What is Yield Strength?
The yield strength of steel refers to the stress level at which plastic deformation begins in the steel under tensile or compressive loading. In the tensile test, the stress-strain curve of the steel shows an obvious linear segment, called the elastic stage, after which the curve begins to bend and enters the plastic stage. Yield strength refers to the point on this curve, that is, the stress value at which the steel begins to undergo sustained plastic deformation.
Detailed Explanation of Yield Strength
Yield strength, also known as yield limit, commonly used symbol δs, is the critical stress value for material yield.
●For materials with obvious yield phenomenon, the yield strength is the stress at the yield point (yield value);
●For materials where the yield phenomenon is not obvious, the stress when the limit deviation of the linear relationship between stress and strain 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 necking occurs after the stress exceeds the yield limit of the material, the strain increases, causing the material to be damaged and unable to 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, some plastic deformation also occurs. When the stress reaches point B, the plastic strain increases sharply and the stress and strain fluctuate slightly. This phenomenon is called yielding. The terms upper yield point and lower yield point refer to the highest and lowest strains at this location, respectively. Since the value of the lower yield point is relatively stable, it is used as an indicator of material resistance, called the yield point or yield strength (ReL or Rp0.2).
Some steels (such as high carbon steel) have no obvious yield phenomenon. 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.
First, explain the force deformation of the material. The deformation of materials is divided into elastic deformation (the original shape can be restored after the external force is removed) and plastic deformation (the original shape cannot be restored after the external force is removed, and the shape changes, elongates or shortens).
Standards for Yield Strength
There are three commonly used yield standards in construction projects:
●Proportional ultimate stress–the highest stress on the strain curve that conforms to a linear relationship. It is often expressed by σp internationally. When σp is exceeded, the material is considered to begin to yield.
●Elastic limit specimen is loaded and then unloaded. The maximum stress at which the material can fully elastically recover is based on the criterion that no residual permanent deformation occurs. Internationally it is usually represented by Rel. When the stress exceeds Rel, the material is considered to begin to yield.
●The yield strength is based on the specified residual deformation. For example, the stress of 0.2% residual deformation is usually used as the yield strength, and the symbol is Rp0.2.
Factors Affecting Yield Strength
Internal Factors
1. Grain Size and Grain Boundaries:
The size of grains and grain boundaries have an impact on the yield strength of the material. Typically, materials with smaller grain sizes and more grain boundaries have higher yield strengths because grain boundaries can hinder the movement of dislocations, thereby increasing the strength of the material.
2. Lattice Defects:
Lattice defects include point defects (such as vacancies, impurities, etc.) and line defects (such as dislocations, etc.). The presence of lattice defects reduces the yield strength of the material because they may serve as starting points for dislocations, making the material susceptible to plastic deformation.
3. Alloying Element Content:
The addition of alloying elements can change the lattice structure and strength characteristics of the material. Usually, the addition of alloying elements will increase the yield strength of the material. For example, adding carbon elements can increase the yield strength of steel.
4. Impurity Content:
The presence of impurities will affect the lattice structure and performance characteristics of the material, thereby affecting the yield strength of the material. Typically, the lower the impurity content, the higher the yield strength of the material.
5. Smelting Process:
The smelting process has an important impact on the grain structure and organizational morphology of the material, which in turn affects the yield strength of the material. Reasonable smelting process can obtain good lattice structure and organizational form, thereby improving the yield strength of the material.
External Factors
1. Temperature:
Temperature is one of the important external factors that affects the yield strength of materials. Generally, as the temperature increases, the yield strength of the material decreases. This is because high temperatures will increase the vibration of atoms or ions in the material, thereby reducing the crystallization strength of the material.
2. Strain Rate:
Strain rate refers to the deformation rate of a material when loaded. High strain rates generally cause the material’s yield strength to increase, whereas low strain rates cause the yield strength to decrease. This is because high-speed loading increases the dislocation density in the material, thereby increasing the yield strength.
3. Humidity and Corrosion:
Humidity and corrosion environment will affect the surface state and chemical composition of the material, thereby affecting the yield strength of the material. Especially for metallic materials, humidity and corrosion will accelerate the corrosion fatigue and hydrogen embrittlement of the material, resulting in a reduction in yield strength.
4. Loading Direction:
The yield strength of a material usually changes with the loading direction. Under unidirectional loading conditions, the yield strength of a material may change with the loading direction, especially for anisotropic materials.
5. Prestress:
Prestress refers to the static stress exerted on the material before loading. Appropriate prestressing can increase the yield strength of the material, giving it a higher load-bearing capacity.
6.Environmental Conditions:
Environmental conditions, such as oxygen content, radiation, etc., can also affect the yield strength of materials. Especially under extreme environmental conditions, such as high altitude, underwater or radioactive environments, the yield strength of materials may be severely affected.
What are the consequences of exceeding the yield strength of steel structure?
1. Increased Plastic Deformation:
After exceeding the yield strength, the steel structure will enter the plastic stage, resulting in increased plastic deformation. This means that the structure will undergo large deformations, which may lead to structural instability or failure.
2. Increased Deformation:
After exceeding the yield strength, the deformation of the structure will increase, which may cause the deflection and deformation of the structure to exceed the design requirements and affect the normal use of the structure.
3. Strength Loss:
After exceeding the yield strength, the strength of the steel may decrease, making the structure prone to failure or collapse under subsequent loads.
4. Local Instability:
After exceeding the yield strength, instability may occur in local parts of the steel structure, such as buckling, buckling instability or buckling-torsion instability, threatening the stability of the entire structure.
5.Cracks and Damage:
After the yield strength is exceeded, cracks may occur in the structure, resulting in local damage or overall failure. In this case, the safety and reliability of the structure will be seriously affected.
How to Control the Yield Strength of Steel in Structure Design?
1. Appropriate Material Selection:
Selecting the appropriate steel material is the first step in controlling the yield strength of the beam steel. According to the design requirements and structural load, steel materials with appropriate yield strength are selected to ensure that the structure can meet the strength requirements under normal use and ultimate states.
2. Control the Cross-Section Size:
Control the yield strength of the beam through appropriate cross-section size design. Increasing the cross-sectional size of a beam can increase its load-bearing capacity and yield strength, allowing it to withstand greater loads.
3. Control the Span and Support Conditions of the Beam:
The span and support conditions of the beam have an important impact on its yield strength. By rationally designing the span and support conditions of the beam, the deflection and stress concentration of the beam can be reduced, thereby controlling its yield strength.
4. Consider Load Combinations:
Consider various load combinations in the design, including permanent loads, variable loads, seismic loads, etc., to ensure that the yield strength of the beam can meet the requirements under various working conditions.
5. Proper Connection Design:
The design of connections between beams and supports, columns, and other components also affects their yield strength. Ensure that the strength and stiffness of the connectors can meet the design requirements to avoid local instability or yield failure.
6. Strict Quality Control:
During the manufacturing and installation process of cross beams, the quality of materials and processing techniques are strictly controlled to ensure that the actual strength of the cross beams matches the design requirements.
