NBSIR 80-2109

A Basis for Traceable NDE

Measurements

D. G. Eitzen

Mechanical Production Metrology Division

Center for Mechanical Engineering and Process Technology

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

H. Berger and G. Birnbaum

Office of Nondestructive Evaluation

National Measurement Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

December 1980

Prepared for: National Bureau of Standards

U.S. Department of Commerce

lA/ashington, DC 20234

.U56

80-2109

1980

NBSIR 80-2109

A BASIS FOR TRACEABLE NDE

MEASUREMENTS

D. G. Eitzen

Mechanical Production Metrology Division

Center for Mechanical Engineering and Process Technology

National Engineering Laboratory

National Bureau of Standards

U S. Department of Commerce

Washington, DC 20234

National Bureau of Standards

Library, E-01 Admin. Bldg.

FEB 2 7 1981

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H. Berger and G. Birnbaum

Office of Nondestructive Evaluation

National Measurement Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

December 1980

Prepared for: National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology, and Innovation

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

Page

Introduction 1

Ultrasonics 1

Acoustic Emission 3

Radiography 4

Eddy Current Testing 4

Magnetic Particles 5

Liquid Penetrants 5

Visual Testing 6

Other NDE Measurement Areas 6

Conclusions 7

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A Basis for Traceable NDE Measurements

+

by

D. G. Eitzen, H. Berger and G. Birnbaum

ABSTRACT

The National Bureau of Standards is beginning to provide a mechanism

for traceability for a number of NDE measurement procedures, an activity that

is expected to have a significant, positive impact on the reproducibility and

accuracy of NDE measurements. Much of the NDE standards activity has been in

ultrasonics and acoustic emission, these efforts leading to calibration ser- vices for ultrasonic reference blocks and ultrasonic and acoustic emission

transducers. Additional NDE standards are also available or are being developed

in radiography, eddy currents, magnetic particles, liquid penetrants and visual

testing.

INTRODUCTION

The National Bureau of Standards (NBS) has been involved in improving

the reproducibility and quantitative aspects of nondestructive evaluation

(NDE) measurements^ in response to a growing need for improvements in this area. This report is an overview of NBS work which is concerned particularly

with the development of NDE measurement procedures that provide a mechanism

for traceability to National Reference Standards. There are several noninterchangeable concepts termed "traceability" which are carefully explored in

ref. 2.

ULTRASONICS

Pulse/echo ultrasonic techniques offer great potential for detecting

and evaluating material defects nondestructively. However, these methods

are sensitive to measurement equipment characteristics and to the condition

of the reference artifacts used. An effort to improve the reliability and

diminish the uncertainty of these techniques has focused on the development

of measurement services for transducers and reference blocks. The measurement services now available from NBS are described below.

+ This is based on a paper presented at the 1979 DARPA/AF Review of

Quantitative NDE, La Jolla, California.
1. Ultrasonic Transducer Power Output versus Frequency. The absolute

total power output of ultrasonic transducers versus frequency is measured over

any part of a range from about 1-20 MHz using a modulated radiation pressure

technique. The transducer undergoes swept continuous wave excitation and the

radiation force of the ultrasonic beam acting on a conical metal target is detected. The uncertainty is frequency dependent but is nominally about +5 percent. In addition to the relative power versus frequency information, the

measurement provides the value of the radiation conductance used to calculate

absolute power output levels. The apparatus, procedure, error analysis and

sample results are discussed in ref. 3.

2. Ultrasonic Transducer and System Power Output by Calorimetry. A

twin, series flow, ultrasonic calorimetric comparator is used to measure the

time-averaged total absolute power output of a transducer or ultrasonic system

for any voltage input waveform, e.g. pulsed, in the range of 1-15 MHz. The

ultrasonic energy is converted to heat by rubber absorbers and this energy

raises the temperature of the working fluid. The uncertainty is approximately

+7 percent. The system, procedures and uncertainties are described in ref. 4.

