Test methods for metals and alloys. Mechanical properties of metals and methods of their testing. See what “test metals” are in other dictionaries

Mechanical testing is of critical importance in industry. In accordance with this, various test methods have been developed to determine the mechanical properties of metals.

The most common tests are static tensile testing, dynamic testing and hardness testing.

Static are tests in which the material being tested is subjected to a constant force or a force increasing very slowly.

Dynamic tests are those in which the metal being tested is subjected to an impact or force that increases very quickly.

In addition, there are tests for fatigue, wear, and creep, which provide a more complete picture of the properties of metals.

Tensile tests. Static tensile testing is a very common method of mechanical testing. For static tests, round samples or flat samples for sheet materials are made ( Fig.20). The samples consist of a working part and heads designed to be secured in the grips of a tensile testing machine. Effective length l 0 take slightly less than the working length l 1 . Sample sizes are standardized. The diameter of the working part of the round sample is 20 mm. Samples of other diameters are called proportional.

Fig.20. Samples for static testing of metals:

1 - round, 2 - flat

The tensile force creates stress in the test specimen and causes it to elongate; when the stress exceeds its tensile strength, it breaks.

On Fig.21 The tensile diagram of mild steel plotted in a rectangular coordinate system is shown. The force is plotted along the ordinate axis R kg, along the abscissa axis - deformation (absolute elongation of the sample  l mm). This diagram is obtained by gradually increasing the tensile force until the sample breaks.

Fig.21. Mild Steel Tensile Chart

Voltage value  at any point in the diagram can be determined by dividing the force R to the cross-sectional area of ​​the sample.

Several characteristic points can be noted on the diagram. Plot OA is a straight line segment and shows that up to the point A sample elongation is proportional to force (load); Each increment of load corresponds to the same increment of deformation. This relationship between the elongation of the sample and the applied load is law of proportionality.

With further loading of the sample, a deviation from the law of proportionality is observed: a curved section appears on the diagram. To the point IN The sample has elastic deformations.

Dot WITH The diagram shows the beginning of the horizontal area, which shows that the sample is lengthening without increasing the load: the metal seems to flow. The lowest stress at which the deformation of the sample continues without increasing the load is called physical yield strength. Yield strength  T determined by the formula

kg mm 2 ,

Where R With .

Fluidity is characteristic only of low-carbon annealed steel and some grades of brass. High-carbon steels and other metals do not have a yield plateau. For such metals, the proof strength is determined at a residual elongation of 0.2%. The stress at which a tensile specimen receives a residual elongation equal to 0.2% of its calculated length is called the proof strength and is designated  0.2

kg mm 2 .

Dot D indicates the highest maximum load that the specimen can withstand. The conditional stress corresponding to the greatest load preceding the failure of the sample is called tensile strength(temporary tensile strength) and is determined by the formula

kg mm 2 ,

Where P .

For a point D elongation  l 3 sample and the narrowing of its cross section occurs evenly along the entire length of the working part. Upon reaching the point D the deformation of the sample is concentrated in the place of least resistance and further elongation  l 4 occurs due to the formation of a neck along which the sample ruptures under load R TO .

At rupture, elastic deformation  l unitary enterprise absolute residual elongation also disappears  l ost consists of a uniform elongation  l 1 and extension of local  l 2 , i.e.

l ost =  l 1 +  l 2 .

To assess the ductility of a metal, it is important to know the relative elongation  and relative narrowing of the cross-sectional area  in percentages.

Relative elongation (in%) is determined by the formula

,

Where l 1 - length of the sample after rupture, mm;

l 0 - estimated length of the sample, mm;

When elongating, the cross-sectional area simultaneously decreases. At the rupture site this area will be smallest. Relative narrowing (in%) is determined by the formula

,

Where F 0 - initial cross-sectional area of ​​the sample, mm 2 ;

F 1 - area at the rupture site, mm 2 .

For brittle metals, the relative elongation  and relative narrowing  close to zero; for ductile metals they reach several tens of percent.

Thus, the static tensile test gives the strength characteristics -  unitary enterprise ,  T (or  0,2 ) and plasticity characteristics -  And  .

Hardness tests .

Hardness tests are carried out by pressing a hard tip.

According to the Brinell method hardened steel ball diameter D (10; 5 or 2.5 mm) is pressed into the test sample by force R (3000;1000; 750kg or less). As a result, an imprint in the form of a spherical segment with a diameter of d (Fig.22). The harder the metal, the smaller the print size. Brinell hardness number NV calculated by the formula

kg mm 2 ,

;

F- size of the print surface, mm 2 .

Fig.22. Brinell test scheme

For small products, balls of smaller diameter are used with less pressing force. The thickness of the metal under the print must be no less than ten times the depth of the print, and the distance from the center of the print to the surface cut must be no less D .

Lever presses are currently mainly used for Brinell hardness testing.

As studies have shown, between the tensile strength of metals  V and Brinell hardness NV there is a dependency:

for rolled and forged steel V = 0.36NV ;

for cast steel...................... V =(0.3-0.4) NV :

for gray cast iron...................... V =0.1 NV .

The Brinell method can test materials with hardness NV up to 450; If the materials are harder, the steel ball may become deformed. This method is also not suitable for testing thin sheet material.

