Tensile strength is the maximum stress that a material can be subjected to before it breaks. If we talk about this indicator in relation to metals, then here it is equal to the ratio of the critical load to its cross-sectional area during the tensile test. In general, strength shows what kind of force is required to overcome and break the internal bonds between the molecules of the material.

How is the strength test done?

Strength testing of metals is carried out using specialized mechanisms that allow you to set the required power during tensile testing. Such machines consist of a special loading element, with the help of which the necessary force is created.

Equipment for testing metals for strength makes it possible to tensile the test materials and set certain values ​​of the force that is applied to the sample. Today there are hydraulic and mechanical types of mechanisms for testing materials.

Types of tensile strength

Tensile strength is one of the main properties of materials. Information on the ultimate strength of certain materials is extremely important when it is necessary to determine the possibilities of their application in certain industrial areas.

There are several separate strength limits of materials:

• when compressed;
• when bending;
• when twisting;
• when stretched.

Formation of the concept of the ultimate strength of metals

At one time, Galileo spoke about the ultimate strength, who determined that the boundary-permissible limit of compression and tension of materials depends on the index of their cross-section. Thanks to the researches of the scientist, a previously unknown quantity arose - the stress of destruction.

The modern theory of the strength of metals was formed in the middle of the 20th century, which was necessary based on the need to develop a scientific approach to prevent possible destruction of industrial structures and machines during their operation. Up to this point, when determining the strength of a material, only the degree of its plasticity and elasticity was taken into account and the internal structure was completely disregarded.

Tensile strength of steel

Steel is the main raw material in most industrial applications. It is widely used in construction. That is why it is very important to select in advance a high-quality, truly suitable type of steel for specific tasks. The result and quality of the work performed directly depends on the correct calculation of the tensile strength of a certain steel grade.

As an example, several values ​​of the ultimate strength indicators of steels can be cited. These values ​​are based on government standards and are recommended values. So, for products cast from structural unalloyed steel, the standard GOST 977-88 is provided, according to which the ultimate value of tensile strength is about 50-60 kg / mm 2, which is approximately 400-550 MPa. A similar steel grade after passing the hardening procedure acquires a tensile strength of more than 700 MPa.

The objective ultimate tensile strength of steel 45 (or any other grade of material, as well as iron or cast iron, as well as other metal alloys) depends on a number of factors that must be determined based on the tasks that are imposed on the material during its application.

Copper strength

Under normal room temperature conditions, annealed commercial copper has a tensile strength of about 23 kg / mm 2. With significant temperature loads on the material, its ultimate strength is significantly reduced. The indicators of the ultimate strength of copper reflect the presence of all kinds of impurities in the metal, which can both increase this indicator and lead to its decrease.

Strength of aluminum

The annealed fraction of technical aluminum at room temperature has a tensile strength of up to 8 kg / mm 2. An increase in the purity of the material increases its ductility, but is reflected in a decrease in strength. An example is aluminum, which has a purity index of 99.99%. In this case, the ultimate strength of the material reaches about 5 kg / mm 2.

A decrease in the tensile strength of an aluminum dough piece is observed when it is heated during tensile tests. In turn, a decrease in the temperature of the metal in the range from +27 to -260 ° C temporarily increases the investigated indicator by 4 times, and when testing the fraction of aluminum of the highest purity - as much as 7 times. At the same time, the strength of aluminum can be slightly increased by the method of alloying it.

Iron strength

To date, industrial and chemical processing has succeeded in obtaining whiskers of iron with a tensile strength of up to 13,000 MPa. Along with this, the strength of technical iron, which is widely used in a wide variety of fields, is close to 300 MPa.

Naturally, each sample of the material, when examined for the level of strength, has its own defects. In practice, it has been proven that the real objective ultimate strength of any metal, regardless of its fraction, is less than the data obtained in the course of theoretical calculations. This information must be taken into account when choosing a specific type and grade of metal for specific tasks.

