On the nature of the yield point phenomenon

Materials in which the yield point phenomenon occurs exhibit a characteristic drop in the stress–strain curve at the end of the elastic (or mainly elastic) region. In most cases the subsequent plastic deformation takes place locally in deformation bands, often termed Lüders bands. Although first described more than 150 years ago, the yield point phenomenon is not yet fully understood. In this paper, a new and simple macromechanical model is presented to explain the yield point phenomenon. It is based on three major points: (i) a realistic or true upper yield point, (ii) typical strain-hardening behaviour common to many materials and (iii) the triaxiality of the stress state that necessarily has to develop at the Lüders front and that determines the stress level at the lower yield strength. The model is evaluated by experimental, analytical and numerical evidence. Combining all evidence, the model basically describes the yield point phenomenon correctly. It may also be applied to related strain-softening phenomena and it explains the comparatively low ductility of some nanocrystalline materials.