Mechanical properties of what. Technical ability. Classification of materials research methods

Topic 3: Investigation of the properties of structural materials.

Classification of materials research methods

Basic properties of metals and methods of their study.

Metals are one of the classes of structural materials characterized by a certain set of properties:

"Metallic luster" (good reflectivity);
plastic;
high thermal conductivity;
high electrical conductivity.

These properties are due to the peculiarities of the structure of metals. According to the theory of the metallic state, a metal is a substance consisting of positive nuclei, around which electrons revolve in orbitals. At the last level, the number of electrons is small and they are weakly bound to the nucleus. These electrons have the ability to move throughout the entire volume of the metal, i.e., belong to a whole set of atoms.

Research methods.

Metals and alloys have a variety of properties. Using one method of researching metals, it is impossible to obtain information about all properties. Several methods of analysis are used.

1. Determination of the chemical composition.

2. Methods of quantitative analysis are used.

3. If high accuracy is not required, then use spectral analysis.

Spectral analysis based on the decomposition and study of the spectrum of an electric arc or spark, artificially excited between a copper electrode and the metal under study.

An arc is ignited, a beam of light through a prism enters the eyepiece for spectrum analysis. The color and concentration of the lines of the spectrum make it possible to determine the content of chemical elements. Stationary and portable steeloscopes are used.

4. More accurate information about the composition is given by X-ray spectral analysis.

It is carried out on microanalyzers. Allows you to determine the composition of the phases of the alloy, the characteristics of the diffusion mobility of atoms.

general characteristics mechanical properties.

This is a set of indicators characterizing the resistance of a material to a load acting on it, its ability to deform at the same time, as well as the features of its behavior in the process of destruction. In accordance with this, voltages are measured (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 cycles of repetitions of the load, mm / kcycle). M. s. m are determined by mechanical testing of samples of various shapes.

V general case materials in structures can be subjected to a variety of loads: work in tension , compression, bending, torsion, shear, etc., or be subjected to the combined action of several types of load, such as tension and bending. Also, the operating conditions of materials are varied in terms of temperature, environment, load application rate and the law of its change in time. In accordance with this, there are many indicators of M. of page. m. and many methods of mechanical testing. For metals and engineering plastics, the most common tests are tensile, hardness, impact bending; fragile structural materials (for example, ceramics, cermets) are often tested for compression and static bending; it is important to evaluate the mechanical properties of composite materials, in addition, during shear tests.

3) Methods of standard tests to determine the physical and mechanical properties and technological indicators of materials and finished engineering products, standard methods for their design.

During operation, machine parts are subject to different types loads. In order to determine the performance of alloys under various loading conditions, they are tested for tension, compression, bending, torsion, etc.

The behavior of metals under the influence of external loads is characterized by their mechanical properties, which allow you to determine the load limits for each specific material, to make a comparable assessment various materials and to carry out quality control of metal in the factory and laboratory conditions.

There are a number of requirements for testing mechanical properties. The temperature-force conditions of the tests should be as close as possible to the service conditions for the operation of materials in real machines and structures. At the same time, test methods should be simple enough and suitable for mass quality control of metallurgical products. Since it is necessary to be able to compare the quality of different materials of construction, methods for testing mechanical properties must be strictly regulated by standards.

The results of determining the mechanical properties are used in computational design practice in the design of machines and structures. The most common are the following types of mechanical tests.

1. Static short-term tests with single loading for uniaxial tension - compression, hardness, bending and torsion.

2. Dynamic tests with determination of impact toughness and its components - specific work of crack initiation and development.

3. Tests with variable load with the determination of the endurance limit of the material.

4. Thermal fatigue tests.

5. Tests for creep and long-term strength.

6. Tests for resistance to crack propagation with determination of fracture toughness parameters.

7. Testing of materials under complex stress conditions, as well as full-scale testing of parts, assemblies and finished structures.

3.2. Material properties

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

Strength- the ability of a metal to resist destruction when acting on it external forces.

