A11100 RfibbbE PUBLICATIONS

V

NBSIR 79-1907

NAT'L INST. OF STAND & TECH

A111DS 4 E R E b

An Appraisal of Current and

Future Needs in Ultrasonic NDE

Standards

G. Birnbaum and D. G. Eitzen

National Bureau of Standards

Washington, D.C. 20234

October 1979

Report to Defense Advanced Research Projects Agency

1400 Wilson Boulevard

Arlington, VA 22209

,U56

79-1907

1979

NBSIR 79-1907

i A « AN APPRAISAL OF CURRENT AND

FUTURE NEEDS IN ULTRASONIC NDE

STANDARDS

SMI&naf- Bureau of •STdnfe as FEB 5 1980

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G. Birnbaum and D. G. Eitzen

National Bureau of Standards

Washington, D.C. 20234

October 1979

Report to Defense Advanced Research Projects Agency

1400 Wilson Boulevard

Arlington, VA 22209

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Luther H. Hodges, Jr., Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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TABLE OF CONTENTS

PAGE

SUMMARY 1

1 . INTRODUCTION 3

2. INPUT DATA 6

2.1 Historical-Background 6

2.2 Workshop on Ultrasonic NDE Standards, October 1977 9

2.3 Documentary Standards 9

2.4 Literature References 10

3. ANALYSIS AND PERSPECTIVE 11

3.1 The Role of Standards 11

3.1.1 General nature of standards 11

3.1.2 Ultrasonic NDE standards 12

3.1.3 Ultrasonic NDE standards organizations in the U.S. 13

3.2 Framework for Assessing Ultrasonic NDE Standards 14

3.2.1 Nature of an ultrasonic flaw detection system 14

3.2.2 Uses of a standard 15

3.2.3 Criteria for evaluating possible standards 16

3.3 An Approach to a Systematic Examination of Ultrasonic

Reference Blocks 17

3.3.1 Introduction 17

3.3.2 Analysis of block design 18

3.3.3 An example of the critique of a proposed standard 22

3.4 The Characterization of Ultrasonic Transducers for NDE 26

3.5 Standards Implications of New Ultrasonic NDE Techniques. 30

4. OUTPUTS OF THIS STUDY 33

4.1

Key Issues 33

4.1.1 What are the standards requirements for 33

quantitative NDE?

4.1.2 What is the impact of computer-based systems and 33

advanced signal processing on standards

requirements?
4.1.3 Hierarchy of standards 34

4.1.4 Is there a "super" block that does everything? 34

4.1.5 Overall versus separate component calibration 34

4.1.6 Why be concerned about trying to improve current 35

standards for pulse echo techniques when better

techniques are around the corner?

4.1.7 What are the tolerances on reference blocks and 36

transducers that are really required?

4.1.8 What are the requirements for transducer 36

calibration?

4.1.9 With the many advances in electronic technology, 37

why are there problems in calibrating the

electronic equipment?

4.1.10 What about standards for measurements of residual 37

stress and materials properties?

it 4.2 Recommendations for Research and Development of 38

Ultrasonic NDE Standards

4.2.1 Improvements in standards for existing systems 38

4.2.2 Proposed methods for standardization and 41

calibration

4.2.3 Standards needs for developing methods; 43

quantitative NDE

4.3 Priorities, Impact, Final Comments 45

4.3.1 There are a great many problems but what are the 45

priorities?

4.3.2 If all the problems are solved, what impact can be 46

expected?

4.3.3 Final comments 46

REFERENCES 48

APPENDIX

A. Workshop on Ultrasonic NDE Standards A-l

B. Qualitative Examination of some Novel Blocks B-l
SUMMARY

The purpose of this study, supported in part by DARPA, is to assess

the current status of ultrasonic NDE standards and calibrations and to

determine current and future needs in this area. The source material

includes surveys of the literature and patents, a study of foreign

practice, surveys of NBS and consensus standards (e.g., ASTM) programs,

discussions, visits and letters, and a Workshop on Ultrasonic NDE Standards: "Current Needs and Future Directions," held October 17-18, 1977

for the purpose of implementing the objectives of this study. This

resource material is critically analyzed: 1. to determine what needs

to be done to improve existing standards; 2. to select for further

evaluation proposed standards and calibration methods; and 3. to assess

the requirements for standards and calibration for developing ultrasonic

systems. Following this analysis, we make recommendations for work in

three areas which should provide the basis for an approach to a system

of ultrasonic NDE standards. In the first part, we recognize that

improvement of accepted standards, e.g. those proposed by ASTM, those

traceable to NBS or those widely used in practice, will have an immediate and cost effective impact on improving the reliability and

more quantitative use of current methods. It is also clear that

improvements in these standards, in the underlying theory and in their

relation to practice will have significant, direct impact on future

systems. The second part deals with proposed methods which are new

rather than improvements to those considered in the first part. The

emphasis here is not on unexplored ideas but rather on the examination

and possible implementation of the concepts for specific standards

needs. The third part deals with standards and calibration needs for

evolving and future ultrasonic NDE methods.

Recommendations for improvement in existing standards will impact

primarily on conventional pulse echo systems and include work on transducers, the electronic system and reference blocks. The fields produced

by transducers are complex and the variability is high. Thus an important aspect of the transducer characterization problem is the need for a better understanding of the experimental and theoretical nature of

transducers to develop minimum specifications to obtain reproducible

results in flaw evaluation. Similar considerations apply in the

characterization of the electronics. What are the right things to be

measured and how should they be measured? The electronics and

transducer calibration problems are not so much how to do it but what to

do. In the areas of blocks, the development of quartz blocks free of

the material problems of metal blocks is a promising future direction.

However, a new problem, that of transferring the calibration to metal

components, must be solved.

1
Many methods of standardization and calibration have been proposed;

these seem to offer logical or practical advantages over currently

accepted methods but are not widely used or used at all. These methods

should be completely analyzed with a proper set of criteria and promoted

if they indeed prove to be significantly advantageous. As an example of

this process, we have examined the possibility of using theoretically

characterized scatterers such as spheres where the theoretical knowledge

of the scattering cross section is to play a crucial role in the calibration process. Although this feature would appear to be attractive,

we find that some of the advantages claimed for the method are really

not important in practice and that further work is necessary. On the

other hand, this study indicates that theory may play a significant role

in aiding the transfer of a calibration to different conditions, for

example, frequency and bandwidth.

We finally consider the standards needs for developing

techniques. Two of the major needs which motivate these developments

are quantitative measurements of flaw geometry, orientation and size and

a decrease in the dependence on operator interpretation. The coupling

of minicomputers to NDE equipment and the use of adaptive and imaging

systems appear to make such hopes real. What are the standards

requirements of such systems? Do such systems need standards? Our

analysis of the problem leads us to suggest that more complex systems

will require more careful characterization and more complex reference

standards. Exactly what these will be is the subject of further

research. However, it appears even at this stage that some of the

necessary standards might not be too different f,rom many that are in

current use.

2
SECTION I INTRODUCTION

The objectives of this study are to examine the present system of

standards for ultrasonic nondestructive evaluation (NDE) measurements,

to assess the standards need of emerging and more quantitative systems

and to present recommendations for the development of an adequate and

integrated system of standards for quantitative ultrasonic NDE.

Although the focus is primarily on pulse echo techniques (as would be

anticipated by their widespread use) we do not neglect other and newer

techniques such as scattering, Fourier transform and imaging systems.

Realizing these objectives has proven to be a difficult task

because of the many uncoordinated standards and calibrations already in

existence, the wide variety of materials, geometries, defect types and

instrumentation. Added to this are the newer more difficult

requirements of quantitative measurements which involve new specialized

techniques. This diversity is compounded by the fact that the quantity

measured in many cases is not a primary quantity such as length but a derived quantity such as reflectivity which is only indirectly related

to (defect) length.

Since meeting the objectives of this study entailed a greater

effort than had been anticipated, we have previously provided several

outputs to meet the needs of the sponsor. These outputs are incorporated in Section 4, which contains Key Issues, Recommendations,

and Priorities, in short a plan for attacking the problem.

We wish to stress that this study is not one of hard science. In

many cases recommendations have had to be based on partial information,

opinions and even educated guesses. Hence, it is not surprising that a number of our recommendations are aimed at developing a better technical

basis for assessing what needs to be done to establish a proper basis

for ultrasonic NDE standards. Finally, it should be emphasized that

this report does not represent our final word in this area. It is hoped

that the comments to be received concerning this work will be used to

update the analysis and findings.

In scope, this report includes a section on various input data

including a history of the problem, a brief discussion of the Ultrasonic

NDE Standards Workshop, discussed in detail in Appendix A, and brief

mention of a review of literature references and documentary standards

which are published separately. However, this by no means represents

the total input to this study which also included numerous conversations

with many workers in the field and the wide experience of many members

of the NBS staff. In Section 3, we deal with the role of standards in

general and with many questions relating to ultrasonic NDE standards in

particular. A new framework for assessing ultrasonic NDE standards 1

1 Developed primarily by N. Hsu.

3
is presented, the question of transducer characterization is examined

and the standards implications of new techniques are considered. Also

included in this section is a discussion of how one might systematically

examine proposed ultrasonic reference blocks for use as possible

standards. In order to gain additional perspective from hands-on

experience with reference blocks, a qualitative study conducted

primarily by N. Hsu and W. Sachse was completed on some novel blocks;

this is reported in Appendix B. Section 4 gives a plan of action,

namely the recommendations for research and their priorities. This

section is introduced by a discussion of key issues which constitutes

the rationale for the plan.

Work not included in this document but considered to be part of

this study includes:

o A Comparison of American and European Ultrasonic Testing

Standards, S. Golan, National Bureau of Standards Interagency

Report, NBSIR 79-1790.

o Ultrasonic Transducers for Materials Testing and Their

Characterization, W. Sachse, N. Hsu, Physical Acoustics, Vol.

14.

o Summary of the Literature on Ultrasonic NDE Standards, D. G.

Eitzen and G. Birnbaum, National Bureau of Standards

Interagency Report, to be published.

These documents grew so large in scope that they required separate

publication in their own right.

A laboratory study on new block concepts was initiated to provide

hands-on experience and to assist in the evaluation of block concepts.

The study was supported by NBS when it was recognized as a beneficial

adjunct to this DARPA project.

This report and the reports cited above are intended to be resource

documents in the area of ultrasonic measurements and particularly in

standards and calibrations. In addition, this report, particularly

Section 4, Output, is meant to provide a basis for planning a unified

approach to the realization of a system of ultrasonic NDE standards.

In the NDE field there are certain terms relating to

standardization, which are used with lack of precision - calibration,

standard, reference standard, etc. The problem of definitions is a serious one. For example, the ASTM glossary on NDE contains about

fifteen definitions of the term "calibration". Our first recommendation

in this report is that the problem of word usage and definitions be

studied and resolved. Organizations such as ASNT and ASTM would seem to

be most stilted for such a task. In the body of this report, we have

used such terms as calibration, standard and product standard with

4
approximately the following meanings: calibration - to compare or adjust instrument readings in accordance with recognized units of

measure, often artifacts and procedures are required to accomplish this;

standard - an artifact, standard procedure or document or a generic term

which may include the calibration process; product standard - an actual

part with a known defect. In addition, the term defect is used

interchangeably with discontinuity although they are clearly not

synonoraous. In any case it is hoped that the context will make

reasonably clear our meaning for such terms.

The major contributors to the overall program are G. Birnbaum, D.

Eitzen, S. Golan, N. Hsu and W. Sachse. However, the responsibility for

this report and the opinions and prejudices therein are due to G.

Birnbaum and D. Eitzen. These authors wish to thank H. Berger for his

continual interest and enthusiastic support of this effort. We also

thank Dr. L. Mordfin for his many helpful comments on this manuscript.

One of us (G.B.) thanks M. Buckley for several stimulating conversations

in which a number of the issues were debated. It is Dr. Buckley who

encouraged the writing of a "gutsy" report and it may not be too much to

hope that this report in some sense meets this challenge.

5
SECTION II

INPUT DATA

2.1. HISTORICAL BACKGROUND

Almost as soon as commercial pulse-echo ultrasonic flaw detection

instruments were available in this country (about 1944), the need for

suitable calibration methods for the ultrasonic measurement system was

recognized. The early uses of ultrasonics were qualitative and the need

for a calibration method was perceived as one for equipment checkout and

for reproducibility of test at a^ site. Many organizations had

responded to this need primarily with internal organizational

solutions. In this country reference blocks with different sized

artificial defects and with the same distance between the ultrasonic

entry surface and the artificial reflector (constant metal travel

distance) were produced by organizations including Sperry, Alcoa,

Grumman and Ultrasonic Testing and Research Labs [l] 2 . These blocks

were primarily used for longitudinal wave testing. Reference blocks for

angle beam and shear wave testing have also been produced. These

include:

o Hemispherical calibration block

o Sulzer calibration block

o Rectangular calibration block

o Aluminum prototype International Institute of Welding (IIW)

block

o CERI calibration block

o British Standards calibration block (BS2704, 1956)

o Rompas block

o IIW calibration block (Dutch block)

Of these blocks the IIW block, evaluated by Commission V of IIW from

1955 to 1959 [2] is probably the most widely accepted for angle beam

testing. Some of these blocks were also designed for use in

longitudinal contact testing. All of the blocks discussed so far were

designed for checking some characteristics of the measurement system and

for setting sensitivity. They were not designed as defect standards and

should not be used to estimate the magnitude of defects [3]

In the 1950's, reference blocks with different metal travel

distances were being produced by several organizations and these blocks

were employed as comparison standards for the acceptance of parts or stock and for a coarse estimate of flaw size. Most organizations

producing these distance/amplitude type reference blocks had agreed on

the use of flat bottomed holes as the reflector for producing a return

2 Figures in brackets indicate the literature references at the end of

the paper.

6
echo for comparison with an actual defect in flaw estimation. Flat

bottomed holes were used because of the "known" area of the reflector

and because the flat reflector was judged more closely to resemble

several types of metallurgical discontinuities. There are, however,

some organizations who still use the cylindrical surface of a side

drilled hole as the reference reflector.

In this country the need for standardizing the systems of reference

blocks being used was recognized by portions of the airframe industry,

DoD, and metals suppliers and in 1951 a task group was set up within

ASTM to standardize the reference system. The need for a primary

reproducible industry-wide standard with which to intercompare different

reference blocks was recognized. By 1958 ball bearings were accepted by

the ASTM task group as a primary standard. Also the details of block

geometry and material for the flat bottomed hole type secondary

reference standards and block evaluation procedure were agreed upon.

The critical part of this evaluation of the secondary reference

standards involves the comparison of the amplitude of the signal

reflected from a specific ball bearing immersed in water with the

amplitude of the signal reflected from the flat bottomed hole in the

immersed reference blocks.

A recommended practice for the fabrication and evaluation of

aluminum ultrasonic reference blocks was published by ASTM (127-58T) [4]

in 1958. In the late 1960's, it became clear to the ASTM committee that

this practice was no longer appropriate or even capable of being

accomplished [5]. Newly fabricated sets of reference blocks did not

behave in the required manner and the differences in response between

nominally identical references were unacceptable. 3 Part of the problem

was thought to be due to changes in the specification of the aluminum

alloy selected. A new revision of the recommended practice was published in 1975 as E127-75 [7]. In the revision the evaluation of the

reference blocks was made much less sensitive by examining the

relationship of the blocks to the ball reflectors at 5 MHz rather than

the previously specified 15 MHz. Also, the tolerance on the allowable

amplitude of response from the reference blocks was increased to _+ 25

percent rather than the previously specified _+ 10 percent. This ASTM

development of aluminum reference blocks in many respects shaped the

development of steel reference blocks embodied in ASTM recommended

practice E428-71 [8]. However, there is no primary standard for the

steel reference blocks and no method for evaluating their ultrasonic

response. Blocks from many other materials are made to the dimensions

of E428-71

.

Effort on the development of flat bottomed hole type reference

blocks has been paralleled abroad. For example, the AQD Labs, (then AID

Labs.) in the United Kingdom began a block development program in

3 Data taken at NBS shows 40 percent differences to be common and that one set differs from the average by 700 percent [6].

7
1954. A set of primary reference blocks were selected in 1962. The

U.K. block geometry and material is somewhat similar to that in ASTM

E127-58 but one particular set is defined as the primary set. Secondary

and working sets are assigned correction factors with an uncertainty of

about + 12 percent by direct comparison with the primary set [9].

The following statement made in 1962 is still true today: "While

it [is] admitted that a number of ultrasonic standards are in use today, none of these has general application; standards either apply to some specific product or material, or they are issued by a particular body or inspection authority and are limited in application" [10].

Despite this bleak picture on the status of ultrasonic NDE

standards, a number of encouraging signs have appeared on the horizon.

With some strong encouragement from the Air Force Materials Lab and with

growing internal awareness, NBS began a program in ultrasonic NDE

standards in 1973 which was later formalized and coordinated through the

Office of NDE. The point here worthy of note is the program focus on NDE standards. Strong attempts were made to get input and coordinate

this standards effort through meetings such as the NBS NDE Public Review

and Workshop in 1974 and the Symposium on NDT Standards sponsored by

NBS, ASTM and ASNT in 1976.

Also, during this general time frame increased interest in

ultrasonic testing at many of the R and D laboratories was apparent;

some of this effort was also devoted to work on standards. By way of

example, the large ARPA/AFML effort at Rockwell International, the work

at NRL and the work at Battelle and SWRI have had identifiable

components relevant to standardization of ultrasonic NDE. It is of

interest to note that in the last two examples, some of the effort

focused not on reference blocks but on transducers. This broadening of

the attention of standardization of ultrasonic testing to areas in

addition to reference blocks is particularly evident in some of the

recent ASTM activity. A comprehensive effort has been launched to

improve the overall standardization of ultrasonic systems,

standardization of electronic subcomponents and transducers and improved

testing practices. The test block effort is still vigorous and there is an active program within ASTM and elsewhere to reduce variability

[11]. This interest in test blocks is certainly reflected in this

report; see Sections 3.2, 3.3, 4.1.4, and Appendix B.

We note that the parallel standardization efforts (e.g. American

Institute of Ultrasound in Medicine) for medical ultrasonic applications

have to a large measure shown a similar broadening to include

transducers and electronic subcomponents. It seems evident that

interactions between these parallel groups on progress in measurement

and standardization efforts could mutually benefit both communities.

Although we do not deal with standards and practices in medical

applications, it is emphasized that interactions between the medical and

NDE communities should be implemented.

8
Finally, at the suggestion of DARPA, this study was undertaken to

take a look at the standards for ultrasonic NDE measurements and to

present a course of action for the development of an adequate system of

standards and calibrations 4 . However, it is interesting to note that

the need for this study was further reinforced by the range of opinions

and diversity of needs which came out of the Workshop on Ultrasonic NDE

Standards conducted as a part of this project.

2.2. WORKSHOP ON ULTRASONIC NDE STANDARDS, OCTOBER 1977

To meet the objectives of this study, it was necessary to promote

first hand exchange of the most current information on ultrasonic NDE

standards. To this end a Workshop on Ultrasonic Nondestructive

Evaluation Standards: Current Needs and Future Directions was held

October 17-18, 1977 at the National Bureau of Standards, Gaithersburg,

Maryland. A number of individuals experienced in various aspects of

ultrasonic testing and research and representing industry, government

and the academic community participated in the workshop. One of the

experts noted that "If this group (workshop attendees) cannot identify

what should be done [about ultrasonic NDE standards] , then there is not

another group in the United States that can." The workshop provided a forum for the discussion of the status of current ultrasonic NDE

standards, ultrasonic NDE needs in industry and government and recent

developments in ultrasonic NDE and their implications for standards.

