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The problem of fatigue in aircraft structures has been under investigation for many years. Much work has been accomplished in testing and evaluating the effects of different cyclic loads primarily upon material coupons, typical joints, and small component parts. Although this type of testing, along with service experience, has established the basis of present-day fatigue knowledge, it does not reproduce the complex load conditions that exist in a full scale aircraft structure. Only in testing of a complete aircraft structure for a given condition can the life relationship of the assembled parts and the influence of these parts on adjoining structure be determined. In recent years, limited repeated load testing of large structures has been accomplished by the use of two test methods: (1) single mass, cantilever, resonate beam type tests, and (2) distributed load application using tension pads and whiffle trees with hydraulic loading cylinders. The single mass resonate cantilever beam type test is simple in setup. A single mass placed on a wing is resonated to give the desired loading. The cyclic rate is fast, dependent only on the resonate frequency of the system. The single concentrated load however imposes a serious limitation to the testing in that the bending moment and shear produced are correct for a given condition at only one spanwise station. Torsion is generally neglected.
Vollmecke, A. R.
Test Engineer, Structures, Convair, Division of General Dynamics Corp., San Diego, Calif.