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

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

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

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

Mechanical properties of metals

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

As a result of mechanical tests, numerical values of mechanical properties are obtained, that is, the values of stresses or strains at which changes in the physical and mechanical states of the material occur.

Property assessment

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

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

Structural strength of metals

The criteria for the structural strength of metallic materials can be divided into two groups:

criteria that determine the reliability of metallic materials against sudden fractures (fracture toughness, work absorbed during crack propagation, survivability, etc.). These techniques, using the basic principles of fracture mechanics, are based on static or dynamic tests of specimens with sharp cracks, which take place in real machine parts and structures under operating conditions (notches, through holes, non-metallic inclusions, microvoids, etc.). Cracks and micro-discontinuities greatly change the behavior of the metal under load, since they are stress concentrators;
criteria that determine the durability of products (fatigue resistance, wear resistance, corrosion resistance, etc.).

Criteria for evaluation

Criteria for assessing the strength of the structure as a whole (structural strength), determined during bench, full-scale and operational tests. During these tests, the influence on the strength and durability of the structure of such factors as the distribution and magnitude of residual stresses, defects in the manufacturing technology and design of metal products, etc., is revealed.

For solutions practical tasks Metallurgy is necessary to determine both standard mechanical properties and structural strength criteria.

Mechanical properties are manifested as the ability of a material to resist all types of external mechanical influences.

Mechanical influences characterize by direction, duration and scope. In the direction of the mechanical action can be considered as linear(stretch and squeeze) and corner(bending and twisting). According to their duration, they are divided into static and dynamic, by scope - by volumetric and superficial.

Mechanical properties determine the change in the shape, size and continuity of substances and materials under mechanical stress, and, consequently, the result of almost any mechanical action on substances and materials that occurs during their production and operation (use).

The main mechanical properties of substances and materials include elasticity, stiffness, elasticity, plasticity, strength, brittleness, toughness and hardness.

Elasticity- the property of materials to spontaneously restore their shape and volume (solids) or only volume (liquids and gases) when external influences cease. Elasticity - due to the interaction between the atoms (molecules) of the substance and their thermal motion.

As a measure of the ability of materials or products to change their size and shape for a given type of load, the concepts "elasticity" and "rigidity".

Elasticity - the ability of a material or product to undergo significant changes in size and shape without destruction with a relatively small acting force.

Hardness - the ability of a material or product to change its dimensions and shape less under a given type of load. The more the stiffness, the less the change.

Elasticity- the ability of solid materials to retain their changed shape and volume without microscopic discontinuities after removing the mechanical loads that caused these changes.

Plastic deformation is associated with the breaking of some interatomic bonds and the formation of new ones. Taking into account the plasticity allows you to determine the safety margins, deformability and stability, expands the possibilities of creating structures with a minimum weight.

Mechanical strength solids - the property to resist destruction, separation into parts), as well as irreversible change in shape under mechanical stress. The strength of solids is ultimately determined by the forces of interaction between their constituent structural units (atoms, ions, etc.).

Fragility- the property of solids to collapse under mechanical stress without significant preliminary changes in shape and volume.

Viscosity (internal friction)- the ability of materials to resist the action of external forces, causing:

V solids- propagation of an existing sharp crack (destruction);

In liquids and gases - flow.

Hardness - the property of materials to resist contact action in the surface layer (indentation or scratching). The peculiarity of this property is that it is realized only in a small volume of matter. Hardness is a complex property of a material, reflecting both its strength and plasticity.

In the absence of mechanical action, the atoms in the crystal are in equilibrium positions. Under mechanical stress, the deformation of the material object occurs.

Deformation- a change in the relative position of many particles of a substance, which leads to a change in the shape and size of the body or its parts and causes a change in the forces of interaction between them. All substances are deformable.

If a compressive load is applied, then the particles of the structure of the substance (for example, atoms) will approach to such a distance at which the internal repulsive forces will balance the external compressive forces. When stretched, the distance between structural particles increases until the forces of attraction balance the external load.

