Tensile strength- this is the same as the temporary resistance of the material. But despite the fact that it is more correct to use the term temporary resistance, the concept of tensile strength has taken root better in technical colloquial speech. At the same time, in regulatory documentation and standards the term “temporary resistance” is used.
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Strength- this is the material’s resistance to deformation and destruction, one of the main mechanical properties. In other words, strength is the property of materials to withstand certain influences (loads, temperature, magnetic and other fields) without collapsing.
TO tensile strength characteristics include the modulus of normal elasticity, proportional limit, elastic limit, yield strength and tensile strength (tensile strength).
Tensile strength- this is the maximum mechanical stress above which destruction of the material subject to deformation occurs; tensile strength is designated σ B and is measured in kilograms of force per square centimeter (kgf/cm2), and is also indicated in megapascals (MPa).
There are:
- tensile strength,
- compressive strength,
- bending strength,
- torsional strength.
Short-term strength (MPa) determined using tensile tests, deformation is carried out until failure. Tensile tests are used to determine tensile strength, elongation, elastic limit, etc. Long-term strength tests are intended primarily to assess the possibility of using materials at high temperatures (long-term strength, creep); as a result, σ B/Zeit is determined - the limit of limited long-term strength for a given service life.
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Strength of metals
Physics of strength founded by Galileo: summarizing his experiments, he discovered (1638) that during tension or compression, the load of destruction P for a given material depends only on the cross-sectional area F. This is how a new physical quantity appeared - voltage σ=P/F- and the physical constant of the material: fracture stress.
Physics of destruction fundamental science of metal strength arose in the late 40s of the XX century; this was dictated by the urgent need to develop scientifically based measures to prevent the increasingly frequent catastrophic destruction of machines and structures. Previously, in the field of strength and destruction of products, only classical mechanics was taken into account, based on the postulates of a homogeneous elastic-plastic solid body, without taking into account the internal structure of the metal. The physics of destruction also takes into account the atomic-crystalline structure of the metal lattice, the presence of defects in the metal lattice and the laws of interaction of these defects with elements of the internal structure of the metal: grain boundaries, second phase, non-metallic inclusions, etc.
Great influence on material strength is influenced by the presence of surfactants in the environment that can be strongly adsorbed (moisture, impurities); the tensile strength decreases.
Targeted changes in the metal structure, including modification of the alloy, lead to an increase in the strength of the metal.
Educational film about the strength of metals (USSR, year of release: ~1980):
Metal tensile strength
Tensile strength of copper. At room temperature, the tensile strength of annealed technical copper is σ B = 23 kgf/mm 2. As the test temperature increases, the tensile strength of copper decreases. Alloying elements and impurities affect the tensile strength of copper in various ways, both increasing and decreasing it.
Aluminum tensile strength. Annealed aluminum of technical purity at room temperature has a tensile strength σ B = 8 kgf/mm 2. As the purity of aluminum increases, the strength of aluminum decreases and its ductility increases. For example, aluminum cast into the ground with a purity of 99.996% has a tensile strength of 5 kgf/mm 2. The tensile strength of aluminum decreases naturally as the test temperature increases. When the temperature drops from +27 to -269°C, the temporary resistance of aluminum increases - 4 times for technical aluminum and 7 times for high-purity aluminum. Alloying increases the strength of aluminum.
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Ultimate strength of steels
As an example, the tensile strength values of some steels are presented. These values are taken from state standards and are recommended (required). The actual values of the tensile strength of steels, as well as cast irons, as well as other metal alloys, depend on many factors and must be determined, if necessary, in each specific case.
For steel castings made from unalloyed structural steels provided for by the standard (steel casting, GOST 977-88), the tensile strength of steel is approximately 40-60 kg/mm 2 or 392-569 MPa (normalization or normalization with tempering), category strength K20-K30. For the same steels after quenching and tempering, the regulated strength categories are KT30-KT40, the values of tensile strength are no less than 491-736 MPa.
For structural high-quality carbon steels (GOST 1050-88, rolled products up to 80 mm in size, after normalization):
- Tensile strength of steel 10: Steel 10 has a short-term strength limit of 330 MPa.
- Tensile strength of steel 20: steel 20 has a short-term strength limit of 410 MPa.
- Tensile strength of steel 45: 45 steel has a short-term strength limit of 600 MPa.
