a i nob chnical Activities 1982

Office of Nondestructive

Evaluation

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Office of Nondestructive Evaluation

Washington, DC 20234

December 1 982

Prepared for U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, DC 20234

national bureau

OF STANDARDS

LIBRARY ( Q

J&H W

nol 0-CX-*

NBSIR 82-2617

TECHNICAL ACTIVITIES 1982

OFFICE OF NONDESTRUCTIVE

EVALUATION

H. Thomas Volken , Editor

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Office of Nondestructive Evaluation

Washington, DC 20234

December 1982

Prepared for U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, DC 20234

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

TECHNICAL ACTIVITIES

TECHNICAL ACTIVITIES

TABLE OF CONTENTS

ULTRASONICS AND ACOUSTIC EMISSION

"EPRI/NBS Joint Program on Acoustic Emission," 4

D. Eitzen, F. Breckenridge, N. Hsu and T. Proctor

Task # 15770

"Further Development of the NBS Conical Transducer," 20

T. Proctor

Task # 15770

"Acoustic Emission for NDE Surveillance and Quality Control," 25

J. Simmons, R. Clough, H. Wadley and R. Mehrabian

Task # 15770

"Ultrasonic Techniques for Nondestructive Evaluation," 42

M. Linzer and S. Norton

Task # 15760

"Transfer Standards for Ultrasonic Power Measurements," 46

S. Fick and C. Tschiegg

Task # 15760

"Improved Ultrasonic Reference Blocks," 47

G. Blessing and D. Eitzen

Task # 15760

"NDE Terms and Definitions: Ultrasonics," 50

D. Eitzen and M. Cadoff

Task # 15770

"Contactless Laser Generation and Optical Interferometric Detection of

Ultrasonic Waves," 51

M. Rosen, S. Fick, F. Nadeau and R. Mehrabian

Task # 15760

ELECTRICAL AND MAGNETIC METHODS

"Eddy Current Standards," 53

A. McAlister

Task # 15750

"Eddy Current Testing," 55

G. Free

Task § 15750

"High Frequency Broadband Eddy Current Probes for Improved Flaw

Characterization," 57

C. Fortunko and S. Padget

Task # 15750

"Theoretical Modelling in Eddy Current NDE," 67

A. Kahn

Task # 15750

i ii
"Magnetic Measurements," 69

L. Swartzendruber

Task # 15750

"Magnetic Particle Inspection Standards," 70

L. Swartzendruber

Task # 15750

RADIOGRAPHIC METHODS

"Development and Application of a Second Generation Image Intensifier

for Real Time Microradiography," 71

M. Kuriyama, W. Boettinger, H. Burdette, R. Spal

Task # 15770

"Radiography ," 73

J. Motz, R. Placious and D. Polansky

Task # 15760

"Design Study of a Facility for Neutron-Activation Autoradiography

of Paintings at NBS Reactor," 79

Y. Cheng and M. Ganoczy

Task # 15760

"Development of High Optical Density Standards," 84

J. Hsia, C. Popenoe and R. Booker

Task # 15760

"Visual Acuity Requirements for NDT," 89

G. Yonemura

Task # 15750

OPTICAL AND INFRARED METHODS

"Optical Measurement of Surface Roughness," 90

E. Teague, T. Vorburger, F. Scire, D. Gilsinn, M. McLay and

C. Trahan

Task # 15750

"Light Scattering from Grooves," 96

G. White

Task # 15750

"Holographic NDE," 100

C. Vest

Task # 15750

"Infrared Methods of Nondestructive Testing 104

J. Cohen

Task # 15760

"Evaluation of Organic Coatings Performance on Metallic Substrates

Using Infrared Thermography 105

M. Knight and J. Martin

Task # 15760

iv
NDE OF MATERIALS PROPERTIES

"Small -Angle Neutron Scattering Facility," 107

C. Glinka

Task # 15760

"Investigation of SiC-Al Composite Material by Neutron Scattering,". . 110

H. Alperin and S. Singhal

Task # 15760

"Ultrasonic Applications to NDT and Materials Evaluation," 112

G. Blessing

Task # 15760

"An Ultrasonic Residual Stress Reference Sample," 113

G. Blessing, N. Hsu and T. Proctor

Task# 15760

"NDE Characterization of Thin Structurally Modified Surface Layers on Bulk Substrates," 115

M. Rosen, F. Nadeau and R. Mehrabian

Task # 15770

"NDE of Composites," 116

H. Ledbetter and J. Moulder

Task # 15760

"Green State NDE," 121

T. Negas

Task # 15770

"Crystall ization Kinetics Study Amorphous Zr-Cu By Ultrasonics and

Microhardness Measurements," 123

M. Rosen, F. Nadeau, and R. Mehrabian

Task # 15770

"Non-Destructive Characterization of Polymer Films via Ultrasonics," . 126

D. Hunston

Task # 15770

"Nondestructive Evaluation of Distributed Damage," 131

E. Fuller, R. Fields, E. Case, S. Singhal and C. Glinka

Task # 15770

"Leak Rate Reference Standards," 141

S. Ruthberg

Task # 15770

"Fluorescent Glass Standards for Use in Dye Penetrant Tests," .... 146

R. Burke and D. Hancock

Task # 15750

"Dye Penetrant Crack Research," 148

D. Lashmore and D. Kelley

Task # 15750

"Wear Condition Monitoring," 152

A. Ruff

Task # 15770

v
"Engine Condition Monitoring Through Oil Analysis," 153

S. Hsu

Task # 15770

"Electromagnetic Noise," 159

U. Bertocci

Task # 15770

"Analysis and Modeling of NDE Round-Robin Data for Engineering

Applications," 161

J. Fong

Task # 15770

"Use of NDE in Rehabilitation and Preservation," 165

J. Clifton

Task # 15770

Appendices

A. NDE Related Conferences 167

B. NDE Seminars Organized 168

C. Invited Talks by NDE Office Personnel 169

D. Awards 170

E. Technical and Professional Committee

Participation and Leadership 172

F. Publications 175
INTRODUCTION

This report provides brief reviews of the technical activities

in nondestructive evaluation (NDE) that were carried out by or for the

National Bureau of Standards (NBS) in fiscal year 1982. Most of this

work was supported and administered by the Bureau's Office of Nondestructive

Evaluation but some of the articles describe activities that were funded

by other NBS units or by non-NBS sources.

Technical activities in NDE have been conducted at NBS for many years

but the NDE Program has been administered as a separate entity only since

1975. Summary reports such as this have been issued annually since 1978.

A parallel series of reports provides annual bibliographies with abstracts

for the Bureau's publications in NDE.

The principal function of these reports is to serve as an introduction

to the Bureau's NDE Program. Many readers will want further details on specific aspects of the work or its outputs, and such inquiries are welcomed

and encouraged, both by the principal NDE investigators (whose names and

affiliations precede each of the articles in this report) and by the

Office of Nondestructive Evaluation. Either can be addressed in care

of NBS, Washington, D. C. 20234, or reached by telephone via (301) 921-

1000. Requests for further information and suggestions for new NDE

projects always receive prompt and careful attention.

There is evidence that the NDE Program continues to gain recognition

in the national scientific and engineering communities. See, for example,

the prestigious awards that were conferred this year upon participants in

the Program ( p . 1 70) . It is especially gratifying that NBS personnel received

an IR-100 award for an NDE-related development for the fourth consecutive

year. Other evidence of the Program's standing derives from the requests

received for NDE expertise. For example, NBS engineers were called upon

to ultrasonically characterize a crack in a titanium billet that had

been destined for NASA's Galileo mission (see d

.

11 2 ) . Also the Office

of the Federal Inspector arranged for an NBS expert to review the weld

radiographs of the Alaska-Canada gas pipeline. And similarly, NBS consultation

was sought in connection with radiographic and liquid penetrant inspections

of debris from the Hyatt Hotel disaster in Kansas City.

1
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Task # 15770

ELECTRIC POWER RESEARCH INSTITUTE/NATIONAL BUREAU OF STANDARDS

JOINT PROGRAM ON ACOUSTIC EMISSION

D. Eitzen, F. Breckenridge, N. Hsu, and T. Proctor

Mechanical Production Metrology Division

Center for Manufacturing Engineering

and

R. Clough, J. Simmons, C. Stockton, and H. Wadley

Metallurgy Division

Center for Materials Science

Acoustic emission (AE) techniques have great potential for determining and

monitoring structural integrity. Acoustic emission signals contain

potentially useful information about the location and intensity of defects and

about the criticality of the defects in a structure under load. However,

current signal reception methods do not preserve all of the available

information and signal processing techniques, such as threshold counting, RMS

recording, energy measurement, peak detection, and spectral analysis, and t^ey often

fail to extract the remaining information unambiguously. The difficulty in

interpreting AE signals lies in the inherent complexity of the generating

mechanisms, the transient wave propagation details, and the physics of the

mechanical-to-electrical conversion process of the sensor.

The objective of the EPRI/NBS Joint Program on Acoustic Emission is to

develop certain of the information needed to form a basis for the AE

monitoring of electric power producing structures and to establish the

feasibility of AE monitoring in simpler structures. The achievements of the

first phase of the work were summarized in (1) and included:

- Contributions to data interchangeability of laboratory and field

measurements through the development of a transducer calibration

servi ce.

- Advancement in the state-of-the-art of AE transducer design.

- Contributions to the reliability of AE field data through the

development of calibrated sources.

- A better understanding of AE source behavior through theoretical

modeling of AE sources.

- A better understanding of the limitations of conventional AE methods

through a critical analysis of these methods.

- A framework for understanding the AE process from source to measured

voltage.

- An analysis of the potential information in AE signals, an understanding of the modification to signals by wave propagation, experimentally

verified theoretical predictions of remote displacement due to an AE

source, and part of the basis for secondary sensor calibrations, all

through the theoretical solution for wave propagation in a plate.

4
- Demonstration of the feasibility of determining the actual AE source function in glass and in steel from remote measurements using

time-domain deconvolution.

Development of a methodology for determining the significance of

defects from received AE signals

- Development of several additional very promising signal processing

techniques.

Some of these developments are being used in industrial applications, and

others are being used by other AE researchers. While these accomplishments

are consistent with the technical plan, additional information must be

developed before utility power plant operators can obtain all the information

from AE that is desired.

In this report we summarize some of our recent work towards quantitative

acoustic emission measurements.

The acoustic emission work we describe here focuses on three areas:

- calibration and sensor activities

- characterization of AE sources

- determination of source significance (activity terminated January 1982).

Significant progress towards implementing a quantitative acoustic emission

measurement system for continuously monitoring structures has been made.

CAL I BRAT I ON ACTIVITIES

In January 1980 we implemented a calibration service for acoustic emission

transducers using the surface-pulse method. Since then almost 300 calibrations

have been performed, about half of these at the request of other laboratories or other NBS units and the rest for internal calibration improvements and quality

control and for the development of the NBS Conical Transducer described below.

This calibration method makes use of the theoretical solution for the normal

displacement of a half-space due to a point-force step-function. This prediction

of normal displacement versus time is shown in Fig. 1 a. The surface-pulse

calibration method also relies on our ability to mechanically produce a point-force step-function (we use the breaking glass capillary method (2-4)) and

it relies on an accurate measurement of actual surface displacement using a capacitive transducer (5). Figure 1 b is a measurement of the surface

displacement on an elastic block due to a breaking glass capillary event using

the standard capacitive transducer.

5
Fig. 1. Surface pulse -- theory and experiment.

A schematic of the calibration facility is shown in Fig. 2 . The large steel

block (A) weighs about 2200 kg and has polished faces. For calibrations by the

surface-pulse method, the source (B) and standard (G) and unknown (H) transducers

are all located on the top surface of the block. We have also started developing

a through-pulse calibration method, in which the source is still located on the

top surface but the unknown sensor is located directly below on the bottom

surface of the block.

1

Fig. 2 . Schematic of the AE sensor calibration apparatus. See (1) for details.

6
Further Developments of the Surface-Pulse System

Repeatability of calibration has been a problem with certain types of

transducers. Although most transducers can be calibrated repeatably without using

any special or unusual mounting practices, there are some which apparently require

special measures to insure repeatability of the calibration. The measures which we have developed (described in last year's report) are, In our experience, sufficient

to guarantee reproducible calibrations, although they are not always necessary.

The reason why some transducers require these measures and others do not, is not

clear to us at this time. We suspect that porosity of the wear face may cause air

entrapment and resulting bubbles in the couplant layer, but this is not known.

Figure 3 shows three calibrations of a transducer with remounting in between.

Previously, for some of the troublesome transducers, subsequent calibrations would

differ by 20 dB or more, and, in general, there would be scant resemblance

between the shapes of the curves. Using the new procedure dramatically

increases the repeatability comparable to that shown in Fig. 3 . Some

transducer manufacturers have been notified of repeatability problems.

SPECTRUM MRGNITUDE: DIV: 328

Fig. 3 . Demonstration of calibration repeatability; three separate calibrations

of a commercial sensor.

7
The transducer frequency response curves in Fig. 3 also exhibit the

aperature effect which results in nulls in sensitivity. The locations of these

nulls are reasonably predicted by the theory for a wave train crossing a circular aperature. The phase response for this transducer (not shown) has

phase jumps at the null locations which are also predicted theoretically.

We have also developed an electronic pulse whose waveform is very similar to

the surface-pulse waveform. This electronic pulse is used as a quality control

tool to verify that the electrical and signal processing parts of the calibration

facility have the proper characteristics and have not degraded.

Calibration Intercomparison

The Nippon Steel Corporation (NSC) offers a calibration service for

acoustic emission tranducers, in addition to the public calibration service offered

by NBS. Both services provide measurement of the amplitude of response as a function of frequency for a transducer subject to predominantly Rayleigh waves on the surface of a steel block (the NBS service also retains the phase information).

The methods used are, however, very different and it was decided that a comparison

of results would be important to the AE community.

The NBS calibration method is outlined above. The method used by NSC is a reciprocity method in which transducers are calibrated in sets of three. Two

transducers at a time are mounted on a large steel transfer block; one of the

transducers is driven by a slowly varying gated continuous wave and the output

from the other transducer is measured. Each of the three possible pairs is

measured in the same way. Using this information and a theoretical

reciprocity parameter the amplitude response of all three transducers can be

determi ned.

For this comparison three transducers each of two different types were

selected; one type is representative of transducers with complex behavior and the

other type is representati ve of transducers with relatively simple response

curves. Both types are sold commercially for use in acoustic emission work. The

six transducers were cal ibrated^ach three times by the surface-pulse method of

NBS and each three times by the reciprocity method of NSC. They were then

recalibrated at NBS to assure that no changes had occurred.

Typical results are shown in Fig. 4 for one type of transducer and in

Fig. 5 for the other type of transducer. In each figure the average of the

three NBS calibrations is compared with the average of the three NSC calibrations.

Reference (6) describes the intercomparison in more detail. The two methods of

calibration obviously yield very similar response curves; the details are in good

agreement. The average of the correlation coefficients relating the two

calibrations for one type of transducer was 0.76 and for the other type was 0.9S5 ; the lower correlation is associated with the type transducer with the more complex response.

8
Fig. 4 . Average of NBS and of NSC calibrations of an AE transducer with more complex response.

Fig. 5 . Average of NBS and of NSC calibrations of an AE transducer with

simpler response.

9
While the qualitative agreement between the methods is quite good, the

reciprocity method yields sensitivities that are lower than those by the surfacepulse method by an average amount of 16%. Efforts have been made to account for

this difference, but, so far, no explanation exists. Further work is planned to

try and narrow the gap between the methods.

Through-Pulse Calibration System

The technique of placing the standard transducer at a location symmetrical to

that of the unknown transducer cannot be used for the through-pulse calibration

method since the input source is located at the top surface of the block and the

unknown transducer is located directly below on the bottom surface (epicenter).

We may, however, compare the output waveform from the unknown transducer

against the theoretical waveform of the thick-plate epicenter solution. The

software for producing this solution has been written, and is available for

incorporation into the calibration system.

Several through-pulse calibrations have been performed on the NBS Conical

Transducer described on page 20 and the results compared with those from

surface-pulse calibrations of the same transducer. For such a transducer, which

has a small aperature, the results should be comparable. One such comparison is

shown in Fig. 6. It is apparent from the figure that there is substantial

SPECTRUM MAGNITUDE: DIV: 239

Fig. 6 • Calibration curve for the NBS Conical Transducer by the surface-pulse

method and by the through-pulse method.

10
agreement between the methods, but that the absolute sensitivity indicated by the

two calibrations differs by 1 dB. Recently, results have been obtained using a quasi-through-pulse configuration as shown in Fig. 7. In this case, both the

standard transducer and transducer under test are located near to and equidistant

from the epicenter. Using this quasi-epicenter configuration the output of the

transducer under test can be compared to the output of the standard transducer.

Measurements on the NBS conical transducer using this configuration^ compare very

well with the surface-pulse calibration of the same transducer. No appreciable

differences result when these results are compared. More experiments of this type

will be required before we can implement a through-pulse calibration service.

QUASI THROUGH- PULSE. CALIBRATION

NBS CONICAL

TRANSDUCER

Fig. 7 . Schematic of quasi through-pulse configuration.

11
AE SOURCE CHARACTERIZATION

This part of our AE program is directed to the deconvolution of the AE source

to determine its character and also to improved system calibration and improved

defect location. The goal of source deconvolution is to remove the effects of the

propagating structure and the receiving transducer on the evolved signal from the

captured voltage-time waveform thus revealing the source characteristics using

causal analysis. In (7) we have shown that for a plate and a capacitive

transducer we can determine the actual source from the received signal using the

inverse transform function. In order to do this source determination in more complex structures, we must experimentally determine the transfer function and

develop general inversion techniques. To carry this out,we must generate a repeatable known source which can be used to determine the impulse response of

complex structures. We also must develop a computational algorithm for inverting

the impulse response to obtain the operator which deconvolves the source waveform.

This work also provides a basis for system calibration and for minimizing flaw

location uncertainty.

Repetitive Short Duration Source

Comparisons have been made between electrical spark impact sources and pulsed

laser sources. The laser source is effective but the spark source has advantages

including portability, simplicity, and low cost. We have perfected the charging,

switching, and triggering circuits so that a few nanosecond, high-voltage current

pulse with very low jitter can be discharged between electrodes. The resulting

simulated AE waveforms in a structure are essentially identical one to another.

The mode of the spark source is controlled by the configuration of the

electrodes. We have emphasized the configuration in which the plate (or

structure) is one electrode and a second electrode is just above the plate

surface. The stress wave thus generated in the structure can be modeled as evolving from local surface plasma heating, the same mechanism as when using a pulsed laser source for radiation heating. From such experiments we conclude

that: the source is extremely reproducible, the detected signals agree well with

theoretical predictions for a dipole source, the surface normal motion has a well

defined negative pulse whose arrival is easily detected (a fact that could be

exploited for precise ultrasonic wave speed measurements), and the rise time for

the stress wave is less than one-half microsecond. Although the source generates

EM signals, these can be easily isolated in the time domain.

This source also generates a shock wave in the air and so current experiments

in a partial vacuum are being carried out to decompose the shock wave and the

thermal effects. We are also beginning to investigate the use of a new commercial

laser diode as a potential source. In order to increase the signal-to-noise ratio

of such measurements, a computer program has been completed which permits signal

averaging of 100 repeated source events in less than 10 seconds.

12
Signal Processing Techniques for Source Characterization

Substantial effort has gone into writing signal processing subroutines for the

dedicated minicomputer based AE signal acquisition and processing system. These

subroutines will be used to deconvolve the AE source function from the received

signal in "complex" structures. These signal processing packages include:

- A fast convolution program with one time series in core memory and one time series on disk.

- A direct, exact time-domain deconvolution program based on error free

matrix inversion.

- A program, based on least-squares fit, for finding the coefficient of

the inverse filter by solving the Toeplitz normal equations.

- A program for power spectra estimation based upon the maximum entropy

method developed by J. P. Burg.

We have also worked on the deconvolution of the digitized signals in the time

domain. In order to evaluate the various approximation techniques, the epicenter

impulse response function and its exact inverse have been used for comparison (7).

Results using a least-square approximate deconvolution are rather disappointing.

While an approximate inverse function is a reasonably "good" approximation of an exact inverse function, the deconvolved displacement is sometimes far from the

input. Further examination by convolving the impulse response and the approximate

inverse reveals that a 10% error is present in the result which accumulates for a step-function source. We have shown that the accuracy of the approximate

deconvolution is signal dependent. It is expected that if the source signal is

cyclic, then the deconvolved result will be much better than for the step-function

source. Similar comparisons have been made for other types of sources for the

epicenter configuration.

Our current conclusions on deconvolution are:

- For the epicenter case one can do an excellent job deconvolving to

obtain an accurate characterization of the source.

- For the source and receiver on the same surface of "a half space," again

an excellent deconvolution is possible.

- For other cases it appears that an approximate solution must be sought

(e.g., least-squares) and one should make use of a priori knowledge about

the source. This can result in an accurate characterization of some sources but not necessarily all sources.

A sequence of controlled experiments have also been designed and started to

determine the significance of information content in the detected signal. The

control variables include different "AE" sources, different source-detector

configurations, and different detectors. Preliminary results show that with a

13
high-fidelity transducer such as the NBS Conical Transducer described on page 20,

the detected signals from different sources are qualitatively different. However,

with commercial resonant transducers, the detected signals appear qualitatively the same even though the frequency content of the sources are completely different.

Further work is planned on analysis of information in the detected signal and

several additional processing schemes will be attempted after the new data

acquisition system is complete.

Data Acquisition System

The new data acquisition system is nearing completion (planned for September

1982). It is centered on a new generation, very high speed mini-computer. Some

of the special features of the system include:

- CAMAC module transient capture, permitting high sample rate (20 MHz),

long duration (128K word record length), transient recording capability

for subsequent processing of an entire AE event.

- Double precision capability, especially needed for theoretical

calculations such as those of the plate solution.

- NBS Bus (and CAMAC interface) for easy flexible interfacing of a wide

range of instruments.

- "High-power" mini-computer, featuring 32 bit words and a 1M core.

While the data acquisition system is essentially complete some additional

software is needed to complete the processing packages.

MATERIALS EFFECTS AND CRACK CHARACTERIZATION IN STEEL PLATES

The milestones of repeatedly producing AE in A533 steel plates and of

developing a multichannel source characterization facility for quantitative

measurements of AE sources have been completed. With the exception of checking

the reproducibil ity, which is best done with a multichannel system, the goal of

reliably producing AE in A533B steel is complete. The development of a multichannel source characterization facility is also complete. The system has

been used to investigate a high-power pulsed laser source. Activities in this

task were terminated January 1982.

Reproducible AE in A533B Steel Plates

The generation of reproducible AE in A533B steel plate has been studied using

the indentation technique described previously (8). This steel is typical of

nuclear reactor pressure vessels and a method of producing repeatable AE in this

material would be very helpful in studying the significance of AE sources. The

as-received A533B steel plate is quite ductile and thus does not readily produce

local stress concentrations which could generate AE through rapid, localized load

drops. Therefore, a systematic study was performed on the effects in A533B

nuclear grade pressure vessel steel of temper embrittlement augmented by

hydrogen charging. The temper-embrittled condition is one which could occur to some extent in an A533 weld region in the field. Hydrogen is a fission product.

14
It was found that this method worked quite well and that a combination of grain

coarsening at 1100 °C, aging at 500 °C up to 20 days, and hydrogen charging

produced a substantial increase in AE activity (9). Our results demonstrate that by

controlling the heat treatment and charging with hydrogen, the relatively quiet

as-received A533B can be converted to a highly emissive material.

Understanding of this phenomenon is further enhanced by a stress analysis of

the indentation. There is a compressive zone immediately under the indenter during

loading which produces a hemispherical region of triaxial tension at one or more diameters away from the indention. Microcracks nucleate during indentation up to

200 kg in temper-embrittled A533B. Micrographs indicate that debonding occurs at

the Mn inclusion interface. No larger microcracks were found so that this is the source of emission in the uncharged material.

In hydrogen-charged material, macroscopic (200 microns) cracks were found. In addition, there is a region of enhanced hydrogen concentration near the surface

which we propose is responsible for microcracks in hydrogen-charged material

being generally either surface-connected or shallow subsurface types. This leads

to the occurrence of cracks at the side, rather than under the indenter as was found previously for tool steels that were not hydrogen charged (9). These

appear to nucleate at MnS inclusion. Although the crack is on the order of 200

microns in size, this is acoustically small relative to the shortest wavelength

examined, here about 2400 microns, so that a crack will appear to be a "point

source".

To summarize, a method has been found which is useful for producing AE in

A533B steel plates. Through controlling the heat treatment and through additional

methods of modifying the material condition, the amount of AE can be controlled

while still keeping the material relevant to conditions in the weld region. This

method is now applicable for use on larger plates of A533B steel, where signal

reproducibility could be checked with the multichannel system.

Multichannel Analysis System

Another unique approach being taken in our AE research program is the

development of a multi-channel AE signal acquisition and processing system to

permit more complete deconvolution of the AE stress drop source. This approach,

which originated at NBS (10), is also being implemented at AERE Harwell and at

other laboratories. The multichannel system is complete and ready for testing.

Two octagonal steel plates, 18 inches across, have been obtained. An 02 tool steel

plate has been hardened by austenitizing and water quenching. Preliminary tests

using such a procedure show that multiple, reproducible AE signals can be obtained

during indentation. A second plate of A533B steel has been machined prior to heat

treatment. The indentation system consists of a pneumatic loading indenter, a Faraday cage to minimize RF noise, and an XV translation table for positioning the

plate specimen.

For the multichannel system a set of six wideband transducers (the new NBS

Conical Transducer design) has been fabricated; also, there are at least six

channels of preamplification, anti-aliasing filters, and amplification up to 100

dB with a bandwidth in excess of 10 MHz available for the system, computer codes

15
for the dynamic plate response have been extended to include all surface dipoles,

and extensive calculation have been made of emission signals on a stainless steel

plate which will be used for additional tests on the wideband transducers.

Computer codes for deconvolution and comprehensive driver programs for our high-speed interface are being prepared on our minicomputer system which, in turn,

is being extensively expanded to carry out novel data processing methods. The

computer interface is now operational with some noise problems being solved. This

completes the current generation multichannel system.

Laser Generated AE

We have begun evaluating the multichannel system and the multichannel

deconvolution approach to determining AE source functions using simulated AE

which has been laser generated. A Nd:YAG laser has been used to produce

predictable, reproducible, thermoelastic dipole sources that generate elastic waves in a large plate. The elastic displacements due to elementary force

dipoles and a force dipole combination representing the thermal expansion for a laser source have been calculated for an isotropic elastic plate at epicenter

and at one and five plate thicknesses on both plate surfaces. Experimental

results show close agreement with the calculated waveforms.

The elastic waves generated on the surface on which the source is located

(Figs. 8 & 9 ) are dominated by a Rayleigh wave whose velocity was 2.89 mm-M S ”‘ for 302 stainless steel. Careful measurements of this velocity could provide

a useful non-contacting method of deducing the surface elastic properties. This

method could also be used during metals processing to detect the occurrence of

surface cracking for NDE applications: the time of the Rayleigh wave arrival at

several transducers would give a very precise location of the source.

Fig. 8 Predicted displacement waveform at

for a 2.55 x lO^Wm-2 flux density

one plate thickness from the source

source.

16
Fig. 9 . Predicted displacement waveform at five plate thicknesses from the

source.

On the opposite surface, at the epicenter (Fig. 10 ), clear longitudinal and

transverse wave arrivals can be seen thereby suggesting the possibility of bulk

microstructure characterization. Away from the epicenter (Figs. 1 1 T<1 2 h the

waveforms for moderate duration sources are dominated by head wave arrivals.

Fig. 10 . Predicted epicenter displacement for a 2.55 x lO^Wm"^ flux density

laser source.

17
Great care has to be exercised with these from an acoustic emission point of

view if source location is attempted because they do not have a well defined

wavespeed. For example, the dominant head wave one plate thickness from epicenter

had an apparent velocity of 2.89 mm*ns-l while that at five plate thicknesses was 5.74 mm*MS -1

. This may be a major reason for inaccurate source location in thick

plates in many current applications of acoustic emission.

The results obtained form a starting point for AE source characterization

studies and this aspect could be more fully developed in following work.

Fig. 11. Predicted displacement one plate thickness from epicenter for a 2.55 x 10l0wm"2 flux density source.