In solids, according to the mechanism of flow, elastic and plastic deformations are distinguished. Elastic deformation deformation is called, the effect of which on the shape, structure and properties of the material is eliminated after the cessation of the action of external forces, and plastic - such part of the deformation that remains after the removal of the load, irreversibly changing the structure of the material and its properties.

All real solids, even with small deformations, have plastic properties, which predetermines mixed mechanisms of deformation - elastoplastic deformation. So, in various parts and structures, plastic deformations cover, as a rule, a small volume of material, the rest undergoes only elastic deformations. If the amount of deformation clearly depends on time, for example, it increases with a constant load, but is reversible, it is called viscoelastic.

Plastic deformation in solids can be carried out, for example, by sliding, which occurs in the crystal lattice of a substance along planes and directions with the densest packing of atoms. Sliding planes and sliding directions lying in these planes form sliding system. In metals, for example, one or several sliding systems can operate simultaneously.

The presentation of the sliding process as the simultaneous movement of one part of the crystal relative to another is purely schematic (Fig), since such a movement would require external loads that are hundreds and thousands of times higher than those at which the process actually proceeds.

In real materials, slip occurs both as a result of displacement of dislocations in one slip plane, and by transition to others. Dislocations moving in a deformed crystalline substance give rise to a large number of dislocated atoms and vacancies.

Most of the work (up to 95%) spent on deformation is converted into heat (heating occurs), the rest of the energy is accumulated in the form of an increased density of lattice defects (vacancies and mainly dislocations). The accumulation of energy is also evidenced by the growth of residual stresses as a result of deformation. In this regard, the state of the plastically deformed material is unstable and can change, for example, during heat treatment.

The simplest deformation elements are:

elongation δ is the ratio of the length increment (/, - / 0) of the sample under the action of the load to its initial value / 0:

δ = (/,-/ 0)/ / 0

relative constriction ψ - ratio of the decrease in the cross-sectional area of the sample under the action of a load (S 0 -S 1) to its original value S 0:

ψ = (S 0 -S 1) / S 0

Resistance to deformation is determined by the resistance to shear of one atomic layer relative to another adjacent one. To estimate the magnitude of this resistance, the concept “ voltage".

Voltage - a measure of internal forces arising during deformation of a material, which characterizes the change in the forces of interaction between particles of a substance during its deformation. The voltage is not measured directly, but is only calculated through the values of the forces acting on the body or is determined indirectly - by the effects of its action, for example, by the piezoelectric effect.

Voltage is a vector quantity; the values of the projection of this vector onto the normal and tangent plane are called normal and shear stresses.

The sliding system under plastic deformation in a particular crystalline substance is characterized by the value of the minimum shear stress, which is necessary for the beginning of sliding. it critical shear stress m 0, which does not depend on the orientation of the sliding plane with respect to the applied load and is one of the fundamental characteristics of a crystalline material.

If sliding in this system begins when the shear stress reaches the critical value m 0, then the continuation of deformation requires a continuous increase in the shear stress, i.e. deformation is accompanied by continuous hardening ( strain hardening, or riveting).

Work hardening- a change in the structure and properties with an increase in the density of crystal lattice defects in substances as a result of plastic deformation. During work-hardening, ductility and impact strength decrease, but hardness and strength increase. Work hardening is used for surface hardening products, but it should be borne in mind that work hardened metals are more susceptible to corrosion and are prone to stress corrosion cracking.

Stresses characterize by source and in relation to the exposure time.

The source of the voltage is divided by mechanical - under mechanical stress, thermal- due to a temperature gradient, for example during rapid heating or cooling between the surface and inner layers, and structural (phase) - during various physicochemical processes occurring in a substance, for example, a change in the volume of individual crystallites during phase transformations.

The magnitude of mechanical stresses in a material sample σ is directly proportional to the magnitude of the external force F, Pa:

σ = F / S,

where S - sample area, m 2.

The main mechanical characteristics resistance of the material to deformation and destruction: Young's modulus, Poisson's ratio, shear modulus, proportional limit, elastic limit, and yield limits and strength.

