Mechanical properties that. Main physical and mechanical characteristics of materials

Mechanical properties characterize the ability of metals and alloys to resist the action of the loads applied to them, and the mechanical characteristics express these properties quantitatively. The main properties of metal materials are; Strength, plasticity (or viscosity), hardness, shock viscosity, wear resistance, creep, etc.
Mechanical characteristics of materials are determined in mechanical tests, which, depending on the nature of the load, the time is divided into static, dynamic and re-variables.
Depending on the method of application of external forces (loads), there are tensile tests, compression, bending, twist, shock bending, etc.
The main mechanical characteristics of metals and alloys.
Temporary resistance (strength limit, tensile tensile tensile strength, corresponding to the greatest load preceding the destruction of the sample.
The true resistance of the rupture (real voltage) is a voltage determined by the reference ratio at the time of breaking to the cross-sectional area of \u200b\u200bthe sample at the break site.
The yield strength (physical) is the smallest voltage at which the sample is deformed without a noticeable increase in the tensile load.
The yield strength (conditional) is the voltage at which the residual elongation reaches a 0.2% length of the sample section, the elongation of which is taken into account when determining the specified characteristic. The limit of proportionality (conditional) is the voltage in which the deviation from the linear dependence between the load and the elongation reaches such a value that the tangent angle of inclination formed by the tangent of the deformation curve (in the point under consideration), with the axis of loads increases by 50% of its value on linear elastic plot. It is allowed to increase tanglex angle of inclination by 10 or 25%.
The limit of elasticity is a conditional voltage corresponding to the appearance of residual deformation. It is assumed to determine the limit of elasticity with tolerances to 0.005%, then accordingly will be designated.
The relative elongation after breaking the increment of the sample length after the gap to its initial settlement length. There are relative elongations obtained by testing on samples with a fivefold and ten-fold ratio of length to diameter. Other relations are allowed, for example 2.5, when testing castings.
The relative narrowing after the break is the ratio of the cross-sectional area of \u200b\u200bthe sample in the gap site to the initial area of \u200b\u200bits cross-section.
These characteristics of mechanical properties are determined when testing materials for stretching according to the methods set forth in GOST 1497-61, on cylindrical and flat samples, forms and dimensions of which are installed in the same standard. Testing tests at elevated temperatures (up to 1200 ° C) are set by GOST 9651-73, on the length of the strength - GOST 10145-62.
The module is normal elasticity of the voltage ratio to the appropriate relative elongation during stretching (compression) within the limits of elastic deformations (the law of the throat).
The shock viscosity is the mechanical characteristic of the viscosity of the metal - is determined by the work consumed for the shock breakage on the pendulum coppe of the sample of this type and the sample referred to the working area of \u200b\u200bthe cross section. Tests for normal temperature It is carried out according to GOST 9454-60, with reduced - according to GOST 9455-60 and with elevated - according to GOST 9656-61.
The endurance limit (fatigue) -maximum voltage at which the sample materials are kept without destroying the specified amount of symmetric cycles (from + p to - p), received for the base. The number of cycles is set by the technical conditions and represents a large number. Metal testing methods are regulated according to GOST 2860-65.
The tensile strength is the ratio of the destructive load to the cross-sectional area of \u200b\u200bthe sample to the test.
The conditional limit of the creep-voltage causing a specified extension of the sample (total or residual) for a set period of time at a given temperature.
Bringel hardness - is determined on a TSh hardness by indulging in a steel hardened ball. Test metal or alloy.
The hardness of Rockwell HRA, HRB and HRC is determined by indulge in the metal ball with a diameter of ~ 1.6mm or cone. (Diamond or carbide) with a duty at a top of 120 ° on a TC hardness. Depending on the definition conditions that are standardized GOST 9013-68, there are three values \u200b\u200bof HR: HRA - for very solid materials (scale A) - the test is performed by pressing a diamond cone; HRB - for soft steel (scale B) - steel ball; HRC - for hardened steel (scale C) - carbide or diamond cone.
The depth of penetration of a diamond cone when testing in the metal is small, which allows you to experience more subtle products than when determining the hardness of the brinel, hardness, but Rockwell is a conditional characteristic, the value of which is counted on the instrument scale.
The hardness of the Vickers HV is determined by pressing the diamond standard regular tetrahedral pyramid. The determination of the number of hardness is performed by measuring the length of diagonals (the arithmetic average of two diagonals) and recalculation by the formula
Standard loads, depending on the sample thickness, received 5, 10, 20, 30, 50 and 100 kgf. Time shutter speed under load is taken for ferrous metals 10-15 seconds, for color - 28-32. Accordingly, the HV 10 / 30-500 symbol means: 500 is the number of hardness; 10 - load and 30 - exposure time.
The Vickers method is used to measure the hardness of parts of small sections and solid thin surface layers of cemented, nitrogenated or cyanted products.

49.Torichny crystallization of metalsSecondary crystallization has a large practical value and serves as the basis for a number of heat treatment processes, aging, etc., significantly changing and improving the properties of alloys. Most of the processes of secondary crystallization are associated with diffusion. Diffusion in solid alloys is possible for a number of reasons. In particular, in substitution solutions, it proceeds by blazing by the presence of unfilled nodes (vacancies) in the lattices. Move can be moved both solvent atoms and solute atoms. In the formation of solutions of implementation, the movement of dissolved atoms occurs through the interstices of the lattices. Diffusion flows the faster than the difference between the concentrate; Under C f e r o and d and z and c i e - the transformation of the elongated crystals into rounded. Both processes proceed due to the desire of the system to reduce free energy. In this case, this is achieved because the ratio of the amount

