When the external force exceeds the elastic limit of the material, the material will be plastically deformed at this time, that is, the residual deformation of the material remains after the material is unloaded. When the external force continues to increase to a certain value, the external force will not increase or decrease and the sample will continue to elongate. On the stress-strain curve, there is a platform or jagged peak and valley. This phenomenon is called yielding. phenomenon. The force at the platform stage is the yielding force. The force before the first drop in the yield of the specimen is called the upper yielding force, and the minimum force in the yielding phase excluding the instantaneous effect is called the lower yielding force. The corresponding strength is the yield strength, the upper yield strength, and the lower yield strength.

Determination of yield strength

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

There are two methods for determining the upper yield strength and the lower yield strength: the graphic method and the pointer method.

1: Graphic method.

The force-clamp displacement map was drawn using an automatic recording device during the test. The ratio of the force axis is required to be less than 10 N/mm 2 per mm, and the curve must be drawn at least to the end of the yield phase. The constant force Fe of the yielding platform is determined on the curve, the maximum force FeH before the first drop of force in the yielding phase, and the minimum force FeL excluding the initial transient effect.

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

Yield strength calculation formula: Re = Fe / S0; Fe is the constant force at yield, S0 is the original cross-sectional area;

Upper yield strength calculation formula: ReH=FeH/S0; FeH is the maximum force before the first drop of force in the yielding stage;

The formula for calculating the yield strength is: ReL=FeL/So; FeL is the minimum force at the yield stage without considering the initial transient effect.

2: pointer method

During the test, the constant force of the first stop of the momentum of the measuring disc or the maximum force before the first rotation of the pointer or the minimum force excluding the initial transient effect correspond to the yield strength, the upper yield strength and the lower yield strength, respectively.

Determination of the upper and lower yield strength:

1: The first peak stress before yielding is judged as the upper yield strength, regardless of the magnitude of the peak stress thereafter.

2: Two or more valley stresses appear in the yielding phase, and the first valley stress is discarded, and the smallest of the remaining valleys is the lower yield strength. If there is only one valley stress, the lower yield strength is taken.

3: The platform appears in the yield stage, and the platform stress is determined as the lower yield strength. If multiple platforms are present and the latter is higher than the former, the first platform stress is taken as the lower yield strength.

4: The correct judgment result is that the lower yield strength must be lower than the upper yield strength.

Meaning of yield strength

The traditional strength design method, for plastic materials, the yield strength is the standard, the allowable stress [σ]=σys/n, the safety factor n is generally 2 or more, and the brittle material is determined by the tensile strength. The allowable stress [σ] = σb / n, the safety factor n is generally taken as 6.

The yield strength is not only of direct use, but also a general measure of some mechanical behavior and process performance of the material. For example, the material yield strength is increased, it is sensitive to stress corrosion and hydrogen embrittlement; the material yield strength is low, cold forming properties and welding performance are good, and so on. Therefore, yield strength is an indispensable indicator of material properties.

Factors affecting yield strength

The intrinsic factors affecting the yield strength are: bond, organization, structure, 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 influence of organizational structure, there are four strengthening mechanisms that affect the yield strength of metal materials, namely solid solution strengthening, deformation strengthening, precipitation strengthening and dispersion strengthening, grain boundary and subgrain strengthening. Among them, precipitation strengthening and fine grain strengthening are the most commonly used means to improve the yield strength of industrial alloys. Among these several strengthening mechanisms, the first three mechanisms improve the strength of the material while reducing the plasticity. Only the grain and subgrain are refined, which can increase the strength and increase the plasticity.

The external factors affecting 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 the body-centered cubic metal is particularly sensitive to temperature and strain rate, which leads to low temperature embrittlement of the steel. The effect of the stress state is also important. Although the yield strength is an essential indicator of the intrinsic properties of the material, the stress state is different and the yield strength values ​​are different. The yield strength of what we usually refer to is generally the yield strength at uniaxial stretching.

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