Definition of basic mechanical tests of metals. Testing of materials and welded joints. Tensile tests

To establish a set of mechanical properties of metals, samples from the material under study are subjected to static and dynamic tests.

Static tests are those in which the load applied to the sample increases slowly and smoothly.

Static tests include tensile, compression, torsion, bending, and hardness testing.

As a result of static tensile tests, which are carried out on tensile testing machines, a tensile diagram (Fig. 6 a) and a conditional stress diagram (Fig. 6 b) of ductile metal are obtained.

Rice. 6 diagram of tensile strength of plastic material; b – diagram of conditional stresses of plastic material

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, elastic deformation is characterized by line OA and its extension.

Above point A, the proportionality between stress and strain is violated. Stress causes not only elastic, but also plastic deformation.

Shown in Fig. 6, the relationship between externally applied stress and the relative deformation caused by it characterizes the mechanical properties of metals:

The slope of straight line OA (Fig. 6a) shows the rigidity of the metal or the characteristic of how a load applied from the outside changes the interatomic distances, which, to a first approximation, characterizes the forces of interatomic attraction; the tangent of the angle of inclination of the straight line OA is proportional to the elastic modulus (E), which is numerically equal to the quotient of the stress divided by the relative elastic deformation (E = s / e);

Stress spc (Fig. 6b), which is called the limit of proportionality, corresponds to the moment of the appearance of plastic deformation. The more accurate the deformation measurement method, the lower point A lies;

Stress supr (Fig. 6b), which is called the elastic limit, and at which plastic deformation reaches a given small value established by the conditions. Residual strain values ​​of 0.001 are often used; 0.005; 0.02 and 0.05%. The corresponding elastic limits are denoted by s0.005, s0.02, etc. Elastic limit is an important characteristic of spring materials that are used for elastic elements of devices and machines;

Stress s0.2, which is called the proof strength and to which a plastic deformation of 0.2% corresponds. The physical yield strength st is determined from the tensile diagram when there is a yield plateau on it. However, during tensile tests of most alloys, there is no yield plateau on the diagrams. The selected plastic deformation of 0.2% quite accurately characterizes the transition from elastic to plastic deformations, and the stress s0.2 is easily determined during testing, regardless of whether there is a yield plateau on the tensile diagram or not. . The permissible voltage used in calculations is usually chosen to be 1.5 times less than s0.2;

The maximum stress sв, which is called temporary resistance, characterizes the maximum load-bearing capacity of the material, its strength prior to destruction, and is determined by the formula

sв = Р max / Fo

The permissible voltage used in calculations is chosen to be 2.4 times less than sv.

The plasticity of a material is characterized by relative elongation d and relative contraction y:

d = [(lк – lо) / lо] * 100,

y = [(Fо – Fк) / Fо] * 100,

where lo and Fo are the initial length and cross-sectional area of ​​the sample;

lк - final length of the sample;

Fк – cross-sectional area at the rupture site.

Hardness is the ability of materials to resist plastic or elastic deformation when a harder body, called an indenter, is introduced into it.

There are different methods for determining hardness.

Brinell hardness is defined as the ratio of the load when pressing a steel ball into the test material to the surface area of ​​the resulting spherical indentation (Fig. 4.7a).

HB = 2P/pD,

D – ball diameter, mm;

d – hole diameter, mm

Rice. 7. Hardness test schemes: a – according to Brinell; b – according to Rockwell; c – according to Vickers

Rockwell hardness is determined by the depth of penetration into the test material of a diamond cone with an apex angle of 120° or a hardened ball with a diameter of 1.588 mm (Fig. 7.b).

The cone or ball is pressed in with two successive loads:

Preliminary Rho = 10 n;

General P = Po + P1, where P1 is the main load.

Hardness is indicated in conventional units:

For scales A and C HR = 100 – (h – ho) / 0.002

For scale B HR = 130 – (h – ho) / 0.002

To determine hardness, a diamond cone with a load of 60 N (HRA), a diamond cone with a load of 150 N (HRC) or a steel ball with a diameter of 1.588 mm (HRB) is used.

Vickers hardness is measured for parts of small thickness and thin surface layers obtained by chemical-thermal treatment.

This hardness is defined as the ratio of the load when a diamond tetrahedral pyramid with an angle between the faces of 136° is pressed into the test material to the surface area of ​​the resulting pyramidal imprint (Fig. 7.c):

HV = 2P * sin a/2 / d2 = 1.854 P/d2,

a = 136o – angle between faces;

d – arithmetic mean of the lengths of both diagonals, mm.