A detailed summary of this workshop, the program and list of

participants are given in Appendix A. In order to encourage free

discussion, it was stated that the authors of the comments would not be

identified. Some of the most significant points discussed and debated

at the workshop form the basis for many of the key issues in Section

4.1. In addition, the proceedings of the workshop are reflected in the

recommendations 4.2 and had significant impact on various other aspects

of this report.

2.3. DOCUMENTARY STANDARDS

Documentary standards were reviewed under this effort in some detail in a separate publication by S. Golan [12]. We expect this

review to be most useful to people interested in developing or changing

documentary standards rather than to the general users of such

standards

.

In this review twenty-seven general ultrasonic standards from nine

industrialized countries and two international organizations, ISO

(International Organization for Standardization) and IIW (International

Institute for Welding), are summarized and evaluated. The standards

which are reviewed cover the following areas: characterization and

evaluation of ultrasonic systems and transducers, standard reference

4 This work was supported by Advanced Research Projects Agency (ARPA

Order Number 3374) and the National Bureau of Standards.

9
blocks and descriptions of methods and procedures. The standards were

evaluated according to the following criteria: simplicity, frequency of

reference in specific product standards and amount and quality of

information. In the summary of the review a discussion of the

"universal" approach versus the "specific" product oriented approach is

given.

2.4. LITERATURE REFERENCES

As resource material for this and future investigations aimed at

developing and improving ultrasonic NDE standards, an examination of the

literature was conducted. The scope of the survey includes journal

references and internal reports on ultrasonic NDE standards, procedures,

calibrations and reference blocks from the 50's to near the present.

Abstracts from all the citations were obtained. Although the collection

is not exhaustive, it grew so large that it is published under separate

cover as "Summary of the Literature on Ultrasonic NDE Standards," by D.

Eitzen and G. Birnbaum [13]. The citations on transducers are to be

found in the article "Ultrasonic Transducers for Materials Testing and

Their Characterization" by W. Sachse and N. Hsu [14]

.

10
SECTION III

ANALYSIS AND PERSPECTIVE

3.1. THE ROLE OF STANDARDS

We present in this section a brief description of the role of

standards in general and more particularly as applied to (ultrasonic)

NDE.

3.1.1. General nature of standards

What is the role of standardization in a measurement? One of the

first requirements is to provide a scale or a metric which defines the

unit of the quantity being measured. In many cases the standard is

provided by an artifact such as the kilogram mass or the early meter

bar. More recently the latter has been replaced by an agreed-upon

wavelength definition of the meter which can be experimentally realized

by following a specified procedure to gain access to this unit of

length. By itself the use of an appropriate standard is not sufficient

to give a reproducible measurement. A procedure for using the standard

and a measurement apparatus is needed to give a signal which is

characteristic of the quantity being measured. If in addition a proper

measurement procedure is followed, then different people at various

times and places can realize similar results in measuring the same

quantity.

The use of an unambiguous unit for length, weight, time, etc.

implies that there be a unique and agreed-upon standard which has been

defined and universally adopted. Then, characteristically it is the

responsibility of one organization in a country to develop, disseminate

and maintain such standards. In the dissemination of the standard,

usually in the form of an artifact (i.e., standard mass), one recognizes

the role of primary and secondary standards. The former is unique and

the latter are carefully-made reproductions which are usually compared

directly with the primary standard and are more widely disseminated.

However, the hierarchy does not stop here. For more routine work,

tertiary or even working or floor standards are compared indirectly with

the primary standard. The relation of the standard actually used to the

primary standard is embodied in the concept of traceability [14a]. The

requirements for traceability are not defined by the standards

organization but by the organization requiring the measurements.

Theory plays an important role in the measurement process. The

theory provides a link between the observed signal and the quantity

being measured. It also plays a crucial role in extrapolating a calibration procedure from given experimental conditions to conditions

that are somewhat different. Another use of theory is to relate the

measured quantity to the fundamental units (e.g., basic SI units).
Also, theory is used to define derived units and standards from the

fundamental quantities, i.e., force from mass, length and time.

It is apparent that the measurement process must be based upon

standards, procedures and theory. Furthermore, the successful

realization of a standard rests on a rational basis for the selection of

a concept and physical embodiment. Having developed a standard an organization for maintaining and disseminating it are needed.

3.1.2. Ultrasonic NDE standards

Nondestructive evaluation using ultrasonic techniques is clearly

much more complex, for example, than mass measurements. What quantity

is being measured in NDE? It certainly isn't defect size, although the

information may be used to infer something about defect size. Strictly

speaking, one appears to measure "ultrasonic reflectivity" i.e., return

energy divided by incident energy. However, even this quantity is not

being measured absolutely since the incident energy is not determined;

rather the return signal is compared with the signal from some more or

less arbitrary reflector. Therefore, ultrasonic measurement systems can

be used at present only in a strictly comparative sense. Even in the

most sophisticated "imaging" system, the output is by no means an absolute image of the defect. The relationship between object and image

size and shape is neither direct nor particularly clear. The

measurement is not a direct dimensional measurement but a measurement of

variations in acoustic impedance and consequently must rely heavily on standards for reliable qualitative and particularly quantitative

interpretation.

Because of the fact that ultrasonic NDE is a comparative

measurement, there is no connection to the fundamental units.

Consequently, the standards that have arisen are in many respects

arbitrary and in fact based on practical considerations. Thus NDE

measurements made with an apparatus calibrated with one set of standards

are not necessarily comparable to measurements made with the same apparatus calibrated with a different type of standard. While

unnecessary diversity should be minimized, there are nonetheless reasons

for the proliferation of standards due to the very wide variety of test

objects and applications in ultrasonic NDE.

Although we can imagine the possibility of calibrating an apparatus

with a limited set of standards, additional standards for a specific NDE

test may be required to ascertain that a given defect can be reliably

detected. Consequently, a wide variety of reference blocks and product

standards (actual parts with a real or artificial defect) have

appeared. It should be noted that some reference blocks have been

designed so that to some degree the reference defect resembles an actual

flaw and the blocks can be used for both calibration of the apparatus

and "defect sizing." Even with such reference blocks, it seems that

product standards will still see great use since presently there is no

12
theory enabling one to calculate flaw size from an arbitrary signal

knowing the signal from a simple artificial standard.

A basic difficulty in ultrasonic NDE is that one measures one or several characteristics of a very complex signal which is affected by

material properties and geometric reflectors around the defect in

addition to the size, shape and orientation of the defect. Since from

the analysis of such signals one would like to deduce the actual size,

shape and orientation of the defect, concern about uniqueness of

interpretation is warranted. Even with multiple signals such as are realized with imaging, scattering and spectral techniques, the relation

between the signals and the defect is so complex that product standards

will likely see continued use. Presently, however, the relationships

between product standards and the more basic standards are obscure. It

would appear that the measurement process yould be strengthened by

providing a link between the product standard and more basic ultrasonic

standards

In sum, in ultrasonic NDE one measures relative ultrasonic

reflectivity. The rather obscure relationship of this to the absolute

dimensions and properties of the defect dictates that the measurement

process make use of standards not only for adjusting the performance of

the apparatus but also for establishing sensitivity and detectability

criteria. Product standards often find use in establishing this

sensitivity and detectability criteria.

3.1.3 Ultrasonic NDE standards organizations in the U . S.

In the United States, ultrasonic NDE standards are arrived at

generally by a consensus process rather than being issued by a central

quasi-government organization as is often the practice abroad.

Organizations in the U.S. such as ASTM, ASNT, and ASME issue standards

documents which describe in detail measurement and calibration

procedures and the use of artifacts, e.g., ultrasonic reference

blocks. These organizations generally do not undertake research and

development on new reference blocks but develop and document a consensus

of good practice based on the experience of the participants and from

the available literature.

Although these standards documents formalize good current practice,

for many applications current practice is insufficient to meet the

demands of quantitative NDE. These demands, as embodied by imaging,

scattering or spectral methods have scarcely been examined with regard

to standards requirements and calibration procedures. Indeed, it is one

of the objectives of this study to focus attention on such needs.

It should be noted that the National Bureau of Standards does not

issue ultrasonic NDE standards documents. It does provide research and

development results in the area of measurements and calibrations and

13
when warranted offers calibration services. It also contributes to consensus standards documents and provides Standard Reference Materials.

How do the roles of NBS and the consensus standards organizations

differ in the area of standards and calibrations? The ASTM procedures

provide, for example, a way for the operator to adjust and use an apparatus employed for NDE inspection. The role of NBS, however, is

characteristically to provide a method for transferring a primary

standard or measurement method from NBS to the user. This is

accomplished through mechanisms such as calibration services or published methods. These procedures are often as different in detail as they are in intent.

When NBS develops a new method or standard which has potential

impact on current practice it, as other laboratories, must bring the

information to the consensus standards organizations for their action

which ultimately is realized in the form of a standards document. Of

course, NBS is not the only organization that brings new developments to

the field of standards and calibrations. In the area of NDE, there are many accepted and proposed procedures that have come from many

laboratories. However, regardless of the origin, meritorious concept is

not enough; a thorough and rigorous time consuming development is

required to go from a concept to a field-worthy standard.

3.2 FRAMEWORK FOR ASSESSING ULTRASONIC NDE STANDARDS

In order to assess the many approaches that have been developed and

the many more that may be proposed for ultrasonic NDE standards, a framework for dealing with these approaches is first established. In

this framework, the nature of an ultrasonic flaw detection system will

first be analyzed; various specific needs which standards must satisfy

will then be discussed; a set of criteria for evaluating possible

standards will be compiled. With this framework, a systematic search of

available alternatives to satisfy the desired usage of the ultrasonic

NDE standards can begin and the alternatives can be evaluated

objectively for final recommendations.

3.2.1. Nature of an ultrasonic flaw detection system

Ultrasonic flaw detection systems may be classified according to

the nature of the interaction between the ultrasonic waves and defects

in the piece under inspection. These include reflection (pulse-echo),

through transmission, scattering, interference and resonance. With the

exception of systems based upon resonance, ultrasonic flaw detection is

a point by point measurement which relies on either scanning manually or automatically, or array type multiple probes to inspect a test object.

Recently developed minicomputer based systems not only control the

automatic scanning, refine the received signal processing and display,

but also use multiple probes to collect the combined information of the

interaction between ultrasonic waves and the defects. However, the

14
basic functions are still the same and the basic interactions are still

reflection, through transmission, scattering, etc.

Although systems for NDE based upon scattering techniques or interferometric techniques, such as acoustical holography, are being

developed, pulse-echo systems are by far in widest practical use. In

most common pulse-echo ultrasonic flaw detection systems, reflections

(echoes) from impedance mismatches produce indications on an oscilloscope screen in addition to the expected back-reflection

pulses. The horizontal locations of the pulses on the screen represent

the arrival times which in turn indicate the travel distances between

indications and the probe; the heights (amplitudes) of the defect pulse

indicate the relative strength of the ultrasonic reflections. Since

these indications are sensitive to instrumental conditions and settings,

transducer characteristics and ultrasonic properties of the test object,

calibration is necessary. Furthermore, because these indications have

fewer parameters (arrival time and amplitude of the pulse-echo) than the

parameters required to specify defects (location, size, shape,

orientation) their quantitative interpretation is "very difficult."

Recent developments of new techniques may alleviate some of the

difficulties of interpretation by refining the use of the above

parameters and making use of more parameters. At the same time, these

new techniques may make additional demands on system performance and

make control through calibration even more crucial. For instance,

instead of using merely the reflected pulse height as the only indicator

of defect size, the entire reflected pulse wave form can be analyzed.

In addition to the direct reflected echo, scattered fields can also be

measured with multiple transducers. The wave form of the reflected and

scattered pulses and their frequency spectra contain more information

that can be used to indicate the characteristics of a defect. However,

a computer based system may be required to analyze the additional

data. In order to make these refined techniques reproducible and

reliable, many key elements of the system especially the transducers and

the pulser/receiver circuits must be well characterized and properly

calibrated.

3.2.2. Uses of a standard^

According to the terminology agreed upon by ASTM [14b], the word

"standard" has two defined meanings:

a. a reference used as a basis for comparison or calibration;

b. a concept that has been established by authority, custom, or agreement to serve as a model or rule in the measurement of a quantity or the establishment of a practice or a procedure.

^See also Section 3.1.

15
While the second meaning usually establishes various documents as "standards," the first meaning clearly defines the "usage" of the

standards, i.e., a standard is used as a basis for comparison or calibration and some existing standards have served well in both

roles

Specifically, in ultrasonic flaw detection, one often uses a block

or a set of blocks for the following purposes:

o to check overall equipment performance,

o to check transducer performance,

o to determine the inspection sensitivity level and its

limitation on resolution,

o to adjust the instrument settings for range,

o to set accept-reject criteria,

o to make system performance evaluation possible in the sense

that test results are reproducible from day to day, article to

article, location to location, instrument to instrument and

laboratory to laboratory.

o to simulate to some degree the geometry of the anticipated

defect

,

o to simulate to some degree the geometry of the part under

inspection.

Some of these purposes, particularly the last two, are also served

by the use of product standards.

Numerous reference blocks have been developed which are intended to

fulfill some of the needs listed above. Because there are different

test articles together with various techniques, and instruments and

probes, numerous reference blocks have been developed. To some degree

the optimum choice of a reference block depends upon the intended usage

so that the question of which block is best can be answered completely

only when this use is fully defined. Nonetheless, some blocks possess

greater versatility than others; see [12]. It may be interesting to

note that it was the search for a versatile and simple "standard

reference" that prompted the adoption of the flat bottomed hole (FBH)

[1]. It is worth emphasizing that the intended use of a block,

particularly a proposed block, should be carefully specified since

blocks are frequently most useful for rather specific applications.

3.2.3. Criteria for evaluating possible standards

Given that there are candidate blocks which meet given requirements

it is necessary to evaluate their relative potential. The following is

a list of criteria which may be useful in evaluating various existing

and proposed ultrasonic standards. For specific applications, other

points may have to be considered in addition to the criteria listed

below. The items are not rank ordered since their importance may depend

on the specific requirements.

16
a. Functional adequacy. Will the standards reliably and

adequately accomplish their intended use?

b. Reproducibility of test results. Can the test results be

reproduced not only within a given organization but also among

different laboratories?

c. Ease of duplication. Can the standards be fabricated at

different laboratories and at a reasonable cost?

d. Ultimate cost of standardization. How much will it cost the users to implement an adequate standardization program? Will

such a program be cost effective?

e. Versatility in providing simulated situations. How many

reference standards are needed to carry out calibration of

various instruments for different test objects?

f. Correlation with real defects. Can real defects be

quantitatively compared with the reference standards?

g. Availability of independent examinations. Are there

independent techniques (such as radiographic techniques) to

verify the standards?

h. Compatibility with existing standards. Is the proposed

standard compatible with or traceable to existing standards?

i. Availability of a theoretical basis. Is there a theoretical

model to establish the standards and to verify and extrapolate

the results?

j. Ease of automation. How readily can the standard system be

automated to reduce reliance on operator performance?

k. Ease of implementation. How easy is it for an operator to use

the standard?

l. Ease of modification to satisfy future needs. How readily can

the standard be adapted to meet future requirements?

3.3. AN APPROACH TO A SYSTEMATIC EXAMINATION OF ULTRASONIC REFERENCE

BLOCKS

3.3.1. Introduction

Numerous reference blocks have been used in NDE for the calibration

of ultrasonic flaw detection systems. These blocks, varying in shape

17 i
and material, are used primarily as a field standard but also as a laboratory standard to check the ultrasonic equipment performance and to

make test results reproducible. In the United States, sets of blocks

with flat bottom holes (FBH) are recommended by ASTM as a general

purpose reference standard for use in, for example: (a) checking

performance of ultrasonic testing equipment, and (b) standardization and

control of ultrasonic tests of metallic products using pulsed

longitudinal waves introduced into test materials either by the direct

contact method or by the immersion method.

Several criticisms have been raised regarding the FBH standard.

The most prevalent is of the variability of these blocks; however,

recent developments provide a mechanism for solving these problems

[11,15]. Another criticism regards their suitability for providing an accept-reject criteria. A further criticism is that they are difficult

to apply outside of the area of qualitative 'inspection and process

control. Finally, it has been stated that the back scattered field of

the FBH lacks a theoretical basis. However, this is clearly not the

case since the short wave length limit for back scattering has been

known for a long time. Moreover, recent theoretical work deals with a method for improving the earlier results by taking into account the

diffraction correction due to the edges [16].

Many new blocks have been proposed and while it is possible to

evaluate each proposed design separately, one must realize that the

selection process is very difficult because of the complex requirements

and infinite variations in block design. To this end, instead of

comparing and evaluating various existing and proposed block designs, we

systematically review generic design parameters and examine how these

parameters effect the block's function. A very limited experimental

study was conducted to supplement our conceptual analysis; see Appendix

B.

3.3.2. Analysis of block design

Broadly speaking, a simple block consists of three generic

components: material, reflector geometry, outer geometry. Perhaps the

simplest example embodying these generic ingredients is a rectangular

aluminum block in which the reflector corresponds to the outside

geometry, namely a back planar surface. At the other end of the scale

of complexity is, perhaps, a landing gear which has a very complicated

outer geometry and contains a fatigue crack as a reflector.

Another example of complexity is compound blocks made of more than

one material and having multiple facets as reflectors. An example is

the IIW block which is made of plastic and metal and has as its

reflector a circular curved surface, a side drilled hole, a rectangular

notch, etc; see [12].

18
In what follows, we separately analyze the three Ingredients of a block and study possible options and implications.

Material

Although an obvious candidate for a block material is that of the

test article, there are at least two reasons for considering another

choice. First, an ultrasonic flaw detection system may be used for a variety of materials; it may not be practical to use a different

standard for each material. Secondly, the variations of ultrasonic

properties (wave speed, acoustic impedance and attenuation

characteristics) among test articles made of nominally the same material

may be large and the selection of one particular material may still

deviate somewhat from the actual test articles. Consequently, the

choice of materials for a standard could be based upon the following

factors:

a. availability,

b. uniformity of acoustic properties,

c. stability of acoustic properties,

d. similarity (transferability; can performance of the ultrasonic

system be inferred by calibrating on the standard block of

different material?)

e. special considerations.

Table 1 lists some possible materials. Note that liquids such as water and viscous fluids are candidates and water has been used in the

calibration process, although no one has suggested literally using water

as a possible block. However, it has many advantages:

a. easily available, stable and uniform;

b. propagating path length in this medium can be changed easily;

c. various geometric reflectors can be implemented as "solid

inclusions";

d. field observation can be implemented - beam profile can be

easily obtained.

A 19
Table 1* Comparison of materials for possible standards.

. Special

Material Nonuniformity Stability Considerations

water none yes transparent

viscous

fluid none some are not transparent

aluminum texture, composition,

grain size some alloys

age harden

steel texture, composition,

grain size

less age

hardening

than A1

medium

attenuation

titanium texture, composition,

grain size, and

secondary phase

less age

hardening

than A1

medium

attenuation

stainless

steel

texture, composition,

grain size

less age

hardening

than A1

medium

attenuation

acrylic batch to batch control of

moisture in

fabrication

High

attenuation,

transparent

glass none yes transparent

,

low attenuation

ceramics depends on type yes transparent

in ir region

The disadvantage of using a liquid is that for many flaw detection

problems there is a lack of similarity between the liquid and the test

article. Furthermore, liquids generally cannot be used to simulate

shear wave techniques. -Finally, the effect of transducer coupling to a solid is not simulated by a liquid test object. However, an important

advantage of a liquid as a "block material" is that it permits direct

inspection of the "flaw" and a direct visualization or mapping of the

transducer field.