The production of rolled products involves the manufacture of a huge number of varieties of structural steels. Structures during operation experience complex loads of tension, compression, impact, bending, or acting simultaneously and in combination. For heavy and difficult operating conditions of structures, mechanisms and structures, it is required to ensure durability, safety and reliability of work, in connection with which increased requirements are imposed on metal, as to the main structural material.

The main thing in the calculation of structures is the desire reduce the cross-section of steel structures modern units to reduce their weight and economical use of material without reducing the bearing capacity of the structure. Depending on the working conditions, the requirements for steels change, but there are standard ones that are important and are applied in the process of design work. Structural steel must meet high strength characteristics with sufficient material ductility.

The yield point is an important conventional physical quantity directly used in the calculation formulas. The use of this indicator as a basis for calculating the strength of the structure is justified, since during operation in the structure irreversible changes in the linear dimensions appear, which leads to the destruction of the shape of the product and its failure. An increase in this characteristic makes it possible to reduce the design cross-sections of the material and the weight of metal structures and allows you to increase the workloads.

The yield point of metals is the characteristic of steel, showing the critical stress, after which it continues material deformation without increasing the load. This important indicator is measured in Pascals (Pa) or MegaPascals (MPa), and allows you to calculate the stress limit for ductile steels.

After the material overcomes the yield point, irreversible deformations occur in it, the structure of the crystal lattice changes, and plastic changes occur. If the tensile value of the force increases, then after passing through the yield area, the deformations of the steels continue to increase.

Often the concept of yield strength of steels is called the stress at which irreversible deformation begins, without determining the difference with the elastic limit. But in real conditions, the value of the yield stress index exceeds the elastic limit by about 5%.

General information and characteristics of steels

Steel is classified as a malleable wrought alloy iron based with carbon and the addition of other elements. Material is smelted from cast iron mixtures with scrap metal in open-hearth, electric and oxygen converter furnaces.

The formed crystal lattice of the metal depends on the amount of carbon contained in them and is determined from the structural diagram in accordance with the processes in this alloy. For example, the lattice of steel, which contains up to 0.06% carbon, has a granular structure and is ferrite in its pure form. The strength of such metals is small, but the material has a high impact strength and yield strength. Steel structures in a state of equilibrium are subdivided:

• ferritic;
• pearlite-ferritic;
• cementite-ferritic;
• cementite-pearlite;
• pearlite;

Effect of carbon content on the properties of steels

Changes in the main components of cementite and ferrite are determined by the properties of the former according to the law of additivity. An increase in the percentage of carbon addition to 1.2% allows increasing strength, hardness, cold capacity threshold at 20 ° C and the yield point. An increase in carbon content changes the physical properties of the material, which sometimes leads to a deterioration in technical characteristics such as weldability, deformation during stamping. Low-carbon alloys have excellent weldability in structures.

Manganese and silicon additives

Manganese is added to the alloy as a processing additive to increase the degree of deoxidation and reduce the harmful effects of sulfur impurities. In steels, it is present in the form of solid components in an amount of no more than 0.8% and does not significantly affect the properties of the metal.

Silicon acts in the composition of the alloy in a similar way; it is added during the deoxidation process in an amount of no more than 0.38%. For the possibility of joining parts by welding, the silicon content should not exceed 0.24%. Silicon in the alloy does not affect the properties of steels.

The limit of sulfur content in the alloy is threshold at 0.06%, it is contained in the form of brittle sulfites. The increased content of impurities significantly impairs the mechanical and physical properties of steels. This is reflected in a decrease in ductility, yield strength, toughness, abrasion and corrosion resistance.

The phosphorus content also deteriorates the quality indicators of metal alloys, the yield stress increases after an increase in phosphorus in the composition, but the toughness and plasticity decrease. The standard impurity content in the alloy is regulated by the range from 0.025 to 0.044%. Phosphorus worsens the properties of steels most strongly with a simultaneous high rate of carbon additions.

Nitrogen and oxygen in the alloy

These substances contaminate steel with non-metallic impurities and impair its mechanical and physical properties. In particular, it is refers to the threshold of viscosity and endurance, plasticity and fragility. The oxygen content in the alloy in the amount of more than 0.03% causes rapid aging of the metal, nitrogen increases brittleness and increases strain aging over time. The nitrogen content increases the strength, thereby lowering the yield stress.