3. Aluminum and Steel Ultrasonic Reference Block Calibration. Sets of

ASTM E-127 type aluminum ultrasonic reference blocks are compared with a block

designated as the NBS Interim Reference Standard using an analytical model and

a well -characterized measurement system. The service provides a mechanism for

comparing sets of blocks with the NBS data base and with other reference blocks

through the NBS ultrasonic system. The service has recently been expanded to

include steel reference blocks. The system and detailed procedures are described

in ref. 5.

4. Loaner Services for Transducers and Transfer Blocks. By arrangement,

carefully characterized ultrasonic source transducers and aluminum block transfer standards can be made available for loan. By employing the accurately

measured ultrasonic source transducers, a user's power or frequency measurement

apparatus can be calibrated in situ . Some of the source transducers are cur- rently being employed in an international intercomparison of ultrasonic power

measurement methods. The transfer aluminum ultrasonic blocks, which have been

carefully compared with the NBS Interim Reference Standard, see item 3, provide

a means for users to compare their reference artifacts with those of NBS on their own ultrasonic system; i.e. they provide a basis for traceability to the

NBS Interim Reference Standard. The transfer blocks can provide the basis for

a measurement assurance program. Calibration of users' blocks can be accom- plished by the user with an uncertainty of a few percent using a procedure

developed at NBS.

Additional work on ultrasonic measurement systems is in progress. An

expansion of the NBS measurement service for ultrasonic reference blocks

to titanium is being developed. The feasibility of developing improved

steel and titanium reference blocks was established in 1979.® Also under

consideration are material independent reference blocks made of amorphous,

low-attenuation material; these could replace much of the present multiplicity

of reference artifacts.

2
The influence of changes in the components or adjustments of the instrument on the variations in the amplitude of response from reflectors has also

been studied in some detail. For example, adjustments in the pulse length

of an ultrasonic instrument resulted in differences in amplitude response

from a reference block by over 13 percent, even after normalization with a ball reflector.® A study of the effects of different (but very similar)

transducers was also conducted"^ which showed variations of over 26 percent

in the response of a given block using different transducers. This study

has raised some important questions, for example, what are the necessary

tolerances on the instrumentation in order to obtain the required reliability

and uniformity in ultrasonic NDE?

Another important activity is the development of methods for determining

the directivity pattern of ultrasonic transducers.® A mathematically rigorous

approach based on probing the amplitude and phase in a plane of the radiated

field of a transducer is being developed as a laboratory method. It is capable

of measuring the absolute value of all important point field parameters of

transducers. Work is also proceeding on the development of techniques more appropriate for the user community. See, for example, ref. 9.

Additional work on standards for quantitative ultrasonic NDE is being

planned. This planning process has been greatly enhanced by a study recently

completed for the Defense Advanced Research Projects Agency (DARPA). The

objective of this project was to examine the present system of standards for

ultrasonic NDE measurements, to assess the standards needs of emerging and

more quantitative systems and to present recommendations for the development

of adequate standards for such NDE systems. In addition to an NBS report

describing the results of this study, outputs of this project include a separate summary of the open literature on ultrasonic NDE standards,

detailed study of foreign and U.S. standards documents^ ^ and a chapter on ultrasonic transducers and their characterization.^®

ACOUSTIC EMISSION

A calibration capability has been developed for acoustic emission (AE)

transducers and is now offered as a measurement service. This activity is

part of an Electric Power Research Institute/NBS acoustic emission program and

is also partially supported by the Office of Naval Research. The objective,

to determine the sensitivity versus frequency of AE transducers over the

approximate range of 100 to 1000 KHz, is accomplished by obtaining the voltage as a function of time from a transducer under test and the NBS standard

transducer, both mounted on a large (2200 Kg) steel block. The input is a

3
point force step function on the same surface of the block as the transducers.

The simulated source and block produce a vertical surface displacement that

is theoretically calculable (Fig. 1). The displacement measured by a capacitive transducer developed at NBS is shown to faithfully reproduce the actual

displacement as shown in Fig. 2. The resulting time histories of the output

voltage are digitized, Fourier transformed and processed to obtain the desired

spectral response. A newly designed and constructed standard transducer

which determines absolute dynamic displacement of the order of a nanometer

with an uncertainty of about 3 percent (Fig. 3) has resulted in further

measurement accuracy.

