RB No. 3H23 NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS WARTIME REPORT ORIGINALLY ISSUED August 19'4-3 as Eestricted Bulletin 3H23 THE EFFECT OF ARTIFICIAL AGING ON THE TENSILE PROPERTIES OF ALCLAD 2I4.S-T AND 21^S-T ALUMINUM ALLOY By Joseph N. Kotanchik, Walter Woods, and George W. Zender Langley Memorial Aeronautical Laboratory Langley Field, Va. NACA w^ WASHINGTON NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribution of advance research results to an authorized group requiring them for the war effort. They were pre- viously held under a security status but are now unclassified. Some of these reports were not tech- nically edited. All have been reproduced without change in order to expedite general distribution. L - 257 I 7- > V 5 2^6^/'/ NATIONAL ADVISORY C0IfflITT3E TOR A3R0RAUTICS RESTRICT 3D BULLSTIN u^ 7 TKS SFF.^GT OF ARTIFICIAL ACtINO Oil TIIS TSNSILS PR0P:^RTIE;S of ALCL^aD 24S-T AND 24S-T ALiaiimM ALLOY By Joseph N. Kotanchik, Walter "Yoods, and George ",7. Zender SUIfflARY An experimontal study was made to determine the effect of arti- ficial fging on the tsnsile properties of alclad 24S-T and 24S-T alw.iinuiD-alloy sheet material. J-'he resiilts of the tests show that certain combinations of aging time and temperature cause a marked increase in the yield strength and a small increase in the ultimate strength; these increases are accompanied by a very large decrease in elongation. A curve is presented that shows the maximum yield strengths that can be obtain'^d by aging this material at various combinations of time and temperature. The higher values of yield stress are obtained in material aged at relatively longer times and lov/or temperatures. lOTRODlTCTION In the design of an airplane, v.-eight control is a loading problem. Every part of the airplane must be so designed or selected as to elimi- nate unnscescpry weight. One of the mo-^t importp.nt items that contri- bute to tne gros.'-' iveight of the airplane is the structure. For this .reason, the airplane stru.oture is examined most critically for elimina- tion of unnecosS'i-ry v/eignt. The 'jontinuing advances in methods of analysis are an important factor for ^ringing about a reduction in weight of the airplane, b.i.t the airplane designer and builder must, in addition, utilize other methods for combating the trend toward increased struc- tural weight. One of th.i most direct m-,'thods is to improve the strength properties of materials already available or in current production. This method has the very important advantage that it .-^an be put into use witli a minimum of delay and interruption to established methods of production. / ni-iniber of r.ircrsft coiapanies are nov; considering the use of 24S-T al'JiTiiuu-i alloy of whic;h thu 5'urongth proportios have been improved by artifid'-al aging. This report present? the resultc of tensile tests on alclad 24S-T and 24S-T aluaunum-alloy sheet material that has been artifi- cially ap-ed at varions combinations of temperature find time. l!AT3RIAL FOK ASTIFICI/,LLY A^2D TEST SPECBI3!-;S J>.s the purpose of these tests is to present the results that can be obtained by artificin.l aging of commercially obtainable material, specimens, of the dimensions shovra in figure 1, v.-ore out from sheets selected from alclad 24S-T and 24S-T aluminum alloy ar. received from the manufacturer. The stretchin;; and rolling operations performed on aluminum alloy at the mills are approximately/ equivalent to the cold work done in giving the sheet a permanent elongation of I percent (reference 1). The extent to which the strength properties or 243-T al'uainum alloy -.lan be improved by arti- ficial E.ging is dependent unon the nmount of cold v/ork performed on the material prior to the aging proooss (reference 1), It is therefore neces- sary th'it the material be of uniform quality v.s reg'rdr; the amount of cold v.'ork performed prior to &gin? in order that consistent results be obtained from the artificial aging. For the tests reported herein, the cutting of the 24S-T and the alclad 24S-T specimens from single sheets of ntch material assured uniformity of the specimens as regards the degree of cold v/ork parforned upon them. That the