§ 4. MECHANICAL PROPERTIES

Rice. 8. Types of deformations:
a - compression, b - tension, c - torsion, d - shear, e - bending

Rice. 9.Stretch Chart:
a - conditional diagram in P-∆l coordinates, b - conditional stress diagram and true stress diagram

Rice. 10. Determination of metal hardness by the Brinell (a), Rockwell (b) and Vickers (c) methods

The properties of metals are often judged only by their hardness, tensile strength and elongation. Based only on these parameters, conclusions are drawn about the capabilities of the metal or different alloys are compared. In fact, this information is absolutely insufficient to decide the suitability of a material for a specific task. In addition to the mentioned parameters, the applicability of metals and alloys is determined by a) structural strength, b) the degree of manifestation of inelastic phenomena, c) wear resistance, d) corrosion resistance and many others.

On this page we will find out what exactly determines the most common parameters of mechanical properties and consider the main indicators of structural strength. Questions are discussed on other pages wear resistance And corrosion resistance.

Content:

1. Elastic and plastic deformations

2. Indicators of elastic and plastic states

2.1. limits of proportionality, elasticity and fluidity

2.2. characteristics of elastic state

2.3. tensile strength

2.4. plasticity and viscosity

2.5. hardness

3. Structural strength indicators

3.1. crack resistance

3.2. fatigue strength

3.3. impact strength

3.4. creep and long-term strength limits

1. ELASTIC AND PLASTIC DEFORMATIONS

The mechanical properties of metals and alloys are determined by how they perceive external loads, i.e. resist deformation and destruction. When they are deformed, two different types of deformations are observed - elastic and plastic - which differ in both external manifestations and internal mechanisms. It is clear that the properties that determine the elastic and plastic state of metals must be described by different characteristics.

Elastic deformations occur due to changes in interatomic distances; they do not change the structure of the metal, its properties and are reversible. Reversibility means that after removing the load the body takes on its previous shape and size, i.e. There is no residual deformation.

Plastic deformations occur due to the formation and movement of dislocations; they change the structure and properties of the metal. After the load is removed, the deformations remain, i.e. plastic deformations are irreversible.

2. INDICATORS OF ELASTIC AND PLASTIC STATE

2.1. LIMITS OF PROPORTIONALITY, ELASTICITY and FLUIDITY.

Is the range of stresses at which only elastic deformation occurs limited by the proportionality limit? pc. In this region, only elastic deformations take place in each grain, and for the sample as a whole, Hooke’s law is satisfied - the deformation is proportional to the stress (hence the name of the limit).

With increasing stress, microplastic deformations occur in individual grains. At such loads, residual stresses are insignificant (0.001% - 0.01%).

The stress at which residual deformations appear within the specified limits is called the conditional elastic limit. In its designation, the index indicates the amount of residual deformation (in percent) for which the elastic limit was determined, for example? 0.01.

The stress at which plastic deformation already occurs in all grains is called the conditional yield strength. Most often, it is determined at a residual deformation value of 0.2% and is designated? 0.2.

Formally, the difference between the limits of elasticity and yield is associated with the accuracy of determining the “boundary” between the elastic and plastic states, which is what the word “conditional” reflects. It's obvious that? pcexactly The limit of proportionality or elasticity determines the degree of manifestation of inelastic properties and the magnitude of the fatigue limit.

The absence of a sharp boundary between the elastic and plastic states means that in the stress range between? pc and? 0.2, both elastic and plastic deformations occur.

The elastic state exists as long as the dislocations in all grains of the metal are motionless.

The transition to the plastic state is observed in a load range at which the movement of dislocations (and, consequently, plastic deformation) occurs only in individual crystalline grains, and in the rest the mechanism of elastic deformation continues to occur.

The plastic state is realized when the movement of dislocations occurs in all grains of the sample.

After the restructuring of the dislocation structure (completion of plastic deformation), the metal returns to the elastic state, but with changed elastic properties.

The given designations of the limits correspond to uniaxial tension, the diagram of which is shown in the figure. Limits of similar meaning are determined for compression, bending and torsion.

The diagram considered is typical for metals in which the transition from an elastic to a plastic state is very smooth. However, there are metals with a pronounced transition to the plastic state. The stress-strain diagrams of such metals have a horizontal section, and they are characterized not by a conditional, but by a physical yield strength. For an example of such a diagram, see the websitehttp://www.physics-words.com/130/207/2770120.html.. Upon reaching the physical yield point, a small increase in stress leads to a sharp elongation of the sample; the metal seems to flow, hence the name of the limit.

2.2. CHARACTERISTICS OF ELASTIC STATE

The most important parameters of the elastic state – elastic limit? y and elastic moduli.

The elastic limit determines maximum permissible operating loads, in which the metal experiences only elastic or small permissible elastic-plastic deformations. Very roughly (and towards overestimation) the elastic limit can be estimated by the yield strength.

Elastic moduli characterize the resistance of a material to a load in an elastic state. Young's modulus E determines resistance to normal stresses (tension, compression and bending), and shear modulus G determines resistance to shear stresses (torsion). The greater the elastic moduli, the steeper the elastic section on the deformation diagram (see figure), the smaller the magnitude of elastic deformations at equal stresses and, therefore, the greater the rigidity of the structure. Elastic deformations cannot be greater than this value? y/E.

Thus, the elastic moduli determine maximum permissible operational deformations(taking into account the elastic limit and product rigidity. Elastic moduli are measured in the same units as stress (MPa or kgf/mm 2).

Structural materials must combine high values ​​of yield strength (withstand heavy loads) and elastic moduli (provide greater rigidity). The elastic modulus E has the same value in compression and tension. However, the compressive and tensile elastic limits may differ. Therefore, with the same rigidity, the ranges of elasticity during compression and tension may be different.

In an elastic state, metal does not experience macro plastic deformations, however, in its individual microscopic volumes local micro plastic deformations. They are the cause of the so-called inelastic phenomena, which significantly affect the behavior of metals in the elastic state. Under static loads, hysteresis, elastic aftereffect and relaxation appear, and under dynamic loads, internal friction occurs.

Relaxation– spontaneous reduction of stress in the product. An example of its manifestation is the weakening of tension connections over time. The lower the relaxation, the more stable the acting stresses. In addition, relaxation leads to the appearance of residual deformation after removal of the load. Susceptibility to these phenomena is characterized by relaxation resistance. It is measured as the relative change in voltage over time. The larger it is, the less the metal is subject to relaxation.

Internal friction determines irreversible energy losses under variable loads. Energy losses are characterized by the damping decrement or the coefficient of internal friction. Metals with a large damping decrement effectively dampen sound and vibration and are less susceptible to resonance (one of the best damping metals is gray cast iron). Metals with a low coefficient of internal friction, on the contrary, have a minimal effect on the propagation of vibrations (for example, bell bronze). Depending on the purpose, the metal must have high internal friction (shock absorbers) or, conversely, low (springs of measuring instruments).

