What is yield strength
What is yield strength?
Yield strength is the yield limit of a metallic material when the yielding phenomenon occurs, that is, the stress to resist a small amount of plastic deformation. For the metal material without obvious yielding, the stress value of 0.2% residual deformation is specified as its yield limit, called the conditional yield limit or yield strength. Greater than this limit of the external force, will make the part permanent failure, can not be recovered. Such as mild steel yield limit of 207MPa, when greater than this limit of external force, the part will produce permanent deformation, less than this, the part will also restore the original appearance.
Some steels (such as high-carbon steel) have no obvious yielding phenomenon, and the stress at which a small amount of plastic deformation (0.2%) occurs is usually used as the yield strength of the steel, called the conditional yield strength.
First, explain the material deformation by force. Material deformation is divided into elastic deformation (the original shape can be restored after the withdrawal of external forces) and plastic deformation (the original shape cannot be restored after the withdrawal of external forces, the shape changes, elongation or shortening).
Construction steel uses yield strength as the basis for design stress.
Yield limit, commonly used symbol σs, is the critical stress value of material yielding.
- (1) For materials with obvious yielding phenomenon, the yield strength is the stress at the yield point (yield value).
- (2) For materials where the yielding phenomenon is not obvious, the limit deviation from the linear relationship with stress-strain reaches a specified value (usually 0.2% elongation of the material occurs) when the stress. Usually used as an indicator for evaluating the mechanical and mechanical properties of solid materials, it is the practical use limit of the material. Because plastic deformation occurs after the stress exceeds the yield limit of the material, the strain increases, causing the material to fail and cannot be used normally.
When the stress exceeds the elastic limit, the deformation increases faster after entering the yielding stage, at which time, in addition to producing elastic deformation, it also produces partial plastic deformation. When the stress reaches the B point, the plastic strain increases sharply, and there are small fluctuations in stress, a phenomenon known as yielding. The maximum and minimum stresses at this stage are called the upper yield point and lower yield point, respectively. As the value of the lower yield point is more stable, so it is used as an indicator of material resistance, known as the yield point or yield strength (ReL or Rp0.2).
a. yieldpointyieldpoint (σs)
The stress when the specimen can continue to elongate (deformation) without an increase in force (remain constant) during the test.
b. Upper yield point upperyieldpoint (σsu)
The maximum stress before the specimen yields and the force drops for the first time.
c. Lower yield point loweryieldpoint (σsL)
When not counting the initial transient effect of the minimum stress in the yielding phase.
Some steels (such as high-carbon steel) without significant yielding phenomenon, usually a small amount of plastic deformation (0.2%) when the stress as the yield strength of the steel, known as the conditional yield strength.
By yielding, it means that after reaching a certain deformation stress, the metal begins to transition from the elastic state to the elastic-plastic state non-uniformly, which marks the beginning of macroscopic plastic deformation.
Types of yield strength
- (1): Silverman yielding: Silver stripe phenomenon with stress whitening.
- (2): Shear yielding.
Determination of yield strength
Metal materials without obvious yielding phenomenon need to measure their specified non-proportional elongation strength or specified residual elongation stress, while metal materials with obvious yielding phenomenon can measure their yield strength, upper yield strength, and lower yield strength. In general, only the lower yield strength is measured.
There are two usual methods for determining the upper yield strength and lower yield strength: the graphical method and the pointer method.
The force-chuck displacement graph is drawn with an automatic recording device during the test. The force axis is required to be scaled to the stress represented by each mm is generally less than 10N/mm2, and the curve should be plotted at least to the end point of the yielding stage. The force Fe at which the yielding plateau is constant, the maximum force Feh before the force first drops in the yielding phase, or the minimum force FeL less than the initial instantaneous effect are determined on the curve.
The yield strength, upper yield strength, and lower yield strength can be calculated according to the following equations.
- Yield strength calculation formula: Re = Fe/So; Fe is the constant force at yielding.
- Upper yield strength calculation formula: Reh = Feh/So; Feh is the maximum force before the force first drops in the yielding stage.
- Lower yield strength calculation formula: ReL=FeL/So; FeL is the minimum force FeL less than the initial instantaneous effect.
During the test, the constant force when the pointer of the force measuring disc stops rotating for the first time or the maximum force before the pointer turns back for the first time or the minimum force less than the initial instantaneous effect corresponds to the yield strength, upper yield strength, and lower yield strength, respectively.
Criteria of yield strength
- 1. Proportional limit stress-strain curve in line with the linear relationship between the highest stress, the international often used σp, more than σp that the material began to yield. There are three kinds of yielding standards commonly used in construction projects.
- 2. The elastic limit of the specimen after loading and then unloading, in order not to appear residual permanent deformation as the standard, the material can be fully elastic recovery of the highest stress. International usually expressed in ReL. Stress exceeds ReL that the material begins to yield.
- 3. Yield strength to provide for the occurrence of a certain residual deformation as the standard, such as the usual 0.2% residual deformation of the stress as the yield strength, the symbol for Rp0.2.
The influence of yield strength factors
The intrinsic factors affecting yield strength are: bonding, organization, structure, and atomic nature.
As comparing the yield strength of metals with ceramics and polymers it can be seen that the influence of bonding is fundamental. From the influence of organization, there can be four strengthening mechanisms affecting the yield strength of metallic materials, which are.
- (1) solid solution strengthening.
- (2) deformation strengthening.
- (3) precipitation strengthening and dispersion strengthening.
- (4) grain boundary and subcrystalline strengthening.
Precipitation strengthening and fine-grain strengthening are the most common means of improving the yield strength of materials in industrial alloys. Of these strengthening mechanisms, the first three increase the strength of the material while also reducing plasticity, and only fine-grained and subcrystalline, which can increase both strength and plasticity.
The extrinsic factors affecting yield strength are: temperature, strain rate, and stress state.
As the temperature decreases and the strain rate increases, the yield strength of the material increases, especially the body-centered cubic metal is particularly sensitive to temperature and strain rate, which leads to low-temperature embrittlement of steel. The effect of the stress state is also important. Although yield strength is an essential indicator of the intrinsic properties of a material, the value of yield strength varies with the state of stress. By yield strength of a material we generally refer to the yield strength in one-way tension.
The engineering significance of yield strength
The traditional strength design method, for plastic materials, the yield strength as the standard, the provisions of the allowable stress [σ] = σys / n, safety factor n depending on the occasion can be from 1.1 to 2 or greater, for brittle materials, the tensile strength as the standard, the allowable stress [σ] = σb / n, safety factor n is generally taken as 6.
It should be noted that, according to the traditional strength design method, it will inevitably lead to the one-sided pursuit of high yield strength of the material, but as the yield strength of the material increases, the material’s resistance to brittle fracture strength is decreasing, and the risk of brittle fracture of the material has increased.
Yield strength not only has direct significance in use, but also in engineering is a general measure of certain mechanical behavior and process properties of the material. For example, the material yield strength increases, sensitive to stress corrosion and hydrogen embrittlement; material yield strength is low, cold working molding performance and welding performance is good, etc.. Therefore, yield strength is an important indispensable index of material properties.
Source: China Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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