NBSIR 75-776

Investigation of Two Tecliniques for

Application in Test Methods for

Detecting Defects in Spacecraft

Thermal-Protection Material

Seymour Edelman, Steven C. Roth, and J. Franklin Mayo-Wells

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington. D. C. 20234

September 26, 1975

Final Report Covering Period

July 1. 1972 to June 30, 1973

Contract T-2979B

Prepared for NASA Lyndon B. Johnson Space Center

Houston, Texas 77058

NBSIR 75-776

INVESTIGATION OF TWO TECHNIQUES FOR

APPLICATION IN TEST METHODS FOR

DETECTING DEFECTS IN SPACECRAFT

THERMAL-PROTECTION MATERIAL

Seymour Edelman, Steven C. Roth, and J. Franklin Mayo-Wells

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

September 26, 1975

Final Report Covering Period

July 1, 1972 to June 30, 1973

Contract T-2979B

This report constitutes an account for the record of exploratory work

undertaken to provide input for technical decisions regarding the

utilization of the techniques in question for proposed test methods.

The results indicated that the techniques were not suitable for further development for the proposed tests, and no refinements of the pre liminary experiments were carried out.

Prepared for NASA Lyndon B. Johnson Space Center

Houston, Texas 77058

U.S. DEPARTMENT OF COMMERCE, Rogers C.B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

ABSTRACT

The feasibility of utilizing either of two nondestructive techniques as a basis

for the development of methods for detecting and locating defects in a spacecraft

thermal shield was investigated. One technique consisted of scanning the surface

of a heated specimen with a polymeric pyroelectric detector. Success with this

technique required that an unflawed specimen be homogeneous with respect to the

transfer of thermal energy along the surface of the material. In the other

technique, conducted at room temperature, two polymeric piezoelectric elements were bonded on opposite sides of a known defect. Analysis of the amplitude and

phase differences between the signal supplied to the exciting element and that

received by the sensing element over a swept range of frequencies was intended to

provide information to locate the defect. Success with this technique required

that in an unflawed specimen the speed of sound along a known direction be

constant or vary in the same manner along two known directions.

Experiments showed that neither technique was unambiguously capable of detecting

previously identified flaws in the specimen thermal -shield material supplied by

the sponsor. The utilization of these techniques as a basis for further development is not recommended.

Key Words: acoustic transmission; nondestructive test; piezoelectric; polymeric

piezoelectric transducer; polymeric pyroelectric sensor; pyroelectric; spacecraft;

thermal scanning; thermal shield.

INVESTIGATION OF TWO TECHNIQUES FOR APPLICATION

IN TEST METHODS FOR DETECTING DEFECTS IN SPACECRAFT THERMAL-PROTECTION MATERIAL

S. Edelman, S.C. Roth, and J.F. Mayo-Wells

1. INTRODUCTION AND SUMMARY

This is the final report of work done under NASA Request Number T-2979, issued by

the L.B. Johnson Manned Spacecraft Center, Houston, Texas. The objective of the

work was to examine two approaches to nondestructive evaluation of the shuttle

[spacecraft] thermal protection system. The effort was to be exploratory in

nature to investigate if utilization of either of the given approaches would

likely result in a useful, reliable method for detecting and locating defects

in thermal -shield material.

This material, which in its intended use is bonded to the spacecraft structure,

is described as a fibrous, ceramic material. It is coated with a glaze said to

provide thermal control, waterproofing, and abrasion resistance. Types of defects

of interest to NASA include water absorption sites, voids or local density variations, sites in which a degradation of thermal conductivity has occurred, and

discontinuities in the glaze coating, including cracks, delaminat ions , and sites

where the glaze has separated from its substrate. Defects in the bond between

the thermal -protect ive material and the shuttle structure are also of interest.

The sponsor supplied a specimen of the glazed material which had several cracks

in the coating, for which the approximate position was known and identified by

a mask. The exploratory work was to be limited to an evaluation of the capability

of the selected approaches to detect these surface cracks.

