7AD-A096 477 ARMY MISSILE COMMAND REDSTONE ARSENAL AL

GROUND EGU--ETC F/ 14/5ATHERMOPLASTIC ACOUSTICAL HOLOGRAPHY RECORDING DEVICE(U

SEP 80 D 8 RIVERSUNCLASSIFIED DRSI/RL-AI-1-TR SBIE-AD-E950 099 NL

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TECHNICAL REPORT RL-81-i

A THERMOPLASTIC ACOUSTICAL HOLOGRAPHYRECORDING DEVICE

Dr. D. B. RiversGround Equipment and Missile Structures DirectorateUS Army Missile Laboratory

DTICELECTE

SMAR 17 198125 September 1980

B

macatcarv Araarifi, Algab13 ixr"Am

Approved for public release; distribution unlimited.

I~l

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1. REPORT NUMBER 2. GOVT ACCESSION N 3R5CIPINJS CATALOG NUMBER

RL-81-1 '6 0 ____ S_ CATALOG__NUMBER

4. TITLE (and Subtie) S. TYPE OF REPORT & PERIOD COVERED

A Thermoplastic Acoustical Holography RecordingDevice Technical Report

S. PERFORMING ORG. REPORT NUMBER

7. AUTHOR(s) S. CONTRACT OR GRANT NUMBER(.)

Dr. D. B. Rivers

DAI L162303A214

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT. PROJECT, TASKCommander AREA & WORK UNIT NUMBERS

US Army Missile CommandATTN: DRSMI-RL AMCMS 6123032140911RmdRI-pnP Ar~qna1l Alahamn 3SR98

II. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATECommander 25 Sept 1980US Army Missile Command IS. NUMBER OFPAGESATTN : DRRMI-RPT1 NMERO AE

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IS. SUPPLEMENTARY NOTES

This work was accomplished through the Laboratory Research Cooperative Program(LRCP) between Dr. D. B. Rivers of the College of Engineering, University ofSouth Carolina, and the US Army Missile Command.

19. KEY WORDS (Continue on reverse side II neceamy mad Identify by block number)

Acoustical holographyReal timeFlaw detection and quantification

Laser and optical reconstruction

2I ABS1" ACTCh - tarn. a A 060Mw d /d.ULfr hy black nb)

This report documents an attempt to define the various variables which affectthermoplastic when used as a recorder of ultrasound. Various devices usingthermoplastic recording media are illustrated. Specific performance data aregiven.

DD 1" 1473 EDITION OF I NOV S IS OBSOLETE

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TABLE OF CONTENTS

Page

I. INTRODUCTION..........................3

11. DISCUSSION OF IDEALIZED SYSTEM. ................ 4

111. ANALYSIS OF AN IDEAL SUBSTRATE. ................ 5

IV. EXPERIMENTATION AND DISCUSSION. ................ 9

A. Initial Recordings ..................... 9

B. Effect of Substrate Thickness...............16

C. Effect of TI' Surface Smoothness..............18

D. Effect of Substrate Surface Roughness .......... 20

E. Heating Device.......................25

F. Additional TI' Recordings ................. 25

V. CONCLUSIONS AND RECOMENDATIONS................28

11o

PREFACE

The author wishes to express gratitude for the aid

of Drs. W. F. Swinson, W. F. Ranson, D. Smith, H. K. Liu,Mr. J. A. Schaeffel, and Mr. Keith Rodgers.

I. INTRODUCTION

The purpose of this report is to describe the analytical and experimental

evaluation of a device to record real-time in situ acoustical holograms. Therecording concept investigated, as shown in Figure 1, consists of a layer ofthermoplastic (TP) spread over the surface of a rigid substrate.

SRADIANT HEAT

TIP

WATER SURFACE

ACOUSTIC SIGNALS

Figure 1. Schematic representation of TP recording device.

This report describes attempts to optimize both substrate and TP. In

addition, an effective heating device for the TP is described.

