i 

NASA FINAL REPORT 

ORBIT TRANSFER VEHICLE ENGINE TECHNOLOGY PROGRAM 

COMBUSTOR WALL CONDITION MONITORING - SUBTASK I1 

TASK E.5 

Prepared By: 

B. L. Szemenyei 

ROCKWELL INTERNATIONAL CORPORATION 

Rocketdyne Division 

Prepared For: 

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 

NASA-Lewis Research Center 

Contract NAS3-23773 

M. Millis, Task Manager 

(ll&SA0Cl?-182275) OW8 COMBUSTOR UALL 189-26899 COIDITIOI lIOIIToPI16 Piwl Report, lor. 1986 

- Sep. 1988 (Rockvell Iaternutiolral C0rp.k 

38 P CSCL 218 Qaclrs 

-. . 

C3/20 = 0323910 - - -_ I 

ROCKETDYNE DIVISION OF ROCKWELL INTERNATIONAL CORPORATION 

6633 Canoga Avenue, Canoga Park, CA 91303 
1. Repon No. 

NASA CR 182275 

7. Author(s1 

Brian Szemenyei 

Robert S. Nelson and S. Barkhoudarian 

2. Government Acwsion No. 3. Recipirnt's Camlog No. 

9. Performing Organization Name and Address 

Rockwell International-Rocketdyne Division 

6633 Canoga Avenue 

Canoga Park, CA 91303 

4. Title and Subtitle 

OTVE Combustor Wall Condition 

Monitoring FInal Report Task E.5 

8. Worming Orpnization Report No. 

RI/RD89-212 

10. Work Unit No. 

5. R~port Date 

6. Worming Omniration cod0 

August 1989 

11. Contract or Grant No. 

12. Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 

Washington, D.C. 20546-0001 

NAS3-23773 

13. Typ of Report and Period Cowed 

NASA Contract Report -- 

%%pkor/d 'Agcy Codd ' 

-.. - 

Uncl assi f i ed 

15. S. Barkhoudarian, B. Szemenyei , R.S. Nelson, R. Pauckert, and 

T. Harmon, "Improved Maintainability of Space-Based Reuseable Rocket Engines" 

presented at the AIAA/ASME/SAE/ASEE 24th Joint Propulsion Conference, 

16. Abstraa 

Unclassified 39 

Conventional ultrasonics, eddy current, and electromagnetic acoustic 

transduction (EMAT) technologies were evaluated to determine their capability 

of measuring wall thickness/wear of individual cooling channels in test 

specimens simulating conditions in the throat region of an OTVE combustion 

Quantitative results are presented for the eddy current 

technology, which was shown to measure up to the optimum 20-mil wall thickness 

with near single channel resolution. Additional results demonstrate the 

capability of the conventional ultrasonics and EMAT technologies to detect a 

thinning or cracked wall. Recommendations for additional eddy current and 

EMAT development tests are presented. 

. chamber liner. 

16. Distribution Statement 

Unclassified - Unlimited I 17. Key Words Iluggested b Author(r)) 

Ultrasonics, eddy current, electromaanet i c acous t i c tr ansduct i on EMAT 

'I wail thickness , cool ing channels 

19. kurily Classif. (of this report) I 20. Security Classif. (of this page) I 21. No. of Pe I 22. Rice' 

For sale by ihe National Technical Information Service. Springfield, Virginia 22151 

NASA-C.168 (Rcv. 0-il) 
OTVE COMBUSTOR WALL CONDITION MONITORING 

FINAL REPORT 

October 10, 1988 

INTRODUCTION 

The scope of this effort was to identify and evaluate 

of condition monitoring technologies capable 

nondestructively measuring wall thickness and the extent of 

cooling channel lgdoghousinglg in test specimens emulating 

conditions which may exist in the throat region of the OTVE 

main combustion chamber after multiple firings. These 

measurements can then be correlated to the remaining 

combustor life, and predict the overhaul schedule and 

requirement. 

SUMMARY 

A literature search was performed resulting in thirteen 

candidate sensor technologies, which were initially 

evaluated against wall thickness measurement criteria such 

as thickness range, thickness and channel resolution, 

accuracy, and the simplicity of the measurement. From these 

criteria, three technologies-- ultrasonics, eddy currents, 

and electromagnetic acoustic transduction (EMAT), a 

noncontacting, couplant-free ultrasonic technique-- were 

selected for laboratory testing. 