The two approaches were based on techniques using polymeric sensors developed at

the National Bureau of Standards [1,2,3,^]-. One technique consisted of scanning

the surface of a heated specimen with a polymeric pyroelectric detector. Success

with this technique required that an unflawed specimen be homogenous with respect

to the transfer of thermal energy along the surface of the material. In the other

technique, conducted at room temperature, two polymeric piezoelectric elements

were bonded on opposite sides of a known defect. Analysis of the ampl i tude-andphase differences between the signal supplied to the exciting element and that

received by the sensing element over a swept range of frequencies was intended

to provide information to locate the defect. Success with this technique required

that in an unflawed specimen the speed of sound along a known direction be constant

or vary in the same manner along two known directions.

Experiments showed that neither technique was unambiguously capable of detecting

previously identified flaws in the specimen thermal-shield material supplied by

the sponsor. The utilization of these techniques as a basis for further development is not recommended.

"Figures in brackets identify references, section 5-
-2-

2. THERMAL-SCANNING EXPERIMENTS

2.1 Apparatus and Procedure

An existing rotating therma 1 -detector system [5] was modified for the thermalscanning feasibility investigation. This detector was originally designed to

provide a thermal signature from the scanned area (such as the walls of a room)

and to detect and locate in azimuth the presence of thermal events causing a specified change in the signature from scan to scan. The modifications included

(1) removal of the optical collection and radiation sensor assembly from the

rotating mount; (2) modification of the optical system to reduce the field of

view to a circle 5 mm in diameter for an optical path length of approximately

30 cm; and (3) construction of new sensors with an electrode geometry providing

a reduced shunt capacitance and hence improved signal-to-noise ratio. These

modifications were intended to permit the detection of a thermal discontinuity

as narrow as 0.02 mm and a few celsius degrees in magnitude. This estimated

performance was based on that of the unmodified system" and was considered

adequate for the purpose of detecting thermal anomalies associated with surface

cracks in the sponsor's specimen.

The detector system, as modified, consisted of a concave spherical mirror of

5-cm focal length, with the pyroelectric polymer film detector located near the

center of curvature and sufficiently off axis to avoid obstruction of the incoming radiation. The sensitive area of the detector film was approximately 3 mm

in diameter and was positioned to receive radiation from a 5-mm-d iameter area

of the scanned surface. The optical system was mounted in an aluminum tube.

The receiving end of this tube was closed by a thin film of infrared-transmitting

polymeric material to shield the detector from air currents; the opposite end was closed by the mirror mount. Attenuation of radiation by the film was approximately 30%, broadband. An FET operational differential amplifier operated at

unity gain was used to match the extremely high impedance of the detector film

(on the order of 10^^ ^) to the input impedance of an oscilloscope or other

instrument used to display and record observations. A storage oscilloscope

was the principal instrument used to observe the sensor output during the

scanning experiments. ,,,,, , ^ :,

The signal from the preamplifier was fed into the storage oscilloscope and the

trace was photographed. Usually, a series of scans was made as rapidly as possible, each trace displaced vertically from the preceding on the oscilloscope

screen, and the series photographed together. The time between scans was about one m i nute . . . i

"The following description of a simple demonstration conducted many times is

presented as a somewhat dramatic indication of the system's capability. An

unmounted automobile tire was supported in the laboratory in a frame that permitted rotation of the tire. The demonstrator placed his thumb on the tire

tread for approximately 10 s. The "heated" area was rotated out of view of

the scanning detector, which was about 30 cm distant, and a baseline scan was developed by the detector and displayed on a storage oscilloscope screen. After

10 min, the demonstrator rotated the tire to bring the heated area into view of

the scanner. The "thermal thumbprint" was then clearly identifiable on the

oscilloscope screen in successive scans.
-3-

Several methods for scanning the specimen smoothly were tried. In the method

found most suitable, the aluminum tube was mounted on the frame of xy recorder,

and the specimen was mounted on the mechanism for traversing the pen so that

the region scanned included at least one known crack.

The specimen on which the tests were made was a rectangular parallelepiped,

approximately 13x9x6 cm, of the shuttle thermal -protection system material

and was stamped LMSC Li-15^2. The sponsor also furnished a mask locating

several defects. In particular, one 13 x 9-cm face was described as having

a crack nearly parallel to the 9-cm edges and another nearly parallel to the

13~cm edges.

A number of scans were performed with the specimen at room temperature and at

several elevated temperatures. The elevated temperatures were produced by

heating the face of the specimen with a heat lamp (to about 90°C) , a Meeker

burner (to about 150°C), or a gas-oxygen flame (to about 300°C) .