According to the Rockwell method The hardness test is carried out by pressing a steel ball with a diameter of D =1.58mm(116 inch) or 120 0 diamond cone.

The steel ball is used to test soft metals (hardness less than 220 on the Brinell scale) at a load of 100 kg, diamond cone - for testing hard metals at a load of 150 kg. The sample is placed on stage 2 of the Rockwell instrument ( Fig.23) and by rotating the flywheel 1, raise it until it comes into contact with the diamond cone 3 (or steel ball). The rotation of the flywheel is continued until the pressure of the cone or ball becomes equal to 10 kg(pre-load), which is indicated by the small arrow of indicator 4. Next, apply the main load using handle 5. Indentation lasts 5-6 sec, then the main load is removed. After this, the large indicator arrow shows the hardness value.

Fig.23. Rockwell press

The indicator dial has two scales: red IN for steel ball testing and black WITH for diamond cone testing.

Rockwell hardness is a conditional value that characterizes the difference in indentation depths. The Rockwell hardness number is designated HR with the addition of an index of the scale on which the test was carried out, for example HR IN or HR WITH. For testing very hard materials, a diamond cone is used at a load of 60 kg. The count is made on a black scale.

Vickers method, which allows you to measure the hardness of both soft and very hard metals and alloys; it is suitable for determining the hardness of thin surface layers (for example, during chemical-thermal treatment).

Using this method, a tetrahedral diamond pyramid with an apex angle of 136 0 is pressed into the sample. Load can be applied from 5 to 120 kg. The fingerprint is measured using a microscope located on the device.

The hardness number is determined by the formula

kg mm 2 ,

;

F - area of ​​the pyramidal imprint, mm 2

Practical value H.V. taken from tables.

Microhardness tests produced by pressing a diamond pyramid with an apex angle of 136 0 under a load from 2 to 200 G; the hardness number is expressed kg mm 2 . Using this method, it is possible to determine the hardness of individual structural components of alloys, small parts, metal threads, oxide films, etc. On Fig. 24, a The PMT-3 device for microhardness testing is shown.

The table 11 and the tube stand 4 rest on the frame 1 of the device. The test object 2 is installed on a table under the lens 9, through which the focus of the microscope is aimed and the threads are installed using an eyepiece microscope 6. Then the diamond pyramid 10 is pressed into the test object for 5-7 sec. After removing the load, measure the diagonal with a microscope d (Fig. 24, b), combining the intersection of the machine threads first with the right corner of the print (dotted lines), and then with the left (solid lines).

Based on the size of the diagonal, the area of ​​the print and hardness are determined using the above formula ( H.V. n ).

Other mechanical tests .Shock testing carried out for parts of machines and mechanisms experiencing shock (dynamic) loads, since some metals with fairly high static strength indicators are destroyed under low impact loads, for example, steel with a coarse-grained structure and cast iron.

Bending impact tests are carried out on standard shaped samples using instruments called pendulum impact testers.

Impact resistance is called impact strength and is measured in kilograms per square centimeter.

Fig.24. PMT-3 device for microhardness testing

Impact strength A n calculated by the formula

kg m cm 2 ,

Where A n - impact work spent on breaking the sample, kg m;

F - cross-sectional area of ​​the sample at the incision site, cm 2 .

Fatigue tests. During operation, many machine parts (engine connecting rods, crankshafts, etc.) are subject to loads that vary in magnitude and direction. Under such repeatedly alternating stresses, the metal gradually changes from a viscous state to a brittle state (gets tired). The brittle state is explained by the appearance of microcracks, which gradually expand and weaken the metal. As a result, destruction occurs at stresses less than the tensile strength.

Microcracks appear and develop from the surface mainly in sections with sharp breaks in the contour line (for example, in the presence of keyways, holes, etc.).

Fatigue tests ( endurance) are produced on various machines. The most common testing machines are:

bending during rotation;

in tension-compression;

when torsion.

For metals operating in difficult conditions, testing machines are equipped with installations and devices that provide testing at elevated and low temperatures, during corrosion and under other special conditions.

Fig.25. Extrusion test

Technological tests (samples). They determine the ability to perform certain technological operations with a given metal.

Extrusion test serves to determine the ability of thin sheet metal to cold stamping and drawing. The test consists of extruding a hole with a rounded head 1 ( Fig.25) until the first crack appears in plate 2, clamped in the annular surface.

The depth of the extruded hole when the first crack appears is a quantitative measure of the sample.

Bend test determines the ability of a metal to withstand repeated bending and is used to assess the quality of sheet material up to 5 mm thick mm, as well as wire and rods.

Settlement test determines the ability of a cold metal to take a given shape when compressed. A cylinder sample, the height of which is equal to two diameters, is considered to have passed the test if, when upsetting to a given height, cracks, tears and fractures do not appear on it.

Weldability test. Two bars of the test metal are welded and tested for bending or tension, after which the results are compared with those corresponding to a solid (unwelded) sample of the same metal. With good weldability, tensile strength weld must correspond to at least 80% of the tensile strength of a solid bar.

Methods of physical and chemical analysis.