The HV value is found from the known d according to the formula or from calculation tables in accordance with GOST 2999-75.

Microhardness, taking into account the structural heterogeneity of the metal, is used to measure small areas of the sample. In this case, the pyramid is pressed in as when determining Vickers hardness, under a load P = 5-500 N, and the arithmetic mean of the lengths of both diagonals (d) is measured in microns. A metallographic microscope is used to measure microhardness.

The resistance of a material to fracture under dynamic loads is characterized by impact strength. It is defined (GOST 9454-78) as the specific work of destruction of a prismatic sample with a concentrator (notch) in the middle with one blow of a pendulum piledriver (Fig. 4.8): KS = K / So (K is the work of destruction; So is the cross-sectional area of ​​the sample at the location of the concentrator ).

Rice. 8. Impact test scheme

Impact strength (MJ/m2) is denoted by KCU, KCV and KCT. The letters KS mean the symbol of impact strength, the letters U, V, T – the type of concentrator: U-shaped with a notch radius rn = 1 mm, V-shaped with rn = 0.25 mm; T – fatigue crack created at the base of the notch; KCU is the main criterion for impact strength; KCV and KCT are used in special cases.

The work expended on the destruction of the sample is determined by the formula

An = P * l1(cos b - cos a),

where P is the mass of the pendulum, kg;

l1 – distance from the axis of the pendulum to its center of gravity;

b - angle after impact;

a - angle before impact

Cyclic durability characterizes the performance of a material under conditions of repeated stress cycles. The stress cycle is a set of voltage changes between its two limiting values ​​smax and smin during the period T (Fig. 9).

Rice. 9. Sinusoidal voltage cycle

There are symmetrical cycles (R = -1) and asymmetrical ones (R varies within wide limits). Different types of cycles characterize different operating modes of machine parts.

The processes of gradual accumulation of damage in a material under the influence of cyclic loads, leading to changes in its properties, the formation of cracks, their development and destruction, are called fatigue, and the ability to resist fatigue is called endurance (GOST 23207 - 78).

A number of factors influence the fatigue of machine parts (Fig. 10).

Rice. 10. Factors affecting fatigue strength

Failure from fatigue compared to failure from static load has a number of features:

It occurs at stresses lower than under static load, lower yield limits or tensile strength;

Fracture begins on the surface (or close to it) locally, in places of stress concentration (strain). Local stress concentrations are created by surface damage as a result of cyclic loading or cuts in the form of traces of processing or environmental influences;

Fracture occurs in several stages, characterizing the processes of accumulation of damage in the material, the formation of fatigue cracks, the gradual development and merging of some of them into one main crack and rapid final destruction;

The fracture has a characteristic fracture structure, reflecting the sequence of fatigue processes. The fracture consists of a fracture focus (the place where microcracks form) and two zones - fatigue and breakage (Fig. 11).

Rice. 11. Diagram of a fatigue fracture: 1 – crack initiation site; 2 – fatigue zone; 3 – dolom zone

Parts of machines and mechanisms operate under different loads: some parts experience constant loads in one direction, others experience impacts, and others experience loads that change in magnitude and direction. Some machine parts are subject to stress at high or low temperatures. Therefore, various test methods have been developed to determine the mechanical properties of metals. There are static and dynamic tests.

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

Dynamic are tests in which a material is subjected to impact loads.

The most common tests are hardness, static tensile, and impact tests. In addition, fatigue, creep and wear tests are sometimes performed, which provide a more complete picture of the properties of metals.

Tensile tests. Static tensile testing is a common method of mechanical testing of metals. During these tests, a uniform stress state is created across the cross-section of the sample; the material is under the influence of normal and tangential stresses.

For static tests, round samples are usually used. 1 (Fig. 2.5) or flat 2 (leaf). The samples have a working part and heads designed to be secured in the grips of a tensile testing machine.

For cylindrical samples, the ratio of the calculated initial length / 0 to the initial diameter (/ 0 /^/ 0) is called sample multiplicity, on which its final relative elongation depends. In practice, samples with a multiplicity of 2.5 are used; 5 and 10. The most common pattern is a multiple of 5.

