What is sigma 0 2. Mechanical properties of metals

The main mechanical properties are strength, elasticity,, . Knowing the mechanical properties, the designer reasonably selects the appropriate material that ensures the reliability and durability of structures with minimal weight. Mechanical properties determine the behavior of a material during deformation and destruction under external loads.

Depending on the loading conditions, mechanical properties can be determined by:

1. Static loading– the load on the sample increases slowly and smoothly.
2. Dynamic loading– the load increases at high speed and has a shock character.
3. Repeated, variable or cyclic loading– the load during the test changes many times in magnitude or in magnitude and direction.

To obtain comparable results, samples and methods of mechanical testing are regulated by GOSTs.

Mechanical properties of metals, steels and alloys. Strength.

Strength– the ability of a material to resist deformation and destruction.

Tests are carried out on special machines that record a tensile diagram expressing the dependence of the elongation of the sample Δ l(mm) from the effective load P, that is, Δ l = f(P). But to obtain data on mechanical properties, they reconstruct: the dependence of the relative elongation Δ l from voltage δ.

Material Tensile Diagram

Figure 1: a – absolute, b – relative;c – scheme for determining the conditional yield strength

Let us analyze the processes that occur in the sample material as the load increases: section oa in the diagram corresponds to the elastic deformation of the material when Hooke's law is observed. The stress corresponding to the elastic limiting strain at a point A, called limit of proportionality.

Mechanical properties of metals, steels and alloys. Limit of proportionality.

Proportionality limit (σ pts) – maximum stress up to which the linear relationship between strain and stress is maintained.

At stresses above the proportionality limit, uniform plastic deformation occurs (elongation or narrowing of the cross-section). Each stress corresponds to a residual elongation, which is obtained by drawing a parallel line from the corresponding point of the elongation diagram oa.

Since it is practically impossible to establish the transition point to the inelastic state, they establish conditional elastic limit, – the maximum stress up to which the sample receives only elastic deformation. The stress at which the residual deformation is very small (0.005...0.05%) is considered. The designation indicates the value of residual deformation (σ 0.05).

Mechanical properties of metals, steels and alloys. Yield limit.

Yield strength characterizes the material’s resistance to small plastic deformations. Depending on the nature of the material, a physical or conditional yield strength is used.

Physical yield strength σ m– this is the stress at which an increase in deformation occurs under constant load (the presence of a horizontal area on the tensile diagram). Used for very plastic materials.

But the majority of metals and alloys do not have a yield plateau.

Proof of Yieldσ 0.2– this is the stress causing residual deformation δ = 0.20%.

Physical or proof stress are important design characteristics of a material. The stresses acting in the part must be below the yield strength. Uniform throughout the entire volume continues up to the tensile strength value. At the point V At the weakest point, a neck begins to form—severe local fatigue of the sample.

Mechanical properties of metals, steels and alloys. Tensile strength.

Tensile strength σ in – stress corresponding to the maximum load that the sample can withstand before failure (temporary tensile strength).

Neck formation is typical for plastic materials that have a tension diagram with a maximum. Ultimate strength characterizes strength as resistance to significant uniform plastic deformation. Beyond point B, due to the development of the neck, the load drops and destruction occurs at point C.

True resistance to destruction – this is the maximum stress that the material can withstand at the moment preceding the destruction of the sample (Figure 2).

True fracture resistance is significantly greater than the ultimate strength, since it is determined relative to the final cross-sectional area of ​​the sample.

True Tension Chart

Rice. 2

F to - final cross-sectional area of ​​the sample.

True stress S i is defined as the ratio of load to cross-sectional area at a given time.

The tensile test also determines the plasticity characteristics.

Mechanical properties of metals, steels and alloys. Plastic.

Plastic – the ability of a material to undergo plastic deformation, that is, the ability to obtain a residual change in shape and size without breaking the continuity. This property is used in metal forming.

Characteristics:

• relative extension :

l o and l k – initial and final length of the sample;

Stress ss in the cross section, at which plasticity first appears. (irreversible) deformations. Similarly, in experiments with torsion of a thin-walled tubular sample, the PT is determined at a shear ts. For most metals ss=ts?3.

