Fatigue Testing of Wing Beam by the Resonance Method Page: 3 of 47
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N.AA.A. Technical Note No. 680
tion of airplanes in flight and by the National Advisory
Committee for Aeronautics in its study of dynamical loads
under flight conditions in a flying boat. The work de-
scribed in this paper is confined to information of the
second type. It may be regarded as an extension to struc-
tural parts of airplanes of the resonance method of fa-
tigue testing developed for airship girders by the Goodyear-
Zeppelin Corporation (reference 1). The experience of the
Goodyear-Zeppelin Corporation was drawn upon in planning
the tests and in procuring the driving units.
The present report describes the application of the
resonance method to wing-beam specimens having various
types of discontinuities such as attachments, access holes,
rivets, bolts, and sharp angles, which may affect the fae-
tigue strength of the wing beam in the finished airplane.
The presence of such discontinuities in the test specimen
is considered particularly, important- since it is well
known that they introduce stress concentrations which may
lower the fatigue strength to a fraction of the value found
in the absence of such concentrations. The effect of the
discontinuities can be studied most conveniently by apply-
ing an alternating axial load to a specimen approximately
uniform in section, so that, except in close proximity to
the discontinuities, the stress is approximately uniform
throughout the specimen.
It is realized that the vibratory stresses in wing.
beams in service will differ from those for this type of
fatigue test in having a steady stress that is generally
different from zero and in being due mainly to flexural
rather than axial loads.
The effect on endurance of steady stresses superposed
on alternating stresses has been the subject of consider-
able study in the case of_-structural materials, and to a
less extent in the case of notched, riveted, and welded
structures (reference 2), but no well-controlled experi-
ments are known which show the effect of steady loads on
the endurance of fabricated aluminum-alloy structures with
local stress concentrations. It ishoped that an estimate
of- ...this effect may be obtained later in this investigation,
The fact that axial alternating stresses rather than
flexural stresses are set up in the wing beam does not
seriously detract from the value of results from axial fa-
tigue tests since the direction of stress in both cases is
the same. Thus the fatigue strength of the flanges of the2
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Bleakney, William M. Fatigue Testing of Wing Beam by the Resonance Method, report, August 1938; (https://digital.library.unt.edu/ark:/67531/metadc54442/m1/3/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.