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 of the length of plastic elongation of the material after breaking to the length of the original sample 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 is, the better the wear resistance is. The commonly used hardness indicators are Brinell hardness, Rockwell hardness and Vickers hardness.
Brinell hardness (HB)
Press a hardened steel ball of a certain size (generally 10mm in diameter) into the material surface with a certain load (generally 3000kg) for a period of time. After unloading, the ratio of the load to the indentation area is the Brinell hardness value (HB).
L Rockwell hardness (HR)
When HB>450 or the sample is too small, the Brinell hardness test can not be used but Rockwell hardness measurement. It uses a diamond cone with a vertex angle of 120 ° or a steel ball with a diameter of 1.59 and 3.18 mm to press it into the surface of the tested material under a certain load, and the hardness of the material is calculated from the depth of the indentation. According to the different hardness of the test material, it can be expressed by three different scales:
HRA: the hardness obtained by using 60kg load and diamond cone indenter, used for materials with extremely high hardness (such as cemented carbide).
HRB: hardness obtained by using 100kg load and 1.58mm diameter hardened steel ball, used for materials with low hardness (such as annealed steel, cast iron, etc.).
HRC: the hardness obtained by using a 150kg load and a diamond cone indenter, used for materials with high hardness (such as quenched steel).
L Vickers hardness (HV)
Press the material surface with a load within 120kg and a diamond square cone indenter with a top angle of 136 °. Divide the surface product of the material indentation dent by the load value, which is the Vickers hardness value (HV)
