J^HPf L-ll^1 y EB No. L5EI5 NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS WARTIME REPORT ORIGINALLY ISSUED May 1945 as Eestrlcted Biaietln L5EI5 INVESTIGATION OF METHODS OF SUPPORTING SINGLE -THICKNESS SPECIMENS IN A FIXTURE FOR DETERMINATION OF COMPRESSIVE STRESS -STRAIN CURVES By Joseph N. Kotaxichik, Walter Woods, and Robert A. Weinberger Langley Memorial Aeronautical Laboratory Langley Field, Va. UNIVERSITY OF FLORIDA DOCUMENTS DEPARTMENT 1 20 MARSTON SCIENCE LIBRARY RO. BOX 11 7011 GAINESVILLE, FL 32611-7011 USA NACA 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 wax 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 - 189 Digitized by tine Internet Arcliive in 2011 with funding from University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation http://www.archive.org/details/investigationofmOOIang ?JACA R3 No. L:>E15 NATIONAL ADVISORY C0KMITTE3 FOR AERONAUTICS RESTRjCTSD BULLETIN IN-/ESTIGATION OF METHODS OF SUPPORTING SIKGLE-THICKI^SS SPEGIIIEN3 IN A FIXTURE FOR DETSRiaNATION OF COMPRESSIVE STRESS-STRAIN CURVES 3y Joseph N. Kotariclilk, V/alter Vn'oods and Robert A. j;elnberger SUMMARY I An investigation w^s made of the methods of sxipporting single-thickness specimens of aluramuin-alloy sheet nic?terlal in a fixture for deternilnation of compreGSive strecs-strain curves. The s-^eciiiiens v/ere supported in the fixture by rollers, offset-grooved plates, ooposite-groovea plrtes, flat brass olstes, or flat wooden plates. - It was found that the measured values of compressive ^ yield sti-ess snd modulus of elasticity obtained with the roller supports wore independent of the sux^porting force , applied to the soociraen provided chat a sufficient minimum force is used to overcome initial deviations from: flatness. The stress -3 brain curves obtained by the use of the plate types of supnort wore affected by an increase in the sup- porting force. Satisfqctcry stress-strain curves were ootsined with p11 types of support; however, Vk'hen the sup- porting force vtss ?:pproximiately ,!+p pc-ands . I The investigation also showed that the com.pression ' fixture, the specimen support, and the single-thickness j specimens must be accurately made in order to obtain accu- j rate stress-strain curves consistently. . I ) INTRODUCTION Compressive stress-strain curx^es for a liominum- policy sheet m.sterial are being obtained in many laboratories , throughout the aircraft industry to provide information on the properties of the materials used in aircraft. It j 2 FACA R3 No. L5EI5 is lirportsnt that the me-';hcd for dsterminctioii of the curves be reliable, pccur^ie, pnd relatively simple. Among the various irethods oroposed, the cingle-thickness ire bhod introduced by W. P. RCon bgcinery of Vought-Si'.-corsky Aircraft apT^eered to be the rr'ost nromifing. In the ineohod proposed by Montgorrery rnd further developed et the Alun'.in^iiii Research Laborrtories of the Aluminum Company of America, a fixture was used for supporting a single thickness of sheet material so rs to prevent buckling. (See reference 1. ) In this fixture, closely spaced rollers en each side of the STDecim.en provided lateral sursport and permitted shortening of the spscimen under load. In order to simplify the Ivlcnt.^omery type of compress icn fixture, the National Bureau of Standards substituted solid brass plates for the rollers. (See reference P..) In experirrentel work at the Langley structures resesrch laboratory, in v.-hich the single-thiclcness method and a Gom:pre3sicn fixture similar to those of references 1 and. 2 were used, great difficulty Vves ezicountered in consistently obtaining accurax-e stress-strain curves. It was therefore decided to maice a thorough investigation of the single- thickness .riet^^cd of detei^mining compressive stress-strain curves for thin sheet material. Various types of support v;ere used in the compression fixture and sti-ess-strsin carves were obtained for specimens thct were supported with dif-^'erent valuos of supporting force. SUPPORTS AI:D GOMPHESSIOTI FIXTURii In the single -thic-oiess method, the function of the compression .■''ixturo is to support a soecim.en of thin sneet material so thpt the stress -straiii curve obtained is the same as vi/culd be obtained for an unsupported speciiaen of the same material hf:ving such pronortlons tnot buckling could not occui-. The degree to .vhich the comoression fixture fulfills its function deg-^nds largely on thu per- formance of the supports, such as the rollers or flat brass plates, that come 5nto contact with the speclmeri. For Ideel oerf crrr.