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Designing against fatigue failure involves so many uncertainties that it is usually necessary to test full-scale structures prior to service to prove their reliability. Such tests should be simple, short, and inexpensive. At the same time, it is necessary to insure that the fatigue response of the laboratory test reflects the actual performance that can be expected of the structure in its service environment.
The nominal stresses anticipated in a structure in service are usually known or can be estimated. Mechanics analyses have been developed which can be used with smooth specimens or smooth-specimen data to determine local stresses and strains in terms of known nominal stresses even when the local behavior is inelastic. Two methods of solving for the critical stresses and strains are reviewed and compared using notched- and smooth-specimen fatigue data for representative aircraft metals. The approach based upon Neuber's Rule is the most satisfactory and can easily be used to provide control conditions which cause smooth specimens to imitate the stress-strain response of the metal at a notch root.
It is now possible to understand and predict many aspects of structural fatigue from relatively simple tests of small unnotched specimens. The emphasis on the local stress-strain behavior is sufficiently different from more traditional ways of approaching structural fatigue that more research is needed. Advantages and disadvantages of this new viewpoint are discussed and possible future developments are indicated.
metal fatigue, engineering structure, notched specimen, smooth specimen, service, environment, reliability, metal behavior, mechanics analysis, nominal, local, stress, strain, design, analysis, simulation, prediction
Professor, University of Illinois, Urbana, Ill.
Scientific Research Staff, Ford Motor Company, Dearborn, Mich.
Professor and Acting Chairman, University of Waterloo, Waterloo, Ont.