REG NACA-TN-3735-1956 Bending tests of ring-stiffened circular cylinders.pdf

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1、.;#Jir.!TECHNICAL NOTE 3735BENDING TESTS OF RING-STIIZFENED CIRCULAR CYLINDERSBy James P. PetersonLangley Aeronautical LaboratoryLangley Field, Va.WashingtonJliiy 1956. - - . . . . . . . . . . . . . . . . . . . . . . . . . - . . . . . .Provided by IHSNot for ResaleNo reproduction or networking permi

2、tted without license from IHS-,-,-TECHLIBRARYKAFB,NMlEb,d.,ImllllllunlllllllnNATIONAL ADVISORY COMMITTEEFOR AEROtiAUTICEtltlLb322mcmmml NOTE3735BENDING TESTS OF RINGSTHTENED cnm CYLINDERSBy James P. Peterson*Twenty-five ring-stiffened circular cylinderswere loaded to fail-ure in bending. The results

3、 are presented“inthe form of design curveswhich are applicable toof lod buckling.The ring-stiffenedtransmittingbemthig orcylinderswith heavy rings that fail as a resultINTRODUCITONshell is efficientshear loads if theas an aircraft fuselage forloading is large. For smallerloadings, a-more efficient s

4、tructure can be obtainedby stabilizingtheshell in some manner such as by the addition of Aringers to the shellor by the use of waffle-like or sandwich-typeplates for the shell Thebending and shear strength of these types of constructionare not knownto the desired degree of accuracy and the designerm

5、ust usually supple-ment his existing knowledge on the subjectwith tests simulatingtheproposed design. Such a scheme rarely leads to the most efficientuseof material and evidently suggeststhat more design data on fuselageconstructionare needed. W order to provide information on one phaseof this probl

6、em, a series of bending tests on ring-stiffenedcircularcylinderswas made at the Langley structuresresearch laboratory. Themain structuralparameters varied in the tests were the ratio of ringspacing to radius and the radius-thiclmessratio. The geometric sizeof the cylinderswas also varied because the

7、 size has been suggestedasa possible contributingfactor in explainingthe disparitybetween theoryand experiment or the discrepancybetween two test series. The ringsused in the test series were heavy in order to eliminate general-instability-typefailures which involve simultaneousfailure of thecylinde

8、rwall and the rings. .The theoretical studies that have been made on the strength of thering-stiffenedshell in eitherbending or compressiondo not adequatelypredict its strength. Considerableprogress has been made, however,which has clarifiedthe importance of the various factors (load-shortening curv

9、e, initial eccentricities,and so forth) that are mainlyresponsible for the poor predictions (see, for example, refs. 1 and 2).Numerous eqerimentsl studieshave been made, but, for the most part,. -. . _ . . - -. -.Provided by IHSNot for ResaleNo reproduction or networking permitted without license fr

10、om IHS-,-,-_._2 NACA TM 3735the studieswere made on compression cylinderswith large radius-thickness ratios and consequentlyare not in the range of loading wherethe ring-stiffenedshell is efficient. Furthermore, these studiesweremade on one-bay cylinders clampedbetween heavy end fixture. This typeof

11、 specimenmaybe an unrealistic counterpartto the case of interest,the ring-stiffenedshell.SYMBOLSzrtEMcr%vcrring spacing, tieradius of cylinder, in.thiclmess of cylinderwall, in.youngs modulus, ksibending mment at cylinderbuckliug; ti-kipsbending mment at cylinder failure, in-kipsPoissons ratiocylind

12、erbuclddng stress, ksiTEST SPECIMENSAND TEST PROCEDURESTest SpecimensA photograph of one of the cyltidersready for testing is shown infigure 1. The dimensions of the cylindersare given in table I. Thesedimensions are nominal except for those Wnensions given for the wall.thickness of the cylinderswhi

13、ch represent the average of a large m.miberof micrometermeasurements. Dimensions of the rings used to stiffenthecylindersare given in figure 2. The location of the sheet splices onthe compression side of the cylinders is indicated schematically-infigure 3 where the upper half of the circle represent

14、s that part of thecylinder in compression. The cylindershad radius-thicknessratios r/tthat varied frmn 120 to 0 and had ring-spacing-radius ratios Z/rof 1/4, 1/2, amd 1 for most of the tests. For one value of r/t(r/t =180), additional cylindersti%h values of Z/r of 2 and 4 werealso tested.1.- -. .-

15、.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-NACA TN 3735 3The specimenswere constructedof 7075-T6 aluminumalloy. Typicalmaterial properties were used in reducing the data. Youngs modulus Ewas taken as 10,500 ksi andPhotographs of typicslThe load