The design length /0 is taken to be slightly less than the working length /. Sample sizes are standardized. Working part diameter

Rice. 2.5.1 - round sample; 2 - flat sample; /1 - length of the working part; /о - initial design length

normal round sample 20 mm. Samples of other diameters are called proportional.

The tensile force creates stress in the test specimen and causes it to elongate. The moment the stress exceeds the strength of the sample, it will rupture.

Before testing, the sample is secured in a vertical position in the grips of the testing machine. In Fig. Figure 2.6 shows a diagram of a testing machine, the main elements of which are: a driving loading mechanism that ensures smooth loading of the sample until it breaks; a force measuring device for measuring the tensile strength of the sample; mechanism for automatic recording of the stretch diagram.

Rice. 2.6.1 - base; 2 - screw; 3 - lower grip (active); 4 - sample; 5 - upper grip (passive); 6 - force measuring sensor; 7 - control panel with electric drive equipment; 8 - load indicator; 9 - control handle; 10 - diagram mechanism; 11 - cable

During testing, the diagram mechanism continuously records the so-called primary (machine) tensile diagram (Fig. 2.7) in load coordinates R; D/ is the absolute elongation of the sample. In the stress-strain diagram of ductile metal materials, three characteristic sections can be distinguished: section OA(straight-line) corresponds

elastic deformation (such a relationship between the elongation of the sample and the applied load is called the law of proportional

Rice.

nality); plot LW(curvilinear) corresponds to elastoplastic deformation with increasing load; plot Sun(curvilinear) corresponds to elastoplastic deformation with decreasing load. At the point WITH the final destruction of the sample occurs, dividing it into two parts.

During the transition from elastic to elastoplastic deformation for some metallic materials, a small horizontal section may appear on the machine stress diagram LL", called the yield plateau. The sample elongates without increasing the load - the metal seems to flow. The lowest stress at which deformation of the test sample continues without a noticeable increase in load is called the physical yield strength.

Fluidity is characteristic only of low-carbon annealed steel, as well as some grades of brass. There is no yield plateau on the tensile diagrams of high-carbon steels.

With increasing elastoplastic deformation, the force with which the sample resists increases and reaches at the point IN its maximum value. For plastic materials, at this moment, a local narrowing (neck) is formed in the weakest section of the sample, where further deformation causes the sample to rupture.

When stretched, the strength and ductility of materials is determined.

Strength indicators materials are characterized by stress a, equal to the ratio of the load to the cross-sectional area of ​​the sample (at characteristic points of the tensile diagram).

The most commonly used indicators of material strength include: yield strength, proof strength, tensile strength.

Yield strength a t, MPa - the lowest stress at which the material deforms (flows) without a noticeable change in load:

A. g = Р T /Р 0,

Where R t - load corresponding to the yield area on the tensile diagram (see Fig. 2.7); P 0 - cross-sectional area of ​​the sample before testing.

If there is no yield plateau on the machine tensile diagram, then a tolerance for residual deformation of the sample is specified and the conditional yield strength is determined.

Conditional yield strength a 02, MPa - stress at which permanent elongation reaches 0.2% of the initial design length of the sample:

a 0.2 = A)2 /^0’

Where R 02 - load corresponding to permanent elongation

D/ 0>2 = 0.002/ 0.

Tensile strength a in, MPa - stress corresponding to the greatest load R tah, preceding sample rupture:

Plasticity index. Plasticity is one of the important mechanical properties of metal, which, combined with high strength, makes it the main structural material. The most commonly used plasticity indicators are:

Relative elongation 5,% - the greatest elongation to which the sample is deformed uniformly along its entire calculated length, or in other words, the ratio of the absolute increment in the calculated length of the sample D/p before loading R max to its original length (see Fig. 2.7):

8 = (D/ r //o)100 = [(/ r - /o)//(,]! 00.

Similar to the limiting uniform elongation, there is a relative narrowing of 1|/ (%) of the cross-sectional area:

y = (A/’ p // , 0)100 = [(/- 0 - r r ur 0 ] T,

Where E 0- initial cross-sectional area of ​​the sample; E r - area at the rupture site.

For brittle metals, the relative elongation and relative contraction are close to zero; for plastic materials they reach several tens of percent.

Modulus of elasticity? (Pa) characterizes the rigidity of the metal, its resistance to deformation and is the ratio of the stress in the metal during tension to the corresponding relative elongation within the limits of elastic deformation:

E= a/ 8.

Thus, during a static tensile test, strength indicators (a m, a 02, a b) and ductility indicators (8 and |/) are determined.

