Mechanical properties of metals and methods for their determination. Testing of materials and welded joints Strength testing of steel

Hooke's law

As is known, various metals and alloys have different mechanical and technological properties, which predetermine the quality of machine parts, as well as metal machinability. These properties of the metal are revealed by appropriate tests for tension, compression, bending, hardness, etc.

Tensile test. To determine the strength of the tensile metal, a sample 1 is made and installed in the clamps (or grips) 2 of the tensile testing machine. For these purposes, machines with a hydraulic power transmission system or with a screw system are most often used.

The tensile force F (Fig. 51) creates stress in the test specimen and causes it to elongate. When the stress exceeds the strength of the sample, it will break.

Rice. 51

The results of the test are usually presented in the form of a diagram. The load F is plotted along the abscissa axis, the absolute elongation?l is plotted along the ordinate axis.

It can be seen from the diagram that at first the sample elongates in proportion to the load. The straight section OA corresponds to reversible, elastic deformations. When unloading, the sample assumes its original dimensions (this process is described by the same straight section of the curve). The curved section AC corresponds to irreversible plastic deformations. During unloading (dashed straight line CB), the sample does not return to its initial dimensions and retains some residual deformation.

From point C, the specimen elongates without increasing the load. The horizontal section of the CM diagram is called the yield plateau. The stress at which the strain increases without increasing the load is called the yield strength.

Studies show that fluidity is accompanied by significant mutual shifts of crystals, as a result of which lines appear on the sample surface that are inclined to the sample axis at an angle of 45°. Having undergone a state of fluidity, the material again acquires the ability to resist stretching (strengthens), and the diagram behind the point M rises up, although much more gently than before. At point D, the sample stress reaches its maximum value, and a sharp local narrowing, the so-called neck, appears on the sample. The cross-sectional area of the neck rapidly decreases and, as a result, the sample breaks, which corresponds to the position of point K on the diagram.

F D - load at which, after a certain period of time, the destruction of the stretched sample occurs, N (kgf); S is the cross-sectional area of the sample in the initial position, m 2 (mm 2).

Usually, when testing various metals and alloys for tension, the relative elongation e is determined - the ratio of the increase in the length of the sample to rupture to the initial length of the sample. Is it determined by the formula? \u003d?l / l 0 -100,

where: ? - relative extension;

L \u003d l 1 - I 0 - absolute elongation; l 0 - the initial length of the sample; l 1 - sample length after testing. It was experimentally established that the stress in the material during elastic deformation increases in proportion to the relative elongation of the sample. This dependence is called the law of Guk.

For unilateral (longitudinal) stretching, Hooke's law has the form o \u003d E-?,

where: o \u003d F / s - normal stress; F - tensile force; s - cross-sectional area;

Relative extension;

E is a constant value depending on the material of the rod.

Note. In the SI system, the unit of stress is Pascal - the stress caused by a force of 1 newton (N), uniformly distributed over a surface normal to it with an area of 1 m 2.

1 Pa \u003d 0.102 10 -4 kgf / cm 2;

1 Pa \u003d 0.102 10 -6 kgf / mm 2;

1 kgf / cm 2 \u003d 9.81 10 4 Pa;

1 kgf / mm 2 \u003d 9.81 10 6 Pa.

Due to the fact that the Pascal unit of stress is very small, it is necessary to use a larger unit - megapascal 1 MPa = 10 6 Pa.

The State Standard allows for the use of the unit newton per square millimeter (N / mm 2). The numerical values of the stresses, expressed in N / mm 2 and in MPa, are the same. The unit N / mm 2 is also convenient because the dimensions in the drawings are in millimeters.

The proportionality factor E is called the tensile modulus or Young's modulus. What is the physical meaning of the modulus of elasticity? Let us turn to the tensile diagram of the sample (see Fig. 51, II). The modulus of elasticity on it is proportional to the tangent of the angle of inclination a to the abscissa axis. This means that the steeper the straight line OA, the stiffer the material, and the more resistance it exerts to elastic deformation.

To characterize the metal, it is important to know not only the relative elongation?, but also the relative narrowing of the cross-sectional area, which also makes it possible to characterize the plasticity of the material.

Naturally, when the sample is stretched, the cross-sectional area decreases. At the break point, it will be the smallest. The relative narrowing is determined by the formula? = (S 0 - S 1) / S 0 100%,

where: ? - relative narrowing;

S 0 - cross-sectional area of the sample before testing; S 1 - cross-sectional area of the sample at the rupture point (in the neck).

The greater the relative elongation and relative narrowing of the cross section of the sample, the more plastic the material.