With increasing temperature, the elastic properties of metals deteriorate. This manifests itself in a narrowing of the elastic region (due to a decrease in the elastic limits), an increase in inelastic phenomena and a decrease in elastic moduli.

Metals that are used for the manufacture of elastic elements and products with stable dimensions must have minimal manifestations of inelastic properties. This requirement is better met when the elastic limit significantly exceeds the operating stress. In addition, the ratio of elasticity and fluidity limits is important. The greater the attitude? y / ? 0.2, the less the manifestation of inelastic properties. When they say that a metal has good elastic properties, it usually means not only a high elastic limit, but also a large value? y / ? 0.2.

2.3. TENSILE STRENGTH

At stresses exceeding the yield strength? 0.2, the metal goes into a plastic state. Externally, this manifests itself in a decrease in resistance to the current load and a visible change in shape and size. After removing the load, the metal returns to the elastic state, but remains deformed by the amount of residual deformation, which can far exceed the limiting elastic deformation. A change in the dislocation structure during plastic deformation increases the yield strength of the metal—its strain hardening occurs.

Typically, plastic deformation is studied under uniaxial tension of the sample. In this case, the temporary resistance is determined? c, relative elongation after rupture? and relative narrowing after rupture?. The picture of tension at stresses exceeding the yield strength comes down to two options presented in the figure.

In the first case (figure in inset), uniform stretching of the entire sample is observed - uniform plastic deformation occurs, which ends with rupture of the sample under stress? V. In this case? in makes sense of the conditional tensile strength, huh? And? determine the maximum uniform plastic deformation.

In the second case, the sample is first stretched evenly, and after the tension is reached? a local narrowing (neck) is formed and further stretching, up to rupture, is concentrated in the neck area. In this case? And? are the sum of uniform and concentrated deformations (see figure). Since the “moment” of determining the tensile strength no longer coincides with the “moment” of sample rupture, then? c determines not the ultimate strength, but the conditional stress at which uniform deformation ends. However, the magnitude? B is often called the conditional tensile strength, regardless of the presence or absence of a neck.

In any case, the difference (? in – ? 0.2) determines the interval of conditional stresses in which uniform plastic deformation occurs, and the ratio? 0.2 / ? B characterizes the degree of hardening. In annealed metal? 0.2/? B =0.5-0.6. and after strain hardening (hardening) it will increase to 0.9-0.95.

The word "conditional" in relation to? V means that it is less than the “true” voltage S V acting in the sample. The point is that tension? is defined as the ratio of the tensile force to the area of ​​the initial cross-section of the sample (which is convenient), and the true stress S must be determined in relation to the cross-sectional area at the time of measurement (which is more difficult). During plastic deformation, the sample becomes thinner and, as it stretches, the difference between the nominal and true stress increases (especially after necking). If you build a tensile diagram for true stresses, then the tensile curve will pass above the curve drawn in the figure and will not have a falling section.

Can metals have the same meaning? c, but if they have different tensile diagrams, the failure of the sample will occur at different true stresses S B (their true strength will be different).

Temporary resistance? B is determined under a load acting for tens of seconds, therefore it is often called the short-term strength limit.

Plastic deformation is also studied under compression, bending, and torsion; the deformation diagrams are similar to those shown in the figure. But for many reasons, uniaxial tension is preferable in most cases. Is it least labor intensive to determine uniaxial tensile parameters? in and?, they are always determined during mass factory tests, and their values ​​are necessarily given in all reference books.

A description of the tensile testing methodology for metals (and the definition of all terms) is given in GOST 1497-73. The compression test is described in GOST 25.503-97, and for torsion - in GOST 3565-80.

2.4. PLASTICITY AND VISCOSITY

Plasticity is the ability of a metal to change shape without compromising its integrity (without cracks, tears, and especially destruction). It manifests itself when elastic deformation is replaced by plastic deformation, i.e. at stresses greater than the yield strength? V.

Is the possibility of plastic deformation characterized by the ratio? 0.2/? V. At? 0.2/? B = 0.5-0.6, the metal allows large plastic deformations (? and? amount to tens of percent). On the contrary, at? 0.2/? At =0.95–0.98, the metal behaves as brittle: the region of plastic deformation is practically absent (? and? are 1-3%).

Most often, plastic properties are assessed by the value of relative elongation at break?. But this value is determined under static uniaxial tension and therefore does not characterize plasticity under other types of deformations (bending, compression, torsion), high deformation rates (forging, rolling) and high temperatures.

As an example, we can cite brasses L63 and LS59-1, which have practically the same values ​​of ?, but significantly different plastic properties. A notched rod from L63 bends at the cut site, and from LS59-1 it breaks off with little force. Wire from L63 is easily flattened without cracking, while wire from LS59-1 cracks after several blows. Brass LS59-1 is easily hot rolled, and L63 is rolled only in a narrow temperature range, beyond which the workpiece cracks.

Thus, plasticity depends on temperature, rate and method of deformation. Plastic properties are strongly influenced by many impurities, often even in very low concentrations.

In practice, to determine plasticity, technological tests are used in which deformation methods are used that are more consistent with the corresponding technological processes.

A common assessment of ductility is the bending angle, the number of bends or twists that a semi-finished product can withstand without cracks or tears.

The test for squeezing the hole out of the tape (analogous to stamping and deep drawing) is carried out until tears and cracks appear.

Good plastic properties are important in metal forming processes. During normal operation, the metal is in an elastic state and its plastic properties do not appear. Therefore, at first glance, it makes no sense to focus on plasticity indicators during normal operation of products.

But if there is a possibility of loads exceeding the yield strength, then it is desirable that the material be plastic. A brittle metal collapses immediately after exceeding a certain limit, while a ductile material is capable of absorbing enough excess energy without collapsing.

The concepts of viscosity and plasticity are often identified, but these terms characterize different properties:

Plasticity - determines the ability to deform without destruction; it is assessed in linear, relative or conventional units.

Viscosity - determines the amount of energy absorbed during plastic deformation, it is measured using energy units

The amount of energy required to fracture a material is equal to the area under the stress-strain curve on the true stress-true strain diagram. This means that it depends on both the maximum possible deformation and the strength of the metal. The method for determining energy intensity during plastic deformation is described inGOST 23.218-84.

2.5. HARDNESS

A generalized characteristic of elastic-plastic properties is hardness.

Hardness is the property of the surface layer of a material to resist the penetration of another, harder body, with its concentrated impact on the surface of the material. The "other, harder body" is an indenter (steel ball, diamond pyramid or cone) pressed into the metal being tested.

The stresses caused by the indenter are determined by its shape and the force of indentation. Depending on the magnitude of these stresses, elastic, elastic-plastic or plastic deformations occur in the surface layer of the metal. In the first case, removing the load does not leave a mark on the surface. If the stress exceeds the elastic limit of the metal, then after removing the load, an imprint remains on the surface.

The smaller the indentation, the higher the indentation resistance and the greater the hardness is considered. By the magnitude of the concentrated force, which does not yet leave an imprint, it is possible to determine the hardness at the yield point (GOST 22762-77).