Steel strength categories
Strength categories of steels (GOST 977-88) are conventionally designated by the indices “K” and “KT”, followed by a number that represents the value of the required yield strength. The index “K” is assigned to steels in the annealed, normalized or tempered state. The “KT” index is assigned to steels after quenching and tempering.
Tensile strength of cast iron
The method for determining the tensile strength of cast iron is regulated by GOST 27208-87 (Cast iron castings. Tensile tests, determination of tensile strength).
Tensile strength of gray cast iron. Gray cast iron (GOST 1412-85) is marked with the letters SCH, the letters are followed by numbers that indicate the minimum value of the cast iron's tensile strength - tensile strength (MPa * 10 -1). GOST 1412-85 applies to cast iron with flake graphite for castings of grades SCh10-SCh35; from here you can see the minimum values tensile strength of gray cast iron in the cast state or after heat treatment they vary from 10 to 35 kgf/mm 2 (or from 100 to 350 MPa). Exceeding the minimum tensile strength of gray cast iron is allowed by no more than 100 MPa, unless otherwise specified.
Tensile strength of ductile iron. The marking of high-strength cast iron also includes numbers indicating the tensile strength of cast iron (tensile strength), GOST 7293-85. The tensile strength of ductile iron is 35-100 kg/mm 2 (or from 350 to 1000 MPa).
From the above it can be seen that nodular cast iron can successfully compete with steel.
Prepared by: Kornienko A.E. (ICM)
Lit.:
Tensile strength is the maximum stress to which a material can be subjected before it fails. If we talk about this indicator in relation to metals, then here it is equal to the ratio of the critical load to its cross-sectional area when conducting a tensile test. In general, strength shows how much force is required to overcome and break internal bonds between the molecules of a material.
How is strength testing performed?
Strength testing of metals is carried out using specialized mechanisms that allow you to set the required power during tensile testing. Such machines consist of a special loading element, with the help of which the necessary force is created.
Equipment for testing metals for strength makes it possible to stretch the materials being tested and set certain values of force that are applied to the sample. Today, there are hydraulic and mechanical types of mechanisms for testing materials.
Types of tensile strength
Tensile strength is one of the main properties of materials. Information about the ultimate strength of certain materials is extremely important when it is necessary to determine the possibilities of their use in certain industrial areas.
There are several separate strength limits of materials:
- when compressed;
- when bending;
- when torsion;
- when stretched.
Formation of the concept of the ultimate strength of metals
Galileo once spoke about the ultimate strength, who determined that the maximum permissible limit of compression and tension of materials depends on their cross-section. Thanks to the scientist's research, a previously unknown quantity arose - fracture stress.
The modern doctrine of the strength of metals was formed in the middle of the 20th century, which was necessary based on the need to develop a scientific approach to prevent possible destruction of industrial structures and machines during their operation. Until this point, when determining the strength of a material, only the degree of its plasticity and elasticity was taken into account and the internal structure was not taken into account at all.
Tensile strength of steel
Steel is the main raw material in most industrial applications. It is widely used in construction. That is why it is very important to select in advance a high-quality, truly suitable type of steel to perform specific tasks. The result and quality of the work performed directly depends on the correct calculation of the tensile strength of a certain steel grade.
As an example, we can cite several values of the ultimate strength indicators of steels. These values are based on government standards and are recommended parameters. Thus, for products cast from structural non-alloy steel, the GOST 977-88 standard is provided, according to which the limiting strength value during tensile testing is about 50-60 kg/mm 2, which is approximately 400-550 MPa. A similar grade of steel, after undergoing the hardening procedure, acquires a tensile strength value of more than 700 MPa.
The objective tensile strength of steel 45 (or any other grade of material, just like iron or cast iron, as well as other metal alloys) depends on a number of factors that must be determined based on the tasks assigned to the material during its use.
Strength of copper
Under normal conditions at room temperature, annealed commercial copper has a tensile strength of about 23 kg/mm 2. With significant temperature loads on the material, its ultimate strength is significantly reduced. The indicators of the ultimate strength of copper are reflected by the presence of various impurities in the metal, which can both increase this indicator and lead to its decrease.
Aluminum strength
The annealed fraction of technical aluminum at room temperature has a tensile strength of up to 8 kg/mm 2. Increasing the purity of the material increases its ductility, but is reflected in a decrease in strength. An example is aluminum, which has a purity of 99.99%. In this case, the ultimate strength of the material reaches about 5 kg/mm 2.
