SWER~R-TR-72-1

0 • FATIGUE PROPERTIES OF GUN BARREL MATERIALS

TECHNICAL REPORT

Dr. Kailasam R. lyer rand

Richard B., Miclot ~'

January 1972

.L41 L ',

RESEARCH D IRECTORATE

WEAPONS LABORATORY AT ROCK -ISLAND

RESEARCH, DEVELOPMENT AND ENGINEERIN5 DIRECTORATE

U. S. ARMY WEAPONS COMMANDRopdtllod by

NATIONAL TECHNICALINFORMATION SERVICE

Spji qfiold. Va 27151

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UnclassifiedSecurity Claiisfication

DOUETCONTROL DAA- R & D•(SccwUiy Cia4*S1ication *I 1I11e, body of abstract and Indexnj annotatlion must be enfered when the overall report Is CeeaiS lcdj

1. ORIGINAI"ING ACTIVITY (Co sp'ela . Au• ath ) I U.. REPORT SECURITY CLASSIFICATIONU. S. Army Weapons Command UnclassifiedResearch, Dev. and Eng. Directorate 2b. GROUPRock Islard, Illinois 61201

3. REPORT TITLE

UFATTGUZ PROPERTIES OF GUN BARREL MATERIALS (U)

4. DESCRIPTýV, NOTES (2.pe oflfepo tand inclualve dateI)

a. AU INORIS)(Fitat name, middle initial, tat nts e)

K. R. Iyer and R. B. Miclot6. REPORT DATE J a. TOTAL NO. OF PAGES Tb. NO. OF REFS

January 1972 32 880. CONTRACT OR GRANT NO. 7a. ORIGINATOR'S REPORT NUMI$ER(iS)

* b. PROJECT NO. SWERR-TR-72-lDA 1T061102B32A

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Approved for public release, distribution unlimited.I I I SUPPLEMENTARY NOTES 12. SPONSORING MILITARY ACTIVITY

U. S. Army Weapons Command

I1. ABSTRACT

A continuing program to determine the fatigue characteristics of gunbarrel materials was ,in'tiated by the Research Directorate, WeaponsLaboratory at Rock Island. The results of this program will lead tothe prediction of fatigue damage in gun barrels and will facilitatethe development of improved barrel materials. The outside surface tem-peratures of an M60, 7.62mm, gun barrel were measured during test firingat various locations along the length of the barrel. The temperaturesacross transverse sections of the barrel were calculated, by the im-plicit finite-difference method. These data will be used to generatethe stress-time-temperature profiles representative of service con-ditions of the barrel. Room temperature fatigue properties of Cr-Mo-Vsteel under axial push-pull and rotating-bending loading conditionswere determined. The data were analyzed to study the effect of pre-stress on ttte fatigue properties of the material and to develop stress-life curves for design purposes. Detailed metallographic examinationof the fatigue fractures was peribrmed. The analysis shows that theeffect of static tensile stress ii to lower the amplitude of the alter-nating stress corresponding to the endurance limit of Cr-Mo-V steel andthat the fatigue fracture characteristics of Cr-Mo-V are similar tothose of fine-grained, quenched, and tempered steels. (U)(Iyer, K. R. and Miclot, R. B.)

14ON M 41' JN 4-WIH-DD, Nov1473 C1""' ot ,A, 64, Unclassified'SoctCulty CI*SSIEIr:a~on . . 7. /

i•:• unqlgS;lfled

Security classification

-KEfy WOROS- - -

ROLE WY ROLE WT ROLE WT

1. Cr-Mo-V Steel

2. Fatigue

3. Temperature Measurement

Unclassified

Sewity Classlfication

AD

RESEARCH DIRECTORATE

WEAPONS LABORATORY AT ROCK ISLAND

RESEARCH, DEVELOPMENT AND ENGINEERING DIRECTORATE

U. S. ARMY WEAPONS COMMAND

TECHNICAL REPORT

SWERR-TR-72-1

FATIGUE PROPERTIES OF GUN BARREL MATERIALS

Dr, Kailasam R, Iyer

and

Richard B. Miclot

January 1972

DA ITO61102B32A AMS Code 501B.11,85500 02

Approved for public release, distribution unlimited.

ABSTRACT

A continuing program to determine the fatigue character-istics of gun barrel materials was initiated by the ResearchDirectorate, Weapons Laboratory at Rock Island. The resultsof this program will lead to the prediction of fatiguedamage in gun barrels and will facilitate the developmentof improved gun barrels.

