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. anxious. 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. A metal material without significant yielding is required to measure its specified non-proportional extension strength or a specified residual elongation stress, and a metal material having a significant yielding phenomenon can measure its yield strength, upper yield strength, and lower yield strength. First, 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.

Determination of 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 stage, 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, then

Take the lower yield strength.

3: The platform appears in the PCT stage, and the platform stress is determined as the lower yield strength. If there are multiple platforms, the latter is higher than the former, and the first platform stress is 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, based on the yield strength, specifies the allowable stress [] = aysn, the safety factor n is taken at 2 or greater, for the brittle material, to resist

For the standard, the allowable stress [] = b / n, the safety factor n - generally take 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 yield strength of the material is increased, the yield strength of the sensitive material is low for stress corrosion and hydrogen embrittlement, the cold forming property and the welding performance are good, and the like. Therefore, the strength of clothing is an indispensable indicator of material performance.

Factors affecting the strength of the clothing affect the intrinsic factors of yield strength: bonding 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 perspective of the influence of organizational structure, there are four strengthening mechanisms that affect the yield strength of metal materials, namely, the hardening strengthening, the deformation strengthening, the precipitation strengthening and dispersion strengthening, the boundary and the subgrain strengthening. Among them, precipitation strengthening and fine grain strengthening are the most commonly used means to improve the yield strength of industrial alloys. In these several strengthening mechanisms

The first three mechanisms improve the strength of the material, but also reduce the nature. Only the grain and subgrain are refined, and the external factors that can improve the strength and increase 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 as the 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 one that reflects the intrinsic properties of the material but the stress state is different, the yield strength values are different. The yield strength of a material we often refer to is the yield strength in uniaxial stretching.