1. Yield point（ σ s）
When the steel or sample is stretched, when the stress exceeds the elastic limit, even if the stress does not increase any more, the steel or sample continues to undergo obvious plastic deformation. This phenomenon is called yield, and the minimum stress value when the yield phenomenon occurs is the yield point. If Ps is the external force at the yield point s and Fo is the sectional area of the sample, then the yield point σ s =Ps/Fo(MPa)。
2. Yield strength（ σ 0.2）
The yield point of some metal materials is very unobvious, which is difficult to measure. Therefore, in order to measure the yield characteristics of materials, it is stipulated that the stress when the permanent residual plastic deformation is equal to a certain value (generally 0.2% of the original length) is generated, which is called conditional yield strength or yield strength for short σ 0.2。
3. Tensile strength（ σ b）
The maximum stress value reached by the material during the tensile process from the beginning to the time of fracture. It indicates the resistance of steel to fracture. The compressive strength and bending strength are corresponding to the tensile strength. If Pb is the maximum tensile force reached before the material is broken, and Fo is the cross-sectional area of the sample, then the tensile strength σ b= Pb/Fo（MPa）。
4. Elongation（ δ s）
The percentage between the length of plastic elongation and the length of the original sample after the material is broken is called elongation or elongation.
5. Yield ratio（ σ s/ σ b）
The ratio of yield point (yield strength) to tensile strength of steel is called yield strength ratio. The greater the yield ratio, the higher the reliability of structural parts. The yield ratio of general carbon steel is 0.6-0.65, and that of low alloy structural steel is 0.65-0.75, and that of alloy structural steel is 0.84-0.86.
6. Hardness
Hardness refers to the ability of a material to resist hard objects pressing into its surface. It is one of the important performance indexes of metal materials. Generally, the higher the hardness, the better the wear resistance. The commonly used hardness indicators are Brinell hardness, Rockwell hardness and Vickers hardness.