-^iice , che supper oc should nold the specimen straight but should not resist changes in length, thickness, or v.idth of the specimen. Rollers, grooved brass pistes, flat brass plates, and flat wooden plates were used in the present investi- gation for supporting the specj.m.ens. These supporcs are shown in figu'^e 1 and are described as follows: MAC A RB No. L5EI5 ^ (1) Steel rollers - In tlie roller type of support, 25 hardened steel rollers O.O925 Inch in diaiaeter nnd spaced 0,10 irxch supported the specimen on each side. The rollers were supported at their conical ends by spring- brass plates that permitted the rollers to roll downward as the specimen shortened daring loading. (2) Grooved brass plates - Two types of grooved brass plate were included in the investigation. In the first type, the grooves of one plate were opposite the grooves of the other plate rna, in the second type, the grooves of one plate were offset so as to be opposite the crests of the other plate (fig. 2). With the offset-grooved plates, there is less rosist-ance to increase of specimen thickness during loading than there is with the opposite- grooved plates. Both types of grooved plate were lubri- cated vtfith a heavy grease (Marfax No. 2). (3) Flat bras? plates - The brass plates were machined to a smooth flat finish. The plates were lubricated with grease in one serj.es of tests and v\?ith a mixture of graphite and oil in another series. ()^-) Flat wooden olates - The plates were made of hardwood (maple) finished to a smooth flat surface and were lubricated with a heavy grease (Mai-fax No. 2). The various types of support were lased in the com- pression fixture that is sho.fn in figure 3» The fixture is composed of a steel holder that is mounted on a hardened steel base ^nd into which are fitted two steel blocks. One of the steel blocks is cylindrically seated to make it self-alining, ^nd the otn^r is a sliding block that permits adjustment of the compression fixture for specimens of various thicknesses. The test specimen and the supports that come into coiitact with the soeclmen are inserted betwec.:i the self -alining and the sliding steel blocks. A leading screw acting upon a loading plate provides a means of 8r)plying the supporting load to the supports for the specimen. TESTS Specimens O.53 inch wide, 2.55 inches long, and 0.06)4- inch thick were cut from one sheet of ZJ+S-T aluminum alloy ;vith the grain of the sheet parallel to the direction h NACA R3 No. L5E15 of loading. These speoi.iuens v/er3 tested in the compression fixture with the four types of suoi^ort c=nd the foar v£:lues of supporting-- force shov/n in tcble I, During routine testing foi- ln-"-eEti2ations other than the present Inves tige.tion it v,-as sometines observed that for spscTrtiens of high-sti'ength material, C.53 inch 'v;ide, there -.v^-c evidence of bending in the plane of the stjeclmen. It v.ps t.Ler3t"'Gre docidei to Increase the width cf the specimens to O.8O inch. The pubs t'^'iiti al increpse in vifldth from 9,5'; to O.SC inch v\?as selected in order that the sr)fc;cimeiis iright be of suitablj dimensions for testing not only the slu;a?num alloy."' with com'oressive yield stresses of sppror.lmatcly oO to So ksi in current use but also for testing alloTs thet n.sy be developed wluh even higher yield stresses. Soecim.ens of ^Ls-T and RJOI-T c'l^amixium ^lloy . 60 inch wide, 2,^1 inches long, and O.064. inch thick were -cested with rollers ^::id oTrset-grooved plates as suo'^orto and with the ss^r.e v&Iugs of supoorting force th^c vjere used for the 0.53-iJ^ch--vc.de s-'ecimens, (See tpble II.) Sxamination o:' column curves for 2l;S-T and R3OI-T slurln^im el leys indj.c&tea thao buckling between rollers mlp:ht occur for soecimens of chin sheet in&terial. Tests accordingly wore nijde of 0. cO-inch-^^lde specimens of 0.025-inch- thick 2li.S-T and 0. 