Macroanalysis. For macroanalysis, a sample-section, or fracture, is prepared, from which the macrostructure-structure of the metal and alloy is revealed, visible to the naked eye or at low magnification up to x 5 times.

Preparation of a polished section consists of leveling and grinding the surface using a grinding machine. Then, the thin section is etched with reagents that dissolve or color parts of the thin section that differ in composition or orientation.

Using macroanalysis, it is possible to detect shrinkage cavities and looseness, voids, cracks, non-metallic inclusions (slag, graphite in gray cast iron, etc.), the presence and nature of the location of certain harmful impurities, such as sulfur.

Microanalysis. A thin section for microanalysis is prepared in the same way as for macroanalysis, but after grinding it is polished to a mirror finish.

Using a metallographic microscope, a microstructure is determined from a thin section: the presence, quantity and shape of certain structural components, contamination with foreign inclusions. The presence and size of pores are determined from unetched sections; To reveal the main structure, the thin section is etched. Since metals are opaque, polished sections can only be studied in reflected light using a metallographic microscope.

On Fig.26 A diagram is presented to explain the visibility of the grain boundaries of an etched section of a single-phase metal. Under the influence of reagents during etching, the metal along the grain boundaries dissolves more strongly, as a result of which micro-beards are formed there. Light rays are scattered in them, so the grain boundaries are darker under a microscope; rays from the flat surface of the grains are reflected and each grain on the thin section appears light, while different colors of the grains are often observed, which is explained by unequal solubility due to anisotropy.

Fig.26. Scheme of ray reflection by an etched thin section

single-phase metal

Along with a conventional light microscope, an electron microscope is widely used, in which electron rays are used instead of light rays: these rays are emitted by a hot tungsten spiral. An electron microscope provides electron-optical magnification up to tens of thousands of times.

X-ray diffraction analysis makes it possible to establish the types of crystal lattices of metals and alloys, as well as their parameters. Determination of the structure of metals, placement of atoms in crystal lattice and the measurement of the distance between them is based on the diffraction (reflection) of X-rays by rows of atoms in a crystal, since the wavelength of these rays is comparable to the interatomic distances in crystals. Knowing the wavelength of X-rays, it is possible to calculate the distance between atoms in a crystal and build a model of the arrangement of atoms.

X-ray analysis(transillumination) is based on the penetration of X-rays through bodies that are opaque to visible light. Passing through metals, X-rays are partially absorbed, and the rays are absorbed more strongly by solid metal than in those parts where gas and slag inclusions or cracks are located. The size, shape and type of these defects can be observed on a luminous screen installed along the path of the rays behind the part being examined. Since X-rays act on a photographic emulsion similarly to light, the luminous screen can be replaced with a cassette of photographic film and a photograph of the object can be obtained.

Thus, X-ray transmission can detect even microscopic defects inside the part.

Thermal analysis comes down to identifying critical points during heating and cooling of metals and alloys and is accompanied by the construction of curves in “temperature - time” coordinates.

If no phase transformations occur in the metal, the cooling (heating) curve will be smooth without kinks or steps; if, when cooling (or heating) a metal, phase transformations occur in it, which are accompanied by the release (when heated, absorption) of heat, horizontal sections or kinks will appear on the curve (i.e. changes in the direction of the curve). These kinks and horizontal sections make it possible to determine the transformation temperatures.

Dilatometric analysis(dilatometry - from Latin to expand) is based on the measurement of volume changes occurring in a metal or alloy during phase transformations, and is used to determine critical points in solid samples. Dilatometric analysis is carried out using dilatometer devices.

Flaw detection.Magnetic flaw detection used to detect defects in parts subject to high alternating stresses. Defects such as cracks, hairlines, bubbles, non-metallic inclusions, etc., under variable load conditions become very dangerous, as they reduce the dynamic strength of parts.

Magnetic testing consists of three main operations: magnetization of products, coating them with ferromagnetic powder, external inspection and demagnetization of products.

In magnetized products with defects, magnetic field lines, trying to go around the defects (due to their reduced magnetic permeability), go beyond the surface of the product and then enter it, forming a non-uniform magnetic field. Therefore, when covering products with magnetic powder, the particles of the latter are located above the defect, forming sharply defined patterns ( Fig.27). The nature of these patterns is used to judge the size and shape of metal defects.

Ultrasonic flaw detection allows you to test any metals (and not just ferromagnetic ones) and identify defects in the thickness of the metal at a significant depth that are not detected by the magnetic method.

To study metal, ultrasonic vibrations with a frequency of 2 to 10 million are used. Hz. At this frequency, vibrations propagate in the metal, like rays, almost without scattering to the sides: they can “shine through” metals to a depth of more than 1 m.

Fig.27. Layout of magnetic field lines on

defective parts

Ultrasound is reflected at the interface between heterogeneous media. Therefore, when propagating in metal, ultrasound does not pass through cracks, cavities, and non-metallic inclusions, thus forming an acoustic shadow ( Fig.28). Here, A-acoustic shadow zone.

Piezoelectric emitters and receivers are used to emit and receive ultrasound, respectively.