There is also a substantial theoretical effort associated with the

ultrasonic and acoustic emission program. Theoretical developments describing

the planar scanning of the radiated field of electroacoustic transducers have

impacted work on determining directivity patterns of transducers. Also, a relationship between the total force on a transducer and its output voltage

suggests future calibration techniques for users. Another development, which

accurately models the behavior of transducers, is making possible more realistic standards procedures (NBS and ASTM). Theoretical advances in wave propagation in structures are leading to the development of primary and

secondary acoustic emission calibration methods.

RADIOGRAPHY

A Standard Reference Material^^ (SRM) available from NBS^^ provides a mechanism for the traceability of radiographic film density measurements.

This is SRM 1001, a radiographic film step tablet covering the H & D density

unit range from 0 to 4.0. Density measurements are reproducible to about

0.02 H & D units.

Additional standards-related work in radiography at NBS includes a recommended practice for thermal neutron radiography (now under consideration

by ASTM, Committee E-7) and work to determine important characteristics of

x-ray film^ (also being done in collaboration with ASTM E-7).

EDDY CURRENT TESTING

Facilities for dc and ac electrical conductivity measurements have been

completed. The NBS is also establishing measurement procedures for conductivity standards over the range of 1-100 percent of the International Annealed

Copper Standard and methods for the calibration of eddy-current test equipment.

Standard Reference Materials are samples which have been characterized

by NBS for some physical or chemical property and are issued with a Certificate that gives the results of the characterization.

4
Initial conductivity calibration services will be offered to accuracies of

0.5%. It is expected, as experience is gained in the measurements, that

accuracy can be improved to 0.1%. In addition to the planned calibration

facilities for electrical conductivity, the measurement of this quantity will

also be traceable by means of SRM conductivity samples now under development.

Theoretical solutions^"^’^® for the field and current distributions

associated with defects in materials will also provide guidance for eddy

current testing and development of artifact standards.

MAGNETIC PARTICLES

Work is under way to measure the brightness of fluorescent magnetic

particles which is aimed at developing an SRM comparison standard for judging

brightness and providing a calibration service. In addition, studies of magnetic flux leakage are in progress. A recent report^^ describes results of

investigations of a test ring for judging the effectiveness of magnetic particles as used in a military specification. A theoretical model developed to

describe the magnetic response of subsurface defects in the ring showed good

correlation with experiment. This work is leading toward improvements in the

testing procedure for general evaluation of magnetic particles. In addition,

NBS is investigating the potentail of a fluorescent glass standard which would

be used to compare brightness of both magnetic particle and liquid penetrant

indications.

LIQUID PENETRANTS

Standards for brightness of fluorescent penetrants follow the same pattern

as indicated above for magnetic particles. In addition, work is in progress to

prepare a relatively inexpensive, well characterized, crack test plate for evaluating liquid penetrants and their consistency during operation as well as com- paring one product or process against another. To fabricate these crack test

plates, alternate layers of copper and nickel are plated. The nickel is merely

a separator between thicknesses of copper (to values as small as 0.3 ym).

When these copper layers are etched, they provide slots or cracks of known

width which can be determined to within 0.15 ym or 50%, whichever is greater.

The depths of the cracks, determined by etching, are somewhat more difficult

to control. However, for shallow cracks, depth to width ratios of about 4, the

minimum crack depth can be determined to an uncertainty of about 0.5 ym. This

plated assembly can be etched, measured and cut into small specimens as indicated

in Fig. 4. It is expected that this approach will result in a relatively inexpensive plate with cracks for determining penetrant sensitivity.

