What are the four indexes to measure the mechanical properties of steel?

The commonly used mechanical properties of steel are generally interpreted as four items, namely strength, hardness, plasticity and toughness.

1. yield point (σs)

When the steel or sample is stretched, when the stress exceeds the elastic limit, even if the stress no longer increases, the steel or sample continues to undergo obvious plastic deformation, which is called yield, and the minimum stress value when the yield phenomenon occurs is the yield point. Let Ps be the external force of the yield point S and Fo be the cross-sectional area of the sample, then the yield point σs =Ps/Fo(MPa).

2. Yield strength (σ0.2)

The yield point of some metal materials is extremely inconspicuous and difficult to measure. Therefore, in order to measure the yield characteristics of materials, the stress when the permanent residual plastic deformation is equal to a certain value (generally 0.2% of the original length) is called conditional yield strength or yield strength σ0.2 for short.

3. Tensile strength (σb)

The maximum stress value of a material from the beginning to the fracture in the tensile process. It represents the ability of steel to resist fracture. Corresponding to tensile strength are compressive strength, bending strength and so on. Let Pb be the maximum tensile Force before material fracture and fo be the cross-sectional area of the sample, then the tensile strength σb= Pb/Fo(MPa).

4. Elongation (δs)

After tensile fracture, the percentage of plastic elongation of the material to the length of the original sample is called elongation or elongation.

5. Yield ratio (σs/σb)

The ratio of yield point (yield strength) to tensile strength of steel is called yield ratio. The greater the yield ratio, the higher the reliability of the structure. Generally, the yield ratio of carbon steel is 0.6-0.65, that of low alloy structural steel is 0.65-0.75, and that of alloy structural steel is 0.84-0.86.

6. Difficulties

Hardness indicates 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. Commonly used hardness indexes include Brinell hardness, Rockwell hardness and Vickers hardness.

The mechanical properties of materials refer to the mechanical properties of materials under various external loads (tension, compression, bending, torsion, impact, alternating stress, etc.). ) in different environments (temperature, medium, humidity).

Generally speaking, the mechanical properties of metals can be divided into ten types:

1. Brittleness refers to the property that a material will not undergo plastic deformation before it is damaged. It is the opposite of toughness and plasticity. Brittle materials have no yield point, but they have breaking strength and ultimate strength, and they are almost the same. Cast iron, ceramics, concrete and stone are all brittle materials. Compared with many other engineering materials, the tensile properties of brittle materials are weak, so compression tests are usually used to evaluate brittle materials.

2. Strength: the ability of metal materials to resist permanent deformation or fracture under static load. It can also be defined as proportional limit, yield strength, fracture strength or ultimate strength. There is no exact single parameter that can accurately define this characteristic. Because the behavior of metal changes with the change of stress and its application form. Power is a very common term.

3. Plasticity: the ability of metal materials to permanently deform under load without being destroyed. When the stress of metal material exceeds the elastic limit and the load is removed, plastic deformation will occur, and the material will retain part or all of the load at this time.

4. Hardness: the ability of the surface of a metal material to resist the invasion of harder objects.

5. Toughness: the ability of metal materials to resist impact load without being destroyed. Toughness refers to the characteristic that metal materials have certain plastic deformation before fracture under the action of tensile stress. Gold, aluminum and copper are tough materials, and they are easily drawn into wires.

6. Fatigue strength: the ability of material parts and structural parts to resist fatigue damage.

7. Elasticity refers to the characteristic that a metal material can recover its original size when the external force disappears. Steel is elastic before it reaches the elastic limit.

8. Ductility Ductility refers to the characteristic that a material bears certain plastic deformation before fracture under the action of tensile stress or compressive stress. Plastic materials generally adopt rolling and forging processes. Steel has both plasticity and ductility.

9. Stiffness is the characteristic that metal materials bear high stress without large strain. The stiffness is evaluated by measuring the elastic modulus e of the material.

10. yield point or yield stress yield point or yield stress is the stress level of metal, in MPa. Above the yield point, when the external load is removed, the metal deformation still exists and the metal material is plastically deformed.

References:

Mechanical properties of Baidu encyclopedia entries