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National Advisory Committee for Aeronautics, Technical Notes - Torsional Strength of Stiffened D-Tubes

The present report covers a series of torsional tests on stiffened
D-tubes of alclad 2hS—T3 aluminum alloy having a cross section similar
to the NACA 0012 airfoil section and a closing web at 30 percent of the
chord. The stiffeners consisted of ribs and stringers. An average-
strength chart has been developed for this type of structure that takes
into account the skin thickness, rib spacing, and stringer spacing. This
chart may also be used for unstiffened D—tubes.

Measurements of unit twist and unit strain were made at a number of
points on most of the Specimens. The unit—twist measurements indicated
substantial agreement between this type of structure and Bredt's theory
below the buckling point. The unit—strain measurements did not provide
data that could be verified by theoretical analysis except for the
strains measured in the webs.

The primary object of this investigation has been the develOpment
of a chart that will permit the estimation of the ultimate torsional
strength of stiffened D-sections. Nearly all airplanes use such a
structure as a primary load—carrying member and an average-strength
chart is considered to be quite desirable. With the advent of high-
speed aircraft the use of symmetrical airfoils should become quite
common and should enhance the value of this report. Other research
projects on thin-shell sections have been carried out previously, as
reported in references 1 and 2; however, only reference 1 deals with the
pure torsion problem and the specimens tested did not have a closing web
as is the case for the present investigation. Similar tests of a multi—
flange box beam are reported in references 3 and h.

Most of the previous work on structures of an elliptical or circular
shape is based on an early investigation made by Donnell (reference 5)
and many of the same factors reported therein were found to apply to this
study.

Although reports on investigations of stiffened D- sections in
torsion were not available, a similar study on stiffened circular
cylinders was found in reference 6.

This investigation was conducted at the Structural Laboratory of
the Department of Aeronautical Engineering, University of Notre Dame,
under the sponsorship and with the financial assistance of the National
Advisory Committee for Aeronautics.