surfaces of grains to their volumes are becoming less. Coagulation and spheroidization take place the easier than the higher the temperature. In fig. 41 shows the alloy state diagram in which the solubility of the second component in the solid solution decreases. On this diagram (in contrast to the figure. 39), an EQ line appears, which characterizes the release of excess component crystals in, which are called secondary (B2), in contrast to the primary crystals (B \\), which are allocated along the CD line. For example, we consider the course of the formation of secondary crystals when cooling solid solutions A with a concentration of K. at a temperature T \\ the structure of one-to-value, when the line EQ is reached, the solution becomes saturated and the excess phase B2 is released from it, the latter can be released on the boundaries of crystals A and Take the type of grid. It also first the formation of germs and then their growth, however, the place of the appearance of embryos and their growth is pre-determined by the surfaces of primary grains. Sometimes the location of the secondary phase in the form of a grid is undesirable, then or prevent it is not formed, or eliminate. Eliminate the grid in different ways, for example, spheroiding annealing. Crystallization in a diagram (Fig. 41) makes it possible to significantly change the properties of the alloy by quenching and leave or by aging.

50.DS alloys with unlimited solubility of componentsBoth component Unlimited soluble in liquid and solid states Inn form chemical compounds.

Components: A, V.

Phases: L, α.

If two component Easternally dissolved in liquid and solid states, then the existence of only two phases is possible - liquid solo Ly solid solo α. Consequently, three phases can not be, crystallization With constant temperature Not observed and horizontal lines on the diagram not.

The diagram shown in fig. 1, consists of three regions: liquid, liquid + solid solution and solid solution.

Line AMV is linie Licolation, A. line ANV - linie Solidus. Processrystallization Depicted Krivoy cooling splava (Fig. 2).

Point 1 corresponds to the beginning crystallization, point 2 - end. Between points 1 and 2 (i.e. between luminalism and solidus) alloy Located in a two-phase state. At two components and two phases system monovariant (C \u003d k-f + 1 \u003d 2 - 2 + 1 \u003d 1), i.e., if the temperature changes, it changes and concentrationComponents in phases; Each temperature correspond to strictly defined composition phases. concentration and the number of phases alloylying between laminsolidus and liquidus are determined rule Segments. So, alloy To B. point And consists of liquid and solid phases. Structure The liquid phase is determined by the projection points b lying on lines Licolation, A. Structure solid phase - projection points with lying on lines Solidus. The amount of liquid and solid phases is determined from the following ratios: the amount of AC / BC liquid phase, the amount of BA / BC solid phase.

In everything intervallexistallization (from points 1do points 2) from liquid alloy,

having source concentration K, crystals are allocated, richer than refractory component. Structure First crystals Determined by the projection s. End crystallization Splan K MUST B. point 2, when the last drop of fluid having Structure l hardens. The segment showing the amount of solid phase was zero in point / when just started crystallization, and the number of total alloy in point 2, when crystallization ended. Structure liquids varies by curve 1 - L, and Composition - By Krivoy s.- 2, and in moment End crystallizationSostavcrystals same as Structure Source fluid.

51.The complex properties of materials For materials, several characteristic temperature points are introduced, indicating the performance and behavior of materials when the temperature changes. Heating resistance - The maximum temperature at which the service life does not decrease. Under this parameter, all materials are divided into heating classes.

Heat resistance - The temperature at which there is a deterioration in the characteristics in its short-term achievement.
Heat resistance - The temperature at which chemical changes are occurred.
Frost resistance - The ability to operate at reduced temperatures (this parameter is important for rubber).
Spray - The ability to ignite, maintain fire, self-ignition is a different degree of flammability. All these concepts determine the characteristic temperatures under which any property of the material changes. There are some temperatures characteristic of all materials, there are temperatures specific for some electrical materials. In which any characteristics change dramatically. Most materials are inherent in melting points, boiling. The melting point is the temperature at which there is a transition from a solid to liquid.Does not have a melting point liquid helium, even at zero Kelvin remains liquid. The most refractory can be attributed to tungsten - 3387 ° C, molybdenum 2622 ° C, rhenium - 3180 ° C, tantal - 3000 ° C. There are refractory substances among ceramics: Hafnium carbide HFC and TAN tantalum carbide have melting points 2880 ° C., Titanium nitride and carbide - more than 3000 ° C. There are materials, mostly, it is thermoplastic polymers that possess a point of softening, but it does not reach the melting, because The destruction of polymer molecules begins at elevated temperatures. The thermosetting polymers even before softening the case does not reach, the material begins to decompose. There are alloys and other complex substances in which the complex melting process: at some temperature, called "Solidus" partial melting occurs, i.e. Transition of a part of a substance into a liquid state. The remaining substance is in a solid state. It turns out something like Cashitz. As the temperature increases, the increasing part goes into a liquid state, finally at some temperature, called "liquidus" there will be a complete melting of the substance. For example, tin alloy and lead for soldering, called simply "solder", begins to melt at about 180 ° C (the point of solidus), and is melted at about 230 ° C (the dial of the liquidus).