Hardness tests. Hardness is the property of a material to resist contact deformation or brittle fracture when a carbide tip (indenter) is introduced into its surface. Hardness testing is the most accessible and common method of mechanical testing. The most widely used in technology are static methods of testing for hardness when indenting an indenter: the Brinell method, the Vickers method and the Rockwell method.

When testing for hardness using the Brinell method, a carbide ball with a diameter of /) is pressed into the surface of the material under the influence of a load. R and after removing the load, the diameter is measured With! imprint (Fig. 2.8, A).

The Brinell hardness number (HH) is calculated using the formula

HB = P/E,

Where R - ball load, N; .Г - surface area of ​​the spherical imprint, mm 2.

A certain load corresponds to a specific hardness value. Thus, when determining the hardness of steel and cast iron,

Rice. 2.8. Brinell hardness testing schemes (A), Vickers (b),

Rockwell (V)

load on the ball P= ZO/) 2 ; for copper, its alloys, nickel, aluminum, magnesium and their alloys - P= 10/) 2 ; for babbitts - P = 2,5/) 2 .

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 edge of the sample must be no less than 1/2).

Lever presses are currently mainly used for Brinell hardness testing.

Using the Brinell method, materials with a hardness of 4500 HB can be tested. If the materials are harder, the steel ball may become deformed. This method is also not suitable for testing thin sheet material.

If Brinell hardness was tested with a ball with a diameter of 10 mm and a load of 29-430 N, then the hardness number is indicated by numbers characterizing the hardness value and the letters “HB”, for example 185HB.

If the tests were carried out under other conditions, then after the letters “НВ” these conditions are indicated: ball diameter (mm), load (kgf) and duration of exposure under load (s): for example 175НВ5/750/20.

This method can test materials with a hardness of no more than 450HB.

When testing for hardness using the Vickers method, a diamond tetrahedral pyramid with an angle of 136° at the apex is pressed into the surface of the material (Fig. 2.8, b). After removing the indentation load, the diagonal is measured c1 x imprint. The Vickers hardness number (VH) is calculated using the formula

NU= 1.854 R/b 2,

arithmetic mean value of the length of both diagonals of the print, mm.

The Vickers hardness number is designated by the letters “NU” indicating the load R and holding time under load, and the dimension of the hardness number (kgf/mm 2) is not set. The duration of exposure of the indenter under load is 10-15 s for steels, and 30 s for non-ferrous metals. For example, 450НУ10/15 means that a Vickers hardness of 450 was obtained at P= 10 kgf applied to the diamond pyramid for 15 s.

The advantage of the Vickers method over the Brinell method is that the Vickers method can test materials of higher hardness due to the use of a diamond pyramid.

When testing for hardness using the Rockwell method, a diamond cone with an angle of 120° at the apex or a steel ball with a diameter of 1.588 mm is pressed into the surface of the material. However, according to this method, the depth of the indentation is taken as a conditional measure of hardness. The Rockwell test diagram is shown in Fig. 2.8, V. First a preload is applied P 0, under the influence of which the indenter is pressed to a depth And (y Then the main load is applied R x, under the influence of which the indenter is pressed to a depth /?,. After this the load is removed R ( , but leave preload R 0 . In this case, under the influence of elastic deformation, the indenter rises up, but does not reach the level And 0 . Difference (AND- /g 0) depends on the hardness of the material. The harder the material, the smaller this difference. The depth of the print is measured by a dial indicator with a division value of 0.002 mm. When testing soft metals using the Rockwell method, a steel ball is used as an indenter. The sequence of operations is the same as for testing with a diamond cone. Hardness determined by the Rockwell method is designated by the letters “H11”. However, depending on the shape of the indenter and the values ​​of the indentation loads, the following letters are added to this symbol: A, C, B, indicating the corresponding measurement scale.

The Rockwell method, compared to the Brinell and Vickers methods, has the advantage that the hardness value according to the Rockwell method is recorded directly by the indicator, eliminating the need for optical measurement of the print size.

Impact strength tests (impact bending). If this or that part of a machine or mechanism, due to its purpose, experiences shock loads, then the metal for the manufacture of such a part, in addition to static tests, is also tested under dynamic load, since some metals with sufficiently high static strength indicators are destroyed under low impact loads. Such metals are, for example, cast iron and steels with coarse-grained structures.