Numerical determination of hardness is carried out using the Vickers, Brinell and Rockwell methods.

In the Rockwell method (GOST 9013-59), hardness is measured in HR units, which reflect the degree of elastic recovery of the indent after the load is removed. Those. The Rockwell hardness number determines the resistance to elastic or small plastic deformations. Depending on the type of metal and its hardness, different scales are used. The most commonly used scale is C. and hardness number H.R.C.

Requirements for the surface quality of steel parts after heat treatment are often formulated in HRC units. HRC hardness most closely reflects the level of performance of high-strength steels, and given the simplicity of Rockwell measurements, it is very widely used in practice. For a detailed description of the Rockwell method with a description of the different scales and hardness of different classes of materials, see http://www.fast-const.ru/articles.php?article_id=2

Vickers and Brinell hardness is defined as the ratio of the indentation force to the contact area of ​​the indenter and metal at maximum penetration of the indenter. Those. hardness numbers HV and HB have the meaning of the average stress on the surface of an unrestored print, are measured in units of stress (MPa or kgf/mm) and determine the resistance to plastic deformation. The main difference between these methods is related to the shape of the indenter.

Application of the diamond pyramid inVickers method (GOST 2999-75, GOST R ISO 6507-1) ensures geometric similarity of pyramidal prints under any load - the ratio of the depth and size of the print at maximum indentation does not depend on the applied force. This makes it possible to fairly strictly compare the hardness of different metals, including the results obtained under different loads.

Ball indenters inBrinell method (GOST 9012-59) do not provide geometric similarity of spherical prints. This leads to the need to select the load value depending on the diameter of the ball indenter and the type of material being tested according to the tables of recommended test parameters. The consequence of this is ambiguity when comparing HB hardness numbers for different materials.

The dependence of the determined hardness on the magnitude of the applied load (small for the Vickers method and very strong for the Brinell method) requires mandatory indication of test conditions when recording the hardness number (see GOSTs), although this rule is often not observed.

The area of ​​influence of the indenter on the metal is comparable to the size of the indentation, i.e. hardness characterizes the local properties of a semi-finished product or product. If the surface layer (clad or hardened) differs in properties from the base metal, then the measured hardness values ​​will depend on the ratio of the indentation depth and layer thickness - i.e. will depend on the measurement method and conditions. The result of a hardness measurement can relate either only to the surface layer or to the base metal, taking into account its surface layer.

When measuring hardness, the resulting resistance to penetration of the indenter into the metal is determined without taking into account individual structural components. Averaging occurs if the size of the print exceeds the size of all inhomogeneities. The hardness of individual phase components (microhardness) is determined using the Vickers method (GOST 9450-76) with low indentation forces.

There is no direct relationship between different hardness scales, and there are no well-founded methods for converting hardness numbers from one scale to another. The existing tables formally linking the various scales are based on comparative measurements and are valid only for specific categories of metals. In such tables, hardness numbers are usually compared with HV hardness numbers. This is due to the fact that the Vickers method allows you to determine the hardness of any materials (in other methods the range of measured hardness is limited) and ensures geometric similarity of prints.

For a graphical relationship between the Rockwell and Vickers scales, seehttp://www.gordonengland.co.uk/hardness/hardness_conversion.gif.

for steels -http://www.grantadesign.com/images/hardness.fe2.gif

The same for non-ferrous alloys -http://www.grantadesign.com/images/hardness.al1.gif

A tabular relationship between all scales for steels is available inhttp://www.freetechnicalcharts.com/images/Steel_hardness_conversion_chart.jpg

There is also no direct connection between hardness and yield or strength, although in practice the relation is often used? в = k НВ. The k coefficient values ​​are determined on the basis of comparative tests for specific classes of metals and vary from 0.15 to 0.5 depending on the type of metal and its condition (annealed, cold-worked, etc.).

Changes in elastic and plastic properties with changes in temperature, after heat treatment, cold hardening, etc. manifest themselves in changes in hardness. Hardness is measured faster, easier, and allows non-destructive testing. Therefore, it is convenient to control the change in the characteristics of the metal after various types of processing precisely by the change in hardness. For example, hardening, increasing? 0.2 and? 0.2/? c, increases hardness, and annealing reduces it.

In most cases, hardness is determined at room temperature using an indenter for less than a minute. The hardness determined in this case is called short-term hardness. At high temperatures, when the phenomenon of creep develops (see below), long-term hardness is determined - the reaction of the metal to prolonged exposure to the indenter (usually within an hour). Long-term hardness is always less than short-term hardness and this difference increases with increasing temperature. For example, in copper, short-term and long-term hardness at 400 o C is 35HV and 25HV, and at 700 o C - 9HV and 5HV, respectively.

The considered methods are static: the indenter is introduced slowly, and the maximum load is applied long enough to complete the processes of plastic deformation (10 - 180 s). In dynamic (impact) methods, the impact of the indenter on the metal is short-term, and therefore deformation processes proceed differently. Various variations of dynamic methods are used in portable hardness testers.

When colliding with the material under study, the energy of the indenter (strike) is spent on elastic and plastic deformation. The less energy spent on plastic deformation of a sample, the higher its “dynamic” hardness should be, which determines the material’s resistance to elastic-plastic deformation upon impact. Primary data are converted into “static” hardness numbers (HR, HV, HB), which are displayed on the device. Such recalculation is possible only on the basis of comparative measurements for specific groups of materials.

There are also hardness ratings based on resistance to abrasive wear or cutting, which better reflect the corresponding technological properties of materials.

From the above it follows that hardness is not a primary property of a material; rather, it is a generalized characteristic that reflects its elastic-plastic properties. At the same time, the choice of method and measurement conditions can primarily characterize either its elastic or, conversely, plastic properties.

3. INDICATORS OF STRUCTURAL STRENGTH

In uniaxial tension, does failure occur when the ultimate strength is reached? c after completion of plastic deformation. However, in real conditions do metals fail at stresses that do not even exceed the yield strength? 0.2. Does this mean the magnitude? does not determine the real strength of metals and other characteristics are needed to describe it.

Practice shows that the durability of a product is determined by 1) structural strength, 2) wear resistance, and 3)corrosion resistance appropriate material under appropriate operating conditions. It is these properties that determine the choice of material in most practical applications.

3.1. CRACK RESISTANCE (FRACTURE TOUGHNESS)

Metals always contain stress concentrators. They are structural inhomogeneities (impurities, strengthening phases), defects (internal and surface cracks), design features of the product (cuts, sudden changes in the cross-section). Fracture mechanisms are associated with microplastic deformations that develop near stress concentrators and lead to crack initiation over time.

Based on the speed of crack propagation, a distinction is made between ductile and brittle fracture. During brittle fracture it reaches a speed of 1000 m/s, and during ductile fracture it is hundreds of times less. Ductile fracture requires significantly more energy because the deformation region covers an area of ​​metal far beyond the crack. In brittle fracture, the deformation is localized in a narrow region at the tip of the crack, so much less energy is required to advance it.