In some materials, with continuous elongation, cylindrical. sample on the diagram of the dependence of normal voltage o on relative. elongation 8 is detected by the so-called. yield tooth, i.e. a sharp decrease in stress before the appearance of plasticity. deformation (Fig., a), and further growth of deformation (plastic) to a certain value occurs at a constant stress, so-called. f i h e s k i m P. t. st.

The horizontal section of the s-e diagram is called. yield area; if its extent is large, the material is called. ideally plastic (non-hardening). In other materials, called hardening, there is no yield plateau (Fig., b) and accurately indicate the stress at which plasticity first appears. deformation is almost impossible.

The concept of conditional P. t. ss is introduced as a stress, upon unloading from which a residual (plastic) deformation of magnitude D is first detected in the sample. Residual deformations less than D are conventionally considered negligible. For example, P.t., measured with a tolerance of D=0.2%, is designated s0.2. (see PLASTICITY).

Physical encyclopedic dictionary. - M.: Soviet Encyclopedia. . 1983 .

in the resistance of materials - stress, at which plasticity begins to develop. deformation. In experiments with tensile cylindrical the sample is determined by the normal stress in the cross section, at which plasticity first appears. (irreversible) deformations. Similarly, in experiments with torsion of a thin-walled tubular sample, the PT under shear is determined. For most metals

In some materials, with continuous elongation, cylindrical. sample on the diagram of the dependence of normal voltage on relative. elongation e is detected by the so-called. yield tooth, i.e. a sharp decrease in stress before the appearance of plasticity. deformation (Fig., c), and further growth of deformation (plastic) to a certain value occurs at a constant stress, so-called. physical P. t. The horizontal section of the diagram is called. yield area; if its extent is large, the material is called. ideally plastic (non-hardening). In other materials, called hardening, there is no yield plateau (Fig. b) and accurately indicate the voltage at which plasticity first appears. deformation is almost impossible. The concept of conditional P. is introduced, i.e. as stress, upon unloading from which a residual (plastic) deformation of magnitude D is first detected in the sample. Residual deformations less than D are conventionally considered negligible. For example, P. t., measured with a tolerance of D = 0.2%, is designated See also Plastic.

IN.

Physical encyclopedia. In 5 volumes. - M.: Soviet Encyclopedia. Editor-in-chief A. M. Prokhorov. 1988 .

See what "YIELD LIMIT" is in other dictionaries:

Yield strength mechanical stress σт, corresponding to the lower position of the upper deviation in the area of ​​the unknown plot of the yield area on the material deformation diagram. If such a platform does not exist, which is typical, ... ... Wikipedia

Yield strength- (physical) this is a mechanical characteristic of materials: stress corresponding to the lower position of the yield plateau in the tensile diagram for materials with this plateau (figure), σТ=PT/F0. The yield point sets the limit... ... Metallurgical dictionary

Yield strength- (physical), N/mm – the lowest stress at which deformation occurs without a noticeable increase in load. [GOST 10922 2012] Physical yield strength is the lowest tensile stress at which the deformation of the reinforcement... ... Encyclopedia of terms, definitions and explanations of building materials

yield strength- Characteristics of the deformation properties of elastic materials, expressed through the stress at which significant plastic deformations occur in the test sample [Terminological dictionary of construction in 12 languages ​​(VNIIIS Gosstroy... ... Technical Translator's Guide

yield strength- 2.12 yield strength: Standard minimum stress value at which an intensive increase in plastic deformation begins (with a slight increase in load) when the pipe material is stretched. Source: STO Gazprom 2 2.1 318 2009:… … Dictionary-reference book of terms of normative and technical documentation

yield strength- takumo riba statusas T sritis fizika atitikmenys: engl. flow limit; yield limit vok. Fließgrenze, f rus. yield limit, f; yield strength, m pranc. limite d’écoulement, f … Fizikos terminų žodynas

Yield strength Yield strength. The stress at which a material exhibits a precisely defined deviation from the proportionality of stress and strain. A deviation of 0.2% is used for many materials, especially metals. (Source: “Metals... Dictionary of metallurgical terms