In addition to scanning the specimen face after raising the specimen temperature,

the specimen was scanned a number of times after the face had been heated and

then sprayed or sponged with water or with a solution of water and detergent.

In some cases, the hot face of the specimen was sprayed or sponged with isopropyl

alcohol. These procedures had been used by the sponsor in other methods for

detecting cracks.

2.2 Results

Typical records are shown in figure 1, experiments A through E. Three scans are shown for each experiment, covering a duration of approximately five minutes.

The top scan was recorded with the specimen at the highest temperature of that

experiment; the specimen was in each case successively cooler for the middle and

bottom scans. Each scan was started with the sensor viewing one face of the

specimen near one end; the large deflections at the extreme right were generated

as the scanned area passed off the opposite end of the specimen. Thus the

horizontal axis corresponds to distance along the selected face. A vertical

deflection of 1 cm typically represented an output change of 0.5 V from the

detector preamplifier; in the figure, deflection downward corresponds to

increasing temperature. Planned calibration of the detector was not carried

out after the nature of the results became evident.

In the scan sets shown and in the others from which these were selected, it was usually possible to identify the deflection due to the known crack except

when the sample had cooled nearly to room temperature. In almost every case,

other deflections were present which were as large as or larger than the deflection due to the known crack. Also, it was noted that the shape of the deflection

due to the known crack varied from one record to another in an unpredictable

manner. On the basis of these observations, it was concluded that thermal scan- ning with a pyroelectric detector is not promising as the basis for a reliable

method for detecting defects in the surface glaze of therma 1 -protect ion material.
3. ACOUSTIC-TRANSMISSION EXPERIMENTS

3.1 Apparatus and Procedure

Two polymeric piezoelectric transducers that had been developed for other work

[6] were mounted on the same surface on either side of a known crack in the

specimen described in 2.1". Each transducer served in turn as an exciting

element and as a sensor of acoustic waves over a swept-f requency range of 20 Hz

to 600 kHz. The exciting waveform was typically sinusoidal, although half-sine

and ramp (sawtooth) pulses with the same range of repetition rates were also

used. The output signals from each transducer were amplified and displayed on the screen of a double-beam storage oscilloscope.

These output signals were found not to exhibit frequency correlation with the

exciting signals. investigation by means of a commercial acce 1 erometer revealed

that the acoustic signal transmitted to the specimen by a single polymeric transducer serving as driver was not strong enough for a reasonable detection signalto-noise ratio. Accordingly, a 1 ead-z i rconate-t i tanate driver was constructed

with a pair of active-element discs connected mechanically in series and electrically in parallel. The acoustic signal transmitted to the specimen by these

drivers was found adequate for the experiments, and the polymeric transducers were found suitable for sensing the signal.

Two polymeric transducers were mounted on the specimen approximately equidistant

from the acoustic source. One transducer was on the same side of a known crack

as the source and the other was on the opposite side of the crack. The general

procedure was to look for differences in the phase and amplitude of the signals

received by the two transducers. Frequencies and waveshapes were as before.

The transducer signals were amplified and displayed on the storage oscilloscope

or fed to a wave analyzer with a range of 20 Hz to 20 kHz, the output of the

analyzer being displayed on the oscilloscope screen.

3.2 Results :. " "

'

Visual comparison of the displayed transducer output waveforms did not reveal

the presence of trace deflections (or differences between the traces for the two

transducers) that could be unambiguously related to the crack. That is, there were deflections that could be attributed to the crack, but these were found amid

a large number of unattr i butab 1 e deflections. The analyzer output exhibited a similar multiplicity of possible signals. As in the case of the thermal scanning

experiments, the indications of the presence of the crack varied from one test

to another and were often smaller than other indications which had no relationship to the crack. It was concluded that no reliable indication of the crack

could be found with the equipment available.

