc

COo

Q.

LU

UJCO

CC

LU>

COz

<

(U T3

• r—

cn • -o

OJ 1 Euro

i/) {/) S- C 03 O r—

U_

l/l -(->' O .Q

1— —

lyl

<+- • O LU c <fOU Ul

00 2

03 E r— O U_

sl<_.

fO

(— ^—-* 03c

CJ) • •I- lyi 00 1—"<

s- s:

O) LtJ s

O l/l D_ (O

lD

03

O)

s_

a

s;!un /^jBJuqjB 'a3M0d IVNOIS

82
the long wavelength region. The presence of flaws B and C is readily

discernable, while neither received-signal voltage nor power positively detect

flaw A.

The scattering results corresponding to reference line 2 are in the lower

half of Figs. 8-4a and 8-5a. There can be no doubt about detecting both slots

D and E lying along this reference line. The two flaws are approximately

25 mm long, and 1.76 and 2.14 mm deep, respectively. It is very interesting

to observe that the scattering signature of flaw C is nearly identical to that

of flaw D. This is not coincidental, since the actual dimensions of the two

flaws are nearly identical. Again, this result shows the repeatable nature of

the SH-wave EMAT inspection system.

Figures 8-4b and 8-5b depict the ultrasonic scattering data collected

along the reference scan lines 3 and 4. Flaws F, G, and H are noteworthy

because they are substantially shorter or longer than the 25 mm of the other

flaws. Their surface lengths are 7.2, 52.3, and 12.8 mm, respectively.

Examining the two figures shows that all three flaws can be positively

detected. We believe that the detection of flaw F and not of flaw A is a

consequence of the former being substantially deeper than the latter, 2.29 mm

vs. 0.48 mm. As with Figs. 8-4a and 8-5a, the flaw signals correspond

faithfully to the actual locations. Furthermore, the surface lengths of

longer flaws G, I, and J can be resolved. On the other hand, the

ultrasonically determined widths of flaws F and H are overestimates. Fortunko

and Schramm (1983) discuss this effect as related to the curvature of the

phase-front illuminating the planar flaw.

The major feature of flaws I and J is that their through-thickness depths

are comparable to a half and full wavelength, respectively. Their reflection

signals are nearly identical and indicate system saturation.

83
To determine the practical flaw detectabil i ty limitations and extent of

regions where long wavelength flaw sizing may be applicable, we reorganized

the experimental results by plotting the reflected signal voltage as a

function of flaw length and through-thickness depth. The results are in Figs.

8-6 and 8-7. These include data from both slot-up and slot-down

configurations

.

In Fig. 8-6 a and b, using peak-to-peak signal voltage and signal power

as detection parameters, we plotted the flaw response as a function of flaw

through-thickness depth for all flaws with approximately 25 mm surface length.

For the top half of Fig. 8-6 the results are for flaws on the same surface as

the transducer; for the bottom half, the flaws were on the opposite side of

the calibration plate. It is evident that both parameters increase

monotonical ly in the region 0.5 mm < flaw-depth <_ 2.5 mm. The latter

dimension corresponds to one third of the ultrasonic wavelength. Above this

value the signal amplitude and power exhibit no further increase and remain

essentially constant. This is in excellent agreement with theory (Datta,

Shah, and Fortunko, 1982).

Figure 8-7 shows the results for flaws approximately 2-mm deep, but whose

surface length varies (D, E, F, G, and H). Because of the finite beam width

in the plane of the surface, it is not possible to decouple completely the

effect of flaw length and depth on the signal voltage. The data collected in

Fig. 8-7 demonstrates this effect. The signal voltage increases monotonical ly

in the region < flaw surface-length < /2xz, where z denotes the initial

transmitter-EMAT-to-flaw separation distance, and then saturates. This effect

is also in yery close agreement with simple theory. (Fortunko and Schramm,

1983). The calculated value of the quantity /2xz is 24 mm, in very good

quantitative agreement with the experimental results of Figs. 8-4 and 8-5.

84
LUa

u. 9

o

-Ul

1 1 —r'^ 1 1 1 1 o E _ o

CM

II

c

CD -1 - \ S _

o\ <0 u.

Ndo

1 1

\

CO o

1 1 —L^^ 1 1 1

1

Q.

LUa

<

o o o o o o o o o o o

o 00 <o t CM o 00 <o * CM

siiun AjBJiiqjB 'a3M0<J 1VN0IS

E

E

l"

1-

+-> Q. rO S- LU +-> O

Q I/) C Q-

<

O 13 O

CO _l ro 5

U. (O • S- n- S- O M- QJ 4- 5

s~ o

x: o Q.

-I-J MCL ' -o E-—

fO S on m

1— O) CU M- ^ CD

+-> fO H- +-^ O C ^ •r- O

C ftl > o E

•1- oj -:^

+-> S- fO O CU a u^ CI. :3 -t-) 1 M- r- o 3 -M

fO 00 1 O) -i^ LO i_ fO (O O)

QJ DI— -C

(TD 1— '^ c n3 en —

E (/) -C E +-> • 5 cr> c

I

1-

D.LU

rt3 d 5

I— QJ O

Ll_ r— T3 a • • vo 1

00

_i QJ U. sCO

HOVnOA IVNOIS XV3dOi->tV3cJ

85
1-o

z

LU O

_l r- ^

<

_l

LL ino

OL

-rn- 1 \ r

E

E

00

II

Q.

0) a

CO

^

o o o O o O O O o o o o 00 CD * CM O 00 CD '^ CM

s»!un AjBJiiqjB 'a3M0d 1VN0IS

X

I-o

z

LU

_l<

E

E o

<t X

1- -M

c: Z -M

111 c/1 S- _l c o

o

o ^

O QZ3 < fO -1 h

for flaws

ower. ^s -i-> s- a.

Si en o

%^ e M- '—

.

o QJ JD

1— (U —E

5 ro

ro CO .— O) M- d) CJl x: fo 4- +-> +J o .— E O

£!•.->

O CD

•.- E ^ +-> CD fO U S- CU E Q- 3 CO 1 <+- -t-J O

r— +-> o (t3 13 1 CD

nal

as

The

res

a)

Peak

E en —

E </5 • O -C • •^ X

1- 5 +-> c

ro Q- 5

r- CU O

o Li_ -O -O z

UJ

_l r^ o ^ CO

Cvi <

_J

CO

en

30VnOA IVNOIS XV3dOi>lV3d

86
Figure 8-7 shows the possibility of resolving the surface length of a

planar flaw when its length perpendicular to the axis of the ultrasonic beam

exceeds /2xz, approximately 24 mm in the present case. Below this value it is

no longer possible to estimate the flaw surface length and through-thickness

depth separately. Furthermore, Fig. 8-6 shows that the voltage of the

reflected signal exhibits a monotonic increase only for a flaw depth less than

a third of a wavelength. Hence, it is not possible to estimate flaw depths

that are larger than that dimension. Fortunately, the above limitations are

not inconsistent with typical flaw sizes that must be reliably detected in

conventional inspections of butt welds, which are most often based on the use

of comparative reference standards (calibration blocks) (Doyle and Scala,

1978).

In a f i tness-for-service approach, it is essential not only to detect

flaws with high reliability, but to size them within some region of concern.

Figure 8-8 delineates the four regions of flaw-length, flaw-depth combiations:

1.) no detection, 2.) detection only, 3.) detection with depth sizing, and 4.)

detection with limited sizing. In the first region, it is not possible to

detect the flaw at all. In the second region, the flaws can be reliably

detected, but their through-thickness depth is only known to exceed x/3. In

the third region, the surface length can be estimated from the transverse scan

while the depth can be estimated from amplitude (voltage or power)

information. In the fourth region, it is still possible to detect the flaw

reliably, but the flaw surface-length and depth are not separable. In the

last case, it may be necessary to employ other inspection techniques (i.e.

conventional ultrasonics or radiography) to estimate the flaw length.

The measurements (Figs. 8-4 through 8-7) on the calibration notches, A

through J, of Table 8-1 determined the approximate boundaries in Fig. 8-8.

87
E

E

X 4

IQ.

LUO

^ 3 <

< 2

I I

O

3

X

•l (flaw detection only)

Limited Sizing

•h^''I" •r

(flaw detection and sizing)

»B /Detection Threshold

(flaw not detected)

i 100 200

FLAW LENGTH, mm

Figure 8-8: Depth and length measurement capabilities and limitations

of the present EMAT system. The letters A through J identify the points indicating the dimensions of the

calibration notches listed in Table 8-1.

The shallow slot A was essentially undetected (Figs. 8-4 and 8-5) and set the

detection threshold. The notches with a 1.8-mm approximate depth, along with

the theoretical value, (Fig. 8-7) defined the vertical boundary at about 25

mm. The very short but deep flaw F established a limit to the curve defining

the undetected region and indicated its general shape. The flaws with a 25-mm

length but various depths, again along with the theoretical value (Fig. 8-6)

indicated the upper bound on the ability to correctly size any detectable

flaw.

88
References

Datta, S. K. , Shah, A. H., and Fortunko, C. M., 1982, Diffraction of medium

and long wavelenath horizontally polarized shear waves by edge cracks, J.

Appl. Phys ., 53:2895.

Doyle, P. A., and Scala, C. M., 1978, Crack depth measurement by ultrasonics

a review, Ul trasonics , 16:164.

Fortunko, CM., and Schramm, R.E., 1983, Evaluation of pipeline girth welds

using low-frequency horizontally polarized waves, J.NDE , 3:155.

89

Chapter 9

SUMMARY AND RECOMMENDATIONS

In this report we have shown the feasibility of constructing a new type

of ultrasonic inspection system for thick, butt welds used in ship

construction. (Thick welds are defined for the purposes of this report as

those having 12.5-nim and greater thickness.) The system makes use of special

ultrasonic probing signals, shear waves that are polarized horizontally with

respect to the plate surface (SH-waves). Using these special probing signals

is beneficial because they generate fewer unwanted ultrasonic signals. Also,

SH-waves can illuminate welds at optimum detection angles because they are

able to propagate with equal efficiency at any angle with respect to the free

surface. It is believed that, for many types of sharp flaws, ultrasonic

illumination at grazing angles with respect to the free surface is

advantageous. However, conventional ultrasonic probing signals, SV-waves and

L-waves, are not at all suitable for this role.

The experimental results described in this report demonstrate that the

new ultrasonic inspection system may be better suited for the role of

inspecting thick welds than conventional ultrasonic systems using piezoelectric transducers and operating at 1 to 10 MHz. The new system's advantages

are special non-contacting transducers and operation at lower ultrasonic

frequencies. The special ultrasonic transducers function on the basis of

electromagnetic-acoustic (EMA) coupling, and can preferentially generate and

detect the SH-wave probing signals. In addition, because they need not

maintain intimate coupling with the surface, the EMA transducers (EMATs) can

operate on relatively rough surfaces that may be at elevated temperatures.