material v/as uniform as regards cold v;ork is in- di-c&ted by the consistent trends established by the test data in figures 2 and 3. The extensive adoption of the artificial aging process would necessitate that close reguletion bo maintained on the uniformity of and the amount of cold vrork porformod on the aliaminum alloj'. TlJGT Sr ilCI'viaJS The test specimens v;erc stamped from single sheets of 0.064-inch alclad 24S-T and 24S-T al\A:ninum alloy by means of a die. In the tost portion of the speci:p.cn, the sViearod edges v/ore carofully hand filed to remove the slightly turned edge left by the stamping die. One specimen was m^.de for each combination of time and temperature investigated. /irtif icial eging of the specimens was performed in an electrically heated o.ir furnace, and the specimens wore cooled in air at room tempera- ture. The test specimens vrere placed in the furnace, v;hioh was at ths desired temperature, end the aging time was taken as the entire pi3riod during which e>-ch sp-^cinen was in the furnace. The rasults are therefore representative of the values obtained Vvith matei-ial that is artificially aged in the same manner. The data for the tensile stress-strain curve v;ere obtained for each specimen to a value of strain beyond that at the 0.2-percent- offset yield sxrcnrth. Strains vrera measured by tv;o Tuckcrraan optical strain ga^es of 2-inch gags length attached to opposite sides of the test specimen. The elongation at failure was determined by measuring, after failure, the increase in length of the initial 2-inch gage length of the test specimen to the nearest J. 01 inch. TEST RES^JLTS JC^D DISOiSSION The results of the tensile tefjts on the artificially aged test spoci-.iens are given in figures 2 to 5. Figure. 2 chov/s the variation i."; the ultimate strength, the yield strength, and t'no elong^ cioa of aleliid 24S-T material that is sub- jected to artificial aging for p.^riods of 2, 4, 3, 7, and 10 hours at temperatures that vary from 100" to GOO^ F. The important information contained in this figui-e is that a substantial increase in yield strength end decrease in elong'-tion occur in tiie material v;hen aged in the temperature range of 340'^ to 400° l\ In this temperature range, a small increase in the ultimate strength of the material also occurs. The most decirrble result obtained from artificial aging is the large increa-e in yield strength. Although this report presents only the results of tensile tests, some compression tests have been per- fori.ied on artificially aged material and thuse tests indicated that the compressive yield strength was increased in essentially the same way as the tensile yield strength. By taking into account the increase in yield strength, the airplrne designer can effect substantial reduc- tions of vrei ght in certain nt.rts of tlie airplane. For example, the full increase in yield strength can be utilized in the design of com- pression members so proportioned and supported thut they v;ill not fail by instability/ before the compressive yield strength is reached. In the case of t'-nsion manbers, only a p^.it of the increase in yield strength cen be utilized und :.r prer;ent design requirements because these requirements specify a definite ratio of allowable yield stress to ultimate stress, and the ultimate strength of the artificially aged material is not increased in t'le najT-e proportion as is the yield strength. The increase in yield strength is accomfisnied by a change in the elongation in 2 inches from about IS percent to about 6 percent. This decrease in elon.-ation v/ill undoubtedly add to the difficulty of for-iinj; the niat=riai during Trbri cation. In many cases, hov/ever, the addod difficulty can be avoided by completing the fonninj operations prior to artificial a^ing. Fi(:urG 2 also chov:s that for any aging time the temperature must be mainbeined v;ithin, a rau^e of "^L^P of the cptiraum valuG in order that the best incraars of yield ?;trongth nay be obtained. This standard of te-T.p-?rature maintenance is readily attain'-bl ; v.i.th modern hoat-tr =atj ng eqii.ipmcnt, Thc! vari&ti^n in yield strongth of alclad 24S-T material that is subjected to artlfl-jial aging ab corstant