NBSIR 78-1433

Low Velocity Performance of a

Bronze Bearing Vane

Anemometer

Prepared for United States Department of the Interior

Bureau of Mines

by

L. P. Purtell National Bureau of Standards

Fluid Mechanics Section

Washington, D C. 20234

February 1978

Task Report

on

Contract No. HOI 661 98

Evaluation of the BEhavior of Mine Anemometers in the NBS Low

Velocity Calibration Facility
DISCLAIMER NOTICE

The views and conclusions contained in this document should not be

interpreted as necessarily representing the official policies or recommendations of the Interior Department's Bureau of Mines
NBSIR 78-1433

LOW VELOCITY PERFORMANCE OF A

BRONZE BEARING VANE

ANEMOMETER

L. P. Purtell

Fluid Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D C. 20234

February 1978

Task Report

Prepared for United States Department of the Interior

Bureau of Mines

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

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science end Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

FOREWORD

This report was prepared by the National Bureau of Standards, Fluid

Mechanics Section, Washington, D. C. 20234, under USBM Contract

Number H0166198. The contract was initiated under the Coal Mine

Health and Safety Program. It was administered under the technical

direction of PM&SRC, with Dr. George H. Schnakenberg, Jr., acting as

the Technical Project Officer. Mr. H. R. Eveland was the contract

administrator for the Bureau of Mines.

This report is a summary of the work recently completed as part of

this contract during the period December 1, 1976 to February 28,

1977. This report was submitted by the author February 1978.

2
'

f V.-
LIST OF SYMBOLS

u velocity measured by laser velocimeter

u

i velocity indicated by anemometer under test

u

if curve fitted to U, U. data

l

m,a,U*,U

o constants involved in curve fit, U . .. if

U group mean velocity

0 standard deviation of U. data from U .

.

l if

0

c a adjusted for known variance in laser velocimeter

measurements (see Appendix)

U ,U. o io

a point on U_^(U)

U ,U. r lr an actual measurement

6U

r U - Ur o 6U,ir

u. - u, ir io

s r (see Appendix)

0 u standard deviation of laser velocimeter measurements

3
LOW VELOCITY PERFORMANCE OF A BRONZE BEARING

VANE ANEMOMETER

L. P. Purtell

1 . INTRODUCTION

The National Bureau of Standards in order to meet the need for a calibration capability with adequate accuracy at low air velocities,

i.e., below 500 feet per minute (fpm) undertook the development of a low-velocity calibration facility for wind speed measuring instruments

which would provide a capability down to 3 meters per minute (approximately 10 fpm) with an accuracy of plus or minus one percent. It was

a natural consequence therefore that when said facility became operational to undertake an evaluation of the state-of-the-art, and to

provide the information needed as to the reliability and performance of

instrumentation for such measurement. Accordingly, a number of prototypes of various types of instruments for low velocity air measurements

are undergoing test at NBS, and this report is concerned specifically

with the results of one such test.

2. TEE INSTRUMENT

The rotary vane anemometer tested for this report is a commercially

available instrument (Davis Instrument Manufacturing Co., Inc., 4-Inch

Low Speed Anemometer, S/N 22889) 1 used in the mining industry and elsewhere

as a portable anemometer. It was supplied for test by the U.S. Mining

Enforcement and Safety Administration (MESA) at the request of the U.S.

Bureau of Mines. The housing is 4 inches in diameter and 1-3/4 inches

deep (Figure 1) . Thin metal vanes without camber or twist mounted on arms drive a rotor linked to a dial indicator by a gear train. The

bearings in this particular instrument are standard bronze bearings (as

opposed to ball bearings). One revolution on the dial represents an indicated passage of 100 feet of air through the instrument. Thus an external timer (not a part of the anemometer) is required to complete a measurement of velocity (an average velocity for the duration of the

measurement)

1 This particular instrument was selected as being representative of

this type of anemometer and its selection does not represent an endorsement

4
3. THE TESTS

The NBS Low Velocity Airflow Facility [1] 2 used to test this

instrument generates a low velocity air stream having a low turbulence

intensity (less than 0.05%) and a large region of uniform flow (at

least 75 x 75 cm). A laser velocimeter is employed as a primary velocity

standard. In addition, it is nonintrusive , has a linear response with

velocity, and has good spatial resolution. Adequate sensitivity is

obtained without the artificial seeding of scattering particles. Thus

the difficulties and inconvenience associated with seeding and the

possible effect of such seeding on the performance of the device under

test are avoided.

The vane anemometer was mounted on the centerline of the tunnel

test section one meter downstream of the entrance to the test section

in a manner to minimize the effect of the support on the air stream

around the anemometer (Figure 1). Since the anemometer itself modifies

the airflow in the tunnel, the velocity should be measured at a location

in the flow which has the same velocity in the presence of the anemometer

as it does in the absence of the anemometer. Thus the velocity upstream

of the anemometer on the centerline was measured to find the position

where deceleration of the flow due to the presence of the anemometer was

no longer perceptible. These measurements were performed at two freestream speeds, 700 and 72 fpm, and as predicted by ideal flow theory

the variation of the ratio of the local velocity to the free-stream

velocity with distance upstream of the anemometer is independent of

free-stream velocity (Figure 2) . A distance of 30 cm upstream of the

anemometer was chosen as the position for velocity measurement by the

laser velocimeter. With no anemometer in the tunnel, variation in

velocity along the centerline is imperceptible over the distance

traversed (30 cm)

.

The air speed indicated by the vane anemometer was computed from

initial and final readings of the dial and of the associated time interval

(around two minutes) . The anemometer runs continuously in the tunnel

since it cannot be accessed while the tunnel is in operation without

disturbing the flow. Thus the readings of the anemometer were performed

with the anemometer in operation. The laser velocimeter measurement of

the air velocity was performed during the time interval for reading the vane anemometer. Five separate test runs were made, each consisting of

about nine such measurements over the range 64 to 690 fpm. The lower

velocity was limited by the starting and stopping speeds of the

instrument. The data are presented in chronological order in Tables

1A to IE.

2 Figures in brackets represent literature references at the end of

this paper.

5
To determine the starting speeds of the instrument, the velocity

in the tunnel was increased from below the starting speed at a smooth

acceleration of approximately 30 fpm/min until movement of the vanes

could be detected by eye. At that moment the air velocity would be

fixed and the laser velocimeter measurements initiated. If the

anemometer continued rotating for at least thirty seconds and did not

decelerate, the measurement of velocity by the laser velocimeter was recorded as the starting speed. Ten such measurements are presented

in Table 2 and have an average of 63.3 fpm and a standard deviation of

3.3 fpm.