To assess the susceptibility of materials to brittle fracture, impact bending tests on notched specimens are widely used, as a result of which the impact strength is determined. Impact toughness is estimated by the work expended on the impact fracture of the sample, divided by its cross-sectional area at the point of the cut.

To determine impact strength, prismatic samples with various cuts are used. The most common are samples with U- and Y-shaped cuts.

Impact strength tests are carried out on a pendulum impact driver (Fig. 2.9). A pendulum of weight C is raised to a height /?, and then released. The pendulum, falling freely, hits the sample and destroys it, continuing its inertial movement to a height of /? 2.

The work spent on impact fracture of the sample is determined by the formula

K=0(And x-L 2),

where C is the weight of the pendulum; /?, is the height of the pendulum before testing; L 2 - the height of the pendulum after testing.

The pointer on the piledriver scale records the work TO.

Impact strength has the designations: KSU and KSI, where the first two letters indicate the symbol of impact strength, the third (V or i) - the type of concentrator (notch). The beat is counting

Rice. 2.9.A- pendulum pile driver; b- location of the sample on the pile driver; 1 - frame; 2 - pendulum; 3 - sample

viscosity as the ratio of work to the cross-sectional area of ​​the sample in the notch:

KS = AG/^o,

Where TO - impact work to fracture the sample; 5 0 - cross-sectional area of ​​the sample at the incision site.

Technological tests or metal testing is carried out to determine the ability of metals to accept deformation similar to that to which it must be subjected under processing or operating conditions. Technological tests of metals are carried out:

• on draft;
• flattening;
• wire winding;
• bend, bend;
• extrusion;
• weldability;
• deployment of shaped material, etc.

Technological samples of metals in many countries (including

including Russia) are standardized. Technological tests do not provide numerical data. The quality of the metal during these tests is assessed visually by the state of the metal surface after the test. For example, to assess the quality of pipes, technological tests are carried out for expansion, flattening, beading, stretching and ring expansion, as well as hydraulic pressure.

In order to evaluate the ability of a metal to plastically deform without violating its integrity during pressure treatment, its technological plasticity (deformability) is determined. Sometimes the ability to deform is called by the name of a specific process: stampability (extrusion test).

Stampability is determined by pressing the punch through sheet material up to 2 mm thick, sandwiched between the matrix and the clamp; serves to determine the ability of a metal to cold stamping and drawing.

Rollability - longitudinal rolling of wedge-shaped samples (rolling onto a wedge), serves to approximately determine the maximum degree of deformation for a given material.

Piercing - helical rolling of conical or cylindrical samples with braking, serves for an approximate (conical sample) or more accurate (cylindrical sample) determination of the maximum compression before the mandrel toe when piercing the workpieces.

Weldability determines the tensile strength of the weld. With good weldability, the tensile strength along the seam should be at least 80% of the tensile strength of the solid sample.

The bend test determines the ability of a metal to withstand bending; used to evaluate the quality of strip and sheet metal, as well as wire and rods.

Upset tests are carried out to determine the ability of a metal to take a given shape in a cold state, without allowing cracks, ruptures, breaks, etc. Such tests are carried out for rivet metals.

The flattening test determines the ability of a metal to deform when flattened. As a rule, sections of welded pipes with a diameter of 22-52 mm with a wall thickness of 2.5 to 10 mm are subjected to such tests. The test consists of flattening the sample under a press, which is performed until a gap is obtained between the inner walls of the pipe, the size of which is equal to four times the thickness of the pipe wall, and the sample should not have cracks.

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 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 surface of the sample 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.

Methods for determining the mechanical properties of metals are divided into:
- static, when the load increases slowly and smoothly (tensile, compression, bending, torsion, hardness tests);
- dynamic, when the load grows at high speed (impact bending tests);
- cyclic, when the load changes repeatedly in magnitude and direction (fatigue tests).

Tensile test

When testing tensile strength, tensile strength (σ in), yield strength (σ t), relative elongation (δ) and relative contraction (ψ) are determined. Tests are carried out on tensile testing machines using standard samples with cross-sectional area Fo and working (calculated) length lo. As a result of the tests, a tensile diagram is obtained (Fig. 1). The abscissa axis indicates the value of the deformation, and the ordinate axis indicates the value of the load that is applied to the sample.
Ultimate strength (σ in) is the maximum load that the material can withstand without destruction, related to the initial cross-sectional area of ​​the sample (Pmax/Fo).