Alloy additives in alloys

Alloyed steels include steels, into which elements are specially introduced in certain combinations to improve quality characteristics. Complex alloying gives the best results. Chromium, nickel, molybdenum, tungsten, vanadium, titanium and others are used as additives.

Alloying increases the yield strength and other technological properties, such as toughness, constriction and the possibility of calcination, and a decrease in the threshold of deformation and cracking.

In order to fully study the properties of the material and determine the yield strength, plastic deformation and strength, metal samples are tested until complete destruction. The test is carried out under the action of loads of the following type:

Determination of the limits of test loads is carried out under standard conditions, using special machines, which are described in the rules of State standards.

Test of a specimen to determine the yield strength

To do this, take a sample of cylindrical shape with a size of 20 mm, with an estimated length of 10 mm and apply a tensile load to it. Pull length refers to the distance between the marks applied on the longer specimen to allow for gripping. To carry out the test, the relationship between the increase in tensile force and elongation of the test piece.

All test readings are automatically displayed as a bar graph for easy comparison. It is called a conditional tension or conditional stress diagram, the graph depends on the initial section of the sample and its initial length. Initially, an increase in force results in a proportional elongation of the specimen. This provision is valid up to the proportional limit.

After reaching this threshold, the graph becomes curved and indicates a disproportionate increase in length with a uniform increase in load. This is followed by the determination of the yield point. As long as the stresses in the sample do not exceed this indicator, then the material with the termination of the load can return to original condition regarding the size and shape. In practice, the test process, the difference between these limits is small and not worth special attention.

Yield point

If you continue to increase the load, then there comes a moment of testing when the change in shape and size continues without increasing the force. This is shown on the diagram by the horizontal yield line (area). The maximum stress is recorded at which the deformation increases after the cessation of the increase in the load. This indicator is called the yield point. For steel Art. 3 yield strength from 2450 kg per square centimeter.

Conditional yield stress

Many metals, when tested, give a diagram in which the yield point is absent or poorly expressed, for them the concept of the conventional yield strength is applied. This concept defines the stress that causes a residual change or deformation in the limit of 0.2%... The metals to which the concept of the conventional yield strength is applied are alloyed and high-carbon steels, bronze, duralumin and others. The more ductile the steel, the greater the residual strain reading. These include aluminum, brass, copper, and mild steels.

Tests of steel samples show that the fluidity of the metal causes significant crystal shifts in the lattice, and is characterized by the appearance of lines on the surface directed to the central axis of the cylinder.

Tensile strength

After a change by a certain amount, the sample transitions to a new phase, when, after overcoming the yield stress, the metal again can resist stretching... This is characterized by hardening, and the line of the diagram rises again, although the increase occurs in a shallower manifestation. Temporary resistance to constant load appears.

After reaching the maximum stress (ultimate strength), a region of sharp narrowing appears on the sample, the so-called neck, characterized by a decrease in the cross-sectional area, and the sample breaks at the thinnest point. In this case, the value of the voltage drops sharply, and the magnitude of the force also decreases.

Steel St.3 is characterized by a tensile strength of 4000–5000 kg / cm2. For high-strength metals, this indicator reaches the limit of 17500 kg / cm3 this.

Plasticity of material

It is characterized by two indicators:

• residual elongation;
• residual constriction at rupture.

To determine the first indicator, the total length of the stretched specimen after rupture is measured. To do this, fold the two halves together. After measuring the length, calculate the percentage of the original length. Strong alloys are less prone to ductility and the elongation rate is reduced to 63 et11%.

The second characteristic is calculated after measuring the narrowest part of the break and is calculated as a percentage of the original section area of ​​the sample.

The opposite property of plasticity is brittleness index... Cast iron and tool steel are considered brittle metals. The division of steels into brittle and ductile is made conditionally, since to determine this indicator, the operating conditions or tests, the rate of increase in the load, and the ambient temperature matter.