This characteristic is of considerable importance in weld inspection.

91
The SH-wave EMAT system is different from conventional ultrasonic systems

in two other respects: 1. it operates at relatively long ultrasonic

wavelengths (7.4 mm), and 2. it uses synthetic-aperture waveform

reconstruction to reduce the contamination of the ultrasonic flaw indications

by unwanted ultrasonic and electromagnetic interferences. The former feature

makes the probability of flaw detection less critically dependent on the

flaw-signal illumination angle and allows use of the reflected signal

amplitude for limited flaw sizing and characterization. The second feature

has only recently become economically feasible due to advances in

digital-computer hardware.

Digital techniques were used extensively to improve the quality of the

ultrasonic signals. They partially compensate for the loss of ultrasonic beam

collimation occurring when wavelengths are large with respect to transducer

size. In addition, extensive digital signal processing allows the use of very

effective algorithms for signal averaging in the time domain and filtering in

the frequency domain. This capability is very important in welding

environments dominated by electromagnetic interferences.

The present SH-wave EMAT system has somewhat limited transverse and

longitudinal resolution, as determined primarily by the wavelength and EMAT

linear dimensions. The wavelength is physically limited by the size of the

samarium-cobalt permanent magnets used in the construction of the transducers.

It is believed that the present operating frequency (454 kHz) represents the

state-of-the-art for systems using the periodic-permanent-magnet EMA

transducer. Therefore, if better resolution is necessary, alternative methods

for generating and detecting SH-waves will be necessary.

The main accomplishment of the present work was the demonstration of the

SH-wave EMAT technology in the context of inspecting thick weldments.

92
Previously, ultrasonic systems using SH-waves were demonstrated in thin-plate

geometries where the propagation of the signals was more appropriately

described in terms of guided wave formalisms. In the present case, freely

diffracting ultrasonic signals were used.

This program was viewed as a technology demonstration. Therefore, a

relatively flexible "brassboard" was developed to serve as a test vehicle to

evaluate the system's inherent sensitivity limits with respect to surface

flaws modeled by thin electrical -discharge-machined notches. The "brassboard"

was designed primarily to investigate performance limits in estimating flaw

size. Although the construction was robust, the design needs critical review

before selecting a configuration for a field-ready prototype. Certain

subsystems should be substantially redesigned. For example, the present

"brassboard" system carries out digital signal processing using decidedly

less-than-optimal hardware and software. Using array-processor technology and

more efficient software driver routines can readily correct this deficiency.

In addition, it is recommended that the transmitter-amplifiers, which

currently use bipolar transistor technology, be redesigned to incorporate

elements of the new high-power FET (field-effect transistor) technology. This

change would result in stronger ultrasonic signals and higher reliability.

The "brassboard" system established a number of important performance

parameters. In particular, it was found that thin surface notches > 0.5-mm

deep and 25-mm long could be positively detected at 454 kHz in 25-mm thick

plates made of standard HY-80 steel. This result is similar to that observed

experimentally in thinner plates (16 mm) (Fortunko and Schramm, 1983).

However, it is believed that this similarity is probably coincidental because,

in the former case, the sensitivity was limited by ul trasonic-mul tipath and

transducer-sidelobe effects. In the latter case, it was limited by an

93
ultrasonic background generated by the weld reinforcement, vjeld root, and

other surface irregularities.

This investigation did not include sensitivity limits with respect to

sharp and blunt interior flaws because suitable calibration specimens were

lacking. However, such limits can be extrapolated from the surface flaw

results on the basis of theory (Datta, Shah, and Fortunko, 1982). Closure

effects on the detectabil ity of sharp flaws were not investigated. Also,

because of the inherent limitations in the "brassboard" system, performance

parameters with respect to transverse weld flaws were not established.

Among the major accomplishments of the present project is the development

of a new SH-wave EMAT, The new EMAT design reduces susceptibility to electromagnetic interferences, an important factor in a welding environment. In

addition, a thin metallic shield was found to protect the fragile parts of the

EMAT from abrasion damage without significantly reducing transduction efficiencies. It was never found necessary to replace the EMATs during the

project despite extensive use. It is believed that the new EMAT design is

particularly suitable for ferromagnetic weldments, which are frequently

covered with layers of rust or paint. Of course, further testing of the new

EMAT design under realistic conditions is desirable.

In addition to physical reliability, the new EMATs have performed well in

conjunction with synthetic-aperture signal processing. This requires good

measurement repeatability in terms of signal phase and amplitude. In a

related, but separately funded development, the new EMAT design was used

successfully to determine residual stress distributions in plates locally

subjected to non-linear deformation exceeding the yield point (King and

Fortunko, 1983). In this case, sufficient phase repeatability was demonstrated to determine values of residual stress with approximately 10 MPa

94
resolution. This level of resolution would not be routinely achievable using

conventional, fluid or grease-coupled ultrasonic transducers.

It should be emphasized that much remains to be accomplished before

SH-wave EMAT inspection systems can be adopted universally as either primary

or back-up inspection tools for thick butt weldments. We recommend that the

laboratory system developed under the present program be used as a test-bed

system for further evaluations of the method's capabilities and weaknesses.

In particular, before making a commitment to a field prototype, it will be

necessary to evaluate system performance using flaw types representative of

particular welding processes. Furthermore, performance limits should be

independently confirmed by appropriate destructive testing procedures. The

results of such a study should be compared with inspection records of the same

weldment obtained with existing inspection techniques: conventional

ultrasonics and radiography.

We believe that the low-frequency, SH-wave EMAT method is more

advantageous for detecting elongated sharp and blunt flaws than radiography.

However, because it uses long ultrasonic wavelengths, it may be less sensitive

to certain categories of blunt flaws (porosity), than conventional ultrasonics

and radiography. Consequently, it is recommended that a study be conducted to

quantify the sensitivity limits of the low-frequency, SH-wave EMAT method to

such flaws. In such work, the emphasis should be focused on an experimental

plan that will make it possible to draw conclusions on the basis of

statistics. In the present work, because of the limited number of calibration

specimens and flaws, statistical methods could not be used.

Finally, it is recommended that a separate study be conducted on the

performance characteristics of the present SH-wave EMATs when operated on

ferromagnetic weldments exhibiting large internal residual and applied

95
stresses. Influences of the material composition on the

electromagnetic-acoustic transduction processes also need study. Past studies

have shown that the presence of stress and compositional variations can

significantly alter the characteristics of the transmitted and received

ultrasonic signals (Thompson, 1978).

References

Datta, S. K. , Shah, A. H., and Fortunko, C. M., 1982, Diffraction of medium

and long wavelength horizontally polarized shear waves by edge cracks, J.

Appl. Phys. 53:2895.

Fortunko, C. M. and Schramm. R. E., 1983, Evaluation of pipeline girth welds

using low-frequency horizontally polarized waves, J. NDE , 3:155.

King, R. B. and Fortunko, C. M., 1983, Surface residual stress evaluation

using horizontally polarized shear waves, to be published in J. Appl . Phys.

Thompson, R. B., 1978, A model for the electromagnetic generation and

detection of guided waves in ferromagnetic metal crystals, IEEE Trans.

Sonics-Ul trasonics, 25:7.

96
Appendix I

SOFTWARE LISTING

These computer programs are written in BASIC and executed on a system

consisting of a CPU with 256 kbytes of memory, a dual floppy disk drive, video

terminal, graphics printer, and an IEEE-488 interface bus. This bus permitted

communication with a digital storage oscilloscope and carried transport

information of a motion control interface card used with motion control

indexer cards and synchronous/stepper motors and two lead screw positioners

(almost 1.58 threads/mm or 40 threads/inch). With appropriate software

changes, other types of equipment could be substituted for those used here.

This software is divided into five parts only because of limitations on the

size of the memory workspace.

SYN - Part 1

Always start here to enter the intial decision and parameters and to

establish the physical position of the EMAT assembly.

100 REM ** SYN *»

110 REM ««»•**»#***»»*###*»»****»***» »»»** **»**********

120 REM ** SYNTHETIC APERTURE BY PHASE SHIFT - PART 1 *»

130 REM »*«*»»»*»*»»»»»»»*«»»*#»«*»#»**#*»*»#**#»**#******

140 REM ** INITIALIZATION *#

150 REM ***»»»«#«»»#*»»»****»

160 COMMON P(20) , I$,P*,A1 ,A2 \ DIM R < 5 1 1 ) , I ( 5 1 1 ) , C* ( 5

)

170 DIM #1,F(20) \ DIM #2,F1(511) \ DIM #3,F*(5)=255

180 DIM #5,R2(511) \ DIM #6,12(511)

190 DISPLAY-CLEAR

200 PRINT 'NEW or OLD DATA? (N or 0) '', 210 GET_CHAR(U$)

220 IF V*= '0' THEN 730

230 IF '.i$<>'N' THEN 210

240 PRINT \ PRINT 'Are you doina to USE THE SCANNER SYSTEM? (Y or N) '; 250 GET_CHAR(S1$) \ IF S1*='N' THEN 300

260 IF S1$<>'Y' THEN 250

300 REM **##*»*»»#*****#**

310 REM *» PARAMETERS ** 320 REM •»#«»**#»**##***##*

330 PRINT \ PRINT \ PRINT 'IDENTIFICATION (2 CHAR MAX) = ", \ INPUT 1$

340 IF LEN(I*)>2 THEN 330

350 PRINT 'date: '; \ INPUT C$(0)

360 PRINT 'TRANSDUCERS: '; \ INPUT C*(l) \ C* ( 1 )=' TRANSDUCERS : '+C$(1)

370 PRINT 'PRF = ' \ INPUT C$(2) \ C*(2)='PRF = '+C$(2)

380 PRINT 'NUMBER OF CYCLES = ", \ INPUT C$(3) \ C$ ( 3 ) =C* ( 3 ) +

' CYCLES'

390 PRINT 'notes: 2 LINES AVAILABLE'

97
400 PRINT 'LINE 11' \ LINPUT C$(4)