The basic acoustical holography system is shown in Figure 2. It wasdeveloped by Irelan, et al. [1] with improvements described by Swinson (2,3].

Additional insight into the problems associated with the TP came from Liu [4).

The use of deformable TP to record optical holograms is well documentedin the previously referenced reports. The idea of applying this method torecord acoustical holograms was introduced by Young and Wolfe [5] in 1967.Such a technique could have widespread nondestructive testing (NDT) applica-

tions where information concerning interior flaws and voids is needed. Of

particular interest to the US Army is the application of this method to de-tect flaws in ceramic radoms and composite materials. There are also potentialmedical imaging applications for such a device.

3

SLIT1~~J~< IRRORS

SPATIAL LASER

SCREEN FILTER

LENS

_____ __ LIQUID SURFACE

OBETACOUSTICAL LENS,'

1 & ACOUSTICAL REFLECTOR

OBJECTTRANSDUCER REFERENCE TRANSDUCER

Figure 2. Acoustical holography layout.

II. DISCUSSION OF IDEALIZED SYSTEM

Although there are many possible arrangements and materials which mightgenerate acoustical holographic recordings, this report deals with just one,the arrangement shown in Figures 1 and 2. Ideally, the system would recordin situ real-time acoustical holograms as follows.

Two identical underwater collimated ultrasonic beams are focused on thesame area of water surface in the imaging tank. One, the object beam, passesthrough the solid object to be viewed and, therefore, is changed spatially bothin amplitude and phase. Tbe second beam, the reference, is unaffected. Whenboth beams combine at the water surface, an interference pattern, the acousti-cal hologram, is formed by distorting the water surface. Although theseacoustical beams are quite powerful, they can only cause microscopic surfacedistortions due to the weight of the water and its surface tension. However,thes- distortions are greater than the wavelength of laser light reflected offthe water surface, so that a diffraction pattern formed at the focal plane ofthe light and, by the use of a slit, the zero, first, second, or higher ordersof the diffraction orders can be selectively viewed on a screen. The mathe-matical formulation has been developed by Swinson [3].

To permanently record this interference pattern, some substance otherthan water must be used since it is obvious that the water levitation willcollapse when the ultrasonic signals are removed. This replacement substancemust be able to form a liquid surface levitation but be capable of retainingthe distortion by some means. The use of TP seems ideal for this applicationsince it is liquid when heated but quickly returns to its solid phase whenthe heat is removed. The need for a substrate is purely structural.

4

III. ANALYSIS OF AN IDEAL SUBSTRATE

Before an acoustical wave pattern can be recorded on the TP surface, thewaves must first pass through two boundaries: from water to substrate; thenfrom substrate to TP. The following is an analysis and discussion of theenergy loss of the ultrasonic waves through these two interfaces.

Consider an incident longitudinal wave of amplitude AiL passing through

a liquid-to-solid interface as shown in Figure 3. There are three waves thatresult:

(1) A reflected longitudinal wave (A rL) back into the liquid,

(2) A transmitted longitudinal wave (A ) into the solid, andtR

(3) A transmitted shear wave (Ats) into the solid.

Brekhovskikh [71 has determined that the amplitudes of these three resultantwaves can be related to the amplitude of the incident wave by the followingratios:

ArL Z2L cos 28 + Z2S sin226 - ZL2L 25 (1)

AiL ZfL cos22a + Z2s sin22B + Z

A tL P1 2Z2L cos2B

. . . .. (2)AIL P2 Z2L cos 22 + Z sin228

and

AtS -P1 2Z sin28 (3)AiL 2 Z 2L cos 28 + Z2S sin 22a + Z1,

where the subscripts 1 and 2 represent medium 1 (water) and medium 2 (solidsubstrate). The acoustical impedances are given by the following:

Z P2 C2L (4)2L cos E

P2 C2SZ2S cos ' (5)

and

0 P1 CIL (6)IL cos a

5

h6. ,. ..- S -- -9

AL ArL

kLIQUID

L IOU I(Medium-i1

\x

SOLID(Medium-2)

DIRECTION OFWAVE PROPAGATION

AtL

Ats

Figure 3. Longitudinal wave propagating from liquid to solid media [6].

where C IL and C2L represent the longitudinal wave propagation velocities in

the water and substrate, respectively, and C2S represents the shear wave

propagation velocity in the substrate.