020-inch- thick RJOl-T alumi- nuiTi E.lloy support 3d in o-ie compression fixture with roller sup^ort3. Similar specimens supported by offset-grooved plates were tested for comparison. In a].l the tests, strains vj-ere measured on the two free edges of each specimen by Tuckermsn optical strain gages of 1-inch gage length. Figure I;, srxows the com- pression fixture with snecimen r.nd strsin gages in position before the te.it. RESULTS Al^D DISCUSS^CI^^ The tests showed that the stras^^-s train curves obtained frox.i siiigle-thickness specimens were deoendent on the type of support used an the com.Tjression fixture, on the magrii- tude of the supporting force ap">lied to the soecin.en, and on the quality of the test apparatus snd the test specimen. The arroanu by which the measured values of compressive NACA RB No. L5E15 5 yield stress pnd modulus of elrsoicj.ty ver-ied v;lth the type of Euoport and the magnitude of the supiDcrting force is illustrated In tpble I and in figure 5 ^^'^ the 0.53- inch-wide specimens and in table 11 and in figure 6 for the 0, 80-inch-v.ide specimens. These tables and figures show that: (1) The values of com.pressive yield stress obtained from tests vvith the roller type of surtport ./ere independent of the Fiagnitude of the supporting force. i-'or supports other then roll3rs, the measured yield stress increased as the supporting force initially applied to the specimen was increased. The greatest increase was obtained with the flat wooden plates. (2) The values of compressive modulus of elasticity obtained from tests with tne rollers and with the offset- grooved plates were independent of the supporting force. For supports other- than rollers and offset-grooved plates, the values of tne modulus of elasticity increased as the supporting force was increased. The greatest increase was obtained with the flat brfss elates lubricated with Marfax No. 2. (?) The stress-strain curves for specimens supported by other than rollers were distorted by increasing amounts as the supnorting force W9S increased. The distortioii was greatest for specimens supported by flat brass plates or by flat vi^ooden plates. As an example of the distortion vjhlch occurred, figure 7 shows that the compressive yield stress, the "orooortional limit, the compressive modulus of elasticity, and the initial part of the stress-strain curve were changed when the specimen was supported between flat bi'ass plates and the supporting force was increased from C.C. to 380 pounds. Tlie stress-strain curves that were obt'^'ined for the roller type of support vcere mora accurate than the curves obtained for the other tynes of support. The roller type of support, however, is m.ore difficult to construct thrin the other types and must be ccnstructsd with precision to ensure accurate results. This fact was emphasized in the aresent investigation when one inaccux^ately constructid roller support had to be discarded bec?use of the unreliable test results that v:ere obtained. The fist- plate and grcoved-plate ty.r.es of suoport are easier to construct than the roller ty.^e but more care m.ust be exercised in their use to obtain an accuraoe stress -strain o NAG A F.B No. L5SI5 curve for the r.ipterial. viflien the supporting force v/gs in the rpnge of 22 to h5 pounu?, the results obtained with the flat-plate or grooved-plate suoports agreed very clocely with the results obtained with the rollers. These results are illustrated for the of fbet-grooved plates by the tyoical stress-strain curves given in figures 8 and 9» The results of the tests of 0»80-lnch-wide specimens with roller support indicated that a supporting force of 22 pounds was inadequate . Although there was no evidence that tne C.020-inch-thick speciK'iens buckled betv;een rollers as the column curver. had indicated they might, examlns-.tion of these speciii.ens after test revealed buckles of wave length greater than the spacing of the rollers. It is believed that this buckling occurred because the supportiiig force of 22. ^oun.is v;as insufficient to "eircve the slight deviations fror' flatness that usually exist in the speci- mens. ?/ith a cuopoi'ting force of i.(-5 pounds, satisf actorj/ stress-strain curves v^e^e obtained consistently with all tyoos of suo;.:iort. ^he biickling of the roller-supported specimens as described indicates that, without sufficient supporting force, tbe roller-siipported specimens may give lonsatisf actory stress-strain curves. The present Investigation and subsequeat I'outine laboratory/' tests showed that accurate stress -strain curves could not be obtained consistently unless the fixture and specimens were accurately made. Variations in the test results v/ere cpusod by specimen defects or inaccuracies si-ich as ends not parallel, not flat, or net perpendicular to the longitiidinal axis of the specimen; free edges of the specimen not straight and parallel; and faces of the specimen scratched or marred so that interference occurred with shoi'tening of the specimen. The accuracy of the single-thickness specimen is so important that production of the specimens by a pionch 3i;d die or by a shearing process is not recommended even if it can be shown that the material properties ai'e not appreciably affected by the cold work in these processes. NACA RB No. L5E15 7 CONCLiJSIOKS An investigation was nade of tlie methods of supporting slngle-thlckne^s soecimens of aluinlnun!