410 PRINT 'LINE 2.'.' \ LINPUT C$(5)

420 PRINT \ PRINT 'NUMBER OF TRANSDUCER POSITIONS = 'J \ INPUT P(0)

440 IF S1$='N' THEN P(19)=-l \ GC TG 460

450 PRINT 'NUMBER OF SCAN SETS = '; \ INPUT P(19)

460 PRINT \ PRINT 'DISTANCES IN MM:'

470 PRINT ' BETWEEN POSITIONS = '; \ INPUT P(l)

480 IF P(19)<2 THEN 510

490 PRINT ' BETWEEN SCAN SETS = '! \ INPUT P(20)

510 PRINT ' BETWEEN TRANSMITTER & RECEIVER = '! \ INPUT P(3)

520 PRINT ' PLATE THICKNESS = '! \ INPUT P(4)

530 PRINT ' FOCAL SPOT LOCATION: ' 532 PRINT ' HORIZONTAL (from initial transmitter position) = '', 534 INPUT P(2)

536 PRINT ' 1,'ERTICAL DEPTH (from transducer side) = 'J \ INPUT P(5)

540 PRINT \ PRINT '# OF REFLECTIONS IN ACOUSTIC PATH = '; \ INPUT P(6)

560 PRINT \ PRINT 'FORWARD OR BACKWARD SHIFT? (F OR E) ", 570 GET_CHAR(D$) \ IF D$='E' THEN 620

580 IF D$<>'F' THEN 570

610 P(7)=l \ PRINT 'ORWARD' \ GO TO 630

620 P(7)=-l \ PRINT 'ACKWARD'

630 PRINT \ PRINT 'FREQUENCY (KHZ) = '; \ INPUT P(8)

640 PRINT \ PRINT 'SOUND VELOCITY (KM/SEC) = '; \ INPUT P(9)

650 PRINT \ PRINT 'NUMBER OF AVERAGES DONE ON SCOPE = ", \ INPUT P(10)

660 Z*='SYi:'+I$ \ OPEN Z$ FOR OUTPUT AS FILE #1

670 ELDCK_MOVE(P(0) ,F(0) ,21 ) \ CLOSE

680 Z$=Z$+'N' \ OPEN Z$ FOR OUTPUT AS FILE #3

690 FOR K=0 TO 5 \ F$(K)=C*(K) \ NEXT K \ CLOSE

700 GOSUB 3000 \ PRINT \ IF V$='0' THEN 720

710 IF P(19)<0 THEN 715

711 PRINT 'Do you want to INITIALIZE TRANSDUCER POSITION? (Y or N)

713 GET_CHAR(M$) \ IF M$='Y' THEN 5000

714 IF M$<>'N' THEN 713

715 CHAIN 'SY0:TEKIN'

720 PRINT 'Do you want to REPROCESS THE DATA'' (Y or N) '; 722 GET_CHAR(E$) \ IF E$='Y' THEN PRINT \ CHAIN 'SY0:CALC1'

723 IF E$<>'N' THEN 722

725 PRINT \ IF P(19)<2 THEN J=l \ GO TO 727

726 PRINT 'Which SCAN SET of ';P(19); \ INPUT J 727 I$=I$+STR$( J) \ CHAIN 'SY0:GRAPH'

730 REM »*•»»«**» s- •»**»««•* *»»*«

740 REM ** HANDLE OLD DATA ** 750 REM -ft**********************

760 PRINT \ PRINT 'OLD DATA PARAMETERS:'

770 PRINT 'IDENTIFICATION (2 CHAR MAX) = T \ INPUT 1$

780 IF LEN(I$)>3 THEN 770

790 W$='SYi:'+I$ \ OPEN W$ FOR INPUT AS FILE #1

aOO BLOCK_MOVE(F(0) tP(0) ,21 ) \ CLOSE

810 W$= 'SYl :

' + I$+'N' \ OPEN W$ FOR INPUT AS FILE #3

820 FOR K=0 TO 5 \ C$(K)=F$(K) \ NEXT K \ CLOSE

830 GOSUB 2000

890 PRINT \ PRINT 'Do you want to CHANGE THESE PARAMETERS? (Y or N)

900 GET_CHAR(W$) \ IF W$='N' THEN 700

910 IF W$= 'Y ' THEN 460

930 GO TO 900

2000 REM -fr*********************

2010 REM ** PARAMETER LIST »* 2020 REM •»**»»•«  »** «**»•»•» **« 2030 PRINT \ PRINT 'ID: ';i$ \ PRINT

2040 FOR K=0 TO 5 \ PRINT C$(K) \ NEXT K \ PRINT

2050 IF P(19)<0 THEN 2080

2060 PRINT P(19);' SCAN SETS'

2065 IF P(19)=l THEN 2080

2070 PRINT P(20);' MM BETWEEN SETS'

2080 PRINT P(0);' TRANSDUCER POSITIONS'

2090 PRINT P(l);' MM BETWEEN POSITIONS'

2110 PRINT P(3);' MM BETWEEN TRANSMITTER & RECEIVER'

2120 PRINT P(4);' MM PLATE THICKNESS'

2130 PRINT 'FOCAL SPOT LOCATION: '

98
2132

2134

2140

2150

2160

2170

2180

2190

2200

3000

3010

3020

3030

3040

3050

3080

3090

5000

5010

5020

5030

5040

5060

5070

5090

5120

5130

5140

5150

5160

5170

5190

5210

5220

5230

5240

5260

5270

5280

5290

5300

5320

5330

PRINT ' "rPiZ)',' MM HORIZONTAL (frotn initial transmitter position)

PRINT ' ';P(5);' MM VERTICAL DEPTH (from transducer side)'

PRINT P(6);' REFLECTIONS IN ACOUSTIC PATH'

IF P(7)=l THEN PRINT 'FORWARD SHIFT'

IF P<7)=-1 THEN PRINT 'BACKWARD SHIFT'

PRINT P(a);' KHZ FREQUENCY'

PRINT P(9);' KM/SEC SOUND VELOCITY'

PRINT P(IO);' AVERAGES DONE ON SCOPE'

RETURN

REM ***«*»»**»»*»****»

REM »» HARD COPY LIST »» REM *#**»»# »#»«»** ***

PRINT \ PRINT 'Do you want a HARD COPY OF THE PARAMETER LIST? (Y or N)

'N' THEN 3090

\ GOSUB 2000 \ PRINT CHR* ( 27 ) ,

' C4i

*#

MOTOR CURRENT OFF **

GET_CHAR(W*) \ IF W*= IF WSO'Y' THEN 3040

PRINT CHR$(27) , '[5i

PRINT \ RETURN

REM **#•» »»** #*»*»**»»**»****»*•»*

REM ** INITIALIZE TRANSDUCER POSITION

REM »*»»»»»****»*#*»»*»*»***#*#«*#*»*

PRINT \ PRINT 'When system is READY push "R" GET_CHAR(H$) \ IF H*<>'R' THEN 5040

REM *» INITIALIZE MOTOR CONTROL

T$=CHR$( 13)+CHR$( 10)

SEND( 'XEYE'+T«,11 ) \ REM

REM *» INPUT PARAMETERS

PRINT \ PRINT 'X or Y coordinate:

GET_CHAR(C$) \ IF C*='X' THEN 5160

IF C*<>'Y' THEN 5140

PRINT \ PRINT 'DIRECTION? (+ OR -)

GET_CHAR(D$) \ IF D$='+' THEN 5210

IF D$<>'-' THEN 5170

PRINT \ PRINT 'MM OF TRAVEL = '; \

P=INT(D«16000/25.4+.5)

PRINT \ PRINT 'Push "E" to EXECUTE

GET_CHAR(E*) N IF E$<>'E' THEN 5240

R*='*'+T* \ REM *# EXECUTE COMMAND

c*= 'c '+c$+'d; '+c*+'M'+d$+str«(P)+'g;

SEND<C$,11) \ SEND(R$,11)

PRINT \ PRINT 'Do you want to MOVE FURTHER? <Y or N> GET_CHAR(G*) \ IF G$='Y' THEN 5130

IF G*<>'N' THEN 5300

PRINT \ CHAIN 'SYO.'TEKIN'

INPUT D

'+C*+'E] '+T$

TEKIN - Part 2

This reads the digital data from the oscilloscope, stores it, and moves

the EMATs in the designated pattern.

100 REM »* TEKIN **

110 REM »»*»#»»»*»#*»»***»»»**«»********»**»*************

120 REM *» SYNTHETIC APERTURE BY PHASE SHIFT - PART 2 **

130 REM *#»#»»***»«»*»«*#**»***»**»»*»******************

140 COMMON P<20) , I*-P$.A1.A2 \ DIM R(511)-I(511)

150 DIM #1,F(20) \ DIM #2,F1(511)

160 DIM #5,R2(511) \ DIM #6,12(511)

170 DISPLAY-CLEAR

171 PRINT 'Press "E" to he^in EXECUTION 'r 172 GET_CHAR(V$) \ IF V$<>'E' THEN 172

173 PRINT

ISO IF P(19)>=0 THEN GOSUB 5000

190 REM *»»**»*»»*»«***»«*»***

200 REM *» START COLLECTION *» 210 REM *»*«**»*»*#•»»****»******

220 S=P(19)-1 \ IF SCO THEN S=0

230 FOR J1=0 TO S \ PRINT \ PRINT

99
240 PRINT 'Scan 'JJl+lP' of ';S+i;' at ';J1»P(20);' mm'

250 FOR J=0 TO P(0)-1

260 PRINT 'Collecting position 'IJ+IJ' of ';P(0);' at ';j»P(l);' mm' 265 IF J>0 THEN 890

270 FOR K=l TO 5 280 RECEK'E(y$, 1 ,6) \ IF •,<$<>',' THEN 280

300 NEXT K 310 FOR K = l TO 6 \ RECE U'E ( 0$ , 1 , 6 ) \ NEXT K 340 REM *»»*»»*»**#**#*»*#*»*#»#

330 REM »« GET TIME INCREMENT ** 360 REM »*»*#«»»»»#*»»*»»»#***»#*

370 GOSUB 4000 \ P (1 1 ) =i.'AL ( 0$ ) / ( 1 0''3 ) 390 FOR K = l TO 6 \ RECE lUE ( i,'$ , 1 , 6 ) \ NEXT K 400 REM **#»**«**»**»*»*

410 REM »» GET X ZERO «» 420 REM *»»»«»»#*»*#*»•»*

430 GOSUB 4000 \ P(12)=0AL(0$)

450 FOR K = l TO 7 \ RECE I ME ( V$ , 1 , 6 ) \ NEXT K 460 REM **»»#**»*»***»»#*»»»»•»•»»*

470 REM ** GET X POINT OFFSET »» 480 REM »•»*##**»»**»*»*#»»»*»»»»#

490 GOSUB 4000 \ P ( 1 3 ) =i.'AL ( U$ ) 510 FOR K=l TO 6 \ RECE IME ( M* , 1 , 6 ) \ NEXT K 520 REM »#«»»»»*•»**»***»*«*»»•»»•*»»»»*»»•»»•»»*•»#»*»

