Why is there a yield point in stress-strain curve of ductile materials like mild steel?
The yield strength or yield point of a material is defined in engineering and materials science as the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed some fraction of the deformation will be permanent and non-reversible. Material like mild steel in its solid state has certain structure which is intact due to forces, each individual atom is experiencing in three dimentional space. The material has some imperfections along the crystal boundaries. When we apply stress, an external force to elongate the material, it creates proportional strain linearly withy in cerain limit. In this limit , the forces binding the matter works like elastic spring which restores it self when force is removed, shape is restored. When force is exceeded beyond the elastic limit, the cracks along the imperfections occure, and the material