43

36

29

22

15

8

0

-6

-13

-20

PRINCIPLE 6- FUNCTION COMPONENTS

Time/ps 20 04 22-92 27-42 31-73 32 3 47- 57 48 9

6 -Function Strength (mm ps per MNmm)

l-89*10'

4 1-32x10' 5

3-26x10"

4 8-76x10'5 -2 44x10'

4 2-02x10”^ 1-8S x!0‘‘

16 20 24 28 32 36 40 44 Time / ps

48 5 2 56 60 64

Fig. 12. Predicted displacement five plate thicknesses from epicenter for a 2.55 x 10l0wm-2 flux density source.

18
ACKNOWLEDGMENT

Much of this work is part of the Electric Power Research Institute/National

Bureau of Standards Joint Program on Acoustic Emission (RP 608). Some of the work

is partially supported by the Office of Naval Research.

REFERENCES

1. D.G. Eitzen, F.R. Breckenridge, R.B. Clough, E.R. Fuller, N.N. Hsu, and J.A.

Simmons. Summmary of Fundamental Developments for Quantitative Acoustic

Emission Measurements . EPRI Report NP-1877, June 198l. (An EPRI Report which

more fully describes this program is in press).

2. F.R. Breckenridge, C.E. Tschiegg, and M. Greenspan. Acoustic Emission: Some

Applications of Lamb's Problem. Journal of the Acoustical Society of America,

576, 1975, pp. 626-631.

3. N.N. Hsu and F.R. Breckenridge. Characterization and Calibration of Acoustic

Emission Sensors . Materials Evaluation, 39, 1981, pp. 60-68.

4. F.R. Breckenridge. Transducer Calibration by Means of the Seismic Surface

Pul se . Journal of Acoustic Emission, Vol . 1, No. 2, April 1982.

5. F.R. Breckenridge. Surface-Wave Displacement: Absolute Measurements Using a Capacitive Transducer. Journal of the Acoustical Society of America, 69,

pp. 1177-1185.

6. F.R. Breckenridge, T. Watanabe, and H. Hateno. Calibration of Acoustic

Emission Transducers: Comparison of Two Methods . Proceedings of the Sixth

International Acoustic Emission Symposium, Japan, Oct. 31-Nov. 3, 1982.

7. N.N. Hsu and D.G. Eitzen. The Inverse Problem of Acoustic Emission--

Explicit Determination of the AE Source Time-Function . Presented at the

AF/DARPA Review of Quantitative NDE, August 1981, Boulder, CO.

8. R.B. Clough and J.A. Simmons. Reproducible Acoustic Emission Signatures

by Indentation in Steels . Materials Evaluation, October 1981.

9. R.B. Clough and H.N.G. Wadley. Indentation Acoustic Emission Studies

of Temper Embrittlement in A533B Pressure Vessel Steel . In preparation.

10.

J.A. Simmons and R.B. Clough. Theory of Acoustic Emission . Dislocation

Modelling of Physical Systems. M. Ashby and J. Hirth, eds. Pergamon

Press, Oxford, 1981, pp. 464-467.

19
Task # 15770

FURTHER DEVELOPMENT OF THE NBS CONICAL TRANSDUCER

Thomas M. Proctor

Mechanical Production Metrology Division

Center for Mechanical Engineering

Figure 13 shows a schematic of one of the configurations of the new NBS Conical

Transducer for acoustic emission (1,2). The transducer measures dynamic normal

surface displacements nearly as faithfully as the standard capacitive transducer

used in the NBS AE calibration facility but is more rugged and is one to two

orders of magnitude more sensitive --about as sensitive as commercial AE

transducers. It consists of a small conical piezoelectric element and a large

backing, usually of brass.

By design, this transducer has an area of contact which is small relative to

Rayleigh wavelengths in the working range (nominally 0.1-1 MHz) thus eliminating

the aperature effect which plagues most AE transducers. The brass backing block

is so large and so lossy that waves entering it via the conical element are largely dissipated by the time they reflect back to the element. Thus the

backing block may be approximated as an elastic solid half-space. On this basis,

it can be shown that, for low frequencies, the transducer functions as a displacement sensor.

Fig. 13. Schematic of one version of the NBS Conical Transducer.

20
Shown in Fig. 14 are two views of the tranducer producing the flattest

response so far. The idea behind the rather complicated backing is to have all

reflecting surfaces at different angles and distances to the piezoelectric cone element so that reflecting waves are not coherent. The response of this

transducer is shown in Fig. 15 and compares very favorably with the

theoretical waveform and with the capacitive transducer response shown in

Fig. 1 a, and b. Its response is much more accurate compared to the response

of a typical commercial tranducer (shown in Fig. 16).

Fig. 14 . Two views of best behaved piezoelectric transducer thus far produced.

Additional evidence of the new transducer's ability to measure dynamic

displacement is presented in Fig. 17 . The inset curve gives the

theoretically predicted normal displacement of a plate for a sensor located two

thicknesses away from and on the same side of a plate as a point-force step

function input. The lower curve is the displacement measured by the NBS Conical

Transducer on a 3 cm thick glass plate with the NBS transducer located 6 cm away

from a breaking glass capillary. The theoretical curve is from Pao et al . of

Cornell and was independently calculated by Simmons, Hsu, and Willis at NBS.

When the transducer was first developed, a satisfactory theory to describe its

function was not available. In order to determine the importance of various

features of the transducer several series of experiments were performed. One

series of response measurements was made as the large backing was reduced in size

in a systematic fashion. The importance of the aperture was investigated by

another series in which the front contact electrode area was reduced in

successive steps. The effects of variation in backing symmetry have been

investigated in a similar fashion. This empirical approach has helped us understand the operation of the transducer. As a result, we have found that

backing size and geometry, small aperture size and conically shaped active

element are all important features of this design.

21
TRANSDUCER OUTPUT: UNK: 311

Fig. 15 . Measurement of surface pulse using the transducer shown in Fig. 13 .

Fig. 16. Detection of surface pulse using a typical commercial transducer.

22
Fig. 17 . Comparison of theory and measurement using the NBS Conical Transducer

located two thicknesses away from and on the same side of a plate as the source.

A preliminary theory of operation was recently developed for the NBS Conical

Transducer. It takes into consideration the mechanical impedance properties of

the three parts of the system, namely the driving block (steel half-space as viewed by the circular area of the contact face), the conical element (treated as

a conical horn), and the backing block (brass half-space as viewed by the

circular area of the cone's base). This theory has had some success in

predicting the response of several transducers which were variations of the NBS

Conical Transducer. Figure 18 shows a comparison of the theoretically

predicted and the measured sensitivity of a transducer constructed with a cylindrical piezoelectric element (for the purpose of this comparison). This

cylindrical element permits a simplified calculation but should be a reasonable

check of the modeling approach. While the results show the approximate theory is

good, an improved theory is being developed. It is hoped that this theory

may aid in optimizing the parameters of transducers of this type and contribute

generally to understanding of AE and ultrasonic transducers coupled to an elastic

medi urn.

As a participant in the DARPA/Naval Air Development Center/Battel le Program on In

Flight AE Monitoring of Aircraft, we have developed a transducer assembly which

uses the regular cylindrical brass backing type transducer but also has an internally contained preamplifier, front electrode protection, and a ground

shield. With the older type backings, the final shielded unit is 2 inches in

diameter and about 2 inches in length.

23
Fig. 18 . Theory and measurement of NBS-type transducer with cylindrical element.

We are also working on special backing materials to permit a smaller transducer

design. We have constructed transducers with 1/4 the volume of earlier models but

with the same fidelity of response. This new "down-sized" transducer is one-inch in

diameter and only 5/8 of an inch long, which makes it comparable in size to most

conventional transducers.

Our future plans include optimization of the design using a new theoretical model

of the transducer. Parameters subject to optimization include material properties of

the backing and piezoelectric element and shape of the element. Plans also are underway to develop a "high-fidelity" displacement transducer for tangential surface

motion.

REFERENCES

1. T.M. Proctor. Improved Piezoelectric Transducers for Acoustic Emission Signal

Reception . J. Acoust. Soc. Am. Suppl. 1, Vol. 68, Fall '80, p. 568.

2. T.M. Proctor. An Improved Piezoelectric Acoustic Emission Transducer . J. Acoust. Soc. Am. /I (5) May 1982, pp. 1163-1168.

24
Task # 15770

ACOUSTIC EMISSION FOR NDE SURVEILLENCE AND QUALITY CONTROL

J. A. Simmons, R. B. Clough, H. N. G. Wadley, and R. Mehrabian

Metallurgy Division

Center for Materials Science

Acoustic emission (AE) refers to a technique for the measurement of

elastic stress waves emitted by rapid dynamic changes of state in materials.

AE can be used to detect, monitor and measure the occurence of plasticity,

cracking, phase transitions and corrosion where they are accompanied by

rapid stress changes. Consequently, AE has unique potential as a passive

surveillance tool for the control of microstructure during materials

processing, monitoring, fabrication of components and detecting, locating

and characterizing flaw growth in structures. It also promises to provide

a new tool for the quantitative measurement of the dynamics of material

changes of state.

The structure of the stress waves emitted during acoustic emission is

complex. Until recently, the general formalism establishing the relationships between stress change sources, the amplitude and temporal

character of the emission they provide was only poorly understood. Recent

work at NBS has addressed these problems. This work demonstrated:

• Existing measurement methodologies that have evolved to date make

use of only a small fraction of the potential information contained

in acoustic emission signals.

• The character of the emission is controlled by synergistic

interactions between metallurgical variables and the stress state.

Nevertheless, the technique is finding increasingly successful application

for monitoring cracking and predicting failure in ceramics, composites and

metal weldments. However, it also has the potential to make a great contribution to the monitoring of metals processing where it could provide essential

data for feedback control improving quality and productivity, but this has

received scant study in the past.

At NBS a unique approach to the measurement of acoustic emission is being

developed, which has as its objective the reconstruction of the source dynamics.

This technique, which is similiar to that currently revolutionizing the

field of seismology, utilizes simultaneous high speed measurements (at

different locations) of the initial wavefronts from individual emissions to

allow quantitative reconstruction of the source stress drop. At least six

observation channels are necessary for reconstruction of the six stress

drop components as a function of time. To implement this approach the

25
following steps have been taken:

• A computer based multichannel AE system has been constructed.

• New experimental methods for controlled generation of AE sources developed.

• New theoretical techniques for forward modelling and inverse

filtering have been devised for signal analysis.

• Critical experiments have been planned and preliminary studies

completed for verification of multichannel AE analysis using

Nd:YAG laser and plate geometry.

• Plans and preliminary studies made to correlate AE signals,

microstructural analysis and thermodynamic parameters of rapidly

solidified electron beam source in aluminum and an aluminum

alloy.

• A set of critical experiments designed to establish the validity

of these new measurements methods.

Parallel studies drawing upon the above techniques, have examined flaw

growth as part of the joint NBS/EPRI program in AE and explored the use of

AE for monitoring rapid solidification as part of the DARPA/NBS project in

Rapid Solidification and NDE.

Our program during the past year can be broken down into four categories:

(1) Theoretical advances.

(2) Source characterization developmetns.

(3) Experimental methods for the controlled generation of AE in

A533B pressure vessel steel

.

(4)

. AE studies of rapid solidification.

(1) Theory advances

The theoretical framework for acoustic emission sources rests

upon a Green's function formalism for stress waves generated by

changes in internal stress induced when a source responds inelastically

to structural loads. The forward formulation, in which the emission

from a source of known character is deduced, has been published (1).

There the representation of an AE source event was first given in

multipolar expansion form and then restricted by two assumptions, to

avoid the technological difficulties associated with high moment

components, namely, the source is infinitesimal in size and all its

components have the same time history. The key restriction for validating

these assumptions is a band limiting process to restrict the frequencies

to a region called the Informative Band. Estimates of the limiting

frequency for the band were given as a function of defect size and

26
shape. Within the information bandwidth one can develop a transfer

function formalism which allows one to consider transmission of the

information frequency by frequency from the source through the structure

to the detector: outside this band specific models of the details of

the AE mechanism are required to deduce AE singals.

The general formulation of acoustic emission from plasticity, cracks,

and phase transformations was then developed within the pseudopoint

approximation using the concept of retarded dipole density to deal

with the problem of time delays of the emission from propagating cracks

and phase change regions. The radiation patterns for defect sources

are highly directional and depend on source type and orientation (see

Figures 19-21). Finally, the concepts of stress controlled and materials

controlled acoustic emission were introduced and the synergistic

relation between the ambient stress field and microstructural contraints

were discussed.

Subsequent work has concentrated on the inverse problem for deducing

the character of the stress drop components from measured acoustic

emission signals. This is one of a class of "remote sensing" problems

in which small amounts of measurement error or noise can obscure the

information about the source signal. Sophisticated signal processing

techniques are demanded.

The transfer function formulism leads to simultaneous convolution

equations describing the acoustic emission signals in terms of the source stress drop components, the structure response, and the transducer characteristics. These simultaneous equations can be reduced to

a deconvolution equation which is, in general, even more illconditioned than deconvolution problems encountered in time series

appl ications.

A singular valued decomposition (SVD) algorithm developed in con- junction with D. O'Leary (Division 711) (2), has been applied to such signals.

Since the full SVD algorithm is prohibitively demanding of computer

resources for problems of the type required in acoustic emission, the

above procedure uses a reduced dimension representation similar to the

Rayleigh-Ritz procedure. The SVD algoithm both inverts and

filters to produce a number of candidate solutions to the inverse

problem. The results obtained, while promising, suffer from the

difficulty that it is difficult to select the "best" approximation

from among the candidate solutions.

To help deal with this problem and to examine the possibility for new pattern recognition techniques for source signatures, an alternate

approach is being studied. This approach uses a modification of the

Z transform of the time series which is to be inverted, which allows

powerful complex variable techniques to be combined with linear

algebraic methods for inverse filtering. In this approach the Z

transform of the time series becomes a polynomial of high degree

27
(typically 512 or higher). An entirely new set of algorithms has

been developed which finds all the zeroes of such high degree polynomials and uses these for both pattern recognition purposes and

to develop new noise filters. These are supplemental to those

obtained in the SVD approach and can be used to greatly reduce

solution noise while allowing "a priori" information about the

frequency content of the solution to be factored in.

These new algorithms, which must be applied to time series of

great length to be usable in acoustic emission inversion, are currently being employed to find robust procedures for decovolution. An example of the types of candidate solutions obtained in

carrying out deconvolution (in this case of a 43 point time series

based on the surface-to-surface Green's function for a half-space)

is illustrated in Figures 22-28.

(2) Experimental verification of source characterization

The objective of source characterization is the measurement of

the magnitude, orientation and time dependence of the stress drop

tensor of acoustic emission sources. The stress tensor provides

the basic information required for defect severity assessment in

NDE and, in the laboratory, promises to provide dynamic information

that would greatly advance our understanding of the dynamics of

metal deformation and phase transformation processes.

A formalism based upon dynamic elasticity has been developed

which, in principle, provides us with a framework for deducing the

stress drop tensor. Instrumentation to implement this approach has

also been developed. A critical series of experiments are underway in which the stress drop tensor is to be measured using acoustic

emission techniques and compared with that deduced by independent

measurement or predicted from well developed theory.

The problem is that there are no available techniques for the

measurement of stress drops when they occur at velocities close to

that of the speed of sound. One possible approach we are following

is to use reproducible cracking under indentation loading which is

discussed below. However, another possible approach is the use of a simulated reproducible acoustic emission.

The ideal simulated source should be composed of dipole forces

of similar magnitude and orientation to those of natural AE and

have a time duration between 10 ns and 10 ys (the range of the

majority of natural emissions). These properties should be predictable

and/or independently measureable. We have set up a laser generated

thermoelastic simulated acoustic emission source for critically evaluating

the performance of multichannel source characterization.

The thermoelastic dilatational strain accompanying absorption of pulsed

laser radiation has been modelled as a combination of force dipoles

28
whose strengths were calculated using thermal diffusion theory. The

temporal behavior of the elastic displacements due to each dipole

component were then evaluated from the dynamic elastic Green's

tensor for an isotropic infinite plate. The displacement waveform

for the complete source was then obtained by linear superposition

of the individual dipole responses.

The thermoelastic source generates longitudinal, transverse

and surface waves. When calculations of displacement are performed

on the same surface as the source, the waveforms are dominated by

surface waves, particularly at large source to receiver distances.

When calculations are made on the surface of the plate opposite the

source, the transient displacement waveforms are dominated by

head waves (critically reflected bulk waves). These latter waves

have a non-unique ray speed and would lead to ambiguous source locations

in acoustic emission monitoring.

A wide bandwidth system has been designed and_constructed that can measure displacement with a sensitivity of 'vlO" 14 m over a 2.5

MHz bandwidth. This has been used to measure the displacement

waveforms from the thermoelastic expansion accompanying the absorption

of Nd:YAG laser pulses of 25 ns duration.

The measured waveforms contain the features predicted by

theory and the dipole magnitudes are, within experimental error, in

agreement with those predicted. From this encouraging basis we are proceeding to apply the deconvolution procedures developed above to

deduce the full tensor description of the source stress change.

(3) Experimental method for the controlled generation of AE in A533B

pressure vessel steel

An objective for the nuclear power industry has been the

development of an experimental technique for localized production

of reproducible AE signals (by indentation) in A533B pressure

vessel steel and the correlation of these signals with the defects

and microstructure at that location. These steels, while in the

normalized condition are nonemissive, can become embrittled and

emissive during fabrication into nuclear reactor pressure vessels

by improper welding and heat treating practices (temper embrittlement)

and can be further embrittled during service due to hydrogen, which

is a fission product. Recently the NRC announced that eight of

these vessels in service are becoming embrittled at a much faster

rate than the original estimate and an expected 40 year lifetime

will be reduced to a few years. It would seem that the above

effects could contribute synergistical ly with the normally present

radiation embrittlement and stress corrosion to reduce lifetimes.

By controlling the tempering and hydrogen charging in A533B we

29
succeeded in producing AE in the range from virtually none in the

normalized condition up to an activity as high as that of borosilicate

glass in the fully embrittled condition. We were further able to

identify the sources of emission as microcrack nucleation (^5

micron) at MnS inclusions (Fig. 29) in the temper embrittled state

and growth to larger grain boundary cracks (^250 micron) with the

addition of hydrogen. A very significant result was the unexpectedly

prominent role played by the MnS inclusions in the temper embrittlement

process, as opposed to the usual attention given to segregated

grain boundaries. This aspect is being followed up by work at

Harwell

.

(4) AE studies of rapid solidification

Rapid solidification is a new technique used for producing

metallic glasses which provide low loss transformer core material.

It is also used to produce superior surface coatings and long

wearing bearing surfaces. AE can be used to monitor rapid solidification surface modification to detect flaws produced during processing. It also promises to provide dynamic information on the

solidification process itself since a typical solidification time for

a surface melt is on the order of 100 ms and stress waves can be

monitored over considerable shorter times (.^100 y»s). Very little

work has been done in this area. However, due to the potential of

AE to shed light on this subject, NBS and DARPA are sponsoring

research on AE monitoring of rapid solidification processing.

•"Because of its commerical applications in welding and surface

modification, the electron beam was used as a directed energy

source for melting. A new technique has been developed for studying

solidification of such melts. First it was verified that the

signals were not electrical noise but were caused by material

changes during solidification. The experimental approach consists

in irradiating a spot on a plate, producing a steady state melt at

2 x 105 Wm" 1 for qa (where q is the energy intensity and a is the source radius), switching off the electron beam to eliminate it as an AE source, then monitoring the amount of energy produced by AE

during solidification.

Our first results show the effects of alloying on resolidification

AE. The resolidified melts in the 2219 aluminum alloy contained

numerous cracks which indicate that hot tearing during solidification

is the cause of the AE in the 2219 alloy (Fig. 30). In contrast, the

1100 aluminum resolidified melts had virtually no cracks (Fig. 31 ).

This is reflected also in the amount of energy emitted acoustically

during resolidification for the two materials (Fig. 32 and 33).

Results of this type are of great interest to the rapid solidification

field. Once the sources of AE in rapid solidification are more fully

ascertained, steps can be made to implement AE monitoring of surface

30
modification processes, and to dynamically observe the solidification

process and its kinetics.

I

REFERENCES

(1) J. A. Simmons and R. B. Clough, "Theory of Acoustic Emission,"

Dislocation Modelling of Physical Systems." Pergamon press, 1981,

p. 464.

(2) J. Simmons and D. O'Leary, "A Bidiagonal ization Regularization

Procedure for Large Scale Discretizations of 111 -Posed Problems,"

SIAM J. Statistics and Computing, Dec. 1981.

31
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38
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39
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41
Task # 15760

ULTRASONIC TECHNIQUES FOR NDE

Melvin Linzer and Stephen Norton

Fracture and Deformation Division

Center for Materials Science

Introduction

Our work in the past year has emphasized the theoretical development of new ultrasonic imaging techniques: (1) A perturbation method to correct for

ray refraction in velocity and attenuation tomography has been devised [1].

This work will allow the practical generation of more accurate images of ultrasonic parameters and hence better characterization of material properties; (2)

A new imaging concept based on an annular aperture has also been developed [2],

Our approach provides a reflectivity image with resolving power equivalent to that

of a full circular aperture twice the diameter of the annulus. This in turn

permits more economical and rapid imaging over a smaller area of a material surface;

(3) The first comprehensive and critical examination of techniques for measuring

ultrasonic attenuation, velocity and scattering has been published [3]; (4) Our

earlier work on a new sonic technique for detecting interrupted cutting during

automated machining has been prepared and accepted for publication [4]; (5)

Finally, a program has been initiated on the development of techniques to improve

the sensitivity and accuracy of acoustic emission measurements. In the account

that follows, we shall review the perturbation techniques for refraction corrections.

Review of Perturbation Technique for Refraction Corrections

The problem of reconstructing ultrasonic refractive index or attenuation

images from ultrasonic transmission measurements is complicated by diffraction

and refraction effects which are nonexistent in x-ray tomography. In ultrasonic

"1 ine-of-sight" or transmission tomography, a number of simplifying assumptions

are customarily made for the sake of tractabil ity. First, diffraction effects

are ignored; this is equivalent to assuming that all but a negligible fraction of

the propagating acoustic energy is confined to a ray connecting the transmitter

and receiver. A second assumption is that this ray is a straight line, i.e., no refraction occurs. Under these conditions, the problem reduces to that of recon- structing an unknown function (e.g., refractive index or attenuation) from its

path integrals over straight lines, for which numerous reconstruction algorithms

may be applied. The failure to take into account diffraction (or scattering) and

refraction effects will inevitably degrade the quality of a reconstructed image

to an extent depending upon the properties of the medium under study, such as the

characteristic scale (correlation distance) and magnitude of the inhomogeneities

that compose it.

In the case of velocity tomography, techniques have been proposed for taking

ray bending into account in an attempt to improve image quality. To date, all of

these have been iterative in nature. A "first-order" reconstruction of a velocity

(or refractive index) map is first obtained under the initial assumption that the

propagation paths are straight lines. An approximation to the actual, or refracted,

path is then computed from this map by numerically integrating the ray equation.

At this stage, one strategy is to use a generalized algebraic reconstruction technique

(ART) in which the path integrals of the parameter of interest (refractive index or absorption coefficient) are recomputed over the calculated ray trajectories. Then,

42
for a given path, the difference between the actual (measured) path integral and the

computed path integral is uniformly distributed back over the ray path in the

fashion of conventional ART (where of course the paths are straight). This process

is performed for all ray paths, and the entire procedure iterated several times

before convergence to an acceptable image results. If desired, further ray tracing

can be carried out on the new image, and the entire process repeated using the

latest estimate of the calculated ray paths. As in conventional ART, the ultimate

objective of this approach is to force the computed path integrals to conform

eventually to the measurements.

Although the above technique may be intuitively appealing, the computational

times required may be unreasonably long because of its iterative nature and the

need to compute a large number of ray trajectories at each iteration. Another

approach to the ray-bending problem, due to McKinnon and Bates [6], exploits an iterative technique based on a simple rearrangement of the eikonal equation. This approach,

which does not require numerical ray tracing, promises to be more efficient computationally than techniques that depend on integrating the ray equation at each

iteration. However, many questions regarding the convergence of the various iterative

schemes, including that proposed by McKinnon and Bates, remain largely unresolved

and deserve further study.

The perturbation approach to the ray-bending problem, which we have developed

in the past year, does not require iteration and also avoids the considerable computational cost of numerical ray tracing. As in the case of the other techniques,

the perturbation approach begins with a first approximation to the velocity distribution obtained by reconstructing a refractive index map under the initial assumption

that the rays are straight. The perturbation corrections are then derived on the

basis of this first map. Our general approach may be described as follows. Suppose

that the index of refraction deviates from its average value (normalized to one for

convenience) by an amount on the order of e, where e<< 1 . Using the initial estimate

of the refractive index, we derive an expression for the ray path whose departure

from the straight line joining the transmitter and receiver is first order in the

small quantity e. This "first-order" path (or ray) closely approximates the true

path when e is small. The expression for this path is then used to obtain a number

of useful perturbation corrections applicable to both velocity (or refractive index)

and attenuation tomography.

In all methods that correct for refraction, the ray path must be computed to

begin at the transmitter and end at the receiver. The perturbation approach has

the significant advantage of solving this boundary value problem automatically.

For techniques that rely on numerical ray tracing, the integration of the ray equation

is inherently an initial value problem, i.e., the initial direction of the ray

path at the source must be specified to get the ray tracing process underway. The

initial condi tions--the source location and the initial ray direction--define a unique path that may or may not intercept the desired receive point. This problem

is typically solved by iteratively adjusting the starting direction of the ray in an attempt to make the ray intercept the receiver. In the perturbation approach,

the ray trajectory is defined once the source and receive points, rather than the

source point and the initial ray direction, are specified. This feature avoids the

exceedingly time-consuming process of computing multiple rays with different initial

conditions in an attempt to find one that coincides with the desired boundary

43
conditions.

The first correction we have considered is the time-delay error that results

when refraction is ignored in a time-of-fl ight measurement. It is shown that

variations in the time-of-fl ight over a straight (unrefracted) path between two

points are first order in e. In other words, the time delay of a pulse propagating

along a straight line may be regarded as a simple retardation, or "longitudinal"

delay, along the path; variations in this "straight-line" delay are first order

in e. If the ray is now permitted to deviate from the straight line due to refraction,

but is constrained to pass through the same transmit-receive points, the original

(straight-line) time-of-fl ight value will change by an amount of orders 2 due to ray

refraction. An explicit formula for this second-order correction, as well as for

the first-order ray path, has been derived.

The time-delay correction that arises due to ray bending can be used to modify

time-of-flight data over refracted paths as follows. For a given path-integral

measurement over a refracted ray, the computed correction term is subtracted from

this value to yield a modified value that is more nearly equal to the path integral

of the unknown refractive index over the straight line joining the transmitter and

receiver. After all of the measurements are modified in this manner, the resulting

"corrected data" are submitted to a conventional straight-line reconstruction

algorithm. The result is a reconstruction less degraded by time-delay errors arising

from refraction.

The first-order ray path and the time-delay correction were derived for both

two and three dimensional refraction. In conventional ray-tracing methods, the

computer time required to calculate large numbers of ray paths in three dimensions

may be so great as to render such schemes computationally impractical, at least in an industrial setting.

Different types of measurement uncertainties that result from using transducers

of finite size have also been analyzed. In a time-of-flight measurement, a finiteaperture receiver or transmitter introduces an uncertainty in the arrival time. In

addition, an attenuation measurement employing an extended, phase-sensitive receiver

is subject to errors because of phase cancellation and beam displacement. Using

the first-order ray path which we have derived, an estimate of the maximum value

of the arrival -time is also computed. Estimates of the attenuation errors are also

obtained, which, in principle, may be exploited to correct for phase-cancellation

and beam-displacement effects. Computer-simulated reconstructions were obtained

with and without the time-delay correction. The simulations show clearly an improvement, at least for the simple examples considered.

Conclusion

The perturbation technique described above is a continuation of our efforts

over the past few years to solve some of the fundamental theoretical problems in

ultrasonic imaging. Previous work in this program includes a comprehensive analysis

of backprojection techniques for reflectivity imaging [6,7] and the first inverse

scattering solution of the wideband pulse-echo imaging problem [8]. Future work will

extend our results on backprojection and inverse scattering approaches in ultrasound

to the acoustic emission area.

44
References

(1) Stephen J. Norton and Melvin Linzer, "Correcting for Ray Refraction in

Velocity and Attenuation Tomography: A Perturbation Approach, " Ultrasonic

Imaging 4, 201-233 (1982).