Application of radioactive isotopes (labeled atoms).In metallurgy and metal science, radioactive isotopes are used for various purposes. For example, radioactive isotopes of phosphorus, sulfur, manganese, etc. are introduced into the slag and the rate of transition of these elements into the metal and the rate of restoration of their equilibrium distribution between the metal and the slag in metallurgical melts are studied when the temperature or composition of the slag changes. The introduction of radioactive carbon into iron during cementation makes it possible to study the rate of diffusion and the distribution of carbon in it.

Fig.28. Scheme of ultrasonic examination of a part

To determine the distribution of tin in nickel, radioactive tin is added to the liquid alloy. The hardened alloy is placed on a cassette with a photographic plate and, after appropriate exposure, the plate is developed.

On Fig.29 A microautograph of such an alloy is presented, from which (from the distribution of darkening) it is clear that radioactive, and with it ordinary tin, borders the nickel grains.

Fig.29. Microradioautography of a nickel-tin alloy

Radioactive isotopes help monitor the wear of refractory masonry in blast furnaces or machine parts.

Strength is the ability of a metal not to succumb to destruction under the influence of external loads. The value of metal as an engineering material, along with other properties, is determined by its strength.

The strength value indicates how much force is needed to overcome the internal bond between molecules.

Testing of metals for tensile strength is carried out on special machines of varying power. These machines consist of a loading mechanism that generates force, produces tension on the test specimen, and indicates the amount of force applied to the specimen. The mechanisms can be mechanical or hydraulic.

The power of the machines varies and reaches 50 tons. In Fig. 7, a shows the structure of the machine, consisting of a frame 2 and clamps 4, with the help of which the test samples 3 are secured.

The upper clamp is fixedly fixed in the frame, and the lower one, using a special mechanism, slowly lowers during testing, stretching the sample.

Rice. 7. Tensile testing of metals:

a - a device for testing metals for tension; b - samples for tensile testing: I - round, II - flat

The load transferred to the sample during testing can be determined by the position of the instrument pointer on measuring scale 1.

Testing of samples should always be carried out under the same conditions so that the results obtained can be compared. Therefore, the relevant standards establish certain dimensions of test specimens.

The standard specimens for tensile testing are the round and flat specimens shown in Fig. 7, b.

Flat samples are used when testing sheets, strip material, etc., and if the metal profile allows, then round samples are made.

The tensile strength (σ b) is the maximum stress that a material can experience before it fails; the tensile strength of a metal is equal to the ratio of the greatest load when testing a sample for tensile strength to the original cross-sectional area of ​​the sample, i.e.

σ b = P b /F 0 ,

where P b is the greatest load preceding the rupture of the sample, kgf;

F 0 - initial cross-sectional area of ​​the sample, mm 2.

In order to safe work machines and structures, it is necessary that during operation the stresses in the material do not exceed the established limit of proportionality, i.e. the highest stress at which deformations are not caused.

Tensile strength of some metals during tensile testing, kgf/mm 2:

Lead 1.8

Aluminum 8

(strength, elasticity, plasticity, viscosity), as well as other properties, are the initial data in the design and creation of various machines, mechanisms and structures.

Methods for determining the mechanical properties of metals are divided into the following groups:

· static, when the load increases slowly and smoothly (tensile, compression, bending, torsion, hardness tests);

· dynamic, when the load increases at high speed (impact bending tests);

· cyclic, when the load changes many times (fatigue test);

· technological - to assess the behavior of metal during pressure treatment (bending, bending, extrusion tests).

Tensile tests(GOST 1497-84) are carried out on standard samples of round or rectangular cross-section. When stretched under the action of a gradually increasing load, the sample is deformed until it breaks. During testing of the sample, a tensile diagram is taken (Fig. 1.36, A), fixing the relationship between the force P acting on the sample and the deformation Δl caused by it (Δl is the absolute elongation).

Rice. 1.36. Tensile Diagram of Low Carbon Steel ( A) and the relationship between stress and elongation ( b)

Viscosity (internal friction) is the ability of a metal to absorb the energy of external forces during plastic deformation and fracture (determined by the magnitude of the tangential force applied per unit area of ​​the metal layer subject to shear).

Plastic— the ability of solids to deform irreversibly under the influence of external forces.

The tensile test determines:

· σ in - strength limit, MN/m 2 (kg/mm ​​2):

0 is the initial cross-sectional area of ​​the sample;

· σ pts — proportionality limit, MN/m 2 (kg/mm ​​2):

Where P pc - load corresponding to the proportionality limit;

· σ pr - elastic limit, MN/m 2 (kg/mm ​​2):

Where R pr - load corresponding to the elastic limit (at σ pr the residual deformation corresponds to 0.05-0.005% of the initial length);

· σ T- yield limit, MN/m 2 (kg/mm ​​2):

Where R t is the load corresponding to the yield point, N;

· δ—relative elongation, %:

Where l 0—specimen length before rupture, m; l 1—sample length after rupture, m;

· ψ — relative narrowing, %:

Where F 0—sectional area before rupture, m2; F- cross-sectional area after rupture, m2.

Hardness tests

Hardness- this is the resistance of a material to the penetration of another, more solid body into it. Of all kinds mechanical test hardness determination is the most common.

Brinell tests(GOST 9012-83) are carried out by pressing a steel ball into the metal. As a result, a spherical imprint is formed on the metal surface (Fig. 1.37, A).