The outside surface temperatures of an M60, 7.62mm,gun barrel were measured during test firing at various lo-cations along the length of the barrel. The temperaturesacross transverse sections of the barrel were calculatedby the implicit finite-difference method. These data willbe used to generate the stress-time-temperature profilesrepresentative of service conditions of the barrel.

Room temperature fatigue properties of Cr-Mo-V steelunder axial push-pull and rotating-bending loading con-ditions were determined. The data were analyzed to studythe effect of prestress on the fatigue properties of thematerial and to develop stress-life curves for designpurposes. Detailed metallographic examination of the fa-tigue fractures was performed.

The analysis shows that the effect of static tensilestress is to lower the amplitude of the alternating stresscorresponding to the endurance limit of Cr-Mo-V steel andthat the fatigue fracture characteristics of Cr-Mo-V aresimilar to those of fine-grained, quenched, and temperedsteel s.

CONTENTS

Page

Title Page i

Abstract

Table of Contents iii

List of Tables iv

List of Figures V

Acknowledgment vi

Introduction

Experimental Procedure 2

Results and Discussion 6

Conclusions 23

Future Work 23

Literature Cited 24

Distribution 25

DD Form 1473 (Document Control Data - R&D) 31

iii

LIST OF TABLES

Table Page

The Composition, Heat Treatment, and 3Tensile Properties of Cr-Mo-V Steel.

II The Effect of the Values of R and 8Static Tensile Stress on theAmplitude of Alternating Stress foran Endurance Limit of lO7 Cycles.

Iv

LIST OF FIGURES

Figure Page

I Sketch of Fatigue Specimen, Push-Pull 4Type, Employed in Sonntag UniversalFatigue Testing Machine

2 Experimental Setup for Fatigue Testing 5under Axial Push-Pull Type Loading

3 Peak Stress-Log Cycles Fatigue Curves 7for Cr-Mo-V Steel

4 Modified Goodman Diagram of Fatigue 9Test Data for Cr-Mo-V Steel

5 Dependence of Fatigue Life of Cr-Mo-V 10Steel on Strain Amplitude

6 Macrographs of Fatigue Fractures of 12Specimens Tested under Rotating Bend-ing Type Loading

7 Scanning Micrograph (Stereoscopic Pair) 14of Fatigue Crack Propagation Region inCr-Mo-V Steel

8 Scanning Micrograph (Stereoscopic Pair) 15of Fatigue Crack Propagation Region inCr-Mo-V Steel

9 Scanning Micrographs of the Final Frac- 16ture Region of Fatigue Failure inCr-Mo-V Steel

10 Recorded Temperatures of the Outside 17Surface of an M60 Barrel During Firing

11 Calculated Values of Bore Surface Tem- 18peratures and Measured Values of OutsideSurface Temperatures of an M60 Barrel

12 Calculated Values of Radial Temperatures 20

13 Bore Surface Temperature of First Round 21on Extended-Time Scale in 7.62mm Gun

14 Calculated Values of Radial Temperatures 22

V

ACKNOWLEDGMENT

The authors wish to thank Dr. Lee Daniels and Dr. FredSchwenk of Battelle Northwest Laboratories, Richland,Washington, for their help in the analysis of fatigue frac-ture specimens.

vi

INTRODUCTION

Erosion of gun barrels refers to the progressive de-terioration and enlargement of the bore. High temperatures(caused by the burning of the propellant), stresses (due topressure, thermal gradients, and the swaging action of theprojectile), abrasive and ablative wear, and the chemicalreactivity of the combustion products contribute to theerosion phenomenon in gun tubes. Metallurgical examinationof test-fired gun barrels has revealed that severe cracking,both tangential and radial, occurs in the bore near thebreech end of the barrel. Cracks that interconnect causematerial removal, whereas those that progress radiallycause catastrophic failure.' Thus, crack nucleation andpropagation, under repetitive-thermomechanical stresses ina reactive atmosphere, cause extensive damage in a gunbarrel. An understanding of the phenomenon of fatigue undercontrolled environmental conditions will lead to better de-sign parameters and will facilitate development of improvedgun barrel materials.