A decrease in the tensile strength of an aluminum dough piece is observed when it is heated during tensile testing. In turn, lowering the metal temperature in the range from +27 to -260 o C temporarily increases the test indicator by 4 times, and when testing the highest purity aluminum fraction - by as much as 7 times. At the same time, the strength of aluminum can be slightly increased by alloying it.
Strength of iron
To date, by industrial and chemical processing it has been possible to obtain whisker-like iron crystals with a tensile strength of up to 13,000 MPa. Along with this, the strength of technical iron, which is widely used in a wide variety of fields, is close to 300 MPa.
Naturally, each material sample, when examined for strength level, has its own defects. In practice, it has been proven that the real objective ultimate strength of any metal, regardless of its fraction, is less than the data obtained during theoretical calculations. This information must be taken into account when choosing a specific type and grade of metal to perform specific tasks.
Rolled products production involves the production of a huge number of varieties of structural steels. During operation, structures experience complex loads such as tension, compression, impacts, bending, or acting simultaneously and in combination. For severe and complex operating conditions of structures, mechanisms and structures, it is necessary to ensure durability, safety and reliability of operation, and therefore increased demands are placed on metal, as the main structural material.
The main thing in design calculations is the desire reduce the cross-section of steel structures modern units to reduce their weight and economical use of material without reducing the load-bearing capacity of the structure. Depending on the operating conditions, the requirements for steels change, but there are standard ones that are important and are used in the process of calculation work. Structural steel must meet high strength characteristics with sufficient ductility of the material.
The yield strength is an important conditional physical quantity that is directly used in calculation formulas. The use of this indicator as a basis for calculating the strength of a structure is justified, since during operation, irreversible changes in the linear dimensions appear in the structure, which leads to destruction of the shape of the product and its failure. Increasing this characteristic makes it possible to reduce the design cross-sections of the material and the weight of metal structures and allows increasing operating loads.
The yield strength of metals is a characteristic of steel that shows the critical stress after which it continues material deformation without increasing the load. This important indicator is measured in Pascals (Pa) or MegaPascals (MPa), and allows one to calculate the permissible stress limit for ductile steels.
After the material overcomes the yield point, irreversible deformations occur in it, the structure of the crystal lattice changes, and plastic changes occur. If the tensile value of the force increases, then after passing the yield plateau, the deformation of the steels continues to increase.
Often the concept of yield of steels is called the stress at which irreversible deformation begins, without defining the difference with the elastic limit. But in real conditions, the value of the yield strength indicator exceeds the elastic limit by about 5%.
General information and characteristics of steels
Steel is classified as a malleable wrought alloy based on iron with carbon and additions of other elements. The material is smelted from cast iron mixtures with scrap metal in open-hearth, electric and oxygen converter furnaces.
The formed crystal lattice of the metal depends on the amount of carbon it contains and is determined from the structural diagram in accordance with the processes in this alloy. For example, a steel lattice, which contains up to 0.06% carbon, has a granular structure and is ferrite in its pure form. The strength of such metals is low, but the material has a high limit of impact strength and fluidity. Steel structures in a state of equilibrium are divided into:
- ferritic;
- pearlite-ferritic;
- cementite-ferritic;
- cementite-pearlite;
- pearlite;
Influence of carbon content on the properties of steels
Changes in the main components of cementite and ferrite are determined by the properties of the former according to the law of additivity. Increasing the percentage of carbon addition to 1.2% allows increasing strength, hardness, refrigeration capacity threshold at 20ºС and yield strength. An increase in carbon content changes the physical properties of the material, which sometimes leads to deterioration in technical characteristics, such as weldability, deformation during stamping. Low-carbon alloys have excellent welding properties in structures.
Manganese and silicon additives
Manganese is introduced into the alloy as a technological additive to increase the degree of deoxidation and reduce the harmful effects of sulfur impurities. In steels it is present in the form of solid components in an amount of no more than 0.8% and does not have a significant effect on the properties of the metal.
Silicon acts in the alloy in a similar way; it is added during the deoxidation process in an amount of no more than 0.38%. To be able to join parts by welding, the silicon content should not exceed 0.24%. Silicon in the alloy composition does not affect the properties of steels.
The limit for sulfur content in an alloy is threshold of 0.06%, it is contained in the form of brittle sulfites. An increased impurity content significantly worsens the mechanical and physical properties of steels. This is reflected in a decrease in ductility, yield strength, impact strength, abrasion and corrosion resistance.
