Mechanical properties of metals. General characteristics of mechanical properties Mechanical properties that

The main mechanical properties include strength, ductility, hardness, impact strength and elasticity. Most indicators of mechanical properties are determined experimentally by stretching standard samples on testing machines.

Strength- the ability of a metal to resist destruction under the action of external forces on it.

Plastic- the ability of a metal to irreversibly change its shape and size under the influence of external and internal forces without destruction.

Hardness- the ability of a metal to resist the introduction of a more solid body into it. Hardness is determined using hardness testers by introducing a hardened steel ball into the metal (on a Brinell device) or by introducing a diamond pyramid into a well-prepared sample surface (on a Rockwell device). The smaller the print size, the greater the hardness of the metal being tested. For example, carbon steel before hardening has a hardness of 100. . . 150 HB (according to Brinell), and after hardening - 500. . . 600 HB.

impact strength- the ability of the metal to resist the action of shock loads. This value, denoted KS(J / cm 2 or kgf m / cm), determined by the ratio of mechanical work A, spent on the destruction of the sample during impact bending, to the cross-sectional area of the sample .

Elasticity- the ability of a metal to restore its shape and volume after the cessation of external forces. This value is characterized by the modulus of elasticity E(MPa or kgf / mm 2), which is equal to the stress ratio a to the elastic deformation caused by it. Steels and alloys for the manufacture of springs and springs must have high elasticity.

Mechanical properties of metals

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

As a result of mechanical tests, numerical values of mechanical properties are obtained, i.e., stress or strain values at which changes occur in the physical and mechanical states of the material.

Property evaluation

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

Criteria determined regardless of the design features and nature of the service products. These criteria are found by standard testing of smooth specimens for tension, compression, bending, hardness (static tests) or impact bending of notched specimens (dynamic tests).
The strength and plastic properties determined during static tests on smooth samples, although they are important (they are included in the calculation formulas), in many cases do not characterize the strength of these materials in real operating conditions of machine parts and structures. They can only be used for a limited number of simple-shaped products operating under static load conditions at temperatures close to normal.
Criteria for evaluating the structural strength of the material, which are in the greatest correlation with the service properties of this product and characterize the performance of the material under operating conditions.

Structural strength of metals

Structural strength criteria for metallic materials can be divided into two groups:

criteria that determine the reliability of metallic materials against sudden failures (fracture toughness, work absorbed during crack propagation, survivability, etc.). These techniques, which use the basic principles of fracture mechanics, are based on static or dynamic tests of samples with sharp cracks that occur in real machine parts and structures under operating conditions (cuts, through holes, non-metallic inclusions, microvoids, etc.). Cracks and microdiscontinuities greatly change the behavior of the metal under load, as they are stress 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 during bench, full-scale and operational tests. During these tests, the influence on the strength and durability of the structure of such factors as the distribution and magnitude of residual stresses, defects in the manufacturing technology and design of metal products, etc., is revealed.

To solve practical problems of metallurgy, it is necessary to determine both the standard mechanical properties and the criteria for structural strength.

Material selection criteria

Properties- this is a quantitative or qualitative characteristic of a material that determines its commonality or difference with other materials.
There are three main groups of properties: operational, technological and cost, which underlie the choice of material and determine the technical and economic feasibility of its use. Performance properties are paramount.
operational name the properties of the material that determine the performance of machine parts, devices and tools, their power, speed, cost and other technical and operational indicators.
The performance of the vast majority of machine parts and products provides a level of mechanical properties that characterize the behavior of the material under the action of an external load. Since the loading conditions of machine parts are diverse, the mechanical properties include a large group of indicators.
Depending on the change in time, the load is divided into static and dynamic. Static loading is characterized by a low rate of change in its magnitude, and dynamic loads change over time at high rates, for example, during shock loading. In addition, loads are divided into tensile, compressive, bending, twisting and shear. The change in load can be of a periodically recurring nature, as a result of which they are called repetitively variable or cyclic. Under the operating conditions of machines, the impact of the listed loads can manifest itself in various combinations.
Under the influence of external loads, as well as structural-phase transformations, internal forces arise in the material of structures, which can be expressed through external loads. The internal forces per unit area of the cross section of the body are called stresses. The introduction of the concept of stresses makes it possible to carry out calculations on the strength of structures and their elements.
In the simplest case of axial tension of a cylindrical rod, the stress σ is defined as the ratio of the tensile force P to the initial cross-sectional area Fo, i.e.