Some materials do not behave as fragile at all under different conditions. For example, cast iron, located so that it is clamped on all sides, does not collapse even with stresses arising inside. Grooved steel characterized by increased fragility. Hence the conclusion that it is much more expedient to test not the limits of fragility, but to determine the state of the material as plastic or brittle.

Tests of steels to determine the physical and technical properties are done in order to obtain reliable data for the performance of work during construction and the creation of structures on the farm.

When a force or a system of forces acts on a metal sample, it reacts to this by changing its shape (deforming). The various characteristics that determine the behavior and final state of a metal sample, depending on the type and intensity of forces, are called the mechanical properties of the metal.

The intensity of the force acting on the sample is called stress and is measured as the total force relative to the area it acts on. Deformation refers to the relative change in the dimensions of the sample caused by the applied stresses.

Elastic and plastic deformation, destruction

If the stress applied to the metal sample is not too great, then its deformation turns out to be elastic - as soon as the stress is removed, its shape is restored. Some metal structures are deliberately designed to deform elastically. So, springs usually require a fairly large elastic deformation. In other cases, elastic deformation is minimized. Bridges, beams, mechanisms, devices are made as rigid as possible. The elastic deformation of a metal sample is proportional to the force or the sum of the forces acting on it. This is expressed by Hooke's law, according to which stress is equal to elastic deformation multiplied by a constant coefficient of proportionality, called the modulus of elasticity: s = ∆ Y, where s- voltage, ∆ - elastic deformation, and Y- elastic modulus (Young's modulus). The elastic moduli of a number of metals are presented in table. 1.

Table 1

Using the data in this table, you can calculate, for example, the force required to stretch a steel bar with a square cross-section with a side of 1 cm by 0.1% of its length:

F= 200,000 MPa x 1 cm 2 x 0.001 = 20,000 N (= 20 kN)

When stresses are applied to a metal sample that exceed its elastic limit, they cause plastic (irreversible) deformation, leading to an irreversible change in its shape. Higher stresses can cause destruction of the material.

The most important criterion when choosing a metal material from which high elasticity is required is the yield strength. The best spring steels have almost the same modulus of elasticity as the cheapest construction steels, but spring steels are able to withstand much higher stresses, and therefore much larger elastic deformations without plastic deformation, since they have a higher yield stress.

The plastic properties of a metallic material (as opposed to elastic) can be changed by fusion and heat treatment. So, the yield point of iron by similar methods can be increased 50 times. Pure iron passes into a state of fluidity even at stresses of the order of 40 MPa, while the yield point of steels containing 0.5% carbon and a few percent of chromium and nickel, after heating to 950 C 0 and quenching, can reach 2000 MPa.

When a metal material is loaded beyond the yield point, it continues to deform plastically, but becomes harder during deformation, so that more stress is required to further increase the deformation. This phenomenon is called deformation or mechanical hardening (as well as work hardening). It can be demonstrated by twisting or repeatedly bending a metal wire. Work hardening of metal products is often carried out in factories. Brass sheets, copper wires, aluminum rods can be cold rolled or cold drawn to the hardness required for the final product.

Bernshtein M.L., Zaimovsky V.A. Mechanical properties of metals... M., 1979
Wyatt O.G., Dew-Hughes D. Metals, ceramics, polymers... M., 1979
Pavlov P.A. Mechanical states and strength of materials... L., 1980
Sobolev N.D., Bogdanovich K.P. Mechanical properties of materials and fundamentals of physics of strength... M., 1985
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Bobylev A.V. Mechanical and technological properties of metals... M., 1987

The yield point is the stress corresponding to the residual value of elongation after removal of the load. Determination of this value is necessary for the selection of metals used in production. If the parameter under consideration is not taken into account, then this can lead to an intensive process of deformation development in an incorrectly selected material. It is very important to take into account the yield points when designing various metal structures.

