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National Advisory Committee for Aeronautics, Technical Notes - Fatigue Investigation of Full Scale Transport Airplane Wings - Summary of Constant Amplitude Tests Through 1953

Results are presented of a fatigue investigation conducted on the
wings of C-h6 HCommando" airplanes. Constant-amplitude tests were con-
ducted by the resonant—frequency method at four different alternating
load levels about a l g or level—flight mean load.

All fatigue failures were classified according to the type of struc-
tural stress raisers in which they originated. Effective stress-
concentration factors were determined for all failures and were found to
vary with the load level. The scatter in fatigue life was analyzed sta-
tistically and found to be comparable to the scatter obtained in tests
of small specimens of the same type of material as that from which the
wing was constructed.

Fatigue-crack propagation was investigated and it was found that
all cracks grew slowly until a certain critical percentage of the struc-
ture had failed, after which the cracks grew rapidly. This critical
percentage was found to vary inversely with the load level. The load-
lifetime relationship of the test structure was established and found to
compare favorably with that of several other full-scale structures which
have been subjected to fatigue tests.

Two appendixes are included which present information on the use
of bonded wires to detect small fatigue cracks and the use of fiber glass
as a method of structural repair.

Few fatigue tests have been conducted on actual airplane structures,
and most of the tests that have been accomplished were directed at the
solution of some particular problem. In View of the general lack of
information regarding the fatigue characteristics of complete airplane
structures, twenty—one C—fi6 "Commando" airplanes were secured for the pur—
pose of carrying out a fatigue investigation of full-scale airplane wings.