Stress Analysis of Monocoque Fuselage Bulkheads by the Photoelastic Method Page: 4 of 56
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NACA Technical.Note No. 870 3
- CONSTRUCTION AND MOUNTING OF MODELS
In order to obtain god results from phot2oelastic tests,
it is' important that the models being tested be properly.
designed and constructed. The faces of the model.perpen-
dicular to the path of the light in the polariscope should
be true plane surfaces, free from scratches, and highly
polished. When tested, the models should, in the unloaded
condition, be free from initial stress or its optical
equivalent. Whenever possible the model should be geomot-
rically similar to the actual structure. In many models,
however, geometric similarity is not possible because of
the requirement that the faces of :the model perpendicular
to the light path be parallel.
In the present investigation of bulkheads or stiffon-
er rings in monocoque structures, the largest stresses in
these rings are those due to bending. If the model is so
designed as to represent the prQper distribution of bend-
ing moment in the bulkheads, the results obtained from the
model tests will be satisfactory. Many full-scale bulk-
heads and stiffener rings are constructed of built-up sec-
tions, I sections, T sections, or other extruded sections
of various shapes. In the design of models of these full-
scale structures it is usually simplest to have the mqdel
of constant thickness and to vary the depth of the cross
section at each point in such a way.-that.-the ratio of the
bending stiffness of any section of,the model to bending
stiffness ot the corresponding full-ecale section is the
same for all cross sections. The shape.of the neutral
axis on the model should then be geometrically similar to
the full-scale original. 'The bending stiffness will be
designated by EI, which is the' product 6f the modulus elas-
ticity and the moment- of inertia of the section in the
plane -of bending, if the model is designed in this fash-
ion, the distribution of bending moment in the model will
be very close to the distribution of bending moment in the
If extremely heavy and stiff bulkheads are being test-
ed in which the shear stresses are large as compared with
the stresses due to bending, then it may be necessary to
build a model of0 varying thickness. .In'most practical
cases, however, such construction may be avoided..
The material for the model, Bakelite BT~-61-893, is
available in sheets of various thickhosses from 1/4 inch
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Ruffner, Benjamin F. Stress Analysis of Monocoque Fuselage Bulkheads by the Photoelastic Method, report, December 1942; (digital.library.unt.edu/ark:/67531/metadc54752/m1/4/: accessed January 23, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.