As long as the crack develops slowly, the product remains functional. But after the crack reaches a certain critical value, its further propagation occurs very quickly and catastrophic destruction of the structure occurs. The slower the crack develops, the greater the structural strength. To characterize structural strength, several values ​​are used (GOST 25.506-85).

The most important parameter of the structural strength of a material is the critical stress intensity factor at the crack tip K 1C (or fracture toughness). It takes into account the length of the crack and the process of its development. Knowledge of it allows one to calculate the maximum permissible load in a structure with a crack of such dimensions that its rapid development to complete destruction has not yet begun. In structural steels, aluminum and titanium alloys, K 1C varies widely - from 15 to 200 MPa*m. The greater its value, the higher the structural strength of the material.

PThere is no strong connection between the fracture toughness K1C and the uniaxial tensile parameters (? 0.2, ? in, ?, ?). At the same time, it significantly depends on the structural features and the presence of impurities.

This can be illustrated by the example of aluminum alloys of the B95 family. Like other heat-strengthening alloys, their structure depends on the quenching and aging regime. There are many examples where metal is preferred am with less strength, but with greater significance K 1C.

3.2. FATIGUE STRENGTH

Cracks in metals originate and develop not only under static loads, but also under the influence of cyclic stresses. A fatigue crack originates in the surface layers (this is its distinctive feature) and slowly develops deeper with each cycle. Fracture occurs when, due to a reduction in cross-section, the effective stresses exceed the destructive ones.

Accumulation of damage means that the more loading cycles, the less the load must be for the metal to “work” without collapsing. The process of gradual accumulation of damage in metal is calledtiredness .

The ability to resist fatigue is calledendurance . Its most important characteristic is its endurance limit. It shows the highest cycle stress at which fatigue failure does not occur after a given number of cycles. More often they use symmetrical alternating cycles (compressive and tensile stresses of equal amplitude act alternately); in such cases, the endurance limit is indicated? - 1 .

Fatigue strength tests are regulated in GOST 25.502.79 and GOST 25.505-85

The second most important endurance characteristic is fatigue life. It determines the number of cycles that a metal can withstand at a given stress. The probability of failure at a given load level and a given number of cycles (or the permissible stress at a given probability of failure) is also determined. An important characteristic of fatigue resistance is the fatigue crack growth rate (CFG) dl/dN and cyclic toughness (cyclic crack resistance) K c 1s. When determining them, the length of the crack is fixed as the number of cycles increases, and loading is carried out at frequencies of 15-20 Hz.

The ability of a metal to work under cyclic loads depends significantly on the loading conditions.

A). At relatively low stresses (which correspond to elastic deformations), the fatigue life is long - the metal retains its integrity over a large number of cycles. Multi-cycle characteristics are determined using a test base of 10 6 – 10 8 cycles at frequencies of 10-300 Hz.

B). Under significant loads (in the area of ​​elastic-plastic deformations), the fatigue life is much less. The parameters of low-cycle fatigue are determined using a test base of up to 5 * 10 4 at frequencies of 3 - 5 Hz.

IN). Cyclic temperature changes under constant stress (or against the background of cyclic loads) are accompanied by elastic-plastic deformations. It leads tothermal fatigue . The ability of a material to resist destruction under conditions of thermal fatigue is called thermal resistance. Thermal resistance indicator is the number of thermal cycles at a given load before failure (GOST 25.502.79).

Obtaining fatigue characteristics is a very expensive and time-consuming process. Therefore, to approximate the fatigue limit, it is often determined through other known characteristics, for example? -1 = k? V. The coefficient k has different values ​​not only for different alloys, but also for different states of the same metal. For example, for annealed aluminum alloys that are not thermally hardened, k = 0.4-0.6, and for heat-strengthened aluminum alloys k = 0.3.

Endurance characteristics depend on a combination of strength, plastic properties and structural features. The endurance of all metals and alloys is negatively affected by impurities and coarse phase inclusions, especially non-metallic ones.

Since fatigue cracks initiate at the surface, the condition of the surface is of particular importance for increasing durability under cyclic loading. Polishing, surface hardening, and absence of corrosion increase the endurance limit.

3.3. IMPACT VISCOSITY

During static tests, the load application speed is 10 -5 – 10 -2 m/s. Their results do not reflect the resistance of the material to loads acting at much higher rates. Therefore, the resistance of a metal to fracture under impact loads is determined in dynamic tests at strain rates of 3–5 m/s.

The main characteristic obtained during impact tests is impact strength (unit of measurement - J/cm 2 ). It determines the energy required to destroy the sample. It is measured by impacting a sample with a previously made cut (GOST 9454-78).

The impact energy is absorbed in a certain volume around the notch. This volume depends on both the strength and ductility of the metal; it is different for different metals and is difficult to estimate. Therefore, the fracture energy is referred not to the volume of the deformed region (which would be correct), but to the cross-sectional area in the notch (which is convenient). For this reason, the value of impact strength is conditional, which must be taken into account when comparing indicators for different metals or different temperatures

Depending on the type of cut (hub), three types of impact strength are determined. Its designation contains a letter indicating the type of concentrator: KST, KSU, KSV (the last letter corresponds to the profile of the cut). The KSV value is used to control materials for critical applications, and KST is used for particularly critical applications. The T-concentrator is a cut with a pre-introduced crack, so in this case the impact energy is spent only on the development of the crack (and not on its formation and development), therefore KST< КСU, КСV. В справочниках часто встречается обозначение ударной вязкости? н, соответствующе КСU.

When determining dynamic viscosity at high or low temperatures, the test temperature designation is additionally introduced, for example KCU -60. Based on such measurements and based on the type of fracture of the sample, another characteristic of the metal is determined - the temperature of the brittle-ductile transition T chr. This is the temperature at which the failure mode changes from ductile to brittle.

3.4. CREEP AND LONG-TERM STRENGTH LIMITS

At stresses below the yield point in metals, the phenomenon of creep is observed. Creep is continuous deformation under constant stress. At low loads and low temperatures it is reversible.

Creep becomes a problem at elevated temperatures (from about 0.4-0.6 Tm) and loads above a certain value (but less than the yield strength). Creep deformation is accompanied by changes in structure and, accordingly, mechanical properties. Unlike plastic deformation, which strengthens the metal, creep deformation leads to its softening. In addition to the constantly increasing deformation and increasing creep rate, cracks begin to appear in the metal and, over time, its destruction occurs.

The concept of heat resistance is associated with the phenomenon of creep. This is the ability to work under load with acceptable deformations and without destruction at elevated temperatures.

A quantitative characteristic of heat resistance is the creep limit (GOST 3248-60) and the long-term strength limit (GOST 10145-81).

The creep limit is used in two ways. In the first, this is tensile stress, at which the deformation reaches a given value in a certain time. In the designation of the limit, the upper index indicates the set temperature, the lower index (through a fraction) indicates the permissible elongation in% and the time during which it is achieved, for example? 900 1/1000.