The acoustic field in glasses and acrylics can also be probed by

making use of photoelastic visualization techniques [17]. Although,

ceramics are opaque they are also transparent in selected portions of

20
the infrared region where the photoelastic effect could be used for

visualization. It is interesting to note that in such materials a point

by point determination of the internal field has not been realized but

may be possible. See [18] for a description of such a realization in

the transparent case.

The selection of materials also points out several existing

problems of the ultrasonic flaw detection system.

i. How does a transducer behave when it is coupled to different

solids? Is it possible to infer the transducer's behavior from

one solid to another?

ii. How can the ultrasonic properties of a solid be

characterized? If these properties are properly measured, will

they allow one to answer question i?

In principle there are no scientific problems in characterizing the

ultrasonic properties of the commonly used structural materials. It

would be possible to develop a data base on velocity, attenuation and

impedance as a function of frequency for these materials. However,

given knowledge of these material properties, couplant and transducer

characteristics, can the calibration of a flaw detection system be

transferred from one material to another. This would seem to be

possible particularly if use is made of current theories. However,

currently available data and current practice still require direct

calibration of the system on a reference made of the same material as

the test item.

Different reflector geometries.

A reflector provides a reference signal for adjusting ultrasonic

systems and also, depending on the objectives of the test, for "flaw

characterization." But, because of the complexity of real flaws, the

reflector in a reference block can only partially simulate a real

flaw. In present practice reflectors are often used to provide a reference for arrival time to indicate flaw depth and a reference for

reflection strength to "indicate" flaw size. It is also important to

obtain information on flaw orientation and type. It is possible that

this additional information may be obtained from the analysis of the

scattered field and spectral response. Imaging is an approach that may

provide all this information but nevertheless will have to rely on reference blocks to some extent.

Reflector geometries that are in use or have been proposed include

planar and curved surfaces, steps, notches, FBH reflectors, cylindrical

reflectors, spheres, ellipsoids and disks, many of these both as voids

or inclusions. The choice of reflector geometry is a subject of some

controversy. Although this matter will not be settled here, we note

some of the current views. Spheres and side drilled holes are said to

21
more closely represent the bulk of the defects found in welds. However,

planar reflectors such as an FBH are said to better represent the more

critical weld flaw - a crack. The scattering spheres and ellipsoids

have a theoretical basis, although theories are also available for discs

and cylinders. Side drilled holes are preferred by some because they

are said to provide a reference not only for longitudinal but also for

angle beam testing. The FBH is said to well represent lamellar defects,

a more common defect in plate material. Spheres also seem to represent

well the small voids which may occur in ceramics. Notches of various

types (V, U, EDM) are often used to represent surface cracks.

A somewhat different and exciting approach in artificial reflectors

is the use of manufactured fatigue cracks [19]. Obviously, such a reference block would offer simulation advantages over other reflectors , for many tests but is more difficult to manufacture and to theoretically

describe.

Outer geometry.

The outer geometry: provides for various metal distances between

the transducer and the reflector, permits varying the angle of

incidence, defines the sound path to the reflector (e.g. the reflector

may be accessible by longitudinal beams but not angle beams), and

simulates the geometry of the test item. In practice a variety of outer

geometries have been proposed or are in use.

3.3.3. An example of the critique of a proposed standard

An important part of the analysis of the improvement of the system

of ultrasonic standards is the critique of current and proposed

standards. Here we give an example of the type of analysis that should

go into the evaluation of ultrasonic standards. The example may be more

important in the demonstration of the methodology than in the particular

conclusions reached. The example chosen is the proposed use of the

scattered field from a spherical void in a metallic block used in a goniometer (see, for example, [19A]). This choice was made because this

proposal has benefited from a large theoretical and experimental

effort. This analysis may change, however, on the basis of additional

considerations or developments.

The standard we consider here consists of:

o A diffusion-bonded titanium polyhedron or sphere with a spherical cavity as the reference reflector. The two halves

of the block each containing a hemispherical cavity are diffusion bonded together.

o A calibration scheme based on: (a) procedure derived from

radar antenna calibrations which relates the received signal

to the transmitted signal by an equation which is a product of

22
the transfer functions of the transmitter, receiver,

transmission media, transducer and scatterer; and (b) a theoretical description of the scattered field of a spherical

reflector. It has been proposed that if the media

characteristics are known, use of the theoretical field will

give the transfer function for the transducer.

Intended uses.

Although this approach has been presented as a more or less general

one intended to replace, for example, flat-bottomed-hole blocks, the

specifics of its intended use have not been made completely clear.

Nonetheless, we here interpret intended uses in terms of published

references [19a-19c] and personal communications. By reference or implication we also perceive that the intended application is for

general systems, at least including scattering and pulse echo systems,

a. to check overall equipment performance - yes,

b. to check transducer performance - yes,

c. to determine inspection sensitivity - yes,

d. to adjust instrument for range - not clear,

e. to set accept-reject criteria - not clear,

f. to provide system reproducibility - yes,

g. to simulate the geometry of the defect - perhaps,

h. to simulate the geometry of the part - no.

Criteria.

Based on the information available, we discuss the proposal in

terms of the criteria for evaluating standards given in Section 3.2.3.

a. Functional adequacy. A spherical reflector may be useful for

pulse-echo systems (see for example ASTM E-127 and [20]).

However, its use in a calibration system which is configured

in a scattering mode with separate transmit and receive

transducers seems most appropriate for test systems also

configured in a scattering mode. In this case the full

benefit of making use of the theoretically predictable changes

in intensity with angle can be affected.

b. Reproducibility of test results. An independent study of one

of the titanium blocks 6 indicated a lack of agreement with

published data. However, this is not surprising since a great

6 W. Sachse, private communication.

23
deal of work is usually required not only on the artifact but

on the measurement procedure and signal interpretation before

even modest reproducibility can be achieved as discussed in

reference [11].

I Ease of duplication. Here, as with any other block, material

uniformity is a potential problem, one that can usually be

overcome with either considerable care or through initial

selection. Given a supply of uniform material, it would seem

that the machining of accurate hemispheres and subsequent

diffusion bonding is a capability that is not yet widely

available. It seems that at least at present such a block

would be somewhat more expensive than some other alternatives.

Ultimate cost of standardization. The cost of implementing a total change from currently accepted reference system to any

new block(s) for all the general flaw detection systems would

in fact be rather large. If the change were intended only for

a class of systems (such as scattering systems) the cost would

be considerably less. Therefore, the cost/benefit ratio

depends heavily on the range of systems for which replacements

is considered.

Versatility in providing simulated situations. It would

appear that by making use of the dynamic range of the

scatterer for setting gain or determining linearity,

significantly fewer blocks would be required in the proposed

system than in the FBH system. However, to realize this

advantage, the ratio of the radius of the sphere to the

wavelength of the ultrasound (in the material) must be chosen

properly. Even so, the dynamic range is compressed

significantly if a broad band source is used owing to

averaging over the range of frequencies. A configuration we

can conceive which has the advantage of large dynamic range

and equally applicable to pulse echo systems is a sphere

located asymmetrically in a polygonal plate to give different

metal-travel-distances

.

Correlation with real defects. Generally, artificial defects

do not simulate real flaws of concern for structural

integrity, although some geometries simulate certain types of

defects better than others. For example, in weld inspection

it is generally agreed that spheres (or side-drilled holes)

better simulate the types of discontinuities most often found

such as porosity and inclusions. However, many also agree

that a FBH somewhat more closely resembles the defects of

greatest interest, i.e. cracks. To reiterate, neither

reflector does the simulation job adequately; this again

suggests additional interest in manufactured fatigue cracks

(see Section 4.22 and [19]). For some inspection applications
the sensitivity required and the defects expected seem

strongly to favor spherical or hemispherical reflectors, e.g.

very high sensitivity inspection of ceramics for porosity

[20]. However, for the bulk of ultrasonic flaw measurements

the size sphere required to give the proper signal strength

makes their use cumbersome.

g. Availability of independent examination. Prior to fabrication

by diffusion bonding, the reflector geometry is amenable to

various kinds of inspection. However after fabrication

independent inspection must be made by radiography. This is

in contrast to say side-drilled holes which can be optically

inspected even after completion. This consideration of post

fabrication inspection also suggests the use of transparent

material, see Appendix B.

h. Compatibility with existing standards. Because the proposed

system is based on scattering and the use of two transducers,

it is not automatically compatible with existing standards.

It would appear, however, that it could be made traceable.

With respect to back reflection, spherical reflectors are already compatible with existing standards since spheres are

the basic standard in e.g., ASTM E-127.

i. Availability of a theoretical basis. For the short wavelength

region, i.e., the regime of geometrical optics, a theoretical

basis is available for many reflectors. However, outside this

region the theory for the proposed system has been extensively

developed [21-24, and the references sited therein]. This

theory would make it possible to verify and extrapolate

measurements and also would provide the theoretical base often

desired for a primary standard.

j. Ease of automation. No obvious advantage for automation is

apparent

.

k. Ease of implementation. There seems to be no obvious

advantage to the operator with this system, although it could

result in a less bulky system.

l. Ease of modification to satisfy future needs. Without

specific future requirements in mind, it is difficult to

evaluate this aspect. For imaging systems, for example, the

proposal seems to have no clear advantage* However, for

requirements leading to the use of say elliptical rather than

spherical reflectors, the concepts of the proposed system stay

mainly intact which would be an advantage.

In conclusion, we see no special advantage in replacing current

standards for pulse-echo systems with the goniometer-sphere proposal.

2 5
However, for scattering systems (and possibly for systems using

transform methods) the proposed system seems to offer some clear

advantages over presently accepted systems.

3.4. THE CHARACTERIZATION OF ULTRASONIC TRANSDUCERS FOR NDE

In this section we deal with the standardization problems

associated with transducers in ultrasonic measurement systems. The need

for dealing with the variability of ultrasonic measurement due to

transducer characteristics have been reemphasized frequently in the open

literature; see for example [25-27]. Two basic questions seem to arise:

1. Which characteristics of the transducer must be measured,

controlled or compensated for at a given level of

standardization or in a given application?

2. How can these measurements be made, keeping in mind the many

types of transducers available and under development and the

large number of applications?

In trying to address these questions and arrive at some partial

answers we have relied, in the main, on the extensive reports of Sachse

and Hsu [14], Miller and Eitzen [28] and Smith, Teller and Swanson

[29]. Reference [14] was an outgrowth of this effort and grew to such

large proportions that it warranted separate publication.

Here we briefly sketch the contents of reference [14]. This work,

which is meant to be rather complete, deals with the analysis of a transducer operating as a source and a receiver, reviews the large

variety of transducer types, reviews acousto-electric techniques for

deliberately modifying the transducer in send or receive mode and

discusses transducer characterization. It is clear that quite general

characterization techniques must be developed or one will be forced into

developing many different techniques due to the many transducer types

and applications. Three levels of transducer characterizations are

given:

1. Complete characterization. This procedure which, in principle,

determines all the parameters of a linear, one-dimensional transducer,

fully characterizes it, so that all of the questions pertaining to the

transduction process can be answered. Here the transduction is

absolutely characterized independently of the test medium or generator/receiver electronic circuitry. This involves the measurement

of the frequency-dependent complex functions which comprise the

transduction matrix associated with a transducer.

2. Partial Characterization. When either the electronic

instrumentation or the test media are fixed, the transduction process

can be represented by a transfer function for the transducer acting

either as a source or a receiver. This simplification from a

26

. i i

i
transduction matrix to a transfer function relies on a specification of

the media or electronics associated with the application. A variety of

methods are available or being developed, including the use of a known

source function, the use of a known scatterer and the use of impedance

measurements

.

3. Transducer Checks. In many ultrasonic measurement applications, the

absolute calibration of a transducer (as in 1 and 2 above) may not be

necessary. What is deemed sufficient are methods for checking to see if

the operation is as expected or is unchanged from a previous

calibration. The parameters so determined may include frequency

response, loop sensitivity, electrical impedance and beam

characteristics

.

Various methods for mapping transducer sound fields are described

in a separate section of [14].

In the final pages, reference is made to the serious assumptions

required for the "complete characterization" referred to in 1 above and

the need for correlating measured field parameters with electrical

excitation parameters. /

We have a number of comments regarding the status of

standardization as reflected by this work. Although the "complete

characterization" gives all of the transduction parameters, the

connection to final performance in field applications is not clear.

Certainly, such a complete characterization is necessary for

understanding performance and helpful for design specifications but in

itself knowledge of the transduction matrix is not sufficient to predict

or verify field performance. We note, in particular, that the onedimensional assumption makes the connection to field performance rather

difficult. It seems to us that the "partial characterization" utilizing

a transfer function may be a practical way of specifying transducer

performance which may correlate more easily with performance in NDE

applications. The necessary specification of material or electronics

may not be overly severe since certain commonality in many applications

exists

.

The "transducer checks" involve the measurement of a variety of

acoustic or electrical parameters, many of these making use of

blocks.7 Taken overall, the three levels of characterization described

in [14] represent a considered account which deserves additional study.

Methods for mapping transducer sound fields range from qualitative

techniques for observing the projection of a beam pattern to complete

determinations of the 3-D field by planar scanning methods [14]. With

^For a detailed account of the accepted methods of using blocks see the

associated work by S. Golan [12].

27

I
regard to the latter, it has been shown rigorously that a scan of phase

and amplitude in one plane is sufficient to reconstruct the entire sound

field in a relatively fast and economical way using modern computational

methods. It should be emphasized that a complete characterization of a transducer utilizing the transduction matrix as in (1) above does not

specify the actual field. For many applications the beam shape,

direction, magnitude of side lobes, symmetry and uniformity are all

important but so far must be determined through beam measurements.

Particularly in quantitative measurements the character and detail of

the interrogating energy must be known or verified.

We next summarize the work by Smith, Teller, and Swanson [29]. The

objective of this work was to characterize a number of commercial

ultrasonic transducers of various types and to examine the correlation

of transducer characteristics with their performance in detecting flaws

of several types. The motivation for the work is the wide variation in

the performance of ultrasonic transducers [25,26]. Its objective is the

correlation of the effect of these parameters with the reliability of

ultrasonic inspection procedures for small fatigue cracks. The scope of

the work included the derivation of 17 descriptive transducer parameters

from measured characteristics for 23 transducers. Flaw detection

performance was limited to longitudinal- immersion examination of an FBH

standard, an elox notch and two fatigue cracks and was found to be

highly variable. The results of the investigation showed good

correlation between "loop sensitivity" ® and the ability to detect

flaws. Other parameters which appear to have reasonable correlation

with flaw detectability include center frequency, beamwidth, spectral

shape and side lobes. The authors conclude that prediction of the flaw

detection performance of individual transducers based on measurable

parameters appears to be a reasonable goal.

We agree with the caveats given in the conclusions to this

report: The sample size could be made statistically more valid; the

restriction to longitudinal immersion conditions is an important one;

and the goal of specifying parameters to predict flaw detectability and

the certification of transducers (for some applications) based on the

parameters appears to be reasonable.

However, while it seems that loop sensitivity can be easily

measured and specified, it does not appear that the work yet provides a preliminary criterion for certification of transducers for service.

Although a strong correlation between transducer loop sensitivity and

flaw detection performance was demonstrated, the results may have been

influenced by the use of low frequency transducers which also had low

loop sensitivity. Further work along these lines would be desirable.

If further investigations show that the loop sensitivity parameter is a good predictor of transducer performance for flaw detection, then it

®Loop sensitivity is related to the transducer send/receive sensitivity

but the exact specification depends on the method of measurement.

28
would also be important to determine the more basic quantities on which

this depends. It then may be possible to eliminate redundant transducer

tests and at the same time to indicate to transducer manufacturers the

precise parameters that need to be controlled. The important point

about the study is the emphasis it gives on determining those transducer

characteristics upon which its performance in an NDE detection mode

depend. We regard such specification as essential for improving NDE

performance in an efficient and cost effective manner.

For the more difficult NDE mode of quantitative flaw measurement

(contrasted with the detection mode discussed above) a similar

correlation approach would likely also be helpful. (In this regard

reference [27] presents a limited study demonstrating this.) For a full

correlation of transducer variables with performance for quantitative

NDE the list of variables may grow somewhat* It is reemphasized that

the pragmatic solution obtained by correlation studies should be

supplemented by an understanding of the basic variables in order to

avoid over-specification. Knowledge of these basic variables also

provides the transducer manufacturers with information needed for design

and quality control.

A rather extensive transducer characterization program under way at

NBS [28] deals with, in part, methods for determining absolute total

power versus frequency, time-averaged power and complete beam patterns

(cw) into a water load. This part of the program is nearly complete.

The physical theory applicable to each method provides the basis for

analyzing the measurement uncertainties. In addition to the work

described in [28], work on field methods for scanning transducers [30], on criteria for specifying transducer parameters [27] and possible

methods for measuring fields in solids [17] are being pursued. There is

also considerable interest in developing laboratory measurements for

pulsed excitation and for angle beam transducers. The emphasis has been

on developing primary measurement methods and services but work is also

devoted to the development of field techniques to be carried out by the

user

.

Thus far in the discussion the transducer type has not been

specified. Although the most common transducers in NDE have used

piezoelectric elements there has been a growing use of electromagnetic

acoustic transducers (EMAT). One of the principle advantages of this

transducer is that it is noncontacting and thus eliminates dependence on acoustic coupling efficiency and allows for use in severe environments. There is a dependence on the gap between the coil and

specimen but this can be minimized by proper design. Further, EMATs may

have some practical advantages in generating polarized waves. From the

point of view of standardization, EMATs could play a valuable role in

characterizing other transducers insofar as the measurements are reproducible and the transduction can be modeled. However, an

29
electrostatic transducer would appear to have these advantages to even a greater degree.9

Based on the literature there are many methods of measuring

something about transducers. In many cases, what is being measured and

to what accuracy is often unclear. Also many methods are limited to

water loads and cw excitation. It appears that the basic transducer

principles are clear, although additional theoretical work in some areas

is needed. More important, though, is a more complete determination of

the most crucial performance parameters (possibly a more comprehensive

study of the type in [29]) and, with quantitative NDE in mind, their

accuracy . Once this is known an effective measurement system for

transducers can be established; many of the necessary techniques are

essentially at hand.

3.5. STANDARDS IMPLICATIONS OF NEW ULTRASONIC NDE TECHNIQUES

Our discussions on calibrations and standards thus far have been

mostly independent of whether the measurement system is a conventional

or an emerging one; however, the conventional systems have influenced

our discussion. In this section we emphasize the possible effects of

the emerging systems in terms of standards and calibrations. The

question of standards implications of new techniques are also raised in

sections 4.1.1, 4.1.2 and 4.1.6.

Possibly, the most important developments have been spectrum

analysis, time delay techniques, several types of imaging systems,

adaptive learning networks and special transducers. In order to improve

signal-to-noise ratio and resolution, signal processing using computers

and microprocessors have been introduced into ultrasonic systems.