Physical characteristic

Yield strength refers to strength indicators. They represent macroplastic deformation with rather low hardening. Physically, this parameter can be represented as a characteristic of the material, namely: the stress that corresponds to the lower value of the yield area in the graph (diagram) of tension of materials. The same can be represented in the form of the formula: σ T = P T / F 0, where P T means the load of the yield point, and F 0 corresponds to the initial cross-sectional area of ​​the sample in question. PT establishes the so-called boundary between the elastic-plastic and elastic deformation zones of the material. Even a slight increase in PT) will cause significant deformation. It is customary to measure the yield points of metals in kg / mm 2 or N / m 2. The value of this parameter is influenced by various factors, for example, the heat treatment mode, the thickness of the sample, the presence of alloying elements and impurities, the type, microstructure and crystal lattice defects, etc. The yield point changes significantly with temperature. Let's consider an example of the practical meaning of this parameter.

Pipe yield strength

The most obvious is the influence of this value in the construction of high-pressure pipelines. In such structures, special steel should be used, which has sufficiently large yield strengths, as well as minimum gap indicators between this parameter and The greater the limit for steel, the higher the allowable working stress should be. This fact has a direct impact on the value of the strength of steel, and, accordingly, the entire structure as a whole. Due to the fact that the parameter of the permissible design value of the stress system has a direct effect on the required value of the wall thickness in the pipes used, it is important to calculate the strength characteristics of the steel that will be used in the manufacture of pipes as accurately as possible. One of the most authentic methods for determining these parameters is to conduct a study on a burst specimen. In all cases, it is required to take into account the difference in the values ​​of the indicator under consideration, on the one hand, and the permissible stress values, on the other.

In addition, you should be aware that the yield point of a metal is always established as a result of detailed reusable measurements. But the overwhelming majority of the system of permissible voltages is taken on the basis of standards or generally as a result of the technical conditions carried out, as well as relying on the personal experience of the manufacturer. In trunk pipeline systems, the entire regulatory collection is described in SNiP II-45-75. So, setting the safety factor is a rather difficult and very important practical task. The correct determination of this parameter depends entirely on the accuracy of the calculated values ​​of stress, load, and also the yield strength of the material.

When choosing thermal insulation for piping systems, they also rely on this indicator. This is due to the fact that these materials directly come into contact with the metal base of the pipe, and, accordingly, can take part in electrochemical processes that adversely affect the state of the pipeline.

Stretching materials

The tensile yield point determines at what value the stress remains unchanged or decreases despite elongation. That is, this parameter will reach a critical level when the transition from elastic to plastic deformation region of the material occurs. It turns out that the yield point can be determined by testing the bar.

PT calculation

In the resistance of materials, the yield point is the stress at which it begins to develop. Let's look at how this value is calculated. In experiments carried out with cylindrical samples, the value of the normal stress in the cross section at the time of the onset of irreversible deformation is determined. The same method in experiments with torsion of tubular specimens is used to determine the shear yield stress. For most materials, this indicator is determined by the formula σ T = τ s √3. In some instances, continuous elongation of a cylindrical specimen in the diagram of normal stresses versus relative elongation leads to the detection of the so-called yield tooth, that is, a sharp decrease in stress before the formation of plastic deformation.

Moreover, further growth of such distortion to a certain value occurs at a constant voltage, which is called a physical PT. If the yield area (horizontal section of the graph) has a large length, then such a material is called ideal plastic. If the diagram does not have a platform, then the samples are called hardening. In this case, it is impossible to specify exactly the value at which plastic deformation occurs.

What is the Conditional Yield Strength?

Let's figure out what this parameter is. In cases where the stress diagram does not have pronounced areas, it is required to determine the conditional PT. So, this is the stress value at which the relative permanent deformation is 0.2 percent. To calculate it on the stress diagram along the axis of determination of ε, it is necessary to set aside a value equal to 0.2. From this point, the initial section is drawn. As a result, the point of intersection of the straight line with the line of the diagram determines the value of the conditional yield strength for a particular material. This parameter is also called technical PT. In addition, the conditional yield strengths in torsion and bending are separately distinguished.

Melt flow

This parameter determines the ability of molten metals to fill linear shapes. Melt flow for metal alloys and metals has its own term in the metallurgical industry - fluidity. In fact, this is the inverse of the International System of Units (SI) expresses the fluidity of a liquid in Pa -1 * s -1.