Mechanical characteristics of materials: voltage corresponding to lower. the position of the yield plateau in the tensile diagram (see figure) for materials that have such a plateau. Denoted bt. For materials that do not have a flow area, a conditional P is accepted... Big Encyclopedic Polytechnic Dictionary

Characteristics of the deformation properties of elastic materials, expressed through the stress at which significant plastic deformations occur in the test sample (Bulgarian language; Български) boundary on the provlachvana (Czech language; Čeština) mez ... Construction dictionary

See Plasticity of clayey rocks... Dictionary of hydrogeology and engineering geology

Books

• Optical method for studying voltages. , Coker E.. The book by Coker and Failon, “The Optical Method for Studying Stresses,” is of very great scientific and practical interest. The authors of this book are prominent experts in the field of elasticity theory and...

Mechanical properties characterize the resistance of a material to deformation, destruction, or the peculiarity of its behavior during the destruction process. This group of properties includes indicators of strength, rigidity (elasticity), ductility, hardness and viscosity. The main group of such indicators consists of standard characteristics of mechanical properties, which are determined in laboratory conditions on samples of standard sizes. The indicators of mechanical properties obtained during such tests evaluate the behavior of materials under external load without taking into account the design of the part and their operating conditions. In addition, they additionally determine the structural strength indicators that are in the greatest correlation with the service properties of a particular product and evaluate the performance of the material under operating conditions.

2.2.1. Mechanical properties determined under static loads

Static tests involve a slow and gradual increase in load applied to the test sample. According to the method of applying loads, static tests are distinguished: tensile, compression, bending, torsion, shear or shear. The most common are tensile tests (GOST 1497-84), which make it possible to determine several important indicators of mechanical properties.

Tensile tests

When stretching standard specimens with a cross-sectional area F0 and working (calculated) length L0, a tensile diagram is constructed in coordinates load - elongation of the sample (Fig. 2.1). Three sections are distinguished in the diagram: elastic deformation before load P(control); uniform plastic deformation from P(control) to P(max) and concentrated plastic deformation from P(max) to P(critical). The straight section is maintained until the load corresponding to the proportionality limit P(pc). The tangent of the angle of inclination of a straight section characterizes the elastic modulus of the first kind E.

In a small area from P(pc) to P(upr), the linear relationship between P and (delta)L is disrupted due to elastic imperfections of the material associated with lattice defects.

Plastic deformation above P(control) occurs with increasing load, since the metal is strengthened during deformation. The strengthening of metal during deformation is called hardening

The hardening of the metal increases until the sample breaks, although the tensile load decreases from P(max) to P(critical) . This is explained by the appearance of a local thinning in the sample - a neck, in which plastic deformation is mainly concentrated. Despite the decrease in load, the tensile stresses in the neck increase until the sample ruptures.

When stretched, the sample elongates and its cross-section continuously decreases. The true stress is determined by dividing the load acting at a certain moment by the area that the sample has at that moment. In everyday practice, true stresses are not determined, but conditional stresses are used, assuming that the cross section F0 sample remains unchanged. Stresses (sigma)Cont, (sigma)T and (sigma)B are standard strength characteristics. Each is obtained by dividing the corresponding load P(urp), P(T) and P(max) per initial cross-sectional area F0.

The elastic limit (sigma) is the stress at which plastic deformation reaches a given value established by the conditions. Typically, residual strain values ​​of 0.005 are used; 0.02 and 0.05%. The corresponding elastic limits are denoted by (sigma)0.005, (sigma)0.02 and (sigma)0.05. Elastic limit is an important characteristic of spring materials that are used for elastic devices and machines.

The conditional yield strength is the stress corresponding to a plastic deformation of 0.2%; it is designated (sigma)0.2. The physical yield strength (sigma) T is determined from the tensile diagram when there is a yield area on it. However, during tensile tests of most alloys, there is no yield plateau on the diagrams. The selected plastic deformation of 0.2% quite accurately characterizes the transition from elastic to plastic deformations, and stress (sigma) 0.2 is easy to determine during testing, regardless of whether or not there is a yield plateau on the tensile diagram.