In any melting processes, the achievement of a certain point is necessary, but not enough melting condition. In order to melt the substance, you need to inform it the energy that is called the warmth of melting. It is calculated for one gram (or one molecule). The boiling point is the temperature at which the transition from the liquid state into the vapor-shaped one occurs. Almost all simple substances boil, they do not boil organic compounds, they decompose at lower temperatures, not reaching the boil. The boiling point has a significant effect of pressure. For example, it is possible to move the boiling point from 100 ° C to 373 ° with a pressure of 225 atm to be shifted. Boiling solutions, i.e. Mutually soluble in each other is a difficult way, they boil two components at once, only in a pair of one substance more than the other. For example, a weak alcohol solution in water drops so that in a pair of alcohol more than in water. Due to this, distillation works and after condensation, the pair is obtained alcohol, but enriched with water. There are mixtures pumping at the same time, for example, 96% alcohol. Here, when boiling, the composition of the liquid and the composition of the pair is the same. After the condensation of the pair, the alcohol is exactly the same composition. Such mixtures are called azeotropic. There are temperatures specific for electrical materials. For example, for ferroelectrics are administered by the so-called. curie point. It turns out that the ferroelectric state of the substance occurs only at reduced temperatures. There is such a temperature for each ferroelectric, above which domains cannot exist and it turns into a paralektrix. This temperature is called the Curie point. The dielectric constant below the point of Curie is large, it slightly increasing as the approach to the Curie point. After reaching this point, the dielectric permeability drops sharply. For example, for the most common ferroelectric: Barium titanate, Curie point is 120 ° C, for lead zirconate 270 ° C, for some organic ferroelectrics, the temperature of the Curie is negative. A similar temperature (and also called the Curie point) is available for ferromagnets. The behavior of magnetic permeability is similar to the behavior of dielectric constant as the temperature and approach to the Curie point increases. The only difference is the drop in magnetic permeability with increasing temperature occurs more sharply after reaching the Curie point. Curie points values \u200b\u200bfor some materials: iron 770 ° C, cobalt 1330 ° C, erbium and golmia (-253 ° C), ceramics - in a wide range of temperatures. For antiferromagnets, a similar point is called point of Neel.

Similar information.

f \u003d F - F Noom [Hz]

f \u003d ± 0.1 Hz - allowable value

f \u003d ± 0.2 Hz - maximum allowable value

f \u003d ± 0.4 Hz - emergency value

Changes in the load of consumers in the network may be different. With a small change, the load requires a small power reserve. In these cases, automatic frequency control of one so-called frequency-adjustable station.

With large load changes, the automatic frequency control must be provided at a significant number of stations. This contains graphs of changes in power stations.

When the powerful power lines are disconnected in the post -avary modes, the system may be divided into separately not synchronously working parts.

In power plants, on which the capacity may not be enough, there will be a decrease in the performance of equipment of their own needs (nutrient and circulation pumps), therefore will cause a significant reduction in the power of the station, up to its failure.

In such cases, ACR devices are provided for preventing accidents, disconnecting a part of less responsible consumers in such cases, and after switching on backup power sources, CAPV devices include disconnected consumers.

Mechanical properties characterize the body's ability to resist strain (elastic and plastic) and destruction. For metals and alloys working as structural materials, these properties are defining. Reveal their tests when exposed to external loads.

Quantitative characteristics of mechanical properties: elasticity, plasticity, strength, hardness, viscosity, fatigue, crack resistance, cold resistance, heat resistance. These characteristics are necessary to select materials and modes of their technological processing, calculations for the strength of parts and structures, control and diagnostics of their strength state during operation.

Under the action of external load in the solid, voltage and deformation occur.

Deformation - This is a change in the shape and size of a solid under the action of external forces or as a result of physical processes arising in the body with phase transformations, shrinkage, etc. The deformation can be elastic(The original sample size is restored after removal of the load) and plastic(saved after removing the load).

The voltage S is measured in Pascal (PA), deformation E - in percent (%) of relative elongation (D l./l.) × 100 or narrowing area (D S./S.) × 100.

With an ever-increasing load, the elastic deformation, as a rule, goes into plastic, and then the sample is destroyed (Fig. 1). Depending on the method of application of the load, methods for testing the mechanical properties of metals, alloys and other materials are divided into static, dynamic and alternating.

Strength- the ability of metals to resist deformation or destruction by static, dynamic or alternating loads. The strength of metals under static loads is tensile, compression, bending and twist. The tensile test is mandatory. The strength at dynamic loads is estimated by a specific shock viscosity, and with alternate loads - fatigue strength.

Strength when tensile test is assessed by the following characteristics (Fig. 1).

Tensile strength(strength limit or time resistance) S B is a voltage that meets the greatest load R Max preceding the destruction of the sample:

This characteristic is mandatory for metals.

Limit of proportionalitys Pz is a conditional voltage R PC , in which the deviation begins on proportional dependence between deformation and load:

Yield strengths T is the smallest voltage R T. , in which the sample is deformed (flowing) without a noticeable increase in load:

Conditional yield strengths 0,2 - voltage, after the removal of which the residual deformation reaches a value of 0.2%.

If on the curve voltage - the deformation for the limit of elasticity is formed by the fluidity platform (Fig. 1), the voltage corresponding to the flow rate of the fluidity is formed.

If after the voltage exceeded S T, it is removed, then the deformation will decrease on the dotted line. Section Oo ¢ shows residual plastic deformation.

The value of S T is extremely sensitive to the deformation rate (duration of the load) and to temperature. If the voltage is applied to the material less than S T for a long time, then it can cause plastic (residual) deformation. This is slow and continuous plastic deformation by exposure to constant loads call creep (cripp).

Plastic- The property of metals is deformed without destruction under the action of external forces and maintain a modified form after removing these forces. Plasticity is one of the important mechanical properties of a metal, which, in combination with high strength, makes it the main structural material. Its characteristics are relative extensionbefore breaking D and relative narrowingbefore breaking y. These characteristics are determined by testing metals for tensile, and their numerical values \u200b\u200bare calculated by formulas (as percent):

where l. 0 I. l. P is the length of the sample before and after the destruction, respectively;

F. 0 I. F. R - The cross-sectional area of \u200b\u200bthe sample before and after destruction.

Elasticity - The property of metals to restore its former shape after removing the external forces causing deformation. Elasticity - property, reverse plasticity.