σ = P/Fo

The action of external forces leads to deformation of the body, i.e. to change its size and shape. The deformation that disappears after unloading is called elastic, and the deformation that remains in the body is called plastic (residual).
The performance of a separate group of machine parts depends not only on the mechanical properties, but also on the resistance to the action of a chemically active working environment, if such an effect becomes significant, then the physical and chemical properties of the material become decisive - heat resistance and corrosion resistance.
Heat resistance characterizes the ability of a material to resist chemical corrosion in an atmosphere of dry gases at high temperature. In metals, heating is accompanied by the formation of an oxide layer (dross) on the surface.
Corrosion resistance- this is the ability of a metal to resist electrochemical corrosion, which develops in the presence of a liquid medium on the surface of the metal and its electrochemical inhomogeneity.
For some machine parts, the physical properties that characterize the behavior of materials in magnetic, electric and thermal fields, as well as under the influence of high energy flows or radiation, are important. They are usually divided into magnetic, electrical, thermophysical and radiation.
The ability of a material to be subjected to various methods of hot and cold working is determined by technological properties. These include casting properties, deformability, weldability and machinability with a cutting tool. Technological properties make it possible to perform form-changing processing and obtain blanks and machine parts.
The last group of basic properties includes the cost of the material, which evaluates the economics of its use. Its quantitative indicator is - the wholesale price - the cost per unit mass of materials in the form of ingots, profiles, powder, piece and welded blanks, according to which the manufacturer sells its products to machine-building and instrument-making enterprises.

Mechanical properties determined under static loads

Mechanical properties characterize the resistance of a material to deformation, destruction, or a feature of its behavior in the process of destruction. This group of properties includes indicators of strength, stiffness (elasticity), plasticity, hardness and viscosity. The main group of such indicators is the 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 operating conditions.
According to the method of application of loads, static tests are distinguished for tension, 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 test. When stretching standard samples with a cross-sectional area Fo and a working (calculated) length lo, a tension diagram is built in the coordinates: load - elongation of the sample (Fig. 1). There are three sections on the diagram: elastic deformation up to load Rupr .; uniform plastic deformation from Rupr. to Рmax and concentrated plastic deformation from Рmax to Рк. The rectilinear section is maintained until the load corresponding to the proportionality limit Rpc. The tangent of the slope of the straight section characterizes the modulus of elasticity of the first kind E.

Rice. one. Ductile metal tensile diagram (a) and diagrams
conditional stresses of ductile (b) and brittle (c) metals.
The diagram of true stresses (dashed line) is given for comparison.

Plastic deformation above R control. goes with increasing load, since the metal is strengthened in the process of deformation. Hardening of a material during deformation is called work hardening.

The work hardening of the metal increases until the sample breaks, although the tensile load decreases from P max to P k (Fig. 1, a). This is explained by the appearance of a local neck thinning in the sample, in which plastic deformation is mainly concentrated. Despite the decrease in load, the tensile stress in the neck increases until the specimen fails.
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 (Fig. 1b). These stresses in everyday practice do not determine, but use the stress conditions, considering that the cross section F o sample remains unchanged.

Stresses σ control, σ t, σ in - standard strength characteristics. Each is obtained by dividing the corresponding load R ex. R t and R max to the initial cross-sectional area F O .

elastic limitσ ex. name the stress at which the plastic deformation reaches values of 0.005; 0.02 and 0.05%. The corresponding elastic limits are denotedσ 0.005, σ 0.02, σ 0.05.

The conditional yield strength is the stress, which corresponds to a plastic deformation equal to 0.2%; it is designatedσ 0.2 . Physical yield strengthσ t determined from the tensile diagram when it has a yield plateau. However, when tested in tension, most alloys do not show a yield plateau on the charts. The chosen plastic deformation of 0.2% quite accurately characterizes the transition from elastic to plastic deformations.

The tensile strength characterizes the maximum bearing capacity of the material, its strength prior to destruction:

σ in \u003d P max / F o

Plasticity is characterized by relative elongation δ and relative contraction ψ:

where lk is the final length of the sample; lo and Fo are the initial length and cross-sectional area of the sample; Fk is the cross-sectional area at the rupture site.
For low-plastic materials, tensile tests (Fig. 1c) cause significant difficulties. Such materials are usually subjected to bending tests.

Bending test. During a bending test, both tensile and compressive stresses occur in the specimen. Cast irons, tool steels, case-hardened steels and ceramics are tested for bending. The determined characteristics are tensile strength and deflection.