In another embodiment, the subscript indicates the permissible rate of steady-state creep.

The long-term strength limit is the conditional maximum stress, under the influence of which a material at a given temperature is destroyed after a given period of time. The designation contains two indices: the upper one indicates the specified temperature, the lower one indicates the specified durability (in hours), for example? 900 1000 . This characteristic determines the ability of a material to resist destruction under prolonged exposure to temperature and load.

Creep strength and long-term strength decrease with increasing temperature and holding time. They should be considered as operating voltage limits at high temperatures.

Heat resistance is often confused with heat resistance - the ability to withstand high temperatures without scaling. Heat resistance can be thought of as resistance to corrosion caused by high temperatures. Its characteristics and methods of determination are given in GOST 21910-76 and GOST 6130-71.

CONCLUSION

From the above material it should be clear that any material is characterized by such a large number of parameters that it is impossible to draw conclusions based on several values ​​about the entire set of properties of the metal and the possibility of its use in certain conditions.

To obtain the necessary completeness of information about properties, it is necessary to use reference literature rather than GOST standards, which contain several easily measured quantities.

Hooke's law

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

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

Tensile force F (Fig. 51) creates stress in the test sample and causes its elongation. When the stress exceeds the strength of the sample, it will rupture.

Rice. 51

The test results are usually presented in graph form. Load F is plotted along the abscissa axis, absolute elongation?l is plotted along the ordinate axis.

The diagram shows that initially the sample elongates in proportion to the load. The straight section OA corresponds to reversible, elastic deformations. During unloading, the sample takes on its original dimensions (this process is described by the same straight section of the curve). The curved section of the AC corresponds to irreversible, plastic deformations. Upon unloading (dashed line SV), the sample does not return to its initial dimensions and retains some residual deformation.

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

As studies show, fluidity is accompanied by significant mutual shifts of the crystals, as a result of which lines appear on the surface of the sample, 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 (is strengthened), and the diagram beyond point M rises upward, although much more hollowly than before. At point D, the stress of the sample reaches its greatest value, and a sharp local narrowing, the so-called neck, appears on the sample. The cross-sectional area of ​​the neck quickly decreases and, as a result, the sample ruptures, which corresponds to the position of point K in the diagram. The tensile strength of the sample is determined by the formula about fc = F D / S, where: S fc - tensile strength;

F D is the load at which, after a certain period of time, failure of the tensile specimen occurs, N (kgf); S is the cross-sectional area of ​​the sample in its original 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 before breaking to the initial length of the sample. Is it determined by a formula? = ?l/l 0 -100,

Where: ? - relative extension;

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

For one-sided (longitudinal) stretching, Hooke’s law has the form o = E-?,

where: o = F/s - normal voltage; 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 measurement for 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 = 0.102 10 -4 kgf/cm 2 ;

1 Pa = 0.102 10 -6 kgf/mm 2;

1 kgf/cm2 = 9.81 10 4 Pa;

1 kgf/mm 2 = 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 MP a = 10 6 Pa.

Gosstandart allows the use of the unit newton per square millimeter (N/mm 2). The numerical values ​​of 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 given in millimeters.

The proportionality coefficient E is called the tensile modulus of elasticity or Young's modulus. What is the physical meaning of the elastic modulus? Let us turn to the sample tension diagram (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 greater its resistance to elastic deformation.

To characterize a metal, it is important to know not only the relative elongation, but also the relative contraction of the cross-sectional area, which also allows one to characterize the plasticity of the material.

Naturally, when the sample is stretched, the cross-sectional area decreases. It will be the smallest at the break point. 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 is the cross-sectional area of ​​the sample at the rupture site (in the neck).

The greater the relative elongation and relative contraction 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 contraction (?), it is possible to determine, using a diagram recorded on a machine, the elastic limit (o y) and the yield strength (o m),

Compression test. To test metals for compression (Fig. 53), presses are most often used in which the compressive force is generated by increasing hydraulic pressure. When a sample made of a plastic material, such as low-carbon steel, is compressed (Fig. 53, I), its transverse dimensions increase, while its length decreases significantly. In this case, the integrity of the sample is not violated (Fig. 54). From the compression diagram (Fig. 53, II) it is clear 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 increase in 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, when a cast iron rod reaches a breaking load, it breaks up 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 compressive modulus of elasticity for most materials is equal to the tensile modulus of elasticity. The only exceptions are some brittle materials - concrete, brick, etc. The analogy in the nature of compressive stress with tensile stress makes it possible to describe these processes using the same mathematical equations.

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

Rice. 55

Let us now imagine imaginary longitudinal fibers in the timber. During bending deformation, the fibers of one zone are compressed, while the other is stretched (Fig. 55, II).

Between the compression and tension zones there is a neutral layer, the fibers of which are not subject to deformation, that is, their length does not change. From Fig. 55 it can be seen that the further the 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 a beam under the influence of internal forces, normal compressive and tensile stresses arise, the magnitude of which depends on the position of the points in question in the section. The highest stresses are usually designated: in the compression zone - ? max, in the stretch zone - ? m ah. At points located on the neutral axis, the voltages 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 using the formula? = (E z) / p,

Where: ? - normal stress;

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

Shear test. When testing for shear (Fig. 56), a metal sample 3, which has a cylindrical shape, is inserted into the hole of a device consisting of a fork 1 and a 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 outer parts. The working area S (cut area) is equal to twice the cross-sectional area of ​​the sample, since the cut occurs simultaneously along two planes.

Rice. 56

When shearing, all points of 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 equal effective stresses.

The magnitude of the stress 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, a tangential stress arises in it, determined by the formula r cf = F/2S, where: r cf - stress value cut;

F - resultant force;

S is the cross-sectional area of ​​the sample. Shear is a destruction resulting from the shear of one part of the material relative to another, which occurs under the influence of tangential stresses. For shear deformation, Hooke's law is valid: in the elastic zone, stresses are directly proportional to relative deformations. The proportionality coefficient is the magnitude of the shear elasticity modulus G. The relative shift (shear angle) is denoted by 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 ​​shifting layers; y - shear 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 motionless 1, the other is rotated using lever 3 (Fig. 57). Torsion is characterized by mutual rotation of the cross sections of a rod, shaft, pipe under the influence of moments (force pairs) acting in these sections. If rectilinear generatrices are applied to the surface of the rod before applying torsional forces (Fig. 57, I), then after twisting these generatrices take the form of helical lines, and each cross section with respect to the adjacent one rotates at a certain angle (see Fig. 57, II) . This means that shear deformation occurs in each section and shear stresses arise. Is the degree of material displacement during torsion determined by the angles of twist? and shift y. The absolute value of torsion is determined by the angle of twist of the section under consideration 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

Twist angle ratio? to the length of section I subject to torsion is called the relative angle of torsion 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 sample 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 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. Surface roughness R z is not lower than parameter 0.100 according to GOST 2789-73. There should be no defects on the surface of the ball that are visible with a magnifying glass at 5x magnification.