Hardness- the ability of metals to resist the penetration of a more solid body into them. Hardness tests - the most affordable and common type of mechanical testing. The most applied in the technique received static methods of hardness testing when indenting an indenter: method Brinell, Method Vickers and method Rockwell. The hardness, according to these methods, is defined as follows.

By Brinell -the test sample with a certain force is pressed hardened steel Ball diameter D. Under the influence of the load P.and after removing the load is measured the diameter of the print d. (Fig.2, but). Number of hardness in Brinell - HB, characterized by the reference ratio P,acting on the ball, to the surface of a spherical imprint M.:

The smaller the diameter of the imprint d., the greater the sample hardness. Sharch diameter D. and load P. Choose depending on the material and thickness of the sample. Method Brinell It is not recommended to apply for materials with a hardness of more than 450 HB, since the steel ball can noticeably deform, which will make an error in the test results.

Vickersa diamond tetrahedral pyramid with an angle at the top A \u003d 136 ° is pressed into the surface of the material (Fig.2, b.). After removing the pressure of the indulgence measured a diagonal of a print d. 1 . Number of hardness in Vickers. HV is calculated as the load ratio Rto the surface of the pyramidal print M:

Number of hardness in Vickers. denoted by the HV symbol indicating the load Rand the exposure time under load, and the dimension of the number of hardness (kgf / mm 2) is not put. The duration of an indenter exposure under load is taken for steels 10-15 C, and for non-ferrous metals - 30 s. For example, 450 HV 10/15 means that the number of hardness Vickers. 450 Received P \u003d.10 kgf (98.1 H) applied to the diamond pyramid for 15 s.

Advantage of the method Vickerscompared to the method Brinell lies in the fact that Vickers You can experience higher hardness materials due to the use of diamond pyramid.

When testing for hardness by the method Rockwellthe surface of the material is pressed a diamond cone with an angle at a vertex of 120 ° or a steel ball with a diameter of 1.588 mm. However, according to this method, the imprint depth is accepted for the conditional measure of hardness. Test Scheme by Method Rockwell shown in Fig. 2, in.First applied preload R 0, under the action of which an indenter is pressed to depth h. 0. The main load is then applied. R 1, under the action of which an indenter is pressed to depth h. 1 . After that, remove the load R 1, but leave pre-load R 0 .

At the same time, under the action of elastic deformation, the indenter rises up, but does not reach the level h. 0 . Difference ( h. - h. 0) depends on the hardness of the material; The harder material, the less this difference. The imprint depth is measured by a time-type indicator with a division price of 0.002 mm. When testing soft metals by the method Rockwell A steel ball is used as an indenter. The sequence of operations is the same as when testing with a diamond cone. The number of hardness determined by the method Rockwellis indicated by the HR symbol. However, depending on the form of the indenter and the values \u200b\u200bof pressing loads, the letter A, C, or B, denoting the corresponding measurement scale, is added to this symbol.

Numbers of hardness Rockwelldetermine in conventional units by formulas:

where 100 and 130 are an extremely predetermined number of fission indicator of the hourly type with a division price of 0.002 mm.

Crack resistance - The property of materials resist the development of cracks in mechanical and other influences.

Cracks in materials can be metallurgical and technological origin, as well as arise and develop during operation. If it is possible to fragile destruction for the safe operation of structural elements, it is necessary to quantify the dimensions of permissible crack-like defects.

Quantitative characteristic of the crack resistance is critical coefficient of stress intensity in flat deformation at the top of the crack K I s.

Many designs during operation experience shock loads. To solve the issue of their durability and reliability under these conditions, the results of dynamic tests are very important (the load is attached with a blow with a lot of force).

The transition from static loading to dynamic causes a change in all properties of metals and alloys associated with plastic deformation.

To assess the tendency of the material to fragile destruction, tests for impact bending of samples with a cut, as a result of which the shock viscosity is determined.

Shock viscosity - The work spent during the dynamic destruction of the abnormal sample, attributed to the cross-sectional area in the place of the cut.

Viscosity - property, inverse brittleness. The shock viscosity of responsible parts should be high.

In addition to the numerical values \u200b\u200bobtained by testing, an important criterion is the character of a break. Fiberated matte bilate without a characteristic metal gloss indicates viscous destruction. Fragile destruction gives a crystalline shiny break.

Impact viscosity depends on many factors. The presence of sharp transitions in cross-section, cuts, cuts, etc. in products, etc. causes an uneven distribution of stresses in cross section and their concentration. The shock viscosity also depends on the state of the sample surface. Risks, scratches, mechanical traces and other defects reduce shock viscosity.

Dynamic loading causes an increase in the limit of elasticity and yield strength, without translating the material into a fragile state. But when the temperature decreases, the impact resistance decreases sharply. This phenomenon is called coldness .

Clamicular metals include metals with a centrified cubic grille (for example, A-Fe, Mo, Cr). For this group of metals at a certain minus temperature there is a sharp decrease in shock viscosity. The non-store metals include metals with a granetable cubic grille (G-Fe, Al, Ni, etc.). Claudeness in coarse material comes at a higher temperature than the fine-grained.

The character of the fall of the shock viscosity resembles the threshold, which led to the expression "threshold of refrigerant."

Temperature at which a certain drop of shock viscosity occurs, is called critical temperature of fragility T kr.

Most of the destruction of parts and structures during operation occurs as a result of cyclic loading. Moreover, in some cases, the destruction occurs at the stresses underlying the limit of elasticity.

Fatigue - The process of gradual accumulation of damage in the material under the action of cyclic loads, leading to the formation of cracks and destruction.

The term "fatigue" is often replaced by the term "endurance", which shows how much load changes can withstand the metal or alloy without destruction. The fatigue resistance is characterized endurance limit s -1. The number of cycles is conditionally adopted for steels equal to 10 7, for non-ferrous metals - 10 -8.