The flexural strength is calculated by the formula:

σ and = M / W,

where M is the largest bending moment; W - section modulus, for the image of a circular section

W = πd 3 / 32

(where d is the sample diameter), and for rectangular samples W = bh 2 /6, where b, h are the width and height of the sample).
Hardness tests . Hardness is understood as the ability of a material to resist penetration into its surface by a solid body - an indenter. 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. A feature of the ongoing plastic deformation is that a complex stress state arises near the tip, close to all-round non-uniform compression. For this reason, plastic deformation is experienced not only by plastic, but also by brittle materials.
Thus, hardness characterizes the resistance of a material to plastic deformation. The same resistance is also estimated by the temporary resistance, in the determination of which a concentrated deformation occurs in the neck region. Therefore, for a number of materials, the numerical values of hardness and tensile strength are proportional. In practice, four methods of measuring hardness are widely used: Brinell hardness, Vickers hardness, Rockwell hardness and microhardness.
When determining the hardness according to Brinell (GOST 9012-59), a hardened ball with a diameter of 10 is pressed into the sample surface; 5 or 2.5 mm under load from 5000N to 30000N. After the load is removed, an imprint is formed on the surface in the form of a spherical hole with a diameter d.
When measuring Brinell hardness, pre-compiled tables are used indicating the HB hardness number. Depending on the indentation diameter and the selected load, the smaller the indentation diameter, the higher the hardness.
The Brinell measurement method is used for steels with hardness < 450 HB, non-ferrous metals with hardness < 200 HB. For them, a correlation has been established between the tensile strength (in MPa) and the hardness number HB:
σ in » 3.4 HB - for hot-rolled carbon steels;
σ in » 4.5 HB - for copper alloys;
σ in » 3.5 HB - for aluminum alloys.
With the standard Vickers measurement method (GOST 2999-75), a tetrahedral diamond pyramid with an apex angle of 139° is pressed into the sample surface. The imprint is obtained in the form of a square, the diagonal of which is measured after the load is removed. The hardness number HV is determined using special tables by the value of the imprint diagonal at a selected load.

The Vickers method is used mainly for materials with high hardness, as well as for testing the hardness of parts with 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.
The Vickers and Brinell hardness numbers for materials with hardness up to 450 HB are practically the same.
Rockwell hardness measurement (GOST 9013-59) is the most versatile and least labor intensive. The hardness number depends on the depth of indentation of the tip, which is used as a diamond cone with an angle at the top of 120 0 or a steel ball with a diameter of 1.588 mm. For various combinations of loads and tips, the Rockwell device has three measuring scales: A.V.S. Rockwell hardness is indicated by the numbers that determine the level of hardness, and the letters HR indicating the hardness scale, for example: 70HRA, 58HRC, 50HRB. Rockwell hardness numbers do not have exact relationships with Brinell and Vickers hardness numbers.
Scale A (tip - diamond cone, total load 600N). This scale is used for particularly hard materials, for thin sheet materials or thin (0.6-1.0 mm) layers. The limits of measurement of hardness on this scale are 70-85.
Scale B (tip - steel ball, total load 1000N). With this scale, the hardness of relatively soft materials is determined (<400НВ). Пределы измерения твердости 25-100.

Scale C (tip - diamond cone, total load 1500N). This scale is used for hard materials (> 450 HB), such as hardened steels. The limits of measurement of hardness on this scale are 20-67. Determination of microhardness (GOST 9450-76) is carried out by pressing a diamond pyramid into the sample surface at low loads (0.05-5N), followed by measurement of the imprint diagonal. This method evaluates the hardness of individual grains, structural components, thin layers or thin parts.

Mechanical properties determined under dynamic loads

During the operation of machine parts, dynamic loads are possible, under which many metals show a tendency to brittle fracture. The risk of destruction is increased by notches - stress concentrators. To assess the tendency of the metal to brittle fracture under the influence of these factors, dynamic impact bending tests are carried out on pendulum impact testers (Fig. 2). The standard sample is placed on two spores and a blow is applied in the middle, leading to the destruction of the sample. On the pendulum copra scale, work is determined TO spent on destruction, and calculate the main characteristic obtained as a result of these tests - percussion viscosity:

KS = K / S 0 1 , [MJ/m 2 ],

where S 0 1, the cross-sectional area of the sample at the notch.

Rice. 2. Scheme pendulum copra (a) and impact test (b):
1 - sample; 2 - pendulum; 3 - scale; 4 – scale pointer; 5- brake.

In accordance with GOST 9454-78, three types of samples are tested: U-shaped (notch radius r=1 mm); V-shaped (r \u003d 0.25 mm) and T-shaped (fatigue crack created at the base of the notch. Accordingly, impact strength means: KCU, KCV, KCT. Impact strength of all mechanical properties is the most sensitive to temperature decrease. Therefore, tests impact strength at low temperatures is used to determine the threshold cold brittleness- the temperature or temperature range in which the impact strength decreases. Cold brittleness- the ability of a metallic material to lose viscosity, brittle fracture with decreasing temperature. Cold brittleness is manifested in iron, steel, metals and alloys having a body-centered cubic (bcc) or hexagonal close-packed (HP) lattice. It is absent in metals with a face-centered cubic (fcc) lattice.