In addition to the reflectivity phenomena utilized in the

conventional pulse-echo technique, other phenomena of interaction

between the acoustic field and the flaw, such as scattering, diffraction

and interference have been studied. Many basic and experimental studies

have been carried out and various sophisticated laboratory systems have

been developed. There is, however, still a wide gap between the

theoretical and laboratory studies and applied NDE. Many of the new

developments are based upon simplified, idealized models for the flaws

and idealized experimental conditions rather than on real conditions and

flaws which are far more complicated. Unfortunately, the bridge between

model and real conditions can not easily be crossed. It is hard to

predict which of the current developments will turn into practical,

quantitative NDE tools. The ideal ultrasonic technique which will

determine the absolute location, size, orientation, shape and stress

^Current work at NBS makes use of a standard source and an electrostatic

transducer as a receiver to a very high absolute accuracy. Report in

progress

.

30
field of a flaw Is a distant goal and requires significant advances In

the state of the art. To further expect this ideal system to be

independent of calibration and standardization does not seem to be

practical.

Each new development offers additional information on flaw

characterization, but also creates new and more complex problems of

interpretation and standardization. A few examples will demonstrate

what new calibration and standardization problems might develop.

The transducer is the key of each ultrasonic system and its

performance is a primary factor in the overall performance of the

system. It is difficult to produce piezoelectric transducers with

preplanned, reproducible characteristics and the performance

characteristic of each transducer must be individually checked. This is one of the main reasons why no primary source of ultrasonic standards

exists and why a universal standard of comparability is difficult to

establish. In the conventional pulse-echo systems single element

transducers are usually used. Standardization and characterization of

these has not yet satisfactorily been accomplished.

In many of the new systems, multi-element transducer arrays are

used. For these the problem of standardization and characterization is

much more complex. Differences between characteristics of the

individual elements and cross-talk between elements are problems which

do not exist in the production and characterization of single element

transducers. Due to the large number of parameters needed to

characterize particularly complex transducers, it seems that computer

analysis of the data for characterization will be required. Some work

on a minimum data set required for transducer characterization has been

done and measurement systems are being developed [28]. Additional work

in this area is needed.

Imaging systems standards will be needed for overall system

evaluation and for flaw characterization. The parameters which

determine the quality of the image in an imaging ultrasonic system are sensitivity and resolution. To verify those parameters special blocks

are required. An aluminum alloy 1100-F test block designed for checking

sensitivity and lateral resolution of a scanning acoustical holographic

system is described in [31]. These blocks, however, are not very useful

for the interpretation of the reconstructed image. Real flaws are much

more complex in their geometries and scattering properties.

Consequently, for correct interpretation the image should be compared to

reference images obtained with known types of flaws such as cracks,

pores, incomplete weld fusion, etc [32].

Another example which will demonstrate the complexity of

standardization and calibration of the new systems is the use of test

specimens in an adaptive learning network (ALN) . The system is "taught"

to recognize the signature of a specific family of flaws in a determined

31
environment and to estimate the size and orientation of the flaws. The

"teaching" process is accomplished with a set of reference reflectors of

the same family to be tested, introduced in an environment (material and

geometry) identical to the real environment. This will best be

demonstrated by a case example [33]. In order to size fatigue cracks in

fastener holes in aluminum plates, sixteen test specimens were used to

"teach" the network. The specimens were made of the same material, had

the same geometry as the real structure and contained fatigue cracks in

the fastener holes of the type expected in the structure. After the

"teaching" process, the system was "smart" enough to deal with this

specific problem, but for a different type of flaw or a different

environment the network has to be "trained" again using a different set

of test specimens to simulate the new situation. This is a highly

"specific" (product oriented) approach to standardization (see [12]).

Moreover, the process of calibration for the ALN system is more complex

than for a conventional pulse-echo system because multi-parameter

response has to be determined, while in the conventional system the only

parameter to be considered is echo amplitude. Additional parameters

such as time delay, frequency spectrum, power density spectrum,

autocorrelation function and the Influence of any combination of these

have to be considered.

In the last decade attempts have also been made to develop computer

automated ultrasonic inspection systems [34, 35] . These have been

developed to make tests more reproducible and more reliable, to reduce

inspection time and to minimize operator intervention in calibration,

setup and data interpretation. In order to evaluate and standardize the

system special test blocks are utilized. Reference [34] describes Air

Force Materials Laboratory Test Block No. 1 for evaluation of a computer

automated system. Even the use of a sophisticated computerized system

must rely on reference standards to evaluate system performance.

Those examples show that the standardization problems of the new

methods are far more complex than those of the conventional pulse-echo

technique.

32

1

I
SECTION IV

OUTPUTS OF THIS STUDY

4.1. KEY ISSUES

This section contains many of the central questions regarding

improved quantitative ultrasonic NDE and the role of standards. Many of

the issues were drawn from discussions in the Workshop on Ultrasonic

Nondestructive Evaluation Standards held at NBS, October 17-18, 1977,

expressly for this purpose. Others are from conversations with NDE

practitioners and those interested in advancing NDE. The discussion of

the issues forms a basis for guiding our recommendations for further

effort on improved ultrasonic NDE; these recommendations are presented

in section 4.2.

4.1.1. What are the standards requirements for quantitative NDE?

One can, in general, distinguish between two NDE functions: a qualitative and a quantitative assessment of material or structural

defects. In quantitative NDE one tries to provide information needed to

determine the absolute integrity of the piece. In order to use the tool

of fracture mechanics, it is necessary to determine one or all of the

features of a flaw such as size, shape, orientation, composition, etc.

Although such a goal is unrealistic in the light of current practice, new developments including imaging systems and adaptive learning

techniques seem to make such a goal closer to reality. However, we can

only repeat here what has been mentioned elsewhere in this report; see,

for example, 3.5. First, the fact that one ’'sees'' an image of a defect

by no means implies that the vision is free of distortion. It seems clear that a set of artifacts will have to be used to ensure that the

apparent size, shape, orientation and location are correct. One must

also deal with the inherent difficulties of using a two-dimensional

image to assess three dimensional attributes such as shape and

orientation, although this problem may be solved in part by future

developments in 3-D imaging. In addition, it is clear that resolution

(range, azimuthal, front surface) must be determined. Thus, as mentioned previously the standards requirements for quantitative NDE

systems appear to be more complex than for the present pulse-echo

methods

.

4.1.2. What is the impact of computer-based systems and advanced

signal processing on standards requirements?

’ From a somewhat extreme point of view the question has been

sometimes asked "With smart systems are standards necessary?" However,

a computer in and of itself does not eliminate the need for standards

and calibrations. It can, however, assist in performing, recording and

evaluating the more difficult calibrations that will undoubtedly be

33
necessary in the use of the more complex systems to be used in

quantitative NDE.

A. 1.3. Hierarchy of standards

The National Bureau of Standards traditionally provides

traceability of standards at a national level and maintains primary

standards. When necessary to disseminate or gain access to the national

standards, calibration, measurement services and standard reference

materials are provided. Often, secondary standards which have a defined

relationship to the primary standards are used as laboratory standards

in a given organization. In addition, there are field standards or product standards which are used in connection with the actual

measurements. Consensus organizations such as ASTM generally focus on procedural standards, secondary or field standards and their use in

calibration. Thus the spectrum of standards includes primary standards,

secondary standards, field standards, calibrations and procedural

standards

4.1.4. Is there a "super" block that does everything?

Many blocks have been developed or proposed (see [12], section 3.3

and Appendix B) for use in the calibration and adjustment of several

aspects of the measuring system. In some cases the blocks in

conjunction with a standards document are capable of a rather complete

system calibration. However, a block which could be used to determine

system function and adjustment, characterize the radiated ultrasonic

field, assure repeatability, provide for interpretation of complex

geometries, assure resolution, provide for assessment of shape, size,

and orientation, is not application-specific and is theoretically

describable does not now exist, nor is it obvious how it could be

realized. However, some of these attributes are or could be embodied in

a reference block. A number of reflectors are theoretically describable

including spheres, cylinders, ellipses and flat-bottom-holes. Blocks

which are less application-specific in the sense that they are material

independent could be developed. For example, fused quartz would provide

a low attenuation, amorphous, transparent reference material which has

desirable attributes independent of the internal reflector geometry. It

is apparent that a "super" block does not exist but that a carefully

designed system of blocks including product standards is an appropriate

goal if the performances of these blocks are defined and related to each

other.

4.1.5. Overall versus separate component calibration.

We imagine that there is available a product standard that has

precisely the size and shape flaw we want to detect. Since the general

requirement is to detect this size flaw or greater. It would appear that

by merely adjusting the response to this flaw the problem of calibration

has been overcome by an overall system calibration. However, because

34
the system may now be in a nonlinear regime, there may be very little

discrimination between flaws of different sizes. Independent of this,

incompatible bandwidths among subsystems may frustrate, for example,

Fourier analysis of the received signal. In these circumstances it will

be necessary to analyze subsystem performance to find the cause. Also,

if the system performance degrades or inadequate performance is apparent

in the first place, it will be necessary to determine the nonfunctioning

parts of the system.

In practice, proper system performance is often achieved by trial

and error with available subcomponents, e.g. transducers. However, when

an appropriate one is located, a basis for obtaining copies of this

component is required. This basis is provided, of course, by measuring

the appropriate component characteristics. Further, when tests are performed under written agreement, the procedure often dictates required

characteristics such as test frequency which cannot be predicted or assured by conventional calibration of the overall system. Finally, it seems clear that with more advanced and complex systems which will

interrogate and process data in a more sophisticated way, the

performance of subsystems and their interdependence are even more

critical and their characteristics will have to be carefully specified

and determined.

If system subcomponents are calibrated separately, do we need any

reference blocks? At the laboratory level the individual component

characteristics may be determined at least on receipt of equipment and

at the time of some malfunction. In field use the equipment must be

checked frequently and with little ancilliary equipment. For this,

reference blocks provide a useful way to adjust and check overall

performance. However, development of an internal test set as part of

the NDE apparatus may relieve some of the pressure on reference blocks

to do more than they reasonably can.

In summary, it is not a question of either subcomponent or system

checks; rather, both seem to be necessary albeit at different places in

the hierarchy of standards and calibrations.

4.1.6. Why be concerned about trying to improve current standards for

pulse echo techniques when better techniques are around the corner?

It seems obvious to us that because of the relative simplicity of

the pulse echo techniques that they will be widely used in some applications for many years to come regardless of the advent of better

(quantitative) techniques. Consequently, it is necessary to make these

techniques more reliable and more quantitative through improved

standards and calibration procedures. When the improved techniques are

developed to the point where they receive wide acceptance, they will

require standards of their own. However, these "new" standards in many

cases may be the old standards used in a different way or will b6 some

rather direct derivative of the old standards. This will, in part.

35
arise because many of the new techniques are pulse echo systems but with

markedly improved display or signal processing. In short, improving

standards for current pulse echo systems does not represent wasted

effort not only because more quantitative methods will not obsolete the

more qualitative methods but also because these improved standards will

quite likely find use in the improved systems.

4.1.7. What are the tolerances on reference blocks and transducers

that are really required?

To clarify the nature of this problem, we cite several examples

involving the question of tolerances. In some consensus documents the

tolerance on transducer probe normality with respect to the part being

inspected is specified on the basis of convenient practice. It would be

preferable to base this tolerance on the effect of misalignment on signal amplitude. The user community might find the effect of

misalignment intolerable if it were known. It also might find the cost

of a smaller tolerance distasteful. Another consensus standard

specified a tolerance on center frequency of transducers that is

perceived as being so stringent that virtually all manufacturers refuse

to supply transducers based on that specification. Here again the

effect of meeting a tolerance requirement in terms of detectability or accept/reject criteria is not known so that a rational decision based on

effect versus requirement and cost cannot be made. Clearly the data for

such decisions should be developed.

4.1.8. What are the requirements for transducer calibration?

It appears that transducer manufacturers and large laboratories may

find it necessary to have some transducers that are very carefully

characterized as laboratory standards. Also, it would seem appropriate

to determine in detail all of the characteristics when developing a new

transducer. The large number of transducer types (focused, wide band,

array, contact, piezo, EMAT) will require an extremely general technique

or techniques. The field test laboratory, on the other hand, may find

it necessary to determine fewer parameters, such as center frequency,

bandwidth and some aspects of the beam pattern. In field use it may

only be necessary to determine that the characteristics previously

established have not substantially changed. In short, the extent or completeness of transducer characterization depends heavily on the

application.

It is constructive to consider the following example because at

first glance it would appear that in this case transducer calibration is

unnecessary. In systems making use of Fourier analysis of received

signals where this signal may be considered a product of the reflector

and subsystems transforms, it is possible to divide out the system

transform, in particular, the transducer frequency response. However,

if the transducer has an inappropriate beam pattern and frequency

response it may result in a very bad signal-to-noise ratio at the place

36
in the spectrum which most strongly affects the accuracy of the

results. Little frequency content in important regions could result in

inaccurate signal processing. As one of the participants in the

Ultrasonic Standards Workshop remarked: "Garbage in equals garbage

out ."

4.1.9. With the many advances in electronic technology, why are there

problems in calibrating the electronic equipment?

There seem to be no scientific problems but rather some

technological and practical problems in the calibration of the

electronics. For example, it may be of interest to determine the peak

voltage amplitude of the pulser in an ultrasonic unit. However, the

impedance of the transducer must be simulated in a specific way which is

really a problem of specification, i.e. what to measure and how to

measure it. It should be noted that both NBS and ASTM Subcommittee

E7.06 are dealing with such problems. It has been suggested that the

electronic calibration or diagnosis problem could be solved by a builtin electronic test set resembling a radar test set or in using the NRL

Electronic Test Block concept in some expanded way. This matter

deserves further attention.

4.1.10. What about standards for measurement of residual stress

and material properties?

There is a growing demand for standards for ultrasonic measurements

of residual stress and material properties such as velocity and

attenuation. Monitoring these characteristics is clearly important for

the assessment of the life and performance of materials and components

and therefore constitutes a valid extension of NDE from defect diagnosis

to determination of material condition. When such measurements are made

by researchers in their laboratory, the accuracy of the apparatus and

measurement technique is assessed and improved in each individual

case. However, when such measurements will be routinely performed in

manufacturing and field environments, there will be a need for

references and standards. The total requirements for standards for

residual stress are still unclear because too little is known about the

final techniques and their sensitivity to material condition. However,

one can also imagine reference artifacts with different residual stress

patterns. In the case of attenuation and velocity measurements one can imagine sets of reference materials with different velocity and

attenutation characteristics for "calibrating" measurement apparatus.

Significant interest has been noted in monitoring elastic modulus of raw

stock by measuring velocity and specific requests for traceable velocity

measurement and certification have been made.

37
4.2. RECOMMENDATIONS FOR RESEARCH AND DEVELOPMENT OF ULTRASONIC NDE

STANDARDS

In this section we present recommendations for research and

development of ultrasonic NDE standards based upon the source material

and their analysis already observed in this report. The recommendations

are organized into the following categories: 4.2.1. What needs to be

done to improve existing standards or standards for existing systems;

4.2.2. To select for further evaluation proposed standards and

calibration methods; and 4.2.3. To assess the requirements for

standards and calibrations for developing ultrasonic systems. In

section 4.2.1., we recognize that improvement of accepted standards,

e.g. those proposed by ASTM, traceable to NBS, or those widely used in

practice, will have an immediate and cost effective impact in improving

the reliability and more quantitative use of current methods. It is

also clear that improvements in these standards, in the underlying

theory and in their relation to practice will have significant, direct

impact on future systems. Section 4.2.2. deals with methods which have

been proposed which are new, rather than improvements of those

considered in section 4.2.1. The emphasis here is not in unexplored

ideas but rather on the examination and possible implementation of the

concepts for specific standards needs. Section 4.2.3. deals with

standard and calibration needs for evolving and future NDE methods.

In considering the standards needs for ultrasonic measurement

systems, the specific system and application must often be considered.

However, as far as possible we have endeavored to make these

recommendations broadly applicable.

4.2.1. Improvements in standards for existing systems

The emphasis in this section is on standards-related work that will

impact conventional pulse-echo systems. The area is further divided

into problems with transducers, reference blocks, and electronics. It

should be recognized that solutions to standards problems of current

systems will often carry over to developing or future systems,

particularly those using pulse-echo techniques. For example, an understanding of the characteristics of the field produced by a transducer is important for conventional pulse-echo techniques; however,

such detailed knowledge is vital for many developing techniques. In

spite of dramatic developments that are forecast for quantitative NDE,

it is expected that the majority of systems that will find use in the near future will still be pulse-echo based systems since many

applications simply will not support the more advanced techniques.

Transducer characterization.

The transducer in an ultrasonic measurement system has probably the

single greatest influence on the data obtained in am evaluation. This

is true not only in present systems but will be even more so in most of

38
the systems contemplated. The need for complete transducer

characterization has been expressed by a large number of the

participants in, for example, the recent ARPA/AFML Reviews of Progress

in Quantitative NDE and the Workshop on Ultrasonic NDE Standards (see

Appendix B). The fields produced by the transducers are complex and the

variability in their characteristics is high. There is a clear need for

a better understanding of the theoretical and actual characteristics of

transducers

1. Necessary and Sufficient Transducer Specifications for Reproducible

Results. The minimum specifications for a transducer to obtain

reproducible results in flaw evaluation, even with the same instrument,

are not known. Some experiments have been performed recently [27] using

six transducers, all of which meet the specifications of ASTM E-127.

Using the same instrument and normalizing the output for each

transducer, differences in amplitude of response from the different

transducers on the same reference blocks were found to average about

25%. The implications for actual inspections are clear.

In developing these criteria it will be useful to determine whether

any procedures (e.g. the very detailed British System, see [12]) are

sufficient to specify transducers. After establishing appropriate

criteria it may be necessary to develop the required measurement

methods

2. Theoretical Models for Transducers. Much of the standards work in

and out of consensus organizations has been hampered by the lack of an appropriate theoretical model for the field produced by ultrasonic

transducers. The usual model, that of a plane piston under continuouswave excitation into water, is clearly not appropriate and leads to a great deal of wasted effort trying to match or explain differences from

"the " (imperfect) theoretical model. This and the item above should

lead to improved transducer design and manufacturing uniformity.

3. Effects of Pulsed Excitation. The response of a system depends on

the characteristics of the exciting pulse. This interaction is not well

understood and leads to differences in measured response in spite of

normalization on the same reflector. This problem deserves further

investigation.

4. Longitudinal Transducers Characterization. Some useful methods for

measuring certain characteristics of transducers have been or are being

developed. These include radiation pressure for absolute power versus

frequency, calorimetry for time-averaged power, and planar scanning for

field quantities. However, since these are all laboratory methods,

techniques for use in the "field" or user laboratory for determining at

least the most important transducer characteristics should be developed.

5. Methods for determining the important characteristics of a transducer with a solid load should also be developed. The European

39
practice of using a block and an EMAT receiver (which is not very

sensitive to coupling variables and doesn't load the surface) should be

investigated in this connection. Non-contact optical methods may also

be useful. In either case a deconvolution of the surface measurement is

needed to determine the field in the solid. (Methods for the

characterization of angle beam transducers should also be Investigated

and developed.)

Reference blocks.

At present ultrasonic NDE is a comparative measurement which relies

on reference blocks for checking and adjusting the equipment, for

forming a basis of agreement (contract between supplier and purchaser)

and for establishing reject criteria. Although such measurements may be

quantitative, they are really a measure of "relative reflectivity" of a defect at a given angle and this often doesn't indicate defect size . The reference blocks are and will continue to be an important part of

the ultrasonic measurement system. Even in future systems, these or similar blocks will be used for comparing performance, demonstrating

resolution, sensitivity, etc.