Because of the anemometer’s angular momentum, stopping speed is more difficult to determine than starting speed. Some preliminary runs indicated that a two minute interval between reductions in air velocity

of approximately 2 fpm was sufficient for the anemometer to come to

rest if the stopping speed had been reached. Ten such measurements

are presented in Table 3 with an average of 54 fpm and a standard

deviation of 2 fpm.

4 . TEST RESULTS

Since a particular air speed in the wind tunnel cannot be exactly

reset from run to run, scatter in the test data is distributed along a curve, thus prohibiting computing the standard deviation of the data

from a simple average. Instead, deviations from a curve fit to the

data were computed and the standard deviation defined to be the r.m.s. value of a group of these deviations. To accomplish this, an expression

was chosen to approximate the relation between the true and indicated

velocities which reflected the linear relationship at the higher speeds

but allowed for deviation from the linear at the lower speeds. This

expression and the four parameters it contains were developed solely

for the statistical analysis of the data and are not meant to represent

any characteristics of the instrument's performance.

A curve of the form

U

if = m(U - U*) { 1 - exp [-a (U - U

q

) ] } (1)

was fit to the data, where U is the true velocity, is the fitted

indicated velocity, and m, a, U* , and U

Q are constants to be determined.

Since asymptotically approaches the line

m(U - U*)

at the higher speeds above 150 fpm, a linear least-squares fit of the

data was employed to determine m and U*. UD and a were then chosen to

6
give a good fit (by eye) to the two lowest groups of data (U < 150 fpm)

.

The resulting values are

m = 1.10

U* = 20.6 fpm

U =53 fpm

oa = .0431 fpm~l

One data point, the first in Table IE, was omitted from these calculations because it lies outside the 3a limit. No explanation for this

large deviation can be offered. The fitted curve and the data are presented in Figure 3.

To determine the standard deviation, a, of the indicated velocity, U.,

about the fitted curve, U.

f

, the squared differences between the U data

and U

f

, i.e. [U (U) - U

, f tu)] 2

, were computed and grouped in the following

five groups

1 1

U < 80 fpm

80 fpm < U < 100 fpm

100 fpm < U < 300 fpm

300 fpm < U < 600 fpm

600 fpm < U.

Since the groups are reasonably compact (small range of U within a group; see Figure 3.), the mean of the squared differences within a group was taken as an estimate of the variance of about U^f within that

group and specified at that group's mean true velocity, U. The square

roots of the estimates, a,_are presented in Figure 4 as velocity and

Figure 5 as percentage of U. Since ± 2 a is extremely close to the 95 percent

confidence interval for one measurement, curves of ± 2a are also included in

Figure 3 as dashed lines.

The scatter in the velocity data includes contributions from the

laser velocimeter as well as the anemometer; however, an estimate of the

standard deviation of the anemometer measurements may be obtained by

accounting for the standard deviation of the laser velocimeter data

estimated from repeated measurements of velocity at a particular fan

setting, thus also including any unsteadiness in the velocity. As shown

in the Appendix, the corrected variance for the anemometer o

2

, may be

computed from

7
a ( 2 ) c

2

> 2 - 0

3U

if

3U ) °

c

2

where ouis the standard deviation of the laser velocimeter measurements,

i.e. .00511 over the range of velocity measured herein. The dashed curves

in Figures 4 and 5 show the values of cr and o /U, respectively. The

adjustment in a is small at the lower velocities indicating that the scatter

at the very low velocities is almost entirely due to the anemometer.

The actual differences between the true and indicated velocities,

U - U^, are presented in Figure 6 and as a percentage of U in Figure 7. The curves shown in each figure have been fitted to the data by using equation (1).

5.

DISCUSSION OF RESULTS

The instrument performed as intended with no erratic behavior. The

starting and stopping speeds were quite consistent considering the difficulty

in determining them with precision. Some general comments concerning application of the instrument follow. With any measurement problem the instrument's capabilities should be matched to the required measurement.

This anemometer is intrusive, i.e., it must be placed in the flow.

This anemometer is entirely mechanical and does not require an outside

source of power.

Many other factors that can affect the suitability of an instrument

for a particular application, such as turbulence or unsteadiness of the air

stream, rough handling (shock and vibration), dirt and other environmental

factors, time, orientation to the velocity and gravity vectors, etc., have

not been tested herein but should be considered.

6SUMMARY

The performance of a 4-inch diameter low speed vane anemometer with

bronze bearings has been evaluated, including starting speed and stopping

speed, at air speeds up to 690 fpm.

The starting and stopping speed measurements are presented and give

an average starting speed of 63.3 fpm and an average stopping speed of

54 fpm.

7

REFERENCES

1. L. P. Purtell and P. S. Klebanoff, The NBS Low Velocity Airflow

Facility, in preparation.

8
APPENDIX

Correction in a for Variance in Laser Velocimeter Measurements

If a great number of velocity measurements were made by the

anemometer under test, U-j^, and the laser velocimeter, U, a curve, U.^(U)

could be drawn through a cross-plot of the data, eg., by a least-squares

fit, representing the expected values of pairs of such measurements.

Thus at a particular setting of the air velocity in the tunnel an expected pair of measurements, U0 and U^0 , could be determined from the curve. Actual velocity measurements by the two instruments, U and U^,

would deviate randomly from the expected values. The r th of N such

measurements could be written

U = U + 6U

r o r U. = U. + 6U.

lr 10 lr

where 6U r is the deviation of Ur from UQ and 6U-^ r is the deviation of Ui r from U-^ 0

. For small deviations from the expected values a one-term

Taylor expansion of U-^f(U) about the point (UQ , UiQ ) leads to (see

figure below)

s r

3U

if

6U

ir

- 'irl 6U

r

where s is the measured deviation of the data point from the fitted curves

(approximately) and (9U-^f/3U) 0 is (3U^f/3U) evaluated at U

Q

.

9
Thus

r =

> 6U.