Transducers for measuring up to a 20-mil thick wall of 

copper with single channel resolution, were custom-designed 

and procured. Transducer fixtures were also designed and 

fabricated to provide for accurate, automated scans of the 

test specimens. 

R I /R 08%- 2 1 2 
Additionally, electromechanical translators were 

designed and fabricated to move the transducers at selected 

speeds across the specimens, and to monitor time and 

amplitude signals. Automated scans were then performed on a 

curved SSME combustor segment and channeled copper and 

NARloy 2 plates with 20-mil land/channel widths and wall 

thicknesses simulating OWE combustor throat conditions. 

Conventional ultrasonics easily distinguished between 

good channels, lands, and cut-through channels, providing a 

high-contrast amplitude signal. However, in its present 

configuration, its immersion or liquid couplant requirements 

inhibit its implementability for space-based applications. 

Eddy current techniques were then tested and found to 

readily resolve a 0-10 mil thick wall thickness, and up to a 

20-mil thick wall when not adjacent to thin-walled channels. 

This technology also provides a high-contrast signal for 

wall thickness measurements, and has the added feature of 

not requiring couplants. 

The couplant-free ultrasonic EMAT technology was then 

tested. Results showed that the EMATs could easily resolve 

signals from a single channel, but that additional data 

processing algorithms are required to correct for acoustic 

interference caused by multiple, consecutive channels. Once 

this is performed, EMAT signal amplitude can be correlated 

with wall thickness to provide a second couplant-free 

combustor wall thickness measurement technique. 

RI/RD89-212 

2 
, 

I 

Continued development of noncontacting measurement 

technologies such as eddy current and EMAT is essential for 

enhanced reliability of rocket engine components. Combined 

with expert systems, these noninferential technologies can 

provide a fast, fully-automated, in-situ inspection of 

space-based rocket engine components, minimizing the need 

for engine disassembly, and reducing turnaround time and 

life-cycle costs. 

The following sections provide technical discussions of 

the technologies tested and the results obtained. 

CONVENTIONAL ULTRASONICS 

A two-transducer ultrasonics technique was selected as 

a candidate measurement technology. This method employed 

two 10-MHz piezoelectric, focusing, immersion transducers in 

a pitch-catch arrangement across alternating channel/land 

sections, as shown in Figure 1. The transducers were 

oriented 30 degrees from the normal of the specimen, and one 

focal length (1/2 inch) from the wall surface. one 

transducer generates an ultrasonic signal which is 

transmitted through the coupling medium into the wall of the 

test specimen. The signal is reflected by the channel/wall 

interface of the test specimen, back into the coupling 

medium, where it is detected by the second transducer. Wall 

thickness can then be determined from amplitude measurements 

of the reflected signals. 

Figure 2 shows the experimental results. When the 

ultrasonic signal is incident on a good cooling channel, the 

RI /RD89-212 

3 
signal is reflected by the back wall of the channel and 

easily detected, as shown by the seven good channels and the 

seven large peaks. When the signal is incident on a 

doghoused or broken-through channel (fourth channel from the 

left), the signal is not reflected, and therefore no signal 

is received by the second transducer. These results show 

that ultrasonic technique provides a high-contrast signal 

for Combustor Wall Condition Monitoring. However, the 

coupling requirements of the immersion technique make the 

technique impractical. Another ultrasonic approach, the 

couplant-free electromagnetic acoustic transduction, which 

will eliminate this impracticality, is discussed later. 

EDDY CURRENT 

The use of eddy currents as a nondestructive testing 

method is frequently used for measuring thickness. It has 

an advantage over conventional ultrasonics in that it 

requires no liquid couplants and no contact with the object 

under inspection. 

An eddy current transducer contains a coil of wire 

through which an AC current is passed, creating an 

electromagnetic (EM) field. The EM field of the transducer, 

when brought near a conducting specimen, induces eddy 

currents in the specimen. These eddy currents produce an EM 

field opposing that of the transducer. This opposing field 

induces eddy currents in the pickup coil of the transducer. 

A measure of thickness is obtained from changes in the eddy 

current magnitude. Data has been collected with two 

RI /RD89-212 

4 
different eddy current transducers which were custom 

designed and fabricated for OWE Combustor Wall Thickness 

measurements. One probe is a two-element, differential 

transducer. The other is a single- element transducer. 

The first eddy current transducer has two elements 

which are separated by approximately one tenth of an inch. 

The coil of each element generates eddy currents in the test 

specimen, which in turn, generates eddy currents in each 

receiver coil. Resistance changes due to channel wall 

thinning cause eddy current amplitude changes, which provide 

wall thickness measurements. Each element measures eddy 

currents at a slightly different location of the test piece. 

