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The development of frequency-of-occurrence distributions or spectra of loadings for an aircraft structure requires the use of all available information on the aircraft's service and on the loadings peculiar to this service. Interest in these loadings is not new, but there is a growing appreciation of the need for detailed information on the load and stress histories that they produce. The need for such information has led to the collection of quantities of service loading data covering a wide range of operational conditions. However, use of the data has generally been restricted to assessments of the adequacy of design load levels and to analytical evaluations of the effects of service loadings on the life of the structure. The direct use of the data in test evaluations of structures is a fairly recent and rather controversial development. The need for this development will be brought out in a brief review of fatigue testing. This review will be followed by a brief discussion of the operational conditions to be considered in the construction of loading spectra. Selection of the most appropriate operational conditions is the first and a very important step in any fatigue evaluation. Inadequate selections can have a major effect on the interpretation of fatigue test results. The generally available data on particular classes of service loadings will then be described in some detail. This description will attempt, by supplying background data, to throw some light on the bases for generalized representations of service loading histories and so provide an indication of their utility. Finally, the use of defined operational conditions and of standardized loading spectra in the construction of test spectra to represent particular aircraft utilizations will be described and illustrated.
McCulloch, A. J.
Lockheed Aircraft Corp., Burbank, Calif.