TECHNICAL REPORT

ISTATES

FRACTURE TOUGHNESS OF DGAC STEEL SHILLELAGH

ROCKET MOTOR CASE

by-

ROBERT N. KATZ

MATERIALS ENGINEERING DIVISION

U. S. ARMY MATERIALS RESEARCH AGENCYWATERTOWN, MASSACHUSETTS 02172 JULY 1964

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NBS 9 64 PROCESSOR:

Rocket motor materials

Steel, high strength

Fracture toughness

FRACTUBE TOUGHNESS OF D6AC STEELSHI LLELAGHI PJCKET MOTOR CASE

Technical Report AMRA TR 64-19

by

Robert N. Katz

July 1964

AMCMS Code 5541.12.25102.05

Burst Tests of Shillelagh Rocket Motor Case

MATERIALS ENGINEEHING DIVISIONU. S. ARMY MATERIALS RESEARCH AGENCY

WATERTOWN, MASSACHUSETTS 02172

U. S. ARMY MATERIALS RESEARCH AGENCY

FRACTURE TOUGHNESS OF D6AC STEELSHILLELAGH ROCKET MOTOR CASE

ABSTRACT

Burst tests and fracture toughness measurements on D6AC Shillelaghmissile motor cases were carried out. The burst testing was done overa temperature range from room temperature down to -200 F. Results ofthese tests showed that the motor cases burst at approximately 250,000psi hoop stress regardless of test temperature, and that all fractureswere 130 percent shear. Fracture toughnss measurements were made oncases containing machined and fatigue-cracked through-the-thicknessnotches, at roorl temperature and -65 F. The results of these measurementswere in good agreement with previously obtained flat sheet data on similarmaterials.

ý•-1L

Physical Metallurgist

APPROVED:

E. N. HEC4E• •ChiefMaterials Engineering Division

CONTENTS

Page

ABSTRACT

INTRODUCTION................... . . . . . . . . . . . . . . .... 3

BACKGROUND................ ....... . . . . . . . . . . . . . 3

MATERIAL .......................... .............................. 3

FABRICATION AND HAT TRhEATMNT.... .. ......... .................. 4

EXPERIMENTAL PROCEDURE ..... ................... . . .. . 5

BURST TESTING .................... . . . . . . . . 5

FRACTRE TOUGHNESS MFASUREMENS ...... . . . . . .... . 6

RESULTS AND DISCUSSION ..... ....................... ... . .. 7

CONCLUSIONS ......................... ............................ 10

ACKNOv'oLEDGME:NT30. ............. .......................... 11

APPENDIX A. FLECTRON bEAM ;iVLDS...... ......... ................ 12

APPENDIX B. END CLOMURE AND PRESSURE PACKING DESIGN ........... .. 14

REFER.FNCES ...................... ............................ ... 16

INTRODUCTION

The initial design of the Shillelagh missile motor case specifiedthe use of Hll steel. In tests conducted by this Agency during October1961 on Shillelagh motor cases fabricated from H1I steel of 0.047-inchthickness, it was shown that brittle fracture occurred at -65 F. A recom-mendation was made to change the material to one which would have highfracture toughness and would behave in a ductile manner at this lowtemperatu:e. It is interesting to note that subsequent to this recommenda-tion, investigations1 ' 2 on the fracture toughness of till steel have shownthat the critical thickness at room temperature for the ductile-brIttletransition for material with yield strengthr ranging from 190 ksl to about230 ksi is approximately 0.040 inch. These findings further validate therecommendation to change t1,e material for the Shillelagh missile motorcase. The material recommended was D6AC steel.

This report will be concerned with the burst tests and fracturetoughness measurements on the Shillelagh missile motor cases. Burst teztEof actual Shillelagh motor cases fabricated from D6AC steel were carriedout. Values of the fracture toughness calculated directly from motor

cases containing through-the-thic,,nfzs notches were obtained and werecorrelated with fracture toughness obtained from flat sheet tensile data.