Brinell hardness is determined by the formula:

— ball diameter, m; d— imprint diameter, m.

The harder the metal, the smaller the print area.

The diameter of the ball and the load are set depending on the metal being tested, its hardness and thickness. When testing steel and cast iron, choose D= 10 mm and P= 30 kN (3000 kgf), when testing copper and its alloys D= 10 mm and P= 10 kN (1000 kgf), and when testing very soft metals (aluminum, babbitts, etc.) D= 10 mm and P= 2.5 kN (250 kgf). When testing samples with a thickness of less than 6 mm, balls with a smaller diameter are selected - 5 and 2.5 mm. In practice, they use a table for converting the area of ​​the imprint into the hardness number.

Rockwell tests(GOST 9013-83). They are carried out by pressing a diamond cone (α = 120°) or a steel ball ( D= 1.588 mm or 1/16", Fig. 1.37, b). The Rockwell instrument has three scales - B, C and A. The diamond cone is used to test hard materials (scales C and A), and the ball is used to test soft materials (scale B). The cone and ball are pressed in with two successive loads: preliminary R 0 and total R:

R = R 0 + R 1 ,

0 = 100 N (10 kgf). The main load is 900 N (90 kgf) for scale B; 1400 N (140 kgf) for scale C and 500 N (50 kgf) for scale A.

Rice. 1.37. Hardness determination scheme: A- according to Brinell; b- according to Rockwell; V- according to Vickers

Rockwell hardness is measured in arbitrary units. The unit of hardness is taken to be the value that corresponds to the axial movement of the tip over a distance of 0.002 mm.

Rockwell hardness is calculated as follows:

HR = 100 - e(scales A and C); HR = 130 - e(scale B).

Size e determined by the formula:

Where h— depth of penetration of the tip into the metal under the influence of the total load R (R =R 0 + R 1); h 0 - tip penetration depth under preload R 0 .

Depending on the scale, Rockwell hardness is designated HRB, HRC, HRA.

Vickers tests(GOST 2999-83). The method is based on pressing a tetrahedral diamond pyramid (α = 136°) into the test surface (ground or even polished) (Fig. 1.37, V). The method is used to determine the hardness of thin parts and thin surface layers with high hardness.

Vickers hardness:

— arithmetic mean of two imprint diagonals measured after removing the load, m.

The Vickers hardness number is determined using special tables along the diagonal of the print d. When measuring hardness, a load from 10 to 500 N is used.

Microhardness(GOST 9450-84). The principle of determining microhardness is the same as that of Vickers, according to the relationship:

The method is used to determine the microhardness of small-sized products and individual components of alloys. The device for measuring microhardness is a diamond pyramid indentation mechanism and a metallographic microscope. Samples for measurements must be prepared as carefully as microsections.

Impact test

For impact testing they are made special samples with a notch, which are then destroyed on a pendulum pile driver (Fig. 1.39). The total energy reserve of the pendulum will be spent on the destruction of the sample and on raising the pendulum after its destruction. Therefore, if we subtract from the total energy reserve of the pendulum the part that is spent on lifting (takeoff) after the destruction of the sample, we obtain the work of destruction of the sample:

K = P(h 1 - h 2)

K = Рl(cos β - cos α), J (kg m),

de P— mass of the pendulum, N (kg); h 1 — height of rise of the center of mass of the pendulum before impact, m; h 2 — take-off height of the pendulum after impact, m; l— pendulum length, m; α, β are the angles of elevation of the pendulum, respectively, before and after the destruction of the sample.

Rice. 1.39. Impact test: 1 - pendulum; 2 - pendulum knife; 3 - supports

Impact strength, i.e., the work expended on the destruction of the sample and related to the cross section of the sample at the notch site, is determined by the formula:

MJ/m 2 (kg m/cm 2),

Where F- cross-sectional area at the site of the sample cut, m2 (cm2).

For determining KS use special tables in which the magnitude of the impact work is determined for each angle β K. Wherein F= 0.8 · 10 -4 m 2.

To indicate impact strength, a third letter is added, indicating the type of cut on the sample: U, V, T. Record KCU means the impact strength of a sample with U-shaped cut, KCV- With V-shaped cut, and KST- with a crack (Fig. 1.40).

Rice. 1.40. Types of cuts on samples for impact strength testing:
A — U-shaped cut ( KCU); b — V-shaped cut ( KCV); V- cut with a crack ( KST)

Fatigue test(GOST 2860-84). The destruction of metal under the influence of repeated or alternating stresses is called metal fatigue. When a metal fractures due to fatigue in air, the fracture consists of two zones: the first zone has a smooth ground-in surface (fatigue zone), the second is a fracture zone; in brittle metals it has a coarse-crystalline structure, and in viscous metals it has a fibrous structure.

When testing for fatigue, the limit of fatigue (endurance) is determined, i.e., the greatest stress that a metal (specimen) can withstand without destruction for a given number of cycles. The most common fatigue test method is the rotational bending test (Figure 1.41).

Rice. 1.41. Rotational bending test setup:
1 - sample; bending moment

The following main types of technological tests (samples) are used.

