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What is yield strength

What is yield strength?

The yield strength is the yield limit when the metal material yields, that is, the stress that resists minor plastic deformation. For metal materials that have no obvious yield phenomenon, the stress value that produces 0.2% residual deformation is specified as its yield limit, which is called conditional yield limit or yield strength.
An external force greater than the yield strength will permanently invalidate the parts and cannot be restored. For example, the yield limit of low-carbon steel is 207MPa. When the external force is greater than this limit, the part will be permanently deformed. If it is less than this limit, the part will return to its original appearance.

  • (1) For materials with obvious yield phenomena, the yield strength is the stress at the yield point (yield value);
  • (2) For materials with insignificant yield phenomena, the stress when the limit deviation of the linear relationship between stress and strain reaches the specified value (usually 0.2% of the original gauge length). It is usually used as an evaluation index of the mechanical properties of solid materials and is the actual use limit of the material. Because after the stress exceeds the yield limit of the material, necking occurs, and the strain increases, which causes the material to be damaged and cannot be used normally.

20210109231259 40353 - What is yield strength
When the stress exceeds the elastic limit, the deformation increases rapidly after entering the yield stage. At this time, in addition to elastic deformation, partial plastic deformation is also generated. When the stress reaches point b, the plastic strain increases sharply and the stress and strain appear small fluctuations. This phenomenon is called yielding.
The maximum and minimum stresses at this stage are called the upper yield point and the lower yield point, respectively. Since the value of the lower yield point is relatively stable, it is used as an indicator of material resistance, which is called the yield point or yield strength (ReL or Rp0.2).
Some steels (such as high-carbon steels) have no obvious yielding phenomenon. Usually, the stress when a small amount of plastic deformation (0.2%) occurs is used as the yield strength of the steel, which is called the conditional yield strength.
First explain the deformation of the material under force. 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, the shape changes, elongation or shortening).
Building steel uses yield strength as the basis for design stress. Yield limit, commonly used symbol σs, is the critical stress value of material yielding.

  • (1) For materials with obvious yield phenomena, the yield strength is the stress at the yield point (yield value);
  • (2) For materials with insignificant yield phenomena, 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 of the 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, and the material fails and cannot be used normally.

Types of yield strength

  • (1) Silver streak yielding: Silver streak phenomenon and stress whitening.
  • (2) Shear yield.

Determination of yield strength

Metal materials without obvious yield phenomena need to measure their specified non-proportional extension strength or specified residual elongation stress, while metal materials with obvious yield phenomena can measure their yield strength, upper yield strength, and lower yield strength. Generally speaking, only the lower yield strength is measured.
Generally, there are two methods for determining the upper and lower yield strengths: graphical method and pointer method.

Graphical method

During the test, an automatic recording device was used to draw a force-chuck displacement diagram. The required 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 of the yield stage. On the curve, determine the constant force Fe of the yielding platform, the maximum force Feh before the first drop of the force in the yield stage, or the minimum force FeL that is less than the initial transient effect.
The 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 upper yield strength calculation formula: Reh=Feh/So; Feh is the maximum force before the first drop in the yield stage.
  • The lower yield strength calculation formula: 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 disc 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 transient effect corresponds to the yield strength, upper yield strength, and lower yield strength, respectively.

Standard of yield strength

  • 1. The maximum stress on the proportional limit stress-strain curve that conforms to the linear relationship is often expressed by σp in the world. When σp is exceeded, the material is considered to begin to yield. There are three commonly used yield standards in construction projects:
  • 2. After the elastic limit sample is loaded and unloaded, the highest stress at which the material can fully elastically recover with no residual permanent deformation as the standard. It is usually expressed in ReL internationally. 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.

Influencing factors

The internal factors that affect the yield strength are: bond, organization, structure, and atomic nature.
If the yield strength of metals is compared with ceramics and polymer materials, it can be seen that the influence of bonding bonds is fundamental. From the perspective of organizational structure, there can be 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 subcrystalline 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 by refining the grains and sub-crystals can both increase the strength and increase the plasticity.
The external factors that affect the yield strength are: temperature, strain rate, and stress state.
As the temperature decreases and the strain rate increases, the yield strength of the material increases, especially body-centered cubic metals are 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 the yield strength is an essential indicator that reflects the intrinsic properties of the material, the value of the yield strength is different for different stress states. What we usually call the yield strength of a material generally refers to the yield strength in uniaxial tension.

The meaning of engineering

The traditional strength design method, for plastic materials, the yield strength is the standard, and the allowable stress [σ]=σys/n is specified, and the safety factor n can be from 1.1 to 2 or more depending on the occasion. For brittle materials, it can resist tension. Strength is the standard, the allowable stress [σ]=σb/n is specified, and the safety factor n is generally 6.
It should be noted that according to the traditional strength design method, it will inevitably lead to one-sided pursuit of high yield strength of the material. However, as the yield strength of the material increases, the brittle fracture strength of the material is decreasing, and the risk of brittle fracture of the material increases.
Yield strength not only has direct use meaning, but also a rough measure of certain mechanical behavior and process performance of materials in engineering.
For example, the higher the yield strength of the material, it is sensitive to stress corrosion and hydrogen embrittlement; the lower the yield strength of the material, the cold forming performance and welding performance are better. Therefore, the yield strength is an indispensable and important indicator of material properties.

Source: Network Arrangement – China Flange Supplier: www.epowermetals.com

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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