AN INVESTIGATION OF THE EFFECT OF SURFACE FINISH ON THE FLEXURE FATIGUE STRENGTH OF 75S-T6 ALUMINUM ALLOY SHEET A THESIS.

Transcription

1 AN INVESTIGATION OF THE EFFECT OF SURFACE FINISH ON THE FLEXURE FATIGUE STRENGTH OF 75S-T6 ALUMINUM ALLOY SHEET A THESIS Presented to the Faculty of the Dvson of Graduate Studes Georga Insttute of Technology In Partal Fulfllment of the Requrements for the Degree Master of Scence In Aeronautcal Engneern.o; Robert Graham Bodford June 1949

2 xvo-c;/d AN INVESTIGATION OF THE EFFECT OF SURFACE FINISH ON THE FLEXURE FATJGUE STRENGTH OF 75S-T6 ALUMINUM ALLOY SHEET Approved: J^TI ~g Date Approved by Charman L*) (U^- 4- ; 1^4-Q \

3 H ACKNOWLEDGMENTS The author wshes to express hs thanks to Professor G. K. Wllams for the dea of the nvestgaton. Sncere thanks are also due to Professor D. W. Dutton for many suggestons, and much encouragement throughout the conduct of the work* The alumnum sheet used was furnshed through the courtesy of Mr, P. V. Faragher, Educatonal Representatve, Alumnum Company of Amerca.

4 TABLE OF CONTENTS PAGE Approval Sheet Acknowledgments Lst of Tables *. v Lst of Fgures v Summary 1 Introducton 2 Materal 5 The Fatgue Testng Machne 7 The Fatgue Specmens.. 7 Preparaton of Specmens.. 8 Applcaton of Scratches... 9 Depth and Nature of Scratches 10 Test Procedure 12 Dscusson of Results 14 Alclad 75S-T S-T S-T3 16 Shape Effect 17 Stress Concentraton Factors., 9 18 Applcaton to Desgn 24 Conclusons 28 BIBLIOGRAPHY 30

5 V PAGE APPENDIX I, Hstorcal Note 34 APPENDIX II, Tables 37 APPENDIX III, Fgures 42

6 v LIST OF TABLES Table I Table II Table III Table IV Mechancal Propertes of Sheet Alclad 75S-T6, 75S-T6 and 24S-T3 from Tenson Tests Maxmum Measured Depths of Abrasve Scratches Values of Flexure Fatgue Strength and Stress Concentraton Factors for Alclad 75S-T6 Wth Varous Surface Condtons Values of Flexure Fatgue Strength and Stress Concentraton Factors for 75S-T6 and 24S-T3 for Varous Surface Condtons PAGE

7 LIST OF FIGURES PAGE Fgure 1 Stress-Stran Curve for Alclad 75S-T6 Sheet Thckness of Inches 43 Fgure 2 Fgure 3 Fgure 4 Fgure 5 Fgure 6 Fgure 7 Stress-Stran Curve for 75S-T6 Sheet of Thckness Inches 44 Stress-Stran Curve for 24S-T3 Sheet of Thckness Inches 45 Front Vew of Sonntag Flexure Fatgue Machne, Model SF-2 46 Sde Vew of Sonntag Flexure Fatgue Machne, Model SF-2 47 Top Vew of Sonntag Flexure Fatgue Machne, Model SF-2 48 Specmen Layout and Mountng Detals 49 Fgure 8 Drll and Router Jg 50 Fgure 9 Fractured Alclad 75S-T6 Specmens 51 Fgure 10 Fractured 75S-T6 Specmens 52 Fgure 11 Fractured 24S-T3 Specmens 53 Fgure 12 Fgure 13 Flexure Fatgue Strength of Alclad 75S-T6 Sheet of Thckness and Inches 54 Flexure Fatgue Strength of Alclad 75S-T6 Sheet of Inches for Polshed Specmens and Specmens Scratched by Crocus Cloth 55

8 Fgure 14 Fgure 15 Fgure 16 Fgure 17 Fgure 18 Fgure 19 Fgure 20 Fgure 21 Flexure Fatgue Strength of Alclad 75S-T6 Sheet of Inches for Polshed Specmens and Specmens Scratched by Grt No. 100 Abrasve Cloth 56 Flexure Fatgue Strength of Alclad 75S-T6 Sheet of Inches for Polshed Specmens and Specmens Scratched by Grt No. 60 Abrasve Cloth 57 Flexure Fatgue Strength of Alclad 75S-T6 Sheet of Thckness Inches for Polshed and Scratched Surface Condtons 58 Flexure Fatgue Strength of 75S-T6 Sheet of Thckness Inches for Polshed Specmens and Specmens Scratched by Grt No. 60 Abrasve Cloth 59 Flexure Fatgue Strength of 24S-T3 Sheet of Thckness nches for Polshed Specmens 60 Flexure Fatgue Strength of Sheet 75S-T6 and Sheet 24S-T3 n Polshed Condton 61 Flexure Fatgue Strength of 24S-T3 Sheet of Thckness Inches for Polshed Specmens and Specmens Scratched by Grt No. 60 Abrasve Cloth 62 Flexure Fatgue Strength of 24S-T3 Sheet of Thckness Inches n Polshed Condton and of 24S-T3 Sheet of Thckness of Inches Scratched by Grt No. 60 Abrasve Cloth Reported by Bond 63

9 1 AN INVESTIGATION OF THE EFFECT OF SURFACE FINISH ON THE FLEXURE FATIGUE STRENGTH OF 75S-T6 ALUMINUM ALLOY SHEET SUMMARY Flexure fatgue tests have been conducted for Alclad 75S-T6, 75S-T6 and 24S-T3 sheet of commercal thckness nches for polshed specmens, and specmens scratched by varous abrasve cloths, Curves of appled stress versus number of cycles to Talure have been plotted, and stress concentraton factors for the ndvdual materals and abrasve grts determned. The flexure fatgue strengths of the materals have been compared, both n the polshed and scratched states, and conclusons have been drawn concernng the relatve merts of the dfferent materals wth respect to ther fatgue qualtes.

