Plasticity refers to the ability of metal materials to produce permanent deformation (plastic deformation) without damage under the action of external force. When metal materials are stretched, the length and cross-sectional area will change. Therefore, the plasticity of metal can be measured by two indexes: the elongation of length (elongation) and the shrinkage of section (reduction of section).
The greater the elongation and area shrinkage of metal material, the better the plasticity of the material, that is, the material can withstand large plastic deformation without damage. Generally, metal materials with elongation greater than 5% are called plastic materials (such as low carbon steel), while metal materials with elongation less than 5% are called brittle materials (such as gray cast iron). The material with good plasticity can produce plastic deformation in a large macro range, and strengthen the metal material due to plastic deformation while plastic deformation, so as to improve the strength of the material and ensure the safe use of parts. In addition, materials with good plasticity can be processed smoothly by some molding processes, such as stamping, cold bending, cold drawing, straightening and so on. Therefore, when choosing metal materials as mechanical parts, certain plasticity indexes must be met.