Temporary tensile strength

Let's look at how this characteristic of mechanical properties is determined. Strength is the ability of a material, under certain limits and conditions, to perceive various influences without collapsing. It is customary to determine mechanical properties using conditional tension diagrams. For testing, reference materials should be used. The testers are equipped with a device that records a diagram. An increase in loads in excess of the norm causes significant plastic deformation in the product. The yield point and ultimate tensile strength correspond to the highest load preceding the complete failure of the specimen. In plastic materials, deformation is concentrated in one area, where a local narrowing of the cross-section appears. It is also called the neck. As a result of the development of multiple slides, a high density of dislocations is formed in the material, and so-called embryonic discontinuities arise. As a result of their enlargement, pores appear in the sample. Merging with each other, they form cracks that propagate in the transverse direction to the tension axis. And at a critical moment, the sample is completely destroyed.

What is a PT for reinforcement?

These products are an integral part of reinforced concrete, usually designed to resist tensile forces. Steel reinforcement is usually used, but there are exceptions. These products must work together with the mass of concrete at all stages of loading a given structure, without exception, have plastic and durable properties. And also meet all the conditions for the industrialization of these types of work. The mechanical properties of steel used in the manufacture of fittings are established by the relevant GOST and technical specifications. GOST 5781-61 provides for four classes of these products. The first three are intended for conventional structures as well as stress-free bars in prestressed systems. The yield point of reinforcement, depending on the class of the product, can reach 6000 kg / cm 2. So, in the first class, this parameter is approximately 500 kg / cm 2, in the second - 3000 kg / cm 2, in the third - 4000 kg / cm 2, and in the fourth - 6000 kg / cm 2.

Yield strength of steels

For long products in the basic version of GOST 1050-88, the following PT values ​​are provided: grade 20 - 25 kgf / mm 2, grade 30 - 30 kgf / mm 2, grade 45 - 36 kgf / mm 2. However, for the same steels, manufactured by prior agreement between the consumer and the manufacturer, the yield strengths may have different values ​​(the same GOST). So, 30 will have a PT in the size from 30 to 41 kgf / mm 2, and grade 45 - in the range of 38-50 kgf / mm 2.

Conclusion

When designing various (buildings, bridges and others), the yield strength is used as an indicator of the strength standard when calculating the values ​​of permissible loads in accordance with the specified safety factor. But for vessels under pressure, the value of the permissible load is calculated on the basis of PT, as well as tensile strength, taking into account the specification of operating conditions.

When the material is stretched in different directions, tensile stress is generated, and as a result, the material breaks. The ultimate value of the force at which rupture occurs is called the tensile strength (tensile strength).

Tensile strength is measured on materials such as alloys, composites, ceramics, and plastics. It is measured in MPa, this is the force applied to the area, i.e. kg / cm 2. The higher this value, the more resistant the material is to tensile forces.

During the pre-failure test, the material goes through the “bell stage” (see Fig. 2).

This test helps you understand the strength of the material.

3. Modulus of elasticity (GPa) / Modulus E / Young's modulus / Flexibility modulus.

The properties of hardness and elasticity of materials are measured in GPa.

The modulus of elasticity reflects the resistance of a material to an external load, in this case bending. No irreversible deformation occurs with the material; after the removal of the external load, it returns to its original state. That is, in this case, unlike other tests, the material is not destroyed.

Three-point bend test. A block of material is installed on 2 supports and a force F is applied to it (Fig. 7 and 8).

The load increases only until the moment when the material begins to bend (see Fig. 9). The higher the value, the more rigid the material.

Rice. eight	Rice. nine

Stiffness is important when choosing a restorative material, since it is not at all necessary for the material to significantly deflect under the influence of stress. A typical example is intrapulpal pins. Its hardness should match the hardness of the dentin.

For elastic impression materials, on the contrary, small values ​​are desirable, since in this case the impression will be easily removed from the patient's mouth.

Flexural strength (MPa)

A three-point test is also used to measure it. In this case, the load is applied until the material collapses (see Fig. 11).