The phosphorus content also worsens the quality characteristics of metal alloys; the yield strength after increasing phosphorus in the composition increases, but the viscosity and ductility decreases. The standard impurity content in the alloy is regulated in the range from 0.025 to 0.044%. Phosphorus most strongly deteriorates the properties of steels with a simultaneous high level of carbon additions.
Nitrogen and oxygen in the alloy
These substances contaminate steel with non-metallic impurities and worsen its mechanical and physical properties. In particular, this refers to the threshold of viscosity and endurance, plasticity and fragility. An oxygen content in the alloy of more than 0.03% causes rapid aging of the metal, nitrogen increases fragility and increases strain aging over time. The nitrogen content increases strength, thereby lowering the yield strength.
Alloying additives in alloys
Alloyed steels include steels into which elements are specially introduced in certain combinations to improve quality characteristics. Complex alloying gives the best results. Chromium, nickel, molybdenum, tungsten, vanadium, titanium and others are used as additives.
Alloying increases the yield strength and other technological properties, such as impact strength, narrowing and the possibility of calcination, reducing the threshold for deformation and cracking.
To fully study the properties of the material and determine the yield strength, plastic deformation and strength, metal samples are tested until complete destruction. The test is carried out under the action of loads of the following type:
Determination of test load limits is carried out under standard conditions, using special machines, which are described in the rules of State standards.
Testing a sample to determine the yield strength
To do this, take a cylindrical sample with a size of 20 mm and a calculated length of 10 mm and apply a tensile load to it. The concept of effective length refers to the distance between the marks marked on a longer sample to allow for gripping. To carry out the test, determine the relationship between the increase in tensile force and elongation of the test piece.
All test readings are automatically displayed as a graph for easy comparison. It is called a conditional tension or conditional stress diagram; the graph depends on the initial cross-section of the sample and its initial length. Initially, an increase in force leads to a proportional elongation of the sample. This provision applies up to the limit of proportionality.
After reaching this threshold, the graph becomes curvilinear and indicates a disproportionate increase in length with a uniform increase in load. Next comes the determination of the yield strength. As long as the stresses in the sample do not exceed this indicator, the material can return to original state regarding size and shape. In practical testing, the difference between these limits is small and not worth much attention.
Yield strength
If you continue to increase the load, then a test moment comes when the change in shape and size continues without increasing force. On the diagram this is shown by the horizontal straight line (platform) of yield. The maximum stress at which the deformation increases is recorded after the load stops increasing. This indicator is called the yield strength. For steel Art. 3 yield strength from 2450 kg per square centimeter.
Proof of Yield
Many metals, when tested, give a diagram in which the yield plateau is absent or poorly expressed; for them the concept of conditional yield strength is used. This concept defines the stress that causes a residual change or deformation within 0.2%. Metals to which the concept of conditional yield strength is applied are alloyed and high-carbon steels, bronze, duralumin and others. The more ductile the steel, the greater the indication of residual deformation. These include aluminum, brass, copper and low-carbon steels.
Tests of steel samples show that the fluidity of the metal causes significant shifts of crystals in the lattice, and is characterized by the appearance on the surface of lines directed towards the central axis of the cylinder.
Tensile strength
After a change by a certain amount, the sample transitions to a new phase, when, after overcoming the yield point, the metal again can resist stretching. This is characterized by hardening, and the line of the diagram rises again, although the rise occurs in a more gradual manner. Temporary resistance to constant load appears.
After reaching the maximum stress (ultimate strength), a sharp narrowing area appears on the sample, the so-called neck, characterized by a decrease in the cross-sectional area, and the sample breaks at the thinnest point. In this case, the voltage value drops sharply, and the magnitude of the force also decreases.
Steel St.3 is characterized by a tensile strength of 4000–5000 kg/cm2. For high-strength metals, this figure reaches a limit of 17500 kg/cm3.
Plasticity of the material
It is characterized by two indicators:
- residual relative elongation;
- residual narrowing at rupture.
To determine the first indicator, the total length of the stretched sample after rupture is measured. To do this, fold the two halves together. After measuring the length, calculate the percentage of the original length. Strong alloys are less susceptible to ductility and the elongation rate is reduced to 63 et 11%.
The second characteristic is calculated after measuring the narrowest part of the fracture and is calculated as a percentage of the original cut area of the sample.