ter:iperfct';res for varying period3 of time ic shown in f'.gure 5. It v^-'^il be noted on this figure that for values of temperature above S-'O^ F thei'e is an aging tim° which will re'^ult in a mnximviiix iucrdase of yield streijgth. For 'iging timer l;ss th';n or 3reatcr th&n the cptimurr. time, lov/er vrlurs of yield stress Vvill bci obtained. /,t tempirttur 3S in exr-ess of 4r5'3 F, the aging; tirr.e hoccmnr v^-ry critii^al. The dp.ng^^r of undoia'^in'5 or cvur- aging of tht. material with conse-^uent iari^e decreifioe ox yield strength .' i? quite s ;rious. For t:;is roi-son, ugiiig tf-'moer; ■'■'jrps in exc'ss of 425° F are not f jasibln for production vrhcire the cnrractor of tht. work is cliant^in,; and vrhcre it is JifficuLt tc detBrminc •*he exrct tine -Jt v/hich the material reaches th'T agin?; tamper a tur-i. At aging teiiiperatures of 40C° F and low;;r, tr.vo important advantages exist; nam;;j.y, higher absolute values nf yield si.rength ct^n be obt-::inod and thr-re. is a consider- able range of ti-ae at v;hi oh aging may be ti'rminated v.l+hout appreciable loss of yijld streni^i;h by reason cf small amount? of underaging or overaging. Figure 4 presents a curve of yield-strength n^aximums for various combinations of time and temperature. Y/ithin the ranr;o of values covered by this investigation, this curve shows that, as aging tempera- ture decreases, the aging time required for o'-st yield strength imreases. The curye also shows that the higher values of yield strength are obtained at the combinations ■if lov."jr temporaturbs and longer aging times shown on this fig'ire. Figure 5 shows the variation of the tensile properties of 24S-T v.'ith temperature for a constant aging time of 2 hours. ViO-ien figure 5 is compared vdth figui e 2, it is evident that 24C-T material responds to artificip.l '^ging in a mannei' simiisr to alclad 24o-T. The tort specimens v/hich were ared for G hours at various temper- atures are sho".m after fracture in figure 6. The change in elongation is clearly sc3n in the final l?ngths of the specimens. The character- istic fr-'-cturer: that occurred at the various temperrtures for each aging time are alro shovai. At temperatures below the range of cri+ical aging temperat^ares, as in the lOO'^ F and 2uO° F specimens on figure 6, the fracture v;as normal to the direction of th^ load but v/as inclined ft 45"^ to the thicknecs of the material. In and brsyond the region of temperature^^ corr'>spouding to the groatast decrease of elongation, tenperat'Jres of 370<^ F and higher, the fracture changed to one that was inclined at 30^ to the width of the specimen. The surface of tlie frecture vas relatively smooth fo^" the material aged below the critical temperatures but it became ver;' rough and irregular in the speciraons aged. St the higher temperatures. In the transition range of tempera- tures, the frecture was pertly of both types as shown by the specimens marked 330° F and 350° F in figure 6. 3TR3SS-STRAIN CIIRVvS The tensile stress-strain curves for the specimens tested are shov:n in figures 7 to 12. The O.ii-percent-ofiset yield strength is indicated on efjdh str';£3-strain curve by a short intercept line. These yield strengths v/ere used in preparing figures 2 to 5. The stress- strain curves show that the modulus of elssticity is substantially the same for the combinations of temperatiu-e and time included in this in- vestigation. It can be observv,-d, hov;ev;jr, thct rs the aging tempera- ture increases t!ie initial part of the stress -strain curve tends to de- velop more curvr.ture, v.'l-.ich indicates a trend tov/ard reduction of the proportional limit at the highir aging temperatures. OOWGLUSIOIIS Artificisl eging of alclad 24S-T aluminum-alloy sheet material in the es-received condition produces a substantial increase in yield strength, a small increase in ultimate strength, and a large decrease in elongation. The artif J 'lial-aging process produ-jes essentially the same effect on 24S-T sheyt .iiaterial as on alclfjd 24S-T sheet material. At any riven temn.-rature there is an optimum value of aging time re- quired to obtain mBxi;iiLun value of yield strength. 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