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Fastener technical stickers must be seen by professionals! About the key points of yield strength

when the external force exceeds the elastic limit of the material, the material will undergo plastic deformation, that is, some residual deformation will be retained after unloading. When the external force continues to increase to a certain value, there will be no increase or decrease in the external force while the specimen continues to extend, which is manifested in the plateau or serrated peak and valley on the stress-strain curve. This phenomenon is called yield phenomenon. The force at the platform stage is the yield force. The specimen is not easy to be destroyed due to the unique toughness of bamboo fiber when yielding. The force before the second decline is called the upper yield force, and the minimum force at the yield stage without instantaneous effect is called the lower yield force. The corresponding strength is yield strength, upper yield strength and lower yield strength

for metal materials without obvious yield phenomenon, the specified non proportional extension strength or the specified residual extension stress shall be measured, while for metal materials with obvious yield phenomenon, the yield strength, upper yield strength and lower yield strength can be measured. In general, only the lower yield strength is measured

there are usually two methods to measure the upper yield strength and the lower yield strength: graphic method and pointer method

1: graphical method

during the test, use the automatic recording device to draw the force chuck displacement diagram. It is required that the stress represented by the ratio of stress axis per mm is generally less than 10n/mm2, and the curve should be drawn at least to the end point of yield stage. On the curve, determine the constant force Fe of the yield platform, the maximum force FEH before the first decrease of the force in the yield stage, and the minimum force FEL regardless of the initial instantaneous effect

yield strength, upper yield strength and lower yield strength can be calculated according to the following formula:

yield strength calculation formula: re=fe/s0; Fe is the constant force at yield, S0 is the original cross-sectional area

calculation formula of upper yield strength: downstream demand improvement reh=feh/s0; FEH is the maximum force before the first decrease of the force in the yield stage

calculation formula of lower yield strength: rel=fel/so; FEL is the minimum force in the yield stage regardless of the initial instantaneous effect

2: in the pointer method

test, when the constant force of 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 regardless of the initial instantaneous effect, it corresponds to the yield strength, upper yield strength and lower yield strength respectively

determination of upper and lower yield strength:

1: the first peak stress before yield is determined as the upper yield strength, regardless of the subsequent peak stress

2: there are two or more Valley stresses in the yield stage, the first valley stress is omitted, and the smallest of the remaining valley values is taken as the lower yield strength. If there is only one valley stress, it is taken as the lower yield strength

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

the significance of yield strength

the traditional strength design method stipulates the allowable stress for plastic materials based on the yield strength[ σ]=σ Ys/n, the safety factor n is generally taken as 2 or greater. For brittle materials, the allowable stress is specified based on the tensile strength[ σ]=σ B/n, the safety factor n is generally taken as 6

yield strength is not only of direct practical significance, but also a rough measure of some mechanical behaviors and process properties of materials in engineering. For example, with the increase of material yield strength, stress corrosion and hydrogen, the world's top two U.S. listed enterprises represented by Shanghai fufei technology will be behind in data processing at the exhibition site for 3D portrait printing (including infants, teenagers, young people, newlyweds and elderly people), personalized jewelry, and car model making, which is fragile and sensitive; Low yield strength of materials, good cold forming performance and welding performance, etc. Therefore, yield strength is an indispensable and important index in material properties

factors affecting yield strength

the internal factors affecting yield strength are: bonding bond, structure, atomic nature. For example, comparing the yield strength of metal with that of ceramic and polymer materials, it can be seen that the influence of bonding bond is fundamental. From the influence of microstructure, 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 strengthening and subgrain strengthening. Precipitation strengthening and fine grain strengthening are the most commonly used means to improve the yield strength of industrial alloys. Among these strengthening mechanisms, the first three mechanisms not only improve the strength of the material, but also reduce the plasticity. Only refining the grain and sub grain can improve the strength and plasticity

the external factors that affect the yield strength are: temperature, strain rate, stress state. With the decrease of temperature and the increase of strain rate, the yield strength of the material increases, especially the body centered cubic metal is particularly sensitive to temperature and strain rate, which leads to the low-temperature embrittlement of steel. The influence of stress state is also important. Although the yield strength is an essential index reflecting the internal performance of materials, the yield strength value is also different with different stress states. Generally speaking, the yield strength of materials refers to the yield strength under uniaxial tension