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.

Figure.1 Four stages in the stretching process of metal materials

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 (R_eL or R_p0.2).
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 F_e of the yielding platform, the maximum force F_eh before the first drop of the force in the yield stage, or the minimum force F_eL 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: R_e=F_e/S_o; F_e is the constant force at yield.
• The upper yield strength calculation formula: R_eh=F_eh/S_o; F_eh is the maximum force before the first drop in the yield stage.
• The lower yield strength calculation formula: R_eL=F_eL/S_o; F_eL is the minimum force F_eL 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 σ_p in the world. When σ_p 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 R_eL internationally. When the stress exceeds R_eL, 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 R_p0.2.

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.

A study on the effect of testing methods on the determination of yield strength of metallic materials

As an important industrial material, metallic materials are widely used in industrial production. To improve the use of metal materials, it is necessary to strictly determine their relevant properties. As the main method of determining the yield strength of metallic materials, the determination of yield point and the selection of the correct yield point control method play an important role in improving the use of materials. It is necessary to study the influence of testing methods on determining the yield strength of metallic materials.
Tensile is an important method for determining the properties of metal materials, which can scientifically determine the properties of metal materials. Yield strength is an important attribute of the mechanical properties of metal materials; the performance of the good and bad directly affect the use of metal materials, and the need to control the yield strength in a reasonable range is an important indicator of the bearing capacity, clear yield strength in the processing technology and construction technology of the differences between the full use of the yield strength size to determine the construction method and technology method is correct. Therefore, increasing the research on the yield strength of metal materials is necessary.

Determination of yield strength of metal materials methods

Upper and lower yield strength position determination principle

• (1) Will yield before the first peak earnings as the key to determining the yield strength, do not have to consider the size of the peak stress behind.
• (2) If there are two or more valleys in the yielding stage, it is necessary to discard the first valley stress and take the smallest of the other valleys as the main factor in determining the yield strength. If there is only one falling valley in the determination process, the valley value should be determined as the yield strength.
• (3) If there is a yield plateau in the yield determination stage, it is necessary to determine the yield plateau as the lower yield strength. If the yield value of the latter is higher than the yield value of the former, and there is more than one yield value of the phenomenon, then the first plateau stress can be determined as the yield strength.
• (4) Do a good job under the yield strength of the correct judgment; the standard yield strength should be higher than the lower yield strength.

Tensile curve in the upper and lower yield point determination method

In recent years, with the rapid development of science and technology, computer technology is widely used in all walks of life, tensile as an effective means of determining the performance of metal materials to ensure the correctness of the determination of the performance of metal materials, it is necessary to microcomputer-controlled electronic universal testing machine used in the tensile test so that the drawing of tensile curve test is more simple and convenient and to enhance the effect of the drawing of the tensile curve. An electronic universal testing machine is used to determine the performance of metal materials, and it can automatically determine yield strength, tensile strength, and other parameters of materials. And in the experimental process, they often appear in the same conditions under the same batch of specimens, yield strength of large differences in the situation, need to increase the consideration of the specimen, clear different specimens produced by the dispersion of different specimens, the difference between the specimens. However, some of the differences in the results of the experimental research cannot be accepted.
Through long-term experimental research, it can be observed that the electronic universal testing machine in the search for the yield point will often appear with more errors, seriously affecting the experimental results. For example, in the textbook of mechanics of materials is the definition of upper and lower yield strength: in the yielding stage of materials, the upper yield limit is generated by the highest stress, and the lowest stress generates the lower yield limit. To enhance the effect of experimental research, it is necessary to strictly follow the requirements of the relevant national standards, with the upper yield strength as the highest stress before the decline, and the lower yield strength as the lowest stress during the yielding period, regardless of the initial transient. The initial transient effect, also known as the inertia effect, mainly refers to the upper yield strength to the yield strength over the transient effect produced by the yield strength. The magnitude of the transient effect is affected by a variety of factors, including the specimen’s yield characteristics, the specimen’s flexibility, the home system’s flexibility, and other factors that have a greater impact. It is difficult to evaluate the transient effect quantitatively, and it is necessary to fully understand the main influence area of the initial transient effect and to do a good job of the excess between the upper yield strength and the lower yield strength. To prevent the initial transient effect on the interval caused by a large impact, we need to do a good job of falling valley stress exclusion, with the minimum stress as the main basis for determining the yield strength. If the result shows a valley, the valley can be determined as the yield strength. However, in the actual experimental process, there are some differences between the results of the determination of the yield strength and the actual experimental results, and the phenomenon of misjudgment is more serious, which affects the results of the yield strength of metal materials. For example, in the determination of the lower yield point, usually point A as the reference point (see Figure 2), in the yield point determination, do not need to consider the initial transient effect caused by the impact of the need to choose the point marked on the chart as the lower yield point. If point A is used as the lower yield point, the measurement results show that the lower yield strength is small relative to the true value. If point B is used as the upper yield point, the point indicated in the diagram needs to be used as the yield point, and the measurement results show that the lower yield strength is greater than the true value (see Figure 3).

