The yield phenomenon in polycrystalline mild steel

Abstract

While Cottbell's theory of the yield point in single crystals is now widely accepted, it has to be combined with some form of grain boundary theory to explain the strength of polycrystalline iron. This paper tries to summarise and interrelate the most important macroscopic features of the yield in polycrystalline iron, and to single out the fundamental properties which must be explained. It appears that the drop in stress after the first yield is responsible for the localisation of plastic flow, and that this localisation may in turn be responsible for the macroscopic shear associated With the Lüders deformation. The existence of this shear influences the pattern of the bands in the specimen and, therefore, the regularity or otherwise of the progress of the yield. It is suggested that the two essential quantities to be derived from any theory are the magnitude of the Lüders strain and the sharpness of its localisation.