2 + f^if1 z 6U 2 - 261J rsu.^ if

irJ [ 8U J o

r lr 3U k / 6U

Summing N such expressions and dividing by N gives

2 N , .2

, N 2 t M

i ?

s r i y NS N

^

r=»l r=l

6U

lr + & i

j 3U N

L . 1 r=l

6U

- 2 3U.*

if

3U

N

h \ au, 6U

N r=l ir r

Since the deviations in the two instruments are independent, the last

term on the right is zero. The term on the left side has been defined

in this report as a

2 (within the Taylor expansion approximation) . The

first term on the right is the corrected value of the variance in

anemometer measurements, a

2 (which would equal a2 if the laser velocimeter had no scatter at al5) , and the summation divided by N in the

second term on the right is the known variance in laser velocimeter

measurements, a

2

. Thus the variance in the anemometer measurements

corrected for a^nown variance in the laser velocimeter measurements is

'3U a

2 = a

2 -

c if

3U

It should be noted that if 3U ^/3U = 1 (approximately

instances) the usual formula involving the difference

obtained

true in some

in variances is

10
Table 1A

Davis Vane Anemometer

Serial No. 22889

Indicated Air Speed,

f pm

True Air Speed,

fpm

730.0

730.0

208.0

479.0

67.8

679.0

687.0

208.0

458.0

94.2

21.0

26.4

28.8

63.6

63.0

76.2

75.0

72.0

94.2

89.4

T = 18.8 to 20.0 °C

B

=' 763.1 mm Hg

Table IB

Davis Vane Anemometer

Serial No. 22889

Indicated Air Speed,

fpm

»•

True Air Speed,

fpm

732.0

208.0

478.0

67.2

37.8

679.0

208.0

451.0

91.8

73.2

37.2

28.8

60.0

63.6

65.4

64.8

90.0

91.2

T = 20.1 to 21.0 °C

B = 763.1 mm Hg

11
Table 1C

Davis Vane Anemometer

Serial No. 22889

Indicated Air Speed,

f pm

True Air Speed

fpm

732.0

211.0

482.0

70.2

39.3

690.0

207.0

458.0

94.8

72.0

36.5

29.6

69.6

68.4

73.2

76.2

96.0

96.6

T = 21.0 °C

B = 755.0 mm Hg

Table ID

Davis Vane Anemometer

Serial No. 22889

Indicated Air Speed, True Air Speed

fpm fpm

735.0 687.0

210.0 214.0

481.0 460.0

69.6 94.2

39.1 75.6

44.0 76.8

45.3 75.6

70.8 96.6

70.8 96.6

T = 21.6 °C

B = 764.0 mm Hg

12
Table IE

Davis Vane Anemometer

Serial No . 22889

Indicated Air Speed,

f pm

True Air Speed,

fpm

710.0

208.0

479.0

67.8

25 .

8

688 .

0

215 . 458 . 95 .

4

74 .

39 .

1

40.9

64 .

8

74 .

4

75 .

0

94 .

2

T

B 21.6 °C

764.0 mm Hg
Table 2

Davis Vane Anemometer

Serial No. 22889

Starting Speed,

fpm

67.2

63.6

63.6

56.4

66.0

Average Starting Speed

63.3 fpm

63.6

66.0

63.6

58.8

64.2

Standard Deviation

3.3 fpm

Table 3

Stopping Speed,

fpm

Davis Vane Anemometer

Serial No. 22889

55.7

51.7

54.1

53.4

57.5

Average Stopping Speed

54 fpm

57.4

51.6

55.8

53.5

54.0

Standard Deviation

2 fpm

14
FIGURE 1. THE ANEMOMETER MOUNTED IN THE TUNNEL.

15
CSI oCNmm

P4WHWaozcao

OeS

PiH

to

CM

S3wuzcH

CO

Z|Z

16

FIGURE

2.

VARIATION

IN

VELOCITY

WITH

DISTANCE

UPSTREAM

FROM

ANEMOMETER
FIGURE 3. INDICATED VERSUS TRUE VELOCITY WITH ± 2a CURVES.

17
I

I

18

FIGURE

4.

STANDARD

DEVIATION,

o,

AND

CORRECTED

STANDARD

DEVIATION

OF

INDICATED

VELOCITY.
uD

D

U P

S'? d|

19

FIGURE

5.

STANDARD

DEVIATION,

a,

AND

CORRECTED

STANDARD

DEVIATION,

AS

PERCENT

OF

GROUP

MEAN

VELOCITY.
I I I I

£5

I a

o.

20

FIGURE

6.

DEVIATION

OF

INDICATED

VELOCITY

FROM

TRUE

VELOCITY.
n

U

(fpm)

FIGURE

7.

PERCENT

DEVIATION

OF

INDICATED

VELOCITY

FROM

TRUE

VELOCITY

.
NBS114A in t v r- > ,i U S. DEPT OF COMM

BIBLIOGRAPHIC DATA

SHEET

1. I’l HI l< A I ION OK l< I l’o|< I NO. NBSIR 78-14332. C Ml V *1 Al l l- SS loll No.

4. I l l I I AND SHUT I I l.l

LOW VELOCITY PERFORMANCE OF A BRONZE BEARING

VANE ANEMOMETER

7. A UTHOR(S)

L. P. Purtell

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

3. Rl l ipiCIIt’s At IT .Moll V

5. Publ ii .it ion Date February 1978

6. Performing Organization < «u]t

8 . Performing Organ. Report Ni NBSIR 78-1433

10. Protect .'Tii.sk Work Unit Ni 2130483

11. < ontracl/Grant No. H0166198

12. Sponsoring Organization Name and C omplete Address (Street , City, State, ZIP)

Office of the Assistant Director - Mining

Bureau of Mines

United States Department of the Interior

Washington, D.C. 20241

13. l ype of Report & Period

( overed Dec. 1 , 1976-Feb . 28,197

14. Sponsoring Agency C

15. SI PPI.EMENTARY NOTES

16. A PS I RA< I (A 200- word or less factual summary of most significant information If document includes a significant

bibliography or literature survey, mention it here.)

Performance of a bronze bearing vane anemometer is evaluated over the speed range

of 64 to 690 feet per minute including starting speed and stopping speed. The tests were performed in the NBS Low Velocity Airflow Facility which provides a uniform flow

of low turbulence and utilizes a laser velocimeter as the velocity standard.

17. KEY WORDS (six to twelve entries, alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons ) Airflow; anemometer; laser velocimeter; low velocity; performance; vane anemometer;

wind tunnel.

18. AVAILABILITY [xk Unlimited

E or Official Distribution. Do Not Release to NT IS

[

. Order Erom Sup. of Doc., U.S. Government Printing Office Washington, D.C . 20402, SD C at. No. Cl C

jj^kTrder Erom National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SEC URITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

24

22. P

USCOMM-DC Z 90 4 2 P 7 *
NBSIR 78-1434

The Macroscopic Detection of

Corrosion in Aluminum Aircraft

Structures With Thermal

Neutron Beams and Film

Imaging Methods

Report Prepared by

Donald A. Garrett

Reactor Radiation Division

Institute for Materials Research

National Bureau of Standards

Washington, D C. 20234

Work Performed by

William Parker, Ph D. Martin Ganoczy

Michael Dorsey

Donald A. Garett

December 7, 1 977

A Report

Submitted to the U.S. Naval Air

System Command, Washington, D.C.

