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(in strength of materials), the capability of materials and structures to resist the action of repeated (cyclic) loading. Damage or failure from the effects of cyclic loading is called fatigue. A distinction is made between low-cycle fatigue (the development of plastic deformations at high load levels) and intrinsic fatigue (the gradual accumulation of latent, irreversible changes in the structure of materials, the subsequent formation of microscopic cracks, and their coalescence into a so-called main macroscopic crack, which results in failure). The relationship between the load level (stresses) σ and the number of cycles N required for failure is represented by a fatigue curve. The fatigue limit σr is the stress corresponding to failure at a specified large number of cycles or at the horizontal asymptote of the fatigue curve. Endurance depends on such factors as the properties of the material, the type of cycle, the type of stressed state, the presence of stress concentrators, the condition of the surface, the properties of the surrounding medium, and the dimensions of the part or structure.
The fatigue limit may prove to be considerably lower than the ultimate strength or the yield limit of a material. The great sensitivity of the fatigue limit to various factors requires special attention in choosing permissible stresses and safety factors in cases of cyclic loading.
REFERENCESSerensen, S. V., V. P. Kogaev, and R. M. Shneiderovich. Nesushchaia sposobnost’ i raschety detalei mashin na prochnost’,2nd ed. Moscow, 1963.
Bolotin, V. V. Statisticheskie metody v stroitel’noi mekhanike, 2nd ed. Moscow, 1965.
Prochnost’, Ustoichivost’, Kolebaniia: Spravochnik, vol. I. Moscow, 1968. Chapter 7.
V. V. BOLOTIN