Bend test(Fig. 1.42) in cold and hot states - to determine the ability of the metal to withstand a given bend; sample dimensions - length l = 5A+ 150 mm, width b = 2A(but not less than 10 mm), where A- thickness of the material.

Rice. 1.42. Technological bending test: A— sample before testing; b- bend to a certain angle; V- bend until the sides are parallel; G- bend until the sides touch

Bend test involves assessing the ability of a metal to withstand repeated bending and is used for wire and rods with a diameter of 0.8–7 mm from strip and sheet material up to 55 mm thick. The samples are bent alternately to the right and left by 90° at a uniform speed of about 60 bends per minute until the sample is destroyed.

Extrusion test(Fig. 1.43) - to determine the ability of the metal to cold stamping and drawing thin sheet material. It consists of pressing a sheet of material with a punch, sandwiched between a matrix and a clamp. A characteristic of the plasticity of the metal is the depth of extrusion of the hole, which corresponds to the appearance of the first crack.

Rice. 1.43. Extrusion test: 1 - leaf; h- a measure of the ability of a material to draw

Test for winding wire with diameter d ≤ 6 mm. The test consists of winding 5-6 tightly fitting turns along a helical line onto a cylinder of a given diameter. Performed only in a cold state. The wire after coiling should not be damaged.

Spark test used when it is necessary to determine the steel grade in the absence of special equipment and markings.

Metals are characterized by high ductility, thermal and electrical conductivity. They have a characteristic metallic luster.

About 80 elements of the periodic table of D.I. have properties of metals. Mendeleev. For metals, as well as for metal alloys, especially structural ones, mechanical properties are of great importance, the main ones being strength, ductility, hardness and impact strength.

Under the influence of an external load, stress and deformation arise in a solid body. related to the original cross-sectional area of ​​the sample.

Deformation – this is a change in the shape and size of a solid body under the influence of external forces or as a result of physical processes that occur in the body during phase transformations, shrinkage, etc. Deformation may be elastic(disappears after the load is removed) and plastic(remains after the load is removed). With an ever-increasing load, elastic deformation, as a rule, turns into plastic, and then the sample collapses.

Depending on the method of applying 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 under static, dynamic or alternating loads. The strength of metals under static loads is tested in tension, compression, bending and torsion. Tensile testing is mandatory. Strength under dynamic loads is assessed by specific impact strength, and under alternating loads - by fatigue strength.

To determine strength, elasticity and ductility, metals in the form of round or flat samples are tested for static tension. Tests are carried out on tensile testing machines. As a result of the tests, a tensile diagram is obtained (Fig. 3.1) . The abscissa axis of this diagram shows the strain values, and the ordinate axis shows the stress values ​​applied to the sample.

The graph shows that no matter how small the applied stress, it causes deformation, and the initial deformations are always elastic and their magnitude is directly dependent on the stress. On the curve shown in the diagram (Fig. 3.1), elastic deformation is characterized by the line OA and its continuation.

Rice. 3.1. Strain curve

Above the point A the proportionality between stress and strain is violated. Stress causes not only elastic, but also residual, plastic deformation. Its value is equal to the horizontal segment from the dashed line to the solid curve.

During elastic deformation under the influence of an external force, the distance between atoms in the crystal lattice changes. Removing the load eliminates the cause that caused the change in the interatomic distance, the atoms return to their original places and the deformation disappears.

Plastic deformation is a completely different, much more complex process. During plastic deformation, one part of the crystal moves relative to another. If the load is removed, the displaced part of the crystal will not return to its original location; the deformation will persist. These shifts are revealed by microstructural examination. In addition, plastic deformation is accompanied by crushing of mosaic blocks inside the grains, and at significant degrees of deformation, a noticeable change in the shape of the grains and their location in space is also observed, and voids (pores) appear between the grains (sometimes inside the grains).

Represented dependency OAV(see Fig. 3.1) between externally applied voltage ( σ ) and the relative deformation caused by it ( ε ) characterizes the mechanical properties of metals.

· straight line slope OA shows metal hardness, or a characteristic of how a load applied from the outside changes interatomic distances, which, to a first approximation, characterizes the forces of interatomic attraction;

· tangent of the angle of inclination of the straight line OA proportional to elastic modulus (E), which is numerically equal to the quotient of stress divided by relative elastic deformation:

voltage, which is called the limit of proportionality ( σ pc), corresponds to the moment of appearance of plastic deformation. The more accurate the deformation measurement method, the lower the point lies A;

· in technical measurements a characteristic called yield strength (σ 0.2). This is a stress that causes a residual deformation equal to 0.2% of the length or other size of the sample or product;

maximum voltage ( σ c) corresponds to the maximum stress achieved during tension and is called temporary resistance or tensile strength .

Another characteristic of the material is the amount of plastic deformation that precedes fracture and is defined as a relative change in length (or cross-section) - the so-called relative extension (δ ) or relative narrowing (ψ ), they characterize the plasticity of the metal. Area under the curve OAV proportional to the work that must be expended to destroy the metal. This indicator, determined different ways(mainly by striking a cut specimen), characterizes viscosity metal

When a sample is stretched to the point of failure, the relationships between the applied force and the elongation of the sample are recorded graphically (Fig. 3.2), resulting in so-called deformation diagrams.