Mechanical properties determined under variable cyclic loads

Many machine parts (shafts, connecting rods, gears) experience repeated cyclic loads during operation. The processes of gradual accumulation of damage in a material under the action of cyclic loads, leading to a change in its properties, the formation of cracks, their development and destruction, are called fatigue, and the ability to resist fatigue - endurance(GOST 23207-78). The ability of materials to work under conditions of cyclic loading is judged by the results of testing specimens for fatigue (GOST 25.502-79). They are carried out on special machines that create multiple loading in the samples (tension - compression, bending, torsion). Samples are tested sequentially at different stress levels, determining the number of cycles before failure. The test results are depicted as a fatigue curve, which is plotted in the coordinates: maximum cycle stress σ max / or σ in ) is the number of cycles. Fatigue curves allow you to define the following endurance criteria:

- cyclic strength, which characterizes the bearing capacity of the material, i.e. the greatest voltage that he is able to withstand for a certain time of work.- cyclic durability- the number of cycles (or operating hours) that the material withstands before the formation of a fatigue crack of a certain length or before fatigue failure at a given stress.

In addition to determining the considered criteria for high-cycle endurance, for some special cases, tests for low cycle fatigue. They are carried out at high voltages (above σ 0.2 ) and low loading frequency (usually no more than 6 Hz). These tests simulate the operating conditions of structures (for example, aircraft) that perceive rare but significant cyclic loads.

Mechanical properties characterize the ability of a material to resist deformation and destruction under the action of applied loads.

According to the nature of the change in time of the acting load, mechanical tests are divided into static (tensile, compression, bending, torsion); dynamic (for impact bending) and cyclic (for fatigue).

According to the effect of temperature on the process, they are divided into tests at room temperature, low-temperature and high-temperature (for long-term strength, creep).

Static tests are carried out when a sample is exposed to a certain speed of a constantly acting load. The strain rate is 10 -4 -10 -1 s -1 . Static tensile tests are among the most common. The properties determined by these tests are given in numerous material specification standards. Static tests include: tension, compression, bending, torsion.

Dynamic tests are characterized by the application of a shock load to the sample and a significant strain rate. The duration of the test does not exceed hundreds of fractions of a second. The strain rate is about 10 2 s -1 . Dynamic tests are most often carried out according to the scheme of impact bending of specimens with a notch.

Cyclic tests characterized by multiple changes in load in magnitude and direction. An example of tests are fatigue tests, they are long and, based on their result, determine the number of cycles to failure at different stress values. Ultimately, the limiting stresses are found that the sample can withstand without destruction for a certain number of loading cycles.

The simplest mechanical property is hardness. Methods for determining hardness are divided, depending on the rate of application of the load, into static and dynamic, and according to the method of its application, into methods of indentation and scratching. Methods for determining hardness according to Brinell, Rockwell, Vickers are static test methods.

Hardness – This is the ability of a material to resist being pressed into it by a harder body (an indenter) under the action of external forces.

When testing for hardness, a pyramid, cone or ball (indenter) is pressed into the surface of materials, in connection with which the test methods are distinguished, respectively, according to Vickers, Rockwell and Brinell. In addition, there are less common hardness testing methods: the elastic rebound method (according to Shore), the comparative hardness method (Poldi) and some others.

When testing materials for hardness, standard special samples are not made, however, certain requirements are imposed on the dimensions, surface of samples and products.

Vickers hardness(GOST 2999-75) is determined by pressing a diamond pyramid into the metal with an apex angle of 136° under a constant load (P): 1; 2; 2.5; 3; 5; 10; twenty; thirty; 50 or 100 kgf and hold under load for 10–15 s. To determine the hardness of ferrous metals and alloys, loads from 5 to 100 kgf are used, copper alloys - from 2.5 to 50 kgf, aluminum alloys - from 1 to 100 kgf. After removing the load, the length of the diagonal of the indentation is determined using a microscope of the device, and the hardness HV is calculated by the formula

where P is the load, kgf; d is the imprint diagonal, mm.

The test standard contains a table of the dependence of hardness on the magnitude of the load and the length of the diagonal. Therefore, in practice, calculations are not made, but a ready-made calculation table is used. Vickers hardness HV is measured in kgf/mm2, N/mm2 or MPa. The Vickers hardness value can vary from HV 2060 to HV 5 at a load of 1 kgf.

By method Brinell a hardened steel ball with a diameter of 10, 5 or 2.5 mm is pressed into the sample or product under the action of loads of 3,000, 1,000, 750, 500, 250, 62.5 kgf and others (GOST 9012-59). The scheme for determining Brinell hardness is shown in fig. 1.20. The resulting round imprint on the sample is measured with a magnifying glass and the Brinell hardness value is found from the tables, the value of which does not exceed 450 HB. The Brinell hardness is almost the same as the Vickers hardness values.

The HB hardness is also the magnitude of the indentation resistance stresses, i.e. physical quantity:

where P is the load, kgf; D is the ball diameter, mm; t is the depth of the imprint segment; d is the indentation diameter, mm.

Rice. 1.20. Scheme for determining Brinell hardness.