To simplify the analysis, it is assumed that the incident longitudinalwave is normal to the water-substrate interface; therefore, a = 00. FromSnell's Law applied to acoustics [2],

sin a sin 6 sin c (7)C IL C22S 2L , L

therefore, a = c = 00. Using these values, Equations 1-3 can be combinedwith 4-6 to get the following:

ArL _ 2 C2 L - L ClL

AiL 2 C2L + P1 C lL

6

AtL 2I C1LAiL 2C2 L + PIC IL

and

Ats

Al= 0 (10)

The energy contained in an acoustical wave may be expressed as [3]:

A.1

EWAVE - Z (1):

where pi' Ai. and Zi correspond to a shear or longitudinal wave in medium i.

Conservation of energy at the interface implies the following:

ElL =ErL + EtL + EtS (12)

An ideal substrate would be one which maximizes the amount of transmittedacoustical energy. By combining Equations 9-12, the following expression isobtained and must be maximized:

Energy Transmitted E tL P41P 2 C1L C2 L (13)

Energy Incident EiL (ilClL + P2C2L)2

Equation 13 may be considered a function of only two variables, p2 and C2L,

since medium 1 will always be water in this experimentation. To maximize thisrelationship, and hence to discover what values of p2 and C2L would cause

maximum acoustical energy transfer from water to substrate, the first partialderivative of the energy ratio with respect to each of the two independentvariables is set to zero. Hence,

Ei 0 (14)

and

3C2L 0 (15)

Both Equations 14 and 15 yield identical solutions of

O2C2 L = PC (16)

7

To determine if Equation 16 represents a maximum or minimum, the secondderivative test is; applied to the solutions of Equations 14 and 15. That is,

for Equation 14,

S ItL <0 (1 7

iCiL

2L C2

The solution to Equation 15 must satisfv

<0 (19)L

and

S>0 (20)

Lo =2 C I I

C2 L=

The solutions to Equations 17 and 19 are, respectively,

0.5 2 2 and -0.5 ( 1

both of which are always negative. Evaluation of Equations 18 and 20,however, both yield results equal to zero. Hence, it cannot be confirmed yetthat satisfying result (Equation 16) will yield maximization of the energytransfer.

By substituting result (Equation 16) into Equation 13, the following

is siown

8

-05L Ad -05,(1

t.. 1.0 (22)

Ei.

This result, when combined with the zero evaluation of Equations 18 and 20,implies that there are an infinite number of values of p2 and C2L which will

maximize the energy transfer into the substrate. All that is required is thatcondition in Euiation 16 be met. In other words, for best energy transfer, anacoustic characteristic impedance match is required. This same type ofreasoning can also be applied to the substrate - TP interface. It is com-plicated by the fact that, as explained later, the TP lower surface is approx-imately equal to the water tank temperature (typically 22*C) while the TPupper surface is heated to improve its recording characteristics. Therefore,the velocity of longitudinal waves in the TP changes as the phase of the TPchanges from a solid to a liquid state. Table I shows impedances of waterand several potential substrates.