10 2 INTRODUCTION The study of fatgue has become more mportant each year. The S-N curves, or Wohler curves as they are called n the G-erman lterature, where the number of cycles to falure (N) Is plotted as a functon of the stress (S), of many dfferent materals have been determned and publshed, More recently, fatgue studes have been carred out to determne the effect of certan stress rasers such as holes, notches, and fllets on the fatgue strengths of certan materals, Por alumnum alloys, wth whch the aeronautcal engneer s especally concerned, the data are meager and, for the recently developed hgh strength alumnum alloys, are practcally non-exstent. The desgn of arcraft elements has been based upon certan lmt loads whch are rarely, f ever, encountered, To these, a small margn of safety s added to determne the desgn loads. However, as the loads on the structure, and consequently the nduced stresses, can be consdered as consstng of a steady, or dead load, and a supermposed dynamc load, the opportuntes for fatgue falures should not be overlooked. It has been clamed by some that the fatgue falures whch have occurred n the past have not been serous n nature. Before 1939, there were no cases on record In whch

11 3 a wng spar falure was caused by fatgue. However, t seems lkely that such falure could possbly have occurred wthout beng detected. More recently falures of ths 2 nature have occurred, and wth great loss of lfe. Fatgue falure n propellers and tal assembles have 3 also caused many fatal accdents. The factors whch contrbute to fatgue falures have by ther nature made the problem more acute each year n spte of the more advanced understandng of the condtons for falure. Some of the factors are: hgher speeds, ncreased wng loadngs, ncreased fre power and maneuverablty, pressurzed cabns, and radcal 4 desgn changes such as jet and rotary-wnged arcraft. The use of new materal wth hgher statc ultmate strength, but not proportonal ncrease n fatgue strength 1 Arnsten, K., Shaw, E.L., "Fatgue Problems n the Arcraft Industry", Metals and Alloys, 10:203-9, July ^Anonymous, "2-0-2 Report," Avaton Week, 49:26, October Staff of Battelle Memoral Insttute, Preventon ^ tne. Fatgue of Metals Under Repeated Stress (New York: "John Wley and Sons^ Inc., 194lT. Jackson, L.R., G-rover, H.J., and McMaster, Battelle Memoral Insttute, "Advsory Report on Fatgue Propertes of Arcraft Materals and Structures," War Metallurgy Commttee, OSRD No. 6600, Seral Number M- 653, March 1, 1946.

12 4 has also been a contrbutng factor. Ths s partcularly true n arcraft snce the great majorty of parts are desgned to operate at a certan percentage of ther ultmate strength. Consder the case of two materals wth equal endurance strengths, but wth unequal ultmate strengths. A certan percentage of the lower ultmate strength for the one materal mght result n a workng stress less than the endurance strength, whle the same percentage of the hgher ultmate strength of the other materal could concevably result n a workng stress greater than the endurance strength. In arcraft use, the alumnum alloys 75S-T6 and 24S-T3 are materals wth physcal propertes smlar to the above condton. The 75S-T6 s the newer of the two alloys and has the hgher ultmate strength. The purpose of ths nvestgaton s to determne the effect of surface fnsh on the fatgue strength of 75S-T6 and to compare ths effect wth that of smlar stress rasers on 24S-T3.

13 5 MATERIAL The materals used for the fatgue tests were Alclad 75S-T6, 75S-T6 and 24S-T3. The Alclad sheet had a core materal of the specfed alloy and a surface claddng of practcally pure alumnum on each sde. For ths type, the word Alclad has been used as a prefx to the alloy desgnaton. The other sheet materal had no claddng on the surface. Ths, n some lterature, s sometmes referred to as "bare". In ths report, however, the alloy desgnaton wth no prefx ndcates that the materal was not of the clad type. Ths notaton has bean followed consstently throughout the report, The nomnal composton of 75S s 1.6 per cent copper, 2.5 per cent magnesum, 5.6 per cent znc, and 0.3 per cent chromum. The balance s alumnum and normal mpurtes. For the alclad sheet, the core s 75S and the claddng materal s 72S, whch has a nomnal composton of 1 per cent znc, wth the remander alumnum and normal mpurtes. Alloy 24S nomnally conssts of 4.5 per cent copper, 0.6 per cent manganese, 1.5 per cent magnesum, balance alumnum and normal mpurtes. ^Anonymous, Alcoa Alumnum and Its Alloys (Pttsburgh, Penna: Alumnum Company of Amerca, 1947), p. 85.

14 6 Followng the alloy desgnaton, are the heat-treat symbols. The "T" ndcates the alloy to be of the heattreatable type, and the number ndcates the heat-treat process. The fnal propertes of the materals are determned by ths process. For 75S-T6, the number sx ndcates a soluton heat-treat followed by artfcal agng. On 24S-T3, the three ndcates a soluton heat-treat followed by stran hardenng, whch n the case of sheet, comes about n a flattenng operaton. All of the sheet used throughout the tests was of the commercal thckness nch. The actual average thckness vared from nches for the Alclad 75S-T6 to nches for the 75S-T6. Although the thckness of the ndvdual sheets vared a few ten thousands of an nch from the actual average, all calculatons and machne settngs were made on the bass of a constant thckness for each ndvdual sheet. One seres of tests for polshed specmens alone was run on nch thck Alclad 75S-T6. Mechancal propertes of the alloys used are shown n Table I. These were determned from tenson tests on standard specmens of two-nch gage length. 6 The values 2Davs, H.E., Troxell, G.E., and Wskocl, C.T. The Testng and Inspecton of Engneerng Materals, (New York! McGraw-Hll book Company, Inc., 1941), p. 80.

15 7 represent an average of two tests for each materal. The average stress-stran curves for the materals used are shown n Fgures 1, 2, and 3. THE FATIGUE TESTING MACHINE The machne wth whch the tests were conducted was a Sonntag Flexure Fatgue Machne, Model SF-2, shown n Fgures 4, 5, and 6. It was of the constant repeatng force type, actng on a cantlever specmen desgned to gve a constant bendng stress throughout the test secton. The varyng vertcal shear stress was neglected as s customary wth ths type loadng. loadng was 1800 cycles per mnute. The speed of The operaton of the machne has been very well 7 8 descrbed n prevous theses. ' THE FATIGUE SPECIMENS The layout of the fatgue test specmen, along wth mountng detals, s shown n Fgure 7«The specmen was mounted as a cantlever and was desgned to produce a constant bendng stress n the area bounded 7 Bond, A.C. "Fatgue Studes of 24S-T and 24S-T Alclad Sheet wth Varous Surface Condtons" (unpublshed Master's Thess, Georga Insttute of Technology, Atlanta, Georga 1948), p Duchacek, Howard, "A Study of the Effect of Thck ness on Fatgue Strength of 24S-T3 Alumnum Alloy Sheet" (unpublshed Master's Thess, Georga Insttute of Technology, Atlanta, Georga 1948), p. 7.

16 8 by the two straght non-parallel lnes and ther ntersecton wth the three-eghths of an nch rad. Preparaton of Specmens: The alumnum sheet was cut nto rectangles on a manually operated shear. The length of the rectangle was three and one sxteenth nches, and the wdth was two nches. The holes were drlled n the jg (shown n Fgure 8), and the test secton cut out on an Onsrud routng machne, as per Fgure 7. The long dmenson of the rectangle was n all cases, n the drecton of rollng for the sheet. The tensle and compressve forces produced by the flexure machne therefore acted n the drecton of the gran of the materal. The tool marks on the edges of the straght sdes boundng the test secton and the adjonng rad were removed by polshng wth 240 grt alumnum oxde cloth, backed up by a three-eghths nch dameter wooden dowel. Ths served to keep the edge square and perpendcular to the flat surface of the sheet. The edge was then polshed n the same manner wth 400 grt alumnum oxde cloth. By frst usng a new pece of abrasve to polsh out the major scratches, from the prevous abrasve cloth, and then a "used" pece of the 400 grt cloth 9, the edge looked absolutely smooth to the unaded eye. SGallaher, E.B. Coated Abrasves, A Handbook and Dgest of Coated Abrasves Technology (Norwalk, Conn: dlover Manufacturng Company, 1945) 36 pp.

