The bondage between the molecules is broken.
In this region, the material can regain its original shape after the removal of the loading or rather the stress. At this juncture, the material obeys Hooke’s law and the stress and strain relate proportionally, and the constant of proportionality is the modulus of elasticity. The structure of the particles is not altered which explains the reason the material regains its original shape after the removal of the stress.
It is the region just after the elastic limit. It is characterized by the graph flattening indicating destroyed proportionality between the stress and strain. Plastic deformations creep in, and the material loses the ability to regain fully its shape and size after the stress has been released. It takes place due to the rearrangement of the molecular or atomic structure that allows for the new shape and size.
Found just after the yielding region. It is characterized by a slight rise of the curve to the ultimate stress point. The behavior is due to the strength of the deformed atoms or molecules having a slight ability to withstand more stress up to the point beyond which complete deformation takes place. The rise of the curve in this region is what is termed as strain hardening.
It is the region beyond the ultimate stress point. In this region, a further increase of the stress leads to a reduction of the cross section of the material only in some parts and not the entire material. The constrictions formed are what is referred to as necking. It is the continuous reduction of the cross-section area that leads to the breakage and thus separation of the material.
Metallic materials are ductile thus has the ability to deform plastically up to some extent. On the graph, it is represented by the region beyond the proportional limit where the elastic limit has been exceeded. .A force beyond the elastic limit affects the mobility of the atoms and dislocations thus the inability to regain the original shape after the force has been removed.