USN Contract No. N0001 9-76-IP-69007

NBSIR 78-1434

THE MACROSCOPIC DETECTION OF

CORROSION IN ALUMINUM AIRCRAFT

STRUCTURES WITH THERMAL

NEUTRON BEAMS AND FILM

IMAGING METHODS

Report Prepared by

Donald A. Garrett

Reactor Radiation Division

Institute for Materials Research

National Bureau of Standards

Washington, D C. 20234

Work Performed by

William Parker, Ph.D.

Martin Ganoczy

Michael Dorsey

Donald A. Garrett

December 7, 1 977

A Report

Submitted to the U S. Naval Air

System Command, Washington, D.C.

USN Contract No. N0001 9-76-IP-69007

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

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director
'
Table of Contents

Page

Abstract 1

1.0 Objectives 2

2.0 Background 2

3.0 Experimental Facilities at NBS 3

4.0 Experimental Procedures: General 3

5.0 Technical Approach < 6

6.0 Exposure Series 1 Through 3: General 6

6.1-

A Exposure Series 1A 9

6.1-

B Exposure Series IB 12

6.1-

C Exposure Series 1: Results and Conclusions 12

6.2-

A Exposure Series 2A 15

6.2-

B Exposure Series 2B 16

6.2-

C Exposure Series 2: Results and Conclusions 21

6.3-

A Exposure Series 3A 21

6.3-

B Exposure Series 3B 22

6.3-

C Exposure Series 3: Results and Conclusions 25

6.4-

A Exposure Series 4 28

6.4-

B Exposure Series 4: Results and Conclusions 29

7.0 Quantification of Corrosion 32

8.0 Remarks and Qualifications 32

9.0 Summary 33

10.0 Recommendations for Future Investigations at NBS 34

10.1 Converter Development 34

10.2 Real Time Imaging Methods for Corrosion Detection 35

10.3 Corrosion Quantification - Standard Penetrameter Development 36

Tables

Page

Table 1A Exposure Series 1A: Radiographic Parameters 9

Table IB Exposure Series IB: Radiographic Parameters 12

Table 2A Exposure Series 2A: Radiographic Parameters 16

Table 2B Exposure Series 2B: Radiographic Parameters 16

Table 3A Exposure Series 3A: Radiographic Parameters 22

Table 3B Exposure Series 3B: Radiographic Parameters 25

Table 4 Exposure Series 4: Radiographic Parameters 28

Table 5 Penetrameter Calibration 29
.4
Figures

Page

Fig. 1 Schematic Diagram of the NBS Thermal Neutron Radiography

Facility 4

Fig. 2 C-2 Aircraft Vertical Fin Section 5

Fig. 3A Aluminum Corrosi on Penetrameter P-1 7

Fig. 3B A1 umi num Corrosion Penetrameter P-1 - Close Up View 7

Fig. 4 A1 umi num Corrosi on Penetrameters P-2-1 through P-2-4 8

Fig. 5 Exposure Series 1A: Radiographic Data 10-11

Fig. 6 Exposure Series IB: Radiographic Data 13-14

Fig. 7 Exposure Series 2A: Radiographic Data 17-18

Fig. 8 Exposure Series 2B: Radiographic Data 19-20

Fig. 9 Exposure Series 3A: Radiographic Data 23-24

Fig. 10 Exposure Series 3B: Radiographic Data 26-27

Fig. 11 Exposure Series 4: Radiographic Data 30-31

Abstract

The primary objective of this investigation was to determine the

feasibility of detecting corrosion in aluminum Naval aircraft components

with neutron radiographic interrogation and the use of standard corrosion penetrameters. Secondary objectives included the determination

of the effect of object thickness on image quality, the defining of

minimum levels of detectability and a preliminary investigation of a means

whereby the degree of corrosion could be quantified with neutron radiographic data.

All objectives were met and may be summarized as follows: (1)

Envi ronment- i nduced corrosion can be detected with high sensitivity by

thermal neutron radiographic interrogation, (2) the fluence at the image

plane required to visualize corrosion with conventional imaging methods

e.g., a gadolinium converter in conjunction with a medium contrast

8 - 2

industrial x-ray film at density of 2.5, is approximately 4 x 10 n-cm .

Assuming that a transportable neutron radiography system were capable of

4 -2 -1

producing a f 1 ux of 1 . 5 x 10 n-cm -sec at an L/D ratio of 40:1, an

exposure of 7.4 hours would be required, (3) at an L/D ratio of 40:1,

the corrosion signatures of both surfaces of a thick object, e.g., a

wing or airfoil, must be interrogated individually. This is due to the

fact that geometrical unsharpness obliterates signature detail on the

surface opposite the cassette. (4) The possibility of corrosion quantification does exist. This conclusion is based on an investigation

with standard corrosion plaque penetrameters , and (5) although the use

of ^LiF/ZnS light emitting converters is the most efficient method by

which thermal neutron beams can be imaged, the results are inadequate

for unambiguous signature analysis with presently available converters.

1
.
1 . 0 Objectives

The primary objective of these investigations was to determine the

feasibility of detecting corrosion in aluminum Naval aircraft components

with neutron radiographic interrogation and the use of standard corrosion

penetrameters. Secondary objectives included the determination of

the effect of object thickness on corrosion image quality, the defining of

minimum levels of detectability and a preliminary investigation of a

means whereby the degree of corrosion could be quantified with neutron

radiographic data.

2. 0 Background

The nondestructive detection and visualization of environmentinduced corrosion in aircraft presents an important maintenance problem

and economic impact in both the civilian and military sectors. Since

there is no reliable instrumental method by which corrosion can be

detected in inaccessible areas of aircraft structures, visual and destructive

inspection methods must presently be employed.

Corrosion in aircraft generally is induced by the presence of

moisture and oxygen. The corrosion products so generated oftentimes,

but not always, contain hydrogen in the forms of hydrated oxides or

hydroxides. In the case of salt-water induced corrosion, chlorine may

also be present. Because of the high attenuation of thermal neutrons by

hydrogen and chlorine, it should be possible to visualize corrosion

areas using neutron radiographic interrogation.

The basic procedures governing the neutron radiographic inspection

methods employed in these studies are outlined in this report. In

addition, the results of preliminary experiments at the National Bureau

O
.