TABLE 1. CHARACTERISTIC IMPEDANCES OF VARIOUS MATERIALS

DENSITY (p) VELOCITY (C L)

MATERIAL g/cm 3 cm/s PCL

Water 1.0 1.45 x 105 1.45 x 105

Plexiglass 1.18 2.68 x 105 3.2 x 105

Plate Glass 2.51 5.77 x 105 15.4 x 105

Pyrex 2.23 5.57 x 105 12.4 x 105

Teflon 2.2 1.35 x 105 3.0 x 105

Nylon 1.20 2.0 x 105 2.40 x 105

Aluminum 2.7 6.4 x 105 17.3 x 105

Steel 7.8 6 x 105 46.8 x 105

IV. EXPERIMENTATION AND DISCUSSION

A. Initial Recordings

An initial attempt to record an acoustical wave pattern on TP wasperformed. Figure 4 shows the test fixture, a plexiglass base plate with114.3-mm square interior wall to contain the TP, and a high outer wall tokeep any water from the imaging tank out of the test fixture. The threeleveling screws and the two bubble levels were added later to aid in thepositioning of the test fixture on the water surface. Figure 5 shows theinitial TP recording made with this device. Only the 3 MHz reference trans-ducer was activated. The 3 MHz object transducer was not activated. TheTP was S-25. This particular TP was sufficiently fluid at room temperature sothat additional heating was not necessary for image formation. The concentric

9

ripire TPreocor(It kit- o fresitel zonie alcllni cal rIn lt ti ii 11""

pat torn oft tilh- F resne I zonto rinlgsiscoa Vs i h F i 'till' t r8ilt'0

011 L 1iOTO iUntO o fter theo rcferenlce benri was tnrned olif TT Ol',,

no( appa~irent d if tf Crone ill t 1101,L' two0 p )L~togrikph i t tsho hI d Kn IfI

rnii, Imt t ern fnkdod , ien if i cant IN a f tr two illinkite o nd was coMp Ilet k, cone

11t 1 theed (11 f iVe llilinntC,-. This, 0magC ( ad inc wa S'1 cas 'V the S. ' Ph

too fl id :k.t room t emperature and indicaIted that T1, wit h I hischer Ill t il n.

ernt a treV001 Ilk he neded for wrinntrcdiis

Figure 6. TP recording of fresnel zone acoustical ring pattern from

3-MHz reference transducer (one minute after removal of

acoustic signal).

Potential problems with higher temperature TP are shown with the fol1owin:

four figures. Figure 7 is the target object used for acoustical imaging in

3 5

Figure 7. Acoustical target.

II

the remainder of the experiments in this report. A flat washer was tapedto a 12.7-mm thick sheet of plexiglass. The washer was surrounded by a tapedtriangle. Since the tape was not expected to image well, a thin strip ofmetal foil lined the unde,-side of the tape. The target was located in theimaging tank according to Figure 2. To get an idea of what kind of TP imageto expect, a water surface image was first produced by filling the 11 4 .3-mmsquare in the recording device with a 6.35-mm layer of water instead of TP,as shown in Figure 8. Only the object beam was activated. The black dots

e 4

Figure 8. Water surface image of object target usingwater laver instead of TP in recording fixturewith square window (object beam only).

superimposed on the image were caused by dust floating on the water surfatceand water drops on the underside of a focusing lens directly above the watersurface. The water in the square window recorder was then removed and rt-placed with a 2.54-mm layer of S-25 TP previously heated to 120'C. This hi:iitemperature was needed to lower the TP viscosity sufficiently so the TP wouldeasily flow and form an even surface across the bottom of the recording de-vice. Previous experimentation had shown that S-25 TP needed to be heated t(,at least 90'C so that air bubbles caused by pouring the TP would not betrapped in the liquid. The heat from the melted TP caused the 1.524-mm thickplexiglass base of the recording device to warp. Consequently, it wasdifficult to align the recording device so that a flat recording area couldbe found. Figure 9 shows the unevenness of the upper surface of the TP priorto activation of the object transducer. This unevenness of the recording TPdistorted the acoustical image when the object beam was activated as shown inFigure 10. However, this recorded image was still superior to any previousattempts and the object target was clearly visible. It also faded approximate-ly five minutes after removal of the acoustic beam.