(2) Stephen J. Norton, "Acoustical Holography with an Annular Aperture,"

J. Acoust. Soc. Am. 71_, 1169-1178 (1982).

(3) Melvin Linzer and Stephen J. Norton, "Ultrasonic Tissue Characterization,"

Ann. Rev. Biophys. Bioeng. 1J_, 303-329 (1982).

(4) F. P. Higgins and M. Linzer, "Sonic Technique for Detecting Interrupted

Cutting During Automated Machining," J. Eng. Ind. (submitted).

( 5 ) G. C. McKinnon and R. H. T. Bates, "A Limitation on Ultrasonic Transmission

Tomography," Ultrasonic Imaging 2 , 48-54 (1980).

(6) Stephen J. Norton and Melvin Linzer, "Ultrasonic Reflectivity Tomography:

Reconstruction with Circular Transducer Arrays," Ultrasonic Imaging 1,

154-184 (1979).

(7) Stephen J. Norton and Melvin Linzer, "Ultrasonic Reflectivity Imaging in

Three Dimensions: Reconstruction with Spherical Transducer Arrays," Ultrasonic Imaging 1_> 210-231 (1979).

(8) Stephen J. Norton and Melvin Linzer, "Ultrasonic Reflectivity Imaging in

Three Dimensions: Exact Inverse Scattering Solutions for Plane, Cylindrical,

and Spherical Apertures," IEEE Trans. Biomed. Eng. BME-28 , 202-220 (1981).

45
Task # 15760

TRANSFER STANDARDS FOR ULTRASONIC POWER MEASUREMENTS

Frank Breckenridge, Steve Fick and Carl Tschiegg

Mechanical Production Metrology Division

Center for Manufacturing Engineering

The rapid growth in medical applications of ultrasound has fostered an increased demand for means by which the total radiated power of medical

ultrasonic transducers can be accurately determined. In addition to its

calibration service for user-supplied transducers, NBS has for some time

offered the loan of custom-built transfer standard transducers. A new type of

transducer has been designed both to improve overall performance and to

preclude certain difficulties now known to be commonplace in field use. This

unit will soon be available for purchase through the Office of Standard

Reference Materials.

The newly-designed standard source is used as a transfer standard for

measurements of the total radiated power sources operating into liquid media

at power levels from 0.1 mW to 1 W in the frequency range of 1-10 MHz. It utilizes a compliantly-mounted piezoelectric crystal in a cylindrical

stainless-steel housing which also contains circuitry to sample the applied rf

voltage. An impedance matching network provided with each transducer relieves

drive requirements by allowing the use of a 50 ohm rf source. Embodied in the

matching network is the additional circuitry necessary to allow the accurate

measurement of rf voltage independent of cable effects which have been found

to significantly degrade the accuracy of such measurements in the field.

Based on preliminary tests, the use of compliant adhesives to mount the

piezoelectric element will offer improved performance by providing greater

acoustical isolation, reducing mechanical stresses, and promoting long-term

stability of the transducer radiation pattern. Research involving the new standard source now includes a study of the feasibility of a "universal

source" capable of useful operation at high-order overtones. Initial results

indicate that the successful implementation of such a transducer is likely.

46
Task # 15760

IMPROVED ULTRASONIC REFERENCE BLOCKS

Gerald Blessing and Donald Eitzen

Mechanical Production Metrology Division

Center for Manufacturing Engineering

The National Bureau of Standards has been asked by the Army Mechanics

and Materials Research Center of the Department of Defense to evaluate the

present procedures for fabricating and testing aluminum alloy ultrasonic

standard reference blocks, and to make recommendations for improving those

procedures (1). These reference blocks, possessing flat-bottom-hole defect

artifacts, are used both for calibration and as reference standards. Here we outline our recommendations for their improvement.

Many sets of aluminum ultrasonic reference blocks have been fabricated

according to the specifications in the ASTM E 127 document, "Standard Practice

For Fabricating and Checking Aluminum Alloy Ultrasonic Standard Reference

Blocks". It is DoD's goal to reduce the present allowable tolerance of +2 and

-3 dB criteria of E 127 to _+l dB, for its own applications. We note that

recommending changes to the ASTM document is not part of this task; rather,

that document serves as a baseline from which to work.

The +1 dB goal represents a substantial reduction in tolerance, and would

impact on all phases of the manufacturing and calibration of reference blocks.

The principal system variables affected may be categorized per reference (1) as follows:

1. Block Material

2. Transducer

3. Pul ser/Receiver

4. Operator Judgment

5. Block Geometrry

A _+l dB tolerance means that the net effect of these system variables be +12

percent or less.

Guided by these constraints, the following are judgments of the optimum

tolerance for each system variable; these tolerances are considered feasible

with present technology.

1. Block Materi al : The recommended approach is first to define a (arbitrary) level of material ultrasonic scattering that is typical

of today's manufacturing process, and which also meets the

requirements of E 127. Secondly, require that the variation in

material scattering as determined by the echo amplitude on the

largest block length, not exceed +5 percent. (A three-measurement

average should be obtained for each block.) This may require that

all bar material be taken from the same manufacturing lot and/or

other restrictions be placed on its fabrication. Hopefully, an effective screening of acceptable material can be achieved by

sampling. The figure of -5 percent represents an educated guess of

present feasibility.

47
2. Transducer : The ability of manufacturers to fabricate like transducers or search units has been very limited. While the best results

have been obtained using quartz, even these transducers show

appreciable variation in performance. However, some success has been

achieved in correcting their acoustical far-field profile variations,

which is the region of significance in calibration work. Based on empirical results using correction factors, a figure of + percent

variation in block response may be ascribed for quartz transducer

variations.

3. Pulser/Receiver : Variations in tuned pulse width may significantly

affect the echo amplitude from a given block. It is not clear how

this variable should be controlled, except that it should at least be

kept constant for a given system. Variations between like systems

will certainly be minimized by using the same pulse width, and as much

as possible, the same pulse shape (and therefore, spectral content).

Receiver linearity of +2 percent or better may be realized with

available solid state circuitry. This, combined with uncertain pulser

variations, contributes approximately a +5 percent variation to the

ultrasonic system.

4. Operator : By eliminating screen reading judgment errors with digital

readout of signal amplitude, the principal operator-dependent

variations would be limited to alignment errors. These should be kept

within a + 2. percent bound. The detection of bubbles, a potential

source of large error, on the transducer and/or block surface is

greatly facilitated by back-lighting an immersion tank constructed

with transparent siding. Finally, to minimize operator-dependent

errors, all reported amplitude values should be an average of three

repeated measurements; and a difference of more than 2 percent within

these three measurements should be investigated.

5. Block Geometry : So long as reference blocks are fabricated by hole

drilling, possible variations of +3 percent in echo amplitude due to

this process will have to be allowed for the smallest diameter - No. 3

size hole. Variations in the larger holes (No. 5 and No. 8) will be

proportionally smaller.

Finally, there is a source of systematic or "system offset" error as

E 127 is presently written. There is a discrepancy between metal ball

diameters, used to set system sensitivity, and the hole size ratios. This

discrepancy must be eliminated in order to use the same table of recommended

echo amplitude values for all hole sizes.

In conclusion, we note that the net tolerance outlined above appreciably

exceeds our+^12 percent (or 1 dB) goal if we assume the worst case conditions.

Eliminating this excess of tolerance would impose more stringent requirements

on the material properties, the transducer, and the pulser. Both the use of

correction factors and the establishment of a central calibration facility

could serve to alleviate these additional requirements.

48
(1) Blessing, G.V. and Eitzen, D.G., U1

Blocks - What Future?" Summary Pape

publ i shed) ASNT Conferences *

.

<*•

'
Task # 15770

NDE TERMS AND DEFINITIONS: ULTRASONICS

Donald Eitzen and Marilyn Cadoff

Mechanical Production Metrology Division

Center for Manufacturing Engineering

The Army Materials and Mechanics Research Center has asked that NBS

assist them in developing terms and definitions suitable for use in Mil

Standards. The specific objectives of the current phase of this activity

are to:

1. Develop appropriate terminology for the broad NDTI primary terms

including: Nondestructive Testing, Nondestructive Inspection,

Nondestructive Evaluation and Nondestructive Examination.

2. Develop appropriate terminology for some of the general terms

associated with nondestructive methods including: Reference

Standards, Calibration Standards, Reference Blocks, and Calibration

Blocks. The metrological use and implications of these terms will

be taken into account in this terminology.

3. Develop appropriate and precise terminology for ultrasonic test

methods and procedures using resources such as ASNT, ASTM, ASME and

military publications with definitions of ultrasonic terms. Also,

identify terms that are necessary but not yet defined.

4. Develop additional general terms associated with nondestructive

methods. The metrological use and implications of these terms

will be taken into account. Also, develop precise terminology for

those specific terms for ultrasonic methods identified earlier as needing definition.

In carrying out these tasks well over 30 draft or published documents were reviewed and competing terms and definitions were organized. A number

of internal workshops were conducted with NBS experts on metrology and on NDE.

As a result, four terms were proposed for the broad NDE primary terminology

such as "Nondestructive Testing". General terms such as "reference standards" were studied and, as a result, 28 general terms were proposed. Also 115

terms were proposed for ultrasonic test methods. This phase of the project

is essentially complete; however, additional editing of some terms will be

undertaken in order to maintain consistency between this effort and some recent NBS comments on proposed MIL-STD-1309C, "Definition of Terms for

Test, Measurement and Diagnostic Equipment".

50
Task # 15760

CONTACTLESS LASER GENERATION AND OPTICAL INTERFEROMETRIC

DETECTION OF ULTRASONIC WAVES

M. Rosen, S. Fick, F. Nadeau and R. Mehrabian

Metallurgy Division

Center for Materials Science

Transient loads applied by rapid energy transfer from a singlepulse Q-switched high-energy laser generate compressive stress

waves that propagate in a material. Propagation of ultrasonic

waves in a medium cause surface displacements that can be measured

optically by exploiting the phase shift of an optical beam reflected

from the surface of the material. When the reflected beam is

approximately combined with a reference optical beam, from a helium-neon laser, phase changes are converted into amplitude

changes, detectable by a suitable photodiode. These amplitude

changes are proportional to the surface displacements on the specimen.

Potential problems arising from the fact that phase changes also

result from relative motion among the optical components of the

system and even from flutuations of the ambient air, are avoided by

appropriate features in the mechanical design of an interferometer.

An arrangement due to Fizeau is inherently compact and mechanically

stable, and thus well -suited to the present application. Two such

Fizeu interferometers can readily be combined so that the two

optical probes may be separated by a convenient distance to allow

accurate measurement of the travel time of an ultrasonic wave in

the specimen over a well-defined distance. Furthermore, the variation

in magnitude of the surface displacements detected by the two

Fizeau interferometers determines the ultrasonic attenuation in the

specimen. This specific system with dual Fizeau interferometers

allows the simultaneous determination of both elastic-wave velocity

and ultrasonic attentuation.

Compared with other sensors, interferometers offer several

advantages. The sensitive area is definite, and can be made as small as a few microns in diameter for highly localized measurements. * A known quantity, displacement, is measured; independent methods of

absolute calibration are applicable which give small overall errors. Bandwidth, determining the fidelity of reproduction of signal

waveshapes, is limited not by the physical nature of the transduction

process but by its associated electronics. In this respect, performance

can exceed that of conventional piezoelectric tranducers. Small

signal resolution and bandwidth are related; displacements of a few

A are detectable at 7 MHz bandwidth. Laser loading produces a stress pulse of high amplitude and short duration so that investigation

of highly attenuating, or very thin, specimens is possible. The

noncontact feature of both generation and detection of ultrasonic

waves is advantageous in situations requiring physical separation

between the measuring system and the specimen. Furthermore, the

noncontact mode of generation and detection precludes interaction

with, and modification of, the wave propagation under study.

51
The two-channel, quadrature dual-laser interferometer developed

and constructed permits highly accurate sound wave velocity measurements

of longitudinal, transverse and Rayleigh waves. From the absolute

surface displacement measurements, it is possible to determine the values

of the attenuation of these waves, as well as their frequency dependence.

The contactless method of generation and detection of ultrasonic

waves permits dynamic measurements to be performed whereby the specimen

is subjected to programmed heat treatments, or is positioned in a hostile

atmosphere. The noncontact mode of generation and detection precludes

interaction or modification of the wave propagation under investigation.

This unique dual -channel laser interferometer is in the process of final

adjustment and calibration.

52
Task # 15750

EDDY CURRENT STANDARDS

A. J. McAlister

Metallurgy Division

Center for Materials Science

The eddy current method is widely used in both the ferrous and non- ferrous metals industries for nondestructive detection of flaws such as cracks, inclusions, and variations in composition and microstructure in metals

and metal parts via local changes in electrical conductivity. Primary con- ductivity standards are needed for reliable execution of this work, but to

date this need has been supplied by industry itself. The necessary range of conductivity for industrial testing is from 5.85 to 58.5 MSm' 1 (1 to 100

percent IACS).

The properties desired in a primary conductivity standards are long term

stability, durability, and low magnetic susceptibl ity. (In testing of high susceptibility steels, normal practice has been to apply a saturating magnetic

field to reduce the differential permeability to a value near that

of vacuum). Hence, elemental metals and alloys of equilibrium phase composition

upon which stable nonconducting surface layers can be formed are desirable.

The surface properties are important not only for durability, but for

reproducible lift-off behavior as well.

Our overall aim is the selection of metals and alloys suitable for primary conductivity reference standards, and the preparation of specifications for

their preparation and fabrication. Progress to date in this work is

summarized in the following Table, in which target conductivity material.

Summary of Conductivity Testing

Target Observed Conductivity

Conductivity and range

(t IACS) Material (w/o) Status Condition (% IACS)

101 OFHC Cu *

* annealed unannealed

101.9 ± 0.1 99.5 ± 0.2

85 1.3 Zn + Cu # as cast 81.1 ± 0.4

75 2.4 Zn + Cu # as cast 70.4 t 0.4

60 Al lldOF + comDlete 60. 0 ± U. Js 48 A1 2024-0 + complete 47.0 ± 0.1 41.5 Al 6061-T + complete 41.0 ± G.05

29 Al 2024-T-351 + complete 30.0 .t 0.1

23 21.5 Zn, 2.5 Al + Cu . T as cast 20.5 ± O.J

17 7 Al + Cu # as cast 15.6 ± 0.1

17 6 Ni + Cu * as received 17.7 ± 0.1

1Z l.b Si + Cu 4 as cast 10.1 ± 0.2

7 3 Si + Cu T as cast 7.5 ± 0.2

7 23 Ni + Cu * as received 6.11 ± 0.01

3TB Ti , Grade 4 unannea ied 1.9b i U Ob

3.5 45 Ni + Cu * as received 3.440 ± 0.002

1.0 8 Al , 1 Mo,

IV + Ti

* unannealed 1.07 ± 0.00/ 0.86 ± 0.012

1.0 6 Al , 4 V + Ti * unannealed 0.97 ± 0.005

Status is indicated as:

+, finished, calibrated standards;

*, commercial products, faced and examined;

#, alloys cast in-house, facad and examined.

Production and calibration of the Al-based standards is complete.

53
condition of the material as examined, observed conductivity and its

extreme variation over the sample faces, and status of the material are

listed.

Apparent conductivity variation over the surfaces of commercial alloys

appears random, and is taken to reflect the variation to be expected in as- received condition. The better uniformity of annealed vs. unannealed Cu is

due to removal of cold work damage. Annealing did not improve Ti or the Ti -based alloys. The major part of the conductivity variation in the

as cast Cu-based alloys is due to solute segregation upon solidification,

and should be largely removed by homogenization.

The decision to focus on Cu-based alloys outside the A1 and Ti ranges, and

to prepare special melts, was not made lightly. As a practical matter, it

is essential that the magnetic permeability of an eddy current conductivity

standard be very close to the vacuum value. Only Cu-based alloys are potentially capable of meeting this criterion in the needed conductivity

ranges. Special melts are indicated because current industrial practice is

to add significant quantities of Fe, Co, and Mn (which can develop full

magnetic moments or form magnetic precipitates in Cu) as grain refiners or

to improve mechanical properties. Currently available Si-bronzes contain

0.8 w/o Fe; Cu pro-nickels, 1.0 w/o Fe; brasses, 0.05% Fe. Yet the brasses,

for example, must contain less than 0.03% Fe if Fe precipitation is to be

avoided during stress relief.

In view of the importance of the magnetic properties, we have assembled and

tested equipment for the measurement of magnetic susceptibility by the Gouy

method. We are currently acting to provide sets of 100 coupons each of 0FHC

Cu and T

i -6-4 alloy.
Task # 15750

EDDY-CURRENT TESTING

George Free

Electrical Measurements and Standards Division

Center for Absolute Physical Quantities

The initial phase of the eddy-current project to make available to

the general public electrical -conductivity Standards Reference Materials

(SRM's) has been completed and three primary standards have been calibrated.

The resistivities of four electrical -conductivity SRM's have been

determined in terms of these three NBS primary standards. The four

values of electrical conductivity cover range 30-60% International Annealed

Copper Standard (IACS) and specifically include SRM's with conductivites

of 31, 41 , 47 and 61% IACS.

The experiment to determine the resistivity of the primary standard

was discussed in the 1981 Annual Report. In an effort to lower the

uncertainty of this measurement, two steps were taken during FY82.

First, the length if the primary standards was reduced to approximately

48.5 cm. The reduction in length makes possible more accurate machining

methods, thereby improving the accuracy and precision of the crosssectional area determination. Second, the dc experiment was modified

by constructing a new bar holder and a potential contact assembly.

In the new system bar temperature was monitored more closely and the

potential contacts were fixed with respect to each other with an approximate

separation of 5.08 cm. Furthermore, the contacts can be moved across or along the bar to measure variations of resistance over the area of

interest. At the time of writing, this experimental arrangement was

still being tested for precision and accuracy. The dc bridge had also

been improved by replacing all critical switches with low thermal

switches. This has reduced the noise in the detector by a factor of 2.

The eddy-current bridge used for comparing the NBS primary standards

with unknown samples was and still is being improved. The basic design

is discussed in the 1981 Report but additional electronics have been

developed for the grounding network in order to improve bridge accuracy

and for reducing the time needed for measurements. The oil bath has

been rebuilt and a secondary control bath has been added to improve

temperature stability and make possible a controlled variable-temperature

bath. Coil holders have been redesigned to minimize coil heating on metal. A new lift-off device has been designed and its components are

being constructed. The new device will make possible more accurate

measurements of coil -metal separation. This data will become part of

the computer algorithm used to calculate conductivity from measurement

data.

A scanner that moves across the test sample with only minor variations

in lift-off was constructed for the eddy-current experiment. With this

scanner, highly accurate measurements of conductivity variation in a given sample can be made.

55
During the year conductivity standards were calibrated for Kaiser

Aluminum and for the Aluminum Association. These calibrations served as

a trial run for the NBS calibration facility.

A paper entitled "Standards for Eddy-Current Nondestructive Testing,"

was coauthored with G. Birnbaum and H. Berger of NBS and V.V. Kljuev and

Y. Fedosenko of the USSR and was presented at the 10th World Conference

on Non-Destructive Testing. This paper discusses some ASTM Eddy-Current

Standards and compares them with their Russian counterparts.

Several new activities are planned for FY83. Theoretical studies

will be initiated on at least two problems related to the methodology

presently used in the calibration of conductivity standards. First, a study of the shortest possible distance between point of entrance of the

dc current and placement of the potential contacts will be made. The

largest component of uncertainty in the primary reference bars is due to

the determination of cross-sectional area. Shorter bars can be machined

and measured more accurately. Thus, the minimum length for the reference

standards is an important quantity. The second problem is to correlate

coil impedance changes with sample geometry. More specifically, the

relationship of coil impedance to coil distance from the sample edge is

known only by experiment. A theoretical model of this problem would aid

in the optimum design of coils.

During FY83 some modifications will be made to both the ac and dc

measurement systems in an attempt to further reduce the uncertainty of

the measurements. Varous aluminum and brass bars will be tested for

electrical uniformity in a continuing effort to improve the electrical

characteristics if the NBS primary standards. A second set of 400

samples in the 30-60% IACS range will be purchased and calibrated as SRM's. Also samples with nominal values of 10Q and 1% IACS may be

obtained and calibrated as SRM's in FY83. The NBS measurement system

will be documented and a technical note written discussing the entire

experiment.

56
Task # 15750

HIGH FREQUENCY, BROADBAND EDDY CURRENT PROBES FOR IMPROVED FLAW

CHARACTERIZATION

C. M. Fortunko and S. A. Padget

Fracture and Deformation Divison

Center for Materials Science

Recent theoretical developments have resulted in a better understanding

of the roles of probe geometry, frequency of operation, and material resistivity

in determining what information about a particular flaw type can be obtained

from the experimental data. The present work was carried out in close collaboration with Dr. B. A. Auld of the W. L. Ginzton Laboratory at Stanford University

and Dr. A. H. Kahn at the Fracture and Deformation Division, Center for

Materials Science in Gaithersburg, MD. The work at Stanford has been mainly

concentrated on very high frequency (microwave) eddy current probes and

theoretical modeling of flaw responses as a function of the probe geometry,

frequency of operation and material resistivity. Because many of the

important theoretical predictions needed verification in a frequency range not

covered by most conventional eddy current inspection systems (100 kHz-2 MHz)

and well below the microwave frequencies, it was decided that a new broadband

probe design was needed that could be used in the 100 kHz-25 MHz frequency

range.

During FY82 the new probe design was developed and a large number of

calibration specimens were prepared that were needed for verification of

specific theoretical predictions. Because the preparation of calibration

specimens involves well known techniques, the present report is focused on the

design and operation of the new eddy current probe system, certain experimental

highlights and a technique, developed by B. A. Auld, for removing unneeded

signal components from the eddy current data.

In the past, it has been pointed out by A. H. Kahn, using an exact theory

for two-dimensional surface cracks, that the phase of the eddy current signals

experiences a significant roll-off in the region 1 < a/6<20, where a is the

crack depth and 6 is the electromagnetic skin depth (1). Later Auld showed

that the phase of the signal in the roll-off region is primarily determined

by the dimensions and shapes of the crack, and by the distribution of the

electromagnetic field beneath the probe. It was then shown that the phase shift

in the eddy current signals in the presence of a flaw can be used under certain

circumstances for inversion of the eddy current data (2). The principal

results of Auld are summarized in Fig. 34(3).

Figure 34 shows the phases of the signals associated with the flaw

and lift-off signals as a function of the parameters a/6, a/c, and r/a. Here

a is the depth of a flaw, and r refers to the mean radius of the coil. It

is seen from Fig. 34 that the maximum change in the flaw signal occurs in the

range a/6 5-50, for rectangular flaws with a unity aspect ratio (a/c^l).

57
The electromagnetic skin depth 6 is given by:

J aiffy y Eq . 1 N 0 where a is the electrical conductivity, f is the frequency, and y is the

permeability of the material under evaluation.

From Eq - _ 1 , it follows that the electromagnetic skin depth for aluminum

(p = 2.655-1

0~

8 ohm-m) at 1 MHz is 8.2 • 10~ 5 m (0.0032-in). Then, it follows

that at 1 MHz the interestinq region in a/6 corresponds to a range of flaw

depths a from 4. MO"4 m (0.0161 in) to 41-10"4m (0.161-in). This range of

flaw sizes can be easily obtained using electron-discharge and conventional

machining techniques.

Since the operating frequency of most conventional eddy current test

instruments does not permit reliable operation above the 1-2 MHz frequency range,

it was decided to design a new probe system that would be operable at least over

the 100 kHz-25MHz frequency range. Concurrently, special eddy_current probes

of spiral geometry (pancake) were designed with a mean radius r of 1.25 mm

(0.050 in).

Figure 35 shows a schematic diagram of the eddy current probe and Fig. 36 shows

a block diagram of the experimental configuration. The key component of the

eddy current probe is a high-frequency, four port hybrid, which functions as a Wheatstone bridge. The hybrid as normally driven from a 50 ohm signal source conected to the "sum" (e) port. The output of the hybrid is obtained from

the "difference" (a) port. Two identical eddy current coils L^and L 2 are attached to the unbalanced, oppositely phased outputs of the hybrid. Because

the eddy current coils exhibit very low impedances, broadband impedance matching

transformers are used to step up the magnitudes of the coil impedances to the

10-50 ohm range. The hybrid and the impedance matching transformers are designed using bifilar winding (#32AWG) techniques to reduce leakage inductance

and minimize interwinding capacitance effects. The winding convention is

indicated by the customary "dots" (4,5,6). The two eddy current coils were made by winding 10 turns #30AWG copper wire in a tight spiral pattern.

Figure 36 shows two experimental set-ups that have been used to measure

eddy current signals. The system used in the frequency range 100kHz to 1MHz is

shown in Fig .36a . An oscilloscope is used to measure the amplitude and phase

of the eddy current probe. The input signals, serving as a reference, also appear

on the oscilloscope display. Figure 36b shows the experimental set-up

for the 1-25 MHz frequency range. In this case, a vector voltmeter is used to

measure the amplitude and phase of the signals appearing at the A port

relative to the signal applied to the e port. A 1MHz overlap region between

the two set-ups can be used for comparative purposes. In both cases 1 volt

(peak-to-peak) input signals were used. At 1 MHz, the voltage level at the

output port (a) in the absence of a flaw and for balanced (0.025-mm) lift-off

distance between the two probes (L^ and l_ 2 ) was approximately 2 mV. At 20 MHz

58
this background signal (caused by the bridge being out of balance) was 4.4 mV

The above results indicate that high levels of common mode rejection

V(CMR) are possible using the probe design of fig. 35. . ”

t The operation of the new eddy current probe was first demonstrated

by scanning a tightly closed crack in an aluminum tension specimen. The

results, for the case of a scan line bisecting the crack at 90° are shown in Figs. 37 and 38. The tightly closed crack was approximately 5-mm

long and 2-mm deep. The phases of the probe output signals are plotted

in Fig. 37 and the amplitudes are plotted in Fig. 38 as a function of probe

position along the scan line. Data are given at the following frequencies

200khz, 500kHz, 1MHz, 2MHz, 5MHz, 10MHz and 20MHz. It is evident

from Fig. 37 that the phase excursion first becomes positive and then

negative in relation to the phase of the background signals associated

with bridge imbalance. The maximum positive excursion corresponds to

the coincidence of the "in-phase" probe with the center of the crack,

and the maximum negative excursion corresponds to the coincidence of the

"out-of-phase" probe with the center of the crack. The spacing between

the two probes (Li and L 2 ) was approximately 33 mm. The results of Fig. 37

and 38 clearly demonstrated the qualitative predictions of Fig. 34. To

facilitate quantitative analysis, it is useful to plot the data in Fig. 37

using a vector-diagram presentation.

Figure 39 shows the outputs from the bridge with the "in-phase"

probe located directly above the center of the crack (broken lines for

flaws and bridge imbalance signals) and far away from the crack (solid

lines for bridge imbalance signals). The data was taken from 200 kHz

and 20 MHz, corresponding to a two-decade frequency range. The flaw

signals are indicated by the solid line vectors drawn between the flaw

and bridge imbalance signals (dotted, solid lines). It is seen from

the construction in Fig.39that the phase of the flaw signal decreases

monotonically with frequency. The rate of decrease is slow initially

(from 200 kHz to 1 MHz), corresponding to an a/6 range of 11-25. However,

the rate of increase is very fast in the frequency range 2-20 MHz,

corresponding to the 34-112 range in a/6. The total phase-shift is

-42°, as shown by the construction in Fig. 39. The above observation is

in a very good quantitative agreement with the theory of Fig. 34.

From the above results it can be concluded that the phase of the

eddy current signals, after subtracting the background signal information,

can be used to infer the depth of tightly-closed cracks. The experimental

results in this report agree with the theory of B. A. Auld, which has

been developed specifically to treat the case of a finite eddy-current

coil above a closed crack of finite dimensions. In the future, it is

planned to investigate, the behavior of the background signals, which

ultimately determine the sensitivity of eddy current techniques to

flaws. In particular, the phases and magnitudes of coil lift-off and

tilt will be investigated as functions of flaw parameters, coil parameters

and skin depths.

59
References

1. A. H. Kahn, R. Spal, and A. Feldman "Eddy-Current Losses Due to a Crack in Conducting Material," J. Appl . Phys. 48 (11), 1977, pp.

4454-4459.

2. B. A. Auld, F. Muennemann, M. Riaziat, and D. K. Winslow "Analytical

Methods in Eddy Current NDE", in the Review of Progress in Quantitative

NDE, ed. by D. 0. Thompson and D. A. Chimenti (Pergamon, New York, 1982),

in press.

3. B. A. Auld, W. L. Ginzton Laboratory, Stanford University, private

communication.