In addition to the three considered characteristics of the mechanical properties of metals: tensile strength (o pch), relative elongation (e) and relative narrowing (?), You can determine, using the diagram recorded on the machine, the elastic limit (o y) and the yield strength (o m),

Compression test. For testing metals for compression (Fig. 53), presses are most often used in which the compressive force is formed by increasing hydraulic pressure. When a sample of a plastic material, such as mild steel (Fig. 53, I), is compressed, its transverse dimensions increase, while the length decreases significantly. In this case, there is no violation of the integrity of the sample (Fig. 54). It can be seen from the compression diagram (Fig. 53, II) that in the initial stage of loading, the deformation increases in proportion to the load, then the deformation increases sharply with a slight increase in the load, then the growth of the deformation gradually slows down due to an increase in the cross section of the sample.

Rice. 52

Rice. 53

Samples made of brittle materials are destroyed under compression (Fig. 54, III). For example, a cast iron rod, when the breaking load is reached, breaks into parts that move relative to each other along oblique platforms (Fig. 53, III).

Rice. 54

For compression, Hooke's law is fully applicable, according to which materials resist compression in proportion to the applied force up to the elastic limit. The modulus of elasticity in compression for most materials is equal to the modulus of elasticity in tension. The only exceptions are some brittle materials - concrete, brick, etc. The analogy in the nature of the compressive stress with the tensile stress makes it possible to describe these processes with the same mathematical equations.

Bending test. When testing for bending, the sample (beam) is laid with its ends on two supports and loaded in the middle (Fig. 55). The resistance of the material to bending is judged by the magnitude of the deflection of the sample.

Rice. 55

Let us now imagine imaginary longitudinal fibers in a beam. When bending is deformed, the fibers of one zone are compressed, while the other is stretched (Fig. 55, II).

Between the zones of compression and tension there is a neutral layer, the fibers of which are not subjected to deformation, that is, their length does not change. From fig. 55 shows that the more fibers are located from the neutral layer, the greater the deformation they experience. Thus, we can conclude that when bending in the cross sections of the beam under the action of internal forces, normal compressive and tensile stresses arise, the magnitude of which depends on the position of the considered points in the section. It is customary to denote the highest stresses: in the compression zone - ? max , in the stretch zone - ? m ah. At points located on the neutral axis, the stresses are zero. Normal stresses arising at points of the cross section of different heights increase in proportion to the distance from the neutral layer and can be calculated by the formula? = (E z) / p,

where: ? - normal stresses;

z is the distance from the fiber of interest to us to the neutral layer; E - modulus of elasticity; p is the radius of curvature of the neutral layer.

Shear test. When testing for a cut (Fig. 56), a metal sample 3, having a cylindrical shape, is inserted into the hole of the device, which is a fork 1 and disk 2. The machine pulls the disk out of the fork, as a result of which the middle part of the sample moves relative to its extreme parts. The working area S (cut area) is equal to twice the cross-sectional area of the sample, since the cut occurs simultaneously in two planes.

Rice. 56

When shearing, all points of the deformable sections, limited by the planes of acting forces, are displaced by equal distances, that is, the material at these points experiences the same deformation. This means that at all points of the section there will be the same effective stresses.

The stress value is determined by dividing the resultant F of the internal (transverse) forces by the cross-sectional area of the rod S. Since the stress vector is located in the section plane, shear stress occurs in it, which is determined by the formula r cf = F / 2S, where: r cf is the stress value cut;

F - resultant force;

S is the cross-sectional area of the sample. A shear is a fracture resulting from the shear of one part of the material relative to another, which occurs under the action of shear stresses. For shear deformation, Hooke's law is valid: in the elastic zone, stresses are directly proportional to relative deformations. The coefficient of proportionality is the value of the modulus of elasticity in shear G. Relative shear (shear angle) is denoted y. Thus, Hooke's law for shear deformation has the form t = Gg, where: r = F/S - shear stress; F - tangential force; S is the area of the shearing layers; y - shift angle;

G is the shear modulus depending on the material of the body.

Torsion test. When testing samples for torsion, one end of the pipe 2 is fixed 1, the other is rotated using the lever 3 (Fig. 57). Torsion is characterized by the mutual rotation of the cross sections of the rod, shaft, pipe under the influence of moments (couples of forces) acting in these sections. If rectilinear generatrices are applied on the surface of the rod before the application of torsion forces (Fig. 57, I), then after twisting these generatrices take the form of helical lines, and each cross section rotates through a certain angle with respect to the neighboring one (see Fig. 57, II) . This means that shear deformation occurs in each section and shear stresses arise. Is the degree of displacement of the material during torsion determined by the angles of twist? and shift u. The absolute value of torsion is determined by the angle of twist of the considered section relative to the fixed section. The greatest angle of twist is obtained at the greatest distance from the fixed end of the rod.

