The behaviour of a single crystal of zinc subjected to alternating torsional stresses

The present experiment has been carried out in the course of a general investigation into the causes and characteristics of the failure of metals under repeated cycles of stress which has been in progress for some years past at the National Physical Laboratory. As this research proceeded, one definite and promising line of investigation suggested itself in the study of the behaviour of single metallic crystals under repeated stress systems, and, as a commencement, attention is being directed to typical metals crystallising in the cubic (face-centred, also body-centred), close packed hexagonal, and rhombohedral space lattice systems. The investigation, when completed, should not only give certain fundamental data concerning fatigue phenomena, but also, when studied in conjunction with the classical researches of Mark, Polanyi and Schmid, Taylor, Elam, and Farren—who have studied in detail, the characteristics of deformation of single crystals under static stressing—afford some contribution towards the discovery of the general laws of deformation of metals subjected to any stress system. Previous reports have described experiments on single crystals of aluminium (face-centred cubic) and iron (body-centred cubic). The present paper describes an experiment on a single crystal of zinc, a metal crystallising in the hexagonal (close-packed) lattice. The deformation of zinc crystals under statical direct stress has received much attention from Mark, Polanyi and Schmid. They showed clearly that the principal slip plane is the basal (0001) plane, the direction of slip being that of the most highly stressed (shear stress) primitive direction (normal to the 1̄21̄0, 1̄1̄20, or 21̄1̄0 plane) contained by the basal plane. In the last stages of deformation, however, they found that deformation was consistent with slip occurring on planes apparently perpendicular to the basal planes, and they suggested that prismatic planes were also slip planes; they were unable, however, to establish this identity.