16、ing frame in figurePoissons ratioTest Procedurestest setups are1 was used whenIL was assumed toshown in figures 1be 0.32.and 4.the expected failing mmuentwas less t 3,000 in otherwise the 1o- frame in figure 4was used. The weight of end fixhres and that part of the test rigwhich would otherwisebe su

17、pportedby the test specimenswas counter-balanced by weights to eliminate stray loads in the test specimens. Thedesired loads were applied to the loading frames by hydraulic jackswhich were accurate to about 1 percent of the applied load.When loading frames such as those shown in figures 1 and 4 are

18、used,the load actually applied to the test specimeqmaybe less than theindicatedload at the jack because of friction in the bearings of theloading frsmea71 b order to obtain a correctionfor this differencebetween indicatedload and applied load, strains at a few locations oneach of the cylinderswere m

19、easured and comparedwith the strains expectedto result from the indicated load. For the frame shown in figure 1, themean error as measured by a large nunber of tests is 4 percent but theerror may be as low as 1 percent or as high as 7 percent. The testresults, as given later, have been correctedby 4

20、 percent; therefore,the error in the results as presented, due to friction in the loadingframe, maybe ashigh as *3 percent. This method of adjusting the loadfor friction was used rather than relying solely on the measured strainsbecause of the uncertainties that may beset any one test and cause locs

21、lchanges in the strain distribution from the expected elementaryvalue.The nuniberof tests made in the loading frame shown in figure 4 is muchsmaller, and the error due to friction has not been establishedas accu-rately as for the loading frame of figure 1. The few tests which lxivebeen made indicate

22、 that the error is much smaller and the resultsobtainedby use of this frame have not been correctedfor friction.TEST RESUILCSValues of the bending moments sustainedby the.test cyltmlersatbuckling are given in table I. These moments represent the maximum “moment as well as the bucklhg moment for thos

23、e cylindersfor whidh thebending moment at failure is not specificallytabulated. Values of thebuckling stresses for the cylindersare given h table I and in figure 5on a plot which has as ordinate and abscissa the parameters obtainedbysmall-deflectiontheory (ref. 3). Also shown in figure 5 is a curve

24、for.-. - . .- -. . .Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-4 NACATN3735the buckling stress of cylinders in compressionas givenby small-deflectiontheory and several other curveswhich have been obtainedbyfairing lower limit curves to the prese

25、nt test data while using thetheoretical cuiwq as a guide.#.On a plot such as used in figure 5, the theoretical curve has aslope of uni for values of-the abscissa greater than about 10 and isgivenby the following familiar equation:(1)When data for a given radius-thicknessratio are parallel to this th

26、eo-retical curve, the effect of ring spacing on the buckling stress isnegligible. A study of the data as plotted in figure 5, or as given intable I, tidicatesthat there is some gain in strengthas the ringspacing is decreasedbut the gain is negligible until a value of l/rof 1/2 has been reached.W con

27、structingthe empirical curves of figure 5, the curveswere ,.drawn parsUel to the theoretical curve for values of the abscissagreater than that value correspondingto a cylinderwith a ring-spacingradius ratio of about 1/2.,The location of curves in this range wasdeterminedly plotting that part of the

28、data from table I which wasobtained from cylinderswith a rhg-spacing-radius ratio greater thanabout 1/2 on a logarithmicplot of acr/E against r/t and then byfitting a straight lower llmit line to the data (see fig. 6). Forsmaller values of the abscissa in figure 5, the curves are faired in tomeet th

29、e theoretical curve for a compression cylinder simply supportedat the rings. The trend of the data suggeststhat a more appropriatefairing of the empirical curvesmight be obtainedby fairing into thetheoretical curve for cylinders clamped at the rings instead of thecurve for cylinders shply supporteda

30、t the rings. However, the testcylindershad heavier rings and therefore a greater clampingaction atthe rings than is usually found in aircraft structures,and it isbelieved that the test data in this range of figure 5 are influencedbythe clsmping of the heavy rings.A photograph of one of the cylinders

31、after being tested to failureis shown in figure 7. The type of buckling =bited is characteristicof that expected for cylinderswith small values of the parameterH= For larger values of the parameter g-. t u-epattern for the test cylinderswas the more familiar type of buckle char- “acterizedby success

32、ive in-and-outbuckles along and around the cylinder.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-NACA TN 3735 5DISCUSSION OF RESULE5One of the factors ofien mentioned when discussing discreciesbetween test data from various investigatorsis the geo

