Abstract
Zinc single crystals with [0001], [10\bar10] and [11\bar20] orientations and zinc polycrystals having a grain size of 130 to 310/cm2 were subjected to the cantilever rotating-bending fatigue test (150 cycles/min) at room temperature. The S-N curve representing the relation between the maximum alternating bending stress, S, and the logarithm of the number, N, of stress cycles required for the failure is linear for single crystals as well as for polycrystals, and its slope is greatest for polycrystals and becomes less in the order of [11\bar20]≅[10\bar10] and [0001] single crystals. The S-N curve for [0001] crystals, which eventually fracture by cleavage along the basal plane when fatigued, is nearly parallel to the abscissa. This indicates that brittle crystals show little fatigue. The S-N curves are nearly the same for [11\bar20] and [10\bar10] crystals, in which the plastic deformation occurs before failure by multiple slips of the (0001) [11\bar20] and {11\bar22} ⟨\bar1\bar123⟩ systems and by {10\bar12} twinning. The endurance limit, coveniently defined as fracture stress at 107 cycles, is 2.0 kg/mm2 for polycrystals, 1.3 kg/mm2 for [11\bar20] and [10\bar10] crystals and 1.1 kg/mm2 for [0001] crystals.
The fatigue data thus obtained is compared with static strength data determined from tensile tests made on similar orientated single crystals. It has been found that, in [0001] crystals, the resolved shear stress operated along the (0001) [11\bar20] direction at the endurance limit coincides with the critical resolved shear stress of the (0001) [11\bar20] slip system in the tensile deformation, and that similar relations also hold for the (0001) ⟨11\bar20⟩ slip, {11\bar22} ⟨\bar1\bar123⟩ slip, and {10\bar12} twinning in [10\bar10] and [11\bar20] crystals. These findings indicate that the endurance limit, below which fatigue failure does not occur, in zinc single crystals is determined by the critical resolved shear stress of the active slip or twinning system.