-A number of methods for bonding the polymeric transducers to the specimen were

tried. It proved impossible to achieve good mechanical coupling between transducer and specimen surface with bonding agents that permitted the transducer to

be removed without damage for re-use. A silver-filled rubber formulation was

found to provide an acceptable bond and also electrical shielding of signal

leads and transducer. This bonding agent was used in the experiments.
-5-

3.3 Experiment with Bricks

A piezoceramic driver was cemented (with alundum cement) to one end of a commercial red building brick with dimensions of approximately 20 x 9 x 5 cm. The brick had a groove about 1 mm wide sawn across the middle of a narrow

face for a depth of 2 cm. The brick was arranged to stand on the driver, and

either a commercial piezoelectric accel erometer or a polymeric piezoelectric

transducer was bonded to the opposite end. The output from the sensing transducer was fed to a wave analyzer, and the output from the analyzer fed to a strip-chart recorder. The driver was excited with sine waves swept very slowly

from about 20 Hz to 20 kHz. The sweep time was on the order of 20 min; chart

speeds varied from 0.5 to 5.0 mm/s. The intent was to provide the best possible

frequency discrimination within the equipment limits. The recorder trace showed

a very large number of deflections over almost the entire frequency range, including several that corresponded closely in frequency to the predicted "resonance"

frequency based on the speed of sound in brick and the dimensions. However, these

deflections were no larger than many others. Further experiments with half-length,

three-quarter length, and whole-length bricks with no obvious surface cracks

resulted in similar traces, with no deflections unambiguously related to brick

dimensions. The results of these experiments suggested that voids and other

i nhomogene i t ies present in the brick produced a pattern of acoustic waves that

could not be interpreted by simple means.

k, CONCLUSIONS AND RECOMMENDATIONS

The results of the experiments with bricks provided a possible explanation of the

poor results achieved in the therma 1 -scann i ng and acoustic-transmission experiments.

Examination of the specimen supplied by the sponsor showed that a small amount of

exfoliation of the glaze surface had occurred along some of the edges (as received,

the specimen was glazed over all surfaces). A small cut was made into the speciment near one of these exfoliation sites to permit examination of specimen construction. It could be seen that the block was covered with an uneven layer about 0.8-mm

thick, that the surface of the main portion of the block was quite irregular, and

that the degree of adhesion of the covering layer to the main portion of the block

varied from place to place. As a working hypothesis, it is assumed that the erratic

results from tests with both techniques were caused by random and accidental combinations of therma 1 -energy or acoustic-energy waves. Wave transmission may have been

affected by local i nhomogene i t i es in the transmission path arising from the unevenness in the two layers of material and their I nhomogeneous coupling.

The results of the work described in this report shov/ that defects in the specimen

therma 1 -sh ie 1 d material cannot be detected reliably and unambiguously with the

techniques and apparatus used.

There appears to be little promise of using thermal scanning techniques with

pyroelectric sensors as the basis for developing a viable method for detecting

such defects, and further development is not recommended.

Theoretical considerations suggest that polymeric piezoelectric transducers might

so be used at frequencies corresponding to wavelengths smaller than the thickness

of the glaze coating. The production of even a few such transducers for feasibility
-6-

test would itself constitute a development effort, and there is no guarantee

that the thermal shield used in flight vehicles will be free from inhomogeneities that do not constitute defects from the point of view of the application,

but which would make interpretation of transducer output signals very difficult.

The geometry of the shielding material as applied to a flight vehicle is also

likely to complicate the analysis task. For these reasons, the acoustic transmission technique as evaluated using polymeric piezoelectric transducers is

considered not promising as a basis for a defect-detection method for the given

application and further development is not recommended. Modified techniques

using higher frequencies and pulses were not evaluated but may offer promise.

5. REFERENCES

[l] Edelman, S., Roth, S.C., and Grisham, L. , Electrical Generation of Motion

in Elastomers, Shook & Vibration Bull. 39_ part 2, pp. 7-9 (I969).

[2] Edel man, S., Grisham, L. , Roth, S.C., and Cohen, J., Improved Piezoelectric

Effect in Polymers, J. Aeous, Soc. Amev. pp. 1040-10A3 (November 1970).

[3] Cohen, J., Edelman, S., and Vezzetti, C.F., Pyroelectr ici ty and Piezoelectricity in Oriented Films of Polyvinyl fluoride and Pol yv i nyl i dene Fluoride,

Book, Electrets, Charge Storage and Transport in Dielectrics, Ed. M.M. Perlman,

pp. 505-516 (The Electrochemical Society, Inc., Princeton, NJ , 1973)-

[A] Cohen, J., and Edelman, S., Polymeric Pyroelectric Sensors for Fire

Protection, AFAPL-TR-7^- 1 6 and NBSIR 75-728 (July 1975).