Figure.2 Lower yield point confusing tensile curve

Figure.3 Confusing tensile curves at the upper yield point

Determination of yield strength of metal materials control mode

Selection of control method

Tensile tests in the process of carrying out by the control mode, different control modes lead to a certain difference in the experimental results, which have a greater impact. Determination of the yield strength of metal materials control mainly includes strain rate control and stress rate control. When testing the yield strength, combining it with the actual situation is necessary to judge and choose the appropriate control method. Specimens in different yielding periods have a greater impact, resulting in their plastic extension of a certain difference; the stress will be accompanied by fluctuations in the effect of up and down. If at this time, to set the stress rate reasonably, the need to increase the effective control of the stress rate to avoid the experimental machine to measure because of the speed of movement and produce large fluctuations in the phenomenon, resulting in inaccurate data measurement. In addition, the larger’s extreme environmental impact often leads to serious loss of control of the test machine phenomenon and then produces a loss of control crisis. For example, the beam moving speed, displacement, and the experimental force displacement drawn on the same graph can see the tensile curve in different stages; the beam speed will occur larger changes. In the elastic stage, it can be seen that the beam is moving at a uniform speed, and the movement is relatively smooth. In the yielding stage, the beam speed increases, and severe and violent shaking occurs. In the plastic phase, the speed of the beam gradually becomes larger, and the speed changes more rapidly in the later phase. For safety reasons, it is necessary to draw the curve well and set the stress rate value in a small range, but in the late plastic phase, it still produces a large value of speed change.

Selection of tensile rate

Reasonable selection of the tensile rate and the correct determination of the yield strength of metal materials have an important role and need to be in strict accordance with the relevant national standards; the yield strength of the lower yield strength of a reasonable measurement, in the parallel length of the sample during the yield strain rate control at 0.00025 – 0.0025/s, to ensure that the parallel length of the strain rate of the persistent constancy. Therefore, through experimental verification, the current state regulations of the experimental rate range are usually broader. The tensile rate experiment shows that the experimental conditions or the actual need for the experimental rate range are relatively broad. Some experiments in the process of carrying out by the experimental rate range of influence, beyond the experimental range of the experimental rate results, will not be used as the basis for the selection of tensile rate. Mild steel as a metal material, in the actual use of the process by the experimental rate of influence, results in a certain difference in yield strength and sometimes even a relatively large difference in the situation. For example, in the same kind of mild steel material in different strain rates for force a displacement curve situation acute analysis can be seen in different strain rates, the yield force will have a big difference, and by the strain rate of the influence of the mild steel under the yield strength will be with the increase of the strain rate and gradually increase.

Displacement rate and strain rate on the determination of the yield strength of metal materials and analysis

(1) Experimental equipment and testing methods. The use of test methods for the determination of the yield strength of metal materials and analysis of the impact of the choice of test equipment mainly includes microcomputer-controlled electronic universal testing machine, electronic extensometer, mild steel, micrometer, mild steel, and duralumin round tensile specimens. To ensure the use of the test machine, the need for test machine for the determination of flexibility needs to be divided into eight groups for the test, of which the first four groups are mainly used in the strain rate control method, the other four groups of displacement rate control method, the specific organization of the situation is as follows.

• Group 1: The displacement rate is controlled at 0.5 mm/min, the modulus of elasticity, upper and lower yield strength, tensile strength, and yield elongation of mild steel are measured, and the relationship between total elongation strength and tensile strength is clarified.
• Group 2: The strain rate was controlled at 0.000225/s, and the determination contents included yield elongation, modulus of elasticity of mild steel, tensile strength, etc. The total elongation strength, non-proportional elongation strength, and tensile strength were determined.
• Group 3: The strain rate control at 0.00025/s, the contents of the measurement, including yield elongation, mild steel modulus of elasticity, tensile strength, etc., for the total elongation strength, non-proportional elongation strength and tensile strength of the determination.
• Group 4: The strain rate control at 0.000275/s, the contents of the measurement, including yield elongation, mild steel modulus of elasticity, tensile strength, etc., to specify the total elongation strength, non-proportional elongation strength and tensile strength of the determination.
• Group 5: The strain rate control at 0.00030/s, the contents of the measurement, including yield elongation, mild steel modulus of elasticity, tensile strength, etc., for the total elongation strength, non-proportional elongation strength and tensile strength of the determination.
• Group 6: The strain rate is controlled at 1 mm/min; the contents of the determination include yield elongation, modulus of elasticity of mild steel, tensile strength, etc., and the total elongation strength, non-proportional elongation strength, and tensile strength are determined.
• Group 7: The strain rate is controlled at 2 mm/min; the contents of the measurement include yield elongation, modulus of elasticity of mild steel, tensile strength, etc., the total elongation strength, non-proportional elongation strength and tensile strength of the determination.
• Group 8: The strain rate is controlled at 5 mm/min, and the contents include yield elongation, modulus of elasticity of mild steel, tensile strength, etc. The total elongation strength, non-proportional elongation strength, and tensile strength are determined.

(2) Analysis of experimental results. The above experiments show that using a strain control rate to determine the metal materials and the measurement results by the lower yield strength factors have a greater impact on the very unstable test results. When the displacement control rate is used to determine the metal material, the measurement results are less affected by the lower yield strength factor, and the test results are more stable. It shows that the strain rate control mode in the material instantaneous yield curve characteristics will be obvious changes; in the displacement rate control mode, the material instantaneous yield curve characteristics are more normal.

Conclusion

In the use of an electronic universal testing machine on the yield strength of metal materials for testing, it can be learned that the yield point of mastery directly affects the yield strength of metal materials test effect, cannot be completely based on the number of machine determination, need to be based on the actual specific circumstances of the yield point determination. To fully consider the impact rate on the material, the same rate measured yield strength is comparable. Therefore, it is necessary to ensure the rigor and prudence of yield point measurement to improve the accuracy of yield strength measurement.
Author: Liu Gang

Source: 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.)

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