17 9 However, to ensure that the edges were as well polshed as practcal, all specmens were nspected durng and after the polshng operaton under an eght power magnfyng glass. Prom ths, and the fact that all specmens were polshed by the author, t was felt that the edges were unform and contaned no major scratches, In the course of processng, the surface of the alclad specmens became very slghtly scratched. Wthn the area of the test secton, mnor mars were removed by buffng n the drecton of rollng. No attempt however, was made to remove all scratches, and the change n thckness due to buffng could n all cases be neglected, In nstances where a remanng scratch was consdered mportant, a note was made of the locaton. If the fatgue fracture occurred at ths poston, the test pont was dsregarded. The surfaces of the 75S-T6 and 24S-T3 specmens were also buffed n order to make the tests as consstent as possble* Applcaton of Scratches; On the Alclad 75S-T6, the scratches for the varous tests were made wth crocus cloth and 100 and 60 grt alumnum oxde abrasve cloths. Alumnum oxde 60 grt abrasve cloth was used for the 75S-T6 and 24S-T3 specmens. For all specmens, a strp of abrasve cloth

18 approxmately one-half nch wde, was held by hand and wth slght pressure, drawn across the specmen test secton several tmes n an attempt to produce a unform number of scratches on the test secton. Ths was done to each sde of the specmen, usng a new pece of abrasve cloth for each sde and each specmen. All scratches were made n a drecton perpendcular to the center lne of the specmen. Ths partcular method of mpartng the scratches to the specmen, and the use of the above mentoned abrasve grades, was chosen n order to make a notch senstvty comparson of the two alloys 75S and 24S wth as few varables as possble. The expermental program for 24S has been prevously conducted by Bond. Depth and Nature of Scratches; In order to determne the depth of scratches, the specmen test secton surface was examned wth the ad of a Baush and Lomb Research Metallograph. For the specmens scratched wth the fner grades of abrasve, a magnfcaton of about X2000 was used. For those specmens scratched wth number 60 grt, a magnfcaton of about X1000 was used. Ths reducton n magnfcaton was Bond, o. clt., pp

19 11 necessary snce a scratch made by 60 grt covered the complete feld of vson at X2000 and the depth could not be measured at the hgher magnfcaton wthout movng the specmen, The actual measurements were made by frst centerng the scratch n the feld of vson. The mcroscope was then focused on the level surface adjacent to the scratch. The readng on the focusng knob was noted. The mcroscope was then focused on the bottom of the scratch. The readng was agan noted. The dfference between the two readngs on the calbrated vertcal focusng knob gave a drect measure of the depth of scratch. The actual scratches vared n depth, wdth and cross-secton shape. Ths was expected, snce for any one abrasve number, the partcles vary to some extent n sze and shape. The observed depths ranged from some hardly notceable up to a certan measured maxmum for each abrasve. A determnaton of average depth over so great a range would not gve a true pcture of the stress concentraton factor operatve n fatgue falure. Further, an average of ths nature would tend to assocate a certan stress concentraton factor wth a certan average depth and ths average, beng composed of many lesser scratches,

20 would be a depth smaller than the major scratches whch are most lkely to cause falure, other factors beng equal. Thus, t would then appear, that a smaller depth of scratch possessed a damagng effect more severe than n realty. In vew of ths, only the maxmum depths observed are recorded n Table II. No doubt, some depths were greater than these, and many slghtly less. The values serve more to establsh the magntude than the exact numercal depth n nches. TEST PROCEDURE The stress appled to each specmen was determned by the settng of the eccentrc mass, shown n Fgure 4. Sample stress calculatons and machne set ups are gven by Bond, Duchacek, and n the oper- 13 atng nstructons for the machne The eccentrc mass settng was made before the specmen was placed n the machne. Ths was done to ensure that the specmen would not be bent or twsted. The test specmen was nserted wth ts center lne perpendcular to the fxed mountng, and the movable yoke was set H-Bond, 2. ct., pp l^duchacek, o_. ct., pp l^anonymous, "Instructons For Installaton, Operaton, and Mantenance of Flexure Fatgue Testng Machne, Model SF-2" (Greenwch, Conn: Manual furnshed by Sonntag Scentfc Co., July 1948) p. 4.

21 parallel to the fxed mountng. possblty of unsymetrcal loadng. Ths elmnated any The followng tests were conducted: : Number Alloy Surface Condton Thckness In Inches 1 Alclad 75S-T6 Polshed Alclad 75S-T6 Polshed S-T6 Polshed Alclad 75S-T6 Scratched by Crocus Cloth Alclad 75S-T6 Scratched by 100 Grt Alclad 75S-T6 Scratched by 60 Grt S-T6 Scratched by 60 Grt S-T3 Polshed S-T3 Scratched by 60 Grt Snce the specmen edges were polshed by hand and the scratches were appled by hand, t was realzed that not all specmens for any one test would be exactly the same, especally snce no more than two or three specmens were gven the fnal surface fnsh each day. In order to mnmze the effect of ths dfference n specmens from day to day, f there were any such effect, the test ponts were not run n order of ncreasng or decreasng values of stress. In other words, one sped-

22 men was run at a hgh stress, the next at a low stress and the next perhaps at some ntermedate value. Ths procedure gave test ponts throughout the complete curve and any dfference n specmens would add to the scatter of ponts on the complete curve rather than establsh a false trend. In general, a maxmum stress settng of about 45,000 pounds per square nch was used to determne the lower lmt of stress applcatons, and an upper lmt of 10 mllon cycles was arbtrarly taken. Wth the machne runnng contnuously, approxmately four days were requred to complete 10 mllon cycles of completely reversed stress. If the specmen remaned unbroken after that number of cvcles, the machne was stopped and the pont plotted as a horzontal arrow, the ordnate of whch ndcates the stress settng wth the arrow orgn located at the number of cycles actually completed. DISCUSSION OF RESULTS Alclad 75S-T6: The S-N curve for polshed Alclad 75S-T6, nches thck s shown n Fgure 12. Also plotted are the tests ponts for the polshed Alclad 75S-T6, nches thck. A comparson of these data reveals that for the small dfference n thckness, there s lttle or no dfference n the fatgue strength.