Rice. 57

Torsion angle ratio? to the length of the section I, subjected to torsion, is called the relative angle of twist Q = ? /Z,

where: Q - relative angle of twist;

twist angle;

Hardness test. When determining the hardness of materials in factory and laboratory practice, two methods are used: the Brinell method and the Rockwell method.

Brinell method. This method is based on the fact that when measuring the hardness of metals, a steel ball 1 with a diameter of 2.5; 5 or 10 mm is pressed into the surface of the test specimen 2 at a given load 3 from 625 N to 30 kN (62.5 to 3000 kgf). After removing the load, the diameter d of the imprint remaining on the surface of the sample is measured (Fig. 58), which is the smaller, the harder the metal.

Rice. 58

Note. The steel ball must be made of heat-treated steel with a hardness of at least HB850. The surface roughness R z is not lower than the parameter 0.100 according to GOST 2789-73. The surface of the ball must be free of defects visible with a loupe at 5x magnification.

The Brinell hardness number is calculated by the formula

D - ball diameter, mm;

d - imprint diameter, mm.

A special table (GOST 9012-59) makes it possible to determine the hardness of the most common metals.

It should be noted that there is a relationship between the Brinell hardness of steel HB and its tensile strength o p for conventional carbon styles, expressed by the formula o p = 0.36 HB.

Therefore, knowing the hardness of steel according to Brinell, it is possible to calculate the tensile strength.

This formula is of great practical importance. The Brinell method usually determines the hardness of non-hardened steels, cast iron, and non-ferrous metals. The hardness of hardened steels is measured using a Rockwell tester.

Rockwell method. When measuring the hardness of metals using this method, a standard type tip (diamond cone for hard metals or a steel ball for softer ones) is pressed into the test sample under the action of two successively applied loads: preliminary (F 0) 100 N (10 kgf) and final (F 1) 1000 N (100 kgf) - for the ball and 1500 N (150 kgf) - for the diamond cone.

Under the action of a preload, the cone penetrates the metal to a depth h 0 (Fig. 59, I); when adding to the preliminary main load, the depth of the imprint increases to h (Fig. 59, II) and after removing the main load remains equal to h 1 (Fig. 59, III).

Rice. 59

The imprint depth h = h 1 - h 0 obtained due to the main load F 1 characterizes the Rockwell hardness. Rockwell tests are carried out with special instruments equipped with an indicator that shows the hardness number immediately after the test is completed.

The indicator has two scales: black (C) for testing with a diamond cone and red (B) for testing with a ball.

Rockwell hardness is measured in conventional units.

An example of the designation of Rockwell hardness: HRC50 (hardness 50 on the C scale).

Determination of hardness with calibrated files. The HRC hardness can be determined using a series of files heat treated to varying cut hardness. Typically, the notch interval ranges from 3 to 5 HRC units. Calibration of files is carried out according to reference tiles, the hardness of which is precisely determined in advance on the device.

The hardness of the part under test is determined by two files with a minimum interval in hardness, one of which can only slide over the part, and the second slightly scratch it. If a file with HRC62 scratches metal, and with HRC59 it only slides over the surface of the part, then the hardness is HRC60-61.

In practice, this method is used to determine the hardness of tools (reamers, cutters, etc.), the hardness of which is difficult to measure in any other way.

There are other methods for determining hardness (Vickers method, electromagnetic methods, etc.), which are not considered in this book.

MECHANICAL PROPERTIES OF METALS AND METHODS FOR THEIR DETERMINATION

Introduction

Mechanical properties determine the ability of metals to resist the effects of external forces (loads). They depend on the chemical composition of metals, their structure, the nature of technological processing and other factors. Knowing the mechanical properties of metals, one can judge the behavior of the metal during processing and during the operation of machines and mechanisms.

The main mechanical properties of metals include strength, ductility, hardness and impact strength.

Strength - the ability of a metal not to collapse under the action of external forces applied to it.

Plasticity - the ability of a metal to receive a residual change in shape and size without destruction.

Hardness - the ability of a metal to resist being pressed into it by another, more solid body.

Impact strength - the degree of resistance of a metal to destruction under impact loading.

Mechanical properties are determined by mechanical tests.

1. Tensile test

These tests determine such characteristics as the limits of proportionality, elasticity, strength and ductility of metals. For tensile tests, round and flat samples are used (Figure 2.1, a, b), the shape and dimensions of which are established by the standard. Cylindrical samples with a diameter of d 0 = 10 mm, having a calculated length l 0 = 10d 0, are called normal, and samples with a length of l 0 = 5d 0 are short. In a tensile test, the sample is stretched under the action of a gradually increasing load and brought to failure.

