Effect of normal pressure on the critical shear stress of curved sheet

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Material Information

Title:
Effect of normal pressure on the critical shear stress of curved sheet
Series Title:
NACA WR
Alternate Title:
NACA wartime reports
Physical Description:
3 p., 9 leaves : ill. ; 28 cm.
Language:
English
Creator:
Rafel, Norman
Langley Aeronautical Laboratory
United States -- National Advisory Committee for Aeronautics
Publisher:
Langley Memorial Aeronautical Laboratory
Place of Publication:
Langley Field, VA
Publication Date:

Subjects

Subjects / Keywords:
Airplanes -- Wings -- Testing   ( lcsh )
Aerodynamics -- Research   ( lcsh )
Genre:
federal government publication   ( marcgt )
bibliography   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )

Notes

Bibliography:
Includes bibliographic reference (p. 3).
Statement of Responsibility:
by Norman Rafel.
General Note:
"Originally issued January 1943 as Restricted Bulletin."
General Note:
"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 previously held under a security status but are now unclassified. Some of these reports were not technically edited. All have been reproduced without change in order to expedite general distribution."

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 003806243
oclc - 124064911
System ID:
AA00009447:00001


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NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS





WARTIME REPORT
ORIGINALLY ISSUED
January 1943 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
120 MARSTON SC!ENJCE LIBRARY
P.O. BOX 117011
GAINESVILLE, FL 32611-701 USA








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 h16


RB Jan. 1943











'N




3 (


NATI ONAL ADVISORY COMMITTEE FOR AERONAUTICS


RESTRICTED BULLETIN


EFFECT OF NORMAL PRESSURE ON THE CRITICAL SHEAR

STRESS OF CURVED SHEET

By Norman Rafel


On an airplane in level flight, the upper surface of
the wing is subjected to an outward-acting pressure 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 been under-
taken. Reference 1 presented the results of compression
tests on unstiffened curved sheet subjected to an outward-
acting normal pressure. The present report gives the re-
sults of torsion tests on two curved-sheet specimens sub-
jected to an outward-acting normal pressure.

The two specimens, 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-capacity
hydraulic jacks were used to apply torque to the specimens,
one in conjunction with a lever to give downward 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 tin 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 buckling loads for the specimen. The
specimen was then repaired by reducing the length of the
end bays to 21"- inches and making new end ribs of heavier-
gage steel. (See fig. 2.) The critical buckling load was
thereafter taken as the load at which buckling occurred
in the bay 30 inches long.











The altered specimen was tested with normal pressures
up to 7 pounds per square inch. At the buckling load for
that pressure, however, very deep buckles and large rota-
tional deflection occurred. After the load was released,
small permanent buckles remained in the sheet. Some addi-
tional tests were made on the specimen in this damaged
state, in order to determine the effect of initial yield-
ing on the buckling load.

In the plotting of test results, the shear stress in
the skin at any torque was assumed to be given by the for-
mula

T T
2At
where

7 shear stress

T applied torque

A area enclosed by median line of skin

t thickness of skin

The results of the tests are presented in figures 4
to 9, from which the following conclusions are drawn:

1. Loading of the specimen until buckling occurred
at normal pressures as high as 6 pounds per square inch
did not appreciably injure the specimen for additional
tests at different pressures, as evidenced by the experi-
mental Doints in figure 4, where the numbers 1, 2, S, and
so forth indicate the order in which the tests were made.
The presence of permanent buckles, however, did lower the
critical shear stress for the specimen, as shown by the
two curves for tne specimen with o0-inch rib spacing.
(See fig. 4.)

2. An outward-acting normal pressure avoreciably
raises the critical shear stress for unstiffened curved
sheet. (See fig. 4.)

3. The absolute increase in critical shear stress
caused by normal pressure is slightly greater 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 was always below the curve of
shear stress against normal pressure at which the buckles
appeared. (See fig. 7.)

5. The relationship between shear stress and normal
pressure at which buckles disappeared is independent of
whether the buckles were made to disappear by increase of
normal pressure or by decrease of shear stress. (See
fig. 7.)

6. The torsional stiffness of the specimens before
buckling was not affected significantly by an outward-
acting normal pressure. (See figs. 8 and 9.) The low
torsional stiffness indicated in figure 8 for normal pres-
sures of 0 and 1 pound per square inch at a shear stress
of 7098 pounds per square inch is explained by the fact
that in these two cases the buckling stress had been
exceeded.


Langley Memorial Aeronautical Laboratory,
National Advisory Committee for Aeronautics.
Langley Field, Va.



REFERENCE


1. Rafel, Norman: Effect of Normal Pressure on the
Critical Compressive Stress of Curved Sheet. NACA
RB, Jtn. 1943.




















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