The property opposite to plasticity is material fragility index. Brittle metals include cast iron and tool steel. The division of steels into brittle and ductile is made conditionally, since to determine this indicator, operating or testing conditions, the rate of increase in load, and ambient temperature are important.
Some materials do not behave as brittle under different conditions. For example, cast iron, positioned in such a way that it is clamped on all sides, does not collapse even with stresses arising inside. Grooved steel characterized by increased fragility. Hence the conclusion that it is much more expedient to test not the limits of fragility, but to determine the state of the material as ductile or brittle.
Testing of steels to determine physical and technical properties is done in order to obtain reliable data for carrying out construction work and creating structures on the farm.
When a force or system of forces is applied to a metal sample, it reacts by changing its shape (deforming). Various characteristics that determine the behavior and final state of a metal sample, depending on the type and intensity of forces, are called mechanical properties of the metal.
The intensity of the force acting on the sample is called stress and is measured as the total force divided by the area over which it acts. Deformation refers to the relative change in sample dimensions caused by applied stresses.
Elastic and plastic deformation, destruction
If the stress applied to the metal sample is not too great, then its deformation turns out to be elastic - as soon as the stress is removed, its shape is restored. Some metal structures are deliberately designed to deform elastically. Thus, springs usually require a fairly large elastic deformation. In other cases, elastic deformation is minimized. Bridges, beams, mechanisms, devices are made as rigid as possible. The elastic deformation of a metal sample is proportional to the force or sum of forces acting on it. This is expressed by Hooke's law, according to which stress is equal to elastic strain multiplied by a constant proportionality factor called the modulus of elasticity: s = ∆ Y, Where s- voltage, ∆ – elastic deformation, and Y– elastic modulus (Young’s modulus). The elastic moduli of a number of metals are presented in table. 1.
Table 1
|
Tungsten |
Iron (steel) |
Aluminum |
||||
Using the data from this table, you can calculate, for example, the force required to stretch a steel rod of square cross-section with a side of 1 cm by 0.1% of its length:
F= 200,000 MPa x 1 cm 2 x 0.001 = 20,000 N (= 20 kN)
When stresses in excess of its elastic limit are applied to a metal specimen, they cause plastic (irreversible) deformation, resulting in a permanent change in its shape. Higher stresses can cause material failure.
The most important criterion when choosing a metal material that requires high elasticity is the yield strength. The best spring steels have almost the same modulus of elasticity as the cheapest construction steels, but spring steels are able to withstand much greater stresses, and therefore much greater elastic deformations without plastic deformation, because they have a higher yield strength.
The plastic properties of a metallic material (as opposed to the elastic properties) can be changed by alloying and heat treatment. Thus, the yield strength of iron can be increased 50 times using similar methods. Pure iron goes into a state of fluidity already at stresses of the order of 40 MPa, while the yield strength of steels containing 0.5% carbon and several percent of chromium and nickel, after heating to 950 C 0 and hardening, can reach 2000 MPa.
When a metallic material is loaded beyond its yield strength, it continues to deform plastically, but becomes harder as it deforms, so that increasing stress is required to further increase the deformation. This phenomenon is called deformation or mechanical hardening (as well as work hardening). It can be demonstrated by twisting or repeatedly bending a metal wire. Strain hardening of metal products is often carried out in factories. Brass sheets, copper wires, and aluminum rods can be cold rolled or cold drawn to the level of hardness required for the final product.
Bernstein M.L., Zaimovsky V.A. Mechanical properties of metals. M., 1979
Wyatt O.G., Dew-Hughes D. Metals, ceramics, polymers. M., 1979
Pavlov P.A. Mechanical states and strength of materials. L., 1980
Sobolev N.D., Bogdanovich K.P. Mechanical properties of materials and fundamentals of strength physics. M., 1985
Zhukovets I.I. Mechanical testing of metals. M., 1986
Bobylev A.V. Mechanical and technological properties of metals. M., 1987
The yield strength is the stress corresponding to the residual elongation after removal of the load. Determining this value is necessary for selecting metals used in production. If the parameter under consideration 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 strengths refer to strength indicators. They represent macroplastic deformation with rather small strengthening. Physically, this parameter can be represented as a characteristic of the material, namely: stress, which corresponds to the lower value of the yield area in the graph (diagram) of tension of materials. This can also be represented in the form of a formula: σ T = P T / F 0, where P T means the yield stress load, and F 0 corresponds to the original cross-sectional area of the sample in question. PT establishes the so-called boundary between the elastic-plastic and elastic deformation zones of the material. Even a slight increase in PT) will cause 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, heat treatment mode, sample thickness, the presence of alloying elements and impurities, type, microstructure and crystal lattice defects, etc. The yield strength changes significantly with temperature. Let's consider an example of the practical meaning of this parameter. 