Rice. 3.2. Diagram "force (tension) - elongation"

The deformation of the sample when the alloy is loaded is first macroelastic, and then gradually and in different grains under unequal loads transforms into plastic, occurring through shear through the dislocation mechanism. The accumulation of dislocations as a result of deformation leads to strengthening of the metal, but when their density is significant, especially in individual areas, centers of destruction arise, ultimately leading to the complete destruction of the sample as a whole.

Tensile strength is assessed by the following characteristics:

1) tensile strength;

2) the limit of proportionality;

3) yield strength;

4) elastic limit;

5) elastic modulus;

6) yield strength;

7) relative elongation;

8) relative uniform elongation;

9) relative narrowing after rupture.

Tensile strength (tensile strength or tensile strength) σ in, is the voltage corresponding to the greatest load R V preceding the destruction of the sample:

σ in = P in /F 0,

This characteristic is mandatory for metals.

Proportionality limit (σ pc) – this is the conditional voltage R pc, at which the deviation from the proportional dependence of the bridge between deformation and load begins. It is equal to:

σ pc = P pc /F 0.

Values σ pc is measured in kgf/mm 2 or in MPa .

Yield strength (σ t) is the voltage ( R T) in which the sample deforms (flows) without a noticeable increase in load. Calculated by the formula:

σ t = R T / F 0 .

Elastic limit (σ 0.05) is the stress at which the residual elongation reaches 0.05% of the length of the section of the working part of the sample, equal to the base of the strain gauge. Elastic limit σ 0.05 is calculated using the formula:

σ 0,05 = P 0,05 /F 0 .

Elastic modulus (E) – the ratio of the increment in stress to the corresponding increment in elongation within the limits of elastic deformation. It is equal to:

E = Pl 0 /l avg F 0 ,

Where ∆Р– load increment; l 0– initial estimated length of the sample; l wed– average increment of elongation; F 0 – initial cross-sectional area.

Yield strength (conditional) – stress at which the residual elongation reaches 0.2% of the length of the sample section on its working part, the elongation of which is taken into account when determining the specified characteristic.

Calculated by the formula:

σ 0,2 = P 0,2 /F 0 .

Conditional limit yield is determined only if there is no yield plateau on the tensile diagram.

Relative extension (after the breakup) – one of the characteristics of the plasticity of materials, equal to the ratio of the increment in the calculated length of the sample after destruction ( l to) to the initial effective length ( l 0) in percentages:

Relative uniform elongation (δ р)– the ratio of the increment in the length of sections in the working part of the sample after rupture to the length before testing, expressed as a percentage.

Relative narrowing after rupture (ψ ), as well as relative elongation, is a characteristic of the plasticity of the material. Defined as the difference ratio F 0 and minimum ( F to) cross-sectional area of ​​the sample after destruction to the initial cross-sectional area ( F 0), expressed as a percentage:

Elasticity – the property of metals to restore their previous shape after removal of external forces causing deformation. Elasticity is the opposite property of plasticity.

Very often, to determine strength, a simple, non-destructive, simplified method is used - measuring hardness.

Under hardness material is understood as resistance to penetration of a foreign body into it, i.e., in fact, hardness also characterizes resistance to deformation. There are many methods for determining hardness. The most common is Brinell method (Fig. 3.3, a), when the test body is subjected to force R a ball with a diameter of D. The Brinell hardness number (HH) is the load ( R), divided by the area of ​​the spherical surface of the print (diameter d).

Rice. 3.3. Hardness test:

a – according to Brinell; b – according to Rockwell; c – according to Vickers

When measuring hardness Vickers method (Fig. 3.3, b) the diamond pyramid is pressed in. By measuring the diagonal of the print ( d), judge the hardness (HV) of the material.

When measuring hardness Rockwell method (Fig. 3.3, c) the indenter is a diamond cone (sometimes a small steel ball). The hardness number is the reciprocal of the indentation depth ( h). There are three scales: A, B, C (Table 3.1).

The Brinell and Rockwell B scale methods are used for soft materials, the C scale Rockwell method is used for hard materials, and the A scale Rockwell method and the Vickers method are used for thin layers(sheets). The described methods for measuring hardness characterize the average hardness of the alloy. In order to determine the hardness of individual structural components of the alloy, it is necessary to sharply localize the deformation, press the diamond pyramid into a certain place, found on a thin section at a magnification of 100 - 400 times under a very small load (from 1 to 100 gf), followed by measuring the diagonal of the indentation under a microscope . The resulting characteristic ( N) is called microhardness , and characterizes the hardness of a certain structural component.

Table 3.1 Test conditions when measuring hardness using the Rockwell method

Test conditions		Designation t firmness
R= 150 kgf
When tested with diamond cone and load R= 60 kgf
When pressing the steel ball and loading R= 100 kgf

The NV value is measured in kgf/mm 2 (in this case, the units are often not indicated) or in SI - in MPa (1 kgf/mm 2 = 10 MPa).

Viscosity – the ability of metals to resist impact loads. Viscosity is the opposite property of brittleness. During operation, many parts experience not only static loads, but are also subject to shock (dynamic) loads. For example, such loads are experienced by the wheels of locomotives and cars at rail joints.