3

PACKGROUND

To provide technical support for the Shillelagh motor case program,this Agency undertook a project which consisted of a thorough investiga-

tion of the smooth and notch tensile properties of 0.040-inch-thick D6AC

sheet material. 3 From this investigation it was learned that D6AC steel,which has been austenitized at 1650 F and quenched and tempered in the

1000 to 1100 F range, maintained adequate fracture toughness (Kc greater

than 175 ksi i/1-.) at temperatures well below -65 F, while at the sametime maintaining high levels of yield and tensile strengths (yield

strength greater than 220 ksi). The relevant smooth and notch tensiledata for D6AC steel sheet 0.040-inch thick are presented in Table I.

MATERIAL

The Shillelagh missile motor cases studied in this investigation

were provided by the prime contractor at the request of U. S. Army Missile

Command. These motor cases were made of D6AC steel forgings whose com-

position is given in Table II.

3

TABLE I. Smooth and Notch Tensile Properties of 0.040-Inch D6AC Sheet

Test Yiele __il I bT•mperature Strength Strength NTS NTS er im-Ib

1000 F Temper

HT 216.5 238.5 197.0 0.83 100 182.5 1110-40 223.5 253.5 193.5 0.77 90 182.5 1110

-120 226.5 256.0 161.0 0.63 85 128.0 545-160 M29.0 262.5 156.0 0.60 4r, 114.0 435

-200 233.5 271.5 147.5 0.55 2S 10.5 380

1050 F TemperlRT 211.5 237.5 207.0 0.87 100 197.0 1295

-40 215.0 247.5 203.5 0.82 100 192.5 1235-)20 229.0 256.0 169.0 0.66 95 141.0 665-160 224.5 2M5.0 154.0 0.58 63 11S.5 445-200 237.5 269.0 143.5 0.53 23 103.0 355

1100 F temperFVT 209.5 232.0 215.5 0.93 100 207.5 1435

-40 218.5 243.0 205.2 0.85 100 188.0 1180-120 219.5 244.5 185.5 0.75 90 169.5 960-160 220.0 251.0 158.0 0.67 95 143.5 685-200 236.5 264.0 138.5 0.53 20 104.0 360

All specimens austenitized 20 minutes at 1650 F. air cooled.All specimens tempered 1+1 hour at temperatures indicated.

TABLE II. Chemical Analysis of D6AC Steel

Elements (weight %)

C 0 Si Cr Ni P S Mo V

Woufacturer's 0.43 0.78 0.26 0.97 0.58 0.008 0.009 0.97 0.08Analysis

Normal Cceposition 0.42- 0.60- 0.15- 0.90- 0.40- 0,015 0.015 0.90- 0.05-Limits 0.48 0.90 0.30 1.20 0.70 ga ax 11.0 0.10

FABRICATION AND HEAT TREATMENT

The Shillelagh missile motor case consisted of three D6AC forgings:one for the gas generator, one for the motor, and one for the bulkheadbetween these two sections. These forgings ranged in minimum sectionthickz.:ss from 0.040 inch in the gas generator to 0.047 inch in themissile motor and had built-up wall thicknesses next to the bulkheads.The three forgings were electron-beam welded (see Appendix A) to form thecoirplete Shillelagh missile notor case unit. A photograph of the crosssection of this motor case is presented in Figure 1. After welding, themotor casez were given the following heat treatment: preh(;at to 900 F;austenitize in an inert bath for '0 minutes at 1650 F; quench in salt at400 F for 5 minutes; air c)ol; temper one hour in salt at 4OO to 450 F;

4

WALL N.•7 INI "HCK WALL O.C[40 tINCY THICK

19-066-1872/AMC-63

Figure I. CROSS-SECTIONAL VIEW OF SHILLELAGH SOLID PROPELLANT MISSILE MOTOR CASE

air cool; and air temper at 1025 F for two hours followed by an air cool.This heat treas'ment rendered the material fully martensitic with a Rock-well C hardness of approximately 45.

EXPERIMENTAL PROCEDURE

The experiments consisted of evaluating the burst strength of theShillelagh missile motor cases at temperatures ranging from room tempera-ture down to -200 F, and of measuring the fracture toughness of the casesat room temperature and -65 F. In ordcr to make these measurements, itwas necessary to devise pressure packings and end closures which couldmaintain their integrity at test temperatures as low as -200 F (fordetails on packings see Appendix B). To measure the fracture toughness,it was necessary to machine through-the-thickness slots in the r,,torcases. Fatigue cracking of some of the motor cases was also attemptedand various means of measuring slow crack growth were investigated.