Pipe yield strength
The most obvious influence of this value is during the construction of pipelines for high-pressure systems. In such structures, special steel should be used, which has sufficiently large yield limits, as well as minimum gap indicators between this parameter and the higher the limit of the steel, the higher, naturally, the permissible value of the operating stress should be. This fact has a direct impact 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 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 manufacture of pipes. One of the most authentic methods for determining these parameters is to conduct research on a discontinuous sample. In all cases, it is necessary to take into account the difference between the values of the indicator under consideration, on the one hand, and the permissible stress values, on the other.
In addition, you should know that the yield strength of a metal is always established as a result of detailed repeated measurements. But the system of permissible stresses is overwhelmingly adopted based on standards or generally as a result of technical specifications, as well as based on the personal experience of the manufacturer. In main pipeline systems, the entire regulatory collection is described in SNiP II-45-75. So, setting the safety factor is a rather complex 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 pipeline 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
The tensile yield strength determines at what value the stress will remain unchanged or decrease despite elongation. That is, this parameter will reach a critical point when a transition from the elastic to plastic region of deformation of the material occurs. It turns out that the yield strength can be determined by testing the rod. 
PT calculation
In the strength of materials, the yield strength is the stress at which it 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 at the moment of occurrence of irreversible deformation is determined. 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 σ T =τ s √3. In some specimens, continuous elongation of a cylindrical sample on the diagram of the dependence of normal stresses on relative elongation leads to the detection of a so-called yield tooth, that is, a sharp decrease in stress before the formation of plastic deformation.
Moreover, a further increase in such distortion to a certain value occurs at a constant voltage, which is called physical DC. If the yield area (horizontal section of the graph) is large, then such a material is called ideally plastic. If the diagram does not have a platform, then the samples are called hardening. In this case, it is impossible to accurately indicate the value at which plastic deformation will occur.
What is proof strength?
Let's figure out what this parameter is. In cases where the voltage diagram does not have pronounced areas, it is necessary to determine a conditional DC. 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 axis of determination ε, it is necessary to set aside a value equal to 0.2. From this point the starting section is carried out. 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, the conditional yield limits for torsion and bending are separately distinguished. 
Melt flow
This parameter determines the ability of molten metals to fill linear shapes. Melt flow for metal alloys and metals has its own term in the metallurgical industry - fluidity. In fact, this is the reciprocal value of the International System of Units (SI) expressing the fluidity of a liquid in Pa -1 * s -1.
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 withstand various impacts without collapsing. Mechanical properties are usually determined using conventional stress-strain diagrams. Standard samples should be used for testing. Test instruments are equipped with a device that records the diagram. Increasing loads above normal causes significant plastic deformation in the product. The yield strength and tensile strength correspond to the highest load preceding the complete destruction of the sample. In plastic materials, 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 dislocation density is formed in the material, and so-called nucleation discontinuities also arise. Due to their enlargement, pores appear in the sample. Merging with each other, they form cracks that propagate transversely to the tensile axis. And at a critical moment the sample is completely destroyed.
What is PT for valves?
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 a given structure without exception, and have plastic and durable properties. And also meet all the conditions for the industrialization of these types of work. The mechanical properties of steel used in the manufacture of fittings are established by the relevant GOST and technical specifications. GOST 5781-61 provides for four classes of these products. The first three are intended for conventional structures, as well as non-prestressed rods in prestressed 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/cm2, and for the fourth - 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/mm 2, grade 30 - 30 kgf/mm 2, grade 45 - 36 kgf/mm 2. However, for the same steels, manufactured by prior agreement between the consumer and the manufacturer, the yield limits may have different values (the same GOST). So, 30 will have a PT in the amount of 30 to 41 kgf/mm 2, and grade 45 - in the range of 38-50 kgf/mm 2.
Conclusion
When designing various structures (buildings, bridges, etc.), the yield strength is used as an indicator of the strength standard when calculating the values of permissible loads in accordance with the specified safety factor. But for vessels under pressure, the permissible load is calculated on the basis of PT, as well as tensile strength, taking into account the specification of operating conditions.