The difference in the intensity of the eddy currents 

produced by the two elements at a particular location is 

proportional to the wall thickness at that location. As the 

transducer scans across alternating channels and lands of 

the test piece, the difference in the eddy current intensity 

produced by each transducer element provides a wall 

thickness measurement. When scanning the channeled test 

piece, the transducer has maximum wall thickness measurement 

sensitivity and channel resolution when oriented such that 

one element is over a channel while the other element is 

over an adjacent land. A Zetec MIZ-17 eddy current tester 

with a visual display and chart recorder output was used to 

make the measurements. 

Figure 3 shows a scan performed manually over a curved 

segment of an SSME Main Combustion Chamber. The results 

RI/RD89-212 

5 
indicate that the two-element transducer can readily resolve 

visibly thin-walled cooling channels in the SSME combustor 

segment, and also suggest that it can detect channels with a 

near-optimum wall. 

Next, a fixture was designed and fabricated to hold the 

transducer. Scanning was automated with a motorized 

actuator to produce accurate and repeatable results. The 

transducer was mounted on a translation stage, and the 

translation stage was moved by the actuator. This automated 

test set-up was used to perform tests on a copper test plate 

and a NARloy Z test plate, both of which had 20-mil 

channel/land widths, simulating throat conditions in the 

OTVE Combustor. 

Figure 4a shows the result of an automated scan of the 

copper test plate with cooling channels simulating wall 

thinning and lldoghousingll. The channels with a 10-mil thick 

wall or less are readily resolved, and have a signal 

strength proportional to remaining wall thickness. The 

channels with a 20-mil thick wall or greater show no 

apparent signal when adjacent to thin-walled channels. 

Figure 4b shows the result of an automated scan of the 

copper test plate with cooling channels having an optimum 

20-mil thick wall, with no thin-walled channels nearby. 

This figure demonstrates the ability of the two-element 

transducer to resolve the optimum wall thickness of OTVE 

Combustor Wall cooling channels. Figure 4c shows a plot of 

eddy current amplitude vs channel wall thickness for the 

RI/RD89-212 

6 
combined scans of figures 4a and 4b, covering a 1.5-20 mil 

thickness range with single channel resolution. 

Figure 5 is the result from a test on a 24-channel 

NARloy Z test plate with three channels simulating wall 

thinning. The three thin-walled channels are easily 

resolved and have good correlation between signal strength 

and wall thickness. Again, the large amplitude signal 

caused by the nearby thin wall overpowers the small 

amplitude signals from the optimum-wall channels. To date, 

the two-element transducer has demonstrated the ability to: 

1) resolve near-optimum-wall cooling channels which are not 

adjacent to cooling channels having a wall thickness of 10 

mils or less; 2) detect and quantify wall thickness of 

isolated thinned channel walls. 

A second transducer, with a single element, was then 

tried to determine its thickness measurement and channel 

resolution capability. This transducer induces eddy 

currents in a test specimen which are proportional to wall 

thickness, similar to the two-element transducer technique. 

As the transducer scans the surface of a test specimen, the 

amplitude of the eddy currents, again, change with the wall 

thickness. Use of this transducer has an advantage since it 

eliminates the necessity of precisely orienting multiple 

transducer elements with respect to the cooling channels. 

To improve the signal strength and channel resolution 

in the data, data collection was automated through an IEEE488 computer interface. Data was collected through the 

RI /RD89-212 
interface from a Tektronix 7D20 digitizer and stored onto 

computer disks. Once on disks, data from several scans can 

be added together to increase signal amplitude and to 

average out noise, thereby enhancing the signal-to-noise 

ratio. The digitizer also reacts much faster to changes in 

signal than conventional chart recorders. The signals from 

the optimum-wall channels, shown in Figure 6, were not 

detected by a chart recorder, which was simultaneously 

recording the data. 

Figure 6 shows the results of three superposed scans of 

the NARloy Z test plate, using the single-element 

transducer. Results indicate that the single-coil 

transducer can readily locate thin-walls of 5.8 and 10.5 

mils, as well as some of the neighboring, but not adjacent, 

optimum-wall channels. The figure shows the two thin 

channels and 5 of 11 of the optimum-wall channels, as shown 

by the solid lines. Undetected channels caused by a nearby 

thin wall are shown by dotted lines. 