Rice. 1. Tension diagram

It should be noted that when stretched, the sample elongates, and its cross-section continuously decreases. The true stress is determined by dividing the load acting at a certain moment by the area that the sample has at that moment. In everyday practice, true stresses are not determined, but conditional stresses are used, assuming that the cross section Fo of the sample remains unchanged.

The yield strength (σ t) is the load at which plastic deformation occurs, related to the initial cross-sectional area of ​​the sample (Рт/Fo). However, during tensile tests, most alloys do not have yield plateaus on the diagrams. Therefore, the conditional yield strength (σ 0.2) is determined - the stress to which a plastic deformation of 0.2% corresponds. The selected value of 0.2% quite accurately characterizes the transition from elastic to plastic deformations.

The characteristics of the material also include the elastic limit (σ pr), which means the stress at which plastic deformation reaches a given value. Typically, residual strain values ​​of 0.005 are used; 0.02; 0.05%. Thus, σ 0.05 = Ppr / Fo (Ppr is the load at which the residual elongation is 0.05%).

Limit of proportionality σ pc = Ppc / Fo (Ppc is the maximum load, under the action of which Hooke’s law is still satisfied).

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

δ = [(lk - lo)/lo]∙100% ψ = [(Fo – Fk)/Fo]∙100%,

where lk is the final length of the sample; lo and Fo are the initial length and cross-sectional area of ​​the sample; Fk is the cross-sectional area at the rupture site.

For low-plasticity materials, tensile tests are difficult, since minor distortions during installation of the sample introduce a significant error in determining the breaking load. Such materials are usually subjected to bending testing.

Hardness test

Regulatory documents:

GOST 8.062-85 “State system for ensuring the uniformity of measurements. State special standard and state verification scheme for hardness measuring instruments on the Brinell scales"

GET 33-85 “State special standard of hardness units on the Brinell scale”

Hardness is the ability of a material to resist the penetration of another, harder body, an indenter. The hardness of the material is determined by the Brinell, Rockwell, Vickers, and Shore methods (Fig. 2).

A	b	V

Rice. 2. Schemes for determining hardness according to Brinell (a), Rockwell (b) and Vickers (c)

The Brinell hardness of a metal is indicated by the letters HB and a number. To convert the hardness number to the SI system, use the coefficient K = 9.8 106, by which the Brinell hardness value is multiplied: HB = HB K, Pa.

The Brinell hardness method is not recommended for use for steels with a hardness of more than HB 450 and non-ferrous metals with a hardness of more than 200 HB.

For various materials, a correlation has been established between the ultimate strength (in MPa) and the hardness number HB: σ in ≈ 3.4 HB - for hot-rolled carbon steels; σ in ≈ 4.5 HB - for copper alloys, σ in ≈ 3.5 HB - for aluminum alloys.

Hardness determination by the Rockwell method is carried out by pressing a diamond cone or steel ball into the metal. The Rockwell device has three scales - A, B, C. The diamond cone is used to test hard materials (scales A and C), and the ball is used to test soft materials (scale B). Depending on the scale, hardness is designated by the letters HRB, HRC, HRA and is expressed in special units.

When measuring hardness using the Vickers method, a tetrahedral diamond pyramid is pressed into the metal surface (being ground or polished). This method is used to determine the hardness of thin parts and thin surface layers that have high hardness (for example, after nitriding). Vickers hardness is designated HV. The conversion of the hardness number HV to the SI system is carried out similarly to the conversion of the hardness number HB.

When measuring hardness using the Shore method, a ball with an indenter falls onto the sample, perpendicular to its surface, and the hardness is determined by the height of the ball’s rebound and is designated HS.

Kuznetsov-Herbert-Rehbinder method - hardness is determined by the damping time of the oscillations of a pendulum, the support of which is the metal under study.

Impact test

Impact strength characterizes the ability of a material to resist dynamic loads and the resulting tendency to brittle fracture. For impact testing, special samples with a notch are made, which are then destroyed on a pendulum impact driver (Fig. 3). Using the pendulum pile driver scale, the work K spent on destruction is determined, and the main characteristic obtained as a result of these tests is calculated - impact strength. It is determined by the ratio of the work of destruction of the sample to its cross-sectional area and is measured in MJ/m 2.