A continuing program with the object of ronducting afundamental investigation of the fatigue behavior of rapid-fire gun barrel materials was initiated by the ResearchDirectorate, Weapons Laboratory at Rock Island. Studieswill invoive the conventional gun barrel material (Cr-Mo-Vsteel) and will encompass materials that are consideredpotentially superior. Naterials will be subjected to therepetitive application of stresses at temperatures simu-lating the conditions relevant to gun barrel application.Characteristics of nucleation and propagation of cracks,type and extent of plastic deformation, and critical cracksizes will be determined by metallographic examination

For the accomplishment of t,$s objective, an assess-ment of the magnitude and direction of the stresses oper-ating on the barrel material and the relevant temperatu*esis necessary. Also, in the case of the current barrel ea.terlal, information about fatigue of this material undersimple conditions of stressing had not been obtained.Therefore, the efforts of the first year concerned primarilytwo definite areas: (1) determination of room temptraturefatigue properties of Cr-Po-V steel and (2) temperaturemeasurements and calculation of thermal stresses, whichform an i-,tegral part of the stress-time profile.

EXPERIMENTAL

Fatigue Testing of Cr-Mo-V Steel

Fatigue tests were conducted at room temperature onheat-treated Cr-Mo-V steel under rotating-bend4 ng and axialloading conditions. The maximum stress-fatigue life (S-N)curves were determined for various values of minimum-to-maximum stress ratio, R. Fatigue tests at R-0.2, 0, -0.2,and -0.4 were performed on a Sonntag machine in which analternating stress (aa) is superimposed on a static stress(a).

The composition, heat treatment, and tensile proper-ties at room temperature of Cr-Mo-V steel are given inTable 1. The test specimens were machined from heat-treatedgun barrel stock in such a manner that the longitudinalaxis of the specimen was parallel to that of the stock.These specimens were then electropolished to remove thework-hardened layer and to provide a reproducible surfacefinish. The geometry and dimensions of the test specimenfor fatigue under axla' load are shown in Figure 1. Fatiguetesting undee conditions of Ra-1 was carried out in anR, R. Moore machine.

The experimental setup and the instrumentation forfatigue testing are shown in Figure 2. 4 strain gage waswelded to the loading springs of the Sonntag machine tomonitor the loading conditions in the specimen during test-ing, After testing, the specimens were metallographicallyexamined to characterize the fracture surfaces.

Temperatur:.Measurements

temperature measurements were made on the outsidesurface of Special M60, 7.62mm, rapid-fire gun barrels (one-piece construction) during test firing under differentSchedules. The barrels were made of Cr-Ho-V st-eel and werein the unplated condition. Chromel-Alumel thermocoupleswere welded to the surface at various locations along thelength of the barrel and the ewf's (which were later con.verted to temperatures) were measured by a high-speed re-corder. From these values, the temperatures at variousradial locations were calculated by the implicit finite-difference method.4

2

TABLE I

COMPOSITION, HEAT TREATMENT, AND TENSILE PROPERTIESOF Cr-Mo-V STEEL

a. Composition in weight percentages

C Mn Cr Mo V Si S PS0.41 0.88 1.19 0.64 0.22 0.217 0.014 0.013

b. Heat treatment

Normalized at 1700'F for 2-1/2 hours, air-cooledHardened at 1575 0 F for 2-1/2 hours, oil-quenched

Tempered at 121O°F for 2-1/2 hours, air-cooled

Stress Rel. at 11000 F for 2-1/2 hours, air-cooled

c. Tensile properties at room temperature

Y. S. (0.2% offset) - 145 ksi

T. S. - 155 ksi

Elongation - 17.6%R. A. - 57.8%

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FIGURE 2 Experimental Setup forFatigue Testing under Axial Push-Pull Type Loading

5

RESULTS AND DISCUSSION

Fatigue Tests

The results of the fatigue tests are presented in thefcrm of S-N curves in Figure 3 for the various values of R.The curve for R = -1.0 is presented in the same diagram.The effect of the values of R and the static stress (a s) on

the stress amplitudes (a ) corresponding to the endurancelimit (I07_.) of Cr-Mo-V steel is shown in Table II. Again,the dat• point for R = -1.0 is included. Direct comparisonof fatigue data under axial and rotating-bending loadingconditions should not be made. When the specimen is testedunder axial loading, the entire cross section is subjectedto the maximum stress, whereas only a small volume of ma-terial on the nerip~ery of the specimen is subjected tomaximum st-ess in a fatigue test conducted under rotating-bending type of loading. This fact illustrates why fatiguestrengths are usually lower in axial push-pull loading thanin rotating-bending.

