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Fatigue hardening of copper single crystals at low stress amplitudes

1967; University of Toronto Press; Volume: 15; Issue: 8 Linguagem: Inglês

10.1016/0001-6160(67)90003-x

1878-0768

P. Kettunen,

Fatigue and fracture mechanics

Abstract Fatigue of copper crystals was studied in the neighborhood of the endurance limit and somewhat above it. The crystal in each case was first pulled slowly to the chosen shear stress and then immediately fatigued with this shear stress as the stress amplitude. If the chosen stress amplitude was taken slightly below the endurance limit, but still such as to cause fatigue striations, it corresponded to a shear stress very high in stage II of unidirectional work-hardening. If the chosen stress amplitude was taken above the endurance limit, the corresponding stress during the first tension was in stage III of Unidirectional work-hardening. This suggests that constant stress amplitudes able to cause fatigue fracture within a finite number of stress cycles correlate best with stage III of unidirectional deformation, as was proposed earlier by Feltham, by Ham and Broom, and others. Fatigue deformation was found to be composed of two stages, viz. the initial hardening and the state of saturation. During the initial hardening the crystal hardens very rapidly and effectively. Below the endurance limit, this leads to a saturation state in which there is no more macroscopic deformation; at the upper limit of the stress range of low amplitude fatigue, the plastic strain amplitude during saturation is of the order of 0.1 % true strain. Metallographie results suggest that the deformation during the initial hardening is rather similar to that during unidirectional deformation. Peculiarities of fatigue, such as striations, extrusions, etc., appear first during the state of saturation. Thus, the phenomena responsible for fatigue fracture seem to occur mainly during the state of saturation; but whether the processes during saturation lead to fracture or not, is controlled by the initial hardening. During the state of saturation the dislocation structure at stress amplitudes close to the endurance limit was composed of mats or carpets along the primary slip plane containing numbers of loops and dislocation dipoles. At higher stress amplitudes, a clear cell-structure was typical. This cell-structure is thought to be due to the approaching change in fatigue mechanism, and not a necessity for the occurrence of the low amplitude fatigue itself.