5
VISUAL TESTING

An important parameter in visual testing is the capability of the inspector

to see details that may be small in size, of low contrast and unsharp. The

visual acuity of inspectors is now measured with a Jaeger chart in which the

inspector is asked to read small black letters on a white background. This

high contrast situation does not always simulate what an NDE inspector may be

called upon to observe. He may be looking for a penetrant indication or a low contrast shading in a radiograph. Therefore, NBS is pursuing the development of a visual acuity measurement procedure that more closely duplicates

the inspector's situation. The new procedure will involve varying the contrast

as well as varying the sharpness and width of indications.

OTHER NDE MEASUREMENT AREAS

There are a number of SRM's available from NBS that impact NDE-related

measurements in addition to those discussed previously. These well characterized

materials provide assistance in several areas. One of these is the determination

of wear through the availability of metallo-organic compounds that permit the

blending of known concentrations of metal in lubricating oil. More than 20 of

these SRM's are available (ref. 15, p. 46).

The measurement of coating thickness by eddy currents, magnetic techniques,

beta (ray) backscatter and x-ray fluorescence methods can be improved by the use of one or more coating thickness SRM's. These are available with magnetic

and nonmagnetic coatings on magnetic or nonmagnetic substrates and gold and

tin coatings on various substrates for radiation measurements. More than 30

SRM's are available for coating thickness measurements (ref. 15, pp. 55-58).

One of the coating thickness SRM's (SRM 1370a) can also be used to

evaluate the ferrite content of austenitic stainless steels. X-ray diffraction standards for powder diffraction measurements and for determination of

iron carbide in steel are available (ref. 15, p. 71). Additional SRM's (ref.

15) include glass specimens for measurement of relative stress optical coefficient (p. 59), alumina for measurement of elastic wave resonances and calculation of elastic moduli (p. 59) and standards for the calibration of the

Mbssbauer isomer shift of iron compounds (p. 71).

Additional SRM's to improve NDE measurements are planned. Possibilities

include calibrated leaks for leak testing in several ranges, reference samples

for wear debris and SRM's for measurement of residual stress.

6
CONCLUSIONS

Several mechanisms for achieving traceable NDE measurements are now available from NBS. These, along with additional measurement services and

improved procedures and techniques, will help industry achieve traceability

and result in more reproducible and meaningful NDE measurements.

ACKNOWLEDGEMENTS

We are indebted to many NBS colleagues for sharing information with

us and for their efforts in forming a basis for traceable NDE measurements.

FIGURE CAPTIONS

Figure 1

.

Theoretical waveform of displacement on the AE calibration

transfer block. The vertical axis is dimensionless time P, S,

and R designate features of the waveform which travel with

speeds of longitudinal, shear and Rayleigh waves.

Figure 2. Displacement of transfer block measured with model 1 NBS

transducer. New NBS transducer gives even better agreement

(not shown) with theory.

Figure 3. Plot showing about 3 percent agreement in dynamic surface

displacement between theory and measurement using new NBS

standard transducer.

Figure 4. Crack sensitivity plate developed for liquid penetrants.

The copper is etched to various depths to simulate surface

cracks.

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REFERENCES

1. H. Berger and L. Mordfin, editors, "Annual Report 1978, Office of Nondestructive Evaluation," NBSIR 78-1581, January 1979.

2. B. C. Belanger, "Traceability - An Evolving Concept," Standardization

News, Vol. 8, 22-28, (Jan. 1980).

3. M. G. Greenspan, F. Breckenridge and C. Tschiegg, "Ultrasonic Transducer

Output by Modulated Radiation Pressure," J. Acoust. Soc. Am., Vol. 63,

1031-1038 (April 1978).

4. T. L. Zapf, M. E. Harvey, N. T. Larsen and R. E. Stoltenberg, "Ultrasonic

Calorimeter for Beam Power Measurements," NBS Technical Note 686

(Sept. 1976).

5. D. J. Chwirut, G. F. Sushinsky and D. G. Eitzen, "Procedures for the

Calibration of ASTM E-1 27-Type Ultrasonic Reference Blocks," NBS

Technical Note 924 (Sept. 1976).

6. D. J. Chwirut, "Recent Improvements to the ASTM-Type Ultrasonic Reference Block System," NBSIR 79-1742 (April 1979).

7. D. J. Chwirut and G. D. Boswell, "The Evaluation of Search Units Used

for Ultrasonic Reference Block Calibrations," NBSIR 78-1454 (Feb. 1978).