Flexural strength is the ability of a material to resist fracture under load. It is measured in MPa, megapascals.

Rice. ten	Rice. eleven

This test is similar to the load on a bridge. A high flexural strength means that the bridge is highly resistant to fracture.

5. Fatigue limit - cyclic loads

First, a flexural strength test is performed to determine the ultimate strength of the material (MPa). Then a load is taken lower than the above tensile strength. In the same three-point loading configuration, the material is sequentially loaded cyclically. Then it is noted how many cycles the material withstands before breakage.

This test simulates chewing loads on a bridge. The more cycles the material withstands, the better.

Rice. 12	Rice. 13
Rice. fourteen	Rice. 15

Fatigue of materials. When exposed to a large number of cyclic loads on the prosthesis, material destruction may occur. The breaking stress (fatigue limit) is then significantly lower than the ultimate strength.
The causes of fatigue are still not entirely clear. Microscopic examination of samples subjected to multiple variable loading showed that after a certain number of loading, a number of dashes appear in the grains of the material, indicating the presence of displacements of parts of the grains. Under the further action of the load, the lines turn into the finest cracks, which merge into a crack. Further destruction is concentrated around it.
The crack grows with each loading, and when the cross-section is sufficiently reduced, failure occurs. The resulting crack acts like a groove, that is, it causes stress concentration and decreases resistance. The moment of destruction is approaching imperceptibly. The structure, which is threatened with collapse, works flawlessly, but finally collapse occurs suddenly, and with a slight load.

Very often, fatigue fractures are caused by abrupt changes in the shape of parts (abrupt transitions in thickness, notches, surface cracks, pores, etc.), which cause stress concentration. Since fatigue cracks appear around these areas, the fight against fatigue, in addition to choosing stronger materials, is to harden its surface. So, for metals this is achieved by chemical-thermal treatment, mechanical treatment (grinding, polishing), quenching with high-frequency currents. These measures allow increasing the fatigue limit by several tens of percent. For plastics, the correct polymerization regime is also of great importance, which does not cause the formation of pores in the prostheses.

The tensile strength of some dental materials:

Elasticity. The ability of a material to change its shape under the influence of an external load and to recover its shape after removing this load is called elasticity. A typical example of the elastic properties of a material is the bending of a steel wire, stretching of a metal spring, compression of a prosthesis made of polyamide plastic, or a piece of hydrocoloid mass. After the force is removed, all these bodies take on their shape. But a return to the previous form can occur only if the applied force does not exceed a certain value, called the elastic limit. The elastic limit is the maximum load at which the material, after deformation and removal of the load, completely restores its shape and size. If the load exceeds the elastic limit, then after removal, its material will not fully recover to its original state, and permanent deformation will appear.
The materials used for the manufacture of dentures and appliances have different elasticity. Some structures must necessarily have elastic properties, since they are constantly under force, and the appearance of permanent deformation makes them unsuitable (clasps, arches, bases of prostheses, etc.).
In other cases, the manifestation of elastic properties interferes with the implementation of some technological steps. So, for example, stamping of crowns is possible if the metal is in the state of the least elasticity.
Metals can exhibit elasticity in different ways depending on their mechanical and heat treatment. Steel increases its elasticity when hammering or pulling, and when hardening.
All materials have elastic properties in certain temperature ranges. For metals, these intervals reach hundreds of degrees; for plastics, they are much smaller. For base plastics, they are measured in tens of degrees.
The elasticity of the material is determined on samples that are strengthened in devices such as hydraulic
press and subjected to loading. The change in the length of the sample is measured at the maximum load that does not cause permanent deformation, after the removal of which the sample returns to its original length. The calculation is carried out for 1 mm 2.