4. H. 0. Granberg "Combine Power Without Compromising Performance",

Electronic Design 28 (12), 198Q, pp. 181-187.

5. R. E. Matick "Transmission Line Pulse Transformers - Theory and

Application", Proc. IEEE 56(1), 1968, pp. 47-62.

6. J. Stevick "Broadband Matching Transformers Can Handle Many Kilowatts,"

Electronics 49 (23), 1976, pp. 123-128.

60
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63
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66

Bridge

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Task # 15750

THEORETICAL MODELLING IN EDDY CURRENT NDE

Arnold H. Kahn

Fracture and Deformation Division

Center for Materials Science

Objectives:

The aim of this project is to provide theoretical support for the

development and improvement of electromagnetic methods of NDE. Applications

include testing and measurement of materials properties, defect detection

and sizing, and contributions to the development of standards.

Accompl ishments:

The following major accomplishments have been achieved to date

since this program was initiated in 1979.

1) A theoretical analysis was made on the nature of eddy currents

in the vicinity of a crack in a conducting material. The applied fields were

considered to be uniform in space, as a starting point. In the analysis it was found that the current distribution could be resolved into characteristic

distributions associated with the corners and tip of the crack, anu that for

cracks sufficiently deep, these patterns could be superimposed to give a satisfactory solution. This approach gives insight into nature of the signals

arising from a crack.

2) In an attempt to develop the theory on a more quantitative basis, we calculated an exact solution for a longitudinal crack in a cylinder surrounded

by a test solenoid. This calculation gave the modification of the impedance

diagram due to a crack, over a wide range of frequency and dimensions of the

cylinder and of the crack. These calculations were of immediate application

in the design of equipment to be used for the production testing of drawn tungsten

filament wires for the manufacture of light bulbs. Independent calculations

by investigators at the General Electric Company, by the finite element method,

were in agreement with the published work by NBS.

3) The analysis of the crack in a plane was extended to cracks of size

comparable to the skin depth, the domain of greatest sensitivity of detection.

The boundary integral equation method was applied. The results gave the

impedance change due to the crack, and the variation of its phase with

frequency. Cracks of arbitrary inclination were included.

In the past year the theory was extended so that the boundary integral

equation method could be applied to eddy current problems in which the applied

excitation field is not uniform in space, as for example excitation by currents

in a pair of parallel wires. This model has some of the characteristics of

a three dimensional coil configuration, but is still computationally tractable.

The necessary computer programs are now being prepared for the production of

numerical results on this part of the program. When completed the results

will be compared with experiments at NBS and at other laboratories.

67
The theoretical work on these eddy current problems is being conducted

in collaboration with Dr. C. Fortunko at the NBS Boulder Laboratories and Dr.

B. Auld of Stanford University.

Future Expectations:

Recent measurements on the phase of the signals from a crack, performed

by Dr. Auld and co-workers, give a dependence of phase on frequency similar

to that predicted by our theory, but moves to frequencies an order of magnitude higher. Dr. Auld attributes this difference to three dimensional aspects

of the measurement system which are not included in our published work. An

important part of the work of the immediate future will be the examination

of the results of the numerical calculations now being prepared, to ascertain

whether spatial limitation of the exciting coil makes a major contribution

to the frequency dependence of the phase of the impedance of a crack.

An important part of the theoretical effort will be the collaboration with Drs.

Fortunko and Auld in the design of coils and apparatus for the purposes of making

critical measurements for testing and theoretical models, and for analyzing

the measured results.

In addition to the study of the non-uniform exciting field, we shall

also extend the same computational methods to treat open cracks. The results

of the calculations may help to explain some of the variability reported

in measurements on artificial and natural cracks.

«

68
Task # 15750

MAGNETIC MEASUREMENTS

Lydon Swartzendruber

Metallurgy Division

Center for Materials Science

Nondestructive evaluation methods utilizing magnetic leakage fields

are widely used for detecting defects in ferromagnetic materials such as iron and steel. These methods involve the magnetization of the material

and the subsequent detection of the leakage fields resulting from any

defects which might be present. A common technique for detection of

leakage fields is magnetic particle inspection (MPI). MPI involves the

spraying or flowing of minute magnetic particles over the magnetized

material. Patterns are formed on the surface which indicate the location

of surface or subsurface defects. This method reveals the presence and

location of defects, but gives limited information about their size, shape, or depth below the surface. Other methods, such as measurements with a Hall probe or moving coil, give more quantitative information about the

leakage field but correlation of the measured field with the shape and

location of the defect also presents considerable difficulty.

In a material of constant permeability, analytical calculations of

leakage fields for various geometries, such as long transverse cylinders, are possible. We have previously shown that measured magnetic leakage

fields above artificial cylindrical defects in a steel sample are considerably

larger than the strength calculated analytically for a material of

constant permeability, however high that permeability might be. Clearly,

the nonlinear B-H relationship has a significant effect in this type of

measurement. Thus, any mathematical model of such a system which incorporates

permeability as a constant is insufficient; permeability must be incorporated

into the model as a function of the field strength. This complication

necessitates numerical solutions which require lengthy calculations for

each specific case.

Utilizing a nonlinear finite difference method, and an analytical

representation of the nonlinear B-H relationship, numerical calculations

have been performed on the Metallurgy Division computer. The numerical

method was used to calculate leakage fields for subsurface defects and

for slits open to the surface. For comparison, linear calculations were also performed. These calculations were then compared with experimental

measurements. The nonlinear numerical calculations are in qualitative

agreement with the experimental measurements and confirm that: (1) the

leakage field perturbation due to the defect continues to increase well

into the saturation region of the magnetization curve; (2) the leakage

fields have a shape nearly that expected from a linear magnetic dipole,

and; (3) nonlinearity causes the defect to appear closer to the surface

than its actual physical distance and stronger than predicted by a linear theory.

69
Task # 15750

MAGNETIC PARTICLE INSPECTION STANDARDS

L. Swartzendruber

Metallurgy Division

Center for Materials Science

This project was initiated by the Specifications and Technical Data

Branch of the Army Materials and Mechanics Research Center, Watertown,

MA. Military Specification MIL-M-1 1 472 , "Magnetic-Particle Inspection,

Process for Ferromagnetic Materials", is being reviewed for proposed

revision or cancellation and replacement by Military Specification

MIL-STD-271, Military Specification MIL-I-6868, or American Society for

Testing and Materials Recommended Practice E709-80. The contents of

these documents have been compared with current state-of-the-art

practice in magnetic particle inspection as revealed by industrial

practice, available literature, current applicable Military Standards,

Aerospace Materials Specifications, and American Society for Testing and

Materials documents. A draft version of a revised MIL-M-1 1472 has been

prepared. Feedback from Department of Defense users will be used to

eliminate or revise portions as deemed necessary.
Task # 15770

DEVELOPMENT AND APPLICATION OF A SECOND GENERATION IMAGE INTENSI FI ER

FOR REAL TIME MICRORADIOGRAPHY

M. Kuriyama, W. J. Boettinger, H. E. Burdette, and R. D. Spal

Metallurgy Division

Center for Materials Science

The resolution of real time radiographic systems is often dictated by

the spatial resolution of viewing detectors. To overcome this difficulty,

an x-ray image magnification technique was developed (1,2). This development does not, however, negate any further improvement of the spatial re- solution of image detectors. As a continuing effort seeking a better detector,

we have employed a double intensified charge injection device (ICID) camera

to develop an x-ray image intensifier. This ICID camera is, in effect, a modified solid-state charge-couple TV camera.

In general, there are two approaches to obtain optical signals from x-ray

photons: a) direct photon-electron conversion in videcons and b) visible

photons converted from x-ray photons by a phosphor, and subsequently viewed

by an optical image intensifier. In this work, we chose the second approach.

Phosphors of different particle sizes were prepared on fiber optics plates

(<30 ym). This ICID camera was tested by a wedge shaped resolution

test pattern after one of the fiber optics plates was mounted tightly in

front of the camera. Typical results are 2.9 line pairs/mm for FG3M phosphor

and 4.5 1 p/mm for P20 phosphor (4.3 mg/cm2

), by visual test. The last value

corresponds to limiting resolution 110 ym.

To determine quantitatively the degree of resolution in this system and

determine quantum efficiency, another set of measurements was made using

this system with the two kinds of phosphors. The ICID camera was operated

in photon counting mode, that is, low flux, high gain mode. The image

output was digitally processed, using a video frame grabber and a digitizer

interfacing LSI 11/23. With the P20 phosphor, the order of 4 pixels in

radius per a single photon was observed. One pixel is 40 ym x 30 ym.

This result agrees with the visual result of 110 ym limiting resolution

(5% MTF)

Quantum efficiency was found to be 100% with a thick phosphor. The thinner

phosphor, the higher the resolution, but less quantum efficiency, as predicted. The decay time of the phosphors was much shorter than the TV

frame period (^30 ms).

This system was used to obtain video tape images from dental teeth,

indentation impressions on a NaCl crystal and a copper crystal in the

microradiography mode. This system was also used to record real time data

from various crystals in the x-ray diffraction topographic mode using

synchrotron radiation.

71
REFERENCES

(1) W. J. Boettinger, H. E. Burdette and M. Kuriyama, Rev. Sci. Instr. 50^ 26 (1979).

(2) 1978 NDE Annual Report.

72
Task # 15760

RADIOGRAPHY

J. W. Motz, R. C. Placious, and D. Polansky

Radiation Physics Divison

Center for Radiation Research

A. RADIOGRAPHIC STANDARDS

1 . Radiographic Detection and Sensitivity

Until recently only one radiographic standard addressed the problem

of image quality. This standard uses an image quality indicator called

the plaque penetrameter. While it is widely accepted and used, it lacks

the ability to accurately measure an x-ray system's real image quality

limits. Moreover, statistical analysis was impossible since it required

that only one "event" be detected (one hole in one plaque).

To allow a better evaluation of image quality, a new standard was developed requiring that a statistically significant number of images

be detected. Moreover, fine graduations were built into the system so that penetrameter sensitivity could be measured over a range of values

instead of one. This new standard was designated "A Standard Method for

Determining Relative Image Quality Reponse of Industrial Radiographic

Film". It now carries an ASTM designation, E-746-80, and NBS can supply it

to users as an SRM. The numerical index resulting from use of this

method is "EQUIVALENT PENETRAMETER SENSITIVITY (EPS)".*

Following this introduction of a new image quality standard, users of high energy radiation (for very thick materials) asked for a similar

capability and we have been working on this problem.

Several NBS designs were tried by users around the country. None of

these proved to be sufficiently suitable for adoption as an industry

standard. Each design, when tested, proved deficient in one respect or another. For example, the high energy system on an absolute basis has

significantly lower EPS capability. This has led to problems in "tuning"

the test device to the proper range of EPS values. The X-Ray Physics

Group of NBS in cooperation with the Office of Nondestructive Evaluation,

is providing the designs, the test devices, and the exposure protocol

for this work. We also collect test data and evaluate the results of the

round robin tests. Our plans are to continue this activity to its con- clusion. We have recently developed a new model image quality indicator

which it is hoped will eliminate some of the previously revealed

difficulties and will prove to be the final version. A document will

then be developed and submitted to ASTM for review and approval.

* See ASTM Standard E-142-77

73
2. Development of a Visual Acuity Standard for Radiographers

Recently a design for testing visual acuity of radiographic

inspectors has been published (1). This should prove to be a useful standard

and will probably be introduced to the NDE community through ASTM or

ASNT.

This design is being developed into a working device by the X-Ray

Physics Group of NBS. A number of test patterns are needed for this

standard. The present approach is to develop the test patterns using a radiographic rather than a photographic method. Reproducing the test patterns

could be done photographical ly; however. So far we have produced one

set of the patterns for the thinnest slit and three specified contrast

levels.

We plan to continue development of this system and will recommend,

if necessary, coordination of a series of round robin tests among radiographic nondestructive laboratories.

3. Unsharpness

Radiographic unsharpness, also called radiographic definition, has

been the subject of many publications in both industrial and medical

radiology. It is well recognized that a high degree of unsharpness can greatly affect the analyses of a flaw or even prevent detection. Voluntary

standards organizations have been reluctant to support work on this subject.

Inasmuch as flaw sizing will eventually become an important factor in NDT, we feel it should be pursued and, if possible, a field method be developed

for measuring total unsharpness (Uy).

We have demonstrated the inadequacy of existing procedures for assessing

unsharpness by a very brief series of tests. The results are given in Table

1. This data demonstrates that the device produced by CERL* for measuring

total unsharpness is not sufficiently sensitive to small variations in Uy

and that the calculated value of U~ (geometric unsharpness) is probably

low by a factor of two or so. This is not surprising since it considers

only the geometric variables of the x-ray system.

Our plans are to continue work in this subject, at a fairly low level

of activity, however, and to gear our effort to a system which will measure

Uy rather that U

G

and which will be relatively easy to adopt to field use.

* Central Electricity Research Laboratories, Surrey, England.

74
Table I

Unsharpness Data

I. kVcp=200, source size= 4x4 MM, SFD=32" (81.3 cm) film type - Kodak AA,

Screens-. 005" front .010" back (Pb)

Test

Film

Density Screens Absorber CERL

Resul ts

UG EPS ( %)

1 2.14 No None .005" 'vO — 2 1.93 Yes None .008" ^0 - 3 1.75 No V steel .008" .002" 3.57

4 1.56 Yes V steel .008" .002" 3.57

5 2.10 No h" steel .008" .003" 2.26

6 2.75 Yes h" steel .013" .003" 2.26

7 1.95 No 3/4" steel .010" .005" 1.51

8 2.27 Yes 3/4" steel .010" .005" 1.51

Same as above except SFD = 64"

1 2.54 No None .004" 'vO _ 2 2.42 Yes None .004" a-Q - 3 2.42 No h" steel .004" .001 2.83

4 2.41 Yes V steel .004" .001 2.83

5 2.51 No h" steel .005" .002 1.79

6 2.57 Yes h" steel .005" .002 1.67

7 2.41 No 3/4" steel .005" .003 1.25

8 2.82 Yes 3/4" steel .005" .003 1.19

Note: SFD is x-ray Source to Film Distance
4.

X-Ray Film Handling

Two documents are being developed in cooperation with and in

collaboration with the film company representatives in the ASTM subcommittee

on radiographic practice. One relates to film processing, the other to film

storage (exposed and unexposed). Our role is as coordinator of these

activites. The technical content is the responsibl ity of the film

representatives.

5. Component Evaluation

There are, at present, two efforts being conducted in cooperation

with ASTM. These are in the area of kilovoltage calibration and x-ray source

size (focal spot) determinations. We are part of the task force involved in

these efforts. While laboratory methods are readily available for this

kind of measurement, good field techniques remain to be perfected. The

consensus of opinion is that kilovoltage calibration will involve some sort of variable x-rf.ty absorption measurement (filter technique) and source size will involve a field application of the pinhole imaging

technique.

We will continue to participate in this development by providing

some of the tests needed to establish the basis of the standard and later,

if needed, calibrations of the field instruments at the NBS facility.

Both of these are high on the ASTM "want" list.

6. The Plaque Penetrameter Standard

As previously mentioned in A-l above, the plaque penetrameter is the

only image quality indicator that has been widely accepted and used by the

industrial radiographic community for many years. It has some deficiencies

and there is a "streamlining" effort now underway for this IQI . An attempt

is being made to reduce, if possible, the number of different sizes

and the number of different materials that are specified in the standard

(E-142-77). NBS is in the forefront of this effort and has participated

in the round robin tests associated with this task. This work will continue

until the final form of this simplification of the plaque penetramenter

is adopted or discarded.

B. RADIOGRAPHIC APPLICATIONS

1 . NDT Support for the Fleet Ballistic Missile System*

The objectives of this program are to (1) provide technical engineering

support to the area of nondestructive testing applications for Trident

missile components, (2) recertify, twice yearly, all facilities involved

in x-ray inspection of Trident rocket motors and (3) proposed evaluate

changes in inspection techniques.

* This work is supported by Strategic Systems Project Office, Dept, of the

Navy.

76
The recertification program of radiographic testing facilities is

carried out twice yearly by an audit that consists either of visits to

installations or submittals of test film for evaluation. This testing

procedure verifies that all facilities are following the required test

procedures and that a high quality of inspection is maintained throughout

the year.

The implementation of real time radiographic testing at all facilities

involved in the missile system necessitated extensive qualification testing of

new equipment and personnel. This work is continuing at the Navy support

bases receive their real time equipment for installation and use. Another

certified measurement that was required was the measurement of the radiation

outputs of the linear accelerators at all facilities so that radiation flux

requirements during inspection can be specified.

Proposed new missile systems being developed for the Department of

Defense have required a reyiew of existing nondestructive test systems

regarding their applicability to these larger missiles. In addition, there

have been proposed new methods of inspection such as computerized axial

tomography. These systems are being evaluated. Review and consultation

on projects involved in tomographic inspection of the Navy and Air Force new missiles will continue.

2. Digital Radiography

Digital radiography involves the scanning, digitizing, and storing in

computer memory of a radiographic image. The image may be from a conventional

radiograph or from a real time imaging system.

This activity is growing rapidly throughout the industrial community.

ASTM has established a subcommittee with responsibility to develop standards

related to real time industrial radiology. NBS is represented on this subcommittee (two participants). We foresee that image quality standards, real

time inspection methods, and (probably) image processing and storage will be

a part of this effort. The first output of this committee is a tutorial

document on real time radiography. D. Polansky contributed one chapter

for this document. Since this is a new group in ASTM and since the technology

is presently undergoing many changes and advancements, the NBS role is not yet

clearly delineated. Our present efforts are slanted toward acquiring expertise

in the use of the systems and their advantages/disadvantages relative to a film

based system.

As an adjunct to this activity, we are devoting our efforts to

acquiring an image processing system and to achieving expertise in image

processing of industrial radiographs and images from real time detectors

systems. We feel strongly that the future of industrial inspection

will be in digital radiography and we are attempting to define the NBS role

and be ready to solve some of the many measurement problems that will arise

as the technology grows.

77
3. High Energy Transmission X-Ray Diffraction

This effort

gators, headed by

REFERENCES

1. NBS Technical

Inspectors in

is being done in collaboration with other NBS investiDr. M. Kuriyama.

Note 1143. "Visual Acuity Testing of Radiograph

Nondestructive Inspection," G. Yonemura, June 1981.

78
Task # 15/60

DESIGN STUDY OF A FACILITY FOR NEUTRON -ACT I VAT I ON AUTORADIOGRAPHY

OF PAINTINGS AT THE NATIONAL BUREAU OF STANDARDS REACTOR

Y. T. Cheng and M. Ganoczy

Reactor Radiation Division

and

I. Schroder

Nuclear Radiation Division

Art conservation laboratories regularly employ techniques such as x-ray radiography and microscopic examination of pigment layer crosssections in the study of paintings. X-ray radiographs, however, show

only the distribution of dense pigments such as lead white and thus

provide only part of the analysis. Relatively recently neutron-activation

autoradiography has been used. It extends the range of information that

can be obtained by being able to measure the distribution of several

pigments other than lead and at the same time identify the elementary

composition of the painting.

A current drawback of the neutron technique is that the radiation

dose to the painting is two orders of magnitude greater at the existing

facilities than a typical x-ray examination. Although this is believed

to be well below the level that would damage the painting, it cannot be

shown that such radiation doses might not result in some deterioration

that would not appear until many years in the future. Therefore, it is

very important to reduce the radiation dose to as low a level as possible.

A proposal was submitted to the Smithsonian Institution and subsequently

a contract was awarded for a design study of a painting autoradiography

facility that would reduce the radiation dose received by a painting by an order of magnitude compared to that available at other reactors. The

possibility of building such a facility is made feasible by the unique

thermal column built into the NBS Reactor. The proposed study will

result in a detailed design of the facility, a cost estimate for its

fabrication, and more detailed measurements of the radiation fields.

• Experiments were carried out to gather information concerning

thermal neutron flux distribution, neutron to gamma ratio, field

nonuniformity, size of usable thermal neutron field, radiation shielding

requirements, etc..

Figures 40 and 41 show the plane view and side view of the proposed

NBSR neutron-activation autoradiography facility, respectively.

The proposed facility will be used for studying oil paintings up to

6' x 6' in size. Flexibility has been built in the design for future

expansion to accommodate paintings of larger sizes and modification of

the graphite thermal column to improve neutron to gamma ratio.

The neutron-activation autoradiography process utilizes neutronactivation analysis to identify elemental contents in the painting

materials and the autoradiographs serve to show the spatial distributions

79
of these elements. Thus, to make the designed facility suitable for

painting studies, a recommended procedure for activation counting and

producing autoradiographs was set up. During the course of gathering

design information, a test panel, two icon paintings, and an early

American painting by Thomas W. Dewing were studied. The study showed

the recommended procedure (Table 1) should be followed if the information

about short-lived elements (A1 , Cu, Ag, etc.) is desirable.

GO
TIME

V 0 End

T

1 5m 1st

II

CsJ

1— 11m 1st

T

3

18m 2nd

1— II 34m 3rd

T

5

lH25m 2nd

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T

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T

8

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EVENT

of neutron activation

film on for 4m

Y-spectroscopic count for 5m

film on for 15m

film on for 45m

Y-spectroscopic count for 30m

film on for 3H

Y-spectroscopic count for 3H

film on for 15H

Y-spectroscopic count for 10H

film on for 3D

Y-spectroscopic count for 3D

film on for 14D

film on for 30D

OPTIMIZATION OF AUTORADIOGRAPHIC EFFECTS

Ag 108 (2.3m)

A1 28 (2.3m)

Cr 55 (3.6m)

S 37 (5.0m)

Cu 66 (5.1m)

Ti 51 (5.8m)

Ca 49 (8.8m)

Co 60m (10.5m)

Mg 27 (9.5m)

Sb 124m (21.0m)

Cl 38 (37.3m)

Cd 111m (49.0m)

Ba 139 (85m)

Mn 56 (2.58H)

Pb 209 (3.3H)

Cd 117m (3. OH)

K 42 (12. 5H)

Cu 64 (12. 8H)

Na 24 (15. OH)

As 76 (26. 7H)

Au 198 (2.7D)

Hg 197 (2.7D)

Sb 122 (2.8D)

Ca 47 (4.7D)

P 32 (14. 2D)

Cr 51 (27. 8D)

Hg 203 (47D)

Fe 59 (45D)

S 35 (87D)

Ca 45 (1 53D)

Ag 110m (253D)

Fe 55 (2.6Y)

Co 60 (5.26Y)

Table I. Recommended Sequence for Gamma Counting and Film Exposure for

Autoradiography.

81
Fig. 40 Top View of the Proposed Painting Autoradiography Facility.

82
Fig. 41 Side View of the Proposed Painting Autoradiography Facility.
DEVELOPMENT OF HIGH OPTICAL DENSITY STANDARDS

Task # 15760

J. Hsia, C. H. Popenoe, and R. L. Booker

Radiometric Physics Division

Center for Radiation Research

The anticipated change in ISO and ANSI standards and continuous

demands for higher optical density standard materials require proper

action from NBS to meet the needs. Optical Density standards are used

to calibrate densitometers. Densitometers are employed to measure ex- posed X-ray films which are used for nondestructive evaluation.

The objective of this project is to develop high optical density

standards (up to optical density of about 5.8), through automation of

Inverse Fourth Apparatus(l ) , characterization of Inverse Fourth Apparatus

and development of high optical density standards.

AUTOMATION

A schematic of the Inverse Fourth Apparatus is shown in Fig. 42 . The flux received by a detector at a distance d from the carriage (CR)

is proportional to the inverse fourth power of the distance d. Light

flux (S) through the source aperture (SA) is irradiated onto the glass

fiber (G) on the carriage (CR). The flux is reemitted as the secondary

source through baffles (B), sample holder (H) and into the entrance

aperture (EA) of the detector system (D). A new sample holder has been

constructed utilizing a stepping motor (M) to move sample step by step

in or out of the light path. A small pressure-vacuum pump or a solenoidactivated mechanical device (SP) will be used to press the sample tightly

against the entrance aperture (EA). The current from the detector is

converted to frequency with the current- to-frequency converter (C/F) and

the frequency is counted by the counter (Cl). The interface (I) receives

signals from the counter (Cl) and the distance position meter (C2). The

interface also relates command from the microcomputer (MC) to the sample

holder motor (M), sample positioning device (SP), and carriage motor.

The experiment is controlled by the microcomputer through CRT and the

data is displayed on a CRT, printed on the printer (PR) and stored in

the disk. Some of the computer softwares are being developed. The goal

of the automation is to increase precision and ease of operation and

characterization. Some detailed results are given below for the characterization of the color temperature of the light source system plus

the glass fiber.

CHARACTERIZATION

Current ANSI specifications^, 3) require that illumination of the

sample be with light having a color temperature of 3000K. Obtaining a light source which emits 3000K light is simple enough; any number of

tungsten filament lamps are avilable that can be operated at a specified

color temperature such as 3000K. However, in the inverse fourth apparatus, radiation emitted from the source is systematically directed by

an optical system that includes a reflector, filter, lenses, diffuser,

and optical fibers. If each of these components is spectrally non- selective, then 3000K radiation from the source will pass spectrally
unmodified to the surface of the optical density step tablet to be

measured. However, manufacturer's data and measurements on similar

components indicate that each of the components departs from spectral

neutrality to some degree. One approach in determining how light from

the source is selectively attenuated would be to measure the spectral

reflectance or transmittance of each of the components and then compute

the individual and net effects on the chromaticity of the source. This

is easier said then done, since the components are non-planar and are imbedded in the instrument. However, since what we want is the net

effect, a procedure that characterizes the nature of the resultant light

from the source system is both acceptable and feasible.

We used a portable Pritchard 1980B spectroradiometer to measure the

spectral radiance of the light from the source system. A color temperature calibration curve on the spectroradiometer was obtained by

characterizing a source at 4 color temperatures in the range of interest

for the inverse fourth source. During calibration of the spectroradiometer, the light from the standard source was reflected from a diffuse,

white Halon plaque. On evaluating the inverse fourth source, this same Halon plaque was used to reflect the light to the spectroradiometer.

During these measurements, the turn-around carriage was positioned in

its full forward position to increase the light flux at the entrance

aperture of the detector port to its maximum value. The detector module

was then removed and the Halon plaque positioned to provide a suitable

target for the focusing optics of the spectroradiometer. The spectroradiometer measured the spectral radiance of the reflected light at 10 nm intervals in the region 400-700 nm and computed the color temperature of

this spectral distribution. The voltage to the lamp source was systematically varied and the process repeated until the lamp voltage that

produced a source system color temperature of 2856 K (CIE Source A) (3) was determined. A calibration voltage for 3000K was also determined in

the process.

The color temperatures measured on the inverse fourth source are actually correlated color temperatures, since, strictly speaking, color

temperature implies a Planckian or grey-body spectral distribution. The

spectral distributions of the inverse fourth source system, at a color

temperature of 3000K, and that of a 3000K blackbody source are shown in

Fig. 43 . Both curves are normalized at 560 nm, which tends to exaggerate

the small differences that appear at both ends of the curve; in fact,

the chromaticity coordinates of these two sources are identical.

Other work planned in the future for characterization of the apparatus includes: detector spectral response, distance measurement ac- curacy, zero distance determination, the uniformity of the power law

dependence of the range of the carriage position, and intercomparison

with inverse square instrument.

DEVELOPMENT

One X-ray company has produced several high optical density step

tablets for us to evaluate. The planned tasks in the future for development of the high optical density standards include: characterization
of candidate materials, calibration of optical density, pilot run with

selected users, intercomparison with other international laboratories,

final selection of standard reference material and preparation of

documentations and certificates.

REFERENCES

1. D. A. Swyt and J. G. LaRock, "Inverse-fourth apparatus for photometric calibration". Rev. Sci. Instrum. 49 (8), 1083-1089 (Aug.

1978).

2. American National Standard, "Conditions for Diffuse and Double

Diffuse Transmission Measurements (Transmission Density)", American

National Standards Institute, 1430 Broadway Ave., New York 10018.

3. International Corrmission on Illumination, "Colorimetry", Publication

CIE No. 15 (E-l.3.1) 1971.

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37
RELATIVE

SPECTRAL

DISTRIBUTION

2.0

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Fig. 43. The relative spectral distribution of the Inverse Fourth

Source system (including the optical fiber) at a color temperature of 3000K compared with the relative spectral distribution of a 3000K bl ackbody source.

38
Task # 15750

VISUAL ACUITY REQUIREMENTS FOR NONDESTRUCTIVE TESTING

G. T. Yonemura

Building Physics Division

Center for Building Technology

The purpose of this project is to improve precision and reliability

between instruments, in this case the instrument being the human eye.