BURST TESTING

Bur-t tosts were accomplished by packing 1oth ends of' the Shillelaghmissile moter case, as described in Appendix B and loading hydrostaticallyby means of a pressurizing fluid. The prcs-;surizi.g fluid:; used werewater-soluble oil for room temperature test:; and ethylene glycol for low-temperature tests. The low-temperature tests were made by imnaersing themotor case containing the press;,;urizing fluid in a bath of' the desiredtemperature and allowing the system tc equlibrate. Temperature readirngswere taken by means of' thrrmocouples both within the motor case and inthe low-temperature bath. Whon temperature rquilibrium was achieved, thhemotor cases were loaded to failure.

FRACTURE TOUGHNESS MEASUREMENTS

To calculate the plane stress fracture toughness Kc of D6AC Shillelaghmissile motor cases, the following formula4 ' 6 for the fracture toughnessof through-the-thickness notched cylindrical pressure vessels was used:

Acr = 1i (• ") wl k\a Ys/

Rearranging terms to solve for Kc,

Kc r (2)

whe re

Kc = plane stress fracture toughness

a = applied stress in vessel normal to crack (hoop stress)

Acr = 1/2 critical crack length (visual, estimate)

C~ys = tensile yield strength (at test temperature).

The hoop stress a was calculated on the basis of the expression:

PD (3)

where

P = burst pressure

D = vessel diameter

t = vessel thickness.

in order to utilize Equation 2 to calculate Kc values for theShillelagh motor cases, through-the-thickness notches were marhined infive of the motor cases. These notches were Elox slots 0.009 inch thickand 0.150 long (one case had a 0.250-inch-long notch). Three of thecases were fatigued to produce natural cracks. Case 110 was cycled froma maximum load of 2000 psi to a minimum load of 200 psi over 375 cyclesand between 1500 and 200 psi over 450 cycles, which produced a fatiguecrack 0.515 Inch long (including the 0.150-inch Elox slot). Case 105 wascycled from niaximum loads of 2000 psi to minimum loads at 200 psi over aperiod of 425 cycles, and at a maximum load of 1500 psi over a period of173 cycles. This produced a fatigue crack 0.610 inch long. Case 113 wascycled between 2200 and 200 psi 60 times, between 2000 and 200 psi 65times, and between 1600 and 200 psi 45 times. This resulted in a fatiguecrack 0.300 inch long.

6

In order to maintain the internal pressure in the presence of athrough-the-thickness notch, it was necessary to use a patch. The inter-nal patch utilized in this study consisted of a layer of copper next tothe notch, backed by a layer of Teflon, which was backed by a layer of'Mylar; these layers were approximately 0.010 inch thick. The layers wereheld In place by neoprene cement.

Several methods of determining the slow crack growth 2Acr were tried.One of these was an acoustical method using microphones and vibration-sensitive transducers to pick up the onset of fast fracture. However,the high level of background noise du• to the mechanical pumping equipmentýused to pressurize the motor cases) precluded the success of thistechnique. Ink staindng also proved unsuccessful due to the ability ofthe pressurizing fluid to dissolve the stain. The most satisfactorytechnique proved to be visual determination on the basis of changes infracture appearance.

RESULTS AND DISCUSSION

The results of the burst tests conducted on the D6AC steel Shilleiaghmotor cases and fracture tougnness measurements on these cases are sum-marized in Table III.

TABLE Ill. Hurst Test Data for D6AC Shillelagh Motor CasesIiie Hoop

Slot Burst Stress Approximote Test4:gt gSAc Pressure U Shear Temperatureas, (inch. 01 1(i 8 k i (ks i) (ks ~i) (% ) (r

112 None 3.8' - -653.7 244 100 70

164.0 253 -100 -55

116 4.0 254 - 10. -65Ill 3.9 249 100 -85104 3.9 - - -200115 0.150 0.148 3.1 183 172 100 70101 0.250 0. 00 2.9 183 156 100 -65

O.15IoDO fatiqged 0.257 1.6 10? 144" 100 70

to 0.5150.150

105 fatiqued 0.305 1.4 88.6 90"" 100 70to 0.610

0.150

113 fatiqued 0.173 2.4 151 168 100 70to 0.300

"oCaoe yielde.1 without fracture. end closure leaked."*Case yielded durinq fatiquing, hence Ke was low due to strain hardening.

