Limits of fluidity of substances. How to determine the yield strength

The yield point is the stress corresponding to the remaining elongation after the load is removed. The determination of this quantity is necessary for the selection of metals used in production. If you do not take into account the parameter in question, then this can lead to an intensive deformation development process in an incorrectly selected material. It is very important to take into account the yield stresses in the design of various metal structures.

Physical Characteristics

Yield strength refers to the strength indicators. They represent a macroplastic deformation with rather small hardening. Physically, this parameter can be represented as a material characteristic, namely, a voltage that corresponds to a lower value of the yield point in the material stretch graph (diagram). The same can be represented in the form of the formula: σ _T = P _T / F ₀ , where P _T stands for the yield stress, and F ₀ corresponds to the initial cross-sectional area of the sample in question. PT sets the so-called boundary between the elastic-plastic and elastic zones of material deformation. Even a slight increase in voltage (above the PT) will cause a significant deformation. The yield strength of metals is usually measured in kg / mm ² or N / m ² . The value of this parameter is influenced by various factors, for example, the heat treatment regime, 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 point varies considerably with temperature. Let us consider an example of the practical value of this parameter.

Pipe yield strength

The most obvious is the influence of this value in the construction of pipelines of high pressure systems. In such designs, special steel should be used, which has sufficiently large yield points, as well as minimum values of the gap between this parameter and the tensile strength. The more steel has a limit, the naturally higher the permissible value of the operating voltage. This fact has a direct effect on the strength of steel, and, accordingly, the entire structure as a whole. Due to the fact that the parameter of the permissible design value of the stress system has a direct effect on the required wall thickness in the pipes used, it is important to calculate the strength characteristics of the steel that will be used in the pipe production as accurately as possible. One of the most authentic methods for determining these parameters is to conduct research on a discontinuous sample. In all cases, it is required to take into account the difference in the values of the index 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 carrying out detailed reusable measurements. But the system of permissible stresses in the overwhelming majority is taken on the basis of standards or, in general, as a result of technical conditions, as well as relying on the personal experience of the manufacturer. In the systems of main pipelines, the entire regulatory collection is described in SNiP II-45-75. So, setting the safety factor is quite a complicated and very important practical task. The correct definition of this parameter depends entirely on the accuracy of the calculated values of voltage, load, and also the yield strength of the material.

When choosing the thermal insulation of piping systems 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 participate in electrochemical processes that adversely affect the pipeline condition.

Stretching of materials

The yield stress at tension determines at what value the stress remains unchanged or decreases, in spite of the elongation. That is, this parameter will reach a critical level when there is a transition from the elastic to the plastic deformation region of the material. It turns out that the yield strength can be determined by testing the rod.

Calculation of PT

In the material resistance, the yield point 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 time of the onset of irreversible deformation. The same method is used in experiments with torsion of tubular samples to determine the shear yield strength. For most materials this index is determined by the formula σ _T = τ _s √3. In some specimens, the continuous elongation of a cylindrical specimen on the diagram of the dependence of normal stresses on the relative elongation leads to the detection of a so-called yielding tooth, i.e., a sharp drop in stress prior to the formation of plastic deformation.

Moreover, further growth of such distortion to a certain value occurs at a constant voltage, which is called a physical FET. If the flow area (horizontal section of the graph) has a long extension, then such a material is called ideal-plastic. If the diagram does not have a platform, then the samples are called hardening. In this case, it is impossible to specify exactly the value at which plastic deformation occurs.

What is the conditional yield strength?

Let's see what kind of parameter it is. In those cases where the stress diagram does not have significant areas, it is required to determine the conditional FET. So, this is the value of the voltage at which the relative residual strain is 0.2 percent. To calculate it on the stress diagram along the axis of determination ε, it is necessary to postpone a value of 0.2. From this point a straight line parallel to the initial segment is drawn . As a result, the point of intersection of the straight line with the chart line determines the value of the conditional yield strength for a particular material. This parameter is also called technical PT. In addition, the conditional yield limits for torsion and bending are separately distinguished.

Melt Flow

This parameter determines the ability of molten metals to fill linear forms. Melt flowability for metal alloys and metals has its own term in the metallurgical industry - fluidity. In fact, this is the inverse of the dynamic viscosity. The international system of units (SI) expresses fluid fluidity in Pa ^-1 * s ^-1 .

Temporary tensile strength

Let's look at how this characteristic of mechanical properties is determined. Strength refers to the ability of a material under certain limits and conditions to perceive various effects without breaking down. Mechanical properties are usually determined using conditional stretch diagrams. For testing, standard samples should be used. The test instruments are equipped with a device that records the diagram. Increasing the loads above the norm causes significant plastic deformation in the product. The yield point and the temporary tensile strength correspond to the greatest load, preceding the complete destruction of the sample. In plastic materials, deformation centers on 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 slides in the material, a high dislocation density is formed, and so-called embryonic discontinuities arise. Because of their enlargements, pores appear in the sample. Fusing with each other, they form cracks that propagate in the transverse direction to the extension axis. And at a critical moment the sample is completely destroyed.

What is PT for reinforcement?

These products are an integral part of reinforced concrete, intended, as a rule, for resistance to tensile forces. Usually use steel reinforcement, but there are exceptions. These products must work together with the mass of concrete at all stages of loading of this structure without exception, and possess plastic and durable properties. And also meet all conditions of industrialization of these types of work. The mechanical properties of steel used in the manufacture of reinforcement are determined by the appropriate GOST and technical conditions. GOST 5781-61 provides for four classes of these products. The first three are designed for conventional designs, as well as non-tensioning rods for prestressed systems. The yield strength of the reinforcement, depending on the class of the product, can reach 6000 kg / cm ² . So, in the first class this parameter is about 500 kg / cm ² , for the second - 3000 kg / cm ² , for the third 4000 kg / cm ² , and for the fourth - 6000 kg / cm ² .

Yield strength of steels

For long products in the basic version of GOST 1050-88, the following values of FP are provided: grade 20 - 25 kgs / mm ² , grade 30 - 30 kgs / mm ² , grade 45 - 36 kgs / mm ² . 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, the grade 30 steel will have a PT in the amount of 30 to 41 kgf / mm ² , and 45 grades in the range of 38-50 kgf / mm ² .

Conclusion

When designing various steel structures (buildings, bridges, etc.), the yield strength is used as an index of the strength standard when calculating the values of the permissible loads according to the specified safety factor. But for vessels under pressure, the permissible load is calculated on the basis of PT, as well as the tensile strength, taking into account the specification of operating conditions.