For design use, data reportad in S-N curves are custom-arily repres~nted in the form of a Goodman diagram. Amnfdified Goodman diagram which represents constant lifecycles under various R values for Cr-Mo-V steel is presentedin Figure 4. Before a Goodman diagram can be used foractual service cases, corrections must be made for suchfactors as geometry, size, surface conditions, stress states,and notches.

Theoretical predictions of fatigue life of materialsare usually made, especially when design on the basis offatigue is warranted and when fatigue data are lacking. Apopular approach is to roly on the tensile properties ofthe material. The method of universal slopes' by Mansonis based on this approach. The fact that the test datacorrelate quite well with predictions by Manson's methodis of interest. The details of calculation are given inReferences 3 and 4. The experimrnntal points for fatiguetests under rotatIng-bendlnS conditions (R = -1.0) areshown in Figure 5. The agreement is considered good. The"universal slopes" in this method have a range of values:-0.4 to -0.8 for the plastic component and -0.08 to -0.16for the elastic component. Values of -0.55 for the 'lasticcomponent and -0.10 for the elastic component fit the ex-perimental data for Cr-Mo-V steel.

6

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TABLE II

THE EFFECT OF THE VALUES OF R AND STATICTENSILE STRESS ON THE AMPLITUDE OF

ALTERNATING STRESS FOR ANENDURANCE LIMIT OF 1O7 CYCLES

.. J R a cym a x -m i n )R amax amin s ma 2

0.2 128 25.6 76.8 51.2

0 121 0 60.5 60.5

-0.2 108 -21.6 43.2 64.8

-0.4 98 -39.2 29.4 68.6

-I.0' 88 -88.0 0 88

*Tests for R - -1.0 were conducted in an R. R. Mooremachine.

All stresses are in ksi units.

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FIGURE 5 Dependence of Fatigue Lifeof Cr-Mo-V Steel on Strain Amplitude

10

Metal lographic Examination of Fatigue Fractures

Before the fracture behavior of the gun barrel steelcan be explained, the characteristics of the material mustbe described so that the observations and the analysis canbe placed in proper perspective. The gun steel is usuallynormalized, quenched, and tempered to achieve a very finegrain size and a high toughness.. The toughness in thelongitudinal direction of the bar stock is about threetimes the toughness in the transverse direction. The speci-mens were machined from the bar stock in such a manner thatthe length of the specimen was parallel to the longitudinaldirection of the bar stock. The crack propagated in thetransverse plane with reference to the gun barrel stock.

In the case of axial push-pull tests, the fracturesurfaces were invariably damaged since, at the end of thetest, the fracture surfaces were impacted against each other.

Macrographic examination of the fatigue fractures underrotating-bending condition revealed the following generalcharacteristics. In all cases, fatigue fracture began nearthe surface and propagated through a major area of the crosssection. The final phase of the failure was a ductilefracture followed by shear rupture. The crack propagationregion was divided into a shiny region and a dull region.The shiny region is the result of crack closure during thecompressive half of the cycle and possible rubbing. Atypical fractograph of the fatigue fracture is shown inFigure 6b. The particular specimen was tested at a peakstress of 95 ksi. The fracture started from both sides,continued inward over an area of about 75 per cent of thecross section by fatigue, and caused failure by shear rup-ture. Comparisons can be made with fractographs (Figures6a and 6c) of specimens which were tested at 105 ksi andgO ksi, respectively. At high stresses, the fracturestarts along three-fourths of the periphery, continuesradially, and causes final failure by rupture. At lowstresses, the fracture starts on one side, propagates overapproximately 85 per cent of the cross section by fatigue,and ends as a ductile fractufe.

