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Yield strength image represents the yield limit of metal materials at the point when yield occurs, that is, the stress required to resist minor plastic deformation. For metal materials that do not exhibit a distinct yield phenomenon, the stress value at which 0.2% residual deformation occurs is defined as their yield limit, which is referred to as the conditional yield limit or yield strength. External forces greater than the yield strength will cause the part to fail permanently, with no possibility of recovery. For example, if the yield limit of low-carbon steel is 207 MPa, then under a force greater than this limit, the part will undergo permanent deformation; whereas if the force is less than this limit, the part will return to its original shape. (1) For materials with a distinct yield phenomenon, the yield strength is the stress at the yield point (yield value) ； (2) For materials in which yield does not occur clearly, the stress at which the maximum deviation from the linear stress-strain relationship reaches a specified value (usually 0.2% of the original gauge length). It is commonly used as an indicator to evaluate the mechanical properties of solid materials, representing the actual limit of use for those materials. Because necking occurs when the stress exceeds the material’s yield limit, the strain increases, leading to material failure and rendering it unusable. Image: When the stress exceeds the elastic limit and enters the yield stage, the deformation increases rapidly; at this point, in addition to elastic deformation, some plastic deformation also occurs. When the stress reaches point b, the plastic strain increases sharply, and there are slight fluctuations in stress-strain; this phenomenon is known as yield. The maximum and minimum stresses at this stage are referred to as the upper yield point and the lower yield point, respectively. Since the value of the lower yield point is relatively stable, it is used as an indicator of the material’s resistance, and is referred to as the yield point or yield strength (ReL or Rp0.2). Some steels (such as high-carbon steel) do not exhibit a distinct yield point; instead, the stress at which a slight amount of plastic deformation (0.2%) occurs is used as the yield strength of such steel, which is referred to as conditional yield strength. First, let’s explain the deformation of materials under stress. The deformation of materials is divided into elastic deformation (the material can return to its original shape once the external force is removed) and plastic deformation (the material cannot return to its original shape once the external force is removed; its shape changes, either by stretching or shrinking). Construction steel uses yield strength as the basis for design stress. The yield limit, commonly denoted by σs, is the critical stress value at which a material yields. (1) For materials with a distinct yield phenomenon, the yield strength is the stress at the yield point (yield value) ； (2) For materials in which the yield phenomenon is not apparent, it is the stress at which the maximum deviation from the linear stress-strain relationship reaches a specified value (usually when the material exhibits an elongation of 0.2%). It is commonly used as an indicator to evaluate the mechanical properties of solid materials, representing the actual limit of use for those materials. This is because plastic deformation occurs when the stress exceeds the material’s yield limit, resulting in an increase in strain that causes the material to fail and become unusable. Type 2 (1): Silver streak yielding: silver streaking phenomenon and stress whitening. (2): Shear yield. Yield strength determination: For metal materials that do not exhibit a distinct yield phenomenon, it is necessary to measure their specified non-proportional strain strength or specified residual elongation stress; whereas for metal materials that do show a clear yield phenomenon, it is possible to measure their yield strength, upper yield strength, and lower yield strength. Generally, only the lower yield strength is measured. Generally, there are two methods for determining the upper yield strength and the lower yield strength: the graphical method and the pointer method. Graphic method: During testing, an automatic recording device is used to plot the force-clamp displacement graph. It is required that the stress per mm represented by the force axis ratio be generally less than 10 N/mm2, and the curve must be plotted at least until 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 drop in force during the yield stage, or the minimum force FeL when the initial instantaneous effect has passed. Yield strength, upper yield strength, and lower yield strength can be calculated using the following formulas: Yield strength formula: Re=Fe/So ； Fe is the constant force at yield. Formula for upper yield strength: Reh = Feh / So ； Feh is the maximum force before the first decrease in force during the yield stage. Formula for calculating the lower yield strength: ReL=FeL/So ； FeL is the minimum force FeL that is less than the initial transient effect. In the pointer method, during testing, the constant force at which the pointer of the force gauge stops rotating for the first time, the maximum force before the pointer starts to move again, or the minimum force before the initial transient effect occurs correspond respectively to the yield strength, upper yield strength, and lower yield strength. 3 Standards 1: The highest stress at which a linear relationship exists on the stress-strain curve of the proportional limit is commonly denoted by σp internationally; once this value is exceeded, it is considered that the material has begun to yield. There are three commonly used yield criteria in construction projects: 2. The elastic limit is the highest stress at which a specimen can return to its original state completely elastically, based on the criterion that no residual permanent deformation occurs after loading and unloading the specimen. Internationally, it is usually denoted as ReL. The material is considered to begin yielding when the stress exceeds ReL. 3. The yield strength is defined by the occurrence of a specified amount of residual deformation; for example, the stress at 0.2% residual deformation is used as the yield strength, denoted as Rp0.2. 4 Influencing Factors The internal factors that affect yield strength include: bonding bonds, microstructure, structure, and atomic properties. By comparing the yield strength of metals with that of ceramics and polymer materials, it can be seen that the effect of bonding bonds is fundamental. From the perspective of the influence of the organizational structure, there are four strengthening mechanisms that can affect the yield strength of metal materials, namely: (1) solid solution strengthening ； (2) Strain strengthening ； (3) Precipitation strengthening and dispersion strengthening ； (4) Grain boundary and subgrain strengthening. Precipitation strengthening and fine-grain strengthening are the most commonly used methods in industrial alloys to increase the yield strength of materials. Among these strengthening mechanisms, the first three reduce plasticity while increasing the strength of the material; only grain and subgrain refinement can enhance both strength and plasticity. The external factors that affect 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. Body-centered cubic metals are particularly sensitive to temperature and strain rate, which leads to the low-temperature embrittlement of steel. The influence of the stress state is also important. Although yield strength is an essential indicator reflecting the intrinsic properties of a material, its value varies depending on the stress state. The yield strength of materials, as we usually refer to it, generally denotes the yield strength under uniaxial tension. 5 Engineering Significance: In traditional strength design methods, for plastic materials, the yield strength is used as a benchmark, with the allowable stress determined as σys/n; the safety factor n can range from 1.1 to 2 or even higher depending on the application. For brittle materials, the tensile strength is used as a benchmark, and the allowable stress is set at σb/n, with the safety factor n typically being 6. It should be noted that, according to traditional strength design methods, there is an inevitable tendency to focus solely on achieving high yield strength in materials. However, as the yield strength of materials increases, their resistance to brittle fracture decreases, thereby increasing the risk of brittle failure. Yield strength not only has direct practical significance but also serves as an approximate measure of certain mechanical properties and processing characteristics of materials in engineering. For example, an increase in the material’s yield strength makes it more sensitive to stress corrosion and hydrogen embrittlement ； Materials with low yield strength have good cold working formability and weldability, and so on. Therefore, yield strength is an essential and important indicator of material properties.