Brinell hardness HB (default) has the dimension kgf/mm 2, for example, the hardness of an aluminum alloy is 70 HB. With a load defined in Newtons, Brinell hardness is measured in MPa.

For example, the hardness of annealed steel is 207 HB at a load of 3,000 kgf, a ball diameter of 10 mm, an imprint diameter of 4.2 mm, or, given the conversion factor: 1 newton = 9.8 kgf, HB = 2028 MPa.

By method Rockwell(GOST 9013-59) a diamond cone is pressed in with an angle at the top of 120° (scales A and C) or a steel ball with a diameter of 1.5875 mm (scale B). In this case, the hardness is determined, respectively, HRA, HRC and HRB. Currently, Rockwell hardness measurement is the most common method, because when using Rockwell hardness testers, it is not necessary to measure the indentation, the hardness number is read from the instrument scale immediately after the main load is removed.

The method consists in pressing the indenter into the test sample under the action of two successively applied loads - the preliminary P 0 and the main P 1, which is added to the preliminary one, so that the total load P = P0 + P1. After holding for several seconds, the main load is removed and the residual depth of penetration of the indenter is measured, which at the same time continues to be under the action of the preload. The movement of the main pointer of the indicator by one division of the scale corresponds to the movement of the indenter by 0.002 mm, which is taken as a unit of hardness.

On fig. 1.21 shows a scheme for measuring hardness using the Rockwell method with a diamond or carbide cone. During testing, the depth of the restored imprint is measured. Scales A and C coincide with each other, since the tests are carried out with the same indenter - a diamond cone, but at different loads: 60 and 150 kgf, respectively. Hardness in this case is defined as

Rice. 1.21. Rockwell hardness determination scheme (indenter - cone).

In practice, Rockwell hardness values are not calculated by formulas, but are read from the corresponding (black or red) scale of the instrument. The HRC and HRA scales are used for high hardness, HRB for low hardness. The Rockwell hardness number is measured in arbitrary units, it is a measure of the depth of indentation of the indenter under a certain load.

Mechanical properties of metals in tension . The tensile test of materials is carried out in accordance with GOST 1497-84 "Tension test methods". The standard establishes methods for static tensile testing of ferrous and non-ferrous metals to determine at a temperature of 20 ° C the limits of proportionality, elasticity, yield strength, tensile strength, relative elongation and relative contraction, modulus of elasticity.

For testing, flat and cylindrical specimens cut from a part or specially made are used. The dimensions of the samples are regulated by the specified standard, they are subject to geometric similarity and can be short and long. For a cylindrical sample, the ratio of the initial working length is taken l 0 and initial diameter d0: l 0 = 5d 0 - short sample, l 0 = 10d 0 is a long sample. For a flat sample, the working length ratio is taken l 0 and cross-sectional area F 0: l 0 = 5.65 F 0 - short sample, l 0 = 11.3 F 0 is a long sample. Cylindrical samples are made with a diameter of 3 mm or more. Samples consist of a working part with a length l 0 , and heads, the shape and size of which corresponds to the grips of the machine (Fig. 1.22).

Rice. 1.22. Cylindrical and flat specimens before and after tensile testing.

Rice. 1.23. Primary stretch diagram.

The sample is stretched on special machines that record the magnitude of the applied load and the change in the length of the sample during stretching.

The same machines allow you to record the change in the length of the sample with increasing load (Fig. 1.23), i.e. primary tensile test diagram in coordinates: load (P), in N, kN and absolute elongation of the sample Δ l in mm.

By measuring the magnitude of the load at the characteristic points of the tensile test diagram (Fig. 1.23), the following characteristics of the mechanical properties of materials are determined:

σ pts - proportionality limit, point R;

σ 0.05 - elastic limit, point e;

σ t - physical yield strength, point s;

σ 0.2 - conditional yield strength;

σ in - tensile strength or tensile strength, point b.

The values of 0.05 and 0.2 in the record of the elastic and yield strength correspond to the value of residual deformation Δ l as a percentage of l 0 when the sample is stretched. Tensile test stresses are determined by dividing the load P corresponding to the characteristic point on the diagram by the initial cross-sectional area F 0 of the working part of the test sample:

The cross-sectional area F 0 is determined as follows:

for a cylindrical sample

for a flat sample, F 0 = a 0 × b 0, where a 0 is the initial thickness, and b 0 is the initial width of the sample. At point k, the fracture resistance stress of the material is determined.

The proportional limit and the elastic limit are determined using a strain gauge (a device for determining the amount of deformation). The physical and conditional yield strength is calculated by determining the load from the tensile diagram. If there is no yield point on the diagram, then to calculate the conditional yield strength, it is necessary to draw graphical constructions on the diagram (Fig. 1.24). First, find the value of residual deformation, equal to 0.2% of l 0 , then mark a segment on the deformation axis equal to 0.2% of l 0 and draw a line parallel to the proportional portion of the stretch diagram until it intersects with the stretch curve. Load R 0.2 corresponds to the point of their intersection. The physical or conditional yield strength characterizes the ability of a material to start plastic deformation, i.e. resistance to small plastic deformation.