7.

All of' the IYuAC Shillelagh rocket motor cases broke in the missilemotor section of' the case, even though the wall thickness in this portionwas 0.047 inch as compared to 0.0240 inch in the gas generator section.However* the gas generator section had a much smaller length-to-dianeterratio than the rocket motor portion. As a result of this condition, thebulkhead imposed more restraint on the gas generator portion than on the

rocket motor portion and therefore failure occurred in the rocket motorportion. Every D6AC Shillelagh motor case which burst exhibited 100percent shear fracture. This was true from room temperature down to -85 F.The burst hoop stresses which were recorded for the unnotched motor caseswere all between 244,000 and 254,000 psi, that is, within )4 percent ofeach other. It is interesting to note that the two cases which yieldedand leaked during testing yielded in this 250,000 psi range. Thus, theD6AC Shillelagh motor case displays an extremely high degree of reproduc-ibility of burst strength even over a wide temperature range.

On' ec (No. 11.5) ;2,•_.ch contained a 0.150-inch-long through-the-thickness notch, but was not fatigue cracked, was tested at 70 F. The Kccalculated from this test was 172 ksi i.., which compares favorably tothe value of approximately 190 ksi j•h which may be interpolated fromthe sheet data presented in Table I. A second Shillelagh motor case(No. 101) containing a 0.250-inch-long through-the-thickness notch withno fatigue crack, was tested at -65 F. This case had a Kc of 156 ksi i-.,which, again, when compared with Table I is low but not more than 10 to

15 percent. A photograph of this failed case together with a fractograph

of the resultant fracture surface is shown in Figure 2. Two cases

COMPOSITE FRACTOGRAPH SHOWING THE EXTENT OF THE SLOW CRACK GROWTH REGION

(BOUNDED BY ARROWS) AND THE FRACTURE SURFACE

9ONTTAt VI! . OF 8HU',T MOTOP CA,! . THE SH[I AN NAIUI•r OF THE FACTUNEI'. FVI0),NT IN THIS PHOTOGRAPH.

Figure 2. CASE NO. 101

8

i Best Available Copy

(Nos. 110 and 105) were fatigue cracked but were stressed beyond the yieldpoint in the vicinity of the crack during fatiguing,* producing localstrain hardening and a resultant change in the fracture toughness in thevicinity of the strain hardening. Thus the values of the fracture tough-ness parameter Kc obtained from these two cases cannot be consideredreliable. However, it is interestinLr to note that even in these two cases(which, as can be seen from Table III, had the lowest burst strengths andKc values encountered) the burst pressures were higher than the serviceoperating pressure of 1100 psi. This behavior in the presence of fatiguecracks in excess of 0.50 inch is, to say the least, very desirable. Aphotograph of Case 110 and its fracture surface are presented in Figure 3.

loxFRACTOGRAPH SHOWING FATIGUE CRACK. NOTE THE DIFFERENCE IN APPEARANCE BETWEEN THIS CRACK

INITIATION AREA AND TOaAT IN THE PRECEEDING FIGURE. NAMELY LACK OF DEFINITIVESLOW CRACK ZONE AND THE FLATNESS OF FRACTURE.

FRONTAL VIEW OF BURST MOTOR CAST. YHE SHEAR NATURE OF THE CRACK (BFYOND THE FAT'GUECRACK) IS AGAIN FVIDENT. SLIGHT YIILI)I(:, (BULGING) IN THE NOTCH VICINITY IS ALS•O EVIDENT.

Figure 3. CASE NO. 110

t 'I e ' I Lo (,IAI tC, t opy trh , ?I o t I' t i t) i t t

Best Available Copy

The fatigue cracking of motor case 113 was completed without the incidenceof plastic deformation in the vicinity of the crack. The Kc value of 168ksi /17h, obtained by testing this motor case at room temperature, maythus be considered to be reliable. It is indicative of the high degreeof reproducibility mentioned above that the room temperature values of Kcfor the Elox and fatigue-cracked specimens (Nos. 113 and 115) differ byIt ksi V17W

Although the agreement between the Kc valh s measured from sheetspecimens and those obtained from actual Shilleiagh missile motor casesis good, the values obtained from the cases are consistently lower thanthose measured from the sheet. There may be several reasons for this.First, the Kc values& measured from the cases were based on visuallyestimated values of the slow crack growth region and are therefore equiv-alent to K., values,* while the sheet data were based on percent shearmeaýurements and thus were Kclt values.* It is known that values based onpercent sV',ar measurements are generally higher than those based on slowcrack growth neasurements. Secondly, the possibility exists that bulgingof the internal patch could act as a wedge at the inner surface of thenotch and thus reduce the measured fracture toughness. Of course, evenin the absence of the above effects, the differences in heat treatmentand processing history between the sheet material and the actual motorcases could account for the observed differences in fracture toughness.

