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Fatigue Evaluation of Wing and Associated Structure on Small Airplanes

It is recognized that, in such a complex problem as fatigue evalua—
tion, new design features and methods of fabrication, new approaches
to fatigue evaluation, and new :onfigurations may require variations
and deviations from the procedures described herein. Engineering
judgment, guided by the extensive literature on the subject’should,
therefore, be exercised for each particular application. The pri—
mary structure of the wing and carry-through_is usually evaluated
both for the original design and after any subsequent design.
changes which affect the loading spectra, internal stresses or
stress concentrations. It should be noted that changes which are
minor from a static strength standpoint‘can have a major effect on
fatigue characteristics.

A basic understanding of fatigue phenomena is necessary for an
adequate fatigue evaluation. In general, fatigue is a progressive
failure of a part under repeated, cyclic, or fluctuating loads.
Any one application of any of these cyclic loads'will not result
in structural failure. The criterion for fatigue (Reference a) is
the simultaneous action of cyclic stress, tensile stress and .
plastic strain.

There are many investigations that collectively help give a better
understanding of fatigue. The pioneering investigations in fatigue
started over a hundred years ago. Yet, at this time, a fatigue?
prediction approach has not yet been developed for a complex
fluctuating stress histories which does not require experimental
support. The various cumulative damage theories are discussed and
presented in their elementary forms in Reference (a) which also
explains the technical reasoning and logic behind their formulation.

The most widely used cumulative damage fatigue theory, in spite of
its limitations, is the one developed by Palmgren—Miner hypothesis
or Miner's Linear Cumulative Damage Theory which will be used in
this report. The basic philosophy of Miner's Theory states
that the fatigue damage introduced by a given stress level
is proportional to the number of applied cycles at that stress level
divided by the total number of cycles to failure at the same stress
level. This ratio is called a cycle ratio and is used to measure
damage.