1. Yield criteria
Three yield criteria commonly used in engineering are:
(1) Proportional Limit – The greatest stress that maintains a linear relationship in the stress-strain curve, represented internationally as σp. The material is considered to begin to yield when the stress exceeds σp.
(2) Elastic Limit – After loading and unloading a test sample, the default is no residual permanent deformation. The highest stress at which the material can recover fully elastically is commonly represented as σel internationally. The material is considered to begin to yield when the stress exceeds σel.
(3) Yield strength – The standard is a specific residual strain, such as 0.2% residual strain stress considered as yield strength, symbolized as σ0.2 or σys.
2. Factors affecting yield strength
Intrinsic factors that affect yield strength include:
Bonding, microstructure, structure, atomic properties. Comparing the yield strength of metal to ceramics and polymers demonstrates the fundamental impact of bonding.
From the point of view of microstructural influences, four reinforcement mechanisms can affect the yield strength of metallic materials:
(1) Strengthening solid solutions;
(2) Strain hardening;
(3) Precipitation strengthening and dispersion strengthening;
(4) Grain boundary and subgrain reinforcement.
Precipitation strengthening and grain refinement are the most common methods for improving the yield strength in industrial alloys. Among these reinforcement mechanisms, the first three decrease plasticity while improving the strength of the material. Only refining grains and subgrains can increase strength and plasticity.
Extrinsic factors that affect yield strength include:
Temperature, strain rate, stress state. As the temperature decreases and the strain rate increases, the yield strength of the material increases. Body-centered cubic metals are particularly sensitive to temperature and strain rate, leading to the phenomenon of steel brittleness at low temperatures.
The effect of stress is also significant. Although the yield strength reflects a fundamental property of the material, different stress states will result in different yield strengths. Typically, when we refer to the yield strength of a material, we are referring to its yield strength under unidirectional stress.
3. Engineering significance of yield strength
Traditional strength design methods use the yield strength as the standard for plastic materials, setting the allowable stress (σ)=σys/n, where the safety factor n is typically 2 or greater. For brittle materials, tensile strength is used as a standard by defining the allowable stress (σ)=σb/n, where the safety factor n is normally 6.
It is important to note that following traditional strength design methods will inevitably lead to an over-emphasis on materials with high yield strengths. However, as the yield strength of the material increases, the fracture toughness of the material decreases, increasing the risk of brittle fracture.
The yield strength not only has direct application significance, but also approximately measures certain mechanical behaviors and process performance of materials in engineering.
For example, an increase in the yield strength of the material makes it more sensitive to stress corrosion cracking and hydrogen embrittlement. If a material's yield strength is low, it will have better cold forming and welding properties. Therefore, the yield point is an indispensable key indicator of material properties.
Once a material begins to yield, continued deformation will cause work hardening.
4. Practical significance of the work hardening index n
The work hardening index n reflects the strain hardening of a material after it begins to yield and continues to deform, determining the maximum stress when necking begins to occur. n also determines the maximum uniform deformation a material can produce, a crucial value in cold forming processes.
For functional parts, it is also necessary for the materials to have certain hardening capabilities.
Otherwise, under occasional overloads, excessive plastic deformation will occur, potentially resulting in local irregular deformation or fracture.
Therefore, the hardening capacity of a material is a reliable guarantee for the safe use of parts.
Strain hardening is an essential means of increasing material strength. Stainless steel has a large hardening index n = 0.5, resulting in a high amount of uniform deformation.
Although the yield strength of stainless steel is not high, it can be significantly improved through cold deformation. High-carbon steel wire, after treatment and isothermal drawing with a lead bath, can reach above 2,000 MPa.
However, traditional stress strengthening methods can only increase strength and at the same time significantly reduce plasticity. In some new materials under development, it is noted that changes in the microstructure and its distribution can improve both resistance and plasticity during deformation.
5. Tensile strength
Tensile strength represents the resistance to fracture when materials do not exhibit necking. When brittle materials are used in product design, their allowable stress is based on tensile strength. What does tensile strength mean for plastic materials in general?
Although tensile strength represents only the maximum resistance to uniform plastic deformation, it indicates the ultimate load-bearing capacity of the material under static stress. The external load corresponding to the tensile strength σb is the maximum load that the sample can withstand.
Although the narrowing is continually developing and the actual tension is increasing, the external load is decreasing rapidly.
The work consumed per unit volume of material from deformation to fracture under static stress is called static toughness. Strictly speaking, it should be the area under the true stress-strain curve.
For engineering simplicity, it is approximated as: For plastic materials, static toughness is a comprehensive indicator of strength and plasticity.
Pure high-strength materials such as spring steel do not have high static toughness, and low-carbon steel with good plasticity also does not have high static toughness.
Only high-temperature quenched and tempered medium-carbon (alloy) structural steel has the highest static toughness.
Hardness is not an independent basic property of metals. It refers to the ability of a metal to resist deformation or fracture at its surface within a small volume.