CONCLUSIONS

1. The J•AC steel is more reliable than HI1 steel for use in theShillelagh missile motor case because of superior fracture toughness atlow temperatures.

2. The IY)AC Shillelagh motor cases tested evidence an extremely highdegree of uniformity and reproducibility of properties.

3. Measurements of the fracture toughness may be made from actualmotor cases using visually estimated slow crack lengths. These measure-ments are in good agreement with sheet data previously obtained formaterial of the same thickness and heat treatment.

4. The Shillelagh rotor case fabricated from DIýAC steel can have athrougsh-the-thicness: raek greater than 1,/2 inch long and still toleratepressures exceeding the design pressure.

,A~ I P%~ to t'~" Orar~ tur to u -Pa a1R te u-~1,' ir 1 2 ont'ý- n PLtC?ý,,an Rpas 2 r i n,? t Po

54.ci~n an, a ~of"7''ttf ion t.'-'. tap o' tAs' t'cc't SQar r on"'Cts for ?'" oc ,, h," ' • , ! 011 tho crar1.

10

ACKNOWLEDGMENTS

The author would like to express his appreciation to Mr. Edward LeMayof the AMRA High Pressur3 Facility for his assistance in conducting theburst testing of the Shillelagh motor cases, and to Mr. Kenneth Abbott,Chief of the Weapons Materials Section, for his help at all stages of thisstudy, but especially in the redesign of the low-temperature pressurepackings.

11

APPENDIX A

ELECTRON BEAM WELDS

Since the fabrication of the Shillelagh missile motor cases includedelectron-beam welding, it would be appropriate to discuss briefly thebehavior of the electron-beam welds. Historically, welds have been the"Achilles' heel" of many solid propellant missile motor cases. It isinteresting to note in the testz; this Agency performed on the Shillelaghmissile motor cases that:

a. no cracks or failures initiated in the welds; and

b. cracks, when running through the welds, would not propagateeither along the weld or in the heat-affected zone.

Figure A-i shows an early electron-beam weld on D6AC material. Aclearly defined weldment and heat-affected zone can be seen. Figures A-2and A-3 show welds cut out of Shillelagh motor cases 115 and ].06. Theseweld segments were taken from the motor case in the vicinity of the crack.It is observed that the weldment and heat-affectcd zone are almost indis-tinguishable microstructurally from the matrix ,;:tcrial and that thefracture is in the base metal, not the heat-affected zone. Simllar micro-structural observations were made by Kern and Lubin' for electron-beamwelded DSAC heat treated after welding (as was the Shillelagh motor case).

Un, f f,!c tvd

8,v•, tHAI Mi,? Welndment

44

NITAL f TCH 50X

Figure A-I. EXAMPLE OF EARLY ELECTRON-BEAM WELD IN D6AC SHEET

12

Welrmnt A? Unaffected Fractureiedw HI Bse (tdl Surface

ry

-~ -,

*IA ETH4o

.96 .~I Vj -

Yk,' 4.. Jv1 *7

WITAL ETCH saxFigure A-2. ELECTRON-BEAM WELD IN CASE 115

APPENDIX B

END CLOSURE AND PRESSURE PACKING DESIGN

The end closure and pressure packing design which was chosen for useat low temperatures (also used at room temperature) consisted of a toppacking support, packing, and bottom packing support. The top packingsupport was comprised of a split-ring packing follower seated on a solidpacking restrainer. The packing follower split ring was made of segmentswhich, when in place, formed a circle with their outer faces and a concavehexagon with their inner faces. This concave hexagon fit over a convexhexagonal surface on the packing restrainer. The hexagonal design of thecontact surfaces between packing follower and packing restrainer allowedsome individual motion of each segment of the follower without segmentcocking which would result in a leak.