23 A smlar concluson for 24S-T3 was drawn by Duchacek n hs nvestgaton of sze effect on that alloy. Fgure 13 s an S-N plot of Alclad 75S-T6, nches thck, n the polshed state and scratched by crocus cloth. The curve for the polshed specmens s repeated on each fgure n order to measure the damage caused by each of the varous abrasve grades. Fgures 14 and 15 show the S-N curves for the Alclad 75S-T6 scratched by numbers 100 and 60 grt. A comparson of results for the varous surface condtons s shown n Fgure S-T6: Fgure 17 shows the result of test made on 75S-T6 of thckness nches. The upper curve represents the results from the polshed specmens, whle the lower curve, those of the specmens scratched by number 60 grt abrasve cloth. of abrasve was used on the 75S-T6. Only ths one grade It was orgnally planned to also run curves for the other two abrasves, However, n vew of the fact that the stress concentraton factors determned were less than those reported by Bond 15, t was felt that a rerun of the 24S materal would be of greater value n formng a comparson of the l^ducbacek, o. ct., p Bond,. ct., p. 30.

24 two materals. 24S-T5: A comparson of two nvestgatons on 24S-T3 s shown n Fgure 18. The S-N curves for 24S-T3 of thckness nches are shown n Fgure 20. Once agan the upper curve s for the polshed specmens whle the lower curve s for those scratched by the number 60 grt abrasve. Fgure 19 shows a drect comparson of the materals 24S-T3 and 75S-T6, both n the polshed state. It can be seen that beyond approxmately fve-hundredthousand cycles, the 75S-T6 has the hgher fatgue strength, whle at the lower number of cycles the 24S-T3 seems to possess the hgher fatgue strength. For the data shown here, ths dfference at the lower number of cycles can only be gven as a trend, snce the tests were not extended beyond a stress settng of 45,000 pounds per square nch for the outsde fber. However, a conforma- T 6 ton of ths trend s reported by G. H. Found It should be noted that for a loadng to falure n one-half cycle, the ultmate strength of the materal s nvolved, and snce the 75S-T6 has the larger ultmate strength, t s necessary for the S-N curves of the two 16 Found, G. H., "The Notch Senstvty n Fatgue Loadng of Some Magnesum Base and Alumnum Base Alloys" A.S.T.M. Proceedngs, Vol. 46, 1946, p. 796.

25 materals to cross at least twce, n the complete range of cycles from one-half cycle to, say fve-hundredmllon cycles. Ths of course, would mean that nether materal s superor to the other n flexure fatgue strength at all numbers of cycles. Ths cannot be shown here, however, snce no S-N curve coverng the complete range of cycles from one-half to fve-hundred-mllon s avalable for the two materals. Shape Effect; For fatgue tests of sheet materal, the values determned are usually less than those found for the same materal usng specmens of a dfferent shape. Tests of the hgh strength alumnum alloy X76S-T ndcate a 30 per cent reducton n endurance strength for rectangular specmens, as compared wth round specmens run on the same vbratory type machne. 17 An even greater reducton was found for square specmens. Ths same effect has been reported by others. The surface hardenng effect due to rollng and straghtenng of sheet materal would tend to gve t a hgher endurance strength. Whle the depth affected by "cold 1 'Dolan, Thomas J. "Effects of Range of Stress and of Specal Notches on Fatgue Propertes of Alumnum Alloys Sutable for Arplane Propellers", N.A.C.A. Techncal Note No. 852, pp Moore, H.F. "Report of the Research Commttee on Fatgue of Metals" A.S.T.M. Proceedngs, 41:133, 1941.

26 workng" of ths nature s small, t becomes a szeable factor when compared to a sheet thckness of several hundredths of an nch. The same depth of strengthened materal on a thckness of several nches would be negl gble. Ths fact has been used to mprove the fatgue 19 lfe of parts by "shot peenng." The sharp edge of the specmen, however, provdes an effectve stress raser, whch s more than enough to overcome any strengthenng due to surface hardenng, and accounts for the lower values of endurance strength. Stress Concentraton Factors: A stress concentraton factor may be defned as the rato of the endurance strength of the materal to the endurance strength of the specmen wth the stress raser, at the same 20 number of cycles. For ths report, the transverse scratches caused by the varous abrasve cloths act as the stress rasers. The actual values of the stress concentraton factors are recorded n tables III and IV. These were determned by dvdng the ordnate of the curve for the polshed specmens by the ordnate of Moore, H.F. "Strengthenng Petals Parts by Shot Peenng" Iron Age, Vol. 158, Nov. 28, 1946, p. 67 and Dec. 5, 1946, p. 8l. 20 Seely, Fred B. Advanced Mechancs of Materals (New York: John Wley and Sons, Inc., Eghth Prntng, 1947), p. 202.

27 19 the curve for the scratched specmens. The ordnates were taken at the same number of cycles of stress applcaton. Ths was done at a number of values of N, and these stress concentraton factors averaged together to form one representatve stress concentraton factor for each abrasve. For the Alclad 75S-T6 scratched by crocus cloth, the factors are practcally constant throughout the range of cycles. Those determned for the other two grts dffer from the average by less than four per cent for the range of cycles covered. The factors determned for 75S-T6 and 24S-T3 vary slghtly more. No explanaton can be gven for the fact that the Alclad 75S-T6 showed the greatest factors near the center of the cycle range, whle the 75S-T6 and 24S-T3 showed the largest factors on both extremes of the range of cycles tested. Prom tables III and IV, t s seen that for the Alclad 75S-T6 and 75S-T6, both scratched by number 60 grt cloth, the stress concentraton factors are very nearly the same. The average factor for the Alclad 75S-T6 s 1.12, whle for the 75S-T6 s It should also be observed that the maxmum and mnmum factors are very nearly the same for the two materals, except that they occur at a dfferent number of cycles as noted above. Ths would seem to ndcate that Alclad

28 75S-T6 and 75S-T6 are almost equally senstve n fatgue to a group of surface scratches. Ths same effect was found by Bond 21 n hs nvestgaton on Alclad 24S-T3, and 24S-T3 wth but one excepton, and that was for the specmens scratched wth grt number 60 abrasve. In ths case the factor for the unclad materal s much larger than that for the Alclad materal. However, t s felt that ths one partcular value from the work of Bond s not n agreement wth hs other results for the followng reason. Of the fve abrasve grades used by Bond on the Alclad and bare materal, for three, the factors determned for the Alclad and bare materal were numercally the same to three fgures, whch are all that can be consdered sgnfcant. For the fourth abrasve, the dfference was lesp than two per cent, and yet for the ffth, grt number 60, the dfference s greater than 10,5 per cent. In vew of ths, t s felt that the stress concentraton factor determned by Bond for 24S-T3 scratched by grt number 60 abrasve s too large, The average concentraton factor determned for 24S-T3 scratched by number 60 abrasve cloth, from table IV, s Ths s n close agreement wth the 1.13 determned for 75S-T6 and 1.12 determned for Alclad Bond, loc. ct.