Tensile machines are equipped with a special self-recording device that automatically draws a strain curve called a stretch diagram. The tension diagram in the coordinates "load P - elongation? l" reflects characteristic areas and points that allow you to determine a number of properties of metals and alloys (Figure 2.1). In the area 0 - Rpc, the elongation of the sample increases in direct proportion to the increase in load. With an increase in load over R pts, in the section R pts - P control, the direct proportionality is violated, but the deformation remains elastic (reversible). In the area above the point P vpr, noticeable residual deformations occur, and the stretching curve deviates significantly from a straight line. Under load P t, a horizontal section of the diagram appears - the yield platform T-T 1, which is observed mainly in parts made of low-carbon steel. There is no yield plateau on the tension curves of brittle metals. Above the point P t, the load increases to point A, corresponding to the maximum load P in, after which it begins to fall, associated with the formation of local thinning of the sample (neck). Then the load drops to point B, where the destruction of the sample occurs. With the formation of a neck, only ductile metals are destroyed.

a, b - standard specimens for tensile testing;

c - tensile diagram of a sample made of plastic material

Figure 2.1 - Tensile test

The forces corresponding to the main points of the tension diagram make it possible to determine the strength characteristics, expressed in megapascals, MPa, according to the formula

where y i - stress, MPa;

P i - the corresponding point of the tension diagram, N;

F 0 - cross-sectional area of the sample before testing, mm 2.

The limit of proportionality at pc is the maximum stress up to which direct proportionality between stress and strain is maintained:

where P c - voltage corresponding to the limit of proportionality, N.

The elastic limit y upr is the stress at which plastic deformations for the first time reach a certain small value, characterized by a certain tolerance (usually 0.05%):

where P control is the stress corresponding to the elastic limit, N.

The physical yield strength y t is the stress, starting from which the deformation of the sample occurs almost without a further increase in the load:

where P t is the stress corresponding to the yield strength, N.

If there is no yield point in the tensile diagram of a given material, then the conditional yield strength y 0.2 is determined - the stress that causes plastic deformation equal to 0.2%.

Tensile strength (tensile strength) y in - stress equal to the ratio of the maximum load preceding the destruction of the sample to its original cross-sectional area:

where P in is the stress corresponding to the tensile strength, N.

According to the results of the tensile test, the ductility characteristics of metals are determined.

Plasticity indicators of metals - relative elongation and relative narrowing - are calculated from the results of sample measurements before and after testing.

The relative elongation d is found as the ratio of the increase in the length of the sample after rupture to its initial estimated length, expressed as a percentage:

where l k is the length of the sample after rupture, mm;

l 0 - estimated (initial) sample length, mm.

The relative narrowing w is determined by the ratio of the decrease in the cross-sectional area of the sample after rupture to the initial area of its cross-section, expressed as a percentage:

where F 0 is the initial cross-sectional area of the sample;

F to - cross-sectional area of the sample at the site of destruction.

2. Methods for determining hardness

The most common method for determining the hardness of metallic materials is the indentation method, in which another, more solid body (tip) is pressed into the test surface under the action of a constant static load. An imprint remains on the surface of the material, the size of which is used to judge the hardness of the material. The hardness index characterizes the resistance of the material to plastic deformation, as a rule, large, with local contact application of the load.

Hardness is determined on special devices - hardness testers, which differ from each other in the shape, size and material of the indented tip, the magnitude of the applied load and the method for determining the hardness number. Since the surface layers of the metal are tested to measure the hardness, in order to obtain the correct result, the metal surface must not have external defects (cracks, large scratches, etc.).

Brinell hardness measurement. The essence of this method lies in the fact that a hardened steel ball with a diameter of 10, 5 or 2.5 mm is pressed into the surface of the tested metal, depending on the thickness of the sample under the action of a load, which is selected depending on the expected hardness of the tested material and the diameter of the tip according to the formulas: P = 30D 2 ; P \u003d 10D 2; P \u003d 2.5D 2 (table 2.1).

Table 2.1 - Choice of ball diameter D and load P

Sample material	Hardness, kgf/mm2	Sample thickness, mm	Ball diameter D, mm	P/D2, kgf/mm2	Endurance under load, s
Ferrous metals (steel, cast iron)
Black metals
Hard non-ferrous metals (brass, bronze, copper)
Soft non-ferrous metals (tin, aluminum, etc.)

An imprint remains on the surface of the sample (Figure 2.2, a), the diameter of which determines the hardness. The diameter of the imprint is measured with a special magnifying glass with divisions.

Hardness is calculated by the formula

where HB - Brinell hardness, kgf / mm 2;

F is the area of the resulting imprint, mm 2 ;

D - tip diameter, mm;

d - imprint diameter, mm.

Figure 2.2 - Hardness measurement by Brinell (a), Rockwell (b), Vickers (c) methods

In practice, they use special tables that give a translation of the indentation diameter into a hardness number, denoted by HB. For example: 120 HB, 350 HB, etc. (H - hardness, B - according to Brinell, 120, 350 - hardness number in kgf / mm 2, which corresponds to 1200 and 3500 MPa).