530 REM * DETERMINE DECIMAL FOR TIME INCREMENT *» 540 REM **«»*»•»»»*»*»««***#«#»»*»»»«»*»*»**»*»«»

550 RECEIOE(X$, 1,6)

560 IF X$='S' THEN P(14)=3

570 IF X$='M' THEN P(14)=0

580 IF X$='U' THEN P(14)=-3

590 IF X$='N' THEN P(14)=-6

600 RECEI','E(0*, 1 ,6) \ IF i.'*<>':' THEN 600

610 REM »-»*»»**»«**»-H-»*«»*»««**»

620 REM ** GET Y MULTIPLIER »* 630 REM -tt*******-****************

640 GOSUB 4000 \ P ( 1 5 ) =OAL ( 1.'$ ) / ( 1 0'^3 ) 660 FOR K=l TO 6 \ RECE I VE ( 0* , 1 , 6 ) \ NEXT K 670 REM •»»»«*»«*»•»*»«•»»•»* 680 REM ** GET Y ZERO ** 690 REM ******#»***»»•»*»#

700 GOSUB 4000

710 P( 16)=i.'AL(0$)

720 RECEIOE(0$, 1 ,6) \ IF !.'$<>':' THEN 720

730 REM *#**»*»«•***#»»#*#»»*

740 REM »« GET Y OFFSET »» 750 REM *-«-*»#*»**»*»*#»»*»»*

760 GOSUB 4000 \ P (17 ) =OAL ( 0$ ) / ( 1 0''3 ) 780 RECEIVE ( 0$, 1 ,6) \ IF USO ' :

' THEN 780

790 REM #•«*»*((•»»#*#»*#»**

SOO REM *# GET Y UNIT ** 810 REM *»*«»**»#»#»*#»»»*

820 RECEIVE (OS, 1,6)

830 IF t.)$='V' THEN P(18)=l

840 IF 0$='M' THEN P(18)=2

850 IF i.'$='U' THEN P(18)=3

870 0$='SYi:'+I$ \ OPEN U* FOR OUTPUT AS FILE #1

880 BLOCK_MOOE(P(0) ,F(0) ,21 ) \ CLOSE

890 RECEIOE(0$, 1,6)

900 IF ASC(0$)<>1 THEN 890

910 REM **«*»**»*»»»*»#»»»•«»»•(!•»

920 REM ** READ SCOPE DATA »» 930 REM ***»»»*•)!«•»»*»»*»» *»**»• 940 FOR K=0 TO 511 \ RECE I OE ( 0$ , 1 , 6

)

950 R(K )=ASC ('.'$) \ NEXT K 960 RECEIOE(0$, ,6)

965 IF J<P(0)-1 THEN GOSUB 6000

970 G$= 'SYl : '+I$+STR*( Jl+1 )+STR$( J+1) \ OPEN G$ FOR OUTPUT AS FILE #2

980 ELOCK_MOi.'E(R(0) ,F1 (0) ,512) \ CLOSE

1000 NEXT J 1010 IF JKS THEN GOSUB 7000

100
1020 NEXT Jl 1030 CHAIN 'SY0:CALC1'

4000 REM **»*»*«»#*#*»*»»**

4010 REM *» READ NUMBER ** 4020 REM *»*»»**»»»»«*»*»»*

4030 U*= ' ' 4040 RECEIUE(X'$, 1,6)

4050 IF X*=',' THEN RETURN

4060 i,'* = y$+X$ \ GO TO 4040

5000 REM ***»«»»**»*•»»*»#*»»•»»»#*»»»»*»»»*

5010 REM »* INITIALIZE MOTOR CONTROLS *

5020 REM *»**»»#**»*»»»»»***•»»•»**»

5030 IF P(19)<0 THEN RETURN

5040 T$=CHR$( 13)+CHR$( 10)

5050 SEND( 'XEYE'+T$, 11 ) \ REM ** MOTOR CURRENT OFF ##

5060 P$=STR$(INT(P( 1 )* 16000/25 . 4+ . 5 )

)

5070 c$= ':xd;xm+'+p$+'g;xe; '+T* \ SEND(C$rii) \ rem ** x motion »* 5090 P$=STR$( INT( (P(O)-l )*P( 1 ) #16000/25 . 4+ . 5 )

)

5090 c$= 'xd;xm-' + p$+ 'g;xe; '+T$ \ send(c$,ii) \ rem * return x motion »» 5100 P$=STR$( INT(P(20)*16000/25.4+.5)

)

5110 C$= 'YD; YM+' + P*+ 'G?YE; '+T$ \ SEND(C$,11) \ REM ** Y MOTION »* 5112 P$ = STR$( I NT (32000/ 25.4+. 5)

)

5114 c$= 'xd;xm- '+PS+ 'g;xm+'+p$+'g;xe; ] '+T* \ send(c$,ii)

5116 SEND( '#4/ '+T*, 11 ) \ PAUSE(3) \ REM *» Ql.'ERSHOOT & RETURN ** 5120 Rl$='#l/'+T$ \ R2$='#2»'+T* \ RETURN

6000 REM *»»*-»»***»**»»#»«-»»******»«»»»»*»»***-»#»«**#*»»***»*#*

6010 REM ** MOUE TO NEXT TRANSDUCER POSITION CX DIRECTION] »#

6020 REM **»***»*#*»**»»*»»*********»*«*****»#*»**»*»»»»*»***

6025 IF P(19)<0 THEN 6040

6030 SEND(R1$,11) \ RETURN

6040 PRINT 'PUSH "G" AFTER MOUING TRANSDUCERS TO NEXT POSITION'

6050 GET_CHAR(G$) \ IF G$<>'G' THEN 6050

6060 PRINT \ RETURN

7000 REM *»*#*»•***»#»*»***»*»»•»»»•*»•«***

7010 REM ** MOME TO NEXT SCAN LINE »* 7020 REM »!(«•»»»#»#»»#»*»»#»**»»#«#**•»«#

7040 SEND (R2*, 11) \ PAUSE (15) S RETURN

CALCl - Part 3

This asks for some (jecisions on the calculations to be done and starts

this process. It includes signal averaging and subtraction, Fourier

transformation, frequency windowing, and power calculations for the individual

data sets.

100 REM ** CALCl *»

110 REM ***•»#»**»*»**»*****#»#»#*»»*«*»*****•»»*»*****»*»*#

120 REM «» SYNTHETIC APERTURE BY PHASE SHIFT - PART 3 *»

130 REM ****»»***»#***#*«»*»*»»•»*»»**## #####**«#»**#»»•»##

140 COMMON P(20) r I$,P«,A1 rA2

145 DIM R( 511 ), 1(511 ) ,W( 511 ) ,R7.(511 ), 17.(511 ) 150 DIM #2,F1(511) \ DIM #3,F*(4)=255

160 IF P(19)<2 THEN I$=I*+'l' \ GO TO 205

170 PRINT \ PRINT 'Perform calculations on WHICH DATA SET (of ';P(19);

180 PRINT ')'; \ INPUT II* \ I$=I$+I1$

205 PRINT \ PRINT \ PRINT 'Window choice:' \ PRINT ' 1. NONE'

210 PRINT ' 2. BARTLETT' \ PRINT ' 3. MANNING'

220 GET_CHAR(D*)

230 IF D*='l' THEN I1*='N0 WINDOW' \ GO TO 270

240 IF D$='2' THEN 1 1 $=' BARTLETT WINDOW' \ GO TO 270

250 IF D$<>'3' THEN 220

260 I1$='HANNING WINDOW'

101
270 D=VAL(D*)

272 PRINT \ PRINT 'Perform calculations on WHICH POSITIONS? (1 to ';P(0);')'

274 PRINT 'Start with position # ' ; \ INPUT Al

276 PRINT ' End with position # ' ; \ INPUT A2

278 I2*= 'POSITIONS #'+STR*(Al)+' TO #'+STR$(A2)

280 PRINT \ PRINT \ PRINT 'Do vou want to SUBTRACT THE AVERAGE? (Y or N) '1 290 GET_CHAR(D$) \ IF D$='Y' THEN I3«= 'AVERAGE SUBTRACTED' \ GO TO 322

300 IF D*<>'N' THEN 290

310 I3$='Na AVERAGE SUBTRACTION'

322 PRINT \ PRINT 'Do you want to CALCULATE POWER SPECTRUM? (Y or N) '; 324 GET_CHAR(P*) \ IF P$='Y' THEN I4*= 'POWER CALCULATIONS DONE' \ GO TO 328

326 IF PSO'N' THEN 324

327 I4*='N0 POWER CALCULATIONS'

328 PRINT \ OPEN 'SYKID' FOR OUTPUT AS FILE #3 \ F$(0) = I* \ F$(1)=I1*

329 F$(2)=I2* \ F$(3)=I3$ \ F$(4)=I4* \ CLOSE

330 PRINT \ PRINT \ PRINT "I'M THINKING ABOUT YOUR PROBLEM NOW!"

340 PRINT 'START 'JCLK*

350 REM *»**#****##»*»**#»»*»»*

360 REM ** CALCULATE AVERAGE »

370 REM *»**»»***»•»•*#»»»»»***»*

380 FOR K=0 TO 511 \ W(K)=0 \ NEXT K 390 IF D$= 'N' THEN 470

400 FOR J=A1 TO A2

410 V$= 'SYl :

' + I$+STR$( J) \ OPEN V$ FOR INPUT AS FILE #2

420 EL0CK_M0VE(F1 (0) ,R(0) ,512) \ CLOSE

430 FOR K=0 TO 511 S W ( K ) =W ( K ) +R ( K ) / ( AZ-Al+1 ) \ NEXT K 440 NEXT J 450 OPEN 'SYIIAVG' FOR OUTPUT AS FILE #2

460 BLOCK_MOVE(W(0) ,F1 (0) ,512) \ CLOSE

470 PRINT 'AVG ';CLK$

480 REM *#*»»»*»»»»*»»**»*«»***»»*»*»»*»»**#»»»»

490 REM ** SUBTRACT AVERAGE & CALCULATE FFT'S »

500 REM »»*»»»»»»»*»***#»»»»*»*»##***»*»»»*»»**

510 Ml = INT(P(8)*(10''3)*512*P(ll)»(10''P(14)) + .5)

520 REM »» Ml IS THE POINT # AT THE CENTER OF THE WINDOW ** 530 W=45 \ REM ** WIDTH OF WINDOW [# OF POINTS] = 2»W=1 ** 540 IF W>M1 THEN W=M1