12

I ~ n T --wtrja i.i ~ r ~

I Ht I I' i'(rd i n)k oF im, ii n j im r .r Lt[ 011 ; 11, 1 r, w I' !

r rltim device 0,Ib e ct beam on 10

IIi~ tII. .ItiI ,

tII t ,i ;I' 'I ,I !V %i th t '~i '

11 t (.,1t t (oh k" f u'' I

ie~~~ 1c oi 5''I 'iii I-- h p1i:I' i IOrd inaw

I 1, I m tti It t a i ii iw tIew refil i( I ti tI t) - r t 11 (2 I i i

it] V, Ut t ' t 1 ii r1lI W, I II r I :, :i s t iI I! , 1 1I I i.u II) t I( d e tt !

I I d ,ic w' , modi T i I iell tU t ru t1it 1) 11 i I 1 hie I

SI T- , '' 2 11- i M I II-ii u o vz VCr i- t I I i w r th[ie i

itt i t ts It inders ide C ) t I I ie' CC'r' i I: r I averI

I I I sunl ar e L','eraItI;d 11'V '1) C l- 1 1 Vl t 1 irCk 1)l1 eXi*ru' Ii, 1 I 1 It I I1I tup to 0 b.3 T!n . oll I ti i) ,L.eit tr.I-,)cdiirr w> I w t<

ee -i 1 rV [1001-Loorr imrniiCe dUe to 'TrC;ltr C17 Lr5Iln iergv i

tihe I.xij Ir -Olli intecrnalI r eIa xat: i on ai sor,)t i on aind fILt t,(2n 1at i o .

Figure 14. Water surface imag~e with no substrate.

To find a more heat resistant substrate, the water imasingeprmn

was repeated using both mylar and glass. Thie .0 8 89 -mm m vIlaIr d e f ormed,(I, sin!I i -

rcant! v, not from the heat but from the water pressure on its lower- - id, wliew

the reco rding fixture was placed in the img np ink * even thloull ilt le my I awas positioned less than 6.35-mm below tilt, Lank w.*ter suirtarew. Ti ii Id ia e1l

t hat r ii, ! i t v o)f t he subs tra te was als "o *'i desi rahble (ia1I i t v. A I . 12-I i

gl Iass p)la te was suf fic ien t Iv r i id hit. , as -;ini I vI i ci! I I% proven i n Sett i on I I It he ',gLass re fl ccted most o I t he itt ra souud so thla t i nsnif f irc jent enerev wais

avalablc ( to orn a wa ter siur face iimag ie.

II0

I' I 'iJ I IS IC~ Witr

Figure 17. Water surface image with 6.35-mm

thick plexiglass substrate.

C. Effect of TP Surface Smoothness

Up to this point, all images were recorded with only one acoustical

transducer activated. By activating both transducers, an acoustical hologram

can be generated at the liquid surface, although there is some argument that

the images already shown were Gabor in-line holograms. Nevertheless, by

activating both transducers, a true interference pattern is formed. Thisinterference image, when viewed at the optical focal plane, produces a bright

dot which represents the zero order diffraction and a series of diffraction

lines above and below the central dot which represent the first, second, and

higher diffraction views as mathematically shown by Swinson [3]. Figures 18

through 20 show the zero, first, and second order diffraction images dis-

cretely. As can be seen, the contrast is significantly improved by vi),\ in

the target at higher diffraction views. These water surface images Vcr<,

recorded at increasing laser power to compensate for the decreasing bright-

ness of each successively higher diffraction order.

To create similar diffraction images on a TP layer, the surface of the

TP was required to be optically flat. All attempts to record diffraict ion

patterns on the TP were unsuccessful due to a lack of flatness which pre-

vented the reflected light from forming a spot on the optical focal ,l;ine.