1. Transfer of Calibration. Even when the material for a reference

block and a part under inspection are the same alloy, different acoustic

properties can cause great difficulty in using the block as a reference

for making measurements in the part. For some steel and titanium alloys

this problem overshadows any uncertainty in reproducibility of the

reference blocks. A method for properly comparing the test and

reference data should be developed. It would likely involve the

development of field methods for the measurement of properties such as attenuation, velocity, and impedance. This problem area also has

implications for Item 3 below, "Quartz blocks."

2. Generic Reference Block Problems. There are certain problems which

are common to nearly all specifications of reference blocks and product

standards, for example, the specifications of surface texture. Although

the present practice specifies only rms finish, evidence indicates that

the return signal can differ greatly from blocks with the same rms surface finish. At least two parameters are necessary to specify

texture but the relation to manufacturing methods are unclear. A need

for appropriate models for the relation of the amplitude of the signal

from a reflector as a function of metal travel distance is another

problem that is common to reference and product standards and, in fact,

to inspection procedures. Some initial work at NBS based on fits of

models to a large data base of reference blocks [36] suggests that such

a model could reduce the number of blocks needed for a distance

amplitude set from about twenty to two or three. Additional work could

result in greater economy, convenience and accuracy.

3. "Quartz" Blocks. The number of different sets of reference blocks

required to do ultrasonic testing is formidable. Usually a different

)

AO
set of test blocks is required for each alloy, sometimes even each

temper or texture. The development of a set of reference blocks

"uncolored" by material properties, along with a compilation of transfer

data for the material under inspection, would lead to a great economy

for any standards effort. A transparent amorphous reference would

eliminate metallurgical texture problems (the greatest single cause of

block variability). Also, for future systems, a need for references

with negligible attenuation was indicated (see ARPA/AFML Review 1975).

Such a conclusion was also expressed at Public Review of the NBS NDE

Program in December 1974.

4. Relation of V, U, and EDM Notches to Cracks. Many product standards

suggest the use of "V", "U" or EDM notches as reference defects for the

evaluation of products or cracks. The quantitative relation of these

reference defects to each other and to actual defects is not

sufficiently clear; see however reference [36a] . These relations should

be established.

Electronics

1. Measurement Methods for Ultrasonic Instrument Components. What are

the right parameters to measure? How should they be measured? When

different systems are operating properly (linear, appropriate

sensitivity, calibrated dB control, etc.) and operating similarly in

terras of resultant measurements there is limited interest in the

characteristics of components; but, given a problem in performance,

these characteristics become central. There are presently no accepted

procedures describing what electrical quantities of the components of a system should be measured or how to measure them. See also 4.1.10.

2. Effects of Instrument on Transducer Performance. Several

laboratories and suppliers have identified a problem associated with the

interaction of the instrument and transducer characteristics. This is a more general problem than the problem of the effect of pulse

characteristics mentioned previously. A symptom of the problem is the

difference in center frequency as measured by a supplier (with his

instrument) and as measured by the user (with the user's instrument).

If instruments within certain categories have the same known electrical

characteristics, it may be possible to develop methods to predict system

performance so that tests can be run at the specified test frequency.

4.2.2. Proposed standards and calibrations

Many methods of standardization or calibration have been proposed

in the recent past [13]. Some of these seem to offer many logical or practical advantages over currently accepted practices but are not

widely used. Selected methods should be more completely analyzed (see

sections 3.2.2., 3.2.3., and 3.3.3.) with logical standards criteria and

promoted if they indeed prove to be significantly advantageous. The

emphasis of the analysis should be on whether the basic concepts are

41

!
sound and whether the technique can be made to work. This process takes more time and effort. than is generally recognized. Independent,

statistically valid procedures and error analyses must follow even the

best technical development in order to properly establish a proposed

standard. In addition, it is frequently necessary to develop an interactive loop with the users.

1. Theoretically Described Scatterers. It is possible to theoretically

describe the reflection, diffraction and scattering of certain

geometrical reflectors of regular shape. When this is possible, what is

the role of such a theoretical description in the development of

standards and calibrations? Can theory be used as another parameter

which characterizes a test block; can it serve as a means for

transferring the calibration of an ultrasonic system from test block to

test block and can it serve as the foundation of a primary, traceable

standard block? Does it give any benefits for an overall systems

check? Existing theories which describe the scattering of infinite

plane ultrasonic waves and pulses from cylinders, spheres and other

smooth obstacles such as ellipsoids should be extended to include the

scattering of non-planar, bounded ultrasonic waves which most closely

resemble an actual ultrasonic testing and calibration situation.

Theoretically described blocks must also be developed and examined in

the context of a criteria such as given in 3.2.2. and 3.2.3.

2. Transducer Field Characterization by Planar Scanning. An elegant

theory has been developed for the very economical determination of the

complete ultrasonic field quantities produced by transducers [37] . At

least two methods of obtaining the required information have been

conceived: one using a receiving hydrophone and one using laser

scanning of a pellicle [14]. If the analysis for deconvolving the free

surface of a solid were developed, the method could be used for

determining the field quantities in a solid. Also, the method has the

potential for examining pulsed fields. This proposed method - planar

scanning - requires additional work before it becomes an established

method. Other sound field visualization techniques such as photoelastic

methods should also be investigated.

Planar scanning is recognized as an advanced laboratory method.

However, it can be done quickly and with great economy and can provide

just the level of information detail required. In any case it is

envisioned that at least every line of manufacturers' transducers and

new design approaches should be analyzed for rather detailed field

patterns as a way of meeting manufacturing design criteria.

3. Well Characterized Fatigue Cracks. The goal of most in-service

inspection is the detection and evaluation of fatigue cracks. In order

to evaluate new measurement systems (and new procedures with

conventional techniques) it has been proposed that fatigue crack

specimens with known characteristics be developed. Some parameters can

be controlled in specimeh production; others must be evaluated in terms

42
of ultrasonic response. Careful design of the loading program can minimize effects of crack topology and crack closure [19]. However, the

details of the functional relationship of stress intensity at the crack

and of relative geometry of the crack and transducer on ultrasonic

response must be determined.

4. Electronic Test Block (ETB) . The use of an electronic test block at

least to partially replace reference blocks has been proposed for

several years [38]. The ETB, which electronically generates and

controls signals which simulate the return signals from reference

defects, offers some clear advantages over presently accepted systems.

However, questions remain such as loading of the transducer, stability

and relation to material under test. Possibly, a more natural and

specialized use of this concept may be in determining and monitoring the

electronic performance of the instrument. One could envision this as being analogous to the radar test set, possibly even incorporated into

the flaw detection instrument.

5. Other Proposed Methods. Other methods for improving the standards

system have been proposed. Some interesting work on blocks simulating

even inclusions has been initiated [39] (see also appendix B) . These

should be critically screened so that the most promising could be

further investigated.

4.2.3. Standards for developing methods; quantitative NDE.

Advanced ultrasonic NDE systems are being developed to make

quantitative measurements of flaw geometry, orientation and size, rather

than "relative reflectivity" and to decrease the dependence on operator

interpretation. However, one must not expect that these developments

will automatically decrease the need for standards. Standards will not

only still be needed but may be even more complex because the amount of

signal extraction, comparison and processing will be even greater. No

matter how quantitative the system, in the end one must validate its

condition and performance. Although functional checks of the overall

system performance may not be more complex, the diagnostic checks

necessarily will be.

It also seems clear that the advent of computer-based systems will

reduce operator dependence in both calibration and the actual NDE

measurements, but still not eliminate the need for standards.

1. Imaging Systems. It has been speculated that standards will not be

needed with imaging systems since a picture of the flaw is presented. A

comparison between x-ray imaging (where standards are required) and

ultrasonic imaging indicates a greater need in the latter because the

use of longer wavelengths in relation to object size degrades image

quality. At least resolution, focus, and grey scale definition must be

established and monitored. As with any reference standard these should

permit relative performance measurement and data interchangeability. A

43
possible standard could have artificial defects of various sizes with

various three dimensional spacings and at various orientations. The

defects could be constructed to have various impedances. A scheme

similar to the one used by the American Institute of Ultrasound in

Medicine for their blocks could be used to simulate materials with

different attenuations [AO] . Some of the newer fabrication techniques

such as diffusion bonding or powdered metallurgy (and including, e.g.,

silica inclusions) may be used to implement these standards. It would

be appropriate to develop such prototype blocks and investigate their

efficacy using state-of-the-art imaging systems in response to this

developing need.

2. Adaptive Systems. Adaptive systems presently make use of many

different variables and c®mbinations and powers of variables in the

correlation of response with defect characteristics. Since this would

seem to make the technique particularly sensitive to the data used in

the learning process, it would seem necessary to provide input using a large variety of characterized defects. Well-characterized fatigue

cracks as described in Section A. 2. 2 and [19] may provide a valuable

teaching aid if implemented. It also may be useful to employ the

arsenal of actual reference defects which were used to develop the ASTM

reference radiographs. Theoretically described defects would also be

very valuable in system training.

3. Fourier Transform Methods. Present ultrasonic methods often employ

only two parameters: time of arrival and amplitude of reflected

signal. At most, therefore, two features of a defect can be determined

from a single measurement, depth of the defect and its relative

reflectivity. Fourier transform methods attempt to use a continuum of

frequencies associated with the return signal in order to determine more

features of a defect. Transform techniques may be either of the pulse

echo or scattering varieties. In the latter case, the use of

theoretically described scatters would be helpful. Workers in this

activity point to a much greater need for knowing the details of the

transmitted signal and for a great need for control of more

characteristics of reference defects used with these techniques.

A. Standards for Material Measurements. There is great interest in

measuring quantities such as residual stress, grain size, hardness,

attenuation and velocity by ultrasonic techniques. Reference artifacts

or procedural standards will certainly play an important part in the

establishment or improvement of these measurements. For example, the

measurement of residual stress relies on establishing a point of

reference, e.g., a zero stress state. Methods and standards to do this

are needed if the techniques are to be used for the measurement of more

than a change in stress state due to applied load.

AA
4.3 PRIORITIES, IMPACT, FINAL COMMENTS

4.3.1. There are a great many problems but what are the priorities?

An integrated approach to the issue of improved ultrasonic

standards requires addressing the three areas in which work is required:

a. improve current standards where the emphasis is on the pulse

echo technique,

b. analyze proposed standards for possible implementation,

c. develop standards for emerging techniques.

While it is convenient for discussion to compartmentalize these

activities, there is much overlap in these areas. Thus, work on proposed standards (item b) should result in implementing improvements

in current standards (item a). Similarly, some proposed standards (item

b) really address forseeable problems in emerging techniques (item c).

We suggest that it is necessary to support and improve current

standards because they will be in use for some time and because it is

cost effective to do so. We do not believe that emerging technology,

e.g. smart and imaging systems, will cause the more primitive pulse-echo

techniques and the standards on which they rely to become obsolete. The

newer techniques will be more complex, costly, difficult to maintain and

will require more complex standardization. Furthermore, there will

continue to be a need for the qualitative tests the present systems can

fulfill. It also seems clear that the standards and calibrations being

developed will be useful for the more complex evolving systems either as they are or by sotae adaptation. Thus, it appears that for current needs

and future applications, improving current standards is necessary and

valuable. Although it is difficult to suggest precise allocations for

the major areas, national needs seem to strongly suggest a balance be

maintained among the major areas of effort.

We now attempt to address a more specific question of priority

regarding relative emphasis for developments in reference blocks,

transducer characterization and measurement of electronic

characteristics. Ultrasonic systems generally are composed of an

electronic apparatus and display, a transducer and frequently some apparatus for determining system performance such as reference blocks.

While some effort on improved reference blocks seems certainly

justified, recent work on reference blocks indicates they can be

successfully fabricated if due attention [11] is paid to material

selection and fabrication techniques. When coupled with an available

calibration service, the problem of variability is solved for one type

of block made of aluminum. While the development of improved blocks and

a more general calibration service is desirable, we caution again that

it may not be reasonable to expect some new "super" block. However,

growing concern has been expressed for a more direct and descriptive

characterization of transducers than can be obtained from reference

45
blocks. In addition, the need for separate electronic calibration has

received increasing attention. The priorities would seem to be first - transducers, second - electronics, and third - reference blocks.

4.3.2. If all the problems are solved, what impact can be expected?

If the problems discussed in Section 4.2 are solved, current NDE

will certainly be made more reliable and quantitative, and in addition

the basis for standards for emerging quantitative systems will be

established. The importance of NDE in conjunction with fracture

mechanics in assuring the integrity of existing and future high

performance structures is obvious and relies heavily on a reliable

quantitative measurement system. As has been shown, the realization of

this goal relies very directly on improved standards and calibrations as well as on new techniques. By anticipating the standards needs of the

future, particularly of intrinsically more quantitative methods, the

time until field application is made will be decreased and the

operational utility of the method will be enhanced.

Although the list of problems in Section 4.2 is meant to be rather

comprehensive, it shpuld not be regarded as a fixed work plan. As some

of the high priority problems are solved, a reevaluation of the

measurement needs may indicate that not all of the other problems need

be solved. On the other hand, it is apparent that unforseen problems

may also arise.

It is difficult to estimate the fiscal benefits of the solution of

the standards problems to ultrasonic NDE measurements. However, a recent study on the "Economic Impact of Acoustic Measurement

Uncertainty" attempts to deal with the fiscal benefits of standards work

on ultrasonic NDE [41]. This report estimates that the cost of

uncertainty for NDE in its many applications might be of the order of

$150-300 million per year. The larger question of measuring the wrong

quantity independent of uncertainty is one of even larger economic

impact, probably by orders of magnitude. Other aspects of the

reliability of ultrasonic measurements such as product liability also

have significant economic impact and a heavy reliance on standards. In

summary, analysis of the problem indicates that better standards and

procedures have high significant economic benefits regardless of whether

the standards are for current, rather qualitative methods or the

emerging, more quantitative methods.

4.3.3. Final comments '

‘ v

As stated in the Introduction, the objectives of this study

include:

o examine the present system of standards,

o assess the standards needs for emerging systems.

46
o recommend a course of action to improve, supplement or replace

the current standards system for ultrasonic NDE.

We have tried to realize these objectives within the constraints of

a very complex measurement system that is very dependent on the wide

diversity of techniques and applications. Although some of the results

could have been foreseen, a number of others may be surprising. The

conclusion may be disappointing that while there may be room for a greatly improved block there is no universal new "super'' block on the

horizon. In addition, the strong evidence that new advanced systems

will place even greater demands on standards may also be surprising.

In addition to this report, this work has a number of other outputs

which are published separately. These include the discussion of US and

European Standards [12], an extensive survey on ultrasonic transducers

and their characterization [14] and a bibliography of the literature

relevant to ultrasonic standards [13]. It is hoped that this report and

the associated documents will serve as a resource document in the area

of standardization of ultrasonic NDE measurements.

Finally, the Workshop on Ultrasonic NDE Standards, summarized in

Appendix A proved to be very valuable in developing this report and

should be useful in future studies.
REFERENCES

[1] Panian, F. C. and Van Valkenburg, H. E., Development of ASTM

Standard Reference Blocks for Ultrasonic Inspection, Proc. of

ASTM Vol. 59, 1959.

[2] Commission V, Use of Reference Blocks for Checking Ultrasonic

Inspection Apparatus, British Welding Journal, April 1960.

[3] Rawlings, H., Ultrasonic Testing Standards, Ultrasonics, Jan.-

March 1963.

[4] Tentative Recommended Practice for Fabricating and Checking

-Aluminum Alloy Ultrasonic Standard Reference Blocks, E127-58T,

1958 Book of ASTM Standards, Part 3.

[5] Burley, C. E., Calibration Blocks for Ultrasonic Testing, in

Nondestructive Testing - A Review, H. Berger, editor, ASTM STP

624, p. 146, 1977.

[6] Eitzen, D. G., Sushinsky, G. F., Chwirut , D. J. , Bechtoldt, C.

J., and Ruff, A. W. , Improved Ultrasonic Standard Reference

Blocks, NBSIR 75-685, April 1975.

[7] Standard Recommended Practice for Fabricating and Checking

Aluminum Alloy Ultrasonic Standard Reference Blocks, E127-75,

1976 Book of ASTM Standards, Part 11.

[8] Standard Recommended Practice for Fabrication and Control of

Steel Reference Blocks Used in Ultrasonic Inspection, E428-71,

1976 Annual Book of ASTM Standards, Part 11.

[9] Millman, I., An Improved Facility for the Calibration of Aluminum

Alloy Ultrasonic Reference Blocks, Report No. AQD/NDT 1369, AQD

Labs, Harefield Uxbridge, Middlesex, U.K.

[10] Weston, H. W., Harrogate Symposium on Standards in NDT, organized

by the British National Committee for NDT, Sept. 1962.

[11] Chwirut, D. J. , Recent Improvements to the ASTM-Type Ultrasonic

Reference Block System, NBSIR 79-1742, April 1979.

[12] Golan, S., A Comparison of American and European Ultrasonic

Testing Standards, NBSIR 79-1790.

[13] Eitzen, D. G. , and Birnbaum, G. , Summary of the Literature on Ultrasonic NDE Standards, NBSIR in preparation.

/

48
[14] Sachse, W. , and Hsu, N., Ultrasonic Transducers for Materials

Testing and Their Characterization, Physical Acoustics Vol. 14,

1978.

[14a] Belanger, B., Traceability - An Evolving Concept, to be published

in ASTM's Standardization News.

[14b] Compilation of ASTM Standard Definitions, p. 600, Third Edition,

1976.

[15] Chwirut, D. J., Sushinsky, G. F., and Eitzen, D. G., Procedures

for the Calibration of ASTM E 127-Type Ultrasonic Reference

Blocks, NBS TN 924, September 1976.

[16] Adler, L. and Lewis, D. K. , Diffraction Model for Ultrasonic

Frequency Analysis and Flow Characterization, Materials

Evaluation, January 1977.

[17] Sachse, W., Hsu, N., and Eitzen, D., Visualization of TransducerProduced Sound Fields in Solids, 1978 Ultrasonic Symposium

Proceedings, IEEE Cat. No. 78CH1344-1SU.

[18] Fick, S. E., An Optical Doppler Probe for Ultrasonic Waves in

Solids, Proceedings of the 1978 Conference on Information

Sciences and Systems, March 29-31, 1978, Johns Hopkins

University.

[19] Chwirut, D. and Eitzen, D., Toward the Development of Improved

Reference Fatigue Cracks for Use in Ultrasonic NDE, to appear in

Inti. Adv. in NDT.

[19a] Tittman, B. R., Thompson, D. 0., and Thompson, R. B., Standards

for Quantitative Nondestructive Examination, in NDT Standards: A

Review, H. Berger, Editor ASTM STP 624, p. 295, 1977.

[19b] Tittman, B. R., and Thompson, D. 0., Approach to a Self

Consistent Calibration Procedure of an Ultrasonic System,

Materials Evaluation Research Supplement, April 1977.

[19c] Tittmann, B. R. , New Procedure for Calibrating Ultrasonic Systems

for Quantitative NDE. Proceedings of the ARPA/AFML Review of

Progress in Quantitative NDE. AFML-TR- 77-44, 1977.

[20] Kino, G., Khuri-Yakub, B., Murakani, Y., and Yu, K-H., Bulk

Defect Characterization in Ceramics, 1978 ARPA/AFML Review of

Quantitative NDE.

[21] Cohen, E. R., Analysis of Ultrasonic Wave Scattering for the

Characterization of Defects in Solids, final report to AF0SR, May

1977, Rockwell International Science Center* Report No. SC579.4;

49
[22] Gubernatis, J. E., Domany, E., Krumhansl, J. A., and Huberman,

H., The Born Approximation in the Theory of Elastic Wave

Scattering by Flaws, J*’ Appl. Phys. _48, 2812 (1977).