The Brinell hardness number is calculated using 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 fp for conventional carbon styles, expressed by the formula o f f = 0.36 nb.

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

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

Rockwell method. When measuring the hardness of metals using this method, a standard type tip (a diamond cone for hard metals or a steel ball for softer ones) is pressed into the test sample under the action of two sequentially applied loads: preliminary (F 0) 100 N (10 kgf) and final (F 1) 1000 N (100 kgf) - for a ball and 1500 N (150 kgf) - for a 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 indentation depth h = h 1 - h 0, obtained due to the main load F 1, characterizes the Rockwell hardness. Tests using the Rockwell method are carried out with special devices equipped with an indicator that shows the hardness number immediately after the end of the test.

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 arbitrary units.

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

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

The hardness of the test part is determined by two files with a minimum interval in hardness, one of which can only slide along the part, and the second can slightly scratch it. If a file with HRC62 scratches the 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 can be difficult to measure in any other way.

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

Material selection criteria

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

σ = P/Fo

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

Mechanical properties determined under static loads

Mechanical properties characterize the resistance of a material to deformation, destruction, or the peculiarity of its behavior during the destruction process. This group of properties includes indicators of strength, rigidity (elasticity), ductility, hardness and viscosity. The main group of such indicators consists of standard characteristics of mechanical properties, which are determined in laboratory conditions on samples of standard sizes. The indicators of mechanical properties obtained during such tests evaluate the behavior of materials under external load without taking into account the design of the part and operating conditions.
According to the method of applying loads, static tests are distinguished: tensile, compression, bending, torsion, shear or shear. The most common are tensile tests (GOST 1497-84), which make it possible to determine several important indicators of mechanical properties.

Tensile test. When stretching standard samples with a cross-sectional area Fo and working (calculated) length lo, a tensile diagram is constructed in the coordinates: load - elongation of the sample (Fig. 1). The diagram distinguishes three sections: elastic deformation before load Rupr.; uniform plastic deformation from Rupr.

to Pmax and concentrated plastic deformation from Pmax to Pk. The straight section is maintained until the load corresponding to the proportionality limit Rpc. The tangent of the angle of inclination of a straight section characterizes the modulus of elasticity of the first kind E. Rice. 1.
Ductile metal tensile diagram (a) and diagrams
conditional stresses of ductile (b) and brittle (c) metals.

The true stress diagram (dashed line) is given for comparison.

Plastic deformation above P control. occurs under increasing load, since the metal is strengthened during deformation. Hardening of a material during deformation is called cold hardening. The hardening of the metal increases until the sample breaks, although the tensile load decreases from P to P k (Fig. 1, a). This is explained by the appearance of a local thinning neck in the sample, in which plastic deformation is mainly concentrated. Despite the decrease in load, the tensile stress in the neck increases until the sample fails.
When stretched, the sample elongates and its cross-section continuously decreases. True stress is determined by dividing the load acting at a certain moment by the area that the sample has at that moment (Fig. 1, b). These stresses are not determined in everyday practice, but stress conditions are used, assuming that the cross section F o sample remains unchanged.

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

Elastic limitσ control called the stress at which plastic deformation reaches values ​​of 0.005; 0.02 and 0.05%. The corresponding elastic limits are denoted byσ 0.005, σ 0.02, σ 0.05.

The conditional yield strength is the stress that corresponds to a plastic deformation equal to 0.2%; it is designatedσ 0.2 . Physical yield strengthσ t determined from the tension diagram when there is a yield plateau on it. However, during tensile tests, most alloys do not have a yield plateau on the diagrams. The selected plastic deformation of 0.2% quite accurately characterizes the transition from elastic to plastic deformations.

Temporary resistance characterizes the maximum load-bearing capacity of a material, its strength prior to destruction:

σ in = P max / F o

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

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

Bend test. During a bending test, both tensile and compressive stresses arise in the sample. Cast iron, tool steel, steel after surface hardening and ceramics are tested for bending. The determined characteristics are tensile strength and deflection.

The bending strength is calculated using the formula:

σ u = M / W,

where M is the greatest bending moment; W – moment of resistance of the section, for an image of a circular cross-section

W = πd 3 / 32

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

The Vickers method is used mainly for materials with high hardness, as well as for testing the hardness of parts of small sections or thin surface layers. As a rule, small loads are used: 10,30,50,100,200,500 N. The thinner the cross-section of the part or the layer under study, the less the load is chosen.
The Vickers and Brinell hardness numbers for materials with a hardness of up to 450 HB are practically the same.
Rockwell hardness measurement (GOST 9013-59) is the most universal and least labor-intensive. The hardness number depends on the depth of indentation of the tip, which is used as a diamond cone with an apex angle of 120 0 or a steel ball with a diameter of 1.588 mm. For various combinations of loads and tips, the Rockwell device has three measuring scales: A.B.C. Rockwell hardness is designated by numbers indicating the level of hardness and by the letters HR indicating the hardness scale, for example: 70HRA, 58HRC, 50HRB. Rockwell hardness numbers do not have exact relationships with Brinell and Vickers hardness numbers.
Scale A (tip - diamond cone, total load 600N). This scale is used for particularly hard materials, for thin sheet materials or thin (0.6-1.0 mm) layers. The limits for measuring hardness on this scale are 70-85.
Scale B (tip - steel ball, total load 1000N). This scale determines the hardness of relatively soft materials (<400НВ). Пределы измерения твердости 25-100.

Scale C (tip - diamond cone, total load 1500N). This scale is used for hard materials (> 450HB), such as hardened steels. The limits of hardness measurement on this scale are 20-67.

Determination of microhardness (GOST 9450-76) is carried out by pressing a diamond pyramid into the surface of a sample under small loads (0.05-5N), followed by measuring the diagonal of the indentation. This method evaluates the hardness of individual grains, structural components, thin layers or thin parts.

Mechanical properties determined under dynamic loads When machine parts operate, dynamic loads are possible, under which many metals tend to undergo brittle fracture. The risk of destruction is increased by cuts - stress concentrators. To assess the metal's susceptibility to brittle fracture under the influence of these factors, dynamic impact bending tests are carried out on pendulum impact drivers (Fig. 2). A standard sample is placed on two spores and a blow is applied in the middle, leading to the destruction of the sample. The work is determined using the pendulum piledriver scale, spent on destruction, and calculate the main characteristic obtained as a result of these tests - percussion viscosity:

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

Where S 0 1, cross-sectional area of ​​the specimen at the notch location.

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

In accordance with GOST 9454-78, three types of samples are tested: U-shaped (notch radius r=1 mm); V-shaped (r=0.25 mm) and T-shaped (fatigue crack created at the base of the notch. Accordingly, impact strength is denoted by: KCU, KCV, KCT. Impact strength of all mechanical property characteristics is most sensitive to temperature reduction. Therefore, testing impact strength at low temperatures is used to determine the threshold cold brittleness– temperature or temperature range in which impact strength decreases. Cold brittleness- the ability of a metal material to lose viscosity and become brittle when the temperature drops. Cold brittleness manifests itself in iron, steel, metals and alloys having a body-centered cubic (BCC) or hexagonal close-packed (HC) lattice. It is absent in metals with a face-centered cubic (fcc) lattice.