The permissible voltage used in calculations is chosen less (sigma)0.2 (usually 1.5 times) or less (sigma)B (2.4 times).

For materials with low plasticity, tensile tests pose significant difficulties. Minor distortions when installing the sample introduce a significant error in determining the breaking load. Such materials are usually subjected to bending testing.

Bending tests

During a bending test, both tensile and compressive stresses arise in the sample. For this reason, bending is a gentler method of loading than tension. Low-plasticity materials are tested for bending: cast iron, tool steel, steel after surface hardening, ceramics. Tests are carried out on long samples (l / h > 10) of cylindrical or rectangular shape, which are mounted on two supports. Two loading schemes are used: a concentrated force (this method is used more often) and two symmetrical forces (pure bending tests). The determined characteristics are tensile strength and deflection.

For plastic materials, bending tests are not used, since the samples are bent without destruction until both ends touch.

Hardness tests

Hardness is understood as the ability of a material to resist the penetration of a solid body - an indenter - into its surface. A hardened steel ball or a diamond tip in the form of a cone or pyramid is used as an indenter. When indented, the surface layers of the material experience significant plastic deformation. After removing the load, an imprint remains on the surface. The peculiarity of the occurring plastic deformation is that it occurs in a small volume and is caused by the action of significant tangential stresses, since a complex stress state close to all-round compression arises near the tip. For this reason, not only ductile but brittle materials experience plastic deformation! Thus, hardness characterizes the resistance of a material to plastic deformation. The same resistance is assessed by the tensile strength, when determining which a concentrated deformation occurs in the neck area. Therefore, for a number of materials, the numerical values ​​of hardness and tensile strength are proportional. This feature, as well as the ease of measurement, allows us to consider hardness tests as one of the most common types of mechanical tests. In practice, four methods of hardness measurement are widely used.

Brinell hardness. In this standard method of measuring hardness, a hardened steel ball with a diameter of 10; 5 or 2.5 mm under loads from 5000 N to 30000 N. After removing the load, an imprint in the form of a spherical hole with a diameter of d. The diameter of the hole is measured with a magnifying glass, on the eyepiece of which there is a scale with divisions.

In practice, when measuring hardness, calculations are not made using the above formula, but pre-compiled tables are used indicating the HB value depending on the diameter of the indentation and the selected load. The smaller the diameter of the print, the higher the hardness.

The Brinell measurement method is not universal. It is used for materials of low and medium hardness: steels with hardness< 450 НВ, цветных металлов с твердостью < 200 НВ и т.п.

Vickers hardness. In the standard Vickers hardness test, a tetrahedral diamond pyramid with an apex angle of 136 degrees is pressed into the surface of the sample. The imprint is obtained in the form of a square, the diagonal of which is measured after removing the load.

The Vickers method is used mainly for materials with high hardness, as well as for testing the hardness of parts of small sections or thin surface layers. As a rule, small loads are used: 10, 30, 50, 100, 200, 500 N. The thinner the section of the part or the layer under study, the less the load is chosen.

Rockwell hardness. This method of measuring hardness is the most universal and least labor-intensive. Here there is no need to measure the size of the print, since the hardness number is read directly from the hardness tester scale. The hardness number depends on the depth of indentation of the tip, which is used as a diamond cone with an apex angle of 120 degrees or a steel ball with a diameter of 1.588 mm. The load is selected depending on the tip material.

Microhardness. Microhardness is determined by pressing a diamond pyramid into the surface of a sample under small loads (0.05 - 5 N) and measuring the diagonal of the indentation. The method of determining microhardness evaluates the hardness of individual grains, structural components, thin layers or thin parts.

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Different materials react differently to an external force applied to them, causing a change in their shape and linear dimensions. This change is called plastic deformation. If the body, after the cessation of the impact, independently restores its original shape and linear dimensions, such deformation is called elastic. Elasticity, viscosity, strength and hardness are the main mechanical characteristics of solid and amorphous bodies and determine the changes that occur with a physical body during deformation under the influence of external force and its limiting case - destruction. The yield strength of a material is the value of stress (or force per unit cross-sectional area) at which plastic deformation begins.