These results show that both the single and the twoelement eddy current transducers can easily resolve up to a 

10-mil thick wall of copper or NARloy Z. Wall thicknesses 

greater than 10 mils and approaching the 20-mil optimum, 

produce much weaker but measurable signals, but are 

undetectable when near thinner-walled channels which produce 

large-amplitude signals. Thus, the eddy current sensors 

have been found to be capable of: 1) locating near-nominal 

channels if remote from thin-wall channels; 2) locating thin 

RI/RD89-212 

8 
wall channels and; 3) quantifying the wall thickness of 

individual thin wall channels. These results also provide a 

high-contrast signal for OTVE Combustor Wall Condition 

Monitoring, and has the advantage of being couplant-free. 

EMAT 

It was shown that a two-transducer ultrasonic technique 

could produce a high-contrast signal for wall condition 

monitoring, but that immersion might inhibit its 

implementation. A recently-invented ultrasonic approach 

uses electromagnetic acoustic transduction (EMAT). The 

EMATs transmit and receive acoustic waves in a conductor 

without the use of liquid couplants. 

An EMAT consists of a periodic array of permanent 

magnets wrapped with wire coils, as shown in Figure 7. When 

placed near a conducting surface, current in the coils 

induce eddy currents in the conductor. The magnetic field 

produced by the permanent magnets exerts a Lorentz force on 

the eddy currents, creating an elastic disturbance in the 

metal lattice. By periodically pulsing the current in the 

coils, the elastic disturbances become acoustic waves. The 

acoustic waves are received by a second EMAT by the opposite 

process. The acoustic wave in the field of the receiver 

EMAT coil induces eddy currents in the pick-up coil, and the 

amplitude of these eddy currents is a measure of wall 

thickness. The wall thickness determines how much of the 

wave is reflected. A thick wall will reflect less of the 

wave than a thin wall. Therefore, the amplitude of the 

RI/RD89-212 

9 
reflected wave indicates the channel wall thickness. For a 

sufficiently thick wall, the wave will not be reflected at 

all. 

An Electromagnetic Acoustic Transducer system was 

custom-designed and fabricated by Innovative Sciences, 

Incorporated (ISI) specifically for OWE Combustor Wall 

Condition Monitoring. Several bread board EMATs were 

initially constructed by IS1 personnel, and they determined 

that the optimum EMATs for wall thickness measurements are 

EMATs which produce horizontally polarized shear waves (SH 

waves). For these SH waves, the motion of the particles 

producing the wave is perpendicular to the direction in 

which the wave travels. Figures 8 and 9 show the setup and 

results, respectively, of a preliminary test performed by 

IS1 personnel when designing the EMATs. The transmitting 

and receiving EMATs were separated by a distance of 38 mm on 

a 20-mil thick copper plate with a single 10-mil thick wall 

between them. The transmitter sends a polarized shear wave 

horizontally towards the receiver. The receiver senses the 

wave as it passes underneath, and the amplitude of this 

initial wave is recorded. 

In Figure 9, A is the first wave received directly from 

the transmitter. B is the reflection from the end of the 

test plate beyond the receiver. As this reflected wave 

travels back toward the transmitter, some of the wave is 

backscattered by the 10-mil channel, producing signal C. 

Finally, D is the reflection of the backscattered wave from 

RI/RD89-212 

10 
the end of the plate. Signal D shows an amplitude decrease 

of nearly 50% of the incident wave caused by reflection 

backscatter from the single channel, indicating that the 

amplitude of these reflections can be used to measure 

combustor wall thickness. 

The system was initially tested at Rocketdyne on the 

copper plate used in earlier eddy current tests. Tests on 

this plate showed strong signals from the transmitted wave 

as well as reflected signals from the edges of the plate 

(Figure lo), confirming results obtained during EMAT 

fabrication. The transmitted signal is greatly attenuated 

when the thin-wall channel of the plate is placed between 

the transmitter and the receiver (Figure 11) , demonstrating 

that the EMAT system can detect thin-wall channels. 

The EMAT system successfully identified thin cooling 

channels on a segment of a curved SSME Main Combustion 

Chamber. Figure 12 shows the test setup, and Figure 13 

shows the signals when the EMAT transmitter and receiver are 

separated by 30 mm. The first signal (A) is from the 

acoustic wave traveling directly from the transmitter to the 

receiver. The second signal (B) is the reflection of thinwalled channels 25 mm away from the receiver. This 

demonstrates the EMAT'S ability to detect wall thinning on a 

J 

curved surface. 