Mechanical properties determined under variable cyclic loads

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

- cyclic strength, which characterizes the load-bearing capacity of the material, i.e. the greatest voltage that it can withstand for a certain operating time.- cyclic durability– the number of cycles (or operating hours) that a material can withstand before the formation of a fatigue crack of a certain length or before fatigue failure at a given stress.

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

Mechanical properties of materials

a set of indicators characterizing the resistance of a material to the load acting on it, its ability to deform in this case, as well as the features of its behavior during the process of destruction. In accordance with this M. s. m. are measured by voltages (usually in kgf/mm 2 or Mn/m 2), deformations (in %), specific work of deformation and destruction (usually in kgf․m/cm 2 or Mj/m 2), the rate of development of the destruction process under static or repeated loading (most often in mm for 1 sec or for 1000 load repetition cycles, mm/kcycle). M. s. m are determined during mechanical tests of samples of various shapes.

In general, materials in structures can be subjected to loads of very different nature ( rice. 1 ): work on stretching , compression, bending, torsion, shear, etc. or be subject to the combined action of several types of load, such as tension and bending. The operating conditions of materials are also varied in temperature, environment, speed of load application and the law of its change over time. In accordance with this, there are many indicators of M. s. m. and many mechanical testing methods. For metals and engineering plastics, the most common tests are tensile, hardness, and impact bending; brittle structural materials (for example, ceramics, metal-ceramics) are often tested for compression and static bending; It is important to evaluate the mechanical properties of composite materials, in addition, during shear tests.

Deformation diagram. A load applied to a sample causes its deformation (See Deformation). The relationship between load and deformation is described by the so-called. strain diagram ( rice. 2 ). Initially, the deformation of the sample (with tension - increment in length Δ l) is proportional to the increasing load R, then at the point n this proportionality is violated, however, to increase the deformation, a further increase in load is necessary R; at Δ l > Δ l c deformation develops without the application of external force, with a gradually decreasing load. The appearance of the strain diagram does not change if stress is plotted along the ordinate axis

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

The resistance of materials is measured by stresses characterizing the load per unit cross-sectional area of ​​the sample

V kgf/mm 2. Voltage

at which the growth of deformation proportional to the load is violated, is called the limit of proportionality. Under load R P n unloading the sample leads to the disappearance of the deformation that arose in it under the action of the applied force; such deformation is called elastic. Slight excess load relative to P n may not change the nature of the deformation - it will still retain its elastic nature. The greatest load that a sample can withstand without the appearance of residual plastic deformation during unloading determines the elastic limit of the material:

Elastic properties. In the elastic region, stress and strain are related by a proportionality coefficient. When stretching σ = Eδ, where E- so-called modulus of normal elasticity, numerically equal to the tangent of the angle of inclination of the straight section of the curve σ = σ(δ) to the deformation axis ( rice. 2 ). When testing a cylindrical or flat sample for tension, a uniaxial (σ 1 > 0; (σ 2 = σ 3 = 0) stress state corresponds to a triaxial deformed state (increase in length in the direction of action of the applied forces and decrease in linear dimensions in two other mutually perpendicular directions): δ 1 >0; δ 2 = δ 3

within the elasticity range for basic structural materials it fluctuates within rather narrow limits (0.27-0.3 for steels, 0.3-0.33 for aluminum alloys). Poisson's ratio is one of the main calculation characteristics. Knowing μ and E, it is possible to determine the shear modulus by calculation

Resistance to plastic deformation. Under loads R > R in Along with the ever-increasing elastic deformation, noticeable irreversible plastic deformation appears, which does not disappear during unloading. The stress at which the residual relative deformation (tensile elongation) reaches a given value (according to GOST - 0.2%) is called the conditional yield strength and is designated

In practice, the accuracy of modern testing methods is such that σ p and σ e are determined with specified tolerances, respectively, for deviation from the law of proportionality [increase in ctg(90 - α) by 25-50%] and for the amount of residual deformation (0.003-0.05%) and talk about conditional limits of proportionality and elasticity. The tensile curve of structural metals can have a maximum (point at rice. 2 ) or break off when the maximum load is reached R in’ . Attitude

characterizes the temporary resistance (tensile strength) of the material. If there is a maximum on the tensile curve in the area of ​​loads lying on the curve to the left V, the sample is deformed uniformly along the entire calculated length l 0, gradually decreasing in diameter, but maintaining the initial cylindrical or prismatic shape. During plastic deformation, metals are strengthened, therefore, despite the reduction in the cross-section of the sample, further deformation requires the application of an ever-increasing load. σ in, like the conventional σ 0.2, σ n and σ e, characterizes the resistance of metals to plastic deformation. In the section of the deformation diagram to the right, the shape of the tensile sample changes: a period of concentrated deformation begins, expressed in the appearance of a “neck”. A decrease in the cross-section in the neck “overtakes” the strengthening of metals, which causes a drop in the external load in the area P in - P k.

For many structural materials, the resistance to plastic deformation in the elastic-plastic region during tension and compression is almost the same. Some metals and alloys (for example, magnesium alloys, high-strength steels) are characterized by noticeable differences in this characteristic under tension and compression. Resistance to plastic deformation is especially often (when monitoring product quality, standard heat treatment conditions, and in other cases) assessed based on the results of hardness tests by pressing a hard tip in the shape of a ball (Brinell or Rockwell hardness), cone (Rockwell hardness) or pyramid (Vickers hardness). Hardness tests do not require violating the integrity of the part and therefore are the most widespread means of monitoring mechanical properties. Brinell hardness (HB) when indenting a ball with a diameter D under load R characterizes the average compressive stress, conventionally calculated per unit surface of a spherical imprint with a diameter d:

Plasticity characteristics. Tensile plasticity of structural materials is assessed by elongation

(Where h 0 And h k- initial and final height of the sample), during torsion - the maximum angle of twist of the working part of the sample Θ, glad or relative shift γ = Θ r(Where r- sample radius). The final ordinate of the deformation diagram (point k on rice. 2 ) characterizes the resistance to destruction of metal S k, which is determined

(Fk- actual area at the rupture site).

Characteristics of destruction. Destruction does not occur instantly (at the point k), but develops over time, and the beginning of destruction may correspond to some intermediate point on the site VC, and the whole process ends when the load gradually drops to zero. The position of point k on the deformation diagram is largely determined by the rigidity of the testing machine and the inertia of the measuring system. This makes the magnitude S k largely conditional.

Many structural metals (steels, including high-strength, heat-resistant chromium-nickel alloys, soft aluminum alloys, etc.) fail in tension after significant plastic deformation with the formation of a neck. Often (for example, in high-strength aluminum alloys) the fracture surface is located at an angle of approximately 45° to the direction of the tensile force. Under certain conditions (for example, when testing cold-brittle steels in liquid nitrogen or hydrogen, when exposed to tensile stresses and a corrosive environment for metals prone to stress corrosion), fracture occurs along sections perpendicular to the tensile force (direct fracture), without macroplastic deformation.

