1、iCq%O_DCqZ(JII/_ 7F_-NATIONAL ADVISORY COMMITTEETECHNICAL NOTE 2662A SUMMARY OF DL%GONAL TENSIONPART II - EXPERLMENTAL EVIDENCEBy Paul Kuhn, ames P. Peterson,and L. Ross LevinLangley Aeronautical LaboratoryLangley Field, Va.WashingtonMay 1952I Itel;,md ucad byNATIONAL TECHNICALINFORMATION SERVICEUS
2、Department o| CommerceSpf_n_held, VA. 22151Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-JgIN O_T I C ETHIS DOCUMENT HAS BEEN REPRODUCED FROM THEBEST COPY FURNISHED US BY THE SPONSORINGAGENCY. ALTHOUGH IT IS RECOGNIZED THAT CER-TAIN PORTIONS ARE IL
3、LEGIBLE, IT IS BEING RE-LEASED IN THE INTEREST OF MAKING AVAILABLEAS MUCH INFORMATION AS POSSIBLE.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-NACA TN 2662CONTENTSsUMMARY “ “ “ “ “ “ “ “ “1iINTRODUCTION .iPLANE-WEB SYSTEMS 2i. Stresses and Deflect
4、ions i.i. General discussion of NACA test procedures . 21.2. Basic data on NACA test beams 441.3. Web buckling 61.4. Upright stresses 1.5. Indefinite-width uprights 91.6. Beam deflections 92. Ultimate Strength i02.1. General discussion . i02.2. Strength of webs tested in pure shear . Ii2.3. Strength
5、 of beam webs 122.4. Upright failure by column buckling . 142.5. Upright failure by forced crippling . 142.6. Web-to-flange rivets 162.7. Upright-to-flange rivets 172.8. Upright-to-web rivets 18CURVED-WEB SYSTEMS . 192O3. Stresses and Deflections 3.1. Test specimens and procedures 203.2. Buckling of
6、 skin 213.3. Stresses in stringers and rings . 223.4. Angle of folds . 233.5. Angle of twist 24 e 3.6. Effects of repeated buckling 254. Ultimate Strength 284.1. Web strength . 284.2. Stringer failure 294.3. Ring failure . 324.4. Riveting 325. Combined Torsion and Compression . . . . 325.1. Test spe
7、cimens . 325.2. Stresses 335.3. Ultimate strength 33iProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-NACATN 2662REFERENCES. 34TABLES 36FIGURES 44iiProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-NATIO
8、NALADVISORYCOMMITTEEFORAERONAUTICSTECHNICALNOTE2662!A SUMMARY OF DIAGONAL TENSIONPART II - EXPERIMENTAL EVIDENCEBy Paul Kuhn, James P. Peterson,and L. Ross LevinSUMMARYMethods of analyzing web systems working in diagonal tension havebeen given in Part I of this paper. Part II presents the experiment
9、alevidence.INTRODUCTIONMethods of analyzing plane or curved shear webs in incomplete diago-nal tension have been presented in Part I of this paper (reference 1).These methods make liberal use of empirical relations, and a ratherlarge amount of space was devoted in Part I to general discussions ofthe
10、 test results in order to furnish the background knowledge that wasfelt to be desirable for anybody concerned with the application of themethods.Part II presents the test information in greater detail. It isintended primarily for those who are interested in improving the methods.It should also be us
11、eful in interpreting specific tests such as mightbe made in the course of demonstrating the strength of a specificairplane.All references to numbered formulas in the text refer to formulasgiven in Part I; a list of symbols is also given in Part I.PLANE-WEB SYSTEMSThe methods for analyzing plane diag
12、onal-tension webs presented inPart I may be considered to consist of a basic stress theory and of astrength theory which is based on the basic stress theory.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-NACATN 2662The experimental evidence concerni
13、ng the stress theory was obtainedmainly from NACAtests on beams, involving extensive strain measurements.These tests are presented in somedetaiL.The experimental evidence on the s_-rength theory is based on NACAtests, including those Just mentioned, _d on tests madeby aircraftmanufacturers. The main
14、 series of NACA_ests comprised about 50 beamsthe manufacturers furnished a total of about 140 tests. Someof thesetest results were given to the NACA with the stipulation that no testdetails be published. For this reason, aund also because a detailedpresentation of the data would be rather _oluminous
15、, the data from manu-facturers tests are presented only in sunnmary form. The cooperationextended by the manufacturers was very v_luable, because many of thestrength formulas are partly or wholly empirical, and the large numberof additional tests greatly increases the confidence that may be placedIn
16、 the formulas.Data from the following manufacturers were used:Boeing Aircraft Ca.Consolidated Aircraft Corp.Douglas Aircraft Co., Inc.The Glenn L. Martim Co.Vultee Aircraft Corp.i. Stresses and DeElectionsi.i. General discussion of NACA test procedures.- The beams testedby the NACA may be divided in
17、to three groups as far as test technique isconcerned: medium-size beams, which formed the largest group, small butheavily loaded beams, and very large beams.A typical test setup for a medium-sime beam is shown in figure i.Beams having depths of 25 and 40 inches were tested in the manner shownas cant