:

- *

'

.
of Standards to establish minimum levels of corrosion detectability and

proposed methods to quantify the degree of corrosion are presented.

3.0 Experimental Facilities at the National Bureau of Standards

The Thermal Neutron Radiography Facility (TNRF) at the NBS 10MW

Research Reactor was employed for these investigations. At a power

7 -2

level of 10MW, the reactor provided a thermal neutron flux of 10 n-cm

-sec at the image plane for an L/D ratio of 40:1. The L/D ratio of

the facility was variable from a minimum of 40:1 to a maximum of 500:1.

The neutron beam is highly thermal i zed, having a gold-cadmium ratio of

500:1. During the course of these studies, the neutron beam shutter was

manually operated. A schematic diagram of the facility is illustrated

in Figure T.

4. 0 Experimental Procedures: General

All radiographic data generated in these investigations was obtained

using direct neutron imaging methods. Manual film development was

performed throughout with Eastman Kodak standard industrial radiography

chemistry. A 7 in. -thick section cut from an uncorroded C-2 aircraft

vertical fin was used as a standard object to be radiographed. The fin,

which was obtained from the NAVAIR Rework Facility at North Island, San

Diego, CA is illustrated in Figure 2.

Standard corrosion penetrameters were fabricated from aircraftgrade aluminum which had been corroded under controlled environments for

predetermined periods of time. The penetrameter samples were obtained

from the Naval Air Development Center, Warminster, PA. Two types of

penetrameters were employed in these investigations.

3
- • .. I n ,
Bismuth

Gamma

Shield

CD

S-3

ro

S-Ola

<T3 S-o

CD

- 4 -

FIGURE

1.

SCHEMATIC

DIAGRAM

OF

THE

NBS

THERMAL

NEUTRON

RADIOGRAPHY

FACILITY

- 5 -

The first was fabricated from aircraft-grade aluminum which had

been subjected to a super-saturated salt water spray for 6.5 hrs.

Nominal step thicknesses were 0.125 in. and 0.87 in. Two views of this

penetrameter are illustrated in Figures 3A and 3B. This penetrameter

will be referred to as P-1 in this report.

The second set of four penetrameters was fabricated from 0.062 in.-

thick aircraft-grade aluminum plates which had been subjected to a salt

water spray/SC^ gas environment for zero, four, eight and fifteen days.

This set which will be referred to as P-2-1 through P-2-4 in this

report, is illustrated in Figure 4. No effort was made to correlate the

corrosion penetrameters with real life conditions. They were employed

only to provide corrosion under standardized conditions.

5. 0 Technical Approach

The tasks outlined in the program objectives were divided into four

systematic experiments, each with one or more objectives. The objectives

of each exposure series at the radiographic parameters employed, and the

experimental results are illustrated.

6. 0 Exposure Series 1 Through 3: General

The general objectives of this series of exposures were:

(1) To determine the feasibility of utilizing neutron radiographic

techniques to visualize corrosion areas in aircraft-grade

aluminum employing several thermal neutron film-converter

imaging techniques and a standard corrosion penetrameter.

(2) To assess the effect of object thickness on the penetrameter

image quality.

(3) To observe the effect of analog edge enhancement on the

penetrameter image quality.

6
'
FIGURE 3A. ALUMINUM CORROSION PENETRAMETER

FIGURE 3B. ALUMINUM CORROSION PENETRAMETER CLOSE-UP VIEW

- 7 -

-8 -

In exposure Series 1 through 3, the penetrameter , object and geometry

outlined below are identical, only the radiographic parameters are

changed in each series. Radiographic exposures were made of:

(A) The penetrameter P-1 alone.

(B) The C -2 vertical fin section alone.

(C) The 02 vertical fin section with penetrameter P-1 facing the

cassette.

(D) The 02 vertical fin with the penetrameter P-1 facing the

collimated neutron source.

(E) Analog edge enhanced image of C above.

(F) Analog edge enhanced image of D above.

6.1-A Exposure Series 1

A

The radiographic parameters employed in the Exposure Series 1A are

outlined in Table 1A below. This series of exposures serves to illustrate

the radiographic data obtained utilizing a reactor-based system having a

high (200:1) L/D ratio and employing medium-contrast (Kodak Type M)

film. The radiographic data are illustrated in Figures 5A through 5F.

Table 1A: Exposure Series 1A: Radiographic Parameters

Converter 0.0005 in. Vapor Deposited Gd

Film Type Kodak Type M

L/D Ratio 200:

1

3

Exposure Time/Fluence 50 min./6 x 10 n-cm

Neutron Beam Cd Ratio 500:

1

Penetrameter P-1

Object C-2 Vertical Fin Section

9
•• -4

‘
Fig. 5: Exposure Series 1A: Radionraphic Data

PENETRAMETER C-2 VERTICAL FIN

(A) (B)

C-2 VERTICAL FIN

PENETRAMETER FACING CASSETTE

(C)

C-2 VERTICAL FIN

PENETRAMETER FACING SOURCE

(D)

- 10 -

Fiq. 5: Continued

ANALOG EDGE ENHANCEMENT PENETRAMETER

FACING CASSETTE

(E)

:.****

ANALOG EDGE ENHANCEMENT PENETRAMETER

FACING SOURCE

(E)

- 11 -

6. 1-B Exposure Series 1

B

The objective of this series of exposures was to determine the

effect on the penetrameter image quality when the L/D ratio was reduced

from 200:1 to 43:1. These experiments provided comparative data of the

results obtainable with a reactor-based system having a high L/D ratio

to a source-based system having a low L/D ratio.

Radiographic parameters employed in Exposure Series IB are outlined

in Table IB below.

Table IB: Exposure Series IB: Radiographic Parameters

Converter 0.0005 in. Vapor Deposited Gd

Film Type Kodak Type M

L/D Ratio 43:

1

Exposure Time/Fluence 90 sec./4 x 10® n-cm ^ (1)

Neutron Beam Cd Ratio 500:

1

Penetrameter P-1

Object C-2 Vertical Fin Section

The radiographic data derived from Exposure Series IB are illustrated

in Figures 6A through 6F.

(1) Significant Parameter Change

6.1-C Exposure Series : Results and Conclusions

(a) Salt water induced corrosion signatures in aircraft-grade aluminum

can be visualized with neutron radiography using a 200:1 L/D ratio.

The effect of the 7 in. -thick fin thickness is to degrade the image

quality. However, the signature can still be discerned as illustrated

in Figures 5C and 5D.