Rice. 1.24. Determination of the yield strength.

The tensile strength can be calculated using the reading of the force meter, according to the maximum load P max at break; or find P max (P in) from the primary stretching diagram. The character of tensile deformation of viscous and brittle materials differs significantly.

Brittle materials, after reaching the maximum load, quickly fail without significant plastic deformation, therefore, σ in for brittle materials is a characteristic of fracture resistance, and for ductile materials, it is a characteristic of deformation resistance.

Fracture stress is defined as true. In this case, the fracture load is divided by the final cross-sectional area of the specimen after fracture (Fc):

All values calculated in this way are characteristics of the strength of the material.

Plasticity, i.e. the ability to deform without destruction, is characterized by changes in the dimensions of the sample. When testing at break, plasticity characteristics are determined: relative elongation

and relative contraction

where l to and F k - respectively, the length of the working part and the cross-sectional area of the sample after rupture.

The calculated characteristics of the mechanical properties after the tensile test are recorded in the protocol.

Mechanical properties of metals under compression . For brittle materials with low tensile strength, a compression test is carried out according to GOST 25.503-97. Cylindrical samples with smooth ends and end grooves are used for testing.

Under compression, the following deformation resistance characteristics are found: proportionality limit
, elastic limit
, physical yield strength
, conditional yield strength
, tensile strength
. Stresses are calculated as the ratio of the corresponding load to the cross-sectional area of the sample before deformation. The tensile strength can be calculated without recording a compression diagram, for other calculations a primary test diagram is needed.

The compression diagram of ductile samples differs from that of brittle samples. Highly ductile materials fail to break under compression and flatten. Therefore, the temporary compressive strength of plastic samples can be determined only conditionally, because after the hardening section, there is a rapid increase in the flattening of the sample. Brittle materials are destroyed at slight deformations and the tensile strength is found in the ratio of the maximum load to the initial cross-sectional area of the sample. Brittle materials such as cast iron have higher compressive strength than tensile strength. Many brittle materials fail under compression due to shearing or shearing along planes at an angle of 45° to the sample axis.

The characteristics of plasticity in compression include ε - the relative shortening of the sample:
where h 0 , h k are the initial and final heights of the sample.

Bending tests . The bending test is carried out according to GOST 14019-80 according to two schemes: with a concentrated load applied in the middle of the span, and with pure bending (Fig. 1.25).

Rice. 1.25. Scheme of bending by a concentrated force ( a) and two symmetrical loads ( b).

As a result of the test, the limit of proportionality, the limit of elasticity, the yield point are found with an accurate measurement of the magnitude of the deformation. Flexural strength is calculated σ izg:
where М izg is the largest bending moment, equal to the first loading scheme М izg = Р l/4, and according to the second scheme - M izg \u003d Ra; W - moment of resistance, characteristic of the cross section of the beam, for samples of round section W = πd 3 /32; for samples of rectangular section W = bh 2 /6, where h is the height of the bar.

Plasticity characterizes f razr (deflection value), deformation, which depends on the material, sample length, moment of inertia, etc.

Dynamic tests . Impact test . An important characteristic of mechanical properties is impact strength, which characterizes the specific work expended on destruction upon impact of a sample with a notch. Impact strength is determined when testing on a pendulum impact tester with a constant pendulum operating margin in accordance with GOST 9454-78 "Impact bending test method at low, room and elevated temperatures." The standard applies to ferrous and non-ferrous metals and alloys and establishes a test method from -100 to +1,000 °C. The method is based on the destruction of a sample with a stress concentrator by impact of a pendulum impact tester. As a result of the test, the total work expended on impact K or the impact strength of the COP is determined.

Rectangular specimens with a U, V, T concentrator (fatigue crack) are used. The most common samples are 55×10×10 mm samples with 2×2 mm U concentrate (Fig. 1.26).

Rice. 1.26. U-notch standard specimen for impact testing.

Only part of the energy of the pendulum is expended on the destruction of the sample by impact, and therefore the pendulum continues to move after the destruction of the sample, deviating by a certain angle. The greater the amount of work expended on the destruction of the sample, the smaller the angle it deviates from the vertical after destruction. The value of this angle determines the impact work K or the work expended on the destruction of the sample. The work of fracture K is related to the cross-sectional area of the sample S0 at the fracture site, and thus KC is determined – impact strength: KS \u003d K / S 0, where K is measured in J (kgf m), S 0 in m 2 (cm 2).

Depending on the type of concentrator, impact strength is designated KCU, KCV, KCT and has the dimensions MJ/m 2 (MJ/cm 2) or kgf m/cm 2 .