33、metric size ofthe specimens (see, for instance, refs. k and 5). order to studythis effect, the geometric size was included as one of the variables inthe present test series. Most of the data presented in figure 5 wereobtained on cylinderswith a diameter of about 30 inches (30-inch cylin-ders), but a

34、 few tests were made on 19-inch, -i.nch, and n-inch cylin-ders as well. A comparisonof the data frcm cylindersof various diam-eters (fig. 5) indicatesthat any size effect that exists must be smaUand is hidden because of the relativelylarge scatter in data frombending tests on cylinders of the same d

35、iameter.The factor usually cited as being mainly responsible for the largedisparitybetween theory and test for cylinders in compressionor bendingis initial imperfectionsor eccentricities. No attempt was made to deter-mine the extent of initial eccentricitiesin the present test seriesexcept to lay a

36、straight edge along the length of the cylinders in theprocess of inspectingthe cylindersprior to testing. The specimensgenerally exhibited an appearance characteristicof good workmanship.However, the sheet splices,which run lengthwise in the cylinders,usuaXLywere bowed in slightlybetween rings. It i

37、s probable that this phenome-non is a result of “pounddngout” the sheet in the neighborhood of thesplice in the process of bucking the rivets, thereby creating an excess ofsurface area in this vicinity. The effect was considerablymore pro-nounced on cylindersth thick skins and large ring spacings,an

38、d, oncylinder25, the sheet splicebowed in about 3/16 inch between rings.When this cylinderwas tested, the eccentricitygrew slightlywith loml,and, when the ncment reached about 2,600 inch-kips,the wall of the cylin-”der at the eccentricity snapped gently inward to form a buckle. The loadwas released

39、and a 1 x 1* x 1 X 3inch Z-section stringerwas rivetedalong each of the four sheet splices in order to pull the splices outand remove the eccentricity. The cylinderwas retested and, this time,sustaineda mment of 4,465 inch-kipsbefore bucklhg. The four stringersadded about 1,500 inches4 to the moment

40、 of inertia of the cylinder (whichis less than 10 percent of the originalmoment of inertia)but were farenough apart circumferentiallyto have a negligible effect on thelocalbuckling stress other than to remove the initial.eccentricity. This typeof eccentricity can be eliminated in design without appr

41、eciableloss inefficiencyby the addition of stringersat sheet splices on the compres-sion side of the fuselage or by eliminating sheet splices on the compres-sion side of the fuselage. The latter canbe accomplished for largefuselagesby using the with-grain direction of the sheet in the circum-ferenti

42、al direction of the fuselage.- . - -. -. . . -Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-. . _ _6 Mm m 3735ExperimentsXlydetermined curves similarto those given in figure “have been pfilished at various times summarizing available data for cyl-i

43、nders testedin compression (see refs. 3, k, and). The object ofsuch Summaries is to provide design data for fuselages sjected tobending loads, but data from bending tests are not used because not muchdata of this type exist. A cylinder in bending can sustain an exhemefiber stresswhich is somewhat gr

44、eater thm the average stress obtati-able in a similar cylinder tested in compression. using theoreticalconsiderations,wshenko (ref. 6) predicts a value of about 1.3 as theratio of the buckling stresses for the two loading cases. Ratios franl.3to 1.8 were obtained in reference 7 in an qerimental inve

45、stigation.The curves of figure 5 give buclddng.stressesas muchas two times thebuckling stresses givenby the correspondingcurves of references 3 to 5but were obtained from data on a differenttype of structureas weU asfor a differentloaiHng condition. The test specimens in the presentinvestigationwere

46、 ring-stiffened cylinderswhich had a shortbufferbay on either end of the test section to help distributethe load inthe neigliborhoodofthe ends of the test section (see fig. 4). The one-bay type of specimensused previously does not have this characteristicand may have stress distributionsin the neigl

47、iborhoodof the ends of thecylinderswhich are rather irregular and which may cause premature 10CSL buckling. The curves of figure 5 are believed, therefore, tobe suitablefor the prediction of the bending strength of ring-stiffened circularcylinders in which th6 rings sre heavy enough so that general-

48、instability-type failures do not occur.CONCLtJDGREMARKS. Design curves for rhg-stiffened circular cylinders subjectedtobending are given: The curves are based on test results from25 cylin-ders with heavy rings in which failure was by local instability (betweenrings) rather them by general instabilit

49、ywhich tivolves simultaneousfailure of the cylinderwall and rings. The design curves generald.ypredict higher failing stressesthan curves now in use which are basedon tests of one-bay compression cylinders.Iangl.eyAeronautical lliboratory,National Advisory Committee for Aeronautics,ey Field, Va., April