29 75S-T6, scratched by the same abrasve. It seems reasonable to conclude, at least as a frst approxmaton, n vew of the lmted data, that 24S-T3 and 75S-T6, are almost equally notch senstve to stress rasers such as those used n ths nvestgaton. A further confrmaton of almost equal notch senstvty for 24S-T3 and Alclad 75S-T6 s found n a report for a sngle notch. 22 The tests were made n bendng and the thckness of the materal was nches. A sngle transverse surface notch on opposte sdes of the specmen n the form of a sxty degree "V" wth mnmum radus of one-thousandth of an nch, and a depth of three-thousandths of an nch was used. At fve hundred thousand cycles, the stress concentraton factor for Alclad 75S-T6 was 1.31, and for 24S-T3 was At ten mllon cycles, the stress concentraton factor for Alclad 75S-T6 was 1.22, and was 1.18 for 24S-T3. From these results, t s notced that the stress concentraton factors vary wth the number of cycles n a manner smlar to that noted for Alclad 75S-T6 by ths expermenter. It should also be observed that at the lower number of cycles, the Alclad 75S-T6 was the least notch senstve whle at the hgher number of cycles, the 24S-T3 was the least notch 22pound, op. ct., p. 715.

30 22 senstve. The theoretcal stress concentraton factor s 2.0 for the above notch, Snce, for Alclad 75S-T6, the nomnal thckness of the 72S claddng on each sde s 4,0 per cent of the total thckness, the depth of notch must have completely penetrated the clad surface. On the tests conducted for ths report, the scratches n no case were through the clad surface, as can be seen from the table of depths, A comparson of the stress concentraton factors for scratched Alclad 75S-T6 wth those avalable for scratched Alclad 24S-T5 24 shows a percentage dfference of from 5 to 10 per cent wth those for Alclad 75S-T6 as the lower of the two. A further comparson of the Alclad 24S-T3 values wth those reported for a sngle notch, whch 25 has been shown to be the more damagng of the two cases, mght ndcate the factors reported by Bond for the scratched 26 Alclad 24S-T3 to be too large. Andrews found for a transverse surface notch on each sde of the sheet made by a ^Anonymous, Alcoa Alumnum and Its Alloys (Pttsburgh, Penna: Alumnum Company of Amerca, 1947;, p ^Bond, o. ct., p Moore, R. R. "Effect of Grooves, Threads, and Corroson Upon the Fatgue of Metals", A.S.T.M. Proceedngs, Vol. 26, Part II, 1926, p Andrews, H. J. and Stckley, G.W. "Effect of Scratches on Fatgue Strength of Alclad Sheet", Avaton June 1943, p. 145.

31 tool wth a sxty degree "V", and a mnmum radus of less than one ten-thousandth of an nch, wth depth equal to 85 per cent of the alclad thckness, an average stress concentraton factor of 1.11 for Alclad 24S-T3. For a notch depth of 55 per cent of the alclad thckness, a factor of 1.06 s reported. One factor whch has not been drectly evaluated s the effect; of the scratch or notch at the edge of the specmen. Snce a great majorty of the fractures began at one edge of the specmen and progressed across, t was felt that the scratch depth was not the most mportant factor n the determnaton of the falure. In Fgure 9, t can be seen that there were many stress rasers along the edge of the specmens whch dd not extend across the surface as scratches. The ntersecton of these, wth the mnute longtudnal scratches whch remaned from the edge polshng operaton, could very well have been the decdng factor for falure, and more mportant than the depth of scratch on the surface of the specmen. The dfference n edge surface fnsh mght also account for the lower values of stress concentraton factors found for Alclad 75S-T6 and 75S-T6. As was noted under preparaton of the specmens, the edge was consdered to be better polshed than those specmens used n prevous tests.

32 One other dfference can be mentoned to account for the hgher stress concentraton factors reported 27 by Bond. The curve for polshed 24S-T3 specmens, whch was used to determne the stress concentraton factors, consstantly runs from 3,000 to 4,000 pounds per square nch hgher, for the same number of cycles, 28 than ether the smlar curve of Duchacek or of the present nvestgaton. Fgure 16 shows the S-N curve determned by Duchacek wth the expermental ponts found by the present author supermposed to demonstrate the extent of smlarty of the two curves. No reason can be found for ths dfference, snce all work was done usng the same machne and smlar specmens. It s possble however, that the frst specmens were slghtly oversze, snce they were not made on the router as were those of Duchacek and the author. Ths last statement can only be consdered as a supposton to explan the dfference as there s no factual evdence to prove they were ether undersze or oversze. APPLICATION TO DESIGN It should be ponted out that not only the low stress (hgh number of cycles) end of the S-N curve, but the whole curve s mportant n desgn work. There 27 Bond, loc. ct. 28 Duchacek, op_. ct., p. 34

33 are component parts n arcraft structures where the number of stress alternatons n the lfetme of the arcraft may be estmated qute closely. One such example s the cycle of stress caused by pressurzng and depressurzng the cabn of modern transport and mltary arcraft. As the art of arcraft desgn and analyss progresses, the number of component parts whose stress hstores can be approxmated, wll be greatly ncreased. In nstances such as these, a much hgher workng stress can be used wth no danger of fatgue falure, snce an Infnte lfe s not requred. It s therefore, also mportant to know the effect of stress rasers on the materal through a wde range of cycles. In general, the stress concentraton factors as determned by usng laboratory test specmens, are not drectly applcable to the desgn or analyss of larger parts. One reason s that wth models, the work hardenng effect at, and near, the surface s n greater proporton than n full-szed objects. Ths tends to gve lov/er stress concentraton factors than would be experenced n the full-szed part. However, the factors determned n ths report could n all probablty be appled wth safety to a smlarly loaded sheet wth a smlar surface roughness snce here the thckness of the specmens s the same as that

34 for the sheets used n ndustry. The stress dstrbuton on a cross-secton should then be the same for specmen or for a sheet n actual use, The stress concentraton factors here determned, should not be drectly appled to parts whch are other than sheets of the same magntude of thckness. For small stress rasers wth hgh theoretcal stress concentraton factors, tests have shown that wth small specmens, the actual stress concentraton factor s less than that predcted by theory. ' ' However, t s beleved that on larger specmens, the stress concentraton factor would approach the theoretcal value. * Therefore, the values usually determned ^ Brueggeman, W.C. and Mayer, M. "Axal Fatgue Tests at Zero Mean Stress of 24S-T and 75S-T Alumnum Alloy Strps Wth A Central Crcular Hole", Techncal Note No. 1611, N.A.C.A. August Found, OJD. ct., p Peterson, R.E., "Model Testng as Appled to Strength of Materals" A.S.M.E. Transactons, 55:79, Peterson, R.E. and Wahl, A.M. "Two and Three Dmentonal Cases of Stress Concentraton, and Comparson Wth Fatgue Tests", Amercan Socety of Mechancal Engneers, Journal of Appled Mechancs Vol. 3, No. 1, 1936, pa-15. ^Tmoshenko, S. "Stress Concentraton and Fatgue Falures" Engneer May 9, 1947, p. 398 and May 16, 1947, p. 421.

35 on small scale tests have recently been subjected to 34 suspcon. Regardless of the numercal value of the stress concentraton factors, the tests conducted here have a very defnte use n comparng the two dfferent alumnum alloys 75S and 24S under a smlar surface condton. ^4Heywood, R.B., "The Relatonshp Between Fatgue and Stress Concentraton", Arcraft Engneer, March 1947, p. 82.