This method is mainly used to measure the hardness of non-hardened metals and alloys: rolled products, forgings, castings, etc.

The Brinell hardness tester can be used if the hardness of the material does not exceed 450 kgf / mm 2. Otherwise, the ball will be deformed, resulting in measurement errors. In addition, the Brinell hardness tester is not suitable for testing thin surface layers and thin section specimens.

Rockwell hardness measurement. The measurement is carried out by pressing a steel ball with a diameter of 1.588 mm or a diamond cone with an apex angle of 120 ° into the tested metal (see Figure 2.2, b). In contrast to the Brinell method, Rockwell hardness is determined not by the diameter of the indentation, but by the depth of indentation of the tip.

The indentation is performed under the action of two successively applied loads - preliminary, equal? 100 N, and the final (total) load equal to 1400, 500 and 900 N. The hardness is determined by the difference in the indentation depths of the prints. Hard materials (e.g. hardened steel) require a load of 1500 N, and steel ball indentation with a load of 1000 N is used to determine the hardness of unhardened steel, bronze, brass and other soft materials. The indentation depth is measured automatically, and the hardness after measurement is calculated on three scales: A, B, C (table 2.2).

Table 2.2 - Tips and loads for scales A, B, C

Hardness (hardness number) according to Rockwell is indicated as follows: 90 HRA, 80 HRB, 55 HRC (H - hardness, P - Rockwell, A, B, C - hardness scale, 90, 80, 55 - hardness number in conventional units).

The determination of Rockwell hardness is widely used, as it makes it possible to test soft and hard metals without additional measurements; the size of the prints is very small, so you can test the finished parts without damaging them.

Vickers hardness measurement. This method allows you to measure the hardness of both soft and very hard metals and alloys. It is suitable for hardness testing of very thin surface layers (up to 0.3 mm thick). In this case, a tetrahedral diamond pyramid with an apex angle of 136 o is pressed into the test sample (see Figure 2.2, c). In such tests, loads from 50 to 1200 N are applied. The measurement of the indentation is carried out along the length of its diagonal, considering the indentation under a microscope included in the hardness tester. The Vickers hardness number, denoted HV, is found by the formula

d is the length of the imprint diagonal, mm.

In practice, the hardness number HV is found according to special tables.

3. Determination of impact strength

Determination of impact strength is carried out on a special pendulum impact tester (Figure 2.3). For testing, a standard notched specimen is used, which is mounted on the copra supports. The pendulum with a certain mass is lifted to a set height H and fixed, and then the pendulum released from the latch falls, destroys the sample and rises again to a certain height h. The blow is applied on the side of the sample opposite the notch. For testing, prismatic specimens with cuts of various types are used: U-shaped, V-shaped, T-shaped (notch with a fatigue crack).

a - test scheme; b - samples for testing.

Figure 2.3 - Impact test

The impact strength of the CS (J / cm 2) is estimated by the work spent by the pendulum on the destruction of a standard notched sample, related to the sample cross section at the notch:

where A is the work spent on the destruction of the sample (determined by the difference in the energies of the pendulum before and after the impact: A 0 - A 1), J;

F - cross-sectional area of the sample at the notch, cm 2 .

Depending on the type of notch in the sample, impact strength is denoted by KCU, KCV, KCT (the third letter is the type of notch).

metal property testing mechanical

Literature

1. Tushinsky, L.I. Materials research methods / L.I. Tushinsky, A.V. Plokhov, A.O. Tokarev, V.N. Sindeev. - M.: Mir, 2004. - 380 p.

2. Lakhtin, Yu.M. Materials science / Yu.M. Lakhtin. - M.: Metallurgy, 1993. - 448 p.

3. Fetisov, G.P. Materials science and technology of metals / G.P. Fetisov, M.G. Karpman and others - M .: Higher School, 2001. - 622 p.

4. Evstratova, I.I. Materials science / I.I. Evstratova and others - Rostov-on-Don: Phoenix, 2006. - 268 p.

5. Markova, N.N. Iron-carbon alloys / N.N. Markov. - Eagle: OrelGTU, 2006. - 96 p.

6. Ilyina, L.V. Materials used in mechanical engineering: reference manual / L.V. Ilyina, L.N. Kurdyumov. - Eagle: OrelGTU, 2007.

Types of loads

The mechanical properties of metals and alloys include those that are able to resist the action of external forces or loads on them. They can be very diverse and are distinguished by their impact:

static, which slowly increase from zero to a maximum, and then remain constant or change slightly;
dynamic - arise as a result of impact and act for a short period.