Instead, an unfocused folded image was created at the focal plane, makingt it

impossible to separate diffraction orders. This uneven tipper TP surface was

caused bv a combination of several separate problems. Besides the rici.iditv

question which has alreadv been raised, the surface of the T11 formed a con-.vex meniscuis after pouring. This tended to expand the reflected light beam

and destroy the potential for separating diffraction orders at the focalplane. Of course, as the distance from the boundaries increased, the sur Ice

18

I,.I~, 9. ri or w.it (T - Ilrl wo h

Figure 20. Second order water surface hologram.

more closely approximated a flat surface. In fact, a previous experiment had

shown that water in the 114.3-mm square recording device formed a less than

flat surface due to meniscus curvature and this made separation of diffractions

more difficult. By filling the recording device with enough water to cover

the 114.3-mm square retaining walls, thereby causing the outer walls to be-

come retaining walls, the boundary effects were sufficiently remote from thecentral recording area and a normal diffraction pattern was observed at the

focal plane.

A new recording device was designed in a further attempt to create a flat

TP recording surface. As shown in Figure 21, the new device differed from

the old in several ways. The plexiglass base was replaced with 6.35-mm thick

aluminum for rigidity. The 114.3-mm square central acoustical opening was

replaced by a 76.2-mm diameter round hole to maximize the rigidity of the

substrate insert. A 76.2-mm diameter window was the smallest acoustical win-

dow thought practical and the round shape of an acoustical substrate insert

minimized deflections due to underside water pressure. The interior retaining

walls were removed so that the outer walls acted as a retaining wall for the

TP and a water barrier as well, thereby causing the TP to approximate a flat

surface near the central acoustical window since this window was far removed

from the boundary effects.

D. Effect of Substrate Surface Roughness

Ptius (76.2 mm in dia.) of various substrate materials were placed in

these new recording devices. Water surface holograms were taken to compare

these materials. Figures 22 and 23 show the zero and first order diffraction

images through a 3.860-mm thick nylon substrate plug covered with 12.7-mm of

water. The qualitv of the images was significantly below that of the water

surface holograms with no substrate due to the relaxation absorption and

20

I- I -oa -1 vcd I

4VC 21 1 P r'ti V: Ui. ii

Figure 23. First order water surface acoustical holocrarthrough 3.860-mm thick nylon substrate.

it tenIIut ion of the ultrasound through the substrate material. Figure- in adrn,25 show the zero and first order images throuwh a similar nvlon pluc, ..had been sanded to a thickness of 1.778-mm. In comparing these with the twoprevious illustrations, it can be seen that no real improvement of the imacesresulted, even though the substrate thicknese had been more than halve ..act, there was some deterioration of image quality in the first order iial.

I' was theorized that substrate surface roughness might account forthese results, since the thinner nylon plug had a rougher surface caused I1wthe s:inding. The wavelength of 3-,MHz ultrasound in water is 0.508-m7.Saindinn the nylon plug with 100-count sandpaper might have caused surfa-evariati[ons in the substrate of the same order of magnitude as the wave]enotStL. ultrasound, therebv causing the image to distort. To test this thocr.

tho thicker nylon plug was sanded with 100-count paper until rough and anet ,rirst rder water surface holograph was attempted. The image aualitv wasc Iv re.duced as shown in Figure 26. In a further test the surfaces ol t i',

thlinn*r nvlon plug were sanded with increasingly fine sandpaper up to 000c( t. The surfaces were then polished with a polishing compound and a firsterder ho loram was imaged, as shown in Figure 27. Some improvement in theinae can he seen, indicating that an ideal substrate should have acousticallvsmooth sur faces.

Figure 28 shows a first order hologram of a 1.600-mm thick teflon sub-strate plug. This image was clearlv superior to anv previously generated,although the teflon surface was not particularly flat. This indicated thatteflon had a lower degree of attenuation and relaxation absorption tlan othermaterials investigated.

2 2

LI-

lLl rL r J r r V lc t c t :I

FiYtire 25. First o)rde~r witer sir fnce, nceu.'sticaI ]l ~ -titroupli I 77 8 -mmi thick nvlon substrate.

Figulre C6. First order water suirface acoustical ho~locrav--

t Ii rouc izh 3. 8 6 l-mm thIiic k nv 1 on s ub st ri te (sa nd o( rati

Figure 27. First order water surface acoustical hologramthrough 1.778-mm thick nylon substrate (polished).