12
«
Fig. 6: Exposure Series IB: Radiograohic Data

PENETRAMETER

(A)

C-2 VERTICAL FIN

PENETRAMETER FACING CASSETTE

(C)

C-2 VERTICAL FIN

PENETRAMETER FACING SOURCE

- 13 - (D)

Fig. 6: Continued

ANALOG EDGE ENHANCEMENT PENETRAMETER

FACING CASSETTE

(E)

ANALOG EDGE ENHANCEMENT PENETRAMETER

FACING SOURCE

(F)

(b) The time required for the exposure was 50 min. for a L/D ratio of

200 : 1 .

(c) With the penetrameter on either side of the fin, analog edge

enhancement aids in identifying the corrosion signature.

(d) When the L/D ratio is reduced to that of a field system, e.g.,

43:1, The corrosion signature can only be visualized when the

penetrameter was placed on the cassette side of the fin as shown

in Figures 6C and 6D, making it necessary to inspect each side of

an airfoil of 7 in. thickness individually.

(e) The time required for the exposure was 90 sec. for a L/D ratio

of 43:1.

(f) With a L/D ratio of 43:1, image edge enhancement aids in

visualizing the corrosion penetrameter when placed on the

source side of the film, however, paint could possibly

produce a similar signature.

6.2-A Exposure Series 2A

In the group 2 series of exposures the 0.0005 in. -thick Gd converter

has been replaced by a Gd£02S converter. The radiographic parameters

employed in this series of exposures is given in Table 2A below.

- 15 -

Table 2A: Exposure Series 2A: Radiographic Parameters

Converter

Film Type

L/D Ratio

Exposure Time/Fluence

Neutron Beam Cd Ratio

Penetrameter

Object

Gd

2

0

2

S ( 2 )

Kodak Type M

200:

3

15 min./2 x 10 n-cm

500: 1

P-1

C-2 Vertical Fin

-2

( 2 )

The radiographic data resulting from exposure series 2A are

illustrated in Figures 7A through 7F.

(2) Significant Parameter Change

6.2-B Exposure Series 2B

The objective of Exposure 2B was to examine the radiographic penetrameter P-1 using a Gd

2

0

2

S converter, when the L/D ratio was reduced

from 200:1 to 43:1. The radiographic parameters employed in this exposure

series are outlined in Table 2B below.

Table 2B: Exposure Series 2B: Radiographic Parameters

Converter

Film Type

L/D Ratio

Exposure Time/Fluence

Neutron Beam Cd Ratio

Penetrameter

Object

The radiographic data from thi

Figures 8A through 8F.

Gd

2

0

2

S

Kodak Type M

43:1 (3)

45 sec./2 x 10^ n-cm ^ (3)

500:

P-1

C-2 Vertical Fin Section

exposure series are illustrated in

(3) Significant Parameter Changes

Fig. 7: Exposure Series 2A Radiogranhic Data

PENETRAMETER

(A)

C-2 VERTICAL FIN

PENETRAMETER FACING CASSETTE

(C)

C-2 VERTICAL FIN

(B)

C-2 VERTICAL FIN

PENETRAMETER FACING SOURCE

(D)

- 17 -

Fig. 7: Continued

- 18 -

Fig. 8: Exposure Series 2B: Radiographic Data

PENETRAMETER

(A)

C-2 VERTICAL FIN

(B)

C-2 VERTICAL FIN

PENETRAMETER FACING CASSETTE

(C)

C-2 VERTICAL FIN

PENETRAMETER FACING SOURCE

(D)

- 19 -

Fig. 8

:

Conti nued

ANALOG EDGE ENHANCEMENT PENETRAMETER

FACING CASSETTE

(E)

'I

ANALOG EDGE ENHANCEMENT PENETRAMETER

FACING SOURCE

(E)

- 20 -

6.2-

C Exposure Series 2: Results and Conclusions

(a) Corrosion signatures can be visualized using Gd^O^S converters

with the penetrameter placed on either side of the 7 in. -thick

fin, using an L/D ratio of 200:1. The apparent contrast was

not as high as that obtained with Gd foil, as illustrated in

Figures 7C and 7D.

(b) The exposure time required using a Gd£02S converter was 15

min. for a L/D ratio of 200:1.

(c) Image edge enhancement aided in identifying the corrosion

signature on either side of the fin with a L/D ratio of 200:1.

(d) When the L/D ratio was reduced to 43:1 using a Gd202S converter,

the corrosion signature could only be visualized with the

penetrameter placed between the cassette and the fin as

illustrated in Figure 8C. All image detail was eliminated due

to geometrical unsharpness when the corrosion penetrameter was

placed on the source side of the fin as shown in Figure 8D.

(e) The exposure required 45 sec. to complete.

(f) Edge enhancement did aid in the visualization of the penetrameter when it was placed between the fin and cassette,

however, played little role in the detection of the corrosion

signature when the penetrameter was placed on the source side

of the cassette.

6.3-

A Exposure Series 3A

The principal objectives of this exposure series was to determine

the effect on penetrameter P-1 image quality when the Gd

2

02S converter

21
-

.
0

and Kodak Type M film were replaced by a LiF/Zns converter and lightsensitive Kodak Type Blue Brand film. The radiographic parameters of

interest in this series of exposures is given in Table 3A below.

Table 3A: Exposure Series 3A: Radiographic Parameters

Converter

Film Type

L/D Ratio

Exposure Time/Fluence

6

Li F/Zns (4)

Kodak Blue Brand (4)

200:

1

5 sec./l x 10^ n-cm

^

Neutron Beam Cd Ratio 500:1

Penetrameter P-1

Object C-2 Vertical Fin Section

Radiographic data from Exposure 3A are illustrated in Figure 9A

through 9F.

(4) Significant Parameter Change

6.3-B Exposure Series 3B

This series of exposure was made to observe the effect of reducing the

system L/D ratio from 200:1 to 43:1 on the image quality of the P-1 penetrameter. The ^LiF/Zns in combination with Kodak Blue Brand film imaging

method remained the same as in Series 3A. The radiographic parameters

employed in these series are presented in Table 3B below.

22

Fig. 9: Exposure Series 3A: Radioaraphic Data

C-2 VERTICAL FIN

PENETRAMETER FACING CASSETTE

(C)

C-2 VERTICAL FIN

PENETRAMETER FACING SOURCE

(D)

- 23 -

Fig. 9: Continued

/

iNALOG EDGE ENHANCEMENT PENETRAMETER

FACING CASSETTE

(E)

,,

•'fv. 4

ANALOG EDGE ENHANCEMENT PENETRAMETER

FACING SOURCE

(F)

- 24 -

Table 3B: Exposure Series 3B: Radiographic Parameters

Converte

Film Type Kodak Blue Brand

L/D Ratio 43:1 (5)

0.2 sec./l x 10^ n-cm ^ Exposure Time/Fluence (5)

Neutron Beam Cd Ratio 500:

1

Penetrameter P-1

Object 02 Vertical Fin Section

The radiographic data derived from these exposures is illustrated in

Figures 10A through 10F.