Mechanical properties of materials

a set of indicators characterizing the resistance of a material to a load acting on it, its ability to deform in this case, as well as the features of its behavior in 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 cycles of repetitions of the load, mm / kcycle). M. s. m are determined by mechanical testing of samples of various shapes.

In general, materials in structures can be subjected to loads of various types ( rice. 1 ): work in tension , compression, bending, torsion, shear, etc., or be subjected to the combined action of several types of load, such as tension and bending. Also, the operating conditions of materials are varied in terms of temperature, environment, load application rate and the law of its change in time. In accordance with this, there are many indicators of M. of page. m. and many methods of mechanical testing. For metals and engineering plastics, the most common tests are tensile, hardness, impact bending; fragile structural materials (for example, ceramics, cermets) 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 the specimen causes its deformation (see Deformation). The relationship between load and deformation is described by the so-called. deformation diagram ( rice. 2 ). Initially, the deformation of the sample (under tension - the increment in length Δ l) is proportional to the increasing load R, then at point n this proportionality is violated, however, to increase the deformation, a further increase in the load is necessary R; at Δ l > Δ l deformation develops without applying external force, with a gradually decreasing load. The form of the deformation diagram does not change if stress is plotted along the ordinate

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

The resistance of materials is measured by stresses that characterize the load per unit cross-sectional area of the sample.

v kgf / mm 2. Voltage

at which the increase in deformation proportional to the load is violated is called the proportionality limit. Under load R P n unloading the sample leads to the disappearance of the deformation that has arisen in it under the action of the applied force; such deformation is called elastic. Slight overload relative to P n may not change the nature of the deformation - it will still retain its elastic character. The highest load that the 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. Under tension σ = Еδ, where E- the so-called. modulus of normal elasticity, numerically equal to the tangent of the slope of the straight section of the curve σ = σ (δ) to the deformation axis ( rice. 2 ). In tensile testing of a cylindrical or flat specimen, a uniaxial (σ 1> 0; (σ 2 = σ 3 = 0) stress state corresponds to a triaxial deformed state (an increase in length in the direction of action of applied forces and a decrease in linear dimensions in two other mutually perpendicular directions): δ 1> 0; δ 2 = δ 3

within the limits of elasticity for the main structural materials 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 design characteristics. Knowing μ and E, it is possible to determine the shear modulus by calculation

Resistance to plastic deformation. Under loads R > P in along with the ever increasing elastic deformation, there appears a noticeable irreversible plastic deformation that does not disappear during unloading. The stress at which the residual relative deformation (in tension - elongation) reaches a given value (according to GOST - 0.2%) is called the conventional yield point and is denoted

Practical accuracy modern methods test is such that σ p and σ e are determined with given tolerances, respectively, for deviations from the proportionality law [increase in ctg (90 - α) by 25-50%] and for the amount of residual deformation (0.003-0.05%) and speak of conditional limits of proportionality and elasticity. The tensile curve of structural metals can have a maximum (point b on rice. 2 ) or break off when the maximum load is reached P in’. Attitude

characterizes the tensile strength (tensile strength) of the material. In the presence of a maximum on the tension curve in the region 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 retaining the initial cylindrical or prismatic shape. During plastic deformation, metals are hardened; therefore, despite the decrease in the sample cross section, an ever increasing load must be applied for further deformation. σ in, as well as conditional 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 stretched specimen changes: a period of concentrated deformation begins, which is expressed in the appearance of a “neck”. A decrease in the section in the neck "overtakes" the strengthening of metals, which causes a drop in the external load in the section P in - P k.