18、ilevers fastened to a heavy universal support. The load is appliedby means of a hydraulic Jack, with rollers interposed in order to givefreedom of extension to the beam flange. Stabilization against torsionalfailure of the beam and against lateral b_ckling of the compression flangeIs effected by hor
19、izontal guide arms (extending to the left in fig. l)which are pivoted at both ends and form a series of parallel-motion guides.The dial gages used to measure beam deflections are supported by asteel truss above the beam. The truss is welded to a vertical post whichIn turn is securely fastened to the
20、 top an_ bottom flanges of the beamat the root end of the test section, where a web doubler plate begins.This method of supporting the dial gages _s found necessary because theangles used to attach the beam to the support deformed under the pull ofProvided by IHSNot for ResaleNo reproduction or netw
21、orking permitted without license from IHS-,-,-tNACA TN 2662 3the tension flange. Although the attachment angles were made of theheaviest steel angles rolled (7/8 in. thick) and were reinforced bywelded gussets, they deformed sufficiently to almost double the deflec-tion at the tip of the beam in som
22、e cases.The beam is shown after failure, and after the strain-gage leadshad been removed the strain gages are not visible in this view. Thefailure is typical of upright failure by column buckling: Although theuprights have large permanent over-all deformations, no local deformationsof the cross sect
23、ions are evident.A typical setup for a small but heavy beam is shown in figure 2.(The beam is 12 in. deep and has a depth-thickness ratio of 120.) Thebeam is simply supported in an inverted position; the reaction supportsare visible above the two ends. Because the beam is heavily loaded, thecompress
24、ion flange is heavily stressed and requires closely spaced supportsto prevent lateral buckling. For beams of the proportions shown (_ _ 120),round steei rods were satisfactory as supports. Heavier beams (_Z 6_were found to twist with sufficient force (due to torsional instability)to set up as much a
25、s 15 percent friction by rubbing of the beam flangesagainst the guide bars. For these beams, the round bars were replacedby square bars, and rollers were placed on each bar to reduce the fric-tion to a negligible amount.A typical setup for a large beam (74 in. deep) is shown in figure 3.The electric
26、 resistance strain gages may be seen on the three middleuprights. Figure 4 shows a different view of the same setup; in thisview, the parallel-motion guide bars may be seen, as well as the structurenecessary to support them.Figure 5 shows column failure of the uprights on one of these largebeams) no
27、 distortion of the cross sections of the uprights is evident.By contrast, figure 6 shows forced-crippling failures of large but frailuprights. The attached legs of the Z-section stiffeners are badly deformed,while the free legs show only a barely visible buckle. It might be notedthat the Junction li
28、ne between the attached leg and the web of theZ-section is also kinked_ while the explanation of forced-crippling failuregiven in Part I stated that this line remains straight. However, the kinkin this line occurred only at the instant of final failure, while theexplanation in Part I referred to the
29、 deformation pattern that begins todevelop as soon as the buckling load of the web is exceeded.On the first beam tested, an extensive strain survey was made withTuckerman strain gages in order to provide a check against the electricresistance gages, which were being introduced at this time. The test
30、sProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-4 NACA TN 2662with Tuckerman gages required loading the beam repeatedly to a fairlyhigh percentage of its ultimate load. On all other beams, electricresistance gages were used exclusively, and no repea
31、ted loads wereapplied; the load was increased in increme_nts until failure occurred.1.2. Basic data on NACA test beams.- The main NACA beam tests weremade over a period of several years in five groups; for convenience ofreference_ the groups have been designated as series I to V.Series I consisted o
32、f beams 40 inches or 25 inches deep with doubleuprights. Series II consisted of 25-inch beams with single uprights.In series I and II, the material was 24S-T3 aluminum alloy (with oneexception as noted).Series IIl consisted of 25-inch beams made of 75S-T6 alloy. Singleas well as double uprights were
33、 used.Series IV consisted of very large beams (74 in. deep) made of24S-T3 alloy.Series V was a series of thick-web beams _ _1201 made of24S-T3 alloy.Each beam carries a code designation _uch as 1-25-4D, with thefollowing meaning:I test series I254approximate depth of beam in inchesnumber of beam wit
