RB Jan . \9hl> 
 
 NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS 
 
 WARTIME REPORT 
 
 ORIGINALLY .ISSUED 
 
 January 194-3 as 
 Restricted Bulletin 
 
 EFFECT OF NORMAL PRESSURE ON THE CRITICAL SHEAR 
 
 STRESS OF CURVED SHEET 
 
 By Norman Rafel 
 
 Langley Memorial Aeronautical Laboratory 
 Langley Field, Va. 
 
 UNIVERSITY OF FLORIDA 
 
 DOCUMENTS DEPARTMENT 
 
 1 20 MARSTON SCIENCE UBRARY 
 
 P.O. BOX 11 7011 
 
 GAINESVILLE, FL 3261 1 -701 1 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 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. 
 
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,io<^^iV( "'"*'*' 
 
 NATIONAL ADTISOEY COMMITTEE FOE AERONAUTICS 
 
 EESTRICTaD 'BULLETIN 
 
 EFFECT OF NOHI/iAL PRESSUES ON THE CRITICAL SHEAS 
 STRESS OF CURVED SHEET 
 By Norman Rafel 
 
 On an airplane in level flight, the i^pper surface of 
 the wing is subjected to an outward-acting pressrire due 
 to the difference in internal and external pressures. In 
 order to determine the effect of this pressure on the 
 critical stresses, an experimental study has oeen under- 
 taken. Reference 1 presented the results of coi;.pre ssi on 
 tests on unstiffened curved sheet subjocted to an outw?ird- 
 acting normal pressure. The present reoort gives the re- 
 sults of torsion tests on two c'^-rved- sheet speciruens sub- 
 jected to an outward-acting normal pressure. 
 
 The two speciraensi shown in figures 1 and 2, are 
 designated by their respective rib spacings of 10 inches 
 and ^30 inches. The specimens were mounted on a rigid 
 vertical abutment in the NACA structures research labor- 
 atory as shown in figure 3. Two 100 ,000-pound-capaci ty 
 hydraiilic jacks were u.sed to apply torque to the specimens, 
 one in conjiinction with a lever to give do'wnward load. 
 Dial-gage readings were taken at the outer edge of each 
 rib station. 
 
 Normal pressure was applied by admitting compressed 
 air through a regulating valve into the specimen. A 
 mercury manometer calibrated in pounds per square inch was 
 used to measure the pressure inside the specimen. 
 
 The specimen with .30-inch rib spacing originally had 
 three bays of equal length. After several runs at differ- 
 ent pressures, however, the buckling in the tit) bay had 
 become severe enough to cause yielding of the flange on 
 the end rib, leaving a permanent buckle in the sheet and 
 resulting in lower buckliiig loads for the specimen. The 
 specimen was then repaired by reducing the length of the 
 end bays to 21-g inches and making new end ribs of heavier- 
 gage steel. (See fig. 2.) The critical Duckling load was 
 thereafter taken as the load at which buckling occurred 
 in the bay 30 inches long. 
 
The altered speciinen was tested with normal pressures 
 ■up to 7 pQ-unds per sq":iare inch. At the 'b-acklirig load for 
 that pressure, however, very deep huckles and large rota- 
 tional deflection occurred. After the load v/as released, 
 suiall permanent buddies remained in the sheet. Some addi- 
 tional tests were made on the specimen in this damp^ed 
 state, in order to determine the effect of initial yield- 
 ing on the buckling load. 
 
 In the plottine:: of test results, the shear stress in 
 the skin at any torque t^s s assumed to 'ce ^iven hy the for- 
 mula 
 
 2At 
 
 where 
 
 T shear stress 
 
 T a"oplied torque 
 
 A area enclosed hy median line of skin 
 
 t thickness of skin 
 
 The results of the tests are presented in fii;ures 4 
 to 9, from which the follo'ving conclusions are drawn: 
 
 1. Loading of the specimen until "buckling occurred 
 at normal TDressures as high as 6 pounds per square inch 
 did not appreciacly injure the specimen for additional 
 tests at different pressures, as evidenced hy the experi- 
 mental points in figure 4, where the numbers 1, 2, 3, and 
 so forth indicate the order in v;hich the tests were made. 
 The presence of permanent buckles, however, did lower the 
 critical shear stress for the s'oecimen, as shown by the 
 two curves for tne specimen with oO-inch rib spacing. 
 
 ( See 
 
 ig' 
 
 4.) 
 
 2. An outward-acting normal pressure a"C"or eciably 
 raises the critical shear stress for \instiffened curved 
 sheet . ( See f ig . 4. ) 
 
 3. The absolute increase in critical shear stress 
 caused by normal pressure is slightly e-^'^ater for the 30- 
 inch rib spacing than for the 10-inch rib spacing. (See 
 fig. 5.) On a percentage basis, however, the increase in 
 critical shear stress caused by normal pressure is consid- 
 erably greater for the 30- inch rib spacing than for the 
 10-inch rib spacing. (See fig. 6.) 
 
4. The curve of shear stress against normal pressure 
 at which buckles disappeared v;as s.lvray s below the curve of 
 shear stress against normal pressure at r/hich the "ouckles 
 appeared. (See fig. 7.) 
 
 5. The relationship "between shear stress and normal 
 pressure at which buckles disappeared is independent of 
 whether the bucklen were made to disappear by increase of 
 normal pressure or by decrease of shear stress-, (Sea 
 fig. ?.) 
 
 6. The torsional stiffness of the specimens before 
 btickling was not affected significantly by an outward- 
 acting normal pressure. (See figs. 8 and S.) The low 
 torsional stiffness indicated in figure 6 for normal pres- 
 sures of and 1 pound per square inch at a shear stress 
 of 7098 pounds per square inch is explained by the fact 
 triat in these two cases the buckling stress had been 
 exceeded. 
 
 Langley Memorial Aeronautical Laboratory, 
 
 National Advisory Committee for Aeronautics, 
 Langley Field, Va. 
 
 EEF3RE1ICB 
 
 1. Rafel, llorman: Effect of Normal Pressure on the 
 
 Critical Compressive Stress of Curved Sheet. NACA 
 as, Jn.n. 1947, 
 
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 constant, ehoar Bti^se increaenn^ 
 
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 Outv/ari-acting non.ial prassure, Ih/sq in. 
 
 JifOire 8.- Effect of norna.l press-are on rotation of spocimon v/itn 
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 Pieare 9.- Effect of norrial pressure on rotation of sp^jcricn with 
 30-inch rih spacing. 
 
, UNIVERSITY OF FLORIDA 
 
 III ill 11 111" " 
 
 3 1262 08106 510 3 
 
 GA\NESV\LLe,FL«3^ 
 
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