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National Advisory Committee for Aeronautics, Technical Notes - Stresses in and General Instability of Monocoque Cylinders with Cutouts - I- Experimental Investigation of Cylinders with a Symmetric Cutout Subjected to Pure Bending

Ten 24S—T alclad cylinders of 20—inch diameter, 45—;-
or 58—inch length, and 0.012—inch wall thickness, reinforced
with BdS—T aluminum alloy stringers and rings were tested in
pure bending. In the middle of the compression side of the
cylinders there was a cutout extending over 19 inches in the
longitudinal direction, and over an angle of 45°, 90°, or 135°
in the circumferential direction. The strain in the stringers
and in the sheet covering was measured with metalectric strain
gages.

The stress distribution in the cylinders deviate con—
siderably from the linear law valid for cylinders without a
cutout. The maximum strain measured was about four-thirds
of the value calculated from the Mc/I formula when I
Was taken as the moment of inertia of the cross section of
the portion of the cylinder where the cutout was situated.

A diagram is presented containing the strain factors defined
as the ratios of measured strain to strain calculated with
the Mc/I formula.

All the 10 cylinders tested failed in general instability.
Two symmetric and one antisymmetric pattern of buckling were
observed and the buckling load appeared to be independent
of the method of manufacture and the length of the cylinder.
The buckling load of the cylinders having cutouts extending
over 45°, 90°, and 135° was 66, 47, and 31 percent. respec—
tively, of the buckling lOad of the cylinder without a cut—
out.

In the past 15 years a great deal of work has been done
in order to clarify the stress distribution in and the failure
of reinforced monocoque structures. Most of this work has
been devoted to the problems of circular cylinders with cross—
sectional properties constant over their length. However.
actual monochue fuselages deviate considerably from this
idealized type of structure, one of the most important deviu
ations resulting from the necessity to cut out portions of
the shell in order to provide openings for doors, windows.
and military equipment.

Two major effects of the cutouts in monocoque cylinders
are to be noted. One is the redistribution of stress in the
neighborhood of the opening. In this report results of strain
measurements are presented which were obtained at the
Polytechnic Institute of Brooklyn with 10 circular cylinders
tested in pure bending. A later report will contain theo—
retical calculations and a comparison of the theory with ex—
periment. It may be mentioned here, however. that good
agreement has been obtained between the theoretical and ex—
perimental axial stresses.