The main type of dynamic tests is impact loading of notched samples under bending conditions. Dynamic impact loading is carried out on pendulum impact drivers (Fig. 3.4), as well as with a falling load. In this case, the work expended on the deformation and destruction of the sample is determined.

Typically, in these tests, the specific work spent on deformation and destruction of the sample is determined. It is calculated using the formula:

KS =K/ S 0 ,

Where KS– specific work; TO– total work of deformation and destruction of the sample, J; S 0– cross-section of the sample at the incision site, m 2 or cm 2.

Rice. 3.4. Impact testing using a pendulum impact tester

The width of all types of specimens is measured before testing. The height of samples with a U- and V-shaped notch is measured before testing, and with a T-shaped notch after testing. Accordingly, the specific work of fracture strain is denoted by KCU, KCV and KST.

Fragility metals at low temperatures are called cold brittleness . The value of impact strength is significantly lower than at room temperature.

Another characteristic of the mechanical properties of materials is fatigue strength. Some parts (shafts, connecting rods, springs, springs, rails, etc.) during operation experience loads that change in magnitude or simultaneously in magnitude and direction (sign). Under the influence of such alternating (vibration) loads, the metal seems to get tired, its strength decreases and the part collapses. This phenomenon is called fatigue metal, and the resulting fractures are fatigue. For such details you need to know endurance limit, those. the magnitude of the maximum stress that a metal can withstand without destruction for a given number of load changes (cycles) ( N).

Wear resistance – resistance of metals to wear due to friction processes. This is an important characteristic, for example, for contact materials and, in particular, for the contact wire and current-collecting elements of the current collector of electrified transport. Wear consists of the separation of individual particles from the rubbing surface and is determined by changes in the geometric dimensions or mass of the part.

Fatigue strength and wear resistance give the most complete picture of the durability of parts in structures, and toughness characterizes the reliability of these parts.

Depending on the method of applying the load, methods for testing the mechanical properties of metals are divided into three groups:

static, when the load increases slowly and smoothly (tensile, compression, bending, torsion, shear, hardness tests);

dynamic, when the load increases at high speed, shock (impact test);

tests under repeated-variable loads, when the load during the test changes many times in magnitude or in magnitude and sign (fatigue test).

The need to test under different conditions is determined by the difference in operating conditions of machine parts, tools and other metal products.

Tensile test. For tensile testing, cylindrical or flat samples of a certain shape and size according to the standard are used. Tensile testing of samples is carried out on tensile testing machines with a mechanical or hydraulic drive. These machines are equipped with a special device on which, during testing (tension), a tensile diagram is automatically recorded.

Considering that the nature of the tensile diagram is influenced by the size of the sample, the diagram is constructed (Fig. 1) in the coordinates stress σ (in N/m 2 or kgf/mm 2) - relative elongation δ (V % ). When testing tensile strength, the following characteristics of mechanical properties are determined: limits of proportionality, elasticity, fluidity, strength, true tensile strength, relative elongation and contraction.

Hardness test.Hardness is the ability of a metal to resist the penetration of another, harder body into it. Hardness testing is the most commonly used method for testing metals. To determine hardness, the manufacture of special samples is not required, i.e. the test is carried out without destroying the part.

There are various methods for determining hardness - indentation, scratching, elastic recoil, as well as the magnetic method. The most common method is to press a steel ball, diamond cone or diamond pyramid into the metal. For hardness testing, special devices are used that are simple in design and easy to use.

Brinell hardness. A hardened steel ball with a diameter of 10, 5 or 2.5 mm is pressed into the surface of the metal being tested with a certain force. As a result, an imprint (hole) is formed on the metal surface. The diameter of the print is measured with a special magnifying glass with divisions. The Brinell hardness number is written in Latin letters HB, followed by a numerical hardness index. For example, hardness according to HB 220. The Brinell method is not recommended for metals with a hardness of more than HB 450, since the ball may be deformed and the result will be incorrect. You should also not test thin materials that are pressed through when the ball is pressed.

Rockwell hardness - hardness test by pressing a cone or ball into the surface of the metal being tested. A diamond cone is pressed at an angle of 120° or a hardened steel ball with a diameter of 1.59 mm. Ball tests are used to determine the hardness of soft materials, and diamond cone tests are used when testing hard materials. The Rockwell hardness number is written in Latin letters HRC (scale C), after which the numerical value of hardness is written. For example, hardness HRC 230.

Vickers hardness - Pyramid indentation hardness test. A tetrahedral diamond pyramid is pressed into the surface of the metal. Based on the load per unit surface of the print, the hardness number, designated HV 140, is determined.

Microhardness test. This test is used to determine the hardness of microscopically small volumes of metal, for example, the hardness of individual structural components of alloys. Microhardness is determined using a special device consisting of a loading mechanism with a diamond tip and a metallographic microscope. The sample surface is prepared in the same way as for microstudy (grinding, polishing, etching). A tetrahedral diamond pyramid (with an apex angle of 136°, the same as the Vickers pyramid) is pressed into the test material under very low load. Hardness is determined by the value N/m 2 or kgf/mm 2.