Knowledge of the mechanical properties of a material is extremely important for the designer who uses them in his work. It determines the maximum load on a particular part or the structure as a whole, if exceeded, plastic deformation will begin, and the structure will lose its strength and shape and may be destroyed. Destruction or serious deformation of building structures or elements of transport systems can lead to large-scale destruction, material losses and even casualties.

The yield strength is the maximum load that can be applied to a structure without deformation and subsequent failure. The higher its value, the greater the load the structure can withstand.

In practice, the yield strength of a metal determines the performance of the material itself and products made from it under extreme loads. People have always predicted the maximum loads that the structures they erect or the mechanisms they create can withstand. In the early stages of industry development, this was determined experimentally, and only in the 19th century did the creation of the theory of strength of materials begin. The issue of reliability was solved by creating a multiple safety margin, which led to heavier and more expensive structures. Today it is not necessary to create a model of a product of a certain scale or full size and conduct experiments on destruction under load on it - computer programs of the CAE (calculation engineering) family can accurately calculate the strength parameters of the finished product and predict the maximum load values.

The value of the yield strength of the material

With the development of atomic physics in the 20th century, it became possible to calculate the value of the parameter theoretically. This work was first done by Yakov Frenkel in 1924. Based on the strength of interatomic bonds, he, through calculations that were complex for that time, determined the amount of stress sufficient to initiate plastic deformation of bodies of simple shape. The value of the yield strength of the material will be equal to

τ τ =G/2π. , where G is the shear modulus , precisely what determines the stability of bonds between atoms.

Calculation of the yield strength value

The ingenious assumption Frenkel made in his calculations was that the process of changing the shape of the material was considered to be driven by shear stresses. For the onset of plastic deformation, it was assumed that it was sufficient for one half of the body to move relative to the other to such an extent that it could not return to its initial position under the influence of elastic forces.

Frenkel suggested that the material tested in the thought experiment has a crystalline or polycrystalline structure, characteristic of most metals, ceramics and many polymers. This structure presupposes the presence of a spatial lattice, in the nodes of which atoms are arranged in a strictly defined order. The configuration of this lattice is strictly individual for each substance, as are the interatomic distances and the forces connecting these atoms. Thus, in order to cause plastic shear deformation, it will be necessary to break all interatomic bonds passing through the conventional plane separating the halves of the body.

At a certain stress value equal to the yield strength , the bonds between atoms from different halves of the body will be broken, and a number of atoms will shift relative to each other by one interatomic distance without the possibility of returning to their original position. With continued exposure, such a microshift will continue until all the atoms of one half of the body lose contact with the atoms of the other half

In the macrocosm, this will cause plastic deformation, change the shape of the body and, with continued exposure, lead to its destruction. In practice, the line of the beginning of destruction does not pass through the middle of the physical body, but is located in the locations of material inhomogeneities.

Physical yield strength

In the theory of strength, for each material there are several values ​​of this important characteristic. The physical yield strength corresponds to the stress value at which, despite the deformation, the specific load does not change at all or changes insignificantly. In other words, this is the voltage value at which the physical body deforms, “flows,” without increasing the force applied to the sample

A large number of metals and alloys, when tested at tensile strength, demonstrate a yield diagram with an absent or weakly defined “yield plateau”. For such materials they speak of a conditional yield strength. It is interpreted as the stress at which deformation occurs within 0.2%.

Such materials include alloy and high-carbon steel alloys, bronze, duralumin and many others. The more plastic the material is, the higher its residual deformation index. Examples of ductile materials include copper, brass, pure aluminum and most low-carbon steel alloys.

Steel, as the most popular mass structural material, is under particularly close attention of specialists in calculating the strength of structures and the maximum permissible loads on them.

During their operation, steel structures are subjected to combined loads of tension, compression, bending and shear that are large in size and complex in shape. Loads can be dynamic, static and periodic. Despite the most difficult conditions of use, the designer must ensure that the structures and mechanisms he designs are durable, reliable and have a high degree of safety for both personnel and the surrounding population.