Tensile strength is the maximum stress to which a material can be subjected before it fails. If we talk about this indicator in relation to metals, then here it is equal to the ratio of the critical load to its cross-sectional area when conducting a tensile test. In general, strength shows how much force is required to overcome and break internal bonds between the molecules of a material.
How is strength testing performed?
Strength testing of metals is carried out using specialized mechanisms that allow you to set the required power during tensile testing. Such machines consist of a special loading element, with the help of which the necessary force is created.
Equipment for testing metals for strength makes it possible to stretch the materials being tested and set certain values of force that are applied to the sample. Today, there are hydraulic and mechanical types of mechanisms for testing materials.
Types of tensile strength
Tensile strength is one of the main properties of materials. Information about the ultimate strength of certain materials is extremely important when it is necessary to determine the possibilities of their use in certain industrial areas.
There are several separate strength limits of materials:
- when compressed;
- when bending;
- when torsion;
- when stretched.
Formation of the concept of the ultimate strength of metals
Galileo once spoke about the ultimate strength, who determined that the maximum permissible limit of compression and tension of materials depends on their cross-section. Thanks to the scientist's research, a previously unknown quantity arose - fracture stress.
The modern doctrine of the strength of metals was formed in the middle of the 20th century, which was necessary based on the need to develop a scientific approach to prevent possible destruction of industrial structures and machines during their operation. Until this point, when determining the strength of a material, only the degree of its plasticity and elasticity was taken into account and the internal structure was not taken into account at all.
Tensile strength of steel
Steel is the main raw material in most industrial applications. It is widely used in construction. That is why it is very important to select in advance a high-quality, truly suitable type of steel to perform specific tasks. The result and quality of the work performed directly depends on the correct calculation of the tensile strength of a certain steel grade.
As an example, we can cite several values of the ultimate strength indicators of steels. These values are based on government standards and are recommended parameters. Thus, for products cast from structural non-alloy steel, the GOST 977-88 standard is provided, according to which the limiting strength value during tensile testing is about 50-60 kg/mm 2, which is approximately 400-550 MPa. A similar grade of steel, after undergoing the hardening procedure, acquires a tensile strength value of more than 700 MPa.
The objective tensile strength of steel 45 (or any other grade of material, just like iron or cast iron, as well as other metal alloys) depends on a number of factors that must be determined based on the tasks assigned to the material during its use.
Strength of copper
Under normal conditions at room temperature, annealed commercial copper has a tensile strength of about 23 kg/mm 2. With significant temperature loads on the material, its ultimate strength is significantly reduced. The indicators of the ultimate strength of copper are reflected by the presence of various impurities in the metal, which can both increase this indicator and lead to its decrease.
Aluminum strength
The annealed fraction of technical aluminum at room temperature has a tensile strength of up to 8 kg/mm 2. Increasing the purity of the material increases its ductility, but is reflected in a decrease in strength. An example is aluminum, which has a purity of 99.99%. In this case, the ultimate strength of the material reaches about 5 kg/mm 2.
A decrease in the tensile strength of an aluminum dough piece is observed when it is heated during tensile testing. In turn, lowering the metal temperature in the range from +27 to -260 o C temporarily increases the test indicator by 4 times, and when testing the highest purity aluminum fraction - by as much as 7 times. At the same time, the strength of aluminum can be slightly increased by alloying it.
Strength of iron
To date, by industrial and chemical processing it has been possible to obtain whisker-like iron crystals with a tensile strength of up to 13,000 MPa. Along with this, the strength of technical iron, which is widely used in a wide variety of fields, is close to 300 MPa.
Naturally, each material sample, when examined for strength level, has its own defects. In practice, it has been proven that the real objective ultimate strength of any metal, regardless of its fraction, is less than the data obtained during theoretical calculations. This information must be taken into account when choosing a specific type and grade of metal to perform specific tasks.
Tensile strength
A certain threshold value for a specific material, exceeding which will lead to the destruction of the object under the influence of mechanical stress. The main types of strength limits: static, dynamic, compressive and tensile. For example, the tensile strength is the limit value of a constant (static limit) or variable (dynamic limit) mechanical stress, exceeding which will rupture (or unacceptably deform) the product. Unit of measurement - Pascal [Pa], N/mm² = [MPa].
Yield strength (σ t)
The amount of mechanical stress at which the deformation continues to increase without increasing the load; used for calculating permissible stresses in plastic materials.