34、hin the seriesD double uprights (S for single uprigluts)The basic data on dimensions and materials are given in tables i and 3;calculated and test data are given in tabla._ 2 and 4. Figures 7 to lOgive information not covered by the tablesA comprehensive report on series I to EV was published as ref
35、er-ence 2, which gives also the original refez_nces. The results forseries V were published in reference 3.1.3. Web buckling.- When the loading r_tio T/Tcr is large, thediagonal-tension factor k is insensitive to small changes in T/Tcr ;even for a ratio as low as i0, a 10-percent change in T/Tcr pro
36、ducesProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-NACATN 2662 5only a 3.5 percent change in k. An accurate e:stimate of the bucklingstress Tcr is therefore not important whent_e beamfails at highloading ratios, and consequently no concerted effort
37、s were madeto meas-ure the buckling stresses for thin webs (with high h/t ratios).Observations in this range were mademostly by two simple methods:(i) Observing the reflections of windows _n the web(2) Checking the web for out-of-flatness with a straight edgeThe first method is a good one under favo
38、rable circumstances, but suchcircumstances often do not prevail in beamtest:s.Another method employed was to take strai_ readings on a 2-inchTuckerman strain gage placed at right angles a_u-oss an expected buckle.The gage, having a rigid body_ measures the gec_metric shortening due tobuckle curvatur
39、e in addition to the strain; thu_, deviation of the load-strain plot from a straight line indicates buckling. This method wasemployed very successfully on curved sheet and _roved satisfactory onflat thick sheet.All measurementsof buckling stress madeom the web have a defect:The first buckle noted ma
40、y be merely a local buckle. This defect couldbe overcome to someextent by using a large number of gages, but thiswas considered an undesirable complication for beam tests.The method considered to be the most desirable one (in general) wasto utilize the measuredupright stresses. As l_ng as the web is
41、 notbuckled, the uprights are unstressed (unless there are bending effectsdue to unsymmetrical construction). The appearaunceof compressive stressesin the uprights marks the beginning of diagonal-tension action and thusindicates that the web has buckled. From a plot of upright stress againstload, i_
42、he buckling load can generally be determined fairly accurately onweb systems for which it needs to be known accu_zately.The curves of empirical restraint coefficients for buckling calcu-lations (Part I, fig. 12(b) were drawn as weighted average curves forbuckling data obtained from beamtests and fro
43、_ miscellaneous other tests.Table 4 lists buckling stresses calculated with the aid of these curvesand buckling stresses determined experimentally by the last-mentionedmethod (appearance of upright stresses) for the beamsof series V. Thesebeamsfailed at loading ratios ranging approximmcely from 1.1
44、to 2.29they are thus in a range where a rather accurate estimate of the bucklingstress is desirable. The fourth column of the t,able shows that the ratioof experimental to calculated buckling stress ravages from 0.77 to 1.24,with an average value of 0.95. For tests on medium-thick webs, the ratiosPr
45、ovided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-6 NACA TN 2662fell in about the same scatter band. For thin webs, on which only simplevisual observations were made, the ratio ranged up to 1.5, but this wasundoubtedly due to inadequate sensitivity of th
46、e test methods underunfavorable conditions.1.4. Upright stresses.- The measurememts of upright stresses wereguided by the following considerations:(i) Built-up structures exhibit more or less irregular stress dis-tributions due to imperfect construction.(2) The stress in an upright varies along the
47、length of the upright.(3) Single uprights are subjected to eccentric loading and thus tobending in addition to compression.(4) Any upright of practical size is subjected to local deformationscaused by the shear buckles in the web; these deformations may becomevery severe at high loads.In vSew of the
48、 first consideration, stresses were measured on threeuprights except on beams with large upright spacing where only twouprights were usable for measurements because the others were adjacentto the stiffened end bays and thus worked under different conditions.In order to take care of the second consideration, a number of gagestations were distributed along the length of each upright. The numberranged from 9 stations for 25-inch beams to 13 for 74-inch beams.The third consideration introduces a difficulty. The calculatedstress sU for a single