Studies on reliability of detecting faults from radiographs indicate a significant difference in the performance of different inspectors within a given installation, and a larger difference in the performance

between installations. There are several general causes for this lack

of precision, but we are addressing one, the visual acuity of the inspectors.

Stimulus and observer parameters that can effect precision of fault detection

are discussed in reference 1. Microdensitometric scans of aircraft

radiographs indicated that defects varied significantly in three variables:

contrast (density differences between defect and background), blur (sharpness of the edges) and size (already considered in current tests). Visual

acuity test parameters based on the above and other considerations have

been recommended in reference 2. These specifications were sent to the

medical departments of the different armed services for comments and

approval. The specifications were approved by the medical departments of the

Army, Navy, and Air Force. Currently master visual acuity test charts are

being made radiographically. The master charts will be used to mass produce

test charts which in turn will be sent out to the field for validation tests

to reflect performance of active radiographic inspectors.

References

1. G. T. Yonemura, "Considerations and Standards for Visual Inspection

Techniques," Nondestructive Testing $tandards--A Review, ASTM STP624,

Harold Berger, ed., Amer. Soc. for Testing Materials, 220-230 (1977).

2. G. T. Yonemura, "Visual Acuity Testing of Radiographic Inspectors in

Nondestructive Inspection," NBS Tech Note 1143 (1981).

89
Task # 15750

OPTICAL MEASUREMENT OF SURFACE ROUGHNESS

E. C. Teague, T. V. Vorburger, F. E. Scire, D. E. Gilsinn, M. J. McLay

and C. Y. Trahan

Mechanical Production Metrology Division

Center for Manufacturing Engineering

There have been, over the last ten years, increasing efforts in the

U. S. and the rest of the world to use light scattering rather than the

traditional stylus instrument for nondestructive evaluation of the surface

roughness of manufactured parts. The theory of light scattering from surfaces

whose irregularity heights are comparable to and larger than the wavelength

of the light is more difficult and much less developed than that for

highly polished surfaces. Light scattering has therefore been used

primarily for comparison measurements of surface roughness. Our work has

been directed towards building the experimental capability and theoretical

expertise to investigate the behavior of light scattering measurements in

terms of their use for both comparator and absolute determinations

of surface roughness.

The basic approach we have taken is to relate light scattering measurements

of a test surface carefully to those of area of 3-D stylus measurements of

the same surface. Comparisons of characterizations i.e., moments of the surface

power spectrum, from the two measurements methods are being used to analyze

and determine the valid ranges of different theoretical models and to determine

the most efficient detector configuration for an instrument design.

The system, described in last year's report, for performing angular

distribution measurements is now assembled and the performance has been

very carefully evaluated. A photograph of the completed system is shown in

Figure44. The system consists of an array of 87 detectors interfaced

through a 100-channel scanner/digital voltmeter to a Hewlett-Packard 9845T

desktop computer. Light scattered from the test surface is picked up with fiber

optics places around a 180 degree yolk above the test surface. This

multidetector design was chosen because it allows rapid measurements of the

angular distribution of scattered light. Some of the system parameters

which have been evaluated and our findings are:

a) Fiber optics transmissivity; uniform to within a three standard

deviation of 18.5 percent.

b) Background noise level of the detector amplifiers; less than 100

microvolts. This noise level results in a useable dynamic range of

104 in input light intensity.

c) Accuracy of angular spacing of fiber optics. Angular position of

fiber optic mounts was 2n + 0.1 degrees, where n = 2 to 88.

SO
d) Optical alignment of the fiber optics. The center of the focal region

of all fiber optics mounted in the yoke was within one millimeer of

yoke center.

e) Detector linearity. Nonlinearity of a typical detector output voltage

is less than five percent over a dynamic range of 105 in input light

intensity. Tracking of the response properties of the fiber optic/

detector/amp! if ier combinations is still being measured. Initial

checks indicate that the voltage outputs of all 87 systems track to

better than 25% over three orders of magnitude change in light

intensity.

Results from one of our first comparisons of light scattering and stylus

meaurements of surface roughness are shown in Figure 45. The test surface

was lapped stainless steel. The dots are experimentally determined values

and the solid line represents the results obtained with a two parameter

nonlinear least squares fit to the experimental data. The theoretical model

used for this fit was a simpler scalar approximation. It had, as parameters,

the root-mean-square deviations of irregularity heights, R , and the spatial

autocorrelation length of the surface roughness. Better models should yield

much closer agreement between theory and experiment both in terms of the agreement between predicted and measured angular distributions and between stylus and

light scattering values for the first moment, R , of the power spectrum.

Due to unacceptable motion errors found in the x-y stage design described

in last year's report, we have designed and constructed a second x-y

stage utilizing flexure pivot bearings. This stage, shown in Figure 46,

provides for almost error free motion over an area of about 5x5 millimeters.

The new x-y flexure stage is now at the point of full mechanical operation.

Positional readout for each axis is achieved by HP plane mirror interferometers.

The scanning axis (x) uses a stepping motor/lead screw drive where the rotational

torque (due to the lead screw) is decoupled using a spring loaded "wobble”

pin. Data acquisition, control of the start and end of the x-axis scan, and

adjustment of the y- axis are all under the control of a minicomputer. The y-axis

is incremented at the end of each x-axis scan using a Burleigh "Inch-Worm". The

steps generated by this device have not proven to be reproducible at the desired

0.1 micrometer level. To overcome this difficulty the step increment is determined

with the HP interferometer and corrected as necessary to achieve the desired

accuracy.

Preliminary testing indicates that the monolithic x-y flexure had roll, pitch,

and yaw values less than 5 arc seconds. The orthogonality of the axes has been

measured to be less that +2 arc seconds which is the specified orthogonality of the

"True Square" which is used for the interferometer mirrors and for the specimen

carrier on the flexure stage.

With both the light scattering apparatus and the 3-D stylus profilometry

now in operation and with both systems' performance carefully evaluated,

our immediate plans are to continue the comparisons of stylus and light

scattering measurements to determine the capability of various models for

91
accurately predicting statistical measures of surface roughness. Surfaces

to be used in the comparisons will range from sinusoidal profile gratinglike surfaces to the random surface of an electron-discharge machined

surface. Another major objective of our work is to determine the

sensitivity of light scattering measurements to the use of different

detector configurations, e.g., for a fixed number of detectors, is a measurement performed more accurately with the detectors closely spaced

in angle around the specular direction or with the detectors more widely

spaced over the full 180 degrees?
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Task # 15750

LIGHT SCATTERING FROM GROOVES

Grady White

Fracture and Deformation Division

Center for Materials Science

Since manufacturing began, scattering of optical radiation from

surface defects has been used qualitatively as the basis for acceptance

or rejection criteria. In recent years, this process has ranged from

manual comparison between scratch and dig standards to automated comparison

of diffraction patterns between acceptable and unacceptable parts.

However, with increasing emphasis on quality control, the need for

quantitative optical NDE methods is greatly increasing. In particular,

the ability to relate specific types of defects to particular scattering

patterns is of growing interest.

To that end, work has been progressing in this lab toward measuring

infrared radiation scattered from grooves of various cross-sectional

shapes and relating the data to theoretical models. Construction of the

apparatus has been detailed in previous reports as have the results of

scattering from shallow rectangular grooves. For such scattering, a Fraunhofer approximation model provided satisfactory agreement with

experimental

)

However, more complicated structures, such as grooves with v-shaped

or trapezoidal shaped cross sections, provide an excellent test for

exact calculations developed by D. Maystre.(2) Accordingly, this past

year, detailed calculations after Maystre were made for scattering from

real v-shaped grooves and results compared with observed scattering from

the same grooves. Figure 47 is a composite of the calculated (solid

line) and the observed (solid circles) scattering patterns from one position on a groove. We found that the theoretical predictions agreed

very well with observation and the discrepencies could be attributed to

finite beam size and lack of longitudinal symmetry of the experimental

groove. As figure 48 shows, diamond stylus profilometer traces of the

groove, spaced lOOym apart, indicate considerable variation in the

groove shapes. Because the theory predicts the scattering from specific

groove profiles, these variations cause the discrepancies observed.

Currently, we are interested in comparing Maystre's theory to a Fraunhofer model in different extremes of groove size. We have

measured scattering from three grooves with trapezoidal cross sections

which vary in width, w, from w^A (x=10.53ymm) to w»A. The first case is

in the resonance domain and the latter should be in the Fraunhofer

region. Because Maystre's theory requires expensive calculations, it is of

considerable interest to know at what point it must be used rather than

the simpler Fraunhofer model. These calculations are about to start.

96
References

1. G. S. White and A. Feldman, Applied Optics, 20, No. 14 (1981), 2585.

2. D. Maystre, private communication.
00

(si;un *qJV) Ajisuejui

00

I

Fig. 47 .Compari son betv/een exoerimental ly observed scattering (solid

circles) from a v-shaped {^ 2 \) groove and theoretical calculations

(solid line) based upon the actual groove profile. Lack of symmetry in

the scattering profiles reflects corresponding lack of symmetry in the

groove.

Scattering

Angle

(Degrees)
Fig. 48 . Diamond stylus profilometer measurements of the v-shaped groove.

Measurements are taken lOOym apart. Positions I and II mark approximate

locations of experimental scatter measurements. Theoretical

calculations were made from groove profiles at 15C0 and 1600ym from the

groove end.
Task # 15750

HOLOGRAPHIC NONDESTRUCTIVE EVALUATION

Charles M. Vest

Department of Mechanical Engineering

The University of Michigan

Objective

This ongoing research project deals with the application of optical

holographic interferometry to nondestructive evaluation of materials and

components. The project has consisted of a continuing evaluative review of

the technical literature of holographic nondestructive evaluation and of

a modest experimental research program. All of this work has been carried

out with the intent of assessing potential future needs for standards in

this area.

Summary of Research Activites and Accomplishments

During the past year ; we continued experimental work to determine the

deformation of both flawed and unflawed circular aluminum disks subjected

to a uniform pressure. The aluminum disks had a variety of "flaws" in the

form of blind-drilled holes or slots machined into their back surfaces. A

special heavy-duty mount was designed which permitted these disks to be

clamped around their periphery with a high degree of uniformity. They

were then pressurized from behind with nitrogen. Double-exposure holographic interferograms of these disks were recorded with a pressure change

between the two exposures. These interferograms had fringe patterns indicative

of the normal deformation of the front surface of the disks under this

differential pressure loading.

In the first phase of this project, the flaws were in the form

of blind-drilled circular holes located at the center of the disks. Most

concepts of holographic nondestructive evaluation require only qualitative

interpretation of such interference fringe patterns, because of the presence

of a flaw in the object is manifested as an anomally in the fringe pattern.

For example, a region of poor bond in the laminate material may give rise

to a local increase in density of fringes when the material is subjected to

appropriate differential mechanical or thermal load. The local high fringe

density makes the presence of the flaw immediately apparent to the observer.

Another example is detection of cracks whose presence causes an abrupt discontinuity

in fringes or fringe curvature. This is not true, however, in the application

at hand, because the pattern is completely symmetric even in the presence

of the flaw. Therefore, a quantitative analysis of the fringe pattern is required.

In addition to pure experimental evaluation, we predicted the surface deformation

with, and without, central flaws of various sizes and depths using finite

element analysis. Good agreement between the finite element calculations

and the experimental results were found.

Another aspect of the experimental w<ji k conducted over the past year dealt

with sensivity of flaw detection in the pressurized plates. For this work

the National Bureau of Standards prepared several aluminum plates with drilled

100
slots in the back surface. These "flaws" had a variety of sizes, shapes

orientations and depths. We found, as would be expected, that the sensitivity

of the technique is highly dependent in these various factors as well as the

precise location of the flaw. Of particular importance is the orientation

of the flaw relative to the fringes in the basic pattern due to the overall

deflection of the disk. In particular, if the flaw is parallel to this

pattern it may be more difficult to detect than if it is perpendicular to this

pattern.

We explored the possibility of forming various types of Moire patterns

from combinations of interferograms in order to provide a more sensitive

and obvious indication of the presence of a flaw. We had some very

interesting successes with this approach. It has been common with

objects which are basically flat to form Moire patterns by overlaying

two transparencies of interference fringe patterns - one of a "master"

component with no flaw, and the other of the object under test. Relatively

subtle differences in fringe pattern then give rise to a Moire fringe

indicative of the flaw. This technique works reasonably well in the

laboratory, but is not very convenient for routine testing procedures

because of the difficulty in attaining identical boundary conditions and

loading in the two interferograms. We have found that excellent results

can be obtained on many flaw configurations by forming a Moire pattern

between two transparencies of a single interferogram of the flawed

component under test. A good example is shown in the accompanying

figures.

Fig. 49 is a photographof a double-exposure holographic interferogram of

a "flawed" aluminum plate. It contains two flaws, one which is oriented

parallel to the basic fringe system and one which is oriented perpendicular to

it. Although the presence of both flaws can be detected in this figure by very

careful inspection, the one which is oriented radially is much easier to

detect than one which is oriented circumferentially. Fig. 50 is a Moire

pattern in which the presence of the flaws are made much more obvious that

in Fig. 49. This Moire pattern was formed by overlaying two transparencies of

the fringe pattern shown in Fig. 49. One of these transparencies is slightly

magnified with respect to the other. In essence the Moire fringes are displays of the radial derivative of normal deformation. Note that in a sense Figs. 49 & 50 are complementary in that the flaw which is difficult to

detect in Fig. 49 is quite evident in Fig. 50, and vice versa.

Fig. 51 is yet another example of the Moire technique which clearly shows

that not only the presence of flaws, but also their orientation, shape and

size can be estimated. This is formed by overlaying transparencies of a "flawed" and an "unflawed" disk subjected to the same differential load.

The work conducted during this year is a logical extension of the work

carried out during the preceding year. In particular, the areas of investigation

were suggested by the results of our evaluative literature review. Although

this experimental research program is modest, the experience being gained in it,

coupled with the ongoing review of the field, is suggesting both new concepts

and giving us insight into how one might go about setting standards for

holographic nondestructive evaluation.

101
Fig. 49. Double-exposure interferogram of a pressurized disk with two

flaws: one oriented parallel to the circumference (in the

upper half of the disk) and one oriented radially (in the

lower half of the disk)

.

Fig. 50. Moire panern formed by overlaying the pattern of Fig. 1

with a slightly magnified transparency of the same pattern.

in?
Fig. 51

, # Moire patterns formed by overlaying transparencies of

interferograms of a flawed and an unflawed disk.

103
INFRARED METHODS OF NONDESTRUCTIVE TESTING

Task # 15760

J. Cohen

Radiometric Physics Division

Center for Radiation Research

Infrared methods of nondestructive testing, especially thermography,

are of widespread and increasing importance in industry. This is a newly established project, begun in March 1982, for the threefold purpose

of 1) developing expertise and maintaining an awareness of these methods

as they are practiced in industry; 2) evaluating the current and future

needs for standards and traceability for these methods; and 3) assessing the

Bureau's role in this area.

The work on this project thus far has been concerned with developing

the necessary expertise and has been concentrated on thermography

and thermal imaging systems, including their performance measures. Thermal

imaging systems are complex, being comprised of diverse, interacting subsystems

such as optical systems, mechanical -optical scanning systems, infrared

detector, signal processing, and display apparatus. The thermogram, or

visual display of an infrared target, depends on the thermal and spatial

characteristics of the target and its surroundings, as well as on the imaging

system per se, thus the desirability for standardized testing of performance

measures. Further, a given test condition may enhance one particular

performance measure, while degrading another performance measure, e.g.,

increasing detector field of view increases temperature sensitivity, but may

decrease spatial sensitivity. Then the values of both performance measures,

not just one, should be disclosed. The writing of a Technical Note, "Elements

of Thermography for Nondestructive Testing", has begun.

In the future, studies of other infrared methods of nondestructive

testing will be undertaken. Also, ASNT and ASTM have just recently

established committees on the title subject, and the principal investigator

intends to become involved with both groups.

104
Task # 15760

EVALUATION OF ORGANIC COATINGS PERFORMANCE ON MATALLIC SUBSTRATES

USING INFRARED THERMOGRAPHY

M. E. Knight and J. W. Martin

Building Materials Division

Center for Building Technology

A rapid, nondestructive testing procedure using infrared thermography

is being developed for detecting corroded areas at the steel /organic

coating interface and for detecting blisters under the organic coating.

The organic coating can be clear or pigmented and the substrate can be

smooth or sandblasted. Defects, visible or invisible to the human eye,

are detected provided that the thermal properties (e.g., emissivity,

thermal conductivity) of the defected area differ from that of the

background.

Experimentally, the procedure is 1) to uniformly heat the test

specimen to a temperature greater than ambient and 2) to measure the

emitted thermal energy using an infrared thermographic camera. In our

case, the test specimens were heated to 45-55°C from the backside using

a collimated light source. A temperature uniformity of greater than

0.5°C was maintained across the panel. The camera used in this study

has a 2-5.6 micron detector with a temperature resolution of 0.2°C, at

30°C. The spatial resolution is 1 mrad.

In determining the defect resolution of the camera, various types

and sizes of defects were artificially fabricated on smooth and sandblasted steel substrates. There were four types of defects that the

procedure could detect: 1) corroded areas under intact films, 2) corrosion

which had erupted through the film, 3) water-filled blisters, and 4)

air-filled blisters. Minor organic film irregularities (e.g., thickness

variations in the film) were not detectable. Panels with localized

areas of corrosion under intact films were prepared by placing drops of

dilute KHSO* solution, having various controlled diameters, on clean

steel substrates. As the solution dried, red iron oxides formed at each

doped location. The panel was then coated. Water-filled blisters were

formed using the same procedure as before except that after the coating

was cured, the panel was immersed in distilled water. Air-filled blisters

were prepared by contaminating small areas of the substrate with a mold

release. The panel was then coated. After the coating had cured, a syringe was used to introduce air under the coating. The coating thickness for all defect types was between 25 to 100 microns.

The presence of a defect is observed as varying grey levels on the

cathode ray tube output. Corrosion under intact films and water-filled

blisters appear hotter that the background; air-filled blisters and

corrosion which has erupted through the film appear cooler than the

background. With the equipment available in this study, defects having

diameters close to 0.5 mm were detectable on smooth substrates. On 50

micron sandblasted panels, defects having a diameter greater that 1 mm were detectable.

105
From a practical standpoint the procedure has already demonstrated

its utility as an evaluation tool for coating performance. Several

steps are being explored to increase its usefulness as both a quality

control device and for studying interfacial corrosion. These include 1)

computer image analysis of the thermographs, 2) improved spatial resolution,

and 3) improved temperature resolution.

106
SMALL-ANGLE NEUTRON SCATTERING FACILITY

Task # 15760

C. J. Glinka

Reactor Radiation Division

Center for Materials Science

A new facility for small -angle neutron scattering (SANS) has, within the

past year, become fully operational at the NBS Reactor. The new facility

performs measurements at smaller angles (thus allowing studies of larger

structures) and with data rates up to 50 times faster than the instrument it

replaces. This enhances the ability of researchers to study the microstructure

of materials such as metals, ceramics, glasses, polymers, biomaterials, and

magnetic materials much more effectively and in greater detail.

The NBS SANS instrument can be ysed to study structural features with

sizes ranging from about JO to 5000 A, a regime of vital importance to understanding the behavior of real engineering materials. In this size regime,

neutron scattering provides information which is complementary with that

obtained by electron microscopy or analogous x-ray methods. The SANS technique

affords the added advantages that the measurements are inherently nondestructive and may be made on relatively thick specimens or on samples in specialized

environments owing to the highly penetrating nature of neutron radiation. The

derived information is thus characteristic of the bulk material and can be

correlated with bulk measurements carried out, in many cases, on the very same samples. This new SANS facility is expected, therefore, to play an important

role in the nondestructive investigation of materials properties.

Figure 52 shows a schematic layout of the SANS facility. The instrument

utilizes a direct beam from the Reactor which is filtered by beryllium and

single-crystal bismuth, then passed through a mechanical velocity selector, to

achieve both low background and tunability of the mean wavelength from 4 to

JO A. The beam is collimated by a series of masks, having nine apertures each,

aligned along an evacuated 4.5 m flight tube preceding the sample. The effect

of the masks is to produce nine independent beams which converge to a point at

the center of the detector thereby increasing the beam area at the sample by

nine without degrading the instrumental resolution. The detector is a 65 x 65 cm position-sensitive proportional counter with a resolution of 5 mm in both

directions. The detector may be positioned at either 2 or 3.5 m from the

sample at the end of an evacuated flight tube which can be rotated about the

sample position to reach larger angles. Measurements can be made at scattering

vectors as small as 0.003 A~2 in fluxes ranging from 104 to 5 x 10 5 n/cm2 -sec,

depending on the colltmation and choice of wavelength.

The sample area at the instrument is large enough to accommodate equipment

such as furnaces, cryostats, magnets, or deformation devices, for example, so that the scattering can be monitored as the sample environment is changed. A

computer-controlled sample changer is also available for ambient measurements

which enables up to four specimens to be examined automatically.

107

A
An interactive color graphics display terminal linked to a high-speed

computer is available at the facility for the direct imaging and detailed

analysis of SANS data. Software has been developed which enables quantitative

results to be obtained on-line to allow true interactive experimental design.

A number of cooperative research projects have already been initiated with

the Metallurgy, Polymers, and Fracture and Deformation Divisions at NBS as well as with other government and industrial laboratories and universities. In

the area of nondestrictuve evaluation, R. Fields and S. Singhal of the Fracture

and Deformation Division have made measurements to detect creep-induced voids in

stainless steels (see p. 132).

108
NBS SANS SPECTROMETER

Fig. 52. Small Angle Neutron Scattering Facility.

109
I

Task # 15760

INVESTIGATION OF SiC-Al COMPOSITE MATERIAL BY NEUTRON SCATTERING

H. A. Alperin, Reactor Radiation Division

S. P. Singhal, Fracture and Deformation Division

Center for Materials Science

We have investigated the feasibility of using neutron diffraction as a tool for the nondestructive evaluation of composite materials. The material

selected. Si C fibers in an aluminum matrix, is ideal for this purpose due to

the very low aluminum neutron absorption cross section. This makes it possible

to observe (111) Bragg reflections from both the fibers and the matrix. Upon

application of increasing compressive stress to a cylindrical sample (^1 cm diameter) while it is undergoing neutron scattering measurements, it is possible to accurately measure the change in d-spacing (strain) as a function of

applied stress (a) independently for both the matrix and for the fibers.

Ad/d = -vo/E = -A(2e)/2tane

Other bulk measurements only give an average for the composite as a whole.

The required accuracy of the measurement derives from the ability to fit

a known function (Gaussian) to the neutron lineshape. With sufficient counting

statistics (^4 hrs for each peak scan) the fit yielded peak positions to an accuracy_of vtO.OOl degrees (2e) which gives a change in d-spacing accuracy of

vtl x 10~ 4 A.

The results are shown in Figure53. For comparison, an identically-shaped

rod of 6061 A1 was also measured. From the slope and assuming a Poisson ratio

v = 0.31 a Young's modulus E = 10.9 x 10 6 psi (7.5 x 10 4 MPa) is obtained;

this is very close to the value (10.5 x 10 6 psi) usually quoted for an A1

alloy. The stiffening of the A1 matrix in the composite is apparent from

Figure53 and gives the value E = 19.5 x 10 6 psi. The Si C fibers show a value

E/v > 300 x 10 6 psi.

Several interesting effects are also observed. One is a relaxation

effect observed in the fibers upon repeated measurements over a period of

8 hours at a constant value of stress. The range of variation in d-spacing

due to this effect is shown by the cross-hatched bars in Figure53. Another

effect is the large initial stiffness shown by both the matrix and the fibers.

Neither of these effects are observed in the A1 alloy sample.

110
S33U93Q (III) D!S -02

in

Fig.

53.

investigation

of

SiC-Al

Composite

Material

by

Neutron

Scattering.
Task # 15760

ULTRASONIC APPLICATIONS TO NDT AND MATERIALS EVALUATION

Gerald Blessing

Mechanical Production Metrology Division

Center for Manufacturing Engineering

1. Titanium Billet Integrity : A large titanium billet, destined to be

part of the payload for NASA's Galileo mission, was ul trasonical ly

interrogated for the extent of a surface-appearing crack. Depth and

profiling showed that the crack extended most of the billet length at

its center.

2. Bridge Weld Specimens : The Department of Transportation provided a series of bridge weld specimens for an evaluation of their integrity.

The specimens were ul trasonical ly scanned with longitudinal and

shear waves to especially study (a) fusion line quality and (b) base

metal laminations.

3. Ultrasonic Calibration System : The performance of a new ultrasonic

system consisting of a commercially available pul ser/recei ver unit

and an externally added tuning box was compared with a previous

generation system that still serves as the industry standard for

reference block calibration work. It is important that a substitute

for the older system be developed since it is no longer manufactured.

4. Surface Roughness Effects on Ultrasonic Echo Amplitude : The effect

of sample surface roughness on ultrasonic echo amplitude was studied

in a set of seven control 1 ed-roughness steel disks. The echo

amplitude was monitored as a function of ultrasonic frequency from 1 to 20 MHz, for both wet and dry back surface conditions. This is

part of a current ASTM round-robin study.

5. Sound Speed in Strontium : The Geophysical Institute of Alaska needs

to know the shock speed in strontium used as a liner in its

shaped-charge devices. The compressional and shear sound speeds were measured for an evaluation of that shock speed.

6. Elasticity of Tungsten Carbide : A need arose in the Temperature and

Pressure Measurements and Standards Division of NBS to measure the

elasticity of tungsten-carbide used as the piston material in a piston-gauge pressure sensing device being developed as a primary

standard for pressure measurement.

7. Composite Rod Wave Speed : An industrial need arose for determining

the wave propagation speed in an 80 percent glass fiber reinforced

polyester matrix composite rod used in well digging applications.

The compressional wave speed was measured for an evaluation of the

Young's mode rod speed.

112
Task # 15760

AN ULTRASONIC RESIDUAL STRESS REFERENCE SAMPLE

Gerald Blessing, Nelson Hsu and Thomas Proctor

Mechanical Production Metrology Division

Center for Manufacturing Engineering

The need for a reliable technique to evaluate a material's residual

stress nondestructively is widely recognized. While ultrasonic elasticity or velocity measurements are frequently applied to such an evaluation, meaningful

measurements are very difficult. A significant part of that difficulty is

the lack of a sample with a known stress state. To satisfy that need for a reference sample, we have taken the approach of fabricating a ring-plug

shrink-fit disk assembly. Figure 54 illustrates the design and summarizes the

stress distribution. Its advantages are that it possesses a well-defined

stress state which is theoretically tractable, is simple to fabricate, and is

amenable to other stress analyses.

In our work to date, the propagation rate of ultrasonic longitudinal and

shear waves has been studied in an aluminum alloy ring-plug disk. Relative

longitudinal wave speeds across the disk diameter have been shown to be in

quantitative agreement with calculated residual stress levels, assuming

literature values for the acousto-elastic coupling coefficient. Relative

shear wave speeds were in qualitative agreement with the theory, quantitative

comparisons having not been made for want of the appropriate acousto-elastic

constants. These results on the stressed disk have been compared to those

obtained on a like-dimensional sample of the same material, constructed

without the addition of internal stresses. This manner of determining the

reference sound speed for zero stress is straightforward, but requires careful

interpretation for the effects of material texturing and inadvertent stresses.

This work represents our initial efforts to develop an ultrasonic

residual stress reference sample. In order to provide a quantitative

i nterpretati on for our shear results, the appropriate acousto-elastic

constants need to be determined. We would like to experimental ly verify the

theory which shows that the unstressed sound velocity may be obtained by a weighted average of the sound speed over the stressed disk volume. This would

represent a significant step forward in determining a material's absolute

stress value. Plans also include fabricating and testing specimens of other

materials, expecially steel, which is of great practical interest. Finally,

more precise and repeatable measurement techniques are being studied for this

application.

113
(a) shrink fit disk Design parameters

• Material properties

Young’s modulus E

Thermal expansion Q

• Geometry

Thickness T

Plug radius Ri

Disk radius R 2

Differential radial interference A

Computed parameters

E(R

2

2 -R?)A

2R 2

At = 7rR 2

A 2 = 7T|R 2

- R? ) Stress distributions

•In plug

<j

rr +odd = -2P

arr - °ee

= 0

‘ lnring

R > 00 A ’ 00 arr +<700-R2_ R

2 2P “ a 2

2P

R? R 2 -1 , °tt~°dd~ R2_R2

' 2f> '

I f2

Figure 54. (a) The design parameters for the shrink fit disk; and the computed

parameters and stress distribution for (b) the sum of the principal

stresses sensed by logitudinal waves, and (c) the difference of the

principal stresses sensed by shear waves.