36 .CONCLUSIONS As a, result of the tests conducted and the prevous dscusson n the man body of ths paper, the followng conclusons seem warranted: 1. The materals Alclad 75S-T6 sheet and 75S-T6 sheet are almost equally notch senstve n flexure fatgue to small transverse scratches. 2. The materals 75S-T6 sheet and 24S-T3 sheet have very nearly the same notch senstvty to small transverse scratches when tested n flexure fatgue, 3. Stress concentraton factors for Alclad 75S-T6 ncrease wth an ncrease n depth of scratch. 4. Stress concentraton factors for Alclsd 75S-T6 and Alclad 24S-T3 are of the same magntude for smlar surface scratches, 5. Nether 75S-T6 or 24S-T3 s superor to the other for the complete range of stress cycles, but 75S-T6 sheet possesses the hgher endurance strength n the range of cycles above one mllon. 6. Arplane parts subjected to repeated loads, or steady loads wth repeatng loads supermposed, should be desgned on a bass of fatgue strength, rather than ultmate strength.

37 7, It s suggested that a seres of flexure fatgue tests be conducted on sheet 75S and 24S materal wth a sngle stress raser of a certan known theoretcal stress concentraton factor, wth the stress raser not extendng to the edges of the specmen,

38 BIBLIOGRAPHY Alcoa Alumnum and Its Alloys, Pttsburgh, Pa: Alumnum Company of Amerca, 1947, 155 pp. Afanasev, N.N., "The Effect of Shape and Sze Factors on the Fatgue Strength", The Engneers 1 Dgest, 5:132, Andrews, H.J. and Stckley, G.W., "Effect of Scratches on Fatgue Strength of Alclad Sheet", Avaton, 42:145, Arnsten, K. and Shaw, E.L., "Fatgue Problems n the Arcraft Industry," Metals and Alloys, 10: , Battelle Memoral Insttute Staff, Preventon of the Fatgue of Metals Under Repeated Stress. &ew York: John Wley and Sons, Inc., 1941, 273 pp. Bond, A.C., "Fatgue Studes of 24S-T and 24S-T Alclad Sheet wth Varous Surface Condtons." Unpublshed Master*s Thess, Georga Insttute of Technology, Atlanta, Georga, 1948, 55 pp. Brueggeman, W.C. and Mayer, M., "Axal Fatgue Tests "at Zero Mean Stress of 24S-T and 75S-T Alumnum Alloy Strps Wth A Central Crcular Hole." Techncal Note No. 1611, N.A.C.A. August 1948, 23 pp. Davs, D.M., "Fatgue Falure of Arcraft Parts Ther Cause and Cure," Automotve Industres, 92:34, Davs, H.S., Troxell, G.E., and Wskocl, C.T. The Testng Inspecton of Engneerng Materals, New York: Mc Graw-Hll Book Company, Inc., 1941, 372 pp. Dolan, Thomas J., "Effects of Range of Stress and of Specal Notches on Fatgue Propertes of Alumnum Alloys Sutable for Arplane Propellers." N.A.C.A. Techncal Note, No Duchacek, Howard, "A Study of the Effect of Thckness on Fatgue Strength of 24S-T3 Alumnum Alloy Sheet." Unpublshed Master 1 s Thess, Georga nsttute of Technology, Atlanta, Georga, 1948, 48 pp.

39 Foppl, 0,, "Stress Concentraton and Fatgue Falures," Engneer, 185:114, Found, G.H., "The Notch Senstvty n Fatgue Loadng of Some Magnesum Base and Alumnum Base Alloys," A.S.T.M. Proceedngs. 46:715 & 796, Frocht, M.M., "Factors of Stress Concentraton Photoelastcally Determned," A.S.M.E. Transactons. 57:A67, Gallaher, E.B. "Coated Abrasves," A Handbook and Dgest of Coated Abrasve Technology. Norwalk, Conn: Clover Manufacturng Company, 1945, 36 pp. Cough, H.J., The Fatgue of Metals. London: Benn Lmted, pp. Ernest Hartmann, E., "The Drect-stress Fatgue Strength of 17S-T Alumnum Alloy Throughout the Range from One Half to Fve Hundred Mllon Cycles of Stress." Techncal Note 865, N.A.C.A. Sept Heywood, R.B., "The Relatonshp Between Fatgue and Stress Concentraton," Arcraft Engneerng, 19:81-85, Hoskns, H., "Fatgue Tests on Duralumn," Arcraft Engneerng, 13:132, "Instructons for Installaton, Operaton, and Mantenance of Flexure Fatgue Testng Machne, Model SF-2." Manual furnshed by Sonntag Scentfc Co., Greenwch, Conn: July pp. Jackson, L.R., and others, "An Evaluaton of the Fatgue Phenomena n Arcraft." New York: A Sherman M. Farchld Publcaton Fund Paper by the Insttute of Aeronautcal Scences, July 19, pp. Jackson, L.R., Groover, H.J., and McMaster, R.C., "Advsory Report on Fatgue Propertes of Arcraft Materals and Structures." War Metallurgy Commttee, OSRD, No. 6600, Seral No. M-653. March 1, 1946.

40 Lee, G. H., "The Influence of Hyperbolc Notches on the Transverse Flexure of Elastc Plates," Amercan Socety Mechancal Engneers, Journal of Appled Mechancs, Vol. 7, No. 2, Lundberg, Bo, "Bear-Up Requrements for Arcraft," Aero Dgest, 55:56, December, Marn, J., Mechancal Propertes of Materals and Desgn. New Y^rk: McGraw-Hll Book~T?ompany, Inc., pp. Moore, H.F., "Report of the Research Commttee on Fatgue of Metals," A.S.T.M. Proceedngs, 41:133, 1941., "Strengthenng Metal Parts by Shot Peenng," Iron A Z Q > 158:81, 1946., Textbook of the Materals of Engneerng!;. New York: The McGraw-Hll Book Company, Inc., pp. Moore, H.F. and Koramers, "An Investgaton of the Fatgue of Metals," Unversty of Illnos Engneerng Experment Staton Bulletn No. 124, Oct. 1921/ 185 pp. Moore, R.R., "Effect of Grooves, Threads, and Corroson Upon the Fatgue of Metals," A.S.T.M. Proceedngs, 26: Part 11:255, " Nagel, C.F., "Vague Specfcatons for Surface Qualty Hamper Arcraft Producton," Metal Progress, 41:323, Peterson, R.E., "Model Testng as Appled to Strength of Materals," A.S.M.E. Transactons, 55:79, Peterson, R.E. and Wahl, A.M., "Two and Three Dmentonal Cases of Stress Concentraton, and Comparson Wth Fatgue Tests," Amercan Socety of Mechancal Engneers, Journal of Appled Mechancs, Vol. 3, No. 1, 1936, pa-15. Seely, Fred B., Advanced Mechancs of Materals. New York: John Wley and Sons, Inc., Eghth Prntng, pp.

