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National Advisory Committee for Aeronautics, Technical Notes - Stresses at Cut Outs in Shear Resistant Webs as Determined by the Photoelastic Method

Stress concentration factors for shear stresses around
cut—outs in the webs of beams having shear resistant webs
were determined by the photoelastic method. Webs with cir-
cular, square, and rectangular cut—outs were tested. The
effect of reinforcements around square and circular cut-outs
was investigated. Results indicate that square and rectan~
gular cut-outs having two sides parallel to the neutral axis
of the beam should be avoided whenever possible. Circular
cut~outs and square out—outs, with sides at 45° to the neu-
tral axis, produced lower stress concentration factors than
the other two types.

Cut-outs in shear webs occur frequently in aircraft
structures. These are often desirable as lightening holes 3
in web areas not highly stressed. In other regions cut-outs “
are necessary for installatidfi of equipment. The purpose of
this investigation was to determine the most efficient shape
of cut-out and to determine the maximum stress concentration
factors induced by various cut—outs. The photoelastic
method was used for the investigation because the region of
maximum stress and the amount of maximum stress is readily
determinable by this method. Values for stress concentra-
tion factors determined by_this method are, however, appli—
cable only in the elastic range.

This investigation, conducted at Oregon State College,
was sponsored by and conducted with the financial assistance
of the National Advisory Committee for Aeronautics.

Beam models were constructed with webs of Bakelite
BT 61—893 and with flanges of brass, The construction of
the models is described in subsequent paragraphs. The meth-
ods used for forming and finishing the Bakelite are described
in detail in reference 1.

In order to obtain physical properties of the materials,
needed in the analysis of beam results, models 1, 2. and 5
were used. The flanges of all models were identical and
were cut from the same brass sheet used for the tension test
specimen, model 1. For the Bakelite webs two different
sheets of Bakelite stock were used. A calibration specimen
was cut from each sheet.