114
Task # 15770

NDE CHARACTERIZATION OF THIN STRUCTURALLY MODIFIED SURFACE LAYERS

ON BULK SUBSTRATES

M. Rosen, F. Nadeau, and R. Mehrabian

Metallurgy Division

Center for Materials Science

Laser and electron beam irradiation techniques are being applied

extensively for the modification of properties of the surfaces of metallic

structures. High intensity power transfer onto the metallic surface produces

a liquid pool that rapidily solidifies under the influence of the chilling

effect of the cold substrate. Formation of hard martensitic layers on a soft pearl itic bulk, solid solution strengthening of the peripheral layer,

grain size refinement, dispersion hardening or formation of an amorphous

layer on a crystalline substrate can be achieved by laser or electron

beam heating. Of particular interest, from both technological and scientific

points of view, are the physical and mechanical properties of the modified

surface layer, as well as its dimensions. In most instances, the thickness

of the modified layer is not accessible to nondestructive testing. However,

the determination of the thickness of a case-hardened steel shaft may be

of prime importance for engineering design. Similarly, it is of interest to

characterize the process of amorphous glazing on a crystalline substrate.

The physical properties of the amorphous layer and its depth may be

important parameters for the evaluation of the efficiency of the glazing

process and determination of the strength of the modified layer.

Microstructural modification in peripheral layers of bulk specimens

was obtained by means of rapid heating and cooling with an electron beam.

Relatively thick amorphous layers (about 1mm) were obtained on crystalline

PdCuSi . In addition, martensitic and bainitic microstructures were produced

on pearl itic steel plates. The NDE characterization is being made by

measuring the Rayleigh surface wave velocity. Therefore the metallurgical

characteristics and the thickness of the modified layer can nondestructively

be determined.

The essence of this research was to develop a technique by which Rayleigh

surface waves may be launched. The Rayleigh waves propagate within the

surface layer only penetrate the substrate at specific wave frequencies.

The Fourier analysis of the frequency content of the Rayleigh

waves allows determination of the thickness of the layer provided its

elastic moduli differ by several percent from those of the crystalline

substrate.

115
Task # 15760

NONDESTRUCTIVE EVALUATION OF COMPOSITES

H. M. Ledbetter, J. C. Moulder

Fracture and Deformation Division, Boulder

Center for Materials Science

and

S. K. Datta

Department of Mechanical Engineering

University of Colorado

Detection of low damage levels in composites is a serious, unsolved

technological problem; appropriate, reliable nondestructive methods do

not exist. Although conventional ultrasonic NDE techniques have proven

successful at detecting macroscopic damage such as delaminations, large

cracks, or bubbles, this type of damage is not likely to occur in a composite material until perilously close to failure. This situation

contrasts sharply with the usual behavior of flaws in metals, where a period of slow, steady crack growth may be expected before the integrity

of a structure is threatened. Thus, methods are required for quantitatively

characterizing microscopic damage throughout a composite, as opposed to severe localized damage. Internal friction, which is a gage of energy

dissipation within a material, should be sensitive to this type of

damage, particularly at low frequencies where the wavelength is comparable

to specimen dimensions.

Last year we reported the results of low-frequency measurements of

Young's modulus and internal friction for specimens of unaxial HMS/934

graphite/epoxy composite both before and after subjecting the composite

to various types of mechanical deformation: tensile loading, thermal

cycling to 76 K, and 10 5 cycles of tension-tension fatigue at 60 percent

of ultimate tensile strength (UTS). These measurements revealed significant

changes in both modulus and internal friction following mechanical

deformation. Furthermore, the induced elastic-property changes were found to be time dependent; the modulus and internal friction showed a tendency to relax with time following deformation. One puzzling aspect

of the results was that although tensile loading to 90 percent UTS

caused Young's modulus to decrease by 1-2 percent and internal friction

toincrease by 20-30 percent, the effect of tension- tension fatigue for

10 5 cycles at 60 percent UTS was just the opposite: Young's modulus

increased by about 1 percent, while the internal friction decreased by

about 10 percent.

To provide a more complete description of this unexplained behavior,

we report here the results of further studies of the effects of fatigue

on Young's modulus and internal friction for graphite/epoxy composites.

Two different graphite/epoxy composites were studied: HMS/934 and

GY70/934, both in a (0°, + 45°, 90°) cross-ply lay-up. The same epoxy

resin was used in both materials but the graphite fibers were different;

116
the gy70/934 formulation had a higher modulus but lower ultimate tensile

strength than the HMS/934. Specimens in the form of circular rods 78 mm

long and 3.8 mm in diameter were machined from a single block of each

material

A Marx three-component composite-oscillator technique was used to

determine Young's modulus and internal friction at a frequency of about

40 kHz; the experimental apparatus has been described in detail elsewhere. (1

)

This technique has considerably better precision and accuracy than the

vibrating-reed pendulum technique used in the previous measurements.

Young's modulus and internal friction were determined for each

specimen before and after a schedule of tension-tension fatigue, usually

consisting of 105 cycles at maximum stresses ranging from 40 to 85

percent of the ultimate tensile strength. Table 1 and Fig. 55 show the

results expressed as the dimensionless ratio of the final value following

damage to the Initial value for each specimen.

The results show that the net effect of fatigue on the GY70/934

composite was to increase Young's modulus by about 2 percent and to

decrease the internal friction by about 20 percent. This behavior,

although unexpected, is consistent with that previously observed in

unixial HMS/934. Contrasting with this behavior, the present results

for cross-ply HMS/934 show a decrease in Young's modulus and an increase

in the internal friction after fatigue. However, the changes induced in

the elastic properties do not increase monotonically with applied stress

as might be expected a priori. Instead, the changes in elastic propreties

caused by high-stress fatigue are less than the changes caused by fatigue

at intermediate stress levels. Another observation arises by comparing

the results for the two HMS/934 specimens fatigued at the same stress

level (58.1 percent UTS) for different numbers of cycles. For these two

specimens, the one subjected to only 5 x 104 cycles of fatigue showed a larger increase in internal friction that the specimen subjected to 10 5 cycles of fatigue, although the modulus of the latter specimen showed a slightly larger decrease after fatigue than did the modulus of the

former specimen.

Results of these experiments suggest two competing mechanisms: one mechanism, dominant during low-stress fatigue and static tensile loading,

acts to reduce the elastic modulus and increase the internal friction.

The second mechanism acts during fatigue at higher stress levels and

reduces the changes in elastic properties assumed to occur during earlier

cycles.

This hypothesis requires further experiments for its confirmation.

In particular, one shouldumonitor both modulus and internal friction of

these materials semicontiniously during fatigue to see if internal friction

first increases and then decreases, as the postulated damage mechanisms

suggest.

Theoretical studies

Collaborating with Prof. S. K. Datta at the University of Colorado,

a general theoretical approach has been used to model composite behavior

117
on the basis of constituent properties. Several properties can be considered

from the viewpoint of plane-wave propagation in composites with either arrayed

or randomly distributed inclusions (fibers or particles). Assuming long wavelengths (longer than the particle diameter), one can derive relationships for

elastic wave speeds and attenuations for both logitudinal and shear waves. These relationships contain two further assumptions: particle distribution

is homogeneous and uncorrelated, and Lax's "quasicrystall ine approximation"

holds. This approximation is to assume that

<U

j > 1j

= <U

j "j

where <u.>.. is the ensemble average of the displacement field at the jthscatter position, keeping the ith and jth scatters fixed in position. We

have already applied these methods successfully to compute the elastic

constants of fiber-reinforced boron-aluminum composites. (2) Recently,

we extended the method to the anisotropic-fiber case and applied -It to

graphite/epoxy composites. (3)

REFERENCES

1. H. M. Ledbetter, Cryogenics 20^, 1980, 637-640.

2. S. K. Datta, H. M. Ledbetter, and V. K. Kinra, in: Composite Materials:

Mechanics, Mechanical Properties and Fabrication (Japan Son. Compos. Mater.,

Toyko, 1981), 30-38.

3. S. K. Datta, H. M. Ledbetter, and R. D. Kriz, Proceedings Fourth

International Conference on Composite Materials, ICCM-4 (Tokyo, October 1982).

11G
Table 1. Effects of cyclic fatigue on Young's modulus, E, and internal

friction, Q , for (0, ±45, 90) graphite/epoxy composites.

Material

Maximum

fatigue stress

(

of ultimate)

Number

of

cycles

E/E

o tVq-J

HMS/934 40.7 10

5 1.001 1.014

(0, ±45, 90) n 58.1 5xl0

4 0.919 1.118

58.1* 10

5 0.914 1.065

62.5 10

5 0.994 1.065

85.

+ 10

5 1.000 1.028

GY70/934 62.0

5 10

3 1.022 0.803

(0, ±45, 90) c 67.0 10

5 1.022 0.773

^Fatiguing performed at 76 K. Subsequently failed after 100,790 cycles.

119
smnaow s.onhoa baiivibu

NOIlOldd ~IVNU3i.NI 3AI1V13U

FATIGUE

STRESS

(%

of

ultimate)

Fig

.

55.

Effects

of

cyclic

fatigue

on

relative

internal

friction,

fT’/Q""

1

,

and

relat

ive

Young's

i-iodu 1

us

,

E/Eg,

for

HflS/934

(0,

*45,

90)

graphi

te/epoxy

composite.
Task # 15770

GREEN STATE NDE

T. Negas

Center for Materials Science

Materials Chemistry Division

Mechanical failure of technical ceramics generally results from

defects introduced into the finished microstructure during any stage of

processing (powder-green compact-sintering). Typical flaws that are generated include voids, surface cracks, pressure (density) gradients,

and inclusions, such as large grains or impurity phases. To insure

structural /chemical reliability, therefore, it is imperative that these

flaws be detected during processing using NDE methods.

Although NDE methods (e.g., radiography, acoustic measurements) can

be utilized to examine finished ceramics for large (» 10 ym) and submicron defects, techniques to evaluate green compacts prior to irrevocable densifi cation have not been developed.

This study has focussed on the feasibility of preparing green

compacts containing pores (> 10 ym) and pore cluster defects. These

constitute a major failure mode in densifi ed ceramics where stress is

concentrated and can interact with adjacent defects. Normally, these

voids result from non-uniform packing of particulates during formation

of the green compact. Potential NDE methods to detect these are being

evaluated in Division 562. In addition, microcracking in one class of

densified ceramics is under investigation in cooperation with Division

562.

Cylindrical green compacts (2.5 cm 0D X 5 cm length, no binders) of

alumina and YCr0 3 have been fabricated using reactive powders < 5 ym.

These were compacted by a local commercial firm by isostatic pressing

(^ 10,000 psi) to avoid large density gradients. Alumina densities to

better than 98 percent of theoretical density (1500 °C) while YCr0 3 requires 1750 °C in a reducing atmosphere (N 2 , 10 percent H 2 ) to obtain

> 95 percent. Attempts to introduce pore clusters in the alumina compacts are in progress. Starch, napthalene, and latex sphere additives

to this powder prior to consolidation are under investigation. All

require a mild "burnout" to develop the necessary voids. The YCr0 3 compacts were pressed between 5-10,000 psi to obtain variable green

densities and, hence, final densities ranging from 83 to > 95 percent of

the theoretical density after sintering. These latter materials con- stitute a series of specimens being examined by elasticity and small

angle neutron scattering measurements, in cooperation with Dr. Eldon Case,

Division 562 (see below §nd p. 134).

Although Division 562 is evaluating potential NDE methods for

ceramic green compacts. Division 561 has also worked closely through

subcontract with Prof. A. G. Evans, Univ. of California, Berkeley, who

suggests (formal report submitted to NBS) the use of low amplitude

121
acoustic waves, x-rays, and neutrons for analysis of green compacts.

Presently, none are used for ceramic processing. To avoid disruption of

the green compact, low frequency, acoustic waves are necessary for

velocity, attentuation , and scattering measurements which could provide

information on powder and void size distributions. X-ray and neutron

methods also should provide similar information from differential absorption due to void spaces and inclusions.

The small angle neutron scattering (SANS) technique is now under

development to detect submicron microfissures in densified YOO3 ceramics, although it is anticipated that application to green compacts is

possible. In this method, a collimated beam of thermal neutrons of

given wavelength is directed onto a specimen. The scattered and transmitted neutrons then impinge on a large array of counters. For scattering centers such as microfissures, the intensity of the scattered neutron beam is proportional to the number density (N) of the cracks, while

a crack size can be derived from analysis of the angular spread of the

beam.* Densified YCr0 3 ceramics have been measured with SANS, and

microfissures in the 175-200 A range have been detected. These data were successfully correlated with conventional elasticity measurements

which relate N and microcrack size to elastic moduli via appropriate

theory. An important feature of the YCr03 materials is that microcracks

can be healed and re-initiated reversibly by appropriate thermal annealing without altering density or grain size. Thus, the same near-perfect

and defect material can be investigated.

The SANS technique was chosen over x-rays, since a larger sample

volume can be sampled. However, for the case of large pores (> 10 yin)

in green compacts, our analysis indicates that most of the information

will be "buried" at very small scattering angles near the neutron beam

stop. Nevertheless, the SANS method offers promise for application to

the processing of ceramic green compacts. With submicron sinterable

powders, agglomeration (25-1 000 A) of particulates is a significant

problem during compaction and sintering. Intra-agglomeration porosity

tends to be very fine, while interagglomerate porosity can be in the

« 10 ym range. Differential densification of partly agglomerated

compacts normally results in detrimental voids or internal surfaces

which cannot be eliminated during the sintering process. The SANS

technique could be developed and correlated with surface area and

porosimetry measurements to detect states of agglomeration, including

abnormal voids (< 5-10 ym) within green compacts.

*Manuscript in preparation, "SANS and Elasticity Measurements on Microfissured YCrO^ Ceramics", E. Case, T. Negas, and L. P. Domingues.

122
Task # 15770

CRYSTALLIZATION KINETICS STUDY AMORPHOUS Zr-Cu BY ULTRASONIC AND

MICROHARDNESS MEASUREMENTS

M. Rosen, F. Nadeau and R. Mehrabian

Metallurgy Division

Center for Materials Science

Metallic glass formation and subsequent crystallization upon heating

has recently become an active research area. Rapidly solidified amorphous

alloys exhibit a variety of desirable physical, chemical and mechanical

properties that are particularly attractive for products in electronics

and power industries. Moreover, functional and economic advantages may be

gained when crystallizing specific alloys into microcrystall ine structures

that can be subsequently consolidated into bulk products. Microstructural

characterization using nondestructive techniques, e.g,, ultrasonics may

thus be attractive both for its contribution to understanding the kinetics and

mechanism of the transformation and for its ultimate application for on- line feedback control of materials processing variables.

Metallic glasses are not thermodynamically stable, and they tend to

structurally relax and finally crystallize upon appropriate heat treatment.

The activation energy for relaxation continuously increases with time, which

may correspond to the removal of quenched-in defects. Associated with

relaxation and crystallization are variations in the elastic and mechanical

properties. Young's modulus E and shear stiffness y generally increase by

20-40%, but the bulk modulus K by only about 7% upon crystallization. The

density of gTassy metals is generally only 0.31-1.5% less than the corresponding

crystalline structures. As-quenched glasses contract approximately 0.5

volume percent during structural relaxation preceding crystallization.

Straightforward elasticity and wave propagation theories enable one to

calculate Young's modulus E. For one-dimensional extensional wave propagation

in a homogeneous, isotropic, linearly elastic solid. Young's modulus is

given by E = v

E /p where v

£

is the extensional wave velocity and p is density

The Cu-Zr metallic glass ribbons were prepared using the melt-spinning

technique. A 2 mm wide, 30 ym thick ribbon was produced. Ultrasonic, x- ray and STEM (Scanning Transmission Electron Microscopy) examinations indicated

that the ribbon was completely amorphous. The ribbons were heat treated

in evacuated capsules at temperatures above the glass transition point, Tg,

for different time intervals in order to obtain information about the

crystallization kinetics and its mechanism.

The objective of the present investigation was to study the crystallization

kinetics of Zr-Cu ribbons (about 50 ym in thickness) by means of lasergenerated and piezoelectrically detected, ultrasonic waves whereby the

extensional wave velocity, and consequently the Young's moduli could be

123
determined as a function of crystallization time. The amorphous-tocrystalline kinetics could thus be determined to a high degree of

accuracy.

The inherent difficulty in measuring sound-wave velocities in thinribbons was overcome by developing a technique applying the load on the

specimen by means of rapid deposition of energy from a single-pulse of

a Q-switched Nd:YAG laser. A state of unbalanced compressive thermal

stress is produced. The stress gradient propagates along the ribbon in

the form of an elastic wave. The piezoelectric crystal, at a distance

from the spot where the last radiation was deposited, detects the local

displacement. Thus, the transit time of the propagating wave could be

determined accurately by means of a transient-pulse recorder. In this

work, ultrasonic extensional waves were detected using the National

Bureau of Standards conical PZT transducer having a flat response between

DC and 2 MHz. Signal amplification was provided by an EMI 6306 chargesensitive amplifier powered by an EMI Power Supply Type 6396. A

silicon photodiode triggered a Nicolet Explorer III digital oscilloscope

so that the elastic wave could be recorded and transit times read directly

from the screen of the oscilloscope. The transit times were determined

to better than 1 part in 104

, and the source-receiver distance to within

+0.3%. All velocity measurements were conducted at room temperature.

Small sections of each heat treated specimen were mounted in epoxy

for Knoop microhardness number (KHN) measurements. To minimize anvil

effects and to ensure that KHN values were taken of the bulk material,

each specimen was cut perpendicular to its length vector, and then was mounted with its cut face exposed. After polishing the mounted ribbons

with 0.05 alumina polishing powder KHN values were determined using a lOg

load and averaging four symmetrical indentations.

The structural relaxation and crystallization effects induced by

isothermal heat treatment on the room temperature ultrasonic extensional wave velocity, Young's modulus, and Knoop microhardness of Cu-Zr ribbons

are summarized below.

Velocity versus log-time curves exhibited isokinetic sigmoidal

behavior. Extensional wave velocity increased approximately 2.0% and

Young's modulus by 4.0% due to structural relaxation. Young's modulus

increased 43% upon transformation from the amorphous to the fully

crystalline state. Knoop microhardness increased to a relative maximum,

equal to 44-88% of the asymptotic KHN value, during structural relaxation. KHN then decreased to a minimum, due to stress relief annealing.

The final _microhardness rise to an asymptotic KHN value of approximately

720 kg mm“ 2 was due to crystallization of a hard, brittle equilibrium

Cu-Zr phase.

Using data derived from the velocity versus log-time curves, the

124
activation energy for 50 % crystallization was found to be 360kJ mol"1 and the n parameter from the Johnson-Mehlr-Avrami equation was approximately

unity. This value is consistent with one-dimensional diffusion-controlled

growth.

An as-spun ribbon contains many quenched-in defects so that the

glass may be viewed as having a continuous distribution of varying

coordination or excess free volume. Reduced coordination in given atom

complexes permits the material to undergo shear transformation without

undue constraint and results in a ductile metall ic glass. Heat treatment

removes quenched-in excess free volume through structural relaxation.

The amorphous material has a rigid close-packed atomic distribution and

thus approaches the ideal glassy state. Consequently, the glass should

harden and become less ductile in the relaxation regime. This behavior was indeed observed in the present study. The main contribution to hardness

was attained during the relaxation of the amorphous phase.
NON-DESTRUCTIVE CHARACTERIZATION OF POLYMER FILMS VIA ULTRASONICS

D. L. Hunston

Polymer Science and Standards Division

Center for Materials Science

Task # 15770

In applications such as adhesives, fiber-reinforced composites, paints,

coatings, etc. the polymer must be fluid in the early stages of fabrication

so that the proper flow, leveling, wetting and spreading at interfaces, etc. can be achieved. The polymer must then harden by cooling, solvent loss, or

chemical reactions to form a rigid material. In most cases the successful

fabrication of a final product requires that associated changes in mechanical

properties take place at the appropriate times and rates. It is desirable

therefore to have nondestructive techniques that can monitor these changes as they occur. Unfortunately, this is generally very difficult, particularly

since many of the applications involve thin films, whose mechanical properties

are difficult to measure and can not usually be predicted from experiments on bulk samples.

To address this problem, an effort is being made to develop new methods

for nondestructi vely characterizing thin films. The work to date has focused

on the investigation of an ultrasonic shear wave propagation technique. A

shear deformation was chosen because the shear properties of liquids and solids

are very different. Ultrasonic frequencies were selected because in this range

the attenuation of shear waves in many polymers is so high that a thin film can

be treated as if it were infinitely thick and this greatly simplifies the

analysis. The devise consists of a thin strip of aluminum or quartz in which

a shear wave is propagated. When the surface is coated with the polymer to be

tested, the wave in the strip generates a shear wave in the coating. Although

the high attenuation in the polymer makes it impossible to observe this wave directly, its generation causes changes in the wave in the strip, i.e., an increase in attenuation and a decrease in velocity measured along the strip.

By examining the wave in the strip before and after the coating is applied, the

shear mechanical properties of the coating can be determined. Although a quantitative evaluation requires the measurement of changes in both velocity

and attenuation, it is sometimes useful simply to follow changes by monitoring

attenuation.

During the last year the apparatus necessary to implement this technique

has been set up and tested by examining several model polymer systems. The

measurements are made using the pulse-echo technique, and to assist in the data

acquisition and analysis, the apparatus has been interfaced to a minicomputer.

The first samples tested were based on an epoxy-type polymer that cures by free

radical initiation. Figure 56 shows a plot of attenuation vs. curing time for

two formulations containing the same epoxy but with different concentrations of

accelerator. The experiments clearly show that there is an initiation period

prior to the onset of cure. The rate of cure influences the width of the

transition while the ultimate attenuation level depends on the structure of

the cured film among other factors. When the accelerator concentration is

changed both the reaction rate and the induction period are altered but the

ultimate attenuation level remains the same. This type of information can be

crucial in designing epoxy-based adhesives and composites.

126
The second model system studied this year involved natural -product

based drying oils that are used in some printing inks (i). Thin films of

these liquids harden by polymerization when exposed to oxygen. The rate

of hardening must be correct if proper performance is to be achieved.

Figure 57 shows the results of a very simple test in which a thin film of the

sample was placed on the quartz strip and the attenuation was monitored as

a function of time. The initial increase in attenuation is rapid but

eventually the attenuation approaches a limit when the film becomes fully

cured. The curve in Figure 57 represents the best fit with a simple first

order rate equation. Data of this type can be very helpful in optimizing

the design of polymer systems.

The results to date indicate that this technique can be used to

qualitatively monitor changes that occur during the 1 iquid-to-sol id

transition in important polymer systems. During the next year an effort

will be made to quantify this test. As part of this effort the measurement

capability will be expanded to permit the determination of velocity change

as well as attenuation. In addition the program will investigate the

possibilities for coupling this technique with other methods such as dielectric tests so that both can be used simultaneously. The information

provided by this coupling would make both measurement techniques more useful.

127
REFERENCES

(l) D. L. Hunston, J. L. Rushford, W. R. Newitt, and B. A. Vaudreuil,

"Rheology of Cure for Intaglio Printing Inks", Organic Coatings

and Plastics Preprints, in press.

128
s

LD CO 1 S) SI SI

<S SI s>

( w o /d © u) N0IlVnN311V

129

Figure

56.

Cure

data

for

the

epoxy

/accelerator

system:

dashed

line

for

uncoated

strip,

X

for

0.25%

accelerator,

and Q

for

0.35%

accelerator.
ts S K)

( W O /d Q U

) NOIlVnNBllV

C3

CO

LD

OJ

s

o

o

CO

S

X

LlI 21H

H130

Figure

57.

Attenuation

vs.

time

plot

for

the

curing

ofa

drying

oil

based

polymer

system.

The

curve

represents

the

best

fit

with

a

first

order

rate

equation.
Task # 15770

NONDESTRUCTIVE EVALUATION OF DISTRIBUTED DAMAGE

E. R. Fuller, Jr., R. J. Fields and E. D. Case

Fracture and Deformation Division

and

S. Singhal and C. J. Glinka

Reactor Radiation Division

Center for Materials Science

Introduction

In a number of instances failure of a structural component results

from an accumulation of damage distributed homogeneously throughout the

component as opposed to the formation and growth of a macrocrack. An example

of such a situation is the nucleation and growth of creep cavities in the

high-temperature components of energy conversion systems. Another example

might be the accumulative damage that occurs in a composite material which

is subjected to a fluctuating service stress. An important design consideration

in the initial materials selection for such applications is an assessment

of the material’s failure life, either creep rupture or fatigue failure in

the examples mentioned above. Of equal importance, however, in assuring

remaining lifetime is a nondestructive assessment of the accumulative damage

that has occurred to a given time (remaining) in the service life. If the

state of progressive degradation in service can be periodically monitored,

the rate of damage accumulation can be determined and actual component lifetime

more accurately predicted. Failure due to distributed damage is not necessarily

restricted to mechanical failure. In material processing, for example, it is

occasionally important to assess the current microstructural status of a component (e.g. the extent of precipitation, microfissuring , sintering, etc.)

before further processing is warranted.

The objective of this research program is to address these issues by an evaluation of various nondestructive inspection techniques that have

potential for monitoring the extent of distributed damage in these materials.

Nondestructive techniques are, in general, more desirable than destructive

ones because they can be applied to actual operating components without having

to replace the component after inspection. However, nondestructive techniques

rarely provide direct measurements concerning the microstructural defect

associated with the distributed damage. We intend to compare the results of

several common nondestructive inspection techniques with direct observational

measurements of the size and abundance of defects, thereby increasing the

scientific base for these NDE standards. During the first year, the research

effort has focused on the detection and characterization of uniformly

distributed microstructural damage through the use of a new small -angle

neutron scattering (SANS) facility at NBS. Two model systems have been

131
developed and utilized for these studies: (1) creep cavitation damage in 304

stainless steel; and (2) uniformly distributed microcracks in a polycrystalline ceramic, yttrium chromite. These studies are described

separately below. The small-angle neutron scattering facility is described

in a separate report by C. J. Glinka. (See p. 107.)

Cavitation Damage in 304 Stainless Steel

304 stainless steel used in high-temperature applications consists of

metal carbides in an austenite matrix. The carbide-austenite interface at a grain boundary becomes a preferential site for cavity nucleation, which at

elevated temperatures can be readily activated with low applied stresses by

creep deformation and/or grain boundary sliding. The resulting cavities then

grow, link and ultimately cause failure. The early stages of this process are extremely slow and the creep cavities, due to their small size, are not

easily detectable or quantifiable. It is these early stages of nucleation

and cavity growth that are examined in this research through the use of

small -angle neutron scattering (SANS).

Since carbide precipitates, as well as creep cavities, give rise to

neutron scattering, two requirements must be satisfied for a quantitative

evaluation of cavitation damage:

(i) The volume fraction of carbide precipitates should not change

significantly during creep; and

(_ii) A reference sample with equivalent carbide structure should be

measured and subtracted from the scattering spectrum of the crept

sample.

The first condition was achieved in the present study by a "stabilizing"

heat treatment of approximately 40 hours at 775°C prior to initiation of

the creep experiments at temperatures of 600°C and below. This allowed a complete precipitation of carbide, thereby stabilizing the volume fraction

of carbides. The second condition was satisfied by a reference specimen

which was only heated (and not stressed) along with the crept specimen.

Although a number of creep specimens were examined in this study, only two

fulfilled this second requirement for a reference sample. These specimens were both crept at 600°C: one (Sample C-21) at a stress of 145 MPa (21 ksi)

for 745 hours and the other (Sample C-18) at a stress of 124 MPa (18 ksi)

for 789 hours. The unstressed reference samples for these specimens are designated R- 21 and R-18, respectively. The neutron wavelength for these

studies was 5.1 A giving a range of scattering vectors, Q, from 0.014

to 0.16 A"J . With this range of Q, contributions from cavities and carbides

in the size range of 40 to 450 A were detected.

The detailed theory and procedure for analyzing the SANS data will be

prescribed in a forthcoming publication. A few salient features, however, are described here. If scattering intensity is denoted by I, the differential

132
scattering cross-section Is given by

dS/dn = cl/t M T

where t Is the sample thickness, M Is the total monitor count (or scattering

time), T Is the sample transmission, and c Is a combination of experimental

constants related to the SANS facility. Creep damage Is related to the

difference In differential scattering cross-sections between the crept sample

(C) and the reference sample (R):

[dS

c

/dfl - dS

R

/dn] = c [I

c /t

cM

cT

c -I^t^T^.