Computer 

automated data 

programs were written which completely 

acquisition and analysis through an IEEE 

interface. Multiple EMAT signals were averaged by a 

RI /RD89-212 

11 
1 

tektronix 7D20 digitizer to improve the signal-to-noise 

ratio, then automatically transferred to computer disks for 

analysis of signal amplitudes and arrival times. The files 

were then transferred to spreadsheet programs for more 

detailed analyses and display. 

Using this method, a test was run to try to resolve 

individual signal reflections from two consecutive thinwalled channels in the copper test specimen. The 

transducers were positioned with a one-inch separation on 

the copper test specimen (Figure 14). Signals from the 

EMATs were sent to the Tektronix 7D20 digitizer, and 

transferred to computer files for analysis and display. 

Figure 15 shows the signal produced for the EMAT 

configuration of Figure 14. The solid lines at 31.6 usec. 

and 38.2 usec. represent the arrival times of signals 

reflected from the edge of the plate nearest the channels. 

The four dotted lines at 29.2 usec., 30.4 usec., 35.8 usec., 

and 37.0 usec. indicate the predicted arrival times for 

reflections from the two thin-walled channels near the edge 

of the plate, but the experimental signals are not 

distinguishable in this single test. 

One factor limiting channel resolution in the previous 

test involves transducer ringing. Upon excitation of the 

EMAT transducer, as in the case of conventional 

piezoelectric ultrasonic transducers, the transducers have 

characteristic rise and decay times associated with them. 

These cause multiple-peak signals, or ringing, as opposed to 

RI/RD89-212 

12 
single signal spikes, decreasing channel resolution. A 

second resolution-limiting factor involves constructive and 

destructive acoustic interference. Phases and amplitudes of 

acoustic waves reflected from multiple channels with 

different wall thicknesses, and reflections from the end of 

the plate, can combine constructively to produce a large 

signal, or they can combine destructively to produce little 

or no signal. It should be noted that in the actual case of 

the circular wall and periodic array of channels in an 

optimum combustor, there would probably be no appreciable 

signal from acoustic waves reflected from the channels, 

reducing the effects of acoustic interference. Signals will 

appear and increase in amplitude only as channels begin and 

continue to erode. 

To assess the effects of transducer ringing and 

acoustic interference, a different test was devised. Signal 

arrival times were obtained at 0.5 mm (20 mils) intervals on 

two sections of the test plate, one section with the two 

consecutive channels, and one section without channels 

(Figure 16). Figures 17 and 18 show the signal arrival 

times as a function of EMAT position for the O-channel and 

2-channel sections of the plate, respectively. 

In Figure 17, the rising slope indicates the increasing 

arrival time from the reflection off of the near edge of the 

plate, while the decreasing slope represents the decreasing 

arrival time as the EMATs approach the farther edge of the 

RI/RD89-212 

13 
plate. These arrival times show a linear, low-scatter 

change, as would be expected. 

In Figure 18, similar curves appear, but with a high 

degree of scatter. The solid lines represent the expected 

arrival time for the reflections from the ends of the plate, 

while the dotted lines represent the expected arrival times 

from the reflections off the two channels. These results 

suggest that additional, more refined data processing 

algorithms are required which compensate and correct for 

transducer ringing and acoustic interference in order for 

the EMATs to adequately resolve signals from consecutive 

channels. Once this is done, tests can be performed to 

correlate EMAT signal amplitudes with known combustor wall 

thickness to complete the EMAT evaluation. 

The EMAT system has shown the ability to easily resolve 

a signal from a single thin-wall channel in a copper plate, 

as well as multiple thin-wall channels in a curved SSME main 

combustion chamber. To resolve individual signals from 

adjacent thin-wall channels, additional data processing 

algorithms, correcting for acoustic interference and 

transducer ringing, must be developed. These algorithms 

will help obtain the single-channel resolution necessary for 

accurate wall thickness measurements with this noncontacting 

couplant-free ultrasonic technique. 

RECOMMENDED ADDITIONAL TESTING/- LUATION 

EDDY CURRENT: Further tests using eddy currents should be 

considered to determine if resolution of the optimum-wall 

RI/RD89-212 

14 
i 

I 

~ 

cooling channels can be improved, especially nearby thinwalled channels. Tests with the single-element transducers 

could include scans at different angles across the channels. 