550 FOR J=A1 TO A2

560 V$= 'SYl : '+I*+STR$( J) \ OPEN V$ FOR INPUT AS FILE #2

570 BLOCK_MOVE(Fl (0) ,R(0) ,512) \ CLOSE

500 FOR K=0 TO 511 \ R'/. ( K ) =R ( K ) -W ( K ) \ IX(K)=0 \ NEXT K 590 FFT ( , 5 1 2 , R7. ( ) , 1 7. ( ) , S7.

)

600 R7. (0)=0 \ REM REMOVE DC LEVEL

610 GOSUB 700 \ GOSUB 1000

620 FOR K = TO 255 \ R ( K ) =R7. ( K ) »2''S7. \ I ( K ) = I 7. ( K ) *2'*S7. \ NEXT K 630 V$= 'SYl :FFT'+STR*( J)+'R' \ OPEN V$ FOR OUTPUT AS FILE #2

640 BLOCK_MaVE(R(0) ,F1 (0) ,256) \ CLOSE

650 V$= 'SYl :FFT'+STR$( J)+'I ' \ OPEN V$ FOR OUTPUT AS FILE #2

660 BLOCK_MOVE( I (0) ,F1 (0) ,256) \ CLOSE

670 NEXT J 680 PRINT 'FFT ' JCLK*

690 CHAIN 'SY0:CALC2'

700 REM »«»»»*#**»»**«»»«*»*»*

710 REM ** WINDOW POSITION *

720 REM **»»»*»»»**»**»»*#**»

730 IF D=l THEN RETURN

740 FOR K=0 TO 255

750 IF K<M1-W THEN 780

760 IF K>M1+W THEN 780

770 GOSUB 830 \ GO TO 790

780 W1=0

790 R7. (K)=R7. (K)»W1

800 17. (K) = I7. (K)*W1

810 NEXT K 820 RETURN

830 REM »»*•»•»•»••»***•«»»)(»**»*##*•»*»»*

840 REM ** CALCULATE WINDOW SIZE ** 850 REM »»»*»«»#»»»»*»#**#*#««»»»***

860 N=K-(M1-W)

102
870 IF D=3 THEN 910

SaO REM •»* BARTLETT WINDOW »

890 IF N<=W THEN W1=N/W \ RETURN

900 IF N>W THEN Wl=2-N/W \ RETURN

910 REM *» HANNING WINDOW ** 920 Wl=.5«( 1-C0S<PI*N/W) ) \ RETURN

1000 REM **#*»»»*»»**»»*»#**

1010 REM

* POWER CALCULATION ** 1020 REM »»**»*»»****»*******»»*

1030 IF P»='N' THEN RETURN

1040 P0WER(256,Ry.() r I7.( ) ,R() ) 1050 FOR K=0 TO 255 \ R ( K ) =SQR ( R ( K ) ) #2''S-/. \ NEXT K 1060 »,'*= '3Y1:PWR'+STR$( J) \ OPEN U$ FOR OUTPUT AS FILE #2

1070 BLOCK_MOVE(R(0) ,F1(0) t256) \ CLOSE \ RETURN

CALC2 - Part 4

This continues the previous part and includes phase shifting, coefficient

av*eraging, reverse Fourier transformation, and power calculations for the

result.

100 REM »» CALC2 *» 110 REM «**»»» »** »»»•»»»»»***»»#»*»#»»»»#*»*»*»»*»*#»

120 REM ** SYNTHETIC APERTURE BY PHASE SHIFT - PART 4 *»

130 REM •»»-tt*«-»-»*»*«*»»-»»*-»-»-»*»*»***»»***#*»»*«*»»»»»»*#»»

140 COMMON P(20) , I$,P$,A1

A2

145 DIM R(511 ) , I (511 ) ,W(255) ,T(255) ,R7. (511 ) , 17. (511 ) 150 DIM #2,F1 (51 1

160 REM *»»*•»*»#**»•»»*»**#*

170 REM *# PHASE SHIFT **

180 REM **»»»»***»*»»»*»»*»

185 IF INT( P(6)/2)=P(6)/2 THEN 195

190 T=( ( P(6)+l )*P(4)-P(5) ) "2 \ GO TO 200

195 T=(P(6)*P(4)+P(5) ) "2

200 IF P(7)=l THEN 230

210 DO=SQR( (P(2)+(A2-A1 )*P(1 )

) "2+T)+SQR( ( P ( 2 ) +P ( 3 ) + ( A2-A 1 )*P( 1 ) )"2+T)

220 GO TO 240

230 D0=SQR(P(2)"2+T)+SQR( (P(2)+P(3) )"2+T)

240 FOR J=A1 TO A2

250 D = SQR( (P(2)+J*P(n) "2 + T)+SQR( (P(2)+P(3)+J»P(1))"2 + T)-D0

260 2=2*PI#D/512/(P(11)*10"P(14))/(P(9)*10"6)

270 <•'$= 'SY1:FFT'+STR$( J) + 'R' \ OPEN i,'$ FOR INPUT AS FILE #2

280 BL0CK_M0UE(F1 (0) ,R(0) ,256) \ CLOSE

290 U$= 'SYl :FFT'+STR$( J)+ 'I ' \ OPEN i.'$ FOR INPUT AS FILE #2

300 ELOCK_MaUE(Fl (0) , I (0) ,256) \ CLOSE

310 FDR K=0 TO 255

320 R = R(K ) \ 1 = 1 (K ) 330 X=K*2 \ C=CaS(X) \ S=SIN(X)

340 REM *# DIi.' BY 2 HERE, THEN ADD 1 TO S7. IN 770-700 «» 350 R(K) = (R*C-I*S)/2 \ I ( K ) = ( I -^C + RwS ) /2

360 NEXT K 370 '.'$= 'SYl :SFT'+STR$( J)+ 'R' \ OPEN V$ FOR OUTPUT AS FILE #2

380 ELOCK_MOyE(R(0) ,F1 (0) ,256) \ CLOSE

390 U$= 'SYl :SFT'+STR$( J)+ 'I ' \ OPEN <.'$ FOR OUTPUT AS FILE #2

400 ELOCK_MO'.'E( I (0) ,F1 (0) ,256) \ CLOSE

410 NEXT J 420 PRINT 'SHIFT ';CLK$

430 REM »»»»»»•»•»•**-»»•»»»#»•«•**«•»*»*»»#*#*»»•»»»

440 REM ** SUM COEFFICIENTS & REVERSE FFT «*

450 REM *•»•»»•#»«•»•»»»•»»»*»«•»»**-»»»»*#•»•»»»#»»»

460 FOR K=0 TO 511 \ R(K)=0 \ I(K)=0 \ NEXT K 470 FDR J=A1 TO A2

480 '.'$= 'SYl :SFT'+STR$( J)+ 'R' \ OPEN '.'$ FDR INPUT AS FILE #2

490 EL0CK_M00E(F1 (0) ,W(0) ,256) \ CLOSE

103
500 FOR K=0 TO 255 \ R < K ) =R ( K ) +W ( K > \ NEXT K 530 U$= 'SYl :SFT'+STRt( J)+'I ' \ OPEN V* FOR INPUT AS FILE #2

540 BL0CK_M00E(F1 (0) ,W(0) ,256) N CLOSE

550 FOR K = TO 255 \ I( K ) = I ( K ) +W ( K ) \ NEXT K 560 NEXT J 570 FOR K=0 TO 255 \ R'/. ( K ) =R ( K ) / ( A2-A1 + 1 ) \ 17. ( K ) = I ( K ) / ( A2-Ai + 1 ) \ NEXT K 680 REM ** NOW FOLD COEFFICIENTS *#

690 FOR K=256 TO 511 \ RV. ( K ) =R7. ( 5 1 2-K ) \ I % ( K ) =-I 7. ( 5 1 2-K ) \ NEXT K 700 FOR K=0 TO 511 \ R(K)=R7. (K) \ I(K) = I7.(K) \ NEXT K 710 y$= 'SYl :CF' + I*+'R' \ OPEN U$ FOR OUTPUT AS FILE #2

720 ELaCK_MOyE(R(0) ,F1 (0) ,512) N CLOSE

730 V*= 'SYl :CF ' + I$+'I ' \ OPEN «,'* FOR OUTPUT AS FILE #2

740 ELOCK_MOME( 1(0) ,F1 (0) ,512) \ CLOSE

745 GOSUB 1000

750 FFT( 'REVERSE' , 512 , R7, ( ) , I 7. ( ) , SI 7.

)

760 REM »* SEE 340 ABOOE ** 770 FOR K=0 TO 511 \ R ( K ) =R7. ( K ) » ( 2*^ (S17.+ 1 ) ) /5 12 780 I (K) = I7.(K)*(2'^{S17.+ 1 ) )/512/1000 \ NEXT K 790 OPEN 'SYKRESULT' FOR OUTPUT AS FILE #2

800 BLOCK_MOOE(R(0) ,F1 (0) ,512) \ CLOSE

830 PRINT 'PHASE 'JCLK*

840 CHAIN 'SY0:GRAPH' i 1000 REM *»»*»»»**»#»»**»»#*»»*###

1010 REM « POWER CALCULATIONS ** 1020 REM *»***»**#*#»#»«##**»»#»*«»

1030 IF P*='N' THEN RETURN

1040 POWER ( 256, R7.( ) , I 7. ( ) , T ( ) ) 1050 FOR K=0 TO 255 \ T ( K ) =SaR ( T ( K ) ) ^2 \ NEXT K 1060 OPEN 'SY1:PWRF' FOR OUTPUT AS FILE #2

1070 BLOCK_MOUE(T(0) ,F1 (0) ,256) \ CLOSE \ RETURN

GRAPH - Part 5

This is for data display and output. It produces graphs and peak-to-peak

amplitudes for the raw, average, and final data, Fourier coefficients, and

power spectrum. It also calculates the total power within a selected

frequency range.

100 REM »# GRAPH **

110 REM »*•l^»**^n^»)^»**#»»*^^#»^^»#•)nn^*^nnnnn(.^t^nnnnnnnnt^n(.^nnnt^^

120 REM ** SYNTHETIC APERTURE BY PHASE SHIFT - PART 5 *#

130 REM ***»»*#»»* + **»»**#*»»»»»*»«*»»»#*»*»**»#»***»»*

140 REM **«»»#*#»#»#»»*»»**»«»»»*

150 REM *» GRAPHICAL DISPLAY »*

160 REM «»***#*******»#»***»»*#»

170 COMMON P(20) , I$,P$,A1 ,A2

175 DIM R(511 ) ,X(511 ) \ DIM #2,F1(511) \ DIM #3,F$(4)=255

180 OPEN 'SYi:iD' FOR INPUT AS FILE #3 \ J$=F$(0)

184 K $='»*####»*»*»»*****#»**»»*»*»»**» ' \ PRINT \ PRINT

185 PRINT K$ \ PRINT 'All calculated files done an ';j$ \ PRINT K*

187 FOR K=l TO 4 \ J$=F$(K) \ PRINT J$ \ NEXT K \ CLOSE

190 PRINT \ PRINT \ PRINT \ PRINT

200 L3*= '

' 210 PRINT 'Display choice."'