24

Figure 28. First order water surface acoustical hologramthrough 1.600-mm thick teflon substrate.

E. Heating Device

A device was designed and constructed to heat high melting point TPto a liquid for image formation. As seen in Figures 29 and 30, the deviceconsisted of a plywood top with a central opening for the spatially filteredlaser beam. Thin (0.254-mm dia.) stainless steel wire was wound between twosets of hooks, which were attached to spring loaded movable supports. Heatwas generated by passing an AC current from a variable transformer throughthe resistive device (57 ohms cold). As the wire heated, significant thermalexpansion occurred, but this was compensated for by the expansion of thesprings on each movable support, thus keeping the wires taut. Experimenta-tion showed that 160V could be tolerated by this device before the wiresbroke (500W). Figure 31 shows the heating device installed in the recordingfixture. The construction of the heater is such that the wires are positionedapproximately 6.35-mm above the TP layer. Figure 11 shows the entire assemblyinstalled in the imaging tank.

F. Additional TP Recordings

The round window recording device was prepared for TP recording.The substrate plug used was Teflon, since it had previously produced the bestwater images. The plug was attached to the aluminum base of the recorderwith silicon rubber so that the Teflon upper surface was level with thealuminum upper surface. Small gaps between the edges of the two materialswere filled with automotive body putty so that the smoothest possible con-tinuous surface could be achieved. The recorder base was then coated with2.54-mm thick layer of S-55 TP, a material which is rigid at room temperature.

The TV had to be heated to 125'(C before pouring in order for it to flow to an

25

Figure 29. Bottom view of new heating device.

Figure 30. Side view of new heating device.

26

L ~" ,. . . ..

- Ci, i

Figure 31. Heating device in recording fixture.

even layer on the recorder. The heating device was then installed. Theassembly was first tested with no acoustic signals to verify that the TPwould adequately liquify with the heater activated. This test was necessarysince it was not known whether or not the effect of having the bottom of theTP coupled through a heat conducting medium (the aluminum base or the thinsubstrate) to a heat sink (the tank water). One could assume that the TPwould be in a multiple phase during heating since the top would liquify fromthe radiant heat but the bottom might always remain close to the temperatureof the tank water and not liquify. The test showed that the TP liquifiedcompletely. However, the square viewing hole had to be covered with anotherwood block so that enough heat could be retained in the vicinity of the TPfor proper liquification. This did not, as one might assume, destroy thepossibility of in situ real-time recording. However, previous experimentationhad shown that the laser light image was temporarily distorted when theheater was activated due to air convection currents. These distortions dis-appeared immediately when the heater was deactivated. By modifying theheater so that the central window was permanently covered with optical gradepyrex, one could use the heater without adding and removing a heat retainingcover.

When the heat was removed, it was observed that the TP surface wasslightly distorted around the perimeter of the substrate disk. Although greatcare had to be taken to insure a smooth continuum between aluminum base andsubstrate disk, there were still slight depth variations. When the TP wasliquified or when water was used as an imaging medium no upper surface im-perfections existed since the shear forces caused by these depth variationscould not be transmitted from the bottom of a liquid imaging media to the top.Consequently, the top of these liquids always was plane. However, as the TP

27

I I h r orI nI !!I ~ I t tot c I c I In n~-t tlt U ir i

I I~'iv t2 w,5 ' I vnr Irer If l oI It Iit I tI I I ' ' I k1 1 i

'ho re-n ot -i of 1 , it I ti''tL~tiv nn I ''t k- t.I It I v X; rn v

IisC i t W k 'I "L i"t ' I I I I tn :i~l w, i neuksr to 11 t~I I is I-101. 1 i' C't t' LI I'lto 111 I I ii ti' Ii I ' t 'oi eV C to I I I I' w I I c I Ion I

ti kn M I vr Ani S CeA i ~ 1't1 w n ri he oeid'' tin prniI ris in1l 'Ih h i II' I t I Wo I- t i~ i I Ih Il I 11,it I dci I i

