Detailed Explanation Of Yield Strength Of Metal Materials

1. Yield Strength

It is the yield limit when a metal material yields, that is, the stress that resists trace plastic deformation. For metal materials with no obvious yield phenomenon, the stress value that produces 0.2% residual deformation is stipulated as the yield limit, which is called the conditional yield limit or yield strength.

External forces greater than the yield strength will cause the parts to permanently fail and cannot be restored. For example, the yield limit of low carbon steel is 207MPa. When exposed to an external force greater than this limit, the part will undergo permanent deformation. If it is less than this, 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 where the yield phenomenon is not obvious, the stress when the limit deviation of the linear relationship between stress and strain reaches a specified value (usually 0.2% of the original gauge length). It is usually used as an evaluation index for the mechanical and mechanical properties of solid materials and is the actual use limit of the material. Because necking occurs after the stress exceeds the yield limit of the material, the strain increases, causing the material to be damaged and unable to 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, some plastic deformation also occurs. When the stress reaches point b, the plastic strain increases sharply and the stress and strain fluctuate slightly. This phenomenon 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 used as an indicator of material resistance, called the yield point or yield strength (ReL or Rp0.2).

Some steels (such as high carbon steel) have no obvious yield phenomenon. The stress when a small amount of plastic deformation (0.2%) occurs is usually used as the yield strength of the steel, which is called the conditional yield strength.

First, explain the material deformation under stress. The deformation of materials is 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 used as the basis for design stress. Yield limit, commonly used symbol σs, is the critical stress value at which the material yields.

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

(2) For materials with no obvious yield phenomenon, the stress when the limit deviation of the linear relationship between stress and strain reaches a specified value (usually 0.2% elongation of the material). It is usually used as an evaluation index for the mechanical and mechanical properties of solid materials and is the actual use limit of the material. Because plastic deformation occurs after the stress exceeds the yield limit of the material, the strain increases, causing the material to fail and cannot be used normally.

2. Type

(1): Silver yield: silver craze phenomenon and stress whitening. (2): Shear yielding.

Yield strength determination

Metal materials without obvious yielding phenomenon need to measure their specified non-proportional extension strength or specified residual elongation stress, while for metal materials with obvious yielding phenomenon, their yield strength, upper yield strength, and lower yield strength can be measured. Generally, only the lower yield strength is measured.

There are usually two methods for measuring upper yield strength and lower yield strength: graphic method and pointer method.

Graphical representation

During the test, an automatic recording device was used to draw a force-chuck displacement diagram. It is required that the force axis ratio is that the stress represented by each mm is generally less than 10N/mm2, and the curve must be drawn at least to the end point of the yield stage. Determine on the curve the constant force Fe at the yield plateau, the maximum force Feh before the first drop in 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.

The formula for calculating the upper yield strength: Reh=Feh/So; Feh is the maximum force before the first force decrease in the yield stage.

Calculation formula of lower yield strength: ReL=FeL/So; FeL is the minimum force FeL less than the initial instantaneous effect.

Pointer Method

During the test, the constant force when the pointer of the force measuring dial stops rotating for the first time or the maximum force before the pointer rotates for the first time or the minimum force less than the initial instantaneous effect corresponds to the yield strength, upper yield strength, and lower yield strength respectively.

3.Standard

1. The highest stress that conforms to the linear relationship on the proportional ultimate stress-strain curve is often expressed by σp internationally. When σp is exceeded, the material is considered to begin to yield. There are three commonly used yield standards in construction projects:

2. The elastic limit specimen is loaded and then unloaded. Based on the criterion that there is no residual permanent deformation, the highest stress at which the material can fully elastically recover. Internationally it is usually represented by ReL. When the stress exceeds ReL, the material is considered to begin to yield.

3. The yield strength is based on the specified residual deformation. For example, the stress of 0.2% residual deformation is usually used as the yield strength, and the symbol is Rp0.2.

4. Influencing Factors

The internal factors that affect the yield strength include: bonding, organization, structure, and atomic nature.

For example, comparing the yield strength of metals with ceramics and polymer materials, it can be seen that the influence of bonding bonds is fundamental. From the perspective of the impact of organizational structure, there are four strengthening mechanisms that affect the yield strength of metal materials, which are:

(1) Solid solution strengthening;

(2) Deformation strengthening;

(3) Precipitation strengthening and dispersion strengthening;

(4) Grain boundary and sub-grain strengthening.

Precipitation strengthening and fine grain strengthening are the most commonly used methods to improve the yield strength of materials in industrial alloys. Among these strengthening mechanisms, the first three mechanisms increase the strength of the material while also reducing the plasticity. Only the refinement of grains and sub-grains can both improve the strength and increase the plasticity.

External factors that affect yield strength include: temperature, strain rate, and stress state.

As the temperature decreases and the strain rate increases, the yield strength of the material increases. In particular, body-centered cubic metals are particularly sensitive to temperature and strain rate, which leads to low-temperature embrittlement of steel. The influence of stress state is also important. Although yield strength is an essential indicator that reflects the intrinsic properties of a material, the yield strength value is also different depending on the stress state. What we usually call the yield strength of a material generally refers to the yield strength when stretched in one direction.

5. Engineering Significance

The traditional strength design method uses the yield strength as the standard for plastic materials and stipulates the allowable stress [σ] = σys/n. The safety factor n can range from 1.1 to 2 or more depending on the situation. For brittle materials, the tensile strength is used as the standard. The strength is the standard, and the allowable stress [σ]=σb/n is specified. The safety factor n is generally 6.

It should be noted that following the traditional strength design method will inevitably lead to the one-sided pursuit of high yield strength of the material. However, as the yield strength of the material increases, the brittle fracture resistance of the material decreases, and the risk of brittle fracture of the material increases.

Yield strength not only has direct usage significance, but also is a rough measure of certain mechanical behaviors and process properties of materials in engineering. For example, if the material yield strength increases, it will be sensitive to stress corrosion and hydrogen embrittlement; if the material yield strength is low, the cold working forming performance and welding performance will be good, etc. Therefore, yield strength is an indispensable and important indicator of material properties.

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