Yield strength: It is the yield limit of a metal material when it yields, that is, the stress that resists a small amount of plastic deformation. For metal materials without obvious yield, it is stipulated that the stress value that produces 0.2% residual deformation is its yield limit, which is called conditional yield limit or yield strength. External force greater than this limit will cause the part to fail permanently and cannot be recovered. For example, the yield limit of the
low carbon steel seamless pipe is 207MPa. When the external force is greater than this limit, the part will be permanently deformed. If it is smaller than this, the part will return to its original state.
(1) For materials with obvious yield phenomenon, the yield strength is the stress at the yield point (yield value);
(2) For materials with insignificant yielding phenomena, the stress at which the limit deviation from the stress-strain linear relationship reaches a specified value (usually 0.2% of the original gauge length). It is usually used as an evaluation index for the mechanical and mechanical properties of solid materials, and is the actual use limit of the material. Because the necking occurs after the stress exceeds the yield limit of the material, the strain increases, and the material is damaged and cannot be used normally.
When the stress exceeds the elastic limit and enters the yield stage, the deformation increases rapidly. At this time, in addition to elastic deformation, partial plastic deformation also occurs. When the stress reaches point B, the plastic strain increases sharply, and there is a small fluctuation in the stress and strain, which is called yielding. The maximum and minimum stresses at this stage are called the upper and lower yield points, respectively. Since the value of the lower yield point is relatively stable, it is used as an indicator of material resistance, which is called yield point or yield strength (ReL or Rp0.2).
Some steels (such as high carbon steels) have no obvious yield phenomenon, and the stress when a small amount of plastic deformation (0.2%) occurs is usually used as the yield strength of the steel, which is called the conditional yield strength.
Construction steel takes the yield strength as the basis for the design stress.
The yield limit, commonly known as σs, is the critical stress value at which a material yields.
a. Yield point (σs)
The stress at which the specimen continues to stretch (deform) without increasing the force (remaining constant) during the test.
b. Upper yield point (σsu)
The maximum stress before the specimen yields and the force decreases for the first time.
c. Lower yield point (σsL)
The minimum stress in the yield phase when ignoring initial transient effects.
Yield strength calculation formula:
Re=Fe/So; Fe is the constant force at yield.
The upper yield strength calculation formula:
Reh=Feh/So; Feh is the maximum force before the first drop of the force in the yield stage.
Lower yield strength calculation formula:
ReL=FeL/So; FeL is the minimum force FeL less than the initial instantaneous effect.