220 PRINT ' 1. RAW DATA 2. COEFFICIENTS or POWER'

230 PRINT ' 3. AUERAGE DATA 4. FINAL DATA'

240 PRINT ' 5. INTEGRATE POWER 6. TERMINATE '; 250 GET_CHAR(A$)

260 IF A$='2' THEN 6200

270 IF A$= '3' THEN 360

280 IF A$= '4 ' THEN 380

104
300 IF A*='5' THEN 7000

305 IF At='6' THEN 8000

310 IF ASO'l ' THEN 250

320 DISPLAY-CLEAR \ L3»= 'RAW DATA' \ PRINT \ PRINT L3t

330 PRINT 'Which INDIVIDUAL RUN (of ';P(0);')';

340 INPUT I \ L3*=L3$+' #'+STR$(I)

350 ','»= 'SYl :

' + I$+STR*( I ) \ GO TO 470

360 DISPLAY_CLEAR S L3*= 'AVERAGE DATA' \ PRINT \ PRINT L3*

370 V*='SY1:AVG' \ GO TO 470

380 DISPLAY_CLEAR \ L3*= 'FINAL DATA' \ PRINT \ PRINT L3*

390 V$= 'SYl :RESULT' \ GO TO 470

400 PRINT 'REAL or IMAGINARY? (R or I) 'J 410 GET_CHAR(C*) \ IF C$='R' THEN 440

430 IF C*<>'I' THEN 410

440 PRINT \ K>* = K>t+Ct 450 IF C$='I' THEN L3*=L3$+' IMAG'

460 IF C*='R' THEN L3*=L3*+' REAL'

470 OPEN V* FOR INPUT AS FILE #2

480 BL0CK_MQVE(F1 (0) ,R(0) ,512) \ CLOSE

500 PRINT 'Do YOU want to SCALE COORDINATES? (Y or N) ", 510 GET_CHAR(S*) \ IF S$='N' THEN 580

530 IF S*<> 'Y' THEN 510

540 PRINT \ PRINT 'X MIN = '; \ INPUT XI 550 PRINT 'X MAX = '? \ INPUT X2

560 PRINT 'Y MIN = ' ; \ INPUT Yl

570 PRINT 'Y MAX = '; \ INPUT Y2

580 PRINT \ GOSUE 4000 \ GQSUB 5000 \ GO TO 190

4000 REM »«»»»«-»#*•«•*»»»#•»•»»»*#»»#*»»»»»*»»»*#»*»»»»

4010 REM *» CALCULATE X's, GRAPH LIMITS, LABELS *

4020 REM »»*»»»»*»*#**«•#»**•»»*»»»*»»»*»*#»»»****»*

4030 IF A«='2' THEN 4240

4040 FOR K=0 TO 511 \ X(K)=K*P(11) \ NEXT K 4044 FOR K=0 TO 511 \ R ( K ) =R ( K ) *P (1 5 ) S NEXT K 4050 IF P(18)=l THEN L2*= 'K ILOMOLTS

'

4060 IF P(ia)=2 THEN L2*= 'MOLTS'

4070 IF P(18)=3 THEN L2*= 'M I LL K'OLTS

'

4080 IF P(14)=3 THEN Ll*='10'>3 SECONDS'

4090 IF P(14)=0 THEN L 1 *= 'SECONDS ' 4100 IF P(14)=-3 THEN LI *= 'MILLISECONDS ' 4110 IF P(14)=-6 THEN LI $= ' M ICRQSECONDS ' 4112 IF S$= 'Y' THEN 4170

4120 IF A*='l' THEN 4140

4130 IF A*<>'3' THEN 4150

4140 Y1=0 S Y2=300»P(15) \ GO TO 4160

4150 Y1=-150»P(15) \ Y2 = -Y1 \ REM *# FOR At= 4 Se 5 ** 4160 X1=0 \ >;2 = 600»P(11 ) 4170 X1*=STR*(X1) \ X2*=STR$(X1+(X2-X1 )/4)

4180 X3*=STR$(X1+(X2-X1 )/2) \ X4$=STR* ( Xl+3* ( X2-X1 ) /4

)

4190 Y1* = STR*<Y1) \ Y2$ = STR$ ( Y 1 + ( Y2-Y1 ) /4 ) \ Y3* = STR$ ( Y1+ ( Y2-Y1 ) /2

)

4200 Y4$ = STR*(Y1+3*(Y2-Y1 )/4) \ Y5» = STR*(Y2)

4210 WINDaW( "EXACT" , X 1 , Yl , X2 , Y2

)

4220 GRAPH( ,512,X(0) ,R(0) ) 4230 GO TO 4340

4240 FOR K=0 TO 511 \ X ( K ) =K /512/P ( 1 1 ) \ NEXT K 4250 IF P*='N' THEN L2$= 'CQEFFIC lENTS ' \ GO TO 4260

4252 IF B$='C' THEN L2*= 'COEFFIC lENTS ' \ GO TO 4260

4254 L2$= 'POWER'

4260 IF P(14)=3 THEN Ll*= 'FREQUENCY , mHz ' 4270 IF P(14)=0 THEN L 1 $= 'FREQUENCY , Hz' 4280 IF P(14)=-3 THEN LI $= 'FREQUENCY , KHz'

4290 IF P(14)=-6 THEN L 1 *= 'FREQUENCY , MHz'

4300 IF S*='Y' THEN 4170

4310 X1=0 \ X2 = 250/500/P( 11 ) 4320 X1*='0' \ X2$=STRt(X2/4) \ X3*=STR* ( X2/2 ) \ X4*=STR$ ( 3»X2/4

)

4330 GRAPH( ,256,X(0) ,R(0)

)

4340 PRINT \ PRINT 'Do you want to calculate the MAXIMUM, MINIM.UM, and

4350 PRINT ' SPAN of the araph? '; 4360 GET_CHAR(H*) \ IF H*='N' THEN PRINT \ RETURN

4380 IF HSO'Y' THEN 4360

105
4390

4400

4410

4420

4430

4440

4450

4460

4470

4480

4490

4492

4497

4500

4510

4530

5000

5010

5020

5030

5040

5050

5060

5070

5080

5090

5100

5110

5120

5130

5140

5190

5200

5210

5220

5230

5240

5250

5260

5270

5280

5290

5300

5310

5320

5330

5340

5350

5360

5370

5380

5390

5400

5410

5420

5430

5450

5460

5480

5490

5500

5510

5550

6200

6210

6220

6225

6230

6240

PRINT \ PRINT 'Within what RANGE?'

PRINT 'MINIMUM X is '; \ INPUT X3

PRINT 'MAXIMUM X is '; \ INPUT X4

REM Y3 IS MINIMUM Y, Y4 IS MAXIMUM Y Y3=R(0) \ Y4=R(0) \ 1=0

IF X(I)<X3 THEN 1=1+1 \ GO TO 4440

IF X(I ) >X4 THEN 4490

IF R(I XY3 THEN Y3 = R(I)

IF R(I ) >Y4 THEN Y4=R(I

)

1=1+1 \ GO TO 4450

PRINT CHR*(27) , 'C5i

PRINT 1$ \ PRINT L3* \ PRINT

'Between ';X3;' and ';X4;' ';L1*

'The MINIMUM UALUE is ';Y3

'The MAXIMUM VALUE is ';Y4

'The SPAN is ';Y4-Y3 \ PRINT CHR* ( 27 )

, ' C4 i

' \ RETURN

SUBROUTINE TO ADD UNITS AND AXIS LABELS TO GRAPHICS

**** THE LABELS FOR THE X-AXIS AND Y-AXIS MUST BE PASSED TO THIS

ROUTINE UIA:

X-AXIS

Y-AXIS

Ll$

L2$

PRINT

PRINT

PRINT

PRINT

REM

REM

REM

REM

REM

REM

REM

REM

B*=CHR$(27)+ 'Plp'

E$=CHR*(27)+'\

K*= 'S(ACO, 479] [767,0] )

' PRINT E$+K*+E*

PRINT B*+'Pi:51 ,479]0C]VC51 , 101]PC711 , 479 ]UC ]0[ 7 1 1 ,101] '+E*

IF A$< >'2 ' THEN 5250

IF S*= 'Y ' THEN 5250

PRINT \ PRINT 'TYPE IN THE Y-ORIGIN AND Y-MAX UNITS'

PRINT 'LIMIT EACH ENTRY TO FIOE CHARACTERS INCLUDING DECIMAL POINT

PRINT \ PRINT 'ORIGIN '; \ INPUT Yl

PRINT 'MAX ' ; \ INPUT Y2

Y1$=STR*(Y1) \ Y2*=STR*(Y1+(Y2-Y1 )/4) \ Y3$=STR$ ( Y1+ ( Y2-Y1 ) /2

)

Y4$=STR*(Y1+3*(Y2-Y1 )/4) \ Y5$=STR$(Y2)

L1=LEN(L1*) \ T17. = 395-Ll/2#20

L2 = LEN(L2*) \ T27. = 294-L2/2#20

ERASE_TEXT

PRINT B$+ 'T(S:9, 15] ) '+E$;

PRINT B*+"PC0,479]T "'+Y5$ + PRINT B*+"PC0,389]T"'+Y4$ + PRINT B$+"PC0,299]T

PRINT B$+"PC0,209]T

PRINT B$+"PCO, 1 19]T

+Y3*+"

+V2$+"

+Y1*+"

"+E$;

"+E$;

"+E*;

"+E$;

"+E$;

"+E*;

"+E$;

"+E$;

"+E*;

\PRINT B$+"PC70,97]T "'+X1$+" "

PRINT B$+"PC228,97]T "'+X2$+"

PRINT B*+"PC388,97]T "'+X3$+"

PRINT B$+"P:548,97]T'"+X4$+"

\

PRINT B$+"P[ "+STR$(T1-/. ) + " , 69 ] T [ +20 , +0 ] (D0S2S2H2) '"+L1$+" "'+E$;

PRINT B$+"PC716, "+STR*(T27. ) + "]TC+0,+20] (D90S2S2H2) "'+L2$+" '"+E*;

\PRINT B$+'TC+9,+0] ( SI , H2 , SC9 , 20] , M C 1 , 2 ] , DO , 10 ) '+E*

PRINT 'Do YOU want to make a HARD COPY'5 GET_CHAR (','$) \ IF <.'$='N' THEN RETURN