Accordingly, all Intensities In the following discussion have been normalized

by sample thickness, transmission and scattering time.

The scattering Intensity for cavities in sample C-21 (l.e., I (C-21)-

I (R-21)). Is shown In Figure50a, and that for sample C-18 in F1gure58b. A

visual comparison of these figures (noting the difference in intensity scale)

indicates that for most scattering vectors Q, the scattering from cavities

Is larger In sample C-21 than In sample C-18, thus indicating the presence of

a higher yolume fraction of cavities in sample C-21. A Guinler analysis can be

performed on these spectra to obtain an average creep cavity size. Gulnier

showed that for randomly distributed, monosized, spherical heterogenleties

scattering curve near Q ^ 0 can approximated by the relation

I = I Q exp (-rV/5)

where I is the intensity intercept at Q

= 0 and R is the cavity radius.

This expression is valid only when the volume fraction of cavities is small

enough to avoid so called "inter-cavity interference".

Guinier plots of the data in Figure 58 qive a straight line behavior

only in the range for Q of 0.01 to 0.025 A' 1 indicating that the cavities formed

during creep cavitation have a range of sizes rather that being one size. To

obtain a cavity size distribution will require further analysis, but an average cavity size has been obtained by a least-squares fit to this low

Q region. The results are

R = 173 + 10 A for sample C-21

and

R = 133 + 7 A for sample C-18

Thus, sample C-21 not only has a higher volume fraction of cavities but also

a larger average cavity size.

133
Common to various theories of creep cavitation discussed in the

literature is the assumption of a two stage process involving nucleation

and growth. In general, the stress level determines both the nucleation

and the growth rate; whereas time under load determines the total volume

fraction of cavities for that rate. For the present studies, an increase

in stress by 21 MPa (3,000 psi) results in larger average cavity size and in

greater volume fraction of cavities. Conclusion regarding nucleation,

however, require further analyses of size distribution.

Although these analyses are continuing, numerous questions and

theoretical obstacles concerning these evaluations have arisen from

physical considerations. Chief among these is the fact that cavities

always occur on carbides. Accordingly, it may be incorrect to assume

that scattering from these two scattering species is independent and

uncorrelated. Secondly, stress induced redistribution of precipitates

is possible and will contribute to the apparent creep damage as deduced

by SANS. This scattering component will not be correctly evaluated if

the scattering cross-section is interpreted as totally due to cavities

These complications have been neglected in the past year's effort

because at present we have no completely satisfactory way to treat them

theoretically or to avoid their influence in 304 stainless steel. However,

the analyses, which have been performed, demonstrate that SANS is sensitive

to the creep damage in an engineering alloy before this damage is detectable

by other techniques, even though an exact Interpretation of the data is

clouded for a complicated system like 304 stainless steel. Accordingly, it

is our intention to better quantify the basic technique by next investigating

a simple single phase system for which scattering variations will be solely

due to the presence of cavities. Such a material might be copper

which is known to be prone to intergranular cavitation. We may then

proceed to the case of a two phase material in which no redistribution

of the dispersed phase is possible. Finally, we will return to the

complicated case of 304 stainless steel having, perhaps, dealt with some of the

theoretical and experimental impediments to a complete analysis of the SANS

spectrum.

Microcrack Damage in Polycrystall ine Ceramics

Microcrack damage in polycrystalline ceramics can degrade the

performance of these materials in terms of both their mechanical integrity

and their chemical stability (through enhanced permeation of environmental

species). Quantitative characterization of this damage is generally done

with surface sensitive techniques. Research in this subtask, however, has

established for the first time the powerful research tool that small -angle

neutron scattering (SANS) provides for the characterization of microcracks and

crack-like voids in polycrystalline ceramics. SANS has two principal

advantages over surface sensitive techniques for this characterization:

(1) surface preparation almost invariably damages the specimen surface,

134
thereby complicating the subsequent analysis; and (2) surface stresses

are not necessarily the same as those in the bulk, so that surface microcrack

sizes and distributions may not be representative. Research has progressed

in the following three areas:

(,U Development of a "Model" Microcracking Ceramic . In order to study

the nature of microcracks with an NDE probe, one must be able to distinguish

the response due to microcracks from that due to other microstructural

features, such as pores or second phase particles. An ideal way to separate

these effects is to have a material for which the microcrack number density

can be varied independently from other microstructural features. YCrO^ is

such a material. It undergoes an apparent phase transformation at ^1 1 00°C so that it can be readily microcracked by annealing at temperatures slightly

above 1 1 00°C ; conversely, these microcracks can be diffusively healed by annealing

slightly below 1100°C. Since the yttrium chromite specimens are initially

sintered around 1750°C, the microstructure (porosity, grain size, etc.)

changes little during these anneals thereby making YCr0

3 an ideal "model"

material for studying microcracking effects in ceramics.

(2) Determination of the Nature of Neutron Scattering . Since relatively

little SANS research has been conducted on polycrystall ine ceramics, it is

important to establish the extent of refractive neutron scattering, before

interpreting scattering results as diffractive in nature, otherwise erroneous

conclusion would result. This distinction can be determined experimentally by

varying either the incident wavelength or the specimen thickness. In both

cases, the scattered intensity versus scattering vector should not change

for diffractive neutron scattering. For the microcracked YCrO^ specimens

considered in this study the neutron scattering was demonstrated to be

predominantly diffractive in nature by varying the neutron wavelength

between 6.25 A and 8.0 A and by comparing the scattering spectrum from specimens

with a factor of five difference in thickness (See Fig. 55 ).

(3) Analysis of Scattering Data . FigureGO shows the neutron scattering

behavior of a YCr0

3 specimen both in its microcracked state (YT1M) and in its

healed state (YT1AT). The difference spectrum is plotted in Figure 61 . Since the spectrum from the healed state subtracts out the background

of microstructural scattering, this difference spectrum represents only the

scattering from microcracks. The solid line in Figure 61 is a least-squares fit

to a model for scattering from a randomly oriented array of thin disks,

I(Q) = (A/Q

Z

) exp (-hV/3),

where I is the scattered intensity for a given scattering vector; A is a constant; and H is the thickness of the disk. This model is an asymptotic

expression for small Q values. The crack-mouth opening, H, as determined from

this fit is 170 A, which is physically reasonable from several independent

estimates of the crack opening in such ceramic systems. Further analyses for

large scattering vectors indicate that the scattering is from sharp-edged

objects, which is consistent with scattering from micro-pores and microcracks.

Additional studies with this and other specimens with various thermal

annealing treatments further support these results.

135
In conclusion, it appears that microcrack damage in YCrOo can be

detected using small angle neutron scattering. Although much more work needs to be done in characterizing the microcrack size, shape

and distribution, the microcrack mouth-opening displacement estimated

from SANS data is in general agreement with other independent measures of

crack opening. Finally, although not discussed here, a rapid method for

assessing total microcrack damage might be through the use of neutron

transmission measurements.

136
[I(C-I8)-I(R-I8)J

/I0 3

[l(C-2l)-I(R-2l)]/lO

Fig. 58Smal 1-angle neutron scattering intensity versus scattering vector Q

for creep cavities (a) in sample C-21 and (b) in sample C-18.

(Note the different intensity scales.)

137
OYTIM

I ‘A)jsueiu| 06bj0av u~|

I or

I JU

Scattering

Vector,

Q

(I/A)

Fig.

59

Small

-angle

neutron

scattering

intensity

versus

scattering

vector,

Q,

for

two

microcracked

polycrystalline

YCr0

3

specimens

of

different

thicknesses.

Specimen

YT1M

is

approximately

five

times

thicker

than

specimen

Y1

Similarity

between

the

two

curves

implies

that

scattering

is

predominantly

diffractive

in

nature.
IV

I1A O

CO

in

CM

_ CM

w

in

in

co

x I ‘A)ISU9)U| e6ejeAv ui

0)

O)

0)

"5o

</>

TD

O)O

03

S-oo

s-o

CO +Jo

-Qe

<uE

•r—o

<uQCOCO o>-

O

10

M i-o

*-cr

S-o

+-> u0J >

QJ>

ITS s

on

S- rd) <c

+-> r—

+-> H03 >-

co

<u

CO=> 03 CO +J

i- CO

<u> -O

<u >>r-

-M 03

•r~ O) CO _e e

<1) CO

+-> -M E *r* r" E

CD T- E

•r- -O

S- E

ai os +J

+-> ^oj s: O rC0 |— O

&- ai

+-> +-> E 03

a> +-> 2T CO oCO

CD
AVERAGE

INTENSITY,

SCATTERING VECTOR, Q (I/A)

Fig.61 .The difference spectrum for the scattering curves plotted in Figure ; . The

solid line is a fit to a scattering model from a randomly oriented array of

thin disks.

140
Task # 15770

LEAK RATE REFERENCE STANDARDS

Stanley Ruthberg

Semiconductor Materials and Processes Division

Center for Electronics and Electrical Engineering

Leak rate reference standards are necessary for providing small

precise flow rates of various gases for the calibration of measurement

apparatus as well as for control purposes. The applications of leak

measurements are steadily increasing into many segments of Industry with

the growth of greater product complexity and the need for greater

reliability. Commercially available reference leaks are generally of two

forms, capillary and permeation. Drawn capillary leaks are affected by

particulates and moisture in the environment, while permeation leaks are normally restricted to helium gas and are temperature sensitive.

Supplied units range in value from ^10" 3 Pa.m3 /s to 10" 10 Pa.m3 /s (1 Pa.m3 /s = 9.869 atm-cm3 /s) with stated accuracies of 10-20 percent, although interlaboratory comparison has indicated up to an order of magnitude deviation

(1). The object of this project is to improve the uniformity and

agreement in leak rate measurements through more accurate reference

standards.

Extensive consideration has been given to possible structures that

could attain the flow precision required. Although porous septums are con- venient and have been used, they require repeated vacuum bake out and

exposure to gas flow conditions for periods of up to one year to reduce

fluctuations in conductance of ^1 0 percent before stabilizing to ^1

percent (2). Drawn glass capillary leaks may not be desirable for the very

small leak range, but specimens made earlier with entrance aperatures

of 1 ym to 25 ym diam were measured repeatedly for the 0.1 to 1 x

10" 7 Pa.m3 /s range and appear to have suitable stabi 1 i ty(3) . Use of such larger

bore capillaries with mixed tracer-carrier gas flows and in_composite structures

for flow division were then considered for the range 1 x

10" 6 to 1 x

10" 10

Pa.m3 /s. Detailed calculations on flow rate and flow mechanism were

made to determine feasibility, constraints, and dimensions. The tracercarrier gas flow procedure did not appear feasible, however, because the

necessary reduction of slip and molecular flow components would require

relatively large capillary diameters with high flow rates. This left

flow divider as a possibility provided it could be implemented in convenient

form.

Our measurements in the 0.1 to 1 x

10" 7 Pa.m3 /s range are made on an absolute basis with a rate-of-rise apparatus (3). The procedure selected

for measurements below this range would use a flow generator for the

calibration of a mass spectrometer which would then be used to measure the

flow rate of a test leak. Again, guidelines for the flow generator are also

derived from capillary analysis.

141
Calculations have now been extended on flow rate and flow composition,

with selected results listed in Table I as relevant to the flow divider

and to a now possible tracer-carrier mixed flow reference.

In principle the flow generator is simply a fixed, stable molecular

flow conductance placed between the mass spectrometer and an accurately

set pressure chamber upstream. The structure between the chamber and mass spectrometer is so designed that it has negligible effect on the

value of the fixed conductance, and the conductance can be removed for

independent measurement. Conductance dimensions and upstream pressure

have to be restricted to allow measurement with the rate-of-rise apparatus

and yet produce the desired range of known flow rate into the mass spectrometer. Box A of Table I lists one selection. Precision bore

glass capillary tubing of 0.1 mm diam is readily available, easily

handled, and somewhat insensitive to plugging. With a 0.2 cm length,

the flow rate can be measured as a function of pressure down to the

level where the conductance is molecular to greater than 99 percent.

Below this value, flow rate would be determined from the upstream pressure

alone. A 1 cm length would experience the same flow compositional

behavior as a function of pressure but extend to smaller flow rates.

Cross comparison would be made in the mass spectrometer.

A mixed gas laminar flow leak would also be restricted to a total

flow rate within the range of the rate-of-rise apparatus and the mass spectrometer. Box B of Table I has several examples. The data on capillaries

of 16 ym and 10 ym diam and others in this range previously cast doubt on the

tracer-carrier concept because of the high flow rates and small laminar

flow fraction. However, the third example shows a possibility with the

long capillaries that are now available for gas chromatography. With the

100 m long, 0.250 mm diam example, the upstream pressure could be dropped to

0.25 atm to produce a laminar flow of ^98 percent that would be small

enough to be handled by the mass spectrometer pumps. Upstream tracer

gas concentrations of 1 percent to 0.1 percent would then yield a tracer flow

rate of ^10" 7 and 10" 8 Pa-m3 /s without significant fractionation. This would

also be used to compare against the flow generator for greater assurance

on generator use.

A flow divider is represented in figure 62. The low pressure Pi

upstream to a molecular flow leak R2 is provided by flow through a preliminary capillary Rj. from 'vatm pressure Pn and a controlled pumpout through

R 3

. Flow division is in R2 and R3

. A 30 cm length of 0.075 mm capillary,

which is readily available, can produce a 98 percent laminar flow with

small enough flow rate to be pumped by a thin, 2 mm diam orifice. With

a 35 s,/s pump below the oriface, the geometrical conductance is the pumping

speed of <3.5 x

10~ 4 m

3 /s. Thus, the 7.2 x 10" 4 Pa*m3 /s flow rate through Ri

would produce a pressure of ^2 Pa at Pj. At 2 Pa above a capillary

of 2 cm length and 0.5 mm diam for R2 , the flow rate for 96 percent molecular is 1 x 10" 6 Pa-m3 /s which is readily reduced with decreasing pressure

for Pj. Note the flow ratio through R 3 /R 2 is >500. Use of a capillary

142
as in Box A would extend the flow range to lower values. The conductances

of all capillary components can be measured independently, while the

conductance of a thin orifice approaches the calculated theoretical

value (4).

An ultra high vacuum mass spectrometer system has now been assembled

and put into operation. The reference vacuum system for the micromanometer

array has been modified, and cold trap refrigeration of the rate-of-rise

pump has been repaired. The rate-of-rise system itself will provide the

stable pressure generation above the flow generator capillary. A leak

holder chamber and manifold has been constructed for insertion between

the rate-of-rise apparatus and the mass spectrometer system.

Capillary leaks have been fabricated to dimensions similar to those

in Table I, and a long chromatographic column has been procured of the

dimensions of the second example in Box B of Table I for use as a mixed

gas laminar flow leak. Other capillary leaks have also been drawn to serve as specimens for repeatability tests. We plan to conclude the present

study with experimental measurements on capillary flow rates as a function

of pressure and tracer concentration following guidelines of Table I. We

also plan to intercompare the tracer-carrier mixed flow against the flow

generator and determine repeatability of measurements at flow rate < 10" 6 Pa.m

3

/s.

References

1. "Leak Rate Reference Standards," S. Ruthberg, NBSIR 80-2162, 56-59

(1980).

2. "The Use of a Silicon Carbide Porous Plug for the Control of Gas Flow

into a Vacuum System down to 1 y£-torr/s," R. G. Christian and J. H. Leek,

J. Sci. Instrum. 43, 229-233 (1966).

3. "Leak Rate Reference Standards, " S. Ruthberg, NBSIR 82-2449, 50-53

(1981).

4. "The Measurement of Conductance to Free Molecular Flow by Substitution

Procedures," S. Ruthberg, J. Vac. Sci. Technol. £, 1457-1469 (1972).

143
Table I

Flow Rate and Composition of Flow for Capillary Leaks

Air, T = 298K, Pdownstream

length diam P

upstream Q lam slip mol

(cm) (Mm) (Pa) Pa*m3 /s* %

A. 0.2 100 266.6 1.66(-5)** 14.0 45.8 40.1

133.3 7. 95( -6) 7.3 35.8 56.9

13.3 8. 05( -7) 0.7 6.9 92.4

6.66 4. 05( -7) 0.36 3.6 96.0

1.33 8. 1 3 ( —8

0.07 0.7 99.2

1.33(-2) 8. 1 3( -1 0)

1 100 133.3 1.59(-6)

1.33 1 . 63( -8)

1.33(-2) 1 . 6(-1 0)

B. 1 16 1.013(5) 4. 9( -5) 92.1 7.7 0.2

10 1.013(5) 7. 4( -6) 87.4 12.1 0.5

2.00(5) 2. 7 (-5) 93.0 6.7 0.2

104 250 1 .013(5) 2.6(-4) 99.4 0.6 0.00

5.066 6. 7

( -5) 98.9 1.1 0.00

2.533(4) 1 . 7(-5) 97.7 2.2 0.02

C. 30 75 1.013(5) 7. 2( -4) 98.1 1.9 0.0

2.026(4) 3.1 (-5) 91.2 8.6 0.3

1.013(4) 8. 5

( -6) 03.7 15.4 0.9

2 500 1.33 1 . 02( -6) 0.4 3.6 96.0

6. 66(-1

)

5. 1 (-7) 0.00 1 .9 98.0

1 »33( -1

1 . 02( -7) 0.00 0.4 99.6

1 ,33(-2) 1 .02(-8)

2. 5( -3) 2 mm 1.33 4. 7

( -4)

* 1 Pa*m3 /s = 9.872 atm* cm /s

** a(-n) = a * 1 0“n

144
Ri Pi R2

Fig. 62. Flow divider network: R, -capillary tube under laminar flow. R

2

- capillary tube under molecular flow. R^-thin oriface under molecular

flow. Pq ^ 1 atm. P 2

j « P-| • Flow rate in R^ >> flow rate in

Rg *

145
Task # 15750

FLOURESCENT GLASS STANDARDS FOR USE IN DYE PENETRANT TESTS

R. W. Burke and D. K. Hancock

Inorganic Analytical Research Division

Center for Analytical Chemistry

Fluorescent dye penetrant testing continues to be one of the most

widely used nondestructive methods for detecting minute cracks and flaws

in metallic surfaces. The basic test, as performed currently, is

essentially the same as originally developed. It entails coating the

part to be tested with the fluorescent dye penetrant, wiping off the excess dye from the exposed surface, applying a developer, and measuring

the fluorescence of the resulting indication. While many improvements

have been made in the formulation of highly sensitive penetrants,

efforts to reproducibly quantify these improvements in terms of their

relative fluorescent brightness and crack-measuring capabilities con- tinue to be thwarted by the nonavailability of suitable fluorescent

standards for calibrating the test equipment.

Our initial efforts to develop fluorescent glass standards for use with various NDE tests were described in last year's report. At that

time, two types of standards were perceived. One consisted of a rec- tangular piece of uranium-doped glass mounted in a metallic heat sink

for enhancing temperature stability; the other type was an assemblage of

glass fibers having a series of graded diameters and uranium concentrations. The first type of standard was intended for the reproducible

calibration of spectrofluorimetric instrumentation while the second type was envisioned as a working standard for improving the reliability with

which the intensities and widths of fluorescent indications could be

measured.

As a result of understanding more clearly the needs of industry and

the problems associated with their "mass production," several changes

in the makeup and design of these standards were considered desirable.

During the past year, efforts have focussed on perfecting these changes.

The most significant change was the decision to seek an alternative

fluorescent glass that could be substituted for the uranium-doped glass

used initially. Two factors were instrumental in this decision. The

predominant factor was that the fluorescent temperature coefficient of

uranium-doped glass is much larger than originally anticipated. Our

measurements have shown that the fluorescence of this glass decreases

approximately 0.9 percent per degree Celsius in the temperature range of

20-40°C. A second factor was a safety issue involved in the in-house

production and fabrication of uranium-containing materials. Subsequent

studies showed that the fluorescence of terbium-doped glass is very

similar to that of uranium-doped glass. Like uranium, terbium-doped

glass fluoresces in the green region of the spectrum and as a conse- quence exhibits excellent coincidence with both the fluorescence emission

146
of most dye penetrants and the mesopic sensitivity of the human eye. In

addition, the range of fluorescent intensities obtainable with terbium-doped

gTass is relatively large since doping levels up to 45 to 50 weight percent

are possible. Its main advantage for use as a standard, however, is the

fact that its fluorescent temperature coefficient is at least an order

of magnitude smaller than uranium-doped glass and thereby displayed the

minimum kind of improvement that we were seeking. The other major

change in our research efforts has concerned the approach to producing a fluorescent standard that can be used in conjunction with the measurement of crack indications. Because of significant handling and mounting

problems, the glass fiber approach has been replaced by a system in

which fluorescent indications of varying widths and relative intensities

are obtained by ruling a series of narrow slits in opaque films that

have been evaporated directly on polished glass flats having different

terbium concentrations. Six to eight slits per side are planned with

widths ranging 5 to 100 micrometers.

Current efforts are concerned with the systematic evaluation of the

various formulations of terbium-doped silicate and phosphate glasses now

on hand and the subsequent selection of one of these as the base formulation that will be used for the preparation of the two types of fluorescent standards being proposed. At the present time, the phosphate

glasses have the apparent advantage because they are intrinsically more

fluorescent. This property may be due to the fact that the phosphate

matrix is more transparent to ultraviolet radiation and hence the energy

reaching the terbium fluorophor is more intense. Once the optimum base

glass composition has been chosen, it will be fabricated into forms that

are easily incorporated in NDE tests. As an instrumental calibrant,

particular emphasis will be placed on providing a configuration and size

that is directly usable in the Turner, Model 111, fluorometer, During the same time, the final decision will be made on which evaporated film

will be used in the production of the simulated crack indication standard.

The goal here is to identify a film that is durable, exhibits a matte

finish which closely approximates that of a typical metal specimen, and

can be properly ruled. Based on the projected progress, it is believed

that sample of both types of standards will be ready for field evaluation

by the end of the calendar year.

147
Task # 15750

DYE PENETRANT CRACK RESEARCH

David Lashmore and David Kelley

Materials Chemistry Division

Center for Materials Science

Introduction

Dye penetrant crack SRM's have been in production for two years and

last year more than 100 were made. This SRM is fabricated using an electro

forming technology and consists of alternative layers of copper and

nickel. There are four layers of copper: the thickest is nominally

2 pm, then next 1 pm, followed by 0.5 pm and 0.2 pm. These copper

layers are then electrochemically removed and the space that the copper

occupied constitutes the crack. This research project was undertaken to

improve various unsatisfactory manufacturing techniques and to investigate several important aspects of the crack geometry that have a bearing on dye pick-up and apparent crack width. Among the topics that

were examined were (a) the extent to which the crack edge was "rounded"

by the etching procedure; (t>) the depth to which the crack penetrated

into the block; (c) the polishing technique which was known to "smear"

the nickel over the copper; and (d) various techniques to automate the

production process.

Results

In order to eliminate as much as possible the rounding of the

crack, an improved etching technique was developed. As shown in figure 63, the nickel is theoretically dissolved at a potential lower (more

cathodic) than is copper. By placing the nickel/copper laminate in a solution of chromic-sulfuric acid and setting the potential in region I,

it theoretical ly is possible to dissolve only the copper and at

the same time avoid producing an appreciable amount of hydrogen. This

potential was accurately determined experimentally. Once this potential

was determined the previously used two electrode system was modified to

include a calomel electrode.

The polishing procedure was changed drastically to reduce to the

maximum extent possible the smearing of the nickel and at the same time

the polishing procedure was automated. Optimal polishing times were determined. The most significant change was in adopting all diamond

polishing abrasives. The previously used aluminum oxide was found to

produce a significant amount of deformation. Since this smearing was accent

uated by hand polishing all hand polishing steps were removed and our automatic metallography station was relied upon for each step in the

surface preparation.

The depth of the crack was determined for the first time. Typical

values for one laminate are shown in Figure64.
Because of the unexpected large depth of the cracks, further research

on the effect of etching on dye pick-up seemed unwarranted. Indeed, it

would seen possible to reduce the etching time even further.

Table 1. Nominal Crack Depths for the Current Dye Penetrant Test Standard

ne No.

Nominal Width M

Actual Width

Cum)

Crack Depth

(urn) D/W

1 2.0 1.89 131 69

2 1.0 0.75 82 109

3 0.5 0.65 26 40

4 0.25 0.18 21 116

Changes in the sample geometry were made to speed up production and to

ensure more uniform polishing. These changes were the inclusion of a stainless

steel ring surrounding the laminate to keep the sample from rounding and changing the epoxy mold geometry so that mounting of the samples in the holder would

result in the sample being exactly parallel to the plane of polishing.

Conclusion and Remarks

1. Procedures have been developed to avoid rounding of the crack edge.

2. The crack depth-to-width ratio has been determined to exceed 40 for all

crack widths and to be as high as 116 so that the effect of etching

time on dye pick-up is most probably negligible after a given critical

etch time has passed.

3. Polishing procedures have been developed to minimize smearing of the

nickel

4. Production has been automated to the maximum extent possible.

The remaining problem with this SRM is the tedious technique used to

measure the crack widths. The first three lines are measured on an old

optical microscope (for each and every crack) which has rather poor resolution.

The finest line is measured on the SEM using a statistically significant

number of samples and assuming that the remainder of standards have the same spacing. This is probably a reasonable assumption as the standards are all

cut from the same sheet.

143
-2-1 0 1 3 4 7 8 9 10 II 12 13 14 15 16

-Z -1 10 11 12 13 14 15 J6 pH

Figure 63. Pourbaix diagram of copper superimposed upon the Pourbaix diagram

for nickel

150
151
Task # 15770

WEAR CONDITION MONITORING

A. W. Ruff

Metallurgy Division

Center for Materials Science

This research involves techniques of acoustic emission and wear

debris recovery from laboratory and field-operating systems that are undergoing wear. In cases of rotating machinery such as gas turbines

and pumps, wear of bearings and other components is frequently the lifedetermining process. Different methods can be used to monitor such

mechanical systems, including thermal measurements, vibrational measurements, etc. More direct methods are very desirable, for example, recovery

of wear debris from the lubricant, since they bear directly on the problem.

However, the capability for early warning of damage, an important issue,

varies greatly with the method. We have worked with many groups outside

NBS on the use of wear debris as an early warning method; this has been

successful in some cases. Project effort is now concentrated on using

acoustic signals.

Acoustic emission detection is being used in a study of metallic

wear in order to monitor particular wear events such as subsurface crack

growth (delamination).. While the noise and vibration background in a typical wear test situation can be complex, it may be possible to remove

such interferences in the detected ultra-sonic signals by using filtering,

synchronous detection schemes, and computer based signal processing. A

laboratory bench wear tester has been instrumented with an acoustic

detection system and recorder. An incremental stressing contact geometry

(consisting of a raised portion on the rotating ring counterface) is

used, and the anticipated emission events are searched for in synchronism

with the load pulses. Three different steels having varying toughness

have been studied; and copper has also been examined briefly, all sliding

under lubricated conditions against 52100 steel counterfaces. Some

unusual emitted waveforms have been detected from the steel specimens, but

they do not occur with every load pulse. Digital pulse recording and

analysis will be tried next in order to obtain better resolution of the

waveform details. This work is carried out in coordination with the other

acoustic emission work in the Metallurgy Division concerned with signal

analysis and source characterization. Plans next year include a wear

study of other crack-sensitive steels, using digital acoustic pulse

detection and characterization techniques. It is planned to accomplish

this work in close collaboration with the acoustic emission effort in the

Metallurgy Division.

152
Task # 15770

ENGINE CONDITION MONITORING THROUGH OIL ANALYSIS

Stephen M. Hsu

Materials Chemistry Division

Center for Materials Science

Highly formulated lubricants are often used to lubricate

various high load, high stress modern engines in various powertrains.

One of the major causes of premature engine failure is lubricant

breakdown which causes corrosion, fatigue, wear, and sometimes

sudden seizure/welding. The object of this project is to develop

test methods to measure and understand the degradation of lubricants

and its relationship with engine wear/failures.

Conventional oil analysis techniques include infrared spectroscopy and wear metal analysis by emission spectroscopy or ferrography.

Emission spectroscopy measures metal particles mostly less

than 1 ym. Ferrographic method is effective only for iron (magnetic)

wear particles at 1 ym or above. All three techniques function

reasonably well in clean oil systems. With normal used oils which are

contaminated by airborne particles, oxidized oil components, and

wear particles, analysis becomes difficult. Recent ASTM round robin

results on oxidation and nitration in used oils by infrared spectroscopy

indicated complete disagreement among a score of oil company labs. One major

technical problem is how to set condemning limits for various parameters that are measured. High levels of wear metals in used oil

indicate incipient seizure and the time to failure may be very short.

