Detailed Explanation of Material Yield Strength

1. Yield strength

Yield strength is the yield limit of metal materials when yield occurs, which is also the stress resisting micro plastic deformation.

For metal materials without obvious yield, the stress value producing 0.2% residual deformation is specified as its yield limit, which is called conditional yield limit or yield strength.

The external force greater than the yield strength will cause the permanent failure of the part and cannot be recovered.

For example, the yield limit of low-carbon steel is 207MPa. When the external force is greater than this limit, the part will have permanent deformation.

If it is less than this limit, the part will return to its original shape.

1) For materials with obvious yield phenomenon, the yield strength is the stress at the yield point (yield value);

2) For materials without obvious yield phenomenon, the stress when the limit deviation of the linear relationship with stress-strain reaches the specified value (usually 0.2% of the original gauge length).

It is usually used as the evaluation index of the mechanical properties of solid materials, and is the actual service limit of materials.

Because necking occurs when the stress exceeds the material yield limit, and the strain increases, the material is damaged and cannot be used normally.

When the stress exceeds the elastic limit and enters the yield stage, the deformation increases rapidly.

At this time, in addition to elastic deformation, there is also some plastic deformation.

When the stress reaches point b, the plastic strain increases sharply and the stress and strain fluctuate slightly, which is called yielding.

The maximum and minimum stresses at this stage are called the upper yield point and lower yield point respectively.

Since the value of the lower yield point is relatively stable, it is taken as an index of material resistance, called yield point or yield strength (ReL or Rp0.2).

Some steels (such as high carbon steel) have no obvious yield phenomenon.

Generally, the stress in case of slight plastic deformation (0.2%) is taken as the yield strength of the steel, which is called conditional yield strength.

First, explain the material deformation under force.

The deformation of materials can be divided into elastic deformation (the original shape can be restored after the external force is removed) and plastic deformation (the original shape cannot be restored after the external force is removed, and the shape changes, elongates or shortens).

The yield strength of construction steel is taken as the basis for design stress.

Yield limit, common symbol σs. is the critical stress value of material yield.

1) For materials with obvious yield, the yield strength is the stress at the yield point (yield value)

2) For materials without obvious yield phenomenon, the stress when the limit deviation of the linear relationship between stress and strain reaches the specified value (usually 0.2% elongation of the material).

It is usually used as the evaluation index of the mechanical properties of solid materials, and is the actual service limit of materials.

Because plastic deformation occurs after the stress exceeds the material yield limit, and the strain increases, the material becomes invalid and cannot be used normally.

2. Type

1. Craze yield: craze phenomenon and stress whitening.

2. Shear yield.

Determination of yield strength

The specified non-proportional elongation strength or the specified residual elongation stress shall be measured for metal materials without obvious yielding, while the yield strength, upper yield strength and lower yield strength can be measured for metal materials with obvious yielding.

In general, only the lower yield strength is measured.

Generally, there are two methods to determine the upper yield strength and lower yield strength: graphical method and pointer method.

Graphic method

During the test, an automatic recording device is used to draw the force collet displacement diagram.

It is required that the stress represented by the force axis ratio of each mm is generally less than 10N/mm2, and the curve should be drawn to the end of the yield stage at least.

Determine the constant force Fe of the yield platform on the curve, the maximum force Feh before the first drop of the force in the yield stage, or the minimum force FeL less than the initial instantaneous effect.

Yield strength, upper yield strength and lower yield strength can be calculated according to the following formula:

Yield strength calculation formula: Re=Fe/So; Fe is the constant force at yield.

Calculation formula of upper yield strength: Reh=Feh/So; Feh is the maximum force before the first drop of force in the yield stage.

Calculation formula of lower yield strength: ReL=FeL/So; FeL is the minimum force FeL without initial instantaneous effect.

Pointer method

During the test, the constant force when the pointer of the force measuring disk stops rotating for the first time, or the maximum force before the pointer rotates for the first time, or the minimum force that does not reach the initial instantaneous effect, corresponds to the yield strength, upper yield strength, and lower yield strength respectively.

3. Standards

There are three yield standards commonly used in construction engineering:

1. The highest stress on the proportional limit stress-strain curve that conforms to the linear relationship is often expressed by σp internationally.

When it exceeds σp, it is considered that the material begins to yield.

2. The elastic limit sample shall be unloaded after loading, and the maximum stress that the material can recover elastically without residual permanent deformation.

It is generally expressed in ReL internationally.

The material is considered to yield when the stress exceeds ReL.

3. The yield strength is based on the specified residual deformation.

For example, the stress of 0.2% residual deformation is usually taken as the yield strength, with the symbol Rpo. 2.

4. Influencing factors

The internal factors affecting the yield strength are bond, structure, structure and atomic nature.

If the yield strength of the metal is compared with that of ceramic and polymer materials, it can be seen that the influence of bonding bond is fundamental.

From the perspective of the influence of structure, there are four strengthening mechanisms that can affect the yield strength of metal materials, namely:

(1) Solution strengthening;

(2) Deformation strengthening;

(3) Precipitation strengthening and dispersion strengthening

(4) Grain boundary and subgrain strengthening.

Precipitation strengthening and fine-grain strengthening are the most common means to improve the yield strength of industrial alloys.

Among these strengthening mechanisms, the first three mechanisms can not only improve the strength of the material, but also reduce the plasticity.

Only by refining the grains and sub crystals, the strength and plasticity can be improved.

The external factors affecting yield strength include temperature, strain rate and stress state.

With the decrease of temperature and the increase of strain rate, the yield strength of materials increases, especially the body centered cubic metal is particularly sensitive to temperature and strain rate, which leads to low temperature embrittlement of steel.

The influence of the stress state is also important.

Although yield strength is an essential index reflecting the internal properties of materials, the yield strength values are different with different stress states.

The yield strength of materials usually refers to the yield strength in uniaxial tension.

5. Project significance

According to the traditional strength design method, for plastic materials, the allowable stress [σ]=σys/n is specified based on the yield strength, and the safety factor n can range from 1.1 to 2 or more depending on the situation.

For brittle materials, the allowable stress [σ]=σb/n is specified based on the tensile strength, and the safety factor n is generally 6.

It should be noted that the traditional strength design method will inevitably lead to the one-sided pursuit of high-yield strength of materials.

However, with the increase of yield strength of materials, the brittle fracture resistance of materials is decreasing and the risk of brittle fracture of materials is increasing.

Yield strength is not only of direct use significance, but also a rough measure of some mechanical behaviors and technological properties of materials in engineering.

For example, when the yield strength of materials increases, they are sensitive to stress corrosion and hydrogen embrittlement;

The material has low yield strength, good cold working formability and weldability, etc.

Therefore, yield strength is an indispensable and important index in material properties.

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