[23] Pao, Y. H., The Transition Matrix for the Scattering of Acoustic

Waves and for Elastic Waves, Modern Problems in Elastic Wave

Propagation , Wiley- Interscience, New York, 1978.

[24] 1978 ARPA/AFML Review of Quantitative NDE.

[25] Liddington, B. H., and Silk, M. G. , The Variability of Ultrasonic

Transducers, British Journal of NDT, November 1972.

[26] Chwirut, D. J. and Eitzen, D. G. , Improving the Reliability of

Ultrasonic Measurements: Recent NBS Progress, ASNT Proceedings

1978 Spring Conference.

[27] Chwirut, D. J. and Boswe-11, G. D., The Evaluation of Search Uni.ts

Used for Ultrasonic Reference Block Calibrations, NBSIR 78-1454,

February 1978.

[28] Miller, E. B. and Eitzen, D. G., Ultrasonic Transducer

Characterization at the NBS, IEEE Transactions on Sonics and

Ultrasonics, Vol. SU-26, No. 1, January 1979.

[29] Smith, V. D. , Teller, C. M., and Swanson, R. K. , Engineering

Services to Determine Acceptance Limits of Ultrasonic Transducers

for Nondestructive Inspection, SWRI Project No. 15-5024, June

1978.

[30] Chwirut, D. J. , A Simple Technique for Visualizing Transmitted or Reflected Sound Fields, to be published in a Materials Evaluation

as a Tech. Note.

[31] Hildebrand, B. P. and Collins, H. D., Evaluation of Acoustical

Holography for the Inspection of Pressure Vessel Sections,

Materials Research and Standards, Vol. 12, No. 12, pp 23-32

(December 1972).

[32] Metals Handbook, Vol. 11, 8th Ed., ASM, p. 231 (1976).

[33] Mucciardi, A. N., Measurement of Subsurface Fatigue Crack Size

Using Nonlinear Adaptive Learning; Proceedings of the ARPA/AFML

Review of Progress in Quantitative NDE, pp. 194-199, AFML-TR-77-

44, September 1977.

[34] Yee, B. G. W. , Kuselman, J. S., Walker, T. C., Gardner, A. H.,

and Robinson, A. R. , Computer-Automated Ultrasonic Inspection

Systems for Aircraft Forgings, AFML-TR-75-82 (May 1975).

50
[35] Sonoray-Vessel-Scanner , Automatic Weld Testing System, Catalog

Rontgen Technische Drenst, 144 Delftweg, 30 46 N Rotterdam,

Holland.

[36] Chwirut, D. J. , NBS, unpublished results.

[36a] Birchak, J. and Gardner, C., Comparative Ultrasonic Response of

Machined Slots and Fatigue Cracks in 7075 Aluminum, Materials

Evaluation, December 1976.

[37] Kerns, D. M., Scattering-Matrix Description and Nearfield

Measurements of Electroacoustic Transducers, J. Acoust. Soc. Am.

Vol. 57, No. 2, Feb. 1975.

[38] Chaskelis, H. H., A Materials and Defect Simulator for

Calibrating Ultrasonic Equipment Used in NDT, NRL Report 6984,

Naval Research Lab., Dec. 1969.

[39] Conn, D. L., New and Improved ASTM Type Ultrasonic Standard

Reference Blocks. Presented at International Symposium on Ultrasonic Material Characterization, NBS, June 7, 1978, to be

published.

[40] Smith, S., Lopez, H. and Stewart, H., Methods and Results of

Dynamic Range Testing of Diagnostic Ultrasound Instrumentation,

Proc. of the Soc. of Photooptical Instrumentation Engineers, Vol

127, Sept. 1977.

[41] Kane, K. M. and Eldred, J. A., Bolt Beranek and Newman Report No

3538.

[42] Sachse, W. H., Golan, S., in Elastic Waves and NDT of Materials,

ASME AMD, Vol. 29, Edited by Y. Pao, p. 17, Fig. 5, 1978.
.
APPENDIX A

Workshop On Ultrasonic NDE Standards

(October 17-18, 1977, NBS, Gaithersburg, MD)

We present here a detailed summary of the proceedings of the

Workshop on Ultrasonic NDE Standards, trying to preserve as much as practical some of the flavor of the free exchange of ideas, criticisms,

comments and suggestions. That there will be numerous contradictory

statements or statements that invite strong disagreement is inevitable

in an area which is intrinsically very complex and where there are

numerous unresolved problems. These, too, have been retained. No

attempt is made here to assess the results of this workshop, although a summary presented at the time of the workshop, but before the general

discussion sessions, is included. Many of the important issues

discussed at this workshop are highlighted in section 4.1 Key Issues.

The program of this workshop and list of attendees are presented as part of this appendix. Instead of following the structure indicated in

the program, it proved more useful to organize the material in the

following way:

1. Test Blocks

2. Transducers

3. Electronic Instrumentation

4. Operator Problem

5. Material Characteristics

6. Standards Implications for Developing Systems

7. Quantitative NDE

8. Directions, Problems, and Approaches

9. ASTM Ultrasonic Standards

10. Role of NBS

1 1 . Summary

A- 1
WHAT FOLLOWS IS A DETAILED SUMMARY BASED ON THE TRANSCRIPTS OF THE

WORKSHOP. THE SUMMARY IS MADE UP OF PARAPHASING TO CLARIFY, SHORTEN OR

IMPROVE THE PRESENTATION AND DIRECT QUOTATIONS. IN NO CASE IN THE

PREPARATION OF THIS SUMMARY DID THE AUTHORS INTERJECT THEIR VIEWS. - — — r

i

1. Test Blocks

The following definitions, distinguishing among the various uses of

blocks, were put forward. A test block is a piece of material with a known defect and resembles as closely as possible the part under

study. A calibration block has known dimensional characteristics or size of discontinuity and is at least reproducible mechanically. A

reference block implies that it has a use as a standardization tool. A

standard reference block imples that the response from the block can be

referenced back to some known target by a prescribed accepted

procedure. The definitions have not been documented.

The need to clarify the above definitions and particularly the

difference between calibration and reference standards and set-up

standards was mentioned.

It was suggested that the large variety of blocks that are in use

and have been proposed be classified according to the material of the

block, the type of reflector, and the outer geometry which provides the

angular relation and distance between the reflector and the search

unit

.

There was much discussion on the widely used flat bottom hole

blocks and their variability. However, it was shown at NBS that

metallurgy is responsible for most of aluminum block variability.

The thought was expressed that the effect of material aging on the

performance of the blocks would quite likely be ten times less than the

changes in a given measurement system.

Questions were raised regarding blocks: are they adequate, what

are the problems with them, is NBS wasting its time offering a calibration service for them? One view is that the use of blocks are bound in specifications, and consequently, the blocks define the desired

performance for inspection. They are also used to assess the quality of

material. They may be inadequate, but they need to be maintained until

something better comes along.

However, if a quantitative NDE system comes along, flat bottom

holes may not be the answer for standards since they do not deal with

the problem of orientation and their scattering cannot be calculated

because they have sharp edges. Disagreement was expressed on the latter

point.

A2
An improved flat bottom hole block was described which is

fabricated by starting with a conventional cylinder and cutting a wafer

out of the block. A hole is drilled in this wafer and the block is

reassembled and diffusion bonded in an inert atmosphere so that the

surfaces of the. wafer do not oxidize. A pressure of 2000 psi at 2000°F

is applied for about 10 to 15 minutes to produce a cylindrical void in

the center of a steel block. Because the wafer need not be centered,

one block can be used for 2 distances. Wafer thickness can be varied.

The void could be sputtered with various substrates, for example, silica

to simulate a silica inclusion matrix. This block is superior to the

conventional flat bottom hole because it is absolutely flat bottomed in

contrast to a drilled hole. Ultrasonic tests show that the bond

boundaries are Invisible.

Targets other than flat bottom holes (spheres, for example) have

been considered, but they become astronomical in size for the

frequencies, search unit diameters, and distances to produce the

sensitivities that are needed for many inspection applications.

On the other hand, the infinite plate (an exactly calculable

reflector) that was suggested for standardizing measurements produces

very large signals that overload the front end of present day

instrumentation when used with present transducers. Angulation is very

critical, moreover, making it very difficult to peak the system on a flat plate.

The point was made that reference blocks frequently do not simulate

real defects.

The standard reference block for examining a pressure vessel for a boiling water reactor according to ASME is a very massive test block

which weighs several hundred pounds, hardly a portable unit and

consequently a disadvantage.

The need for correction factors for curved surfaces was made clear

in a report (due to Serabian) which showed that if flat blocks were used

for calibration for tests on curved surfaces without a correction,

errors as large as 42 dB may be encountered.

The problem with reference standards for ceramics arises from the

fact that fracture toughness calculations have shown that flaws of the

order of 10 to 100 microns may be critical. The problem of building

standard reference blocks with flaws of that size is enormous. However,

a block with inclusions of six different materials and ranging in size

from .060 to .0001 inch serves as a kind of standard for NASA. In most

tests, however, NASA is not concerned with rejecting materials as much

as with ranking the materials because they are later going to be tested

and broken.

A- 3
The need for standards in the testing of composites was highlighted.

A great deal of work needs to be done to produce artificial flaws,

(i.e. reference fatigue cracks) that are more accurately representative

of the type of void that is being encountered in materials or structures

in service.

A cemmon standard or reference for set-up to detect fatigue cracks

includes a triangular slot in an object simulating the structure under

scrutiny. The operator proceeds to interrogate the actual structure

after assuring himself that the equipment has responded properly to the

simulated flaw in the standard.

A problem lies with the fact that we do not know what is

transmitted so that what comes back doesn't tell us too much.

Therefore, we use reference blocks. Many years ago, it was felt that

there were too many reference blocks for this purpose, and this led to

the idea of an electronic test block (ETB) . It receives the acoustic

pulse and later generates another acoustic pulse which is proportional

to what is received. The proportionality is adjustable. The electronic

test block has not been successful because we do not know enough about

the transmitted beam. With any reference block, we must have some a_ priori knowledge about the beam. A more successful use of the ETB is as an instrument evaluation tool because there is control of time and

amplitude which allows one to check for resolution and instrumental

linearity.

2. Transducers

Many participants suggested that the transducer is the most

critical element in the calibration procedure.

One needs to know the characteristics of the acoustic beam if one

is to avoid variances in testing blocks. Once the acoustic beam is

characterized for its pressure amplitude and spatial location, one can

use some universal test block such as the ETB where amplitude and time

is controlled.

Evidence was presented to show that the response function of the

search unit is different when driven CW than when pulsed. Although

under CW excitation the search unit may act as a piston, it may not do

so under pulsed excitation. Nevertheless, using a microprobe, no difference was observed between CW and burst CW down to perhaps 9

cycles

Other transducer problems were noted. When a typical 5 MHz quartz

crystal unit is pulsed, its frequency shifts. Although it was claimed

that a great deal of experimental work agrees "reasonably" well with

theory if we assume a piston motion for transmission, there is evidence

Athat the transducer does not act as a piston on reception since adjacent

sections of the crystal vibrate essentially independently of each

other. It appears that the search unit that is going to be used for the

calibration of blocks should be theoretically defined and designed. For

example, the 3/8-inch 5-MHz crystal that is used to standardize

reference blocks gives results not in conformity with theory. Is it

proper to use such a procedure? There appears to be some question

regarding transducer linearity and stability especially in ceramic

transducers. There is a problem in obtaining the desired center

frequency of a transducer since the electrical pulse and loading at the

manufacturing facility and that of the user are not the same.

Everyone is aware of the variability of transducers. We need to

know more precisely the fundamental frequency, bandwidth, energy level

and beam profile. The question of how components interact is

important. For example, it is recognized that the type of pulse created

by the pulser can affect the way the transducer reacts. Also, the

pulser puts out a different type signal when it is in the unloaded

condition. Different transducers may make the pulser react

differently. This, then, raises the question of what are the conditions

under which components should be evaluated to make the results

meaningful.

There is a need for characterizing transducer variables such as beam spread, damping characteristics and frequency for peak response.

These quantities are not well-defined in ultrasonic NDE practice in the

nuclear industry which is based on the ASME Boiler and Pressure Vessel

Code. Furthermore, there is no provision for keeping track of them from

one examination to the next; e.g., large temperature variations can have

an effect on transducer characteristics.

If one knows the electronic components, which are easier to

quantify than characterizing the transducer, then one could know if the

transducer is degrading the performance of the system.

A suggestion was made for transducer calibration using an ideal

transducer, one whose characteristics are known and is broadband for use

as a standard sound source and receiver.

Transducers should be calibrated in conjunction with the medium

with which they will be used since there is a serious question whether

the sound profile in a solid is the same as in water, the medium used

for many calibrations.

The use of EMATS whose design and performance may be described by

theory was suggested for the calibration of other types of transducers

such as the piezoelectric type. Since the frequency characteristics of

EMATS are also well-behaved and predictable, one may be able to use them

to test the electronic system.

ARegarding the NBS role in the development of suitable transducers,

it was said that NBS is not going to market transducers or compete with

industry in providing transducers. NBS is, however, deeply concerned

about different transducers giving the same results and examining

various methods for measuring transducer properties. NBS has considered

building transducers that may be regarded as standards in the sense of a standard reference material for various applications such as to check

the results of spectral analysis, measurement of the power spectrum,

etc. Although NBS is not in the manufacturing business, it was suggested that it should somehow get some good design information to the

manufacturers. The question regarding the NBS role was again: does

industry wish NBS to offer reference transducers or calibration

services? Are there other properties of interest besides total power,

frequency response, beam profile and electrical characteristics? It was noted that although NBS work on transducers thus far has concentrated on

CW use, plans are under way to attack the pulse problem.

If "suitable" standard transducers were available, an imaging

system's performance could be determined without the complicated problem

of transducer characterization. Emphasis on absolute performance

standards (e.g. resolving reference defects) rather than on procedural

standards would then be possible at a great savings.

It was also suggested that NBS develop simpler field laboratory

techniques for transducer evaluation.

3. Electronic Instrumentation

The point was stressed that it is very important to know the

frequency and bandwidth of various parts of the system such as the

transducer, pulser and amplifier. If one, in addition, considers that

the ultrasonic responses from flaws have a variety of frequency

dependencies, the net result of all these parts of the system, if

misaligned or not compatible, can have strange effects on the output

signals. The pulser, in particular, is a very important part of the

system which affects the search unit and is affected by it. It is

necessary to look at the specifications of the electronics and search

unit and ascertain whether instrumental adjustments are consistent with

these specifications. Despite this presentation, someone commented

later that if you could set-up on the standard provided and if there is

any reasonable dynamic range, the problem is not primarily with the

electronic equipment.

The need to build instrumentation with greater uniformity of

performance was mentioned.

A number of people emphasized the importance of developing a standard procedure or recommended practice for evaluating ultrasonic

systems and their components. These standards should enable the user as well as the manufacturer to evaluate the equipment performance by a

Acommon procedure. Many electronic characteristics of an ultrasonic

system vary considerably from one system to another as well as with

time. These characteristics include: voltage, rise time and width of

the pulse, receiver bandwidth, linearity of amplifier response over useful frequency range, bandwidth of gating circuit, and proportionality

of the signal passing through the gate to the recorder. The phase

response also should be added to this list in some applications. A

basic problem regarding the electronics is to know how the instrument

perturbs the incoming signal. Is this signal actually proportional to

the signal received by the transducer? How do the components when

connected interact and influence each other? What are the conditions

under which components should be evaluated to make the results

meaningful?

The potential of what could be learned from experience with radar

was mentioned. In particular, should we have an ultrasonics test set

analogous to the radar test set, a neat little package for measuring the

performance of the electronic instrumentation?

Another way of going about the calibration problem is in analogy

with testing high fidelity equipment and is basically the method of

using a test sound recording for overall system evaluation. The

distinction was made between much specific technical qualifications of

the various parts of the equipment versus a standard which tests the end

use. However, if one takes the latter approach to the extreme, then

problems of reproducibility arise. You may get the sound you want but

if something breaks down, how do you replace it with a unit of similar

characteristics? The problem then reduces to determining the minimum

number of critical tests, purely electronic, which would satisfactorily

describe the electronic performance. In any case, if we had a minimum

number of tests which are accepted by industry, the vendor could list

these specifications when he sells the equipment. If one knows what to

ask for, the Air Force as a major buyer of equipment, for example, could

require bids giving those specifications.

4. The Operator Problem

The seriousness of the operator problems was highlighted by the

following study conducted by the Air Force Logistics Command. A common

reference for set-up to detect fatigue cracks uses a triangular slot in an object simulating the structure under scrutiny. The operator

proceeds to iterrograte the actual structure after assuring himself that

the equipment has responded properly to the simulated flaw in the

standard. The operator is conditioned to a "can't miss" frame of mind

when he sets up the equipment on the standard. What he sees is usually

a clean signal using a well specified set of conditions on a simulated

flaw in a known location. Contrast those conditions with the actual

flaw detection with ragged signals of varying structures confused by

noise which may or may not contain flaws in any of a number of possible

locations. It is very easy to miss a flaw in the real world

A7
situation. Reliability of detection was emphasized in this study rather

than precision in assessing flaw characteristic.

The Air Force Logistics Command has gained some insight to the

difficulty of the problem through a field evaluation of its

capabilities. This program takes structural hardware to the field and

depot environment where technicians conduct NDI tests on this hardware

with their own equipment in the conditions they normally work. Detailed

procedures are supplied and each participant can ask any questions about

the task and seek help in setting up the equipment and help in finding

out how to perform the task, which is to detect fatigue cracks and

report their locations. The procedure calls for ultrasonic shear wave

NDI at 5 MHz with a 70-degree transducer. A wide variability in results

was obtained. The best performer detected 33 of 42 flaws in a section

of wing box containing slightly over 500 inspection sites, while the

three poorest performers detected none. The results of this study

showed that the most significant influence on detection capability may

be attributed to the operator himself. The base that found the most

flaws has a good certification program and is selective in the selection

of testing personnel.

The demands of calibration and the set-up of equipment which is

automated will require the use of skilled operators. Yet the statement

was made that we must get rid of the inspector.

5. Material Characterization

The need to know material parameters was emphasized a number of

times during the course of the workshop. This may be of particular

importance, for example, in working with dispersive materials such as graphite (nuclear industry) which strips off the high frequencies.

Anisotropy in welds creates significant problems for ultrasonic

examination and problems for standardization. Scattering within the

weld due to anisotropic variations or grain structure variations may

mask a defect or create a false indication.

Standards are needed not only for performance tests of equipment,

but also those related to material performance. The need to specify

surface conditions, such as roughness and waviness, was stressed.

Raw materials vary significantly in properties, in particular,

attenuation. There are data to show that errors up to 20 dB may occur

even using the concept of transfer of attenuation to try to account for

such material variations. Large temperature variations as encountered

in the nuclear industry can have a large effect on the sound attenuation

in materials.

A- 8
NASA has to deal with a wide range of materials, nickel-base alloys

(superalloys), titanium alloys, aluminum-based alloys, and a wide range

of composites, metal and non-metal matrix composites. Such a wide

variety of materials makes the use of ultrasonic standards difficult.

Nevertheless, NASA uses conventional flat bottom holes whenever it is

necessary. to set up some sort of accept/reject criteria.

Composite materials are very dispersive and tend to vary quite a bit with moisture content. These factors make the use of standard

reference blocks difficult.

NASA has a program whose intent it is to develop the preliminary

means of estimating fracture toughness using ultrasonic measurements

such as attenuation and velocity. Such measurements are used to monitor

the onset and extension of fatigue cracks and fracture toughness during

tests.