Control questions and tasks

1. What types of crystal lattices are typical for pure metals?

2. Draw the crystal lattices of bcc, fcc, hcp, indicate their coordination number and packing density.

3. What types of bonds are typical for metals Al, Cu, Fe; semimetals Bi, Si and non-metallic materials?

4. Describe the typical signs of a metallic condition.

5. What defects in the crystal structure are present in real crystals?

6. Describe the structure of plastics and other non-metallic materials.

7. Describe the main methods for researching materials.

8. What is the macro analysis of materials?

9. What can be determined in the study of the microstructure?

10. How to prepare research objects for macro- and microanalysis?

11. Describe the advantages of electron microscopy in the study of materials.

11. What problems can be solved by using X-ray methods of analysis to study materials?

12. What are the requirements for the choice of material in the manufacture of products?

13. Describe the chemical properties of materials.

14. What types of corrosion are possible in materials during their operation in aggressive environments?

15. Describe the physical and thermal properties of materials.

16. Describe the mechanical properties of materials.

17. How is Brinell, Rockwell and Vickers hardness determined?

18. Write down the Brinell, Rockwell, and Vickers hardness units.

19. What test methods of mechanical properties are classified as static, dynamic and cyclic?

20. Draw a primary tensile diagram for a ductile material.

21. How to determine the tensile strength and yield strength from the tensile diagram?

22. What types of samples are used to find elongation and relative contraction?

23. What characteristics are determined during compression and bending tests?

24. What characteristics are calculated when testing for impact bending?

25. What is the difference between impact strength, denoted KCU, KV, KST?

Mechanical properties characterize the ability of materials to resist the action of external forces. The main mechanical properties include strength, hardness, impact strength, elasticity, ductility, brittleness, etc.

Strength - This is the ability of a material to resist the damaging effects of external forces.

Hardness - this is the ability of a material to resist the introduction of another, more solid body into it under the action of a load.

Viscosity is the property of a material to resist fracture under the action of dynamic loads.

Elasticity - this is the property of materials to restore their size and shape after the termination of the load.

plasticity called the ability of materials to change their size and shape under the influence of external forces, without collapsing at the same time.

X brittleness - this is the property of materials to collapse under the action of external forces without residual deformations.

Hardness is the resistance of a material to penetration into its surface by a standard body (indenter) that does not deform during testing.

Wide distribution is explained by the fact that special samples are not required.

This is a non-destructive testing method. The main method for assessing the quality of heat-treated products. The hardness is judged either by the depth of penetration of the indenter (Rockwell method), or by the size of the impression from the indentation (Brinell, Vickers, microhardness methods).

In all cases, plastic deformation of the material occurs. The greater the resistance of the material to plastic deformation, the higher the hardness.

The most widespread methods are Brinell, Rockwell, Vickers and microhardness. Test schemes are shown in fig. 3.1.

Rice. 3.1. Hardness determination schemes: a- according to Brinell ; b- according to Rockwell; v- according to Vickers

Brinell hardness(GOST 9012)

The test is carried out on a Brinell hardness tester (Fig. 3.1 a)

A hardened steel ball with a diameter of D 2.5 is used as an indenter; 5; 10 mm, depending on the thickness of the product.

Load P, depending on the diameter of the ball and the measured hardness: for heat-treated steel and cast iron - , cast bronze and brass - , aluminum and other very soft metals - .

Exposure time: for steel and cast iron - 10s, for brass and bronze - 30s.

The resulting print is measured in two directions using a Brinell loupe.

Hardness is defined as the ratio of the applied load P to the spherical surface of the indentation F:

Standard conditions are D = 10 mm; P = 3000 kgf; = 10 s. In this case, the Brinell hardness is designated HB 250, in other cases the conditions are indicated: HB D / P /, HB 5/ 250 / 30 - 80.

Rockwell method ( GOST 9013)

Based on indentation into the surface of the tip under a certain load (Fig. 3.1 b)

The indenter for soft materials (up to HB 230) is a steel ball with a diameter of 1/16 "( 1.6 mm), for harder materials - a diamond cone.

Loading is carried out in two stages. First, a preload (10 kf) is applied to bring the tip into close contact with the sample. Then the main load P 1 is applied, for some time the total working load P acts. After the removal of the main load, the hardness value is determined from the depth of the residual indentation of the tip h under load.

Three hardness scales are used depending on the nature of the material.

Rockwell hardness scales

Vickers method

Hardness is determined by the size of the imprint (Fig. 3.1 c).

As an indenter, a diamond tetrahedral pyramid with an apex angle of 136 o is used.

Hardness is calculated as the ratio of the applied load P to the surface area of the impression F:

The load P is 5…100 kgf. Print Diagonal d measured using a microscope mounted on the instrument.