After passing the yield point, irreversible changes are observed in the metal structure: the crystal lattice is rearranged, and significant plastic deformations appear. At the same time, self-strengthening of the metal occurs and after the yield point, the deformation increases with increasing tensile force.
This parameter is often defined as “the stress at which plastic deformation begins to develop,” thus identifying the limits of yield and elasticity. However, it should be understood that these are two different parameters. The yield strength values exceed the elastic limit by approximately 5%.
Endurance limit or fatigue limit (σ R)
The ability of a material to withstand loads that cause cyclic stress. This strength parameter is defined as the maximum stress in a cycle at which fatigue failure of the product does not occur after an indefinitely large number of cyclic loads (the basic number of cycles for steel is Nb = 10 7). The coefficient R (σ R) is taken to be equal to the cycle asymmetry coefficient. Therefore, the fatigue limit of the material in the case of symmetrical loading cycles is denoted as σ -1, and in the case of pulsating ones - as σ 0.
Note that fatigue tests of products are very long and labor-intensive; they involve the analysis of large volumes of experimental data with an arbitrary number of cycles and a significant scatter of values. Therefore, special empirical formulas are most often used that connect the endurance limit with other strength parameters of the material. The most convenient parameter is considered to be the tensile strength.
For steels, the bending endurance limit is usually half the tensile strength: For high-strength steels, you can take: 
For ordinary steels during torsion under conditions of cyclically changing stresses, the following can be accepted:
The above ratios should be used with caution, because they were obtained under specific loading conditions, i.e. during bending and torsion. However, when tested in tension-compression, the endurance limit becomes approximately 10-20% less than in bending.
Proportionality limit (σ)
The maximum stress value for a particular material at which Hooke’s law still applies, i.e. The deformation of the body is directly proportional to the applied load (force). Please note that for many materials, reaching (but not exceeding!) the elastic limit leads to reversible (elastic) deformations, which, however, are no longer directly proportional to stress. In this case, such deformations may be somewhat “lag” relative to the increase or decrease in load.
Diagram of the deformation of a metal sample under tension in the coordinates elongation (Є) - stress (σ).
1: Absolute elastic limit.
2: Limit of proportionality.
3: Elastic limit.
Tensile strength or stress at break expressed in dynes/cm2. The elastic limit always lies below the breaking stress. The process of drawing materials, i.e. making the wire increases the tensile strength, and the thinner the wire, the greater the tensile strength. In gold, when it is processed, an increase in tensile stress is usually found due to its ductility.
Technical properties of materials (i.e. breaking stress, fatigue, fluidity, etc.) at normal or elevated temperatures.
To bring values expressed in dynes/cm 2 to approximate values in kgf/mm 2, the first must be divided by 10 8; to convert to lbf/sq.in., divide by 7*10 4 ; to ton-force/sq.in. values – divide by 1.5*10 8 .
Table of tensile strength values of materials and substances
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Material, substance |
Tensile strength 10 9 dynes/cm2. |
Material, substance |
Tensile strength 10 9 dynes/cm2. |
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Aluminum (cast) |
Leather belt |
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Aluminum (sheet) |
Hemp rope |
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Magnesium (cast) |
Silk thread |
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Magnesium (pressed) |
Quartz thread |
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Copper (cast) |
Thermoplastic plastics |
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Copper (sheet) |
Thermoset |
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Wires |
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Welding iron |
Aluminum |
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Cast steel |
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Soft steel (0.2% C) |
Copper (cold drawn) |
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Spring steel |
Copper (annealed) |
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Tempered steel |
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Nickel steel, 5% Ni |
Iron (on charcoal) |
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Chrome-nickel steel |
Cold drawn iron |
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Lead (cast) |
Annealed iron |
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Tin (cast) |
Steel for ornaments |
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Zinc (sheet) |
Tempered steel |
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Brass (66% Cu) cast |
Cold drawn steel |
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Brass (34% Cu) sheet |
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Phosphor bronze (cast) |
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Gunmetal (90% Cu, 10% Sn) |
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Soft solder |
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Non-metals: |
Phosphor bronze |
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Nickel silver |
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Duralumin |
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Ash, beech, oak, teak, mahogany |
Tungsten |
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Fir, resinous pine |
Palladium |
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Red or white spruce boards |
Molybdenum |
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White or yellow pine |
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Annealed zirconium |
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Cold drawn zirconium |
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