(5) Significant Parameter Change

6.3-C Exposure Series 3: Results and Conclusions

verter in conjunction with Kodak Blue Brand film and a L/D

ratio of 200:1, it was possible to visualize the corrosion

signature when the penetrameter was placed between the vertical

fin and cassette. The image quality was so poor however, that

it could easily be confused with the scintillator mottling as

illustrated in Figure 9C.

(b) The exposure time required was 5 sec.

(c) When the corrosion penetrameter was placed on the source side

of the fin section, it was impossible to differentiate accurately

between the corrosion signature and the background mottling.

(d) Edge image enhancement provided no useful purpose in the

visualization of the corrosion signature with the penetrameter

on either side of the film.

(a) Using a Nuclear Enterprise NE 421

6 LiF/Zns light-emitting con25
.

-

-
Fig. 10: Exposure Series 3B: Radiographic Data

PENETRAMETER

(A)

C-2 VERTICAL FIN

(B)

C-2 VERTICAL FIN

PENETRAMETER FACING CASSETTE

C-2 VERTICAL FIN

PENETRAMETER FACING SOURCE

( c ) -26-

(D)

Fig. 10: Continued

ANALOC EDGE ENHANCEMENT PENETRAMETER

FACING CASSETTE

(E)

ANALOG EDGE ENHANCEMENT PENETRAMETER

FACING SOURCE

(F)

- 27 -

(e) When the L/D ratio was reduced to 43:1, it was impossible to

accurately distinguish between the corrosion signature and the

background mottling with the penetrameter on either side of

the vertical fin.

(f) The exposure time required for this radiograph was approximately

0.2 sec.

6.4-A Exposure Seri es 4: General

These investigations were conducted in an attempt to define the

lower limit of corrosion detectability in aircraft-grade aluminum utilizing

reactor-based neutron radiographic procedures. A set of four calibrated

corrosion penetrameters P-2 were fabricated from 0.062 in. -thick aluminum

plates which had been subjected to a salt water spray/SO^ gas environment

for 0, 4, 8 and 15 days. These penetrameters are illustrated in Figure 4.

Thermal neutron radiographs were made of the seven inch thick 02 vertical

fin with calibrated penetrameters placed on the cassette side of the 02

vertical fin.

The radiographic parameters employed for this set of exposures are

given in Table 4 below.

Table 4: Exposure Series 4: Radiographic Parameters

Converter 0.0005 in. -Thick Vapor Deposited Gd

Film Type Kodak Type M

L/D Ratio 100:1

Q

Exposure Time/Fluence 30 min./18 x 10 n-cm

Neutron Beam Cd Ratio 500:1

Object C-2 Vertical Fin

Penetrameters P-2-1 through P-2-4

The penetrameter calibrations are outlined in Table 5 below.

28 -

Table 5: Penetrameter Calibration

Penetrameter Number Exposure Period to Salt Water

P-2-1

Spray/SO^ Gas Environment (Days)

0

P-2-2 4

P-2-3 8

P-2-4 15

The radiographic data illustrated in Figures 11 A, 11C, HE and 1 1

are video reproductions of penetrameters subjected to the corrosive

environment for periods of 0, 4, 8 and 15 days respectively. The data

illustrated in Figures 11B, 11D, HE and 1 1 G illustrate the edge enhanced

reproductions respectively of these radiographs. It should be noted that

the artifacts which appear as white dots in the neutron radiographic

images were the result of dirt or other foreign neutron absorbing matter

on the inside of the airfoil and were not images of the corrosion penetrameter.

6.4-B Exposure Series 4: Results and Conclusions

(a) Thermal neutron radiographs of the four calibrated corrosion

penetrameters P-2-1 through P-2-4 are illustrated in Figures

11A, 11B, 11C and 1 1 D . These penetrameters were corroded with a

salt water spray/SO^ gas mixture for periods of 0, 4, 8 and 15

days respectively.

(b) In the corrosion radiographs illustrated in Figures 11 A and 11C,

the corrosion cannot be distinguished from the background

noise as shown in the edge enhanced illustrations in

29 -
'
Fig. 11: Exposure Series: Radiographic Data

- 30 -

Fig. 11: Continued

4

(E) (F)

(C) (H)

-31

Figures HE and 11F. It is likely that the mottled background

noise is due to the neutron response to the fin paint.

(c) Environment induced corrosion can be observed without image

enhancement in the 8 and 15 day samples illustrated in Figures

11C and 11D respectively. Image enhancement serves to render

the corrosion images more observable as illustrated in Figures

11G and 11H.

7. 0 Quantification of Corrosion

Based on the experience gained in these preliminary investigations,

empirical quantification of corrosion appears to be a possibility,

either by isodensity scanning of neutron radiographs and comparison of

scans of the corrosion areas with sound aluminum, or by digital image

enhancement techniques. This conclusion is based on the fact that

differences in corrosion density in the P-1 penetrameter could be visualized and that a minimum level of detectability could be established with

the plaque penetrameters P-2-1 through P-2-4. Penetrameter P-1 was more

easily seen then the P-2 series because of the greater degree of induced

corrosi on.

8. 0 Remarks and Qualifications

It should be noted that the approach taken in these investigations

was a very practical one in that the radiographic data obtained using

ideal laboratory conditions was taken as a base, e.g., relatively high

resolution imaging methods were employed. Comparative radiographic data

were then obtained by varying the radiographic parameters to simulate a

low-resolution system as might be the case in a transportable field unit

neutron radiography system.

32 -

In the preparation of this report the conclusions reached were

derived from actual radiographic data and not from photographic reproductions.

Photographic reproduction of high density radiographs was very difficult to accomplish for this report, e.g. , the overexposure of Figure 5

compared to Figure 6 is not real, but is due to the photographic reproduction.

The data obtained in this work was limited to the use of filmconverter imaging systems. No attempt was made to evaluate scintillatorcoupled real time imaging systems.

9. 0 Summary

The significant results obtained from these investigations at the

National Bureau of Standards may be summarized conclusively in the

following statements.

(1) Environment- induced corrosion in aircraft-grade aluminum can

be detected with a high degree of sensitivity with thermal

neutron radiographic interrogation.