Oil analysis usually measures oxidation degradation and sometimes

contamination, but their relationship to wear and failure are not

clear.

During the past year, some key experiments were performed. A

carefully selected lubricant with known engine performance was degraded in the laboratory under controlled conditions. Various

state-of-the-art measurement techniques as well as conventional oil

analysis methods were used to follow the degradation pathway of the

oil. Periodic samples were withdrawn and tested in NBS developed

laboratory engine simulation tests. We found a definite correlation

between additive depletion and increase in wear. However, secondary

interactions between additive decomposition products and base oil

oxidation products were able to provide additional wear protection

but not the load carrying capability of the lubricant. Results are shown in Figures 65 and 66.

153
In Figure65, (from left to right) the viscosity of the oil

remains relatively stable until about 120 hours. At that time, the

viscosity increases sharply to over 800% of the original oil viscosity.

The antioxidant capacity is measured by the Ford free radical titration

method (1) which measures the ability of the oil to decompose free

radicals. We see that the antioxidant capacity drops sharply under

oxidation at about 50 hours (antioxidant depletion/degradation).

However, it rises again to about the half of the original values.

This suggests secondary reactions between the base oil and additive

degradation products to regenerate anti -oxidant species. The seizure

load (2) indicate the capacity of the lubricant to carry a certain

load without seizure. The data in Figure 65 suggest that the seizure

load decreases by 30% in the first 50 hours coincidental with the

additive depletion. After 50 hours, test results suggest frequent

seizures and erratic friction traces and a clear seizure load was difficult to determine. The amount of wear increases about 30%

and remains level after 50 hours. Differential infrared results

on the phosphorus sulfur bond (characteristic bond of the anti-wear

additive zinc di thiophosphate) suggests that the antiwear additive

is exhausted in the first 50 hours. This confirms the observed wear

and seizure load measurement.

In Figure 66 , additional parameters measured in the same experiment are presented. The viscosity increase and the antioxidant

capacity are shown for reference. The oxidation induction times

are measured with the thin film oxygen uptake test

(TF0T) which has demonstrated correlation with engine oxidation

results (3). The oxidation induction time drops quickly and reaches

a minimum at about 20 hours. This decrease in oxidation stability

suggests additive depletion and is in agreement with the antioxidant

titration results. After 20 hours, the oxidation stability of the

oil increases gradually, again suggesting antioxidant regeneration.

Differential infrared measurements on carbonyl (carbon oxygen bond)

suggest a continuous rise in the oxidation process independent of other

processes. The total acid number measures the organic acidic species

formed under oxidation. The data in Figure 66 indicate stepwise

increases with different slopes. The acidic species begin to form

at about the 75th hour with steep slope. At about the 120th hour,

the formation rate decreases and the viscosity increase begins to rise

sharply. Average molecular weights as measured by gel permeation

chromatography (GPC) indicate a constant level until about the 130th

hour.

These results are interesting and significant. Contrary to

conventional theories, drastic failure did not occur when all the

additives were exhausted. In fact, the original additives were exhausted in the first 40 hours, yet the viscosity failure did not occur until about 120 hours. Coincidental with the viscosity increase,

the acidic species in the oil reach a certain level (TAN = 12). The

load carrying capacity, however, appears to correlate with the additive

depletion.

154
Controlled experiments with a single lubricant degradation suggest

some condemning limits on used oil properties. Three new test procedures

have been developed and proven useful in oil analysis for engine

condition monitoring. Future work will test other lubricants and

additives in an oxidation wear coupled system.

155
REFERENCES

(1) Mahoney, L. R. et al , "The Determination of the Antioxidant

Capacity of New and Used Lubricants: Method and Application,"

Ind. Eng. Chem. , Prod. Res. and Develop., 17, 250, 1978.

(2) Gates, R. S. and Hsu, S.M., "Development of a Four-Ball

Wear Test Procedures to Evaluate Automotive Lubricating

Oils," ASLE preprint no. 82-AM-6A-4,

Presented at the ASLE meeting, Cincinnati, Ohio, May, 1982.

(3) Ku, C.S. and Hsu, S.M., "A Thin-Film Oxygen Uptake Test for

the Evaluation of Automotive Lubricants," Presented

at the ASLE/ASME joint conference, Washington, D.C., October 7-9,

1982.
rao/sav ‘s = & hiq

i.-, K.'iOlIX 1 [SOJ TA / 157

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w

9

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Fig.

65

.Effects

of

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on

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tmSjaft JB-[no3-j;on sSeaBAy

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Fig.

66.

Effects

of

Oxidation

on

Oil

Properties
ELECTROMAGNETIC NOISE

Task # 15770

U. Bertocci

Materials Chemistry Division

Center for Materials Science

Electrochemical noise measurements have been taken on Fe-Cr alloys

of three different Cr concentrations, in order to examine random fluctuations of the current under potentiostatic conditions when these alloys

are in passive conditions, both in the absence and in the presence of

chloride ions in solution, which can cause pitting. Noise measurements

were taken before and after pit initiation. These measurements were

carried out with a computer-controlled apparatus, based on a two-channel

spectrum analyzer, which can distinguish between fluctuations of the

current due to changes in the electrode characteristics and noise induced by the input signal. See Fig. 67.

No noise could be detected in the absence of chlorides, while

significant fluctuations were observed in their presence, both before

and after pit initiation. The results indicate that the most important

role of chlorides is to induce breakdown in the passive film, rather

than affecting the repassivation kinetics.

These results also show that noise analysis, as an NDE method, has

the feasibility to detect significant changes in the amplitude before

pitting actually starts.

Another application of noise measurements in electrochemistry, the

determination of the electrode impedance using a white noise input

signal, has been used to study the transition from passive to transpassive dissolution in stainless steels. Comparison with more traditional methods has shown that impedance measurements employing noise are at least as accurate and considerably easier to carry out.
Fe~12ZCr

Alloy

s (9 (9 s ca 19 ca ca

00 ca (\J (O CD ca CM

1 H H H H H C\J (\1

i 1 1 1 l 1 1

ca

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i

N

I

>-

U

z

LU

Z)

a

LU

tY.

Ll

CD

a

Z H >>t UJO /VSP ‘dBMOd 3SI0N

160

Fig.

67.

Noise

power

spectrum

for

Fe-12%

Cr

alloy

in

pitting

and

non-pitting

regimes.

Line

indicates

noise

level

contributed

by

instruments.
Task # 15770

ANALYSIS AND MODELING OF NDE ROUND-ROBIN DATA FOR ENGINEERING

APPLICATIONS

J. T. Fong

Mathematical Analysis Division

Center for Applied Mathematics

This project was initiated tn 1978 to answer the following question:

"When a flaw is detected by two or more NDE teams with different numerical

answers for its location and size, is there a rigorous method to compute

a single set of numbers to characterize the flaw with a confidence interval

to permit the assignment of an engineering safety factor?"

The answer was no when NBS undertook in 1976 a study on the structural

integrity of 612 welds of the Trans-Alaska Oil Pipeline. The answer today

is still in the negative because controlled experiments in nondestructive

evaluation of flaws are rarely conducted and human factors are significant

in almost all NDE processes.

Nevertheless, it is possible to develop a data analysis and variability

modeling methodology such that round-robin NDE data from carefully planned

and well-conducted test programs can yield useful information for engineering

appl i cat ions.

For example, as early as 1965, the Pressure Vessel Research Conmittee

(PVRC) of the Welding Research Council recognized the need to assess the

reliability of weldment inspection techniques through an industry cooperative round-robin test program. A total of 12 plate-weld specimens containing

intentional flaws of various types were fabricated and inspected by industry

teams using techniques and procedures prescribed by PVRC. Upon the completion

of the round-robin test program, each specimen was or will be sectioned to

verify the true location and size of each flaw as well as to study the

reliability of each NDE procedure.

At the invitation of Mr. H. C. Graber, then Chairman of PVRC Subcommittee on NDE of Pressure Components, Fong began the application of an analysis methodology to a complete set of sectioning and round-robin data

for PVRC Specimen 251 -J in Sept. 1981. The analysis methodology has been

under development by Fong as part of the objective of this project. The

availability of the PVRC data with true values of the flaw location and

size provides for the first time an opportunity to address both the precision

and the accuracy questions of an NDE procedure.

The analysis methodology consists of five basic steps: (a) Data Representation; (b) Multi-Level Data Reduction; (c) Variance Analysis; (d)

Variability Modeling; (e) Data Retrieval and Display. An analysis of variance

technique due to Mandel (1977) and first applied to fatigue and NDE data

by Fong (1979, 1981) was implemented in this methodology for generic applications. Example of some preliminary application of the methodology

1C1
(steps (a) and (e)) to PVRC data is given in Figures 68, 69, 70Five sets

of data for PVRC Specimen 251 -J are currently being analyzed by Fong and

his colleagues with partial financial support from the industry-based

Pressure Vessel Research Committee.
251J/A-XY PVRC-251J (YUKAWA) FLAW A - YZ PROJECTION

Fig. 68. 2-dim. Computer Graphics Based on Flaw Sketches Furnished

by Yukawa of G.E. -Schenectady (1981).

Fig. 69. 3-dim. Computer Graphics for Flaw and Weldment (Fong, 1982).

163
Fig. 70. Enlarged View of Flaw and Weldment for Variability Modeling

(Fong, 1982).

164
Task # 15770

USE OF NDE IN REHABILITATION AND PRESERVATION

J. Clifton

Building Materials Division

Center for Building Technology

An important phase of the rehabilitation or preservation of concrete

and masonry structures is deciding whether to repair, reinforce, or replace concrete and masonry materials and building components. Information

on the condition of the concrete and masonry materials, including any

internal metal reinforcement, is usually required before such decisions

are made. The traditional process of condition assessment includes making

a visual inspection of the structure, characterizing the important properties

of a material using cored and other samples, and making an engineering

analysis of structural components. For condition assessment purposes,

the use of nondestructive evaluation (NDE) methods can be beneficial. For

example, while NDE does not entirely eliminate the need for taking samples

and analyzing or mechanically testing them, it does substantially reduce

the amount of such testing.

NDE techniques applicable to determining the condition of concrete and

masonry in structures being rehabilitated have been identified and assessed.

The assessment of these techniques were based on consideration of usefulness

of information obtained, reliability, and expertise required to use the

equipment and to interpret the results. The results of the assessment are given in a paper submitted to the American Concrete Institute for publication

in a special publication dealing with rehabilitation and preservation of

concrete and masonry structures.

165

A
I

I

APPENDICES

166
A. NDE Related Conferences

1. Deformation, Fracture, Wear and Nondestructive Evaluation of Materials,

Nov. 23-24, 1981, New Orleans, LA.

2. Time-Dependent Failure Mechanisms and Assessment Methodologies, 35th

Mechanical Failures Prevention Group Meeting, April 20-22, 1982,

National Bureau of Standards.

3. Seventh International Symposium on Ultrasonic Imaging & Tissue

Characterization, June 6-9, 1982, National Bureau of Standards.

4. Process Control Sensors for the Steel Industry, July 27-28, 1982,

Gaithersburg, MD.

167
B. NDE Seminars Organized

Speaker

L. W. Kessler

Sonoscan, Inc.

J. A. Swets

Bolt Beranek &

Newman, Inc.

Date

March 9., 1982

April 23, 1982

Topic

Acoustic Microscopy: Methods,

Applications, Outlook

Evaluation of NDE Systems

ICO

nniWUWlfBUMDUIMfl
c. Invited Talks by NDE Office Personnel

1. "Condition Monitoring of Machines," L, Mordfin, American Physical Society,

New Orleans, LA, November 24, 1981.

2. "Recent Developments in Non-Destructive Evaluation Measurements at the NBS,"

G. Birnbaum, Howard University, January 8, 1982.

3. "Calibration, Reference Standards, and Traceability to NBS in NDE,"

L. Mordfin, Symposium on Validity of NDE Measurements, Butler County College,

Butler, PA, February 3, 1982,

4. "NDT Measurements Traceable to NBS," G. Birnbaum, 10th World Conference on

NDT, Moscow, USSR, August 23, 1982.

5. "Future Directions of Ultrasonic NDE Standards in the U.S,," G. Birnbaum,

10th World Conference on NDT, Moscow, U.S.S.R., August 23, 1982.

6. "Some Comparisons of Standards for Eddy Current Nondestructive Testing,"

10th World Conference NDT, Moscow, U.S.S.R., August 26, 1982.

7. "Advanced Diffraction Techniques for the Nondestructive Evaluation of

Internal Residual Stress," L. Mordfin, Seventh International Conference on Experimental Stress Analysis, Technion Israel Institute of Technology,

Haifa, Israel, August 25, 1982,

8. "Standards for Quantitative NDE," L, Mordfin, Israel Aircraft Industries,

Lod, Israel, August 29, 1982.

169
D. Awards

Industrial Research, IR-1Q0 Awards

Kenneth Yee and Donald Blomquist of the NBS Automated Production Technology

Division won an IR-100 Award for inventing a device that monitors the behavior

of machines engaged in drilling holes and sends a warning signal when it senses that a drill is about to break. The "Drill-up" monitors the vibrations in the

workpiece while it is being drilled. The device compares the vibration

"signature" over a period of time and detects a periodic disturbance in the

yibration that the researchers have found is characteristic of a drill that

is about to break. "Drill -up" assures that the disturbance is not a random

event and signals the machine control system to retract the drill and wait for

a replacement.

Dr. Roald Schrack, Dr. C. Bowman, and Mr. J. Behrens, of the NBS Nuclear Radiation

Division won an award for the design of a two-dimensional imaging system for

resonance neutron radiography that can selectively display the position and

concentration of any isotope or element in a complex specimen. The detector

is designed for use with higher-energy pulsed neutron sources. The entire device

has a sensitive area with a diameter of 2.5 cm. The information is stored

by a computer-based data gathering system. The stored data can be used to

reconstruct a neutron radiograph image "tuned" to any desired resonance,

highlighting any element or isotope with that resonance.

NBS Silver Medal Award

Dr. R. Schrack, Dr. C. Bowman and Mr. J. Behrens were cited for developing

Resonance Neutron Radiography which is described in more detail in the IR100 Awards above.

NBS Bronze Medal Award

Dr. Nelson N. Hsu was cited for his outstanding contributions to the

science and technology of experimental and theoretical mechanics and their

resulting applications in research and industry. His work on acoustic

emission and ultrasonics testing have greatly affected the quality

and safety of structures.

NBS Applied Research Award

Dr. Donald G. Eitzen was cited for his unique contributions in applying

research to the development of quantitative acoustic emission measurements.

Work in his group has resulted in calibration services, major breakthroughs in design and development of an acoustic emission transducer

and major advances in the theoretical understanding of stress wave propagation in structures and solids.

ASNT Gold Medal

Mr. Harold Berger received the distinguished American Society for Nondestructive

Tesing Gold Medal in recognition of his many years of continuing meritorious

service to ASNT.

170
IEEE Fellow

Mr. George C. Hannon of the Center for Electronics and Electrical Engineering

has been elected Fellow of the Institute of Electrical and Electronics Engineers.

Mr. Hannan was named for development of process control and screening procedures

for microelectronic welding and bounding.

ASCE Awards

Mr. Jimmy James, a co-op student from Prairie View A & M Univeristy, is the

recipient of two separate American Society of Civil Engineers scholarships.

Mr. James received the Samuel Fletcher Tapman Scholarship and the B. Charles

Tiney Memorial Scholarship.

171
E. Technical

Name

H. Berger

G. Blessing

Don Eitzen

and Professional Committee Participation and Leadership

Parent

Organ. Activity

ASTM E007 Nondestructive Testing

SC. 01

Radiographic Practice and Penetrameters

SC. 05

Neutron and Radiography

SC. 91

USA Committee for IS0/TC135 (NDT)

SC. 92

Editorial Review

SC. 98

New Methods Review

ASNT

American Nuclear

Society

American Society

for Metals

Materials Evaluation Committee (Chairman)

Technical Publications Division

Penetrating Radiation Comm.

Energy Systems Component Corm.

Isotopes & Radiation Division

Awards Funding Committee

Nondestructive Testing Committee (Chairman)

American Society Pressure Vessels and Piping Division

of Mechanical Decade of Progress Committee

Engineers

APS member

Brit. Inst, of NDT member

Sigma Xi

ASTM

National Material

Advisory Board

ASTM

member

E7.06 on Ultrasonic NDE

E28.13 on Residual Stress

Committee on NDE of Metal Matrix Composites

E7.06.02

A1 Reference Blocks

E7.04.02

AE Sensors

172
Parent

Name Organ. Activity

D. Garrett ASTM E7.05

J. Hsia International

CIE Subcommittee

Chairman

N. Hsu ASTM E7.04

Acoustic Emission

L. Mo rdfin ASTM D20. 10.23

Residual Stress

E7.03

Magnetic Particle and Pentetrant Testing

E7.08

Leak Testing

E7. 10.04

Infrared Methods, Chairman

E9.02

Residual Stress Effects in Fatigue

E28.13

Residual Stress Measurements, Chairman

ASNT Liaison Division

Utilities Contnittee

Visual Methods Committee

Infrared & Thermal Methods Committee, Chairman

SESA Residual Stress Measurement Conmittee

SAE Nondestructive Test Methods Division

X-Ray Fatigue Group

ASTM E7.01.03

Radiographic Practices and Methods

ANSI PH2.34

Industrial X-Ray Film Sensitometry

ASTM E-7

E-10

F-12

ANSI PH 2-34

R. Placious

D. Polansky

173

A
Name

Parent

Organ. Activity

T. Proctor ASTM E28.13.Q5

Ultrasonic Measurements

C. Teague ANSI B46

Surface Finish Measurements

T. Vorburger ANSI B46.1

Profile Methods

H. Yolken ASM NDT Committee

ASTM Ad Hoc Committee

Medical Device Standards

174
F. Publications

1

Journal Publications

1. "R & D Summary: A New Look at the NBS Programme in NDE," H.

Berger, NDT International , Vol. 14, No. 5, pg. 292-293 (Oct. 1981).

2. "Radiation-Induced Acoustic Cavitation Threshold vs. Temperature

for Some Liquids,” M. Greenspan and C. E. Tschiegg, J. Acoust. Soc.

Am. Res. Brief, 1982.

3. "Surface-Wave Dispalcement: Absolute Measurements Using a Capacitive

Tranducer," F. Breckenridge and M. Greenspan, J. Acous. Soc. Am. 69(4).

Pg. 1177-1185, April 1981.

4. "Acoustic Emission Transducer Calibration by Means of the Seismic

Surface Pulse," F. Breckenridge, J. Acous. Emission 1(2), pg. 87-94,

1982.

5. "An Improved Piezoelectric Acoustic Emission Tranducer," T. Proctor,

J. Acous. Soc. Am. 71(5), pg. 1163-1168, May 1982.

6. "Noise Measurements in Electrochemical Systems,” U. Bertocci and

Ye Xiang-Yang, Corrosion Protection (People's Republic of China),

1982

7. "Optical Techniques for On-line Measurement of Surface Topography,"

T. V. Vorburger and E. C. Teague, Precision Engineering, 3 (1981),

pg. 6.

8. "Light Scattering from Manufactured Surfaces," E. C. Teague, T. V.

Vorburger and Daniel Maystre, Annals of the CIRP 30 (1981), 563-570.

9. "Dyadic Green Functions for the Time-Dependent Wave Equation,” Egon

Marx and Daniel Maystre, Journal of Mathematical Physics 23 (1982)

pg. 1047-1056.

10.

"Elastic Constants of Polycrystalline Copper at Low Temperatures:

Relationship to Single-crystal Elastic Constants, H. M. Ledbetter

Phys. Stat. Sol i di (a) 66^ (1981) pg. 477-484.

12. "Predicted Single-crystal Electric Constants of Stainless-Steel 316,"

H. M. Ledbetter, Brit. J. of Nondestructive Test. 3^, 1981, pg. 286-287.

2.

Conference Proceedings

1. "What is NBS Doing in NDE?," H. Berger and L. Mordfin, 36th Annual

Quality Congress Transactions, pg. 929-933 (1982).

2. "The 1976 NBS Study of Girth Welds in the Trans-Alaska Oil Pipeline,"

H. Berger , ASNT Paper Summaries, ASNT Fall Conference, Atlanta, GA

(Oct. 1981).

3. "An overview: New Ideas in Nondestructive Evaluation," H. Berger,

Rubber Chemistry and Technology , Vol. 54, No. 5, pg. 996-1002

(November 1981).

4. "Standards for Residual Stress Measurement," L. Mordfin, Residual

Stress Effects in Fatigue, ASTM STP-776, pg. 2-12 (1982).

175
5. "Introduction to Residual Stress Measurement," L. Mordfin, Proceedings

of DoT Symposium on NDT Measurment of Wheel/Axle Residual Stress,

Cambridge, MA, 1981.

6. "Advanced Diffraction Techniques for the Nondestructive Evaluation

of Internal Residual Stress," L. Mordfin, Proceedings of the 7th

International Conference on Experimental Stress Analysis, pg. 602-603,

(Aug. 1982).

7. "Toward the Nondestructive Characterization of Fatique Damage in

Composite Materials," L. Mordfin, Damage in Composite Materials,

ASTM STP 775, pg. 7-15 (1982).

8. "Standards for Eddy Current Nondestructive Testing," G. Free, G.

Birnbaum, H. Berger, V. Kljuev, Y. Fedosenko, 10th World Conference

on Nondestructive Testing, Vol. 7-4, pg. 262-266 (1982).

9.

"NDT Measurements Traceable to NBS," H. Berger, G. Birnbaum, D. G. Eitzen,

10th World Conference on Non-destructive Testing, Vol. P, pp. 58-65 (1982).

10. "Future Directions of Ultrasonic NDE Standards in the U.S.," G. Birnbaum,

D. G. Eitzen, 10th World Conference on Non-destructive Testing, Vol. 7-5,

pp. 267-272 (1982).

11. "The Inverse Problem of Acoustic Emission -- Explicit Determination of AE

Source-Time Functions," N. Hsu and D. Eitzen, DARPA/AF Review of Quantitative

NDE, Boulder, CO, August 1981.

12.

"Traceable NDE Standards," G. Birnbaum, H. Berger, and D. Eitzen, Proc. of the

13th Symp. on Nondest. Eval , April 21-23,1981, San Antonio, TX. (Also to appear

in Materials Evaluation.).

13.

14. .

15.

16.

17.

18.

"NBS Developments in Quantitative Acoustic Emission Measurements," D. Eitzen,

F. Breckenridge, R. Clough, N. Hsu, T. Proctor, and J. Simmons, Proc. of the

DARPA/AF Review of Quant. NDE, Boulder, CO, Aug. 1981 (in press).

"Dynamic Monitoring of the Precipitation Hardening Process in 2219 Aluminum

Alloy by Means of Sound Velocity and Attenuation Measurements," M. Rosen, E.

Horowitz, S. Fick and R. Mehrabian, Proc. of the Ultrasonic Symp, Chicago,

Oct. 1981.

"Ultrasonic Standard Reference Blocks - What Future?" G. Blessing and D Eitzen, ASNT Conf. Proc., Columbus, OH, (to be published Oct. 1982).

"Calibration of Acoustic Emission Transducers:

Breckenridge, T. Watanabe, and H. Hatano, Proc.

Emission Symp., Japan, Oct. 31-Nov. 3, 1982.

Comparison of Two Methods," F.

of the 6th Int. Acoust.

“Noise in Electromagnetic Systems," U. Bertocci, 6th International Conference

on Noise in Physical Systems. NBS Special Publication 614 (1981).

"Development of In Situ Techniques for the Detection of Corrosion of Copper

Concentric Neutrals of Electric Cables in Underground Environments," E. Escalante,

U. Bertocci, J. Mullen, M. Cohen, and J. Kruger, IEEE/Power Engineering Society

1981 Transmission and Distribution Conference, September 20-25, Minneapolis,

Minnesota (1981).

176
19. "Evaluation of Methods for Characterizing Surface Topography of Models for High

Reynolds Number Wind-Tunnels," E. C. Teague, T. V. Vorburger, F. E. Scire,

S. M. Baker, S.W. Jensen, C. Trahan, and B. B. Gloss, Proceedings of the AIAA

12th Aerodynamic Testing Conference, March 22-24, 1982, Williamsburg, VA

p. 246-251.

"Elastic Constrants and Internal Friction of Fiber-reinforced Composites,"

H. M. Ledbetter, Composite Materials: Mechanics, Mechanical Properties and

Fabrication, (Japan Soc. Compos. Mater. Tokyo, 1981), p. 65-70

21. "Single-crystal Elastic Constants in Nondestructive Evaluation of Welds,"

H. M. Ledbetter, in Review of Progress in Quantitative Nondestructive Evaluation

(Plenum, New York, 1982)_.

22. "Stainless-steel Elastic Constants at Low Temperatures: A Review," H. M.

Ledbetter, Proceedings ICMC, (Kobe, May 1982).

3.

Reports

1. "Technical Activities 1981 - Office of Nondestructive Evaluation," H. Berger

and L. Mordfin, Eds., NBSIR 82-2449 (Dec. 1981).

2. "NDE Publications: 1979," L. Mordfin, Ed., NBSIR 81-2351 (Oct. 1981).

3. "NDE Publications: 1980," L. Mordfin, Ed., NBSIR 81-2364 (Oct. 1981).

4. "Fundamental Developments for Quantitative Acoustic Emission Measurements,"

D. Eitzen, F. Breckenridge, R. Clough, E. Fuller, N. Hsu, and J. Simmons, EPRI

NP-2089, Proj. 608-1, Palo Alto, CA: Electric Power Research Institute, Oct.

1981, 242 p.

5. "Electric Power Research Institute/National Bureau of Standards Joint Program

on Acoustic Emission," D. Eitzen and J. Quinn RP608-3, Sect. 35 in

Nondestructive Evaluation Program Progress in 1981. Coe Corp., eds.,

NP-2088-SR. Palo Alto, CA: Electric Power Research Institute, Dec. 1981,

35-1 - 35-18.

6. "Summary of Fundamental Developments for Quantitative Acoustic Emission

Measurements," D. Eitzen, F. Breckenridge, R. Clough, E. Fuller, N. Hsu, and

J. Simmons, EPRI NP-1877, Res. Proj. 608-1. Palo Alto, CA: Electric Power

Research Institute, June 1981, 86 p.

7. "Ultrasonic Research Summary Report and Literature Guide to the National

Bureau of Standards/Office of Naval Research Program," M. Greenspan and D.

Eitzen, Nat. Bur. Stand. (U.S.), NBSIR 82-2529, June 1982.

8. "Ultrasonic Measurements of Titanium 6211 Wield and Plate," G. Blessing, Nat.

Bur. Stand. (U.S.) NBSIR 82-2500, May 1982.

177
9. "Nondestructive Characterization of the Precipitation Hardness Process in

Aluminum Alloys, M. Rosen, S. Fick, C. Friant, and E. Horowitz, Johns Hopkins

U., Center for Matls. Res. Report No. CMR-NDE-2, Sept. 1981, 49 p.

10. "Correlation of Ultrasonic Data with Hardness after Aging 2024 Aluminum Alloy Under T4, TR351, and T851 Conditions," M. Rosen, S. Fick and E. Hororowitz,

Johns Hopkins U., Center for Matls. Res. Report No. CMR-NDE-4, Nov. 1981.

"Magnetostructively-Driven Ultrasonic Device for Sound Velocity Measurements

in Thin Ribbons," F. Nadeau, S. Fick, M. Rosen and E. Horowitz, Johns Hopkins

U., Center for Matls. Res. Report No. CMR-NDE-3, Nov. 1981.

12. "Integral Equations for Transient Electromagnetic Fields," Egon Marx, NBS Technical

Note 1157, February 1982.

178
NBS-114A (REV. 2-80) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR-82-2617

2. Performing Organ. Report No, 3. Publication Date

December 1982

4. TITLE AND SUBTITLE

Technical Activities, 1982, Office of Nondestructive Evaluation

5. AUTHOR(S)

H. T. Yolken

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State. ZIP)

10.

SUPPLEMENTARY NOTES

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

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

A review of nondestructive evaluation programs at NBS, for FY1982 is

presented in this annual report.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Acoustic emission, eddy currents, imaging, leakage testing, magnetics, material

parameters, nondestructive evaluation, optics, penetrants, radiography, and

ultrasonics.

13. AVAILABILITY

|X | Unlimited

| | For Official Distribution. Do Not Release to NTIS

| Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

3T] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

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