For many structural materials, the resistance to plastic deformation in the elastic-plastic region under tension and compression is practically 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 (during product quality control, standardized heat treatment modes, etc.) assessed by the results of hardness tests by indenting a hard tip in the form of a ball (Brinell or Rockwell hardness), a cone (Rockwell hardness) or a pyramid (Vickers hardness). Hardness tests do not require breaking the integrity of the part and therefore are the most widespread means of controlling mechanical properties. Brinell hardness (HB) for ball indentation D under load R characterizes the average compressive stress, conventionally calculated per unit surface area of a spherical indentation with a diameter d:

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

(where h 0 and h k- initial and final height of the sample), in torsion - the limiting angle of twisting of the working part of the sample Θ, glad or a relative shift γ = Θ r(where r- sample radius). End ordinate of the deformation diagram (point k on rice. 2 ) characterizes the resistance to fracture of the metal S k which is determined

(F k is the actual area at the break point).

Destruction characteristics. The destruction does not occur instantly (at the point k), but develops in time, and the beginning in destruction can correspond to some intermediate point on the site VC, and the whole process ends with the load gradually dropping to zero. The position of the 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.) are destroyed by stretching 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 about 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 (straight 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 varying in magnitude), schemes for the application of external forces (uniaxial tension, biaxial, with bending, etc.), operating temperature, environment... The dependence of the strength and plasticity of metals on the shape is characterized by the so-called. notch sensitivity, usually assessed by the ratio of the tensile strengths of a notched and a smooth specimen

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

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

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

When assigning a sample thickness t and crack size 2 l tr proceed from the following requirement

Stress intensity factor TO takes into account not only the value of the load, but also the length of the moving crack:

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

Impact bending tests of prismatic specimens with a notch on one side are very common to assess the quality of metal. At the same time, impact strength is evaluated (see Impact strength) (in kgf․m / cm 2 or MJ / m 2) - the work of deformation and destruction of the sample, conditionally referred to the cross-section at the notch. Impact bending tests of specimens with an artificial fatigue crack at the base of the notch are widely used. The work of destruction of such samples and that is generally in satisfactory agreement with such a fracture characteristic as K 1C, and even better with an attitude

Time dependence of strength. With an increase in the duration of the load, the resistance to plastic deformation and the resistance to fracture decrease. At room temperature, this becomes especially noticeable in metals when exposed to a corrosive (stress corrosion) or other active (Rebinder 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 estimated by the conditional creep limit - most often by the stress at which plastic deformation is over 100 h reaches 0.2%, and designate it as σ 0.2 / 100. The higher the temperature t, the more pronounced the phenomenon of creep and the more the resistance to fracture of the metal decreases over time ( rice. 4 , b). The last property is characterized by the so-called. the ultimate long-term strength, that is, the stress that, at a given temperature, causes the destruction of the material in a given time (for example, σ t 100, σ t 1000, etc.). Have polymer materials the temperature-time dependence of strength and deformation is more pronounced than that of 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. Another important mechanical property of materials is associated with creep - the tendency to stress relaxation, i.e., to a gradual drop in stress under conditions when the total (elastic and plastic) deformation remains constant at a given value (for example, in tightened bolts). Stress relaxation is due to an increase in the proportion of the plastic component of the total deformation and a decrease in its elastic part.

If a load acts on 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-variable loading. For | 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; therefore, 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 application of the load. The resistance of materials under conditions of low frequency and high values of repeated loading (slow, or low-cycle, fatigue) is not unambiguously associated with fatigue limits. In contrast to static loading, under repeated alternating loads, notch sensitivity is always manifested, i.e., the fatigue limit in the presence of a notch is below the fatigue limit of a smooth specimen. For convenience, notch sensitivity in fatigue is expressed as

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

Lit .: Davidenkov N. N., Dynamic testing of metals, 2nd ed., L. - M., 1936; Ratner SI, Destruction at repeated loads, M., 1959; Serensen S.V., Kogaev V.P., Shneiderovich R.M., Bearing capacity and strength calculations of machine parts, 2nd ed., Moscow, 1963; Applied questions 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, t. 2, M., 1974.

S. I. Kishkina.

Rice. 3. A specimen with a specially created fatigue crack at the notch tip to determine K 1C. Eccentric (a) and axial (b) tensile tests.

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

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

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

Deformations can be: elastic and plastic.

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

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

The main mechanical properties are:

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

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

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

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

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

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

Methods for measuring hardness.

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

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