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Different theories of fatigue crack initiation for time-independent fatigue are briefly reviewed. The micromechanics theory for time-independent fatigue proposed by Lin and Ito is shown to be applicable also for time-dependent fatigue. Three closely located parallel thin slices in a most favorably oriented crystal located at a free surface of a polycrystal are assumed to have small initial resolved shear stresses of opposite signs. From the slip rate versus resolved shear stress data of aluminum single crystals, the increases of local creep strains in the slices with cycles of stress loading were calculated by using the analogy between creep strain gradient and body force. This calculation was made for loading frequencies of 0.1 to 1000 cpm. It was found that the creep shear strain in the slices depends on the time of loading for frequencies greater than 10 cpm and becomes more dependent on cycles of loading at lower frequencies. The method is applicable also to loadings with given alternate strains instead of stresses.
crack initiation, creep, elevated temperature, extrusion, fatigue, intrusion, resolved shear stress, slipbands, slip lines
Professor, University of California, Los Angeles, Calif.
Manager, The Aerospace Corp., El Sequndo, Calif.