IF V$<> 'Y ' THEN 5460

PRINT \ PRINT CHR$(27) , 'C5i ' \ PRINT

PRINT \ PRINT \ PRINT CHR* ( 27 )

, ' [ 4i

DISPLAY-COPY

RETURN

REM **##*#***#*»*»-ft-**»*»»*#**#»

REM »* HANDLE COEFFICIENTS «* REM »»«•»«#*#»«»»*##»»**»«*#*##*

DISPLAY-CLEAR

PRINT \ IF P$='N' THEN 6290

PRINT 'COEFFICIENTS or POWER? (C or P)

(Y or N)

\ PRINT I* \ PRINT L3*

106
6250

6260

6270

6280

6290

6300

6310

6320

6330

6340

6350

6360

6370

6380

6390

6400

6410

6420

6430

6440

6500

6510

6520

6530

6600

6610

6615

6620

7000

7010

7020

7030

7050

7060

7070

7080

7090

7100

7110

7120

7130

7140

7150

7160

7170

7180

7190

7200

7210

7220

7230

7240

7250

7260

7270

7280

7290

8000

\ L3$='FINAL' +L3$ \ GO TO 6380

L3*= ''SHIFTED' +L3$ \ GO TO 6370

L3*= •'RAW'+L3«

6600 \ RETURN

GET_CHAR(B*) N IF B*='C' THEN 6290

IF B$<>'P' THEN 6250

PRINT \ PRINT \ L3$= 'POWER' \ PRINT \ PRINT L3*

GOSUB 6500 \ GO TO 470

PRINT \ L3*=' COEFFICIENTS' \ PRINT \ PRINT L3*

PRINT 'RAW, SHIFTED, or FINAL? (R, S, or F) '7 GET_CHAR(C$) \ IF C*='R' THEN 6360

IF C$= 'S' THEN 6350

IF C*<>'F ' THEN 6310

PRINT \ ».'$= 'SYl :CF' + I* PRINT \ t.'*= 'SYl :SFT' \ PRINT \ 0$='SY1:FFT' \

GOSUB 6600

PRINT 'REAL or IMAGINARY? (R or I) '; GET_CHAR(C$) \ IF C$='R' THEN 6410

IF C$<> 'I ' THEN 6390

PRINT \ V*=0$+C$

IF C*='I' THEN L3*=L3$+' IMAG'

IF C*='R' THEN L3*=L3*+' REAL'

GO TO 470

PRINT 'INDIVIDUAL or FINAL? (I or F)

GET_CHAR(C$) \ IF C$= ' I

' THEN GOSUB

IF C$<>'F' THEN 6510

i.'$= 'SYl :PWRF' \ L3$ = L3$+' FINAL' \ PRINT \ RETURN

PRINT \ PRINT 'Which INDIVIDUAL RUN (of ';P(0);')';

INPUT I \ L3$=L3$+' - #'+STR$(I)

IF B*='C' THEN V$ = i.'*+STR$( I ) \ RETURN

'.'$= 'SYl :PWR'+STR*( I ) \ RETURN

REM **»»**»#*#«*»»****•»»**»»

REM ** POWER INTEGRATION ** REM *»*«**»»•»» »** *»»»#*»»

F0=0 \ F2=0 \ A$="" \ B$=""

PRINT \ PRINT 'INTEGRATED POWER'

PRINT 'INDIVIDUAL OR FINAL (I or F) '! GET_CHAR(A$) \ IF A$='F' THEN 7100

IF A*<> 'I ' THEN 7070

PRINT \ PRINT 'Which INDIVIDUAL RUN (of ';P(0);')';

IF P(14)=3 THEN Ll*='(.iHz' IF P(14)=0 THEN Ll*='Hz'

IF P(14)=-3 THEN Ll$='KHz'

IF P(14)=-6 THEN Ll*='MHz'

PRINT \ PRINT 'What are the LIMITING FREQUENCIES, i PRINT 'STARTING FREQUENCY'; \ INPUT FO

PRINT 'FINAL FREQUENCY'; \ INPUT F2

K1=F0»512»P( 11 ) \ K2=F2»512*P( 11

)

IF A$= 'I ' THEN 7200

V$= 'SYl :PWRF ' \ GO TO 7210

v$= 'SYl :pwr '+STR*( I

)

OPEN V* FOR INPUT AS FILE #2 \ BLOCK_MOVE ( F 1 ( ) , X ( ) FOR K=K1 TO K2 \ P0=P0+X(K) \ NEXT K PRINT CHR$(27) , 'C5i ' \ PRINT

I

' THEN 7260

\ PRINT 'The FINAL INTEGRATED

\ PRINT 'The INTEGRATED POWER

between ';F0;' and ';F2;L1$

\ INPUT I

;li*; '?

IF A$ = PRINT

PRINT

PRINT

PRINT

GO TO

DISPLA

POWER for ';i$;

for position #'

?56) \ CLOSE

';P0; \ GO

is ';po;

TO 7270

\ PRINT CHR$(27) , 'C4i

190

Y_CLEAR \ END

107

Appendix II

SOFTWARE SAMPLE RUN

The following represents a typical session of data collection, analysis,

and output. All keyboard entries are circled, and printer output is

delineated by the square brackets ([). All other prompts and information

appear on the screen of the video display terminal (VDT). Those entries

marked by * are used for information recording only and do not enter into any

calculations. Other entries are used for decision making, scanner control,

and data processing.

(run syn) ,,,-v NtW or'TILD DATA"? (N or 0) (n)

Are you aoin^ to USE THE SCANNER SYSTEM-^ (Y or N ) (y)

IDENTIFI CATION (2 CHAR MAX) = » date: •? (9-6-83)

» transduc ers : ? (gimeal)

W PRF = -^ (999)

""^ * NUMBER OF CYCLES = ?

Q

notes: 2 lines AUAILAf.LE

LLU^^J -V » ?(THIS is a SAMPLE PRINTOUT)

L;Nr r: =zi ^

«

'^EDM NOTCH ON SAME SIDE OF PLATE AS EMATSJ

NUMBER OF TRANSDUCER POSITIONS = ?(5)

NUMBER OF SCAN SETS = ?(T)

IN MM. ^.^ POSITIONS = -^(2)

TRANSMITTER & RECEIUER = -?f 32.5

DISTANCES IN MM: BETWEEN

BETWEEN

PLATE THICKNESS = -^(25

FOCAL SPOT location:

HOR

IHORIZONTAL (from initial transmitter positjoi-i) = "^(^o)

VERTICAL DEPTH (frow transducer side) = ?(o^

# OF REFLECTIONS IN ACOUSTIC PATH = -^(0)

FORWARD OR BACKWARD SHIFT"? (F OR B) (FORWARD

« FREQUENCY (KHZ) = ?(454)

SOUND t.'ELOCITY (KM/SEC) = ^ (3.2J

» NUMBER OF AVERAGES DONE ON SCOPE

109
Do YOU want a HARD COPY OF THE PARAMETER LIST? (Y or N

)

(y)

id: Y2

9-6-83

transducers: dimbal

PRF = 999

4 CYCLES

THIS IS A SAMPLE PRINTOUT

EDM NOTCH ON SAME SIDE OF PLATE AS EMATS

1 SCAN SETS

5 TRANSDUCER POSITIONS

2 MM BETWEEN POSITIONS

32.5 MM BETWEEN TRANSMITTER & RECEIVER

25 MM PLATE THICKNESS

FOCAL SPOT LOCATION:

40 MM HORIZONTAL (from initial transmitter position)

MM VERTICAL DEPTH (from transducer side)

REFLECTIONS IN ACOUSTIC PATH

FORWARD SHIFT

454 KHZ FREQUENCY

3.2 KM/SEC SOUND VELOCITY

16 AUERAGES DONE ON SCOPE

Do v'OLi want to INITIALIZE

When system is READY push

X or Y coordinate:

DIRECTION-^ (+ OR -, MM OF TRAVEL = '^(Ts)

Push "E" to EXECUTE (E

Do 70 u want to MO'.'E FtfRTHER

Press "E

TRANSDUCER

"R@ POSITION? (Y or N)

to faeain EXECUTION(e)

or N)

©

®

Scan 1 of 1 at CoIlectir(g position

Collecting position

Collectina positioi-i

Collecting position

Collectina Positioi-i

rii m

1 of 5 at mm

2 of 5 at 2 m fTi 3 of 5 at 4 (II (ii 4 of 5 at 6 fTi m 5 of 5 at 8 mm

Window choice:

1

.

NONE

2. EARTLETT

3. HANNING

VPerform calculations oi Start with position #

' End with position # '

iHICH POSITIONS^ (1 to 5

Do YOU want to SUBTRACT THE Ai.'ERAGE-^ (Y or N; Do von ;^ant to CALCULATE POWER SPECTRUM? (Y c ©

I'M THINKING ABOUT YOUR PROBLEM NOW

START 14:13:27

A'.'G 14:13:5a

FFT 14:15:20

SHIFT 14:16:24

PHASE 14:17:20

110
^^nnnl.^nnnnnnn^***)^*»^!^^*^^*»#•t^*#»***

All calcalated files done on YZl

»***»*»**»***»»***»*************

NO WINDOW

POSITIONS #1 TO #5

AVERAGE SUBTRACTED

POWER CALCULATIONS DONE

Display choice;

1 . RAW DATA 2, 3. AVERAGE DATA 4, 5. INTEGRATE POWER 6. RAW DATA

Which INDIVIDUAL RUN (of

COEFFICIENTS or POWER

FINAL DATA

TERMINATE (T)

5 )

Do v'ou want to SCALE COORD I NAT (Y or N) ©

(Data plotted on VDT)

Do you want to calculate the MAXIMUM, MINIMUM, and SPAN of the araph? (V)

Within what RANQE"^

MINIMUM X is "@.^

MAXIMUM X is ?(3Di

YZl

RAW DATA #1

Between

The MINIMUM

The MAXIMUM

The SPAN is

and 30

VALUE is

VALUE is .21

MICROSECONDS

. 172

.382

Do •ou want to make a HARD COP'i (Y or N)

YZl

RAW DATA #1

L

R I

.45

,15

^-r

;\^; Wn^vV^^^^^

15 I 30 MICROSECONDS

45

111
Display choice:

1. RAW DATA Z. COEFFICIENTS or POWER

3. AMERAGE DATA 4. FINAL DATA

5. INTEGRATE POWER 6. TERMINATE (V)

FINAL DATA ^-^ Do v'ou want to SCALE COORDINATES? (Y or N) (N^

(Data plotted on VDT) Do you want to calculate the MAXIMUM, MINIMUM, and SPAN of the araph? (7)

Within what RANfiE?

MINIMUM X is ?@.