Detailed metallographic examination of the fatiguespecimens tested under rotating-bending conditions wasiarriedout with a scanning electron microscope. In thecrack propagation region, 'ductile" regions (in whichstriations could be discerned) and "brittle" transgranular

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regions were present. The relative area of ductilestriated region decreases as the crack propagates. Theinterstriation distance remains the same or decreasesslightly as the crack advances. This behavior is to be ex-pected since, in rotating-bending fatigue tests, the stressis not uniform across the section, but decreases radiallytoward the center. The tendency of the striated region tochange planes, as the crack propagates, causes microscopictear. Some of these features can be seen in Figures 7and 8 which are stereoscopic pairs of the crack propagationregions in two specimens. In some striated regions, ortho-gonal to the striations, a "hill and valley" type of struc-ture can be seen. Such a feature has been observed in theferritic regions of fatigue fractures of steels- 6 The finalfracture region of a typical fatigue failure of Cr-Mo-Vsteel is shown in Figure 9. A mixture of small and largedimples are found in the ductile fracture region. The finalshear rupture is characterized by small dimples.

In all respects, the general fatigue characteristicsof Cr-Mo-V steel at room temperature are similar to thoseof quenched and tempered fine grained low alloy steels.7,

Temperature Measurements

Various attempts were made to determine the bore sur-face as well as the outside surface tem eratures of differ-ent gun barrels during this program. The details and theanalyses of these tests will be published separately, laterA few of these results, pertinent to the M60 barrel, willbe presented in this report.

The recorded temperatures of the outside surface ofthe Cr-Mo-V steel, one-piece bar-el at the various locationsindicated on the curves, are shown in Figure 10. The firtngschedule required six continuous bursts of 125 rounds withan intermediate cooling for 10 seconds. Bore surface tem-peratures were calculated by the implicit finite-differencemethod based on the measured outside surface temperaturevalues. The bore surface and the outside surface tempera-tures as a function of time for the region 3 75 inches fromthe breech end (1.75 inches from the origin of rifling),are shown in Figure 11.

13

IIII

F~IGURE 7 Scanning M1crograph(Stereoscopic Pair) of Fatigue Crack

Propagation Region In Cr-Ho-V Steel. 100OX.

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FIGURE 8 Scanning Micrograph(Stereoscopic Pair) of Fatigue CrackPropagation Region In. Cr-Ho-V Steel

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In CrN- Ste. 100 .X

FIGUR 9 4nlgHcorp$o h

Final Vracture Region of Fatigue Failurein Cr-No-V Steel. lO00X

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160 BARREL TEMPEFqTURES AS A FUNCTION OF TIME

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"i. UUIER LNr!S i!T( s 0.502. ORPRL HATERM~ C 0.M.V.37.3. AMBIENT TPIPqPETLIPE a 70.W) tri, F)4. F(R(NG IlE 6 60 R*JIN0B PR HINUr

S. IEWEPATES TAIKEN AT 3.75 JIS FRIt THE UBUI.5. &iPER ttPVC 15 THE 913W TE'U'PfE.R7. LMeR CURVE !5 Tit UUTSJE SURMCE TEI9EWRI1.6 PfRWN6 SCHEDULE t 125 RIIJNOb FULLDED BY I0 bECOO• r COIN•I.9. OUR rE4P. T.N ArE RT 12S MUM FIRMG WD0 TER 10 SEM. C.INM.10. 5UJCRE TEW. TR(EN AFER 125 qOS.FIILNG.S SEC. COUUL6 IN O 1,M. C.•LNG.

0.00 1.0D .Wo :. oo 4IE 1' (.1 6.007.00 L L OD Io.TIME (SEC)

FIGURE 11 Calculated Values of Bore SurfaceTem peratures and Mleasured Values of

Outside Surface Temperatures of an M60 Barrel

18

Some explanation is necessary about the phrase "cal-culated bore surface temperature." The temperatures atvarious radial locations after four bursts of 125 roundswith an intermediate cooling for 10 seconds are shown inFigure 12. Curves like this were generated for severalpoints along the firing schedule. The implicit finite-difference method is applicable to regions very close tothe bore surface. A small region still exists whereaccurate calculations are possible only if the nature ofthe slope can be defined. The bore surface temperaturesquoted in this report do not refer to the "skin" of ma-terial at the bore surface. Also, each time a shot isfired, the bore surface temperature reaches a peak andquickly drops to a stable value, Attempts have been madein this laboratory to measure this temperature by carefullydesigned thermocouples positioned at the bore surface, Arecord of the measurements is shown in Figure 13. Thesepeak temperatures are probably of value to study the chem-ical and the metallurgical reactions occurring in the bore;but, for mechanics considerations, only the lower end ofthe envelope is relevant because of the very short decaytimes.