41 33 Tmoshenko, S., "Stress Concentraton and Fatgue Falures," Engneer, 183:398 & 421, "Two-O-Two Report," Avaton Week, 49:26, Oct

42 APPENDIX I Hstorcal Note

43 35 HISTORICAL NOTE For many years the subject of fatgue, or progressve falure, n metals has been under study, and each passng year has shown t to be of ncreasng mportance. In the mddle of the nneteenth century when wrought ron was begnnng to replace stone and brckwork as a major buldng materal, t was suggested by some, that snce the prncples of desgn were not 35 too well understood, experments should be carred out. It was only then dscovered that a repeated load could cause falure, and that the falure was due to the repeated load rather than due to any reducton of the ultmate statc strength of the materal wth age. Experments on bult-up wrought ron grders were carred out n England n 1864 by Farbran. He determned that n the case of completely reversed stress, that Is, from a tensle to an equal compressve value, the maxmum stress should not be greater than onethrd the ultmate statc strength. The really outstandng poneer, n the study of fatgue was Herr A. Wohler, Chef Locomotve Super- Gough, H. J., The Fatgue of Metals (London: Ernest Benn Lmted, 1926)"/ p7 4. S^Marn, j. t Mechancal Propertes of Materals and Desgn (New York: McGraw-Hll Book Co77 Inc., 1942), p. 117

44 ntendent of the Royal Lower Slesan Ralway, who n 1862 undertook to determne the cause of falure of axles on ralway cars and locomotves. Hs extensve seres of tests contnued for over ten years and covered the varous methods of repeated loadng wth several 37 dfferent specmen shapes. In one group of tests, the topc of stress concentraton n fatgue work was approached. A specmen shape wth a rapd change of secton was tested both wth and wthout fllets. The specmen wth the fllets wthstood twelve tmes the number of load applcatons as the one wthout the fllet. Ths was, n effect an expermental determnaton of a stress concentraton factor for that partcular fllet. Snce the tme of Wohler, much work has been done to establsh the endurance strengths of varous materals and some studes made on the effect of certan stress rasers on the endurance strength. The nature of fatgue has not yet passed the TO phenomena stage n spte of the work whch has been done. 37 An account n Englsh of Wohler*s work can be found under "Wohler 1 s Experments on the Fatgue of Metals", Engneerng (London, March 1871), 11:199. ^Jackson, L.R. and others, "An Evaluaton of the Fatgue Phenomena n Arcraft" (New York: A Sherman M. Farchld Publcaton Fund Paper by the Insttute of Aeronautcal Scences, July 1946), p. 30.

45 APPENDIX II, Tables

46 TABLE I MECHANICAL PROPERTIES OF THE ALUMINUM ALLOYS USED IN FATIGUE TESTS Ultmate Tensle Yeld Strength Modulus of Strength, Kps (0.2% Off set),kps Elastcty, Kps Materal Per Square Inch Per Square Inch Per Square Inch Alclad 75S-T ,000 Per Cent Elongaton n 2 Inches S-T ,200 24S-T ,

47 TABLE II MAXIMUM DEPTH OP SCRATCHES Materal Abrasve Depth of Scratch n Alclad 75S-T6 Alclad 75S-T6 Alclad 75S-T6 75S-T6 24S-T3 Crocus Cloth Grt No Grt No Grt No Grt No

48 TABLE III.VALUES OF FLEXURE FATIGUE STRENGTH AND STRESS CONCENTRATION FACTORS FOR ALCLAD 75S-T6 WITH VARIOUS SURFACE CONDITIONS Polls hed Crocus Cloth 100 Grt 60 Grt Cvcles Stress* Factor Stress-* Factor Stress* Factor Stress-* Factor x x x ? Average Stress Concentraton Factor *Note: Values of stress are gven n kps per square Inch. o

49 TABLE IV VALUES OF FLEXURE FATIGUE STRENGTH AND STRESS CONCENTRATION FACTORS FOR 75S-T6 AND 24S-T3 FOR VARIOUS SURFACE CONDITIONS 75S-T6 24S-T3 Polshed 60 Grt Polshed 60 Gr lt Cycles Stress* Stress * Factor Cycles Stress-* Stress!-J Factor 2 x x x x x x x x Average Stress Average Stress Concentraton Concentraton Factor 1.13 Factor 1.12 *Note: Values of stress are gven n kps per square nch. ^

50 APPENDIX III, Fgures

51 ._ : - : : :! T::~ -.::.:. _ : _. * -T "-- : : - - : : " -C o a a> u. a cr w 60 *H (D 0. W a U2 co w OS - CO MI m. :..:. ~:f ;!: ;: Ln[[ -.-r. ' > ' TTT; t '!::: f -frr- ^r. ':P -UL- - : 1 ::,',!,. ~rr --- t» -r: :;'::.; T; : : :-r- TT : : :; TT : r : '; : : r *:; rrr "77 - '-[ ' :: : : : : : - :r:r :':: : -r - : :~ '.' r'rr ^ ;r...:. _.;. l.:^ - ; - ; : ~: :.':r '-: :':' : ~~.. - ~^, ~H., ~ ~- -.:... --: L - - rrrr T 1 :-? ' T- --: - > *^ :-: ' \ *! ' ', ' ''.'.' T- --: *~ r : :- < :!:? -:-: : ' '! ' ; - -P : j ::: " : r :: ;: - : :...::. \ -rrrf * ~ : - '-ro m : ~ IAT * w SII.IT 5E fcb* SI! r. 74,200 IW> :PJK r SQVAB1 3 1 :::: ffj 8t - : : ::. urn rvtr f A 7 IT r ( IMIT ATI mj L>ulj JUS 171 t LA >T CI. "ft - - :: r / ; :::. $5 ATI TV ss= [SO 0 1 *fls ;:- : :1»'B» S<UARj -ftm /TD' - _-. ~~: ^ ' ; To' "H^- " T-TT -:~r _ - "-rr -r: : - "TT : ---.!:: J : ~..:., --:- -r L:.... T- : :;-:... --: : : _ : :. :, -r~....: : ] : :-\-~- - ^ : : :; I 1 BTCH" r-.- r - LA >T CI..1 l-e s ^ t.y. X "V- -4 L- _ J : FIGURE 1. STRAIN, Inches Per Inch TENSILE STRESS-STRAIN CURVE FOR INCH ALCLAD 75S-T6 ALUMINUM ALLOY SHEET

