Yield stress is the stress corresponding to the residual value of elongation after the load is removed. The determination of this value is necessary for the selection of metals used in production. If this parameter is not taken into account, this can lead to an intensive process of deformation development in an incorrectly selected material. It is very important to consider yield strengths when designing various metal structures.
Physical characteristics
Yield strength refers to strength indicators. They represent a macroplastic deformation with a rather small hardening. Physically, this parameter can be represented as a characteristic of the material, namely: stress, which corresponds to the lower value of the yield point in the graph (diagram) of the stretching of materials. This can also be represented as a formula: σT=PT/F0, where P T means the yield stress load, and F0 corresponds to the origin althe cross-sectional area of the sample under consideration. PT establishes the so-called boundary between the elastic-plastic and elastic deformation zones of the material. Even a slight increase in stress (above DC) will cause significant deformation. The yield strength of metals is usually measured in kg/mm2 or N/m2. The value of this parameter is influenced by various factors, for example, the heat treatment mode, the thickness of the sample, the presence of alloying elements and impurities, the type, microstructure and defects of the crystal lattice, and so on. The yield strength changes significantly with temperature. Consider an example of the practical meaning of this parameter.
Yield strength of pipes
The most obvious is the influence of this value in the construction of pipelines of high-pressure systems. In such structures, special steel should be used, which has sufficiently large yield strengths, as well as minimal gap values between this parameter and the tensile strength. The greater the limit of steel, the higher, naturally, should be the indicator of the permissible value of the operating voltage. This fact has a direct impact on the value of the strength of steel, and, accordingly, the entire structure as a whole. Due to the fact that the parameter of the allowable design value of the stress system has a direct impact on the required value of the wall thickness in the pipes used, it is important to calculate as accurately as possible the strength characteristics of the steel that will be used in the manufacturepipes. One of the most authentic methods for determining these parameters is to conduct a study on a discontinuous sample. In all cases, it is required to take into account the difference between the values of the indicator under consideration, on the one hand, and the allowable stress values, on the other.
In addition, you should know that the yield strength of the metal is always set as a result of detailed reusable measurements. But the system of permissible voltages is overwhelmingly adopted on the basis of standards or in general as a result of the technical conditions carried out, as well as based on the personal experience of the manufacturer. In trunk pipeline systems, the entire regulatory collection is described in SNiP II-45-75. So, setting the safety factor is a rather complicated and very important practical task. The correct determination of this parameter entirely depends on the accuracy of the calculated values of stress, load, and the yield strength of the material.
When choosing thermal insulation for piping systems, they also rely on this indicator. This is due to the fact that these materials directly come into contact with the metal base of the pipe, and, accordingly, can take part in electrochemical processes that adversely affect the condition of the pipeline.
Stretching materials
Tensile yield strength determines the amount at which stress will remain the same or decrease despite elongation. That is, this parameter will reach a critical point when there is a transition from elastic toplastic deformation region of the material. It turns out that the yield strength can be determined by testing the rod.
Fri calculation
In the resistance of materials, the yield strength is the stress at which plastic deformation begins to develop. Let's look at how this value is calculated. In experiments carried out with cylindrical samples, the value of the normal stress in the cross section is determined at the moment of occurrence of irreversible deformation. Using the same method in experiments with torsion of tubular samples, the shear yield strength is determined. For most materials, this indicator is determined by the formula σT=τs√3. In some instances, continuous elongation of a cylindrical sample in a normal stress vs. elongation diagram results in the discovery of a so-called yield tooth, i.e. a sharp decrease in stress before plastic deformation occurs.
Moreover, further growth of such distortion to a certain value occurs at a constant voltage, which is called a physical FET. If the yield area (horizontal section of the graph) has a large extent, then such a material is called ideally plastic. If the diagram does not have a platform, then the samples are called hardening. In such a case, it is impossible to accurately specify the value at which plastic deformation will occur.
What is the conditional yield strength?
Let's figure out what this parameter is. In cases where the stress diagram does not have pronounced areas, it is required to determine the conditional FET. So, this is the stress value at which the relative permanent strain is 0.2 percent. To calculate it on the stress diagram along the definition axis ε, it is necessary to set aside a value equal to 0, 2. A straight line is drawn from this point, parallel to the initial section. As a result, the point of intersection of the straight line with the line of the diagram determines the value of the conditional yield strength for a particular material. This parameter is also called technical PT. In addition, conditional yield strengths in torsion and bending are separately distinguished.
Melt flow
This parameter determines the ability of molten metals to fill linear shapes. Melt fluidity for metal alloys and metals has its own term in the metallurgical industry - fluidity. In fact, this is the reciprocal of dynamic viscosity. The International System of Units (SI) expresses the fluidity of a fluid in Pa-1c-1.
Temporary tensile strength
Let's look at how this characteristic of mechanical properties is determined. Strength is the ability of a material, under certain limits and conditions, to perceive various influences without collapsing. Mechanical properties are usually determined using conditional tension diagrams. For testing, standardsamples. Test instruments are equipped with a device that records the diagram. Increasing loads in excess of the norm causes significant plastic deformation in the product. The yield strength and tensile strength correspond to the highest load that precedes the complete destruction of the sample. In ductile materials, the deformation is concentrated in one area, where a local narrowing of the cross section appears. It is also called the neck. As a result of the development of multiple slips, a high density of dislocations is formed in the material, and so-called nucleating discontinuities also arise. As a result of their enlargement, pores appear in the sample. Merging with each other, they form cracks that propagate in the transverse direction to the tension axis. And at the critical moment, the sample is completely destroyed.
What is a rebar PT?
These products are an integral part of reinforced concrete, intended, as a rule, to resist tensile forces. Usually steel reinforcement is used, but there are exceptions. These products must work together with the mass of concrete at all stages of loading this structure, without exception, and have plastic and durable properties. And also meet all the conditions of industrialization of these types of work. The mechanical properties of the steel used in the manufacture of fittings are established by the relevant GOST and technical conditions. GOST 5781-61 provides for four classes of these products. The first three are intended for conventional structures, as well as non-stressed bars at pre-stressed systems. The yield strength of reinforcement, depending on the class of the product, can reach 6000 kg/cm2. So, for the first class, this parameter is approximately 500 kg/cm2, for the second - 3000 kg/cm2, for the third 4000 kg/cm 2, while the fourth has 6000 kg/cm2.
Yield strength of steels
For long products in the basic version of GOST 1050-88, the following PT values are provided: grade 20 - 25 kgf/mm2, grade 30 - 30 kgf/mm 2, brand 45 - 36 kgf/mm2. However, for the same steels, manufactured by prior agreement between the consumer and the manufacturer, the yield strengths may have different values (the same GOST). So, grade 30 steel will have a PT in the amount of 30 to 41 kgf/mm2, and grade 45 will be in the range of 38-50 kgf/mm2.
Conclusion
When designing various steel structures (buildings, bridges, etc.), the yield strength is used as an indicator of the strength standard when calculating the values of allowable loads according to the specified safety factor. But for pressure vessels, the value of the allowable load is calculated on the basis of PT, as well as tensile strength, taking into account the specification of operating conditions.