Types of deformation

Deformation is a modification of the configuration of a solid body under the influence of loads applied to it (external forces). Deformations after which the material returns to its previous shape and retains its original dimensions are considered elastic, otherwise (the shape has changed, the material has lengthened) - plastic or residual. There are several types of deformation:

Compression. The volume of the body decreases as a result of the action of compressive forces on it. Such deformation is experienced by the foundations of boilers and machines.
Stretching. The length of a body increases when forces are applied to its ends, the direction of which coincides with its axis. Cables, drive belts are stretched.
Shift or cut. In this case, the forces are directed towards each other and, under certain conditions, a cut occurs. Examples are rivets and tie bolts.
Torsion. A pair of oppositely directed forces acts on a body fixed at one end (shafts of engines and machine tools).
bend. Change in the curvature of the body under the influence of external forces. Such an action is typical for beams, booms of cranes, railway rails.

Determination of metal strength

One of the main requirements that is imposed on the metal used for the production of metal structures and parts is strength. To determine it, a metal sample is taken and stretched on a testing machine. The standard becomes thinner, the cross-sectional area decreases with a simultaneous increase in its length. At a certain moment, the sample begins to stretch in only one place, forming a "neck". And after a while there is a gap in the region of the thinnest place. This is how exceptionally ductile metals behave, brittle: solid steel and cast iron are slightly stretched and they do not form a neck.

The load on the sample is determined by a special device, which is called a force meter, it is built into the testing machine. To calculate the main characteristic of the metal, called the tensile strength of the material, it is necessary to divide the maximum load sustained by the sample before rupture by the value of the cross-sectional area before stretching. This value is necessary for the designer in order to determine the dimensions of the manufactured part, and for the technologist to assign processing modes.

The strongest metals in the world

High-strength metals include the following:

Titanium. It has the following properties:
- high specific strength;
- resistance to elevated temperatures;
- low density;
- resistance to corrosion;
- mechanical and chemical resistance.

Titanium is used in medicine, military industry, shipbuilding, and aviation.

Uranus. The most famous and durable metal in the world, is a weak radioactive material. It occurs in nature in pure form and in compounds. It belongs to heavy metals, flexible, malleable and relatively ductile. Widely used in manufacturing areas.
Tungsten. The calculation of the strength of the metal shows that it is the most durable and refractory metal that is not amenable to chemical attack. It is well forged, it can be pulled into a thin thread. Used for filament.
Rhenium. Refractory, has a high density and hardness. Very durable, not subject to temperature changes. Finds application in electronics and engineering.
Osmium. Hard metal, refractory, resistant to mechanical damage and aggressive environments. Used in medicine, used for rocket technology, electronic equipment.
Iridium. In nature, it is rarely found in free form, more often in compounds with osmium. It is poorly machined, has high resistance to chemicals and strength. Alloys with metal: titanium, chromium, tungsten are used to make jewelry.
Beryllium. Highly toxic metal with a relative density, having a light gray color. It finds application in ferrous metallurgy, nuclear power engineering, laser and aerospace engineering. It has high hardness and is used for alloying alloys.
Chromium. Very hard metal with high strength, white-blue color, resistant to alkalis and acids. The strength of metal and alloys allows them to be used for the manufacture of medical and chemical equipment, as well as for metal-cutting tools.

Tantalum. The metal is silvery in color, has high hardness, strength, has refractoriness and corrosion resistance, is ductile, and is easy to process. It finds application in the creation of nuclear reactors, in metallurgy and the chemical industry.
Ruthenium. Belongs to Possesses high strength, hardness, refractoriness, chemical resistance. Contacts, electrodes, sharp tips are made from it.

How are the properties of metals determined?

To test metals for strength, chemical, physical and technological methods are used. Hardness determines how materials resist deformation. Resistant metal has greater strength and parts made from it wear out less. To determine the hardness, a ball, diamond cone or pyramid is pressed into the metal. The hardness value is set by the diameter of the imprint or by the depth of indentation of the object. Stronger metal is less deformed, and the depth of the imprint will be less.

But the tensile specimens are tested on tensile machines with a load gradually increasing during tensile. The standard may have a circle or a square in cross section. To test the metal to withstand impact loads, impact tests are carried out. An incision is made in the middle of a specially made sample and placed opposite the percussion device. Destruction must occur where the weak point is. When testing metals for strength, the structure of the material is examined by X-rays, ultrasound and using powerful microscopes, and chemical etching is also used.

Technological includes the most simple views tests for destruction, ductility, forging, welding. The extrusion test makes it possible to determine whether the sheet material is capable of being cold formed. Using a ball, a hole is squeezed out in the metal until the first crack appears. The depth of the pit before the appearance of fracture will characterize the plasticity of the material. The bending test makes it possible to determine the ability of a sheet material to accept desired shape. This test is used to assess the quality of welds in welding. To assess the quality of the wire, a kink test is used. Pipes are tested for flattening and bending.