Upon the application of pressure the packing restrainer is forced up-ward. This upward motion of the packing restrainer results in radialmotion of the packing follower ring segments. To prevent yielding of themotor case as a result of the radial movement of the packing followersegments, it was necessary to use heavy restraining rings around the motorcase ends.

The packing consisted of a layer of Thiokol-impregnated leather, ap-proximately 1/8-inch thick; a layer of silicon rubber, approximately 3/8-inch thick (this material has a glass transition temperature of about -110C, which allows it to maintain its rubber elasticity at temperatures wellbelow other rubbery polymers); and a layer of Teflon, approximately 1/8-inch thick. This packing, with top restrainer and follower on top of it,was supported by the bottom restrainer which consisted of a simple disk.The entire assembly was held together by a rod which was locked with anut at either end of the packing. On one packing this rod contained acenter bore allowing admission of pressurizing fluid to the motor case.The end closure assembly is shown in Figure B-I. After insertion into thecase, the restraining ring was placed around the case in the vicinity of

the packing.

14

TOP VIEW OF TOP PACKING SUPPORT SIDE VIEW OF TOPRESTRAINER AND FOLLOWER PACKING SUPPORT

II

r'---r, .... -. | -- • ---

CROPS-SECTIOFAL, 'TO OF CROSS-SPCTPIDAL VIEW OF PACKING IN MOTOR

ASSEMBLED PACKING CASE wITH RESTRAINING qING IN PLACE

Figure 3-1

15

REFERENCES

1. SLINEY, JOSEPH L., and SCHMID, FRED. The Notch Properties and ShearTransitional Behavior of H1l Steel. Watertown Arsenal Laboratories,WAL TR 766.5/2, February 1963.

2. MATAS, S. J., HILL, M., and MtNGER, H. P. Current and Future Trendsfor Steels with High Strength and Toughness'. Metals EngineeringQuarterly, v. 3, no. 3, August 1963, p. 7.

3. KATZ, ROBERT N. Effect of Tempering on Notch Properties of D6AC AlloySteel Sheet. U. S. Army Materials Research Agency, AMRA TR 63-35,November 1963.

4. CARMEN, C. M., et al. Effect of Solidification Practice on theProperties of High-Strength Steels. Presented at the Fall Meeting ofthe Metallurgical Society of AIME, 1963.

5. TIFFANY, C. F., and LORENZ, P. M. Notes for Meeting of ASTM FractureToughness of High Strength Materials Committee, August 1962.

6. AMERICAN SOCIETY FOR TESTING AND MATERIALS. Fracture Testing of High-Strength Sheet Materials. Report of a Special ASTM Committee, ASTMBulletin, January 1960, p. 29.

7. KERN, W. I., and LUBIN, L. E. Electron Beam Welding of AerospaceMaterials. Directorate of Materials and Processes, ASD-TDR-63-132,Aeronautical Systems Division, Air Force Systems Command, February 1963.

16

U. S. ARMY MATERIALS RESEARCH AGENCY

WATERTOWN, MASSACHUSETTS 02172

TECHNICAL REPORT DISTRIBUTION

Report No.: AMRA TR 64-19 Title: Fracture Toughness of D6AC SteelJuly 1964 Shillelagh Rocket Motor Case

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Headquarters, Aeronautical Systems Division, Wright-PattersonAir Force Base, Ohio 45433

1 ATTN: ASRCN-11, Mr. F. J. Fechek2 ASRCEM-1, Mr. R. F. Klinger2 ASRCT, Manufacturing and Technology Division1 ASRCNP, Dr. Tambc.rski2 ASRCEE-I-2, Mr. H. Zoeller

6593 Test Group (Development), Edwards Air Force Base, California 925.Z1 ATTN: Solid Systems Division, DGSC

1 U. S. Atomic Energy Commission, Office of Technical InformationExtension, P. 0. Box 62, Oak Ridge, Teanescee

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Philco Corporation, Aeronutronics Division of Ford Motor Company,Ford Road, Newport Beach, California

1 ATYN: Aeronutronics Library2 Mr. Charles Martin

Commanding Officer, U. S. Army Materials Research Agency,Watertown, Massachusetts 02172

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106 TOTAL COPIES DISTRIBUTED

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