The phenomenon of fatigue is observed in bending, tap, stretching compression and with other methods of loading.

Microscopic heterogeneity, non-metallic inclusions, gas bubbles, chemical compounds, as well as cuts, risks, scratches, the presence of an anti-housing layer and corrosion traces on the surface of products, which lead to the uneven voltage distribution, and reduce the material resistance to re-alternating loads and reduce the material resistance to re-alternating loads.

Wear resistance - resistance of metals wear due to friction processes. Worn consists in the separation from the rubbing surface of its individual particles and is determined by changing the geometric sizes or mass of the part.

The fatigue strength and wear resistance give the most complete picture of the durability of parts in structures, and the shock viscosity and crack resistance characterizes the reliability of these parts.

Heat resistance- The ability of metals and alloys to resist the beginning and development of plastic deformation and destruction under the action of permanent loads at high temperatures. Limit of short-term strength, creep limit and long-term strength limit - numerical characteristics of heat resistance.

Mechanical properties of materials

the totality of indicators characterizing the resistance of the WHO material on it acting on it, its ability to deform at the same time, as well as the features of its behavior in the process of destruction. In accordance with this, M. s. m. Measure voltages (usually in kgf / mm 2 or MN / m 2), deformations (in%), specific work of deformation and destruction (usually in kgf․m / cm 2 or MJ / m 2), speed of development of the process of destruction during static or reloading (most often in mM. for 1 sec or for 1000 loot repetition cycles, mm / Kcycle). M. s. m. Defined in mechanical testing of samples of various shapes.

In general, materials in structures may be subjected to most different loads ( fig. one ): work on stretching , Compression, bending, crash, sliced, etc., or undergo a joint action of several types of load, such as stretching and bending. Also varied conditions of operation of materials and temperature, environment, the speed of the application of the load and the law of its change in time. In accordance with this there are many indicators of M. s. m. And many methods of mechanical testing. For metals and structural plastics, tensile tests are most common, hardness, impact bending; Fragile structural materials (for example, ceramics, metal ceramics) are often tested for compression and static bending; The mechanical properties of composite materials is important to evaluate, in addition, when testing for a shift.

Deformation diagram. The load applied to the sample causes its deformation (see deformation). The ratios between load and deformation are described by t. N. deformation diagram ( fig. 2. ). Initially, the deformation of the sample (with tension - the increment of the length Δ l.) proportional to the increasing load Rthen at the point n. This proportionality is violated, however, to increase the deformation, it is necessary to further improve the load R; at Δ. l. > Δ l. The deformation is developing without an application from the outside, with gradually falling load. The form of the deformation chart does not change if the ordinates are put off the voltage

(F 0. and l 0. - Accordingly, the initial cross-sectional area and the estimated length of the sample).

Material resistance is measured by stresses characterizing the load per unit cross-sectional area of \u200b\u200bthe sample

in kgf / mm 2. Voltage

in which the proportional load is violated, the growth of deformation is called the proportionality limit. With load R P N Unloading of the sample leads to the disappearance of the deformation that occurred in it under the action of the applied force; Such deformation is called elastic. A slight excess of the load relatively P N. It may not change the nature of the deformation - it will still retain the elastic character. The greatest load that the sample withstands without the appearance of residual plastic deformation during unloading, determines the material of the material of the material:

Elastic properties.In an elastic region, the voltage and deformation are associated with the coefficient of proportionality. When stretching σ \u003d eδ, where E. - T. N. The module of normal elasticity, numerically equal to the tangent angle of the rectilinear section of the curve σ \u003d σ (δ) to the deformation axis ( fig. 2. ). When tensile testing of a cylindrical or flat sample of uniaxial (σ 1\u003e 0; (σ 2 \u003d σ 3 \u003d 0), a three-way deformed state corresponds to a stress state (the increment of length in the direction of the applied forces and a decrease in linear dimensions in two other mutually perpendicular directions): Δ 1\u003e 0; δ 2 \u003d δ 3

within elasticity for the main structural materials, it varies in quite narrow limits (0.27-0.3 for steels, 0.3-0.33 for aluminum alloys). Poisson coefficient is one of the main calculated characteristics. Knowing μ I. E., can be calculated by defining the shift module

Resistance to plastic deformation. With loads R > R B. Along with an ever-increasing elastic deformation, a noticeable irreversible, not disappearing plastic deformation during unloading, appears. The voltage in which the residual relative deformation (with stretching - elongation) reaches a given value (according to GOST - 0.2%), is called the conditional yielding limit and is indicated

Almost accuracy modern methods Tests are such that σ n and σ e are determined with a given tolerances according to the deviation from the law of proportionality [an increase in CTG (90 - α) by 25-50%] and the value of residual deformation (0.003-0.05%) and speak of conditional limits of proportionality and elasticity. Stretching curve structural metals can have a maximum (point in on fig. 2. ) or break when the highest load is reached R B. '. Attitude

characterizes the time resistance (strength limit) of the material. If there is a maximum on the stretching curve in the area of \u200b\u200bloads lying on the level of the left in, the sample is deformed evenly over the entire calculated length l. 0, gradually decreasing in diameter, but maintaining an initial cylindrical or prismatic form. With plastic deformation, the metals are reinforced, therefore, despite the decrease in the sample cross section, it is necessary to apply an increasing load for further deformation. σ B, as well as the conditional σ 0.2, σ n and σ e, characterizes the resistance of plastic deformation metals. In the area of \u200b\u200bdeformation diagram, it changes to the form of a stretched sample: a period of concentrated deformation, expressed in the appearance of "neck". Reducing the section in the neck "overtakes" the hardening of metals, which causes the fall of the external load on the site P in - P K.