The strength of materials realized in structural elements depends not only on the mechanical properties of the metal itself, but also on the shape and size of the part (the so-called shape and scale effects), the elastic energy accumulated in the loaded structure, the nature of the acting load (static, dynamic , periodically changing in magnitude), schemes for the application of external forces (uniaxial, biaxial tensile, with superimposed bending, etc.), operating temperature, environment. The dependence of the strength and ductility of metals on the shape is characterized by the so-called. sensitivity to notching, usually assessed by the ratio of the tensile strengths of notched and smooth samples

(for cylindrical samples the cut is usually made in the form of a circular recess, for strips - in the form of a central hole or side cutouts). For many structural materials this ratio under static load is greater than unity, which is associated with significant local plastic deformation at the apex of the notch. The sharper the cut, the smaller the local plastic deformation and the greater the proportion of direct fracture in the failed section. A well-developed direct fracture can be obtained at room temperature in most structural materials in laboratory conditions, if samples of a massive cross-section (the thicker the more plastic the material is) are subjected to stretching or bending, providing these samples with a special narrow slot with an artificially created crack ( rice. 3 ). When a wide, flat sample is stretched, plastic deformation is difficult and is limited to a small area of ​​size 2 r y(on rice. 3 , b shaded), immediately adjacent to the tip of the crack. Direct fracture is usually characteristic of operational failures of structural elements.

Indicators such as the critical stress intensity factor for plane strain, proposed by the American scientist J.R. Irwin as constants for conditions of brittle fracture, have become widespread. K 1C and fracture toughness

In this case, the destruction process is considered in time and indicators K 1C(G 1C) refer to the critical moment when the sustainable development of a crack is disrupted; a crack becomes unstable and propagates spontaneously when the energy required to increase its length is less than the energy of elastic deformation arriving at the crack tip from neighboring elastically stressed zones of the metal.

When assigning sample thickness t and crack sizes 2 l tr based on the following requirement

Stress intensity factor When machine parts operate, dynamic loads are possible, under which many metals tend to undergo brittle fracture. The risk of destruction is increased by cuts - stress concentrators. To assess the metal's susceptibility to brittle fracture under the influence of these factors, dynamic impact bending tests are carried out on pendulum impact drivers (Fig. 2). A standard sample is placed on two spores and a blow is applied in the middle, leading to the destruction of the sample. The work is determined using the pendulum piledriver scale takes into account not only the load value, but also the length of the moving crack:

(λ takes into account the geometry of the crack and the sample), expressed in kgf/mm 3/2 or Mn/m 3/2. By K 1C or G 1C one can judge the susceptibility of structural materials to brittle fracture under operating conditions.

To assess the quality of metal, bending impact tests on prismatic samples with a notch on one side are very common. In this case, impact strength is assessed (See Impact strength) (in kgf․m/cm 2 or Mj/m 2) - the work of deformation and destruction of the sample, conventionally assigned to the cross section at the notch location. Impact bending tests on samples with an artificially created fatigue crack at the base of the notch have become widespread. The work of destruction of such samples and that one is generally in satisfactory agreement with such destruction characteristics as K 1C, and even better with attitude

Time dependence of strength. As the load duration increases, the resistance to plastic deformation and fracture resistance decrease. At room temperature in metals, this becomes especially noticeable when exposed to a corrosive (stress corrosion) or other active (Rehbinder effect) environment. At high temperatures, the phenomenon of creep is observed (See Creep), i.e., an increase in plastic deformation over time at constant stress ( rice. 4 , A). The creep resistance of metals is assessed by the conditional creep limit - most often the stress at which plastic deformation exceeds 100 h reaches 0.2%, and is designated σ 0.2/100. The higher the temperature t, the more pronounced the phenomenon of creep and the more the resistance to destruction of the metal decreases over time ( rice. 4 , b). The last property is characterized by the so-called. limit of long-term strength, i.e. stress, which at a given temperature causes destruction of the material in a given time (for example, σ t 100, σ t 1000, etc.). In polymeric materials, the temperature-time dependence of strength and deformation is more pronounced than in metals. When plastics are heated, highly elastic, reversible deformation is observed; starting from a certain higher temperature, irreversible deformation develops associated with the transition of the material to a viscous-flow state. Creep is also associated with another important mechanical property of materials - a tendency to stress relaxation, i.e., to a gradual drop in stress under conditions when the general (elastic and plastic) deformation maintains a constant specified value (for example, in tightened bolts). Stress relaxation is caused by an increase in the proportion of the plastic component of the total deformation and a decrease in its elastic part.

If a load is applied to the metal, periodically changing according to some law (for example, sinusoidal), then with an increase in the number of cycles N load its strength decreases ( rice. 4 , c) - the metal “gets tired”. For structural steel, such a drop in strength is observed up to N= (2-5) ․10 6 cycles. In accordance with this, they talk about the fatigue limit of structural steel, usually meaning the stress amplitude

below which steel does not fail under repeated varying loads. At |σ min | = |σ max | the fatigue limit is denoted by the symbol σ -1. The fatigue curves of aluminum, titanium and magnesium alloys usually do not have a horizontal section, so the fatigue resistance of these alloys is characterized by the so-called. limited (corresponding to a given N) limits of fatigue. Fatigue resistance also depends on the frequency of load application. The resistance of materials under conditions of low frequency and high values ​​of repeated loading (slow, or low-cycle, fatigue) is not clearly related to fatigue limits. In contrast to a static load, under repeated variable loads sensitivity to a notch always appears, i.e., the fatigue limit in the presence of a notch is lower than the fatigue limit of a smooth sample. For convenience, sensitivity to notch during fatigue is expressed by the ratio

characterizes the asymmetry of the cycle). In the fatigue process, one can distinguish a period preceding the formation of a source of fatigue failure, and following it, sometimes quite long, the period of development of a fatigue crack. The slower the crack develops, the more reliable the material in the structure works. Fatigue crack propagation rate dl/dN is associated with the stress intensity factor by a power function:

Lit.: Davidenkov N.N., Dynamic testing of metals, 2nd ed., L. - M., 1936; Ratner S.I., Failure under repeated loads, M., 1959; Serensen S.V., Kogaev V.P., Shneiderovich R.M., Load-bearing capacity and strength calculations of machine parts, 2nd ed., M., 1963; Applied issues of fracture toughness, trans. from English, M., 1968; Fridman Ya. B., Mechanical properties of metals, 3rd ed., M., 1974; Methods of testing, control and research of engineering materials, ed. A. T. Tumanova, vol. 2, M., 1974.

S. I. Kishkina.

Rice. 3. A sample with a fatigue crack specially created at the top of the notch to determine K1C. Eccentric (a) and axial (b) tensile tests.

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

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