Plasticity refers to the property of a material that, when subjected to stress exceeding the yield limit, undergoes significant deformation without immediate fracture. The residual deformation after unloading is called "plastic deformation".

Common plasticity indicators are the elongation after fracture and the reduction of area after fracture.

(1) Elongation after fracture (mechanical symbol: A; English abbreviation: EL).Elongation after fracture is the ratio of the total elongation length of a material from plastic deformation to fracture to its original length, i.e.,A = [(L₁ - L₀)/L₀] × 100% (2-9)Where:

Elongation after fracture is an indicator for measuring the plasticity of metal materials. In engineering, materials are usually divided into two categories based on their elongation after fracture: Materials with an elongation after fracture greater than 5% are called ductile materials, such as low-carbon steel and austenitic honed cylinder steel tubes; materials with an elongation of less than 5% are called brittle materials, such as cast iron and high-carbon martensitic honed cylinder steel tubes. Generally, brittle materials have low tensile strength, poor plasticity, high compressive strength, and low cost.

A high elongation of metal materials after fracture means they allow a large degree of plastic deformation and have good fracture resistance, which is beneficial for various deformation processes such as deep drawing, flanging, and bulging. Different applications of metal materials have different requirements for plasticity: For example, stamping materials require better plasticity, while structural parts require higher strength to meet the stability requirements of the structure.