In many structural materials, the resistance of plastic deformation in the elastic-plastic region during tension and compression is almost the same. For some metals and alloys (for example, magnesium alloys, high-strength steel) are characterized by noticeable differences in this characteristic when tensile and compression. The resistance of plastic deformation is particularly often (when controlling the quality of products, the standard of thermal processing modes and in other cases) is estimated according to the results of solidness tests by depressing a solid tip in the shape of a ball (solidity in Brinell or Rockwell), cone (solidity in Rockwell) or pyramid (Hardness on Vickers). Tests on hardness do not require disorders of the integrity of the part and therefore are the most massive means of controlling mechanical properties. Brinell hardness (HB) when giving a ball with a diameter D. under load R characterizes the average compressive voltage, conditionally calculated per unit surface of the ball imprint diameter d.:

Characteristics of plasticity. Plasticity with tension of structural materials is estimated by elongation

(Where h 0. and h K. - the initial and final height of the sample), with a crash - the limit angle of twisting the working part of the sample θ, glad or relative shift γ \u003d θ r. (Where r. - sample radius). End of the deformation chart (point k. on the fig. 2. ) characterizes the resistance to the destruction of the metal S K.which is determined

(F K. - the actual area at the breakdown).

Characteristics of destruction. Destruction is not instantly (at the point k.), but develops in time, and the beginning in destruction can correspond to some intermediate point on the plot vC, and the whole process end with gradually falling up to zero load. The position of the point K on the deformation diagram is largely determined by the rigidity of the test machine and the innercity of the measuring system. This makes the magnitude S K. To a large extent conditional.

Many structural metals (steel, including high-strength, heat-resistant chromonichel alloys, soft aluminum alloys, etc.) are destroyed when tensile after significant plastic deformation with the formation of cervix. Often (for example, high-strength aluminum alloys), the destruction surface is located at an angle of about 45 ° to the direction of the tensile force. For certain conditions (For example, when testing colder steels in liquid nitrogen or hydrogen, when exposed to tensile stresses and a corrosion medium for metals, prone to corrosion under stress), the destruction occurs in cross sections perpendicular to the tensile strength (straightflow), without macroplastic deformation.

The strength of materials implemented in elements of structures depends not only on the mechanical properties of the metal itself, but also on the shape and size of the part (so-called. The effects of form and scale), elastic energy accumulated in a loaded design, the nature of the active load (static, dynamic , periodically varying in magnitude), the scheme of the application of external forces (stretching uniaxial, two-axis, with the imposition of bend, etc.), the operating temperature, the environment. The dependence of the strength and plasticity of metals from the form is characterized by T.N. Sensitivity to the incision, estimated usually in relation to the limits of the strength of the abnormal and smooth samples

(In cylindrical samples, the incision is usually performed in the form of a circular shading, in the strips - as a central opening or side cuts). For many structural materials, this ratio under static load is greater than the unit, which is associated with a significant local plastic deformation at the top of the outbreak. The sharper incision, the smaller the local plastic deformation and the greater the proportion of the direct break in the destroyed section. Well-developed straightforward can be obtained at room temperature in most structural materials in laboratory conditions, if stretching or bending expose the samples of a massive section (the thicker than plastic material), supplying these samples with a special narrow slot with an artificially created cracker ( fig. 3. ). When stretching a wide, flat sample, plastic deformation is difficult and limited to a small area of \u200b\u200bsize 2 r y. (on the Fig. 3. , B is shaded), directly adjacent to the tip of the crack. The straight break is usually characteristic of the operational destruction of structural elements.

Wide distribution received offered by American scientist J. R. Irvin as a constant for the conditions of fragile destruction indicators such as the critical stress intensity coefficient with flat deformation K 1C. and viscosity of destruction

In this case, the process of destruction is considered in time and indicators K 1C.(G 1C.) refer to the critical moment when the sustainable development of the crack is disturbed; The crack becomes unstable and distributed spontaneously when the energy required to increase its length is less than the energy of the elastic deformation coming to the vertex of cracks from the neighboring elastic stress zones of the metal.

When appointing sample thickness t. and crack size 2 l Tr. proceed from the following requirement

Voltage intensity coefficient TO Recalls not only the value of the load, but also the length of the moving crack:

(λ takes into account the geometry of the crack and sample), is expressed in kgf / mm 3/2 or MN / m 3/2. By K 1C. or G 1C. You can judge the tendency of structural materials to fragile destruction under operating conditions.

To assess the quality of the metal, the test on the bending of prismatic samples having an incision on one side is very common. In this case, the shock viscosity is estimated (see shock viscosity) (in kgf․m / cm 2 or MJ / m 2) - the work of deformation and destruction of the sample, conditionally attributed to the cross section in the place of the cut. Widely distribution was obtained by testing for shock bending of samples with artificially obtained in the base of the end of the crack fatigue. The work of the destruction of such samples and T. is generally in satisfactory according to such a characteristic of destruction as K 1C., and even better with attitude

Temporary dependence of strength. With an increase in the load time, the resistance of plastic deformation and resistance to the destruction is reduced. At room temperature, metals becomes particularly noticeable at the effects of corrosion (corrosion of stress) or other active (rebeider effect) of the medium. At high temperatures there is a phenomenon of creep (see creep), i.e., the growth of plastic deformation over time at a constant voltage ( fig. four , but). The resistance of creecement metals is evaluated by the conditional belt of creep - most often with a voltage at which plastic deformation for 100 c.reaching 0.2%, and denote it σ 0.2 / 100. The higher the temperature t., the stronger the phenomenon of creep and the more decreases in time the resistance to the destruction of the metal ( fig. four , b). The last property is characterized by T.N. The limit strength limit, i.e. with a voltage, which at a given temperature causes the destruction of the material for a given time (for example, Σ t 100, σ t 1000, etc.). W. polymeric materials The temperature-temporal dependence of the strength and deformation is expressed more than that of metals. When heating plastics, highly elastic reversible deformation is observed; Starting at a certain higher temperature, irreversible deformation is developed, associated with the transition of the material into a viscous as well. The creep is connected and others. An important mechanical property of materials is a tendency to relaxation of stresses, i.e., to a gradual drop in voltage under conditions when the total (elastic and plastic) deformation retains a constant specified value (for example, in tightened bolts). The relaxation of stresses is due to the increase in the share of the plastic component of the general deformation and the decrease in its elastic part.