Therefore, increased demands on mechanical properties are placed on steel. From the point of view of economic efficiency, the company strives to reduce the cross-section and other dimensions of its products in order to reduce material consumption and weight and thus increase performance characteristics. In practice, this requirement must be balanced with the safety and reliability requirements set out in standards and technical specifications.

The yield strength of steel is a key parameter in these calculations because it characterizes the ability of a structure to withstand stress without permanent deformation or failure.

Influence of carbon content on the properties of steels

According to the physicochemical principle of additivity, the change in the physical properties of materials is determined by the percentage of carbon. Increasing its proportion to 1.2% makes it possible to increase the strength, hardness, yield strength and threshold cold capacity of the alloy. A further increase in the proportion of carbon leads to a noticeable decrease in technical indicators such as weldability and ultimate deformation during stamping operations. Steels with low carbon content exhibit the best weldability.

Nitrogen and oxygen in the alloy

These non-metals from the beginning of the periodic table are harmful impurities and reduce the mechanical and physical characteristics of steel, such as the viscosity threshold, ductility and brittleness. If oxygen is contained in amounts greater than 0.03%, this leads to accelerated aging of the alloy, and nitrogen increases the fragility of the material. On the other hand, the nitrogen content increases strength by reducing the yield strength.

Manganese and silicon additives

An alloying additive in the form of manganese is used to deoxidize the alloy and compensate for the negative effects of harmful sulfur-containing impurities. Due to its similar properties to iron, manganese does not have a significant independent effect on the properties of the alloy. Typical manganese content is about 0.8%.

Silicon has a similar effect; it is added during the deoxidation process in a volume fraction not exceeding 0.4%. Since silicon significantly worsens such a technical indicator as the weldability of steel. For structural steels intended for welding, its proportion should not exceed 0.25%. Silicon does not affect the properties of steel alloys.

Impurities of sulfur and phosphorus

Sulfur is an extremely harmful impurity and negatively affects many physical properties and technical characteristics.

The maximum permissible content of this element in the form of brittle sulfites is 0.06%

Sulfur impairs the ductility, yield strength, impact strength, wear resistance and corrosion resistance of materials.

Phosphorus has a dual effect on the physical and mechanical properties of steels. On the one hand, with an increase in its content, the yield strength increases, but on the other hand, viscosity and fluidity simultaneously decrease. Typically the phosphorus content ranges from 0.025 to 0.044%. Phosphorus has a particularly strong negative effect with a simultaneous increase in the volume fraction of carbon.

Alloying additives in alloys

Alloying additives are substances intentionally introduced into the composition of an alloy to purposefully change its properties to the desired levels. Such alloys are called alloy steels. Better performance can be achieved by simultaneously adding several additives in certain proportions.

Common additives are nickel, vanadium, chromium, molybdenum and others. With the help of alloying additives, the values ​​of yield strength, strength, viscosity, corrosion resistance and many other physical, mechanical and chemical parameters and properties are improved.

Metal melt fluidity

The fluidity of a metal melt is its ability to completely fill the casting mold, penetrating into the smallest cavities and relief details. The accuracy of the casting and the quality of its surface depend on this.

The property can be enhanced by placing the melt under excess pressure. This physical phenomenon is used in injection molding machines. This method can significantly increase the productivity of the casting process, improve the surface quality and uniformity of castings.

Testing a sample to determine the yield strength

To carry out the standard tests, a cylindrical sample with a diameter of 20 mm and a height of 10 mm is used, fixed in the test apparatus and subjected to tension. The distance between the marks applied on the side surface of the sample is called the calculated length. During the measurements, the dependence of the relative elongation of the sample on the magnitude of the tensile force is recorded.

The dependence is displayed in the form of a conditional stretch diagram. At the first stage of the experiment, an increase in force causes a proportional increase in the length of the sample. Upon reaching the limit of proportionality, the diagram turns from linear to curvilinear, and the linear relationship between force and elongation is lost. In this section of the diagram, when the force is removed, the sample can still return to its original shape and dimensions.

For most materials, the values ​​of the proportional limit and the yield strength are so close that in practical applications the difference between them is not taken into account.