The advantage of this method is that it is possible to measure the hardness of any materials, thin products, surface layers. High accuracy and sensitivity of the method.

Microhardness method- to determine the hardness of individual structural components and phases of the alloy, very thin surface layers (hundredths of a millimeter).

Similar to the Vickers method. The indenter is a pyramid of smaller dimensions, the indentation loads P are 5 ... 500 gf

scratching method.

With a diamond cone, pyramid or ball, a scratch is applied, which is a measure. When scratching other materials and comparing them with a measure, the hardness of the material is judged.

It is possible to inflict a scratch with a width of 10 mm under a certain load. Observe the amount of load that gives this width.

Dynamic method (according to Shore)

A ball is thrown onto a surface from a given height and bounces back a certain amount. The larger the rebound value, the harder the material.

As a result of dynamic tests for impact bending of special samples with a notch (GOST 9454), the viscosity of materials is estimated and their tendency to transition from a ductile state to a brittle one is established.

Viscosity - the ability of a material to absorb the mechanical energy of external forces due to plastic deformation.

It is an energy characteristic of the material, expressed in units of work. The viscosity of metals and alloys is determined by their chemical composition, heat treatment and other internal factors.

Also, the viscosity depends on the conditions in which the metal works (temperature, loading rate, the presence of stress concentrators).

impact strength is determined by the work A spent on the destruction of the sample, referred to its cross-sectional area F; J/m2:

The tests are carried out by hitting a special pendulum impact tester. For testing, a standard notched sample is used, mounted on the supports of a copra. A pendulum of a certain mass strikes on the side opposite to the notch.

Technological properties determine the ability of materials to undergo various types of processing. Casting properties characterized by the ability of metals and alloys in the molten state to fill the cavity of the mold well and accurately reproduce its shape (fluidity), the amount of volume reduction during solidification (shrinkage), the tendency to form cracks and pores, the tendency to absorb gases in the molten state.

Ductility - this is the ability of metals and alloys to undergo various types of pressure treatment without destruction.

Weldability is determined by the ability of materials to form strong welded joints.

Machinability is determined by the ability of materials to be processed by a cutting tool.

Mechanical properties - the ability of a metal to resist the effects of external forces, loads. Therefore, when choosing a material, it is necessary, first of all, to take into account its basic mechanical properties. These properties are determined from the results of mechanical tests in which the material is subjected to external forces (loads).

The load induces stress and deformation in the solid body. Voltage- the magnitude of the load, referred to the unit area of the cross section of the test sample. Deformation- the ability of a material to change its shape and size under the influence of applied external forces (loads). In the direction of action of forces (loads), tensile, compression, bending, twisting and shearing deformations occur. In practice, as a rule, forces act on a part or product not separately, but in combination with each other, in which case complex deformations occur.

Deformations can be: elastic and plastic.

Elastic deformation- after the load is removed, the sample returns to its original position.

Plastic deformation- after the load is removed, the sample does not return to its original position.

The main mechanical properties are:

1) Hardness. Hardness - the ability of a metal to resist the introduction of another harder body into it;

2) Strength. Strength - the ability of a metal to resist destruction;

3) Viscosity. Viscosity - the ability of a metal to resist impact or impact dynamic loads;

4) Plasticity. Plasticity - the ability of a metal to resist deformation.

5) Fatigue. Fatigue is the ability of a metal to resist the effects of repeated alternating stresses. In the process of fatigue, there is a gradual accumulation of damage to the material under the influence of repetitively alternating stresses, leading to the formation of cracks and destruction.

6) Endurance. Endurance is the ability of a material to resist fatigue. The endurance limit is the maximum stress that a metal can withstand without destruction for a given number of loading cycles. The endurance limit is determined by bending and tension-compression.

Methods for measuring hardness.

Methods for determining hardness	Marked.	Formula	Indenter or tip	Notes
T Brinell hardness (Brinell)	HB	HB=P/F 0	Art. temper. ball. D: 2.5	>6 3-6 <3	P=KD 2 K=factor K = 30 black Me. K = 10 colors Me. K=2.5 of antifriction materials	P-load F 0 - ball imprint area D-ball diameter
Rockwell hardness (Rockwell)	HRB HRC HRA		Me. ball D=1.58 diamond. cone. With< при вер.120 0		100+900=1000N 100+1400=1500N 100+500=600N	P \u003d P 0 + P 1 P 0 \u003d 100H-const. P - total load P 0 \u003d 100N-const P 1 - additional load
Vickers hardness (Vickers)	HV	HV=1.85P/D2	Diamond. pyram. With< при вер.136 0		From 5 to 120 kgf.	P-load D-arithmetic mean of two diagonals of the diamond pyramid imprint
Microhardness	H0	H 0 \u003d 1.85P / D 2	Diamond pyramid With< при вер.136 0		From 5 to 500 gs.

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