Why does low carbon steel have an upper and lower yield point?
The term yield stress refers to the stress for continuous yielding of a material. But in low carbon steel there’s anomaly so that there are two yield points. The first yield point (upper yield point) is higher than the second and the yield drops dramatically after the upper yield point. The transition is somewhat analogous to the transition from static friction to sliding friction. At the start of this process critical number of dislocations (generated by the initial elastic elongation) are reached and plastic yielding begins, dropping the stress necessary for elongation. The deformation occurring throughout the yield-point elongating is heterogeneous in a way that a discrete localized bands of plastic deformation, often readily visible, appear at a stress concentration such as fillet. In typical cases, several bands form at several points of stress concentration. These bands are generally at approximately 45° to the tensile axis. They are usually called Luder bands, Hartmann lines or