The following general comments are given with respectto heat transfer characteristics in the barrel: The tem-perature gradient across the barrel section becomes in-significant at the end of each cooling interval (Figures12 and 14). The fact that the outside surface graduallybecomes heated as the firing continues and thus the tem-perature gradient is gradually decreased is consideredimportant because of the following reasoning: The thermalstresses and the pressure stresses operate in oppositedirections. When the outside surface is cold, the thermalstresses partially nullify the pressure stresses. But,when the outside surface is hot, the nullifying effect isdecreased. Furthermore, this action occurs at a time whenthe barrel is mechanically weak. During service, thebarrel is heated and cooled between the ambient temperatureand temperatures as high as 14001F. From this observation,the importance of the study of thermal fatigue in gun bar-rels is evident.

19

CALCULATED RADIAL TEMPERATURES FOR THE 46II BARREL

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0~

'* 7. OUTER ENIX!VI1T.LO••2: BARE MTERIF0.t 0JMNV.$T.

1. AMBIENT TE•DIRAT.M T70.0 Mrs= F.S. FIRING OT!, RO8 Mi PEN MINUTE4. TEWEARTUJ'f WrTEA SOU SOL3. TERPCMIM TqKLN Rr 3.75 INHES FNCI TI 46. FIfING SMOItCLEt 125 IM FIOLL±[O eY 10 lWON OF WJNr

SI I I I I

t.50 2.0U 2.50 3.M RFO 1 'RLO 1 STRN qN IN +N) 1.5M 0 5OU S. L5t

FIGURE 12 Calculated Values ofRadial Temperatures. The Radial

Distances are Measured from the Axis of the Barrel Tube

20

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"CALCULATED RADIAL TEMPERATURES FOR THE M60 BARREL

7. IJ1! ENINIVITULO.

(". IUSET DIP MI-7'0.0 MON F.S. P111114 1141 am Fil Mj N11MLI IEI9MI1UICWA ME 17% I. FM T6 1 M W.LfI3. FIRING1 TNO~ INT Fa RHUM P10M WI MIND1. P 1 SOIIE 1 1I 1WLE NT 15 1f 1 .

1.50 LOD 1.0 U 1 18 W M LW

FIGURE 14 Calculated Values ofRadial Temperatures. The Radial

Distances are Measured from the Axis of the Barrel Tube

22

½..

CONCLUSIONS

1. The effect of static tensile stress is to lowerthe amplitude of the alternating stress corresponding tothe endurance limit of Cr-Mo-V steel.

2. The method of "universal slopes" is applicable tothe prediction of fatigue life of Cr-Mo-V steel.

3. The fatigue properties and the characteristics offatigue fracture of Cr-Mo-V steel are similar to those ofother quenched and tempered, fine-grained, low-alloy steels.

FUTURE WORK

Thermal stresses in the 7.62mm barrel during firingwill be calculated as a function of time from the data ob-tained during the first year. The coupling of these datawith pressure stresses will result in a stress-time profilerepresentative of gun barrel service. Three different kindsof fatigue testing are planned for the future. They are:l I thermal fatigue studies based on the temperature data,

fatigue tests under programmed-loading and constanttemperatures up to 1500 0 F, and (3) fatigue testing underprogrammed temperatures and constant stress amplitude. Theinvestigation will be carried out toward the understandingof the phenomenon of fatigue in gun-barrel materials.

23

LITERATURE CITED

1. Ebihara, W. T., "Erosion in 7.62mm Machine GunBarrels," AD-721890.

2. Clausing, A. M., Technical Report 0100-1, "Descrip-tion of Computer Program HT-2A."

3. Manson, S. S., Experimental Mechanics 5 (7), p. 193-226, (1965).

4. Tetelman, A. S., and A. J. McEvily, Jr., Fracture ofStructureal Materials, p. 377, John Wiley and Sons,Inc. (1967).

5. MIL-S-46047A(MR) dated 5 December 1963.

6. Waldron, G. W. J., A. E. Inclke, and P. Fox, Pro-ceedings of the Third Annual Scanning ElectfronMicroscope Symposium, p. 297, IITRI, ChicagoIllinois, (1970).

7. O'Connor, H. C., and J. L. L. Morrison, InternationalConference on Fatigue of Metals, p. 102,'Inst."Mech. Engrs., (1956).

8. Dahlberg, E. P., Trans. Am. Society for Metals, p. 46,Vol. 58, (1965).

24