MAXIMUM X is ?(3q)

YZl

FINAL DATA

Between and 30 MICROSECONDS

The MINIMUM '.'ALUE is -.0777266

The MAXIMUM UALUE is .0753516

The SPAN is .153078

Do you want to maKe a HARD COPY? (Y or N) (7)

YZl

FINAL DATA

-.15

^-

15 I 30 I 45 MICROSECONDS

Display choice:

1 . RAW DATA

3. AVERAGE DATA

5. INTEGRATE POWER

COEFFICIENTS or POWER"^ (C or P)

2. COEFFICIENTS or POWER

4. FINAL DATA

6. TERMINQ^TE (T)

POWER

INDIVIDUAL or FINAL? (I or F

) (?) Do you want to SCALE COORDINATES? (Y or N) ®

(Data plotted on VDT)

112
Do you want to calculate the MAXIMUM- MINIMUM, and SPAN of the 3raph7 (7)

Within what RANJjE?

MINIMUM X is ?r^

MAXIMUM X is "^ (?)

YZl

POWER FINAL

Between and 1 FREQUENCY, MHz

The MINIMUM UALUE is The MAXIMUM UALUE is 2709.79

The SPAN is 2709.79

TYPE IN THE Y-ORIGIN AND Y-MAX UNITS

LIMIT EACH ENTRY TO FK'E CHARACTERS INCLUDING DECIMAL POINT,

ORIGIN

MAX

Do you want to make a HARD COPY? (Y or N) (y)

Y21

POWER FINAL

2250

1500

750

la 1.25 12.5 FREQUENCY,

3.75 MH:

COEFFICIENTS or POWER

FINAL DATA

TERMINATE (s)

Display choice!

1. RAW DATA 2. 3. AVERAGE DATA 4, 5. INTEGRATE POWER 6. INTEGRATED POWER

INDIVIDUAL OR FINAL (I or F)

What are the LIMITING FREQUENCTES, in MHz STARTING FREQUENCYTKr

FINAL FREQUENCY?(T)

©

c

The FINAL INTEGRATED POWER for YZl is 11627.9 between and 1 MHz

113

APPENDIX III

TECHNICAL PAPERS RESULTING FROM THIS PROGRAM

Several papers based on various aspects of this work or using equipment

and ideas developed in the course of the program have been published.

Fortunko, C. M. , and Schramm, R. E., 1983, An analysis of

electromagnetic-acoustic transducer arrays for nondestructive evaluation of

thick metal sections and weldments, jj2_: "Review of Progress in Quantitative

Nondestruction Evaluation, Vol. 2A," D. 0. Thompson and D. E. Chimenti, eds..

Plenum Press, New York, p. 283.

King, R. B., and Fortunko, C. M. , 1983, Evaluation of residual stress states

using horizontally polarized shear waves, ui.' "Review of Progress in

Quantitative Nondestructive Evaluation, Vol. 2B," D. 0. Thompson and D. E. Chimenti, eds., Plenum Press, New York, p. 1327.

King, R. B., and Fortunko, C. M., 1983, Determination of in-plane residual

stress states in plates using horizontally polarized shear waves, J. Appl

.

Phys. , 54:3027.

King, R. B., and Fortunko, C. M., 1983, Acoustoelastic evaluation of arbitrary

plane residual stress states in non-homogeneous, anisotropic plates.

Ultrasonics 21:256.

King, R. B., and Fortunko, C. M., 1984, Evaluation of surface residual

stresses using horizontally polarized shear waves at grazing incidence, to be

published in J . Appl . Phys

.

Schramm, R. E., Fortunko, C. M., and Moulder, J. C, 1984, Advanced methods

for noncontact inspection of welds using electromagnetic-acoustic transducers,

submitted to: "Review of Progress in Quantitative Nondestructive Evaluation,

Vol. 3," D. 0. Thompson and D. E. Chimenti, eds.. Plenum Press, New York.

Schramm, R. E., Moulder, J. C, and Fortunko, C. M., 1984, Nondestructive

evaluation of thick austenitic stainless steel weldments by shear horizontal

acoustic waves, j_n: "Advances in Cryogenic Engineering, Vol. 30," A. F. Clark

and R. P. Reed, eds.. Plenum Press, New York, p. 119.

115
116
APPENDIX IV

ACKNOWLEDGMENTS

The authors are grateful for the sponsorship of the U.S. Department of

Defense, Department of the Navy, Naval Sea Systems Command through the Energy

and Minerals Resources Research Institute, Ames Laboratory, Iowa State

University. We wish especially to thank our Navy monitor. Dr. Hans

Vanderveldt, for his thoughtful interest.

Many people in our laboratory have contributed to various aspects of this

work and we wish to acknowledge their considerable efforts. Dr. Richard King

provided insights into the vagaries of ultrasonic propagation and diffraction.

Earl Ballinger, Dave Brannon, Al Bennett, Jim Newman, and Shelly Dalton aided

in various parts of equipment design and construction or testing and data

collection. Nick Sanchez managed to make our artwork intelligible and

readable. Diane Shields, JoAnne Wilken, and Peggy Vigliotti led us through

the myraid administrative details and survived our many text revisions. To

all these, and yet more, we offer our gratitude and thanks.

117

NBS-n4A (REV. 2-ac) U.S. DEPT. OF coMM. 1. PUBLICATION OR

„,„,.««,,.„.« ^^«-r/, REPORT NO. BIBLIOGRAPHIC DATA

SHEET (See instructions) NBS TN-1075

2. Performing Organ. Report No. 3. Publication Date

May 1984

4. TITLE AND SUBTITLE

Electromagnetic-Acoustic-Transducer/Synthetic-Aperture System for Thick-Weld

Inspection

5. AUTHOR(S)

C. M. Fortunko, R. E. Schramm, J. C. Moulder, J. D. McColskey

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No. Al -4572

A2-1699, SC-83-051

8. Type of Report & Period Covered

Final

7-1-81 to 12-31-83

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Ames Laboratory

Energy & Mineral Resources Research Institute

Iowa State University

Ames, Iowa 50011

10. SUPPLEMENTARY NOTES

I^J Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here) This report describes a system based on electroma

as an approach to automated nondestructive evalua

Good siqnal-to-noise ratios, often a problem with

careful design of the transducers and associated

of signal averaging. At 454 kHz, the transducers

of approximately 7-mm wavelength in steel. The 1 of through-thickness flaw depth from the amplitud

A minicomputer controlled transducer positioning

sonic waveforms for synthetic aperture processing

further improved signal quality and yielded flaw

thickness. Measurements on artificial flaws demo

of 0.5 mm (through thickness) and sizing ability

theoretical predictions. Details include the des

as well as the mechanical positioner, signal proc

computer program listings.

gnetic-acoustic transducers (EMATs)

tion of thick weldments (>25 mm).

EMATs, were possible through

electronic circuits and the use produce shear-horizontal waves

ong wavelength permits determination

es of scattered ultrasonic waves,

and acquired the digitized ultra-

. The synthetic aperture technique

localization through the weld

nstrated a detectabil ity threshold

up to 2.5 mm, in agreement with

ign of the transducers and electroni

essing algorithms, and complete

cs

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

electromagnetic-acoustic transducer; flaw detection; nondestructive evaluation;

S-H waves; synthetic aperture; ultrasonics; weld inspection

13. AVAILABILITY

[Xl Unlimited

I I For Official Distribution. Do Not Release to NTIS

[^ Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C

20402. Q^ Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

130

15. Price

USCOMM-DC 6043-P80

U.S. GOVERNMENT PRINTING OFFICE: 1984—779-422 9337 REGION NO

NBS TECHNICAL PUBLICATIONS

PERIODICALS

JOURNAL OF RESEARCH—The Journal of Research of the National Bureau of Standards reports NBS research and develop- ment in those disciplines of the physical and engineering sciences in which the Bureau is active. These include physics, chemistry,

engineering, mathematics, and computer sciences. Papers cover a broad range of subjects, with major emphasis on measurement

methodology and the basic technology underlying standardization. Also included from time to lime are survey articles on topics

closely related to the Bureau's technical and scientific programs. As a special service to subscribers each issue contains complete

citations to all recent Bureau publications in both NBS and non- NBS media. Issued six times a year. Annual subscription; domestic

$18; foreign $22.50. Single copy, $5.50 domestic; $6.90 foreign.

NONPERIODICALS

Monographs—Major contributions to the technical literature on

various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with in- terested industries, professional organizations, and regulatory

bodies. Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other special publications

appropriate to this grouping such as wall charts, pocket cards, and

bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and

studies of special interest to physicists, engineers, chemists,

biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a worldwide program coordinated by NBS under

the authority of the National Standard Data Act (Public Law

90-396),

NOTE: The principal publication outlet for the foregoing data is the Journal of Physical and Chemical Reference Data (JPCRD)

published quarterly for NBS by the American Chemical Society

(ACS) and the American Institute of Physics (AIP). Subscriptions,

reprints, and supplements available from ACS, 1 155 Sixteenth St., NW, Washington, DC 20056.

Building Science Series—Disseminates technical information developed at the Bureau on building materials, components,

systems, and whole structures. The series presents research results,

test methods, and performance criteria related to the structural and

environmental functions and the durability and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures

published by the Department of Commerce in Part 10, Title 15, of

the Code of Federal Regulations. The standards establish nationally recognized requirements for products, and provide all concerned interests with a basis for common understanding of the

characteristics of the products. NBS administers this program as a supplement to the activities of the private sector standardizing

organizations. Consumer Information Series—Practical information, based on NBS research and experience, covering areas of interest to the con- sumer. Easily understandable language and illustrations provide

useful background knowledge for shopping in today's tech- nological marketplace. Order the above NBS publications frort\: Superiniendem of Documents. Government Printing Office. Washington. DC 20402. Order the following NBS publications—FIPS and NBSIR's—from

the National Technical Information Service, Springfield, VA 22161

Federal Information Processing Standards Publications (FIPS

PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The Register

serves as the official source of information in the Federal Govern- ment regarding standards issued by NBS pursuant to the Federal Property and Administrative Services Act of 1949 as amended.

Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 1 1717 (38 FR 12315, dated May II, I973)and Part 6

of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or

final reports on work performed by NBS for outside sponsors

(both government and non-government). In general, initial dis- tribution is handled by the sponsor; public distribution is by the National Technical Information Service , Springfield, VA 22161,

in paper copy or microfiche form.
U.S. Department of Commerce

National Bureau of Standards

Washington, D.C. 20234

Official Business

Penalty for Private Use $300

POSTAGE AND FEES PAID

U S DEPARTMENT OF COMMERCE

COM-215

SPECIAL FOURTH-CLASS RATE BOOK