Mechanical properties of metals and alloys

Metal includes the following:

Strength. It lies in the ability of a material to resist destruction under the influence of external forces. The type of strength depends on how external forces act. It is divided into: compression, tension, torsion, bending, creep, fatigue.
Plastic. This is the ability of metals and their alloys to change shape under the influence of a load without being destroyed, and to keep it after the end of the impact. The ductility of a metal material is determined when it is stretched. The more elongation occurs, while reducing the cross section, the more ductile the metal. Materials with good ductility are perfectly processed by pressure: forging, pressing. Plasticity is characterized by two values: relative contraction and elongation.
Hardness. This quality of the metal lies in the ability to resist the penetration of a foreign body into it, which has a greater hardness, and not to receive residual deformations. Wear resistance and strength are the main characteristics of metals and alloys, which are closely related to hardness. Materials with such properties are used for the manufacture of tools used for metal processing: cutters, files, drills, taps. Often, the hardness of the material determines its wear resistance. So hard steels wear out less during operation than softer grades.
impact strength. The peculiarity of alloys and metals to resist the influence of loads accompanied by impact. This is one of important characteristics the material from which the parts that experience shock loading are made during the operation of the machine: wheel axles, crankshafts.
Fatigue. This is the state of the metal, which is under constant stress. Fatigue of the metal material occurs gradually and may result in the destruction of the product. The ability of metals to resist fracture from fatigue is called endurance. This property depends on the nature of the alloy or metal, the state of the surface, the nature of the processing, and the working conditions.

Strength classes and their designations

Regulatory documents on the mechanical properties of fasteners introduced the concept of metal strength class and established a designation system. Each strength class is indicated by two numbers, between which a dot is placed. The first number means the tensile strength, reduced by 100 times. For example, strength class 5.6 means that the tensile strength will be 500. The second number is increased by 10 times - this is the ratio to the tensile strength, expressed as a percentage (500x0.6 \u003d 300), i.e. 30% is the minimum yield strength of the tensile strength for stretching. All products used for fasteners are classified according to the intended use, shape, material used, strength class and coating. According to the intended use, they are:

Lemeshnye. They are used for agricultural machines.
Furniture. They are used in construction and furniture production.
Road. They are attached to metal structures.
Engineering. They are used in the machine-building industry and instrument making.

The mechanical properties of fasteners depend on the steel from which they are made and the quality of processing.

Specific strength

The specific strength of the material (formula below) is characterized by the ratio of the tensile strength to the density of the metal. This value shows the strength of the structure for a given weight. It is of greatest importance for industries such as aircraft, rocket and spacecraft.

In terms of specific strength, titanium alloys are the strongest of all technical materials used. twice the specific strength of metals related to alloy steels. They do not corrode in air, in acidic and alkaline environments, are not afraid of sea water and have good heat resistance. At high temperatures, their strength is higher than that of alloys with magnesium and aluminum. Due to these properties, their use as a structural material is constantly increasing and is widely used in mechanical engineering. Flaw titanium alloys lies in their low machinability. It has to do with physical and chemical properties material and the special structure of the alloys.

Above is a table of the specific strength of metals.

Use of plasticity and strength of metals

Highly important properties metal are ductility and strength. These properties are directly dependent on each other. They do not allow the metal to change shape and prevent macroscopic destruction when exposed to external and internal forces.

Metals with high ductility are gradually destroyed under the influence of a load. At first, they have a bend, and only then it begins to gradually collapse. Ductile metals easily change shape, so they are widely used for the manufacture of car bodies. The strength and ductility of metals depends on how the forces applied to it are directed and in which direction the rolling was carried out during the manufacture of the material. It has been established that, during rolling, metal crystals elongate in its direction more than in the transverse direction. For sheet steel, strength and ductility are much greater in the direction of rolling. In the transverse direction, the strength decreases by 30%, and plasticity by 50%; these figures are even lower in the thickness of the sheet. For example, the appearance of a fracture on a steel sheet during welding can be explained by the parallelism of the axis of the weld and the direction of rolling. According to the plasticity and strength of the material, the possibility of using it for the manufacture of various parts of machines, structures, tools, and devices is established.

Normative and design resistance of metal

One of the main parameters that characterize the resistance of metals to the effects of force is the normative resistance. It is set according to design standards. The design resistance is obtained by dividing the normative by the appropriate safety factor for this material. In some cases, the coefficient of operating conditions of structures is also taken into account. In calculations of practical importance, the calculated resistance of the metal is mainly used.