If the load is valid for the metal, periodically varying on any law (for example, sinusoidal), then with an increase in the number of cycles N. load its strength decreases ( fig. four , B) - Metal "Tired". For structural steel, such a drop in strength is observed before N. \u003d (2-5) .10 6 cycles. In accordance with this, they talk about the limit of the fatigue of structural steel, understanding the voltage amplitude usually under it

below which steel is not destroyed by re-variable load. With | Σ min | \u003d | Σ MAX | The fatigue limit is denoted by the symbol σ -1. The fatigue curves of aluminum, titanium and magnesium alloys usually do not have a horizontal portion, therefore the fatigue resistance of these alloys is characterized by T.N. limited (relevant specified N.) limits of fatigue. The fatigue resistance also depends on the frequency of the load application. The resistance of materials under low frequency conditions and high reloading values \u200b\u200b(slow, or low-cycle, fatigue) is uniquely connected with the limits of fatigue. In contrast to the static load, with re-variable loads, the sensitivity to the incision is always manifested, i.e. the fatigue limit in the presence of an end is below the fatigue limit of a smooth sample. For convenience, the sensitivity to the incision at fatigue express the attitude

characterizes asymmetry of the cycle). In the process of the setting, the period precedes the formation of the focus of fatigue destruction, and the next, sometimes quite a long, period of development of fatigue cracks. The slower the crack develops, the more reliable the material works in the design. The rate of development of fatigue crack dL / DN. Bind with a power intensity coefficient by power function:

LIT: Davidenkov N. N., dynamic tests of metals, 2 ed., L. - M., 1936; Ratner S. I., destruction during re-loads, M., 1959; Serensen S. V., Kogaev V. P., Schnederovich R. M., carrying the ability and calculations of parts of machines for strength, 2 ed., M., 1963; Applied questions viscosity of destruction, per. from English, M., 1968; Friedman Ya. B., Mechanical properties of metals, 3 ed., M., 1974; Test methods, control and research of machine-building materials, ed. A. T. Tumanova, vol. 2, M., 1974.

S. I. Kishkin.

Fig. 3. Sample with a fascinated fatigue specially designed in the top for determining K 1C. Tests for horsepower (a) and axial (b) stretching.

Big soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

Watch what is "mechanical properties of materials" in other dictionaries:

Mechanical properties of materials, such as strength, resistance to destruction, hardness, etc. are in many cases determining to make a decision on the use of material. Methods for testing mechanical properties It should be noted the following ... Wikipedia

Materials Reaction material on the applied mechanical. Load. OSN. Characteristics of mechanical. Properties are voltages and deformation. Voltage characteristics of forces, to rye refer to a single sample section of a material or product, designs from ... ... Physical encyclopedia

Materials, such as strength, resistance to destruction, hardness, etc. are in many cases determining to make a decision on the use of material. Methods for checking mechanical properties It should be noted the following basic methods ... ... Wikipedia

Mechanical properties - - reflect the ability of the material to resist the power, thermal, shrinkage or other internal stresses without disturbing the established structure. Mechanical include deformal properties: strength, hardness, abrasibility, ... ...

Mechanical properties of rock - - Properties characterizing the occurrence, distribution and change of mechanical stresses and deformations in rocks when exposed to mechanical loads. [GOST R 50544 93] Topic Termin: Properties rock breed Headings Encyclopedia ... Encyclopedia Terms, Definitions and Explanations of Building Materials

Properties of materials - Terms Headings: Material Properties Aggregation Material Activation Materials Actric Activities Analysis Real ... Encyclopedia Terms, Definitions and Explanations of Building Materials

Mechanical properties estimate the body's ability to resist mechanically loads, characterize the performance of products.

Mechanical Properties are called, which are determined when testing under the action of external loads, the result of these tests are the quantitative characteristics of mechanical properties. Mechanical properties characterize the behavior of the material under the action of stresses (leading to deformation and destruction) operating both in the process of manufacturing products (casting, welding, pressure processing, etc.) and operation.

Standard characteristics of mechanical properties are determined in laboratory conditions on samples of standard sizes by creating an irreversible plastic deformation or destruction of samples. Tests are carried out under the conditions of exposure to external loads: stretching, compression, tapping, blow; In conditions of alternate and wear loads. The values \u200b\u200bof the features are usually given in reference books.

An example is the characteristics:

Resistance to the destruction, estimated by the strength limit, or time resistance, is the maximum specific load (voltage), which the material can withstands prior to destruction during its tension;

The resistance of plastic deformation, estimated by the yield strength, is the voltage at which the plastic deformation of the material under tension begins;

Resistance to elastic deformations, estimated by the elastic limit - is the voltage, above which the material acquires residual deformations;

The ability to withstand plastic deformations, evaluated by the relative elongation of the sample when tensile and the relative narrowing of its cross section;

The ability to resist dynamic loads, estimated by a shock viscosity;

The hardness estimated by the resistance of the injection of the indenter (reference sample).

The mechanical properties of materials are determined in static and dynamic loading conditions.

Elasticity characterizes the elastic properties of the polymer, the ability of the material to large reversible changes in the form at low loads due to fluctuations in the links and the ability of the macromolecule.

