Knowledge

-Yield strength-

When the external force exceeds the elastic limit of the material, the material will undergo plastic deformation, that is, after unloading, the material retains some residual deformation. When the external force continues to increase to a certain value, the phenomenon occurs where the sample continues to elongate even though the external force does not increase or even decreases. This is manifested on the stress-strain curve as a plateau or sawtooth-shaped peaks and valleys, and this phenomenon is called the yield phenomenon. The force at the plateau stage is the yield force. The force before the first drop when the sample yields is called the upper yield force, and the minimum force during the yield stage without considering the instantaneous effect is called the lower yield force. The corresponding strengths are yield strength, upper yield strength, and lower yield strength.

 

-Determination of Yield Strength-

For metallic materials without a distinct yield phenomenon, the specified non-proportional extension strength or the specified residual elongation stress should be measured. For metallic materials with a distinct yield phenomenon, the yield strength, upper yield strength, and lower yield strength can be measured. Generally, only the lower yield strength is determined.

 

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

 

1. Graphical method

During the test, an automatic recording device is used to plot the force-head displacement graph. The force axis scale should be such that each mm represents a stress generally less than 10 N/mm², and the curve should be plotted at least to the end of the yield stage. On the curve, determine the constant force Fe at the yield plateau, the maximum force Feh before the first force drop in the yield stage, or the minimum force Fel that is not affected by the initial instantaneous effect.

 

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

Yield strength calculation formula: Re = Fe / So, where Fe is the constant force at yield.

Upper yield strength calculation formula: Reh = Feh / So, where Feh is the maximum force before the first drop in force during the yield stage.

Lower yield strength calculation formula: Rel = Fel / So, where Fel is the minimum force less than the initial instantaneous effect.

 

2. Pointer Method

During the test, the constant force at which the pointer of the measuring force dial first stops moving, the maximum force before the pointer first rotates back, or the minimum force less than the initial instantaneous effect, respectively correspond to the yield strength, the upper yield strength, and the lower yield strength.

 

-Determination of Upper and Lower Yield Strengths-

 

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

 

If two or more minimum stress values occur during the yielding stage, the first minimum stress value should be discarded and the smallest of the remaining minimum stress values should be taken as the lower yield strength. If only one minimum stress value exists, it should be taken as the lower yield strength.

 

If a plateau appears during the yielding stage, the stress at the plateau is determined as the lower yield strength. If multiple plateaus occur and the stress of the latter is higher than that of the former, the stress of the first plateau is taken as the lower yield strength.

 

The lower yield strength must be lower than the upper yield strength.

 

- The Significance of Yield Strength -

In traditional strength design methods, for plastic materials, the yield strength is taken as the standard, and the allowable stress [σ] is defined as σys / n, with the safety factor n generally set at 2 or higher. For brittle materials, the tensile strength is used as the standard, and the allowable stress [σ] is defined as σb / n, with the safety factor n typically set at 6.

 

Yield strength not only has direct practical significance but also serves as a rough measure of certain mechanical behaviors and technological properties of materials in engineering. For instance, an increase in a material's yield strength makes it more susceptible to stress corrosion and hydrogen embrittlement; conversely, a lower yield strength indicates better cold working formability and weldability, etc. Therefore, yield strength is an indispensable and crucial indicator among material properties.

 

- Factors Affecting Yield Strength -

The intrinsic factors influencing yield strength include: bonding, microstructure, structure, and atomic nature. Comparing the yield strength of metals with that of ceramics and polymers reveals that the type of bonding is fundamental. From the perspective of microstructure and structure, there are four strengthening mechanisms that affect the yield strength of metallic materials, namely solid solution strengthening, work hardening, precipitation strengthening, and dispersion strengthening, as well as grain boundary and subgrain strengthening. Among these, precipitation strengthening and grain refinement are the most commonly used methods in industrial alloys to enhance material yield strength. Among these strengthening mechanisms, the first three not only increase the strength but also reduce the plasticity of the material. Only grain refinement and subgrain strengthening can enhance both strength and plasticity.

 

The external factors that affect the yield strength include: temperature, strain rate, and stress state. As the temperature decreases and the strain rate increases, the yield strength of the material rises. Especially for body-centered cubic metals, they are particularly sensitive to temperature and strain rate, which leads to the low-temperature embrittlement of steel. The influence of stress state is also significant. Although the yield strength is an essential indicator reflecting the intrinsic properties of materials, the yield strength value varies with different stress states. The yield strength of materials we usually refer to generally means the yield strength under uniaxial tension.

 

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