yield stress is called a voltage corresponding to the residual elongation after unloading.Determination of this quantity necessary for selection of metals used in manufacturing.If not taken into account the considered parameter, this can lead to extensive development deformation incorrectly chosen material.A very important consideration in the design flow limits of various metal structures.
The yield related to the indicators of strength.They represent macroplastic deformation with relatively low hardening.Physically, this parameter can be represented as a characteristic of the material, namely, a voltage which corresponds to the lower value of the yield plateau in the chart (diagram) stretching materials.This can also be represented as the formula: PT = ReH / F0, where PT is the yield point load and F0 corresponds to the original cross-sectional area of the treated sample.PT establishes a so-called boundary between the elastic-plastic deformation zones and elastic material.Even a slight increase in voltage (above PT) will cause significant deformation.The yield of metal is generally measured in kg / mm2 or N / m2.The magnitude of this parameter is influenced by various factors such as thermal treatment, the thickness of the sample, the presence of alloying elements and impurities, the type and microstructure lattice defects and other.Yield varies significantly with temperature.Consider the example of the practical value of this parameter.
The most obvious is the effect of this value in the construction of high-pressure piping systems.In such designs should be used special steel, which are large enough yield strength and minimum performance gap between this parameter and tensile strength.The higher limit of the steel, so naturally, the higher must be outside the allowable value of operating voltage.This has a direct impact on the value of strength of steel, and accordingly, the entire construction.Due to the fact that the option is valid the calculated value of the system voltage has a direct impact on the value of the necessary thickness of the walls of the tubes used, it is important to accurately calculate the maximum strength characteristics of steel that will be used in the manufacture of pipes.One of the most authentic methods for determining these parameters is to conduct research on a tensile specimen.In all cases it is required to take into account the difference of values of this indicator, on the one hand, and the valid values of stress - on the other.
In addition, you should know that the yield strength of the metal is always set as a result of detailed measurements reusable.But the system of allowable stress in the vast majority take on the basis of standards or even as a result of technical specifications, as well as relying on the personal experience of the manufacturer.The main pipeline system all standard collection is described in SNIP II-45-75.Thus, the installation safety factor - is quite complicated and very important practical problem.The correct definition of this parameter is entirely dependent on the accuracy of the calculated values of voltage, load, and the yield strength of the material.
When selecting insulation piping systems are also based on this figure.This is due to the fact that these materials directly come in contact with the metal base tube, and, accordingly, may take part in the electrochemical processes that adversely affect the condition of the pipeline.
tensile yield strength determines the value at which the voltage will remain unchanged or decline, in spite of the extension.That is the parameter reaches a critical point when the shift from an elastic to plastic deformation of the material field.It turns out that the yield point can be determined by testing rod.
The strength of materials yield point is the stress at which plastic deformation begins to develop.Let's look at how this value is calculated.In experiments conducted with the cylindrical samples, determine the value of the normal stress in a cross section at the time of the irreversible deformation.The same method in the experiments with the samples produced torsion tube defining the yield strength in shear.For most materials, the figure given by the formula ReH = τs√3.In some instances a continuous extension of the cylindrical sample in the graph depending on the normal stress elongation leads to the discovery of so-called tooth point, i.e. the sharp voltage drop to form a plastic deformation.
Moreover, further growth of such distortion occurs to a certain value at a constant voltage, which is called the physical Fri.If the yield plateau (horizontal portion of the graph) has a greater extent, such a material is called a perfectly plastic.If the chart has no grounds, the samples referred reinforcing.In this case, it is impossible to accurately specify the point at which plastic deformation occurs.
What is the yield strength?
Let's see what kind of option.In cases where the graph does not have a pronounced stress areas, need to define a conditional Fri.So this is the voltage at which the relative residual strain is 0.2 percent.To calculate the chart axis voltage determining ε should be delayed an amount equal to 0.2.From this point of a straight line parallel to the primary site.As a result, the intersection point with the line chart determines the value of yield strength for a particular material.Also, this option is called technical Fri.In addition, separately isolated yield strength in torsion and bending.
melt flow rate
This parameter determines the ability of molten metal to fill the linear forms.The melt flow rate for the metal and metal alloys has its term in the steel industry - fluidity.In fact, it is the inverse of the dynamic viscosity.The International System of Units (SI) expresses the fluidity in Pa-1 * s-1.
Let's look at how this is determined by the mechanical properties.The strength is the ability of the material under certain limits and conditions to take a variety of effects, not destroyed.Mechanical properties usually determined by means of conventional stress-strain diagram.For testing, use standard samples.Testers are equipped with a device that records a chart.Increased loads in excess of causing substantial plastic deformation in the product.Yield point and the tensile strength corresponds to the highest load, the total destruction of the previous sample.In plastic material deformation is concentrated on one area where there is a local constriction of the cross section.It is also called the neck.As a result of multiple slides in the material forms a high density of dislocations, as well as a so-called germ discontinuity.Because of their consolidations occur in the sample pores.Merging together, they form cracks that extend transversely to the axis of stretching.And at the critical moment the sample was completely destroyed.
What is PT for reinforcement?
These products are an integral part of the concrete, is meant as a rule, to resist tensile forces.Typically, a steel reinforcement, but there are exceptions.These products need to work together with the concrete mass at all stages of loading, without exception, this structure has ductile and strong properties.And also to meet all the conditions of industrialization of these types of operations.The mechanical properties of the steel used in the manufacture of valves, installed complies with GOST and technical conditions.GOST 5781-61 provides four classes of these products.The first three are intended for conventional designs, as well as Free of tension rods from pre-stressed systems.Yield strength of reinforcement depending on the class of the product can reach 6000 kg / cm2.Thus, in this first class parameter is approximately 500 kg / cm2, and at the second - 3000 kg / cm2, and a third 4000 kg / cm2, and the fourth - 6000 kg / cm2.
steels for long products in the basic version of GOST 1050-88 provides for the following values of PT and brands 20 - 25 kgf / mm2, mark 30 - 30 kgf / mm2, mark 45 - 36 kgf / mm2.However, for the same steel, manufactured by the prior consent of the consumer and the manufacturer, the yield strength can have different values (the same GOST).For example, steel grade 30 will have a PT in the amount of 30 to 41 kgf / mm2, and the mark 45 - within 38-50 kgf / mm2.
Conclusion When designing various steel structures (buildings, bridges, etc) yield strength is used as an indicator of the strength of the standard values in the calculations of allowable loads, respectively, said the safety factor.But for vessels under pressure, the load capacity rating is calculated based on the PT, as well as tensile strength, taking into account the specifications of the operating conditions.