Static testing also includes compression tests, twisting, bending and other types of loading.

The general disadvantage of static methods for determining the physicomechanical properties of materials is the need to destruction of the sample, which eliminates the possibility of further use of the part for the direct purpose as a result of a sample cut from it for testing.

Determination of hardness. This is a method of non-destructive testing of the mechanical properties of the material under static load. The hardness is assessed mainly in metals, since for most non-metallic materials, the hardness is not a property determining their performance.

The hardness is assessed by the resistance of the material to penetration into it with the static load of the foreign body of a proper geometric shape having a reference hardness (Fig. 14).

Fig. 14 Determination of material hardness: but - loading scheme; b. - measurement of chapel hardness; in - Vickers hardness measurement

Pressing the reference sample into the test sample is performed on special devices, of which the devices of Brinenel, Rockwell, Vickers apply.

The brinnel method is the most common - a ball of tempered steel is pressed into the sample. Diameter Print d. OTO is measured using a magnifying glass with a scale. Next, the tables find the hardness of the material. In the tests according to the Vickers method, a diamond cutter is used, and according to the Rockwell method - a diamond cone.

Luminescence (fluorescence and phosphorescence) are the effects of the luminescence when the energy of the incident light, mechanical effects, chemical reactions or heat.

The optical properties of substances have a huge applied value. The refraction of light is used to manufacture the lenses of optical instruments, the reflection is thermal insulation: the selection of the corresponding coatings can be influenced by the properties of the materials in order to absorb or reflect the heat radiation, but passing visible light. Window glasses have a characteristic color for air conditioning.

Self-searchable glasses-chameleons, fluorescent lamps and oscilloscope screens. Metal coatings (anodized aluminum) are used for decorative purposes (the value has the reflectivity of the material), precision mirrors of metallized surfaces.

Decorative properties Materials are determined by them external species and depend on their outer pattern, design, texture, structures, the method of surface treatment, from the presence of coatings and reliefs.

Biological properties materials are determined:

Their impact on environment, the degree of their toxicity for living organisms;

Their suitability for the existence and development of any organisms (fungi, insects, mold, etc.).

The main mechanical properties include strength, plasticity, hardness, shock viscosity and elasticity. Most mechanical properties are determined experimentally stretching standard samples on test machines.

Strength - The ability of the metal to resist the destruction of the external forces on it.

Plastic - Metal ability to irreversibly change its shape and sizes under the action of external and internal forces without destruction.

Hardness - The ability of the metal to resist the introduction of a more solid body into it. The hardness is determined by the help of hardnessers by the introduction of a steel hardened ball into the metal (on the Brinell device) or the introduction of a diamond pyramid into a well-prepared surface of the sample (on the Rockwell instrument). The smaller the imprint size, the greater the hardness of the test metal. For example, carbon steel Before hardening, has a hardness 100. . . 150 HB (for brinell), and after quenching - 500. . . 600 HV.

Shock viscosity - Metal ability to resist the action of shock loads. This value is indicated Ks. (J / cm 2 or kgf m / cm) is determined by the ratio of mechanical work BUT,spent on the destruction of the sample during shock bending, to the cross-sectional area of \u200b\u200bthe sample .

Elasticity - Metal ability to restore the form and volume after the cessation of the actions of the external forces. This value is characterized by a modulus of elasticity. E. (MPa or kgf / mm 2), which is equal to the ratio a K. caused by the elastic deformation. High elasticity should have steel and alloys for the manufacture of springs and springs.

Mechanical properties of metals

Under mechanical properties understand the characteristics that determine the behavior of the metal (or other material) under the action of the applied external mechanical forces. The mechanical properties usually include the resistance of the metal (alloy) of deformation (strength) and resistance to the destruction (plasticity, viscosity, as well as the ability of metal not to collapse in the presence of cracks).

As a result, mechanical tests receive numerical values Mechanical properties, i.e., the values \u200b\u200bof stresses or deformations in which changes in the physical and mechanical states of the material occur.

Property evaluation

When evaluating the mechanical properties of metal materials, several groups of their criteria are distinguished.

1. Criteria defined independently of the design features and nature of the product service. These criteria are located by standard tests of smooth stretching samples, compression, bending, hardness (static tests) or shock bending with cuts (dynamic tests).
2. Strength and plastic properties defined in static tests on smooth samples although they have important (They are included in the estimated formulas) in many cases do not characterize the strength of these materials in real conditions of operation of machine parts and structures. They can only be used for a limited number of simple in the form of products operating under conditions of static load at temperatures close to normal.
3. Criteria for assessing the structural strength of the material that are in the greatest correlation with service properties this product and characterize the performance of the material under operating conditions.

Design strength of metals

The criteria for the structural strength of metal materials can be divided into two groups:

• criteria that determine the reliability of metallic materials against sudden destruction (viscosity of destruction, work absorbed in the propagation of cracks, vitality, etc.). At the heart of these techniques using the main provisions of destruction mechanics, there are static or dynamic tests of samples with sharp cracks, which take place in the real parts of machines and structures under operating conditions (cuts, through holes, non-metallic inclusions, micro-ducts, etc.). Cracks and micro-links strongly change the behavior of metal under load, since there are voltage concentrators;
• criteria that determine the durability of products (fatigue resistance, wear resistance, corrosion resistance, etc.).

Criteria for evaluation

Criteria for assessing the strength of the structure as a whole (structural strength), determined in poster, inventory and operational tests. With these tests, the effect on the strength and durability of the design of such factors, as the distribution and value of residual stresses, defects of manufacturing technology and the design of metal waste, etc.

For solutions practical tasks Metal science must be determined both standard mechanical properties and criteria for structural strength.