To designate impact strength, use the letters KS and add a third, which indicates the type of cut on the sample: U, V, T. The notation KCU means the impact strength of a sample with a U-like notch, KCV - with a V-like notch, and KCT - with a crack , created at the base of the cut. The work of destruction of a sample during impact tests contains two components: the work of crack initiation (Az) and the work of crack propagation (Ar).

Determining impact strength is especially important for metals that operate at low temperatures and exhibit a tendency to cold brittleness, that is, a decrease in impact strength as the operating temperature decreases.

Rice. 3. Scheme of a pendulum pile driver and impact sample

When performing impact tests on notched samples at low temperatures, the cold brittleness threshold is determined, which characterizes the effect of a decrease in temperature on the tendency of the material to brittle fracture. During the transition from ductile to brittle fracture, a sharp decrease in impact strength is observed in the temperature range, which is called the temperature threshold of cold brittleness. In this case, the structure of the fracture changes from fibrous matte (ductile fracture) to crystalline shiny (brittle fracture). The cold brittleness threshold is designated by a temperature range (tb. – txr.) or one temperature t50, at which 50% of the fibrous component is observed in the fracture of the sample or the value of impact strength is reduced by half.

The suitability of a material for operation at a given temperature is judged by the temperature margin of viscosity, which is determined by the difference between the operating temperature and the transition temperature of cold brittleness, and the larger it is, the more reliable the material.

Fatigue test

Fatigue is the process of gradual accumulation of damage to a material under the influence of repeated alternating stresses, which lead to the formation of cracks and destruction. Metal fatigue is caused by the concentration of stress in its individual volumes (in places of accumulation of non-metallic and gas inclusions, structural defects). The ability of a metal to resist fatigue is called endurance.

Fatigue tests are carried out on machines for repeated-alternating bending of a rotating sample, fixed at one or both ends, or on machines for testing tension-compression, or for repeated-alternating torsion. As a result of the tests, the endurance limit is determined, which characterizes the material’s resistance to fatigue.

Fatigue limit is the maximum stress under which fatigue failure does not occur after a basic number of loading cycles.

The endurance limit is denoted by σ R, where R is the cycle asymmetry coefficient.

To determine the endurance limit, at least ten samples are tested. Each specimen is tested at only one stress to failure or at a base number of cycles. The basic number of cycles must be at least 107 loads (for steel) and 108 (for non-ferrous metals).

An important characteristic of structural strength is survivability under cyclic loading, which is understood as the duration of operation of a part from the moment of initiation of the first macroscopic fatigue crack of 0.5...1 mm in size until final destruction. Survivability is of particular importance for the operational reliability of products, the trouble-free operation of which is maintained through early detection and prevention of further development of fatigue cracks.

Chemical testing usually consists of using standard methods of qualitative and quantitative chemical analysis to determine the composition of the material and determine the presence or absence of undesirable and alloying impurities. They are often supplemented by an assessment of the resistance of materials, in particular those with coatings, to corrosion under the influence of chemical reagents. In macroetching, the surface of metallic materials, especially alloy steels, is selectively exposed to chemical solutions to reveal porosity, segregation, slip lines, inclusions, and gross structure. The presence of sulfur and phosphorus in many alloys can be detected by contact imprinting, in which the surface of the metal is pressed against sensitized photographic paper. Using special chemical solutions, the susceptibility of materials to seasonal cracking is assessed. The spark test allows you to quickly determine the type of steel being tested.

Spectroscopic analysis methods are especially valuable because they allow for rapid qualitative determination of small amounts of impurities that cannot be detected by other chemical methods. Multichannel photoelectric recording instruments such as quantometers, polychromators and quantizers automatically analyze the spectrum of a metal sample, after which an indicator device indicates the content of each metal present.

Mechanical methods.

Mechanical tests are usually carried out to determine the behavior of a material under a specific stress state. Such tests provide important information about the strength and ductility of the metal. In addition to standard types of tests, specially designed equipment can be used to reproduce certain specific operating conditions of the product. Mechanical tests can be carried out under either gradually applied stress (static load) or shock loading (dynamic load).

Types of stress.

Based on the nature of their action, stresses are divided into tensile, compressive and shear. Torsional moments cause a special type of shear stress, and bending moments cause a combination of tensile and compressive stresses (usually in the presence of shear). All of these different types of stress can be created in a sample using standard equipment that allows the determination of maximum permissible and failure stresses.

Tensile tests.