(2) The fluence (integrated neutron flux) at the image plane

required to visualize corrosion with conventional imaging

methods e.g., gadolinium converter in conjunction with a

medium contrast industrial x-ray film at density of 2.5, is

8 *2

approximately 4 x 10 n-cm . Assuming that a transportable

4

neutron radiography system were capable of producing 1.5 x 10

-2 -1 n-cm -sec at a L/D ratio of 40:1, an exposure of 7.2 hrs

would be required.

(3) At a L/D ratio of 40:1, the corrosion signature of both

surfaces of a thick object, e.g., a wing or airfoil, must be

33
r

'
interrogated individually. This is due to the fact that

geometrical unsharpness obliterates signature detail on the

surface opposite the cassette.

(4) The possibility of corrosion quantification does exist, based

on investigation with standard corrosion plaque penetrameters.

0

(5) Although the use of LiF/ZnS light emitting converters is the

most efficient method by which thermal neutron beams can be

imaged, the results are inadequate for unambiguous corrosion

signature analysis using presently available converters. The

reasons for this are described below.

10.0 Recommendations for Future Work at NBS

10.1 Converter Development

The most efficient light-emitting neutron imaging system in so far

as speed is concerned consists of a physical mixture of ^ Li F and ZnS

powders. The components are held in suspension with a binder and settled

on a thin aluminum substrate. The nuclear reaction which takes place is

^Li(n,T)a, the tritons and alpha particles producing light in the ZnS

sci nti 1 1 ator.

The neutron radiographic images produced when this type of imaging

screen is placed in intimate contact with light-sensitive film is quite

grainy, exhibiting poor resolution. This is due, in fact, to several

factors which include:

(a) The fact that the matrix is composed of powders having a

finite grain size. If this grain is too coarse, a granular

image will result.

34
_

-
(b) The

6 LiF/ZnS is partially transparent to light generated

within the matrix, making it possible for light generated at

one point to be scattered and emitted at a neighboring site.

(c) Light sensitive films generally tend to exhibit greater grain

than industrial x-ray films.

We propose that a funded R & D program be initiated at NBS to

develop a neutron-sensitive scintillating converter which will overcome

the aforementioned problems of the presently available continuous ^LiF/ZnS

thermal neutron imaging screens. Specifically, (1) we propose to size

the ^LiF/ZnS components so that only the finest grains available are

employed to mix the final matrix using a fluorocarbon binder and (2) to

break the initial continuous screen into a myriad of discrete information

centers, or cells in order to eliminate the cross-talk generated as a

result of the light transmission within the scintillator screen matrix.

We have in mind to deposit the converter matrix in individual cells

using chemically milled meshes having cells of approximately 0.001 in.

on a side separated by 0.005 in. walls to minimize cross-talk.

10.2 Real-Time Imagi ng Methods for Corrosion Detection

Pending the outcome of the screen-development work, we propose that

an evaluation of state-of-the-art real-time imaging systems be conducted

for corrosion detection under field conditions. It would be most cost

effective to evaluate the available real-time imaging systems on a lease

basis.

This method has the advantage of speed based on the assumption that a

suitable fast imaging screen can be developed which would provide the required resolution.

35

10.3 Corrosi on Quanti f ication Standard Penetrameter Development

The quantification of detectable corrosion could produce an economic

impact on military aircraft maintenance costs based on the assumption that

short exposure times can be achieved. By being able to quantify the

corrosion, it should be possible to predict the time at which a defective

area of skin or component should be replaced, rather than replacement on

a preventative maintenance basis. This is contingent on the fact that

a fast, high resolution imaging screen can be developed which would

result in reasonable exposure times.

We propose that several avenues of investigations towards this end

be pursued, e.g.

(1) Automated radiographic image analysis based on the isodensity

pattern comparison with standard corrosion penetrameters or

edge enhanced shadow analysis.

(2) Microchemi stry of actual aircraft corrosion samples to aid in

the fabrication of standard corrosion penetrameters for use

as Standard Reference Materials.

(3) Subtraction of x-ray data from neutron data to eliminate the

radiographic interferences caused by structural airframe

thickness variation.

36
-
(J .S. DEPT. OF COMM

BIBLIOGRAPHIC DATA

SHEET

1. PUBI.K AT'ION OK REPORT NO.

NBSIR 78-1434

2. Ciov’t Acccssio

No.

3. Recipient's Accession No.

TITLE AND SUBTITLE

THE MACROSCOPIC DETECTION OF CORROSION IN ALUMINUM AIRCRAFT

STRUCTURES WITH THERMAL NEUTRON BEAMS AND FILM IMAGE METHODS

5. Publication Date

6. Performing Organization Code

AUTHOR(S) 8. Performing Organ. Report No. Donald A. Garrett

PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

Sponsoring Organization Name and Complete Address (Street, City, State, ZIP) 13. Type of Report & Period Covered

14. Sponsoring Agency Code

S. SUPPLEMENTARY NOTES

6. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

The primary objective of this investigation was to determine the feasibility of

etecting corrosion in aluminum Naval aircraft components with neutron radiographic

interrogation and the use of standard corrosion penetrameters . Secondary objectives inluded the determination of the effect of object thickness on image quality, the defining

if minimum levels a dectability and a preliminary investigation of means whereby the deiree of corrosion could be quantified with neutron radiographic data.

All objectives were met and may be summarized as follows: (1) Environment-induced

:orrosion can be detected with high sensitivity by thermal neutron radiographic interrogation, (2) the fluence at the image plane required to visualize corrosion with conventional imaging methods e.g., a gadolinium converter in conjunction with a medium contras

industrial x-ray film at density of 2.5, is approximately 4x10^ n-cm"^. Assuming that a transportable neutron radiography system were capable of producing a flux of 1.5xl04

n-cnr^-sec" ' at an L/D ratio of 40:1, an exposure of 7.2 hours would be reauired, (3) at

an L/D ratio of 40:1, the corrosion signatures of both surfaces of a thick object, e.g.,

awing or airfoil, must be interrogated individually. This is due to the fact that geometrical unsharpness obliterates signature detail on the surface opposite the cassette.

(4) The possibility of corrosion quantification does exist. This conclusion is based on

an investigation with standard corrosion plaque penetrameters, and (5) although the use

of °LiF/ZnS light emitting converters is the most efficient methpQl by which thermal

neutron beams can be imaged, the results are inadeauate for unambiguous signature analys

yith pres ently availabl e conve rters. — : I/. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Aircraft corrosion; corrosion; corrosion characterization; corrosion detection;

corrosion quanti

-radiography

i with neutron beams ; neutron

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(THIS REPORT)

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