[5] Scanning Thermal Detector Developed, Nat. Bur. Stand. Tech. News Bull.j

NBSTAR 56 (12), p. 277 (December 1972).

[6] Edelman, S., Piezoelectric Polymer Stress Gages, Proc. Symposium on Advancement in Instrumentation for Civil Engineering Applications, Technical

Report No. AFWL-TR-73- 1 86 , Air Force Weapons Laboratory, Kirtland Air Force

Base, NM, pp. 221 et seq. (1973).
Figure 1; Drawings made from photographs of oscilloscope screen showing traces

FOR three scans OF PYROELECTRIC SENSOR ACROSS ONE FACE OF SPECIMEN THERMAL-SHIELD MATERIAL, FOR FIVE EXPERIMENTS, ThE HORIZONTAL AXIS CORRESPONDS TO DISTANCE ALONG THE SELECTED FACE; TRACE DEFLECTION

DOWNWARDS CORRESPONDS TO INCREASIIiG TEMPERATURE. In EACH CASE. THE CRACK LOCATIor.i is INDICATED B'^' THE ARROWHEAD. EaCH SCAN WAS STARTED WITH THE SENSOR VIEWING ONE END OF THE SPECIMEN; THE LARGE" VERTICAL

TRACE DEFLECTION AT THE RIGHT REPRESENTS THF OTHER END OF THE SPECIMEN. Over the time of the scans shown for each experiment (about I MIN), the SPECIMEN WAS AT ITS HIGHEST TEMPERATURE WHEN THE TOP TRACE

WAS RECORDED AND SUCCESSIVELY COOLER FOR THE MIDDLE AND BOTTOM TRACES. Experiment A; crack parallel to specimen 15-cm edges; specimen heatkd

TC about lEO°r, WIPED WITH DETERGENT SOLUTION/ AND REHEATED BRIEFLY. Experiment E; crack parallel to specimen 9-cm edges; specimen heated

TO about 150°r, WIPED WITH DETERGENT SOLUTION, AND REHEATED BRIEFLY. Experiment C; crack parallel to specimen 13-cm edges; specimen heated

to about 150°r., wiped with water, and reheated briefly,

Experiment H: crack parallel to specimen 13-cm edges; specimen heated

TO ABOUT 150°C. Experiment E: crack parallel to specimen 9-cm edges; specimen heated

TO ABOUT 300°C, wiped WITH DETERGENT SOLUTION, AND REHEATED BRIEFLY.
NBS-n4A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 75-776

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Investigation of Two Techniques fo r Appl icat ion in Test

Methods for Detecting Defects in Spacecraft Thermal

Protection Material

5. Publication Date September 26, 1975

6. Performing Organization Code

7. AUTHOR(S)

Edelman, S., Roth, S.C., and Mayo-Wells, J.F.

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. ^253552

11. Contract/Grant No. T-2979B

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

Lyndon B. Johnson Space Center

National Aeronautics and Space Administration

Houston, TX 77058

13. Type of Report & Period Covered

Final 7/1/72-6/30/73

14« Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.)

The feasibility of utilizing either of two nondestructive techniques as a basis for the

development of methods for detecting and locating defects in a spacecraft thermal shielc was investigated. One technique consisted of scanning the surface of a heated specimen

with a polymeric pyroelectric detector. Success with this technique required that an unflawed specimen be homogeneous with respect to the transfer of thermal energy along

the surface of the material. In the other technique, conducted at room temperature,

two polymeric piezoelectric elements were bonded on opposite sides of a known defect.

Analysis of the amplitude and phase differences between the signal supplied to the

exciting element and that received by the sensing element over a swept range of

frequencies was intended to provide information to locate the defect. Success with

this technique required that in an unflawed specimen the speed of sound along a known

direction be constant or vary in the same manner along two known directions.

Experiments showed that neither technique was unambiguously capable of detecting

previously identified flaws in the specimen thermal-shield material supplied by the

sponsor. The utilization of these techniques as a basis for further development is

not recommended.

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 ) Acoustic transmission; nondestructive test; piezoelectric;

polymer piezoelectric transducer; polymeric pyroelectric sensor; pyroelectric;

spacecraft; thermal scanning; thermal shield. -

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