8. E. B. Miller and D. G. Eitzen, "Ultrasonic Transducer Characterization

at NBS," IEEE Trans. Sonics and Ultrasonics, Vol. SU-26, 28-37

(Jan. 1979).

9. D. J. Chwirut, "Technical Note: A Simple Technique for Visualizing

Transmitted or Reflected Sound Fields," Materials Evaluation, Vol. 37,

29-33 (Dec. 1979).

10. G. Birnbaum and D. G. Eitzen, "An Appraisal of Current and Future Needs

in Ultrasonic NDE Standards," NBSIR 79-1807 (Oct. 1979).

11. D. G. Eitzen and G. Birnbaum, "Summary of the Literature on Ultrasonic

NDE Standards," NBSIR in preparation.

12. S. Golan, "A Comparison of American and European Ultrasonic Testing

Standards," NBSIR 79-1790 (June 1979).

13. W. Sachse and N. Hsu, "Ultrasonic Transducers for Materials Testing and

Their Characterization," Physical Acoustics, Vol. 14, 277-407, Academic

Press, (1978).

+++

National Bureau of Standards Report

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14. F. R. Breckenridge, C. E. Tschiegg and M. Greenspan, "Acoustic Emission

Some Applications of Lamb's Problem," J. Acoust. Soc. Am., Vol . 57,

626-631 (March 1975).

15. "Catalog of NBS Standard Reference Material," NBS Special Publication

260 (1979-80).

16. R. C. Placious, "ASTM Activity in Industrial Radiographic Film

Characterization," ASNT Fall Conference, St. Louis, MO, Summary Book,

pp. 86-87, (October 1979).

17. A. H. Kahn, R. Spal and A. Feldman, "Eddy Current Losses Due To A

Surface Crack in Conducting Material," J. Appl . Phys., Vol. 48, 4454-

4459 (Nov. 1977).

18. S. Spal and A. H. Kahn, "Eddy Currents in a Conducting Cylinder With

A Crack, J. Appl. Phys., Vol. 50, pp. 6135-6138 (Oct. 1979).

19. L. J. Swartzendruber, "Magnetic Leakage and Force Fields for

Artificial Defects in Magnetic Particle Test Rings," 12th Symp. on NOE, Southwest Research Inst., San Antonio (April 1979).

20. 6. T. Yonemura, "Considerations and Standards for Visual Inspection

Techniques," in Nondestructive Testing Standards -- A Review, pp. 220-

230, STP 624, ASTM, Philadelphia (June 1977).

13
U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 80-2109

4. TITLE AND SUBTITLE

A Basis for Traceable NDE Measurements

5. Publication Date December 1980

AUTHOR(S)

D. G. Eitzen, H. Berger and G. Birnbaum

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS fSfreet. city, state, zip)

8. Performing Organ. Report No.

11. Contract/Grant No.

13. Type of Report & Period Covered

14. SpMMoriitg Asency Code

15. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200^word or leoa /actual auirunary of moot altfrii/icant information. If document includes a significant bibliography or litoratuTO auTvey, mention it here,)

The National Bureau of Standards is beginning to provide a mechanism for

traceability for a number of NDE measurement procedures, an activity that is

expected to have a significant, positive impact on the reproducibility and

accuracy of NDE measurements. Much of the NDE standards activity has been in

ultrasonics and acoustic emission, these efforts leading to calibration ser- vices for ultrasonic reference blocks and ultrasonic and acoustic emission

transducers. Additional NDE standards are also available or are being developed

in radiography, eddy currents, magnetic particles, liquid penetrants and visual

testing.

17* KEY WORDS /six to twelve entries; alphebeticat order; capitalize only the firat tetter of the firat key word unless a proper nam^-, separated by semicolons^ Acoustic emission; calibration; eddy currents; magnetic particles; nondestructive

evaluation; radiography; traceability; ultrasonics; and visual testing.

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