52 STRAIN, Inches Per Inch FIGURE 2. TENSILE STRESS-STRAIN CURVE FOR INCH 75S-T6 ALUMINUM ALLOY SHEET ^

53 STRAIN, Inches Per Inch FIGURE 3. TENSILE STRESS-STRAIN CURVE FOR INCH 24S-T3 ALUMINUM ALLOY SHEET o

54 * FIGURE 4. SONNTAG FLEXURE FATIGUE MACHINE MODEL SF-2

55 FIGURE 5. SONNTAG FLEXURE FATIGUE MACHINE MODEL SF-2

56 o FIGURE 6. SONNTAG FLEXURE FATIGUE MACHINE MODEL SF-2

57 STATIONARY MOUNT \ n W m > & W 49 FRELSEAIL r-load YOKE / mm, V////M { CROSS SECTIONAL VIEW OF SPECIMEN MOUNTING Lulu DRILL NO. II (Q191) I HOLE ~ DRILL 2 HOLES SPECIMEN DETAILS ALL DIMENSIONS IN INCHES FIGURE 7. SPECIMEN AND MOUNTING DETAILS

58 50 9 ( Q TEMPLATE CLAMP c * M > «1 I ROUTER JIG DRILL JIG FIGURE 8. PHOTOGRAPH OF DRILL JIG AND ROUTER JIG

59 51 B FIGURE 9. FRACTURED ALCLAD 75S-T6 SPECIMENS, SURFACE FINISH: A. POLISHED. B. CROCUS CLOTH, C. GRIT NO. 100, D. GRIT NO. 60.

60 : _ A B A FIGURE 10. FRACTURED 75S-T6 SPECIMENS SURFACE FINISH: A. POLISHED B. GRIT NO. 60 m

61 A. y ' / FIGURE 11. FRACTURED 24S-T3 SPECIMENS. SURFACE FINISH: A. POLISHED, B. GRIT NO. 60 m w

62 10 10 J 10 10' CYCLES TO FAILURE FIGURE 12. COMPARISON OF FATIGUE FLEXURE STRENGTHS FOR ALCLAD 75S-T6 SHEET OF THICKNESS AND INCHES S

63 CYCLES TO FAILURE FIGURE 13. FLEXURE FATIGUE STRENGTH FOR INCH ALCLAD 75S-T6 SHEET 1 01

64 !.! " TCP*: I X3 O C I ( <U I: ~ 40 \p \ \t A V > s u a cr QQ - * ' < ; Per CO rh!rf CO.20 to u3 S ;; QJ E- CO - -. j. ~ I ::^ N s; : I... t.: 10 j... : N ^ z *N nl:; -* f s < ).. ^NX < ' - x "" " ^. ~K^ - t... ' IgELXJIHlrj vmttmta. sffc?; 'Mms SCHAI CH EI> unt flrt* Wft'.'l nc A^AS.IVE oor.n- 1 :......! '.... lp Tl t... j. ' I I!......j... j j.... n g; ; '.'". :~-" : 'Z ' '.'.'.'.... -^j 10 10" CYCLES TO FAILURE a FIGURE 14. FLEXURE FATIGUE STRENGTH OF INCH ALCLAD 75S-T6 SHEET 1

65 - \ ' - I CD J-, o3 3 <y n n OK) u Q) cu tfj a 1/2 a: w N or.7.. j... V.... N ;.;; \ 5 V :.:; : ;: L. ]..::. 4 \ s. N? fcj ; - I ' 1'.j.....! F '.'. '. I I-.... Ls^ Fv I. - N^,, M-,. I N.... j J.. j '! " J... ; :.:.... I.-*-- **..... I : I j 7 s 7. *. j * " ~^ r... I ; rr ;..:.J:... a ' " J.? ' r ~~~ I! - "' J ' ] ' J I folishel)..... t IS A I Jt '! ' '.'' - j! '! -!...j... S^FCII IP2 rs 'w-l h : sdfa«torn 2B 33t no 1 IS^ASEV 15! CLO TH. "t'. ::;...;. I. I I I, :..: "..I-- W { < " 10' CYCLES TO FAIL URE 9 m " 1 " ' [ ;.. _^ j : 1 J'.1. j 1 1! -1--:! 1 1 FIGURE 15. FLEXURE FATIGUE STRENGTH OF INCH ALCLAD 75S-T6 SHEET

66 I a> tn c3 P (H a. a; co o- CO CO bo ~ 1 " r... I... Spr 1 -! ' r "U K f! apj Wn WENS tv^ S] ^q m ra : \ -s HW3 S< T'CH-J M4 * pjmk < Itw:: 'Urn ^ ^ > T" l! SHKCTF {jabe UK s<.. Luj_., mm OH! 1 % Jt 0 T s I: J X... 1 j m j vj, ' ' ' ' 1 ' M ' 1 1 " j ' sjcjlat) :KF D w cm ql^c )C us jcto M 1 4 r ta L.J u 1 1'... # ITN( >. do ' I 1 MffTH (GF.'lJT m. 1 "m J......J... I 1 r.... '!... j : 1 ;. j : ] j 1 1 o H CO 10 n... J : :. nl -j-if \m ht-f * M a.. 4 I J ::II ;. ; L j. HS?, J 1 3a j... UJj ;±: j j - I... 10' 10! 10' 10' CYCLES TO FAILURE. 1 ll] :;;;: U _ - FIGURE 16. COMPARISON OF FLEXURE FATIGUE STRENGTHS FOR INCH ALCLAD 75S-T6 SHEET SCRATCHED WITH VARIOUS ABRASIVES 1 1 s

67 - :- h a^c nb».3 qutctaaa pjrps 6_0_ AlhA^TV^ CYCLES TO FAILURE FIGURE 17. FLEXURE FATIGUE STRENGTH OF INCH 75S-T6 SHEET T

68 1! Q) U d 3 cr u <D On Q, C/2 OS E-< W 50 U 30 2U I J....J.. j 1 pe "1 J ]... - j- u r 11...J.! T 1 rv* rs^...j......j , m 1! 1 T" j 4 j jo 1 O 6- o + j M4H 1 j *r*44 p '_ J J.. J. m II " - *--,L '' -L 1 '. J r l l 1 ' 1 JlroHuH, d drst T '-IP I... dfflnff IMSte I* SB J 1 1 E3I 1 I. jj j! ( I Wlffl SIP ttecimeh tek.^ 1 3 D.T NTS... j ' 1 "j '' Jv.. Ul j 1 1 ' l ' '... Bt,..., j * 10' CYCLES TO FAILURE FIGURE 18. COMPARISON OF TEST RESULTS FOR TWO INVESTIGATIONS IN FLEXURE ON 24S-T3 SHEET as. d 8 _j FATIGUE 4^ :* o

69 TTTT I j 50 o S 40 QJ t- a cr IQ f~> a; cu c/3 O, 30 CQ Ex) as 20 H CO 10*- FIGURE CYCLES TO FAILURE COMPARISON OF FLEXURE FATIGUE STRENGTHS FOR SHEET 24S-T3 AND 75S -T6 : -

70 CYCLES TO FAILURE FIGURE 20. FLEXURE FATIGUE STRENGTH OF INCH -< 0

71 ' 10' CYCLES TO FAILURE FIGURE 21. FLEXURE FATIGUE STRENGTH OF INCH 24S-T3 SHEET A oo