This is one of the most common types of mechanical testing. The carefully prepared sample is placed in the grips of a powerful machine, which applies tensile forces to it. The elongation corresponding to each tensile stress value is recorded. From these data, a stress-strain diagram can be constructed. At low stresses, a given increase in stress causes only a small increase in strain, corresponding to the elastic behavior of the metal. The slope of the stress-strain line serves as a measure of the elastic modulus until the elastic limit is reached. Above the elastic limit, plastic flow of the metal begins; elongation increases rapidly until the material fails. Tensile strength is the maximum stress that a metal can withstand during testing.

Impact tests.

One of the most important types of dynamic tests is impact testing, which is carried out on pendulum impact drivers with notched or unnotched specimens. Based on the weight of the pendulum, its initial height and the height of rise after destruction of the sample, the corresponding impact work is calculated (Charpy and Izod methods).

Fatigue tests.

Such tests are aimed at studying the behavior of the metal under cyclic application of loads and determining the endurance limit of the material, i.e. stress below which the material does not fail after a given number of loading cycles. The most commonly used machine is flexural fatigue testing machine. In this case, the outer fibers of the cylindrical sample are exposed to cyclically changing stresses - sometimes tensile, sometimes compressive.

Deep drawing tests.

A sheet metal sample is clamped between two rings and a ball punch is pressed into it. The depth of indentation and time to failure are indicators of the plasticity of the material.

Creep tests.

In such tests, the combined effect of prolonged application of load and elevated temperature on the plastic behavior of materials is assessed at stresses not exceeding the yield strength determined in short-duration tests. Reliable results can only be obtained with equipment that provides precise control of sample temperature and accurate measurement of very small dimensional changes. The duration of creep tests is usually several thousand hours.

Determination of hardness.

Hardness is most often measured by the Rockwell and Brinell methods, in which the measure of hardness is the depth of indentation of an “indenter” (tip) of a certain shape under the influence of a known load. On a Shore scleroscope, hardness is determined by the rebound of a diamond-tipped striker falling from a certain height onto the surface of the sample. Hardness is a very good indicator of the physical condition of a metal. Based on the hardness of a given metal, one can often judge with confidence its internal structure. Hardness tests are often used by technical control departments in production. In cases where one of the operations is heat treatment, continuous hardness control of all products leaving the automatic line is often provided. Such quality control cannot be achieved by other mechanical testing methods described above.

Fracture tests.

In such tests, a sample with a neck is destroyed with a sharp blow, and then the fracture is examined under a microscope, revealing pores, inclusions, hairs, flakes and segregation. Such tests make it possible to approximately estimate the grain size, the thickness of the hardened layer, the depth of carburization or decarburization and other elements of the gross structure in steels.

Optical and physical methods.

Microscopic examination.

Metallurgical and (to a lesser extent) polarizing microscopes often provide reliable judgment of the quality of a material and its suitability for the application in question. In this case, it is possible to determine structural characteristics, in particular the size and shape of grains, phase relationships, the presence and distribution of dispersed foreign materials.

Radiographic control.

Hard X-ray or gamma radiation is directed at the part under test from one side and recorded on photographic film located on the other side. The resulting shadow X-ray or gammagram reveals imperfections such as pores, segregation and cracks. By irradiating in two different directions, the exact location of the defect can be determined. This method is often used to control the quality of welds.

Magnetic particle testing.

This control method is suitable only for ferromagnetic metals - iron, nickel, cobalt - and their alloys. Most often it is used for steels: some types of surface and internal defects can be detected by applying magnetic powder to a pre-magnetized sample.

Ultrasonic testing.

If a short ultrasound pulse is sent into metal, it will be partially reflected from an internal defect - a crack or inclusion. The reflected ultrasonic signals are recorded by the receiving transducer, amplified and displayed on the screen of an electronic oscilloscope. From the measured time of their arrival at the surface, it is possible to calculate the depth of the defect from which the signal was reflected, if the speed of sound in a given metal is known. Control is carried out very quickly and often does not require taking the part out of service.

Special methods.

There are a number of specialized control methods that have limited applicability. These include, for example, the method of listening with a stethoscope, based on changes in the vibration characteristics of the material in the presence of internal defects. Cyclic viscosity tests are sometimes carried out to determine the damping capacity of a material, i.e. its ability to absorb vibrations. It is estimated by the work converted into heat per unit volume of material during one complete cycle of stress reversal. For an engineer involved in the design of structures and machines subject to vibration, it is important to know the damping capacity of structural materials.