There is a need to establish a program to make quantitative

measurements on the acoustic properties of materials, particularly

attenuation and scattering.

6 . Standards Implications for Developing Systems

Imaging systems

The wavelengths, generally speaking, are the same order of

magnitude as the defects we are trying to look at in imaging systems;

therefore, there is a resolution problem. There Is certainly an identification problem because things do not look the way we are accustomed to seeing them look. Consequently, what is needed for

calibrating imaging systems is a set of reference images, something like

the radiographers use. Size and resolution calibrations are required.

Side drilled holes at various depths and different spacings have been

used to test the resolution and quality (focus) of imaging systems. A

s( et of balls whose spacing is varied has been used to demonstrate

spatial resolution.

Although two years ago it was said that for imaging no standards

are required (except perhaps for size calibration), a position taken

again by some at this workshop, the present view is that ultrasonic

imaging has a long way to go in sophistication before we get to that

point. So meanwhile, we must still rely for calibration purposes on the

flat bottom hole, side drilled hole or some other artifact.

Scattering methods

In order to utilize scattering information properly, a suitable

calibration procedure has to be capable of using multifrequency

information. It should be amenable to calibration of either a pulse

echo or pitch-catch system. A key aspect to the solution of this

A9
problem is the use of theoretically characterized scattering objects,

although one could use empirical characterization. However, when one or few standards are desired for many frequencies and experimental

situations, the number of measurements may become prohibitive, whereas

theory can be used to generate efficiently the required solutions.

Given a spherical scatterer in a solid, one can develop a selfconsistent calibration procedure in the following way. One can formulate a transfer function which in the frequency domain is just the

product of the transfer functions of the individual components of the

system. The purpose of the calibration procedure is to isolate and

define separately the transducer, the medium, the scatterer, etc. so that the frequency dependence of the scattering object can be separated

from the rest of the system. Thus, the absolute efficiency of the

transducer is determined, the attenuation and diffraction losses of the

material are measured and the electronic system is calibrated, the

latter by purely electronic techniques. Now, to calibrate the complete

system to determine whether it gives the correct answers, one determines

whether the system gives the theoretically known scattering for the

sphere. A sphere embedded in glass can be fabricated at low cost and

provide a lightweight standard which is portable and durable. It should

have very low attentuation if properly made.

There were a number of comments regarding this approach. The

problem of dealing with attenuation in the range equation was cited.

Ideally, the attenuation should have a small effect on the signal

compared with the target reflection. The effect of the difference of

the loading of the transducer on glass and on the metal of the test

object must be taken into account. Although a good test of the

goniometer device using the spherical scatterer would be to make pulse

echo measurements from all sides to determine the variability of the

response, this has not been done.

i 4 < Automation

Computer-automated systems to automatically scan parts with complex

geometry have been used in the aerospace industry. For testing such

systems, a beveled block with flat bottom holes was used. Between scans

of parts, the transducer is sent back to a certain set of position and

rotation coordinates over the reference block. The computer makes sure

a strong top surface reflection is received, and the back surface

reflection is of reasonable amplitude. The transducer is then sent (by

the computer) to another area of the block that has a flaw embedded in

it and makes sure the time base is calibrated by checking the timing

between the top surface reflection and that from the flaw. The

transducers are checked by using the Fourier transform capability of the

system.

A- 10
The use of advanced product standards, particularly for checking

the performance of automated systems, was discussed. It was stressed

that these have been very expensive to develop. Whether the flaw is

natural or artificial, but particularly in the latter case, it is

important to find out as much as possible about the flaw by a variety of

methods if it is to serve as a standard.

For computer controlled systems where the computer is in total

control, it will be particularly important to educate the computer.

Since we have to know that the data that are being programmed are

correct, a standardization procedure is a must.

Adaptive learning techniques

A difficulty with the adaptive learning technique is that it takes a great deal of data for training the computer. Where a defect is

theoretically characterized, the network is easily trained on theoretical solutions and can be tested against experimental ones. As

an example of this, the network may be trained with spherical scatterers

and tested against ellipsoidal scatterers.

Some general remarks

If computer interpretation is to be reliable, the following must be

accomplished: the means for measurement of pulser, search unit, energy

transmitted, character of the defect as a function of what it does to

the sound energy and the energy received by the search unit and

amplified energy. In complex signal processing systems, it was stressed

that the most important problem is to understand what one is doing so

that the information derived from the processing is in reality

meaningful. When using more and more features of a signal it may be

necessary to control or standardize more and more features.

7. Quantitative NDE

A view was expressed that people have not asked the right

questions in the area of standards. Given a better qualitative tool,

improved standards will not make an incremental improvement in the

overall capability in that the best that will come out is still

qualitative. No matter how good standards are, a truly quantitative

capability giving flaw size is not attainable using conventional pulse

echo systems. Better standards alone do not make NDE more quantitative,

although they make it more reliable. Imaging systems to replace pulse

echo systems of the same size, flexibility and cost are now feasible.

This will be a major step toward quantitative NDE because one can now

see the defect and know its orientation. Assuming that quantitative NDE

is going to have a major impact in the whole of NDE, what does this mean

regarding standards? The speaker said he did not know the answer and

I A- 1 1
continued; this is what we are hoping the NBS/ARPA program 10 will

resolve; hopefully, something gutsy that says what the role of

ultrasonic standards would be in the future.

What then is the function of standards if we have a quantitative

capability? The calibration of the instrument is one function. Should

the standard be a collection of defects? Does it have to be absolute or relative? Costs, of course, must be a factor to consider. Should the

standards be based on theoretical models? It would appear that they

should provide a more solid foundation on which to base measurements.

Finally, if after the approach is decided, what fabrication techniques

are reproducible and affordable? There is a diffusion bonding technique

that is very attractive in many ways. However, is there something

cheaper which is satisfactory? In short, it is necessary to define the

goals and that is what NBS should do. It is worth emphasizing that if we can really measure size, shape, and orientation, do we really need

standards?

Current problems should disappear when NDE really becomes

quantitative as it will in a few years. Such problems as are traceable

to the operator will disappear when his current inferior tools are replaced with much better ones. Granted that ultrasonics is moving heavily in the imaging area to

obtain more quantitative results, the previous discussion was countered

by the thought that standards will nevertheless be required regardless

of how far ultrasonics or quantitative NDE advances. For example, it seems unlikely that quantitative NDE will advance beyond the stages of

photography for quantitative results. Yet, we use standards to measure

spatial resolution, the grey scale response, etc.

The universality of imaging system usage was challenged. For

example, because of the inaccessibility of steam generating tubing, one must test this item in situ with pulse echo equipment. Although it is

certainly helpful to know the size and character of a flaw with high

precision, it first must be detected. The detection of flaws is a primary objective of NDI in the maintenance and production

environment. However, with the advent of fracture mechanics, there is

increasing need to know the exact size of a discontinuity.

The question was raised whether within the framework of

quantitative NDE there is a basic difference between flaw detection

versus resolution of the flaw itself. Is it a foregone conclusion that

if you have a better method of resolving the flaw, you have a better

method of detecting it? The answer to this question was that it depends

on the sensitivity of the method. However, within the sensitivity

range, if it is quantitative, there is a way of narrowing the curve for

probability of detection.

10 Author's comment: the speaker is referring to the subject of this

report

A- 12
The opinion was expressed that a series of graded flaws would not

be an approach to quantitative NDE. Trying to duplicate actual flaws

has an infinite number of possibilities. Once we have a quantitative

system, such as an imaging system, a test of its resolution will pretty

well define the performance of the instrument. With such a test of

resolution, standards will become well defined and rather clear.

However, maintaining that performance or diagnosing a problem is another

matter. But until that point is reached, we do not have quantitative

NDE. Thus it may not be worthwhile emphasizing the systems that are presently in use, and consequently, standards for this system. Contrary

to this thought, it was mentioned that it might take 20 years to make

these systems reliable and by that time we will have something better,

i.e. quantitative.

8. Directions, Problems and Approaches

There are two approaches to the problem of calibrating an ultrasonic NDE system: to measure the characteristics of all components

of the system separately, or to calibrate the whole system at once using

some appropriate block.

The issue was raised several times, which parameters of an ultrasonic system are crucial for the detection and characterization of

flaws? Why should time be wasted in measuring 15 parameters if only 5

are, in fact, crucial?

Role of standards. Is the role of the ultrasonic standard to setup and to calibrate? Is it to be a basis for defect detection? Is it

to be a basis for defect characterization or the basis for common comparison and communication? It was suggested NBS can offer a benchmark measurement to help a user calibrate a system.

The question of standards and codes in the German nuclear industry

was discussed. In this country, we have committee (consensus) action

which results in a code with minimal requirements. In Germany, there is

a private company, under contract to the government which enforces the

nuclear and other codes. The German codes are based on the DGS system

of standards and calibrations, which was referred to favorably more than

once in this workshop. The German requirements are based on a combination of theory and practice. For example, these requirements

consider what happens if the search unit is not quite at the correct

frequency. Also, a nomogram has been developed for obtaining the

response of a flat bottom hole relative to that from a side drilled

hole.

The Air Force indicated that it is committing over 1/2 million

dollars from 1978 through 1980 for the development of new standards, the

technology for producing them and standards for manufacturers. A

problem area of real concern for the Air Force is that although it

A- 13
look at the 95% confidence level, the probability of detecting flaws at

that level is not very good in the field.

The Army noted that there seems to be an extension of the consumer protection act to the G.I. As a consequence, 100% inspection is

required and automation is being heavily used. This means that built-in

self-checks on the equipment are a must, also that actual standardized

flaws would be helpful.

According to an account of the needs of the Nuclear Regulatory

Commission, the problem of detecting and characterizing flaws arises

from insufficient standardization and variation in ultrasonic equipment

and procedures. For example, methods for measuring transducer

characteristics such as beam spread, damping characteristics and

frequencies for peak response are not well defined and there are no provisions for keeping track of them from one examination to another in

the ASME code. Large temperature changes can have an effect on the

transducer characteristics and on sound attenuation in the material.

Similarly, pulser, receiver, amplifier, display screen size or linearity

may vary from one inspection to another and influence the magnitude of

the flaw indication. The needs are for well-defined ultrasonic

procedures and standards. Present practice is based on the ASME Boiler

and Pressure Code which has no standards for either ultrasonic systems

or components.

Given the knowledge of all the characteristics of the transducer

and all the properties of the electronic system, it may still not be

possible to infer the actual performance of the NDE system. The biggest

impact one could have in the short term would be to produce

specifications for the electronics. The next thing would be to produce

a standard which is not only reproducible, but would also work in

calibrating an imaging system. On a lower level of priority, one should

calibrate transducers simply and try to improve them. (Others thought

this should be a very high priority.) It is not going to be worthwhile

to do an extensive transducer calibration because it is complicated and

expensive, and moreover too difficult to relate electrical-acoustical

properties to actual performance. To this view should be contrasted the

view that if a proper transducer calibration has not been accomplished,

and if the transducer happens not to be stable, it will not be possible

to know what has changed. It was also pointed out that the calibration

of transducers may not be such a monumental task with improving methods,

such as a simplified approach to obtaining field patterns. Again, the

thought was expressed that such information cannot be related to actual

performance. However, it was speculated that in an imaging system, the

capability of the instrument could easily be specified.

9. ASTM Ultrasonic NDE Standards

Although the current ASTM activity has been generated almost

entirely empirically, it has much in common with the theoretically

A- l 4
derived system which requires, among other things, the following:

targets which are disk shaped, perpendicular to the axis of the sound

beam, and have constant and essentially 100-percent reflectivity, disk

diameters that are small compared to the beam diameter and usually

larger than a half wavelength in the material, and target distances

which correspond to the part thickness.

These conditions are essentially met for most of the commonly used

search unit sizes, frequencies and test conditions by the test blocks

provided in the ASTM distance-amplitude and area-amplitude test block

set. Echo amplitude for constant target distance varies as the target

area. For constant diameter, it varies essentially as the inverse

square of the distance.

The disk-like targets in flat bottom hole blocks approximate

typical flattened discontinuities actually found in forgings and raw

stock such as plate material. The disk diameters produce echo

amplitudes of the same order of magnitude. In other fields of

inspection, for example, pipe and tubing and weldments, many other

configurations of artificial defects have been used such as notches and

side drilled holes. However, in general, these do not meet the original

ASTM criteria established to provide a relatively neat physical target

and to meet the sensitivity and amplitude requirement for test blocks.

Test blocks have the following advantages. They check a complete

system: instrument, search unit, cable, connections and couplant for

sensitivity and noise level; allow a meaningful resolution check under

actual operating conditions with a variety of search units and they

establish initial set-up conditions. They can provide direct comparison

of echo amplitude for determining material acceptability on a customerto-vendor basis. Test blocks have a number of disadvantages: they may

be difficult to reproduce precisely; they do not easily allow a separation of search unit and instrument characteristics; it may be

costly and time consuming to perform standardization and calibration

using test blocks; even with the same block, reproducibility may be

difficult depending on operator capability; and, finally, there is a certain lack of versatility.

The ASTM aluminum blocks come closest to representing from the ASTM

viewpoint a general purpose reference block. They meet the target size

and distance requirements, and use a material that is more or less

uniform. The blocks are made from a reasonably hard alloy so that they

can withstand both contact and immersion use. They can be anodized

against corrosion. If one wanted to test a steel part using these

aluminum blocks and the attenuation was ruled to be negligible for short

distances then only an impedance mismatch factor would have to be taken

into account.

It was hoped that a large group of blocks could be made and held to

within 10 percent tolerance, a goal which was really never met even when

A- 15
the test frequency was lowered from 15 to 5 MHz. Although a spread of 2 db for amplitude blocks is unacceptable, 2 db tolerance at 5 MHz is

presently written in this standard.

Many people have felt that standardization of the ultrasonic system

using test blocks and ball targets is not enough. It is necessary to be

assured that acceptable techniques for separately measuring transducers

and the electronic parameters of the instrument are available. For this

purpose, ASTM Section E07.06.09 was formed. Group A establishes and

writes procedures or recommended practices for measuring the various

transducer parameters. Group B establishes procedures for measuring the

electronic characteristics of the various modules of the ultrasonic

instrument: the pulser, receiver, video section and alarm circuits.

Group C develops the characteristics of instruments for use in hostile

environments. Group D reviews various international documents on standardization for input to the ISO TC 135 Group, which reports to ANSI

through ASTM. Group G is dealing with the identification of vertical

linearity of the instrument. The present practice is covered by ASTM

E-317. Group E is preparing an improved document that can be used for

looking at a total system in the field. The document provides a relatively quick check for the average technician or engineering aid to

ascertain whether the system is functioning properly.

The impact of the ASTM output will be that, for the first time, the

engineer and instrument designer will have a set of specifications to

guide instrument design. He will have measurement methods so that he

can specify values for such parameters as resolution, sensitivity,

signal-to-noise , vertical and horizontal linearity, etc. The intent of

the ASTM Group will be not only to provide a recommended practice useful

to the instrument manufacturers, but also to the customer of the

instrument. As part of the program, the procedures for testing the

various components will be outlined in such a way that any well-equipped

electronics laboratory can make its own measurements. There again, we

feel that this will be the first time that there will be agreement on

test procedures and the evaluation of the various parameters.

When the above is accomplished there should be no excuse for the

instrument being an unknown quantity, which has been mentioned a number

of times at this workshop. This program will impact not only on the

reliability of a particular product but also on the liability that so many companies are worried about. Consequently, the implication of this

standardization program to industry as a whole will mean safer and more

reliable products.

It was noted that the pattern followed in this workshop very

closely resembled the pattern of discussion in ASTM Subcommittee

E7.06. However, this should not be surprising since these are the same problems that experts have been thinking about for a number of years.

A- 16
There still are, of course, some problems with some ASTM

specifications. Data indicates changing search units can introduce a variability of more than 25 percent in tests with blocks, yet all of the

six search units involved in a test at NBS were acceptable according to

current ASTM specification E127-75. Obviously, improvements in this

specification are needed to meet the ASTM goals. Blocks shorter than

1/2 inch metal distance cannot be checked using ASTM E127 specification

because of near surface resolution. Yet ASTM E317 recommends the use of

a 1/8 inch metal travel block for checking resolution.

Section E7.06.10 is addressing the problem of steel reference

blocks and applying correction factors to ultrasonic examination of

steel with cylindrically curved surfaces.

10. The Role of NBS

Many statements, suggestions, needs, etc. were raised regarding the

role of NBS in ultrasonic NDE standards.

1. The suggestion was raised that perhaps NBS should act as a clearinghouse for developments in NDE and medical diagnosis to provide a cross

path for ideas in one area to get to the other.

2. The complaint was raised several times that standards in general are

not traceable to NBS.

3. NBS does very little in the way of standards setting, although it

does work very closely with the standards organizations in helping them

introduce standards which industry recognizes and uses. One of the

basic functions Qf NBS is to service measurement systems, which

distinguishes it from a regulatory role. NBS defines proper

measurements and provides the means for making such measurements. It is

necessary to define an acceptable uncertainty in the measurements with

the same material and equipment and with different pieces of equipment

at various locations.

4. There is a hierarchy of calibrations, some which NBS alone might

use, and others that might be used in field laboratories.

5. In a proper measurement system it is helpful to deal with obstacles

that are theoretically characterized; this provides an understanding of

the standards and calibration procedures at a basic level. However, the

proper role of theory in ultrasonic NDE standards has not been

clarified. For example, is it more useful to know the absolute

scattering or rather to use theory as a means of transferring a calibration or standard from one situation to another?

6. There is an important question regarding the conditions under which

components should be evaluated in order to make the results

meaningful. This question arises, in particular, in assessing how

A- 17
components interact with one another and how different transducers

affect the pulser. This was suggested as an area for NBS involvement.

7. NBS should provide industry with a recommended practice for field

measurement of the electronic instrumentation and transducer

characteristics, a difficult problem. The NBS approach at present,

however, is to offer a transducer calibration service. This calibrated

transducer can be compared against others and determine, for example,

what properties one wants from that transducer. It was emphasized again

that NBS is not a regulatory agency.

8. NBS should develop a standard for imaging which would serve as an all-purpose one and have the following properties. (a) It is based on a theoretical model so that one can understand what is happening. (b) It

would simultaneously check the lateral, transverse and depth resolution

of a system. (c) It would have a range of structure inside of it so that you could get an acoustic impedance ratio to check on the

contrast. It might be possible for one block to be useful in the range

from 1 to 25 MHz. It would be necessary to work out the ranges on impedance ratios to span a range of contrasts. It might be necessary to

have one block for each impedance ratio. (d) The program should treat

the three-dimensional problem.

In the short term, the problem is simply to get meaningful

standards. For example, the electronic problem can probably be handled

today, and its solution could go a long way to satisfying some of the

current needs. In the 5 to 10 year period, however, near and far term

solutions would provide the capability for a quantitative system which,

moreover, would serve as a standard for people with simpler problems.

9. If quantitative NDE capability is going to have a major impact in

the whole role of NDE, what is the role of standards? This is a question that the NBS/ARPA program will address. Do we even need

standards if with imaging systems we can really measure the size, shape,

and orientation of defects?

