Plasticity refers to the ability of metal materials to deform (plastic deformation) without being destroyed under the action of external loads. When the metal material is stretched, the length and cross-sectional area are changed, so the plasticity of the metal can be measured by the elongation of the length (elongation) and the contraction of the section (shrinkage of the section).

The larger the elongation and section shrinkage of the metal material, the better the plasticity of the material, that is, the material can withstand large plastic deformation without damage. Generally, metal materials with an elongation greater than 5% are called plastic materials (such as low carbon steel, etc.), and metal materials with an elongation less than 5% are called brittle materials (such as gray cast iron, etc.). Good plastic material, it can produce plastic deformation in a large macro range, and at the same time make the metal material due to plastic deformation and strengthen, so as to improve the strength of the material, to ensure the safe use of parts. In addition, the plastic material can be smoothly processed by some forming processes, such as stamping, cold bending, cold drawing, straightening and so on. Therefore, when choosing metal materials for mechanical parts, certain plasticity indicators must be met.