Ways to increase the strength of metal

There are several ways to increase the strength of metals and alloys:

Creation of alloys and metals having a defect-free structure. There are developments for the manufacture of whiskers (whiskers) several tens of times higher than the strength of ordinary metals.
Obtaining volumetric and surface hardening artificially. When metal is processed by pressure (forging, drawing, rolling, pressing), volume hardening is formed, and knurling and shot peening gives surface hardening.
Creation using elements from the periodic table.
Purification of metal from impurities present in it. As a result, its mechanical properties are improved, the propagation of cracks is significantly reduced.
Elimination of roughness from the surface of parts.

Titanium alloys, the specific gravity of which exceeds aluminum by about 70%, are 4 times stronger, therefore, in terms of specific strength, alloys containing titanium are more profitable to use for aircraft construction.
Many aluminum alloys exceed the specific strength of steels containing carbon. Aluminum alloys have high ductility, corrosion resistance, are excellently processed by pressure and cutting.
Plastics have a higher specific strength than metals. But due to insufficient rigidity, mechanical strength, aging, increased brittleness and low heat resistance, textolites and getinaks are limited in their use, especially in large-sized structures.
It has been established that in terms of corrosion resistance and specific strength, ferrous, non-ferrous metals and many of their alloys are inferior to glass-reinforced plastics.

The mechanical properties of metals are the most important factor in their use in practical needs. When designing some kind of structure, part or machine and selecting a material, be sure to consider all the mechanical properties that it has.

Mechanical testing of metals. Strength, determination of the strength of the metal.

The choice of metal for the manufacture of machine parts and structures is determined by design, operational, technological and economic requirements.

The metal must have the necessary strength, the ability to deform, meet the operating conditions (corrosion resistance, thermal and electrical conductivity, etc.) and have a minimum cost.

Strength is the main requirement for any metal used for the manufacture of machine parts and metal structures.

Strength is the ability of a material to withstand, without collapsing, external loads. The measure of strength is the load that each square millimeter (or centimeter) of the section of the part can withstand.

The strength of the metal is determined by stretching samples of a certain shape and size on a testing machine. When stretched, the cross-sectional area of the sample decreases, the sample becomes thinner, and its length increases. At some point, the stretching of the sample along its entire length stops and occurs only in one place, the so-called neck is formed. After some time, the sample breaks at the site of the "neck" formation.

The tensile process proceeds in this way only for viscous materials, for brittle ones (hard steel, cast iron) the sample breaks with a slight elongation and without the formation of a “neck”.

When dividing the maximum load that the sample withstood before rupture (the load is measured by a special device - a force meter included in the design of the testing machine), by its cross-sectional area before stretching, the main characteristic of the metal is obtained, called the tensile strength (σ in).

The designer needs to know the tensile strength of each metal to determine the dimensions of the part, the technologist - to assign processing modes.

At elevated temperatures, short-term tensile tests are performed on conventional testing machines, only a furnace (usually an electric muffle) is built into the machine to heat the sample. The furnace is mounted on the machine frame so that the axis of the muffle coincides with the axis of the machine. The sample to be tested is placed inside the oven. For uniform heating, the oven must be 2-4 times longer than the sample, and therefore fixing it directly in the grips of the machine is impossible. The sample is fixed in special heat-resistant steel extensions, which, in turn, are fixed in the grips of the machine.

To obtain stable results, the sample must be held at the test temperature for 30 minutes. The value of the tensile strength of the heated metal is significantly affected by the tensile rate: the higher the speed, the greater the value of the tensile strength. Therefore, for a correct assessment of the heat resistance of steel, the duration of the tensile test should be 15-20 minutes.

Strength is the ability of a metal to resist destruction under the influence of external loads. The value of metal as an engineering material, along with other properties, is determined by 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 various capacities. These machines consist of a loading mechanism which generates a force, stretches the test specimen and indicates the amount of force applied to the specimen. Mechanisms are mechanical and hydraulic action.

The power of the machines is different and reaches 50 tons. 7, a shows the device of the machine, consisting of a frame 2 and clamps 4, with which the test samples 3 are fixed.

The upper clamp is motionless fixed in the frame, and the lower clamp slowly lowers during testing with the help of a special mechanism, 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 transmitted during testing on the sample can be determined by the position of the arrow of the device on the measuring scale 1.

Samples should always be tested under the same conditions so that the results can be compared. Therefore, the relevant standards establish certain sizes of test specimens.

The standard specimens for tensile testing are specimens of round and flat sections shown in fig. 7b.

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 greatest stress that a material can experience before it is destroyed; the tensile strength of the metal is equal to the ratio of the maximum load when testing the specimen for rupture to the initial cross-sectional area of the specimen, i.e.

σ b = P b / F 0 ,

where R b - the highest load preceding the rupture of the sample, kgf;

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

For the safe operation of 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 in a tensile test, kgf / mm 2:

Lead 1.8

Aluminum 8