V C Vt I r;t i I to 1 l'rtii iil i i ion, 1i L' ' ', I-I' kl t, I Ifo O

!wiII Iit I' iIis k, r S Io t ist ito . Ol ns t hnI hi

TP ;,. ro I' e r rI,,I o IIc IIist I cI - :i

CL( V,0I N R O '1N A I N

!P ro order ini;wk, thirtv iimit rk--o ;f Iw if1\.

energy might be transmitted to the TP. For a similar reason, the substrateshould be as thin as possible. However, the thinner a given substrate, theless rigid. Experimentation showed that slight surface deformation caused bywater pressure and TP weight had a severe impact on the ability of the re-corder to reproduce diffraction order views. The recorded image was alsbsubject to distortion if the substrate and TP surfaces where not acousticallypure; i.e., if the surface roughness was not at least an order of magnitudebelow the wavelength of the ultrasound.

The TI' layer needed to be no thicker than that necessary to free thecentral recording area from boundary effect distortions. Thickness of TI'above this caused a lessening of the visual image since the TP itself exhibits

significant relaxation absorption and attenuation of ultrasonic signals.The type of TP used was shown to be a trade-off between a sufficiently high

melting point (so that the TP was rigid at room temperature) and some maximumtemperature (dependent on the substrate) so that the substrate would notthermally deform when the TP was heated to a liquid during holographicrecording.

Recommendations for system modification include the following. Smallerdiameter transducers of higher power would enhance the images by allowing thesystem to operate in the "cleaner" Fraunhofer zone as opposed to the fresnelzone, where the large diameter transducers currently being used operate. Theadditional power would also allow the use of a substrate with a poorercharacteristic impedance match but greater rigidity. Therefore, a recordingfixture with a separate acoustical window would not be necessary so that nolocalized substrate boundary effects would affect the TI' surface smoothness.Thus, the TP would be able to record and reproduce the higher order diffrac-tion images. In addition, higher power transducers would allow the use of ahigher frequency ultrasonic signal, which would improve the resolution of thesystem.

In addition, the'use of thin film (thickness 0.254-mm) TI' should be in-vestigated. Although this has been tried previously without applying a suf-ficiently smooth and level coating of TP, the technology does exist toperform this coating successfully.

The effect of TP surface modification during cooling should also beinvestigated, since failure of the TP to retain the acoustical interferencepattern correctly would obviously eliminate it from consideration as a re-cording medium.

30

REFERENCES

i. Irelan, V. G., B. R. Mullinix, J. G. Castle, "Real-Time AcousticalHolography Systems," US Army Missile Research and Development Command, Tech-nical Report T-78-10, Redstone Arsenal, AL, October 1977.

2. Swinson, W. F., "Optimizing a Real-Time Acoustical Holography System,"US Army Missile Command, Technical Report RL-80-2, Redstone Arsenal, AL,1 October 1979.

3. Swinson, W. F., "Improving a Real-Time Acoustical Holographic System forFlaw Detection," US Army Missile Command, Technical Report RL-80-3, RedstoneArsenal, AL, 35809, 1 October 1979.

4. Liu, H. K., "A Thermoplastic Device for Real-Time In-Situ Recording ofAcoustical Holograms," US Army Missile Command, Technical Report RL-80-4,Redstone Arsenal, AL, 1 October 1979.

5. Young, J. D., J. E. Wolfe, "A New Recording Technique for AcousticalHolography," Applied Physics Letters, Vol. 11 (1967), p. 294.

6. Schaeffel, J. A., Jr., "Acoustical Speckle Interferometry," TechnicalReport T-79-39, US Army Missile Research and Development Command, RedstoneArsenal, AL, March 1979.

7. Brekhovskikh, L. M., Waves In Layered Media, Academic Press, New York,1960.

8. Hildebrand, B. P., B. B. Brenden, An Introduction to Acoustical Holography,Plenum Press, 1972.

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