Only scans with the transducer moving perpendicular to the 

channels have been performed thus far. Varying the scan 

angle to values other than 90 degrees would create a longer 

scan, making the channels appear wider and possibly easier 

to resolve. Tests could also be run at different 

frequencies to maximize channel resolution. Additional 

tests with the two-element probe should include varying 

frequency and detector angle while scanning the test 

specimens to determine if the transducer can resolve both 

optimum and thin-wall cooling channels which are adjacent. 

Additional tests are also necessary to determine how 

the curvature of the OTVE Combustion Chamber effects the 

eddy currents and the scan procedure. To best determine the 

effects of curvature, tests should be performed on the 

combustion chamber throat of the OTV\ICE. While no absolute 

wall thicknesses could be determined for this component, the 

tests would at least provide qualitative and rough 

quantitative wall thickness information on an actual 

component. If the ICE component is unavailable, an 

alternative specimen would be a 1.5-2.0 inch diameter copper 

tube with channels cut on the outer diameter to various 

depths. Scans would then be performed on the inside of the 

tubes to determine if the transducers can resolve cooling 

channels on this representative curved surface. 

RI/RD89-212 

15 
EMAT: Additional testing should be performed with the EMAT 

system using more refined algorithms which correct for 

acoustic interference and transducer ringing effects present 

in the data obtained to date. Additionally, tests would be 

performed on the OTV/ICE combustion chamber component, or 

perhaps a test calorimeter or channeled copper tube. It 

should be noted that with the cylindrical chamber test 

piece, a periodic array of channels and lands would minimize 

acoustic interference caused by the discontinuities (plate 

edges) of existing specimens. It is expected that signals 

from the optimum chamber would be small until the wall of a 

channel begins to erode, the thinning wall producing a 

larger reflection signal. Such tests need to be performed 

to thoroughly evaluate the EMAT technology. 

CONCLUSION 

Thirteen technologies were evaluated for measuring wall 

thickness of the OTVE combustion chamber liner wall, and 

three technologies were selected for laboratory evaluation. 

Conventional ultrasonics provided a high-contrast 

signal, easily distinguishing good channels, cracked 

channels, and lands. While liquid couplant requirements 

limit this method's implementability, results suggested a 

different ultrasonic approach using couplant-free EMATs. 

Eddy current techniques were tested and found to 

resolve up to a 20-mil thick wall over channel/land widths 

of 20 mils, the optimum OTVE Combustor conditions. Signals 

from one or two optimum-walled channels adjacent to a thinR I /R 08%- 2 1 2 
walled channels are not resolvable, but this will not effect 

the detection of a thinning wall. These results demonstrate 

that eddy current techniques can provide the necessary wall 

thickness measurements, and the techniques are couplant-free 

and fully automatable. 

An EMAT system was tested and found to be able to 

resolve a signal from a single thin-walled channel, but 

requires that additional signal processing algorithms be 

developed to correct for acoustic interference from multiple 

consecutive channels. Amplitude and arrival times will then 

provide wall thickness and channel locations, respectively. 

RI /RD89-212 

17 
RI/RD89-212 

18 
ORIGIN'ilL PAGE 

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FIGURE 2: ULTRASONIC TEST RESULTS 

RI/RD89-212 

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R: I /RD89- 2 12 

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FIGURE 5: DIFFERENTIAL EDDY CURRENT SCAN OF NARLOY Z SPECIMEN 

SHOWING STRONG SIGNALS FROM THE THREE THIN-WALL CHANNELS 

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RI/RD89-212 

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RI/RD89-212 

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FIGURE 9: EMAT SIGNAL FROM SINGLE CHANNEL IN COPPER 

PLATE. 

A: DIRECT SIGNAL FROM T TO R 

B: REFLECTION FROM E N D O F PLATE 

C: PARTIAL REFLECTION FROM CHANNEL 

D: SECOND REFLECTION FROM END OF PLATE 

RI/RD89- 2 12 

28 
A B C 

FIGURE 10: NO CHANNELS 

A: INCIDENT PULSE (TRIGGER) 

B; SIGNAL FROM T TO R 

C: REFLECTION FROM END OF PLATE 

A B C 

FIGURE 11: WITH CHANNELS 

A: INCIDENT PULSE (TRIGGERR) 

B,C: CRACKED CHANNEL BETWEEN T AND R CAUSES 

SIGNAL ATTENUATION 

RI /RD89-212 

ORIGINAL PAGE 

BLACK AND WHITE PHOTOGRAPH 29 
R I/RD89- 21 2 

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FIGURE 14: ORIENTATION OF EMATS USED TO DETERMINE 

CONSECUETIVE 

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0 

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0 

0 

a 

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0 

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