10. Another area which would fit the NBS charter well and be meaningful

to everybody is to establish a program to make quantitative measurements

on the acoustic properties of materials, particularly on attenuation and

ultrasonic scattering. Very little technology has been developed to

make such quantitative measurements in the frequency domain of

ultrasonic NDE. One cannot go to a handbook and obtain acoustic

attenuation and scattering values to determine, for example, that a number one hole in 304 stainless steel could be used to provide a specified reflection. Such measurements cannot be made with ordinary

flaw detection equipment. The attenuation should be measured as a function of frequency and also orientation because anisotropy should be

considered. Such a compilation would be rather complicated but

useful. The ultimate object may be to establish a signal-to-noise ratio

A- 18
for materials, i.e., a detectability number related to a disc or spherical scatterer or whatever as a function of frequency and distance.

11. NBS plans to address forward-looking problems. Nevertheless, it

would be a mistake to totally address problems that might need to be

solved in the future and ignore near-term problems that will impact

conventional systems (and future systems) destined to be used for many

years

12. An overlap in the deliberations of this workshop and ASTM

Subcommittee E-7.06 was noted but not found to be surprising that a group of experts should consider similar directions. Regarding this

overlap in ASTM and NBS, close cooperation is needed to avoid

duplicating efforts on the one hand and to make more rapid progress in

these complex areas than would otherwise be possible on the other hand.

The following near term goals of the NBS ultrasonic program were

stated:

o Work will continue to provide calibrations for flat bottom hole

reference blocks in aluminum and pursue the development of

suitable blocks for steel and titanium.

o Develop fatigue cracks that are well characterized with regard

to ultrasonic response.

o Pursue field point characterization of pulsed transducers.

o Provide a theoretical model of the transducer. The model of the

infinite baffle piston transducer is not accurate enough. If

for CW excitation, we suppose zero velocity at the edge, the

results of the axial profile are much different than for uniform

velocity.

o There is a definite need for the evaluation of transducers when

loaded, if not by the material actually inspected, at least by

something with a reasonably similar impedance.

o At present, there is no agreement on what variables should be

measured in an instrument or how to measure those variables.

The NBS role will not be to specify instrument accuracy, it will

be to determine what kind of measurements are appropriate.

Some future directions were suggested:

o Preparation of samples to make reliable measurements of residual

stress

o Develop clear analytical procedures that could be used in the

design of transducer and possibly improve their performance and

reliability.

o Develop product standards by developing a procedure for

producing cracks by applying a given stress function for a given

loading program to produce a reasonably uniform fatigue crack.

o Make attenuation measurements.

A- 1 9
o The NBS role in the imaging atea is not at present clear.

However, there is the possiblity of reference images such as used in radiography, for example.

11. Summary of the Workshop1 *

1. It is necessary to clarify the distinction between calibration,

reference and sef-up standards, and develop a common usage.

2. It seems that one type of block, electronic standard or one type of

instrument in NDE, or for that matter in any field, cannot be expected

to do all .things for all people. Since a variety of types has to be

available, the question then is how many different varieties.

3. Regarding European versus U.S. Standards, perhaps we should examine

our system especially with regard to the use of minimal requirements.

4. There was a great deal of discussion on transducers as the most

critical,, non-reproducible component, although more comment was made

regarding the need to standardize the operator.

5. If one has a thorough understanding of the theoretical concepts

involved in any process, that is certainly an assistance in making the

measurements. Conversely, since the measurements are required to

substantiate the theory, there is a great deal of interplay.

6. For most commercially available transducers that are conventionally

used, the plane wave approximation works. The vibrating piston type of

theoretical treatment seems to give a fairly good representation of

measurements in a number of cases. However, examination of the

vibration of a piezoelectric element shows that it acts quite unlike a vibrating piston. On a time average the piston idea may work fairly

well, although it does not really vibrate as a piston. The piston

approach was said not to be correct for the receiver mode.

7. Regarding beam profiles, the technique using a microprobe is

superior to that using the sphere in water. In either case, it should

be emphasized that the wave fields in solids are quite different from

those in water.

8. As a result of work at NBS, it was found that metallurgical

variations were responsible for most of the variability of results in

blocks tested in a round robin survey. With metals, one has to be

concerned about composition, homogeneity, texture and surface

conditions. Although making uniform, homogeneous metal in quantity is

very difficult, it might be possible to reach such a goal by suitable

process control.

rr Presented at the workshop by Prof. R. Green.

A- 2 0
9. The wide variety of blocks which have been used or proposed were

discussed. These include: flat bottom hole, an angle flat bottom hole,

side drilled hole, diffusion bonded sphere, steel ball in lucite, steel

ball in water and triangular slot. In the opinion of this reviewer, a specific defect standard, although very difficult to fabricate in

general, would be the best of all.

10. The great variability in the performance of instruments, even the same type of instrument by the same manufacturer, was the subject of

much discussion.

11. One will need a set of reference images for ultrasonic imaging

systems similar to those currently used in radiography.

12. The point "garbage in equals garbage out" scored. The computer

cannot possibly help this situation but only make it worse.

13. The operator competence and reliability problem looms as a very

large one.

14. We should pay attention to the list of criteria being developed to assess the wide variety of blocks that are being used or will be

proposed.

A- 2 1
ULTRASONIC WORKSHOP PARTICIPANTS

Mr. Harold Berger

National Bureau of Standards

Washington, DC 20234

Dr. George Blrnbaum

National Bureau of Standards

Washington, DC 20234

Mr. J. E Bobbin

Krautkramer Branson Inc. 250 Long Beach Blvd.

Box 408

Stratford, CT 06497

Dr. M. Buckley

ARPA

1400 Wilson Blvd.

Arlington, VA 22209

Mr. R. I. Buckley

Texas Instruments Inc.

P. 0. Box 941

Attleboro, MA 02703

Mr. C. E. Burley

Reynolds Metals Company

4th & Canal Streets

Richmond, VA 23261

Mr. H. H. Chaskelis

Naval Research Laboratory

Code 8435

Washington, DC 20375

Mr. Daniel J. Chwirut

National Bureau of Standards

Washington, DC 20234

Dr. Roger B. Clough

National Bureau of Standards

Washington, DC 20234

Mr. D. L. Conn

Research Center

Armco Steel Corp.

Middletown, OH 45042

Mr. Ben Cross

Rockwell International

Rocky Flats Division

P. 0. Box 888

Golden, CO 80401

Mr. George A. Darcy

Industrial Applications Branch

Army Materials and Mechanics

Research Center

Watertown, MA 02172

Dr. Donald G. Eitzen

National Bureau of Standards

Washington, DC 20234

Prof. J. R. Frederick

2046 E. Engineering Bldg.

Dept, of Mechanical Engineering

University of Michigan

Ann Arbor, MI 48103

Dr. Harold Frost

Applied Research Laboratories

The Pennsylvania State Univ.

P. 0. Box 30

State College, PA 16801

Dr. V. S. Goel

USG Nuclear Regulatory Comm.

Washington, DC 20555

Mr. Sam Golan

National Bureau of Standards

Washington, DC 20234

Dr. Robert Green

Johns Hopkins University

Mech. & Matls. Science Dept.

Baltimore, MD 20218

Dr. Logan E. Hargrove

Office of Naval Research

Code 421

800 N. Quincy Street

Arlington, VA 22217

Mr. John Cope Mr. S. D. Hart

Newport News Shipbuilding & Drydock Naval Research Laboratory

Department X03 Code 8435

Newport News, VA 23607 Washington, DC 20375

Dr. James Couchman

General Dynamics Corp.

Applied Research

P. 0. Box 7258

Fort Worth, TX 76101

A-22

Dr. Nelson Hsu

National Bureau of Standards

Washington, DC 20234
Dr* L. C. Ianniello

Chief, Mtlurgy & Cer. Bra*

Materials Science Office

Division of Physical Research

U.S* Energy Res. & Dev. Admin.

Washington, DC 20545

Mr. Stanley Klima

NASA Lewis Research Center

21000 Brook Park Road

Cleveland, OH 44135

Mr. W. E. Lawrie

Babcock & Wilcox

P. 0. Box 1260

Lynchburg, VA 23305

Dr. Hassell M. Ledbetter

National Bureau of Standards

Boulder, CO 80302

Dr. Melvin Linzer

National Bureau of Standards

Washington, DC 20234

Mr. Robert W. McClung

Oak Ridge National Laboratory

P. 0. Box X

Oak Ridge, TN 37830

Mr. P. C. McEleney

U. S. Army Materials & Mechanics

Research Center

Arsenal Street

Watertown, MA 02172

Dr. Eric Miller

National Bureau of Standards

Washington, DC 20234

Dr. Leonard Mordf in

National Bureau of Standards

Washington, DC 20234

Mr. G. J. Posakony

Battelle Northwest

Box 999

Richland, WA 99352

Mr. Charles F. Raatz

Specialist Engineer

Boeing Commerical Airplane Co.

Seattle, WA 98109

Mr. Richard Rowand

Technical Manager for NDT

Air Force Materials Laboratory

Wright-Patterson AFB, OH 45433

Dr. Wolfgang Sachse

National Bureau of Standards

Washington, DC 20234

Mr. Ronald Seiner

Universal Technology Corp.

1656 Mardon Drive

Dayton, OH 45432

Prof. Steven Serablan

Mechanical Engineering Dept.

Lowell Technological Institute

Lowell, MA 01854

Mr. S. I. Shelton

Lead Engineer - NDT

Vought Corporation

P. 0. Box 5907

Dallas, TX 75222

Dr. John Simmons

National Bureau of Standards

Washington, DC 20234

Mr. William H. Sproat

Materials Development Laboratory

Lockheed - Georgia Co.

Marietta, GA 30063

Dr. Donald Thompson

Director, Structural Materials

Center for Advanced NDE

Rockwell International Science

Center

1049 Camlno Dos Rios

Thousand Oaks, CA 91360

Dr. R. Bruce Thompson

Rockwell International Science

Center

1049 Camino Dos Rios , Thousand Oaks, CA 91360

Dr. H. H. Vanderveldt

Naval Sea Systems Command

Code 03522

Department of the Navy

Washington, DC 20362

Dr. E. C. van Reuth

Assistant to Director,

Materials Research

Defense Advanced Research Projects

Agency

1400 Wilson Blvd.

Arlington, VA 22209

Mr. H. E. Van Valkenberg

Automation Industries Inc.

P. 0. Box 3500

Danbury, CT 06810

A-23
PROGRAM

Welcome and Introduction - H.. Berger, National Bureau of Standards

1. View of Current Ultrasonic NDE Standards -- Moderator: D. G. Eitzen, National Bureau

of Standards

a. Rationale for Existing ASTM Standards — H. E. Van Valkenberg, Automation

Industries, Inc. b. Current NBS Program — D. J. Chwirut, National Bureau of Standards

c. Problems in Ultrasonic Testing and Standards — G. J. Posakony, Battelle

Northwest Laboratory

d. Some Criteria for a System of Ultrasonic NDE Calibrations and Standards —

N. Hsu, National Bureau of Standards

e. European Philosophy, Practices and Standards —- W. E. Lawrie, Babcock and Wilcox

2. Recent Developments in Ultrasonic NDE: Implications for Standards — Moderator: D. 0. Thompson, Rockwell International Corp.

a. Scattering Methods - R. B. Thompson, Rockwell International Corp.

b. Transducers and Arrays — W. H. Sachse, Cornell University and National Bureau

of Standards

c. Imaging Systems — S. Hart, Naval Research Laboratory

d. Signal Processing — M. Linzer, National Bureau of Standards

e. Automation — J. C. Couchman, General Dynamics Corporation

3. Ultrasonic NDE Needs in Industry: Implications for Standards — Moderator: R. H. Seiner, Universal Technology Corporation

a. Aerospace — W. H. Sproat, Lockheed-Georgia Corporation

b. Nuclear — R. W. McClung, Oak Ridge National Laboratory

c. Raw Materials — S. I. Shelton, Vought Corporation

^d. NDE Equipment — R. I. Buckley, Texas Instruments

4. Ultrasonic NDE in Government: Current Problems, Future Needs and Implications for

Standards — Moderator: E. Van Reuth, ARPA.

a. Air Force — R. Rowand, Air Force Materials Laboratory

b. Navy — H. H. Vanderveldt, Naval Sea Systems Command

c. Army — G. Darcy, Army Materials and Mechanics Research Center

d. Advanced Research Projects Agency — M. Buckley, ARPA

e. Nuclear Regulatory Commission — V. S. Goel, NRC

f. Energy Research and Development Administration — L. C. lanniello, ERDA

g. National Aeronautics and Space Administration — S. Klima, NASA Lewis Space

Flight Center

5. Summary, Comments and Critique — Moderator: G. Birnbaum, National Bureau of

Standards

a. Problems and Needs for Standards — R. E. Green, Johns Hopkins University

b. Future Directions — D. G. Eitzen, National Bureau of Standards

A-24

,|
APPENDIX B

Qualitative Examination of Some Novel Blocks

At the outset of this study it was thought worthwhile to initiate

some very limited laboratory effort related to alternative ultrasonic

standards. It was decided that hands-on experience with several novel

blocks which could be easily obtained would be useful. Three types of

blocks were chosen for qualitative examination.

A. Spherical Reflectors - There has been continuing interest in

incorporating spherical reflectors in reference blocks. The

interest in spheres is not particularly new since in fact the

primary reference for the ASTM aluminum reference block system as published by ASTM in 1958 is a sphere in water. Tests for

ultrasonic equipment for medical applications also make use of a sphere. The key advantages that ASTM recognized in the sphere were

the clean theoretical description available (a linear relationship

between amplitude and ball diameter) and the reproducibility and

availability of the ball bearings used as reflectors in water.

However, for a field standard it is convenient to have the sphere

embodied in some material with a definite geometry. We consider

here in a cursory way three embodiments of this geometry:

1. a spherical void in a titanium cylinder fabricated by diffusion

bonding,

2. a spherical void in glass,

3. a steel ball bearing in an acrylic cylinder.

Note that two of these are in transparent material with a view

toward the design of material-independent blocks.

B. Side Drilled Hole - A side-drilled hole in a polygonal glass plate

was fabricated. The interest here was in another embodiment of a theoretical describable reflector in a transparent material. The

side-drilled hole is favored by some workers since it easily serves

for both longitudinal and angle beam testing.

C. Conical Block - Blocks of aluminum with truncated conical ends were

fabricated. This geometry preserves the advantages of (primarily)

a disc reflector in a block which is easier to machine and

dimensionally to verify than a flat-bottomed-hole.

Experimental results

Essentially qualitative tests were conducted using a wide band

laboratory type pulser-receiver and a wide band 10-MHz transducer.

B-l
A. Spherical Reflectors

1. Titanium diffusion-bonded block with a spherical void* 2

. Since the

spherical void was located midway along the cylindrical axis, the

reproducibility of the return from two faces could be determined.

It was necessary to gate close to the back scattered waveform from

the sphere because of nonreproducible , nonidentif iable signals,

probably due to grain boundary scattering. It was noted that the

apparent attenuation was quite high. The amplitude of the back

scattered signal of the sphere obtained from the two faces did not

agree very well. However, the integral of the power spectrum

obtained by a fast Fourier transform of the gated reflections from

the sphere gave agreement between the signals from the two ends

within about 3 dB.

The results from this particular specimen were disappointing

since even the agreement of the integrated power spectrum was unsatisfactory. Nonetheless, better results could likely be

obtained with additional fabrication or material controls.

2. Spherical void in glass

An Orrefors paperweight (designed by Olle Alberius) contains a spherical void in glass. The diameter of the sphere is estimated

to be about 7/16 in and consequently much larger than the

wavelength at 10 MHz. The upper part of the "block" containing the

void is a pentagonal pyramid. It was found that the reflections

obtained from each of the sides of the pyramid agreed extremely

well. Moreover, the reflected waveform was in close agreement with

theoretical predictions [42]. It was easy to visualize the sound

field due to an ultrasonic pulse reflecting from the hole in glass

using photoelastic methods [17]. On the basis of these results and

in light of the low cost, it appears that such a glass block could

be considered for possible development. However, reproducibility

of sphere size, for example, was not determined since only one specimen was investigated.

3. Ball bearing in acrylic

An easily formed specimen makes use of a ball bearing in a polyresin cylinder. The sphere is cast in the resin. Of course

other outer geometries are easily obtained. Fabrication is

extremely simple with the reflector geometry being very

reproducible. However, there are questions of resin variation,

moisture control and long term stability. Also, the resin is

12 Obtained by W. Sachse from B. Tittman.

B - 2
rather lossy. A controlled fabrication process may improve the

reproducibility. Despite the disadvantage of high attenuation, its

transparency, ease of fabrication and low cost make a case for

further consideration of such a specimen, possibly with a lower

attenuation resin.

B. Side-drilled hole in a glass plate

Another manifestation of a transparent block easily fabricated is a hole drilled through a plate with polygonal sides. The specimen

used had nine sides which were equidistant from the side-drilledhole. One would expect very good agreement among the reflections

from the various sides. Agreement only within about 1 dB in the

amplitude of the reflected signal was obtained. The side-drilledhole was observed to be less than perfect and could account for some of the variations. On the basis of these very preliminary

measurements, this geometry could also merit further study.

C. Truncated cone

Several specimens were prepared from aluminum in the geometry of a block with a conical truncated end. This provides a disc reflector

for waves which go through the block and reflect from the truncated

surface. The metal travel distance and disc size were fabricated

to correspond to a few of the blocks in the ASTM-type FBH

- block

series. The return signal from the disc appeared to be somewhat

distorted by reflections and interference from the adjoining

conical surface. In addition, this effect might also lead to a relationship between disc area and reflection amplitude which is

not nearly as nice as the very linear one associated with FBH

blocks. It was noted that the amplitude of the reflection was significantly different from that of an FBH block with the same

size disc reflector and metal travel distance. The economies of

fabrication and dimensional checks are apparent. While this

geometry is interesting, additional work would certainly be

necessary before this block could be given serious consideration

for use in NDE measurements.

Conclusions

.

This laboratory effort was of such limited scope that no conclusions based on data can be drawn on the relative merits of the

different block concepts. No statistical information was developed

since only one specimen of each type was examined. Where possible,

however, multiple paths were examined and in one case these showed

rather large differences. The one conclusion that can be drawn is that even in the case of very simple geometries considerable care and effort

are required to obtain reproducible results.

B- 3

MBS-U4A (rev. 9-78

j*. Accession No.

8. Performing Organ. Report No,

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

. TITLE AND SUBTITLE

An Appraisal Of Current And Future Needs In Ultrasonic

NDE Standards

7. AUTHOR'S)

George Birnbaum and Donald G. Eitzen

5. Publication Date

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contrdct/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, state, ZIP) 13. Type of Report & Period Covered

15. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or less factual summary of moat significant information. If document includes a significant bibliography or

literature survey, mention it here.) The purpose of this study, supported in part by DARPA, is to assess the

current status of NDE ultrasonic standards and calibrations and to determine

current and future needs in this area. The source material includes surveys

of the literature and patents, a study of foreign practice, surveys of NBS

and consensus standards (e.g. ASTM) programs, discussions, visits and letters,

and a Workshop on Ultrasonic NDE Standards: "Current Needs and Future Directions",

held October 17-18, 1977 for the purpose of implementing the objectives of

this study. Recommendations for improvement in existing standards will

impact primarily on conventional pulse echo systems and include work on transducers, the electronic system and reference blocks.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalise only the first letter of the first key word unless a proper name;

separated by semicolons) ~ Calibrations; flaw geometry; future needs; orientation; pulse echo systems;

quantitative measurements; reference blocks; ultrasonic NDE standards; and

ultrasonic transducers.

18. AVAILABILITY Unlimited

For Official Distribution. Do Not Release to NTIS

I I Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003-

I | Order From National Technical Information Service (NTIS), Springfield,

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