- alloy sheet material in n fixture for deterjninstion of coiiipressi ve stress -strain curves. As a general conclusion, the results showed that for the roller type of support tfx3 accuracy of the stress- strain curves obtained was independent of the supporting force used. Plate types of support, however, also gave satisfactory results provided that the supporting force ap'olied to the specimen was aoproximately Ii-5 pounds. For the types of support e/as Increased, stress- strain curves obtained with plate types of supports were distorted and the compressive yield stress and modulus of elasticity were raised by increasing amounts. I)-. A supporting force sufficient to overcome the slight deviations from flatness that usually exist in the specimens m^ust be used. 5» The compres^^ion fixture, the specimen supports, and the slnr;le-thic!-ness specimens must be accurately made. Langley Mertiorlal Aeronautical Laboratory National Advisorjr Coram.ittee for Aeronautics Langley Field, Va. 8 KACA R3 ITo. L5E15 REFERENCES 1. Paul, B. A., Hov;ell, r. T;!. , and Grleshaber, H. E. : Coirparison of Stress-Strain Curves Obtained by Sir.gle-Thickness snd Pack Methods. NACA TI\' No. 8iq, 19U1. 2. Anon.: Oor-press: ve Tests of Siieet Metal with Solid Guides for Lateral Support. Strucj. rv^eino. No. 10, Bur. Aero., I-Iavy Dept., Fcv. 10, 19^2- NACA RB No. L5E15 a M H rt W t, T) E-< O r-i (H »H o m >5 1-1 M (S 3 0) •-H > o a -H Sfc > M tD ml 1 gg SM a> a E OH E-H kJoC .. O o o >HE> sO ^ HhJ O W . 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O u t, cd tx^ a cc ta cc K bL. a or. bC CD / ■*-* 1 a 1 s 1 S 1 £ / ^ p ■p -P p / o o x: 0) -C (U £ (U^ / '^ 01 P yl -P n -P 0] p / p- ;- ^ 1_ -H <;-. -H V. ^ / ^ i- « ^ s c^ S (-. S / ^ o o- o o o >^ 5 o z CO o M (E > M 9^ K •-1 O <&. O M M M < H I. a E o NACA RB NO. L5E15 Fig. la-d (q) Steel rollers. (b) Grooved brass plates, (c) Flat brass plates. (d) Flat wooden plates. NATIONAL ADVISORY COMMITTEE FM AERONAUTICS Figure I. -Types of support used in the fixture for determination of compressive stress- strain curves. NACA RB No. L5E15 Fig. 2a, b 0.53-inch-widc ^™^ — specimen (a) Opposite-grooved brass plates. 0.80- inch-widft S-<- specimen (b) Offset -Qrooved brass plates. NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS Fi9ure 2,.- Opposite- and orf3Bt- grooved supporting plate/5. Sections are perpendicular to direction of load application. * NACA RB No. L5E15 Fig. 3 CO o a to Ui 0) o 0) 3 ♦J « (m C O •H CQ CO (U e o o I ^ o o to - 48- 46 AA-< 4^- t: 1 100 200 300 Supporting force, lb Flat wooden plates pposite-grooved brass plates Flat brass plates (Morfax No.2,) Flat brass plates (graphite and oil) Rollers 400 en UJ o o 1,400 1,000 10.600 100 200 300 Supporting force, lb Flat brass plates (Ma rf ax No.2) Flat brass plates (graphite and oil) Flat wooden plates Opposite -grooved brass plates Ro I lers 400 NATIONAL ADVISORY COHHITTEE FOB AERONAUTICS Figures.- influence of type of support and supporting force on compressive yield stress and modulus of elasticity. 245-T aluminum alloy 0.064 inch thick and 0.53 inch wide. NACA RB No. L5E15 Fig. 6 6E -^ 60 ^^ 59 - 44 43 d ,-L 245-T J^ L in o _a en o 10,800 10,600 r 10,800 10,600 — 01^ -q^^i- R30I-T 245-T 100 200 300 Supporting force, lb _D o Rollers n Offset- grooved brass platas 400 NATIONAL ADVISORY COMMITTEE FOP AERONAUTICS Figure 6.' Influence oT type of support and supporting force on compressive yield stress and modulus of elasticity. Specimens 0.064 inch thick and 0.80 inch wide. NACA RB No. L5E15 Fig. 7 c 3 -4-3 o o a s^ n h- 3 iD >D ^ a) (M a n Q> cD > tO 3 a JO c _^ t> rt I-.— <.') iT) vO tO -.J <-»> i_ o -^ Q. O t . 3 o o c Q) — '^ to i— r o Q> Q.-0 cvj S ^ o V O" 2: q> — ■ — c3~: (\) 00 ^ o to tNj 03 ^ O 1/-) ^ ^ ^ m oO (VJ t\J IVI is>l 'ssaj'^g NACA RB No. L5E15 Fig. 8 O O rvj iNj — — ie>) '€eaj']<5 NACA RB No. L5E15 Fig. 9 O "^ <^ -»J in -'-' Q- d ^ ^ 0"Q- I— uJ t^uO Ci> r iD o _&^ O CO o 1 CJ £ r o -►J -o 3 s O c_ 1— o) t- ^ 'o Ci^ Cl- io ' 1 o s c_- a in ? c\J o c. O O o l_ r cP « g 8 /O 3 (i. O I/) c .^ p -o -17^ in o in Cl_ ■ T i,o J-> ^ -g C3 ■Q- r~i .>^'*i 1— 1 rT> CTi C I4J 2 3 n ,^ U- 3 U-) o o to ^ ^ o 00