It is clear that when determining the loads allowed on different parts of the prosthesis, knowledge of the elastic limit of the material from which it is made is absolutely necessary, since the load above the elastic limit leads to a change in the shape of the prosthesis, and, consequently, to the impossibility of using it.
If you continue to load the sample, then it gradually begins to lengthen, and its cross-section becomes smaller, and when the load is removed, the sample does not return to its previous dimensions. The more the sample is able to lengthen, and its cross-section narrows, the more ductile the material.
In contrast to ductile materials, brittle materials break under stress without changing their shape. Fragility, as a rule, is a negative property, therefore, in orthopedic dentistry, not only durable and elastic materials are most often used, but also plastic to a certain extent.

Plastic. The ability of a material, without collapsing, to change its shape under the action of loads and to maintain this shape after the load ceases to act. This property is possessed by many impression materials, wax, plaster, metals.
Thus, all plastic materials have a pronounced permanent deformation. Plasticity is necessary for impression materials, metals used to obtain products by stamping, plastics, from which the bases of prostheses are formed, and filling materials.
Sometimes a material is chosen only due to its property of acquiring a plastic state. This applies primarily to impression materials, plastics. To obtain maximum plasticity of the metal, it is subjected to a special heat treatment - annealing, wax and impression masses are heated, gypsum is mixed with water, etc. Usually, processing that increases plasticity reduces resistance to deformation and vice versa.
Viscosity. The ability of a material to stretch under tensile loads. This type of deformation is characterized by the fact that the sample under study increases in size in the direction of the applied force (usually along its length) and narrows in cross section.
Some materials are highly viscous (gold, silver, iron, etc.). Others do not have this ability (cast iron, porcelain, etc.). They belong to the group of fragile materials.
Thus, fragility is the opposite of viscosity.
When testing various materials, in particular plastics, the method of determining the impact strength is widely used. Specific impact strength is the work expended to break a specimen divided by its cross-sectional area. Determination of impact strength is carried out on a pendulum impact machine MK-0.5-1. The device consists of a massive base on which a pendulum-type device is mounted. A pendulum with a removable weight (10-15-30 kg), fixed on the axis of the bed, is fixed at a certain height with a clip. When the clamp is released, the pendulum falls freely and strikes the sample. The stronger the sample, the lower the height the pendulum rises after impact, i.e., the more work was spent on impact destruction of the sample. The lower the toughness, the more brittle the material is.

The given mechanical properties of materials make it possible to determine the rigidity of materials. The ability of structural elements to resist deformations under the influence of external forces is called rigidity.
It should be remembered that when calculating the required dimensions of structural parts under the expected load, the rule is always adhered to that the material should not only collapse, but also deform. Therefore, calculations are always based on a fourfold safety factor, i.e. if the tensile strength of carbon steel is 90 kg / mm2, then the permissible load should be 22-23 kg / mm2. If the workload exceeds these figures, then the dimensions of this part should be increased. So, for example, if we know that the force applied to the denture at the moment of chewing is 60 kg, and the tensile strength of the plastic is 1000 kg / cm2, then the plate should have a width of 2.5 cm in the smallest part, with a thickness 1 mm.

Literature:

1. Popkov V.A. Dental materials science: Textbook / V.A. Popkov. O. V. Nesterova, V. Yu. Reshetnyak, I. N. Avertseva. // M. - MEDpress-inform. - 2009 .-- 400s.

2. Craig R. Dental materials: properties and application / R. Craig, J. Powers, J. Wataga // - 2005. - 304s.

3.http: //article-factory.ru/medicina/zubotehnicheskoe-materialovedenie/139-mehanicheskie-svojstva.html

4.www.infodent.ru

Similar information.

Tensile strength is the same as the temporary resistance of the material. But despite the fact that it is more correct to use the term temporary resistance, the concept of tensile strength has better taken root in technical colloquial speech. At the same time, the term "temporary resistance" is used in regulatory documents and standards.

ICM (www.site)

Strength is the resistance of the material to deformation and destruction, one of the main mechanical properties... In other words, strength is the property of materials, without collapsing, to perceive certain influences (loads, temperature, magnetic and other fields).

TO tensile strength characteristics include the modulus of normal elasticity, proportional limit, elastic limit, yield point and ultimate strength.

Tensile strength- this is the maximum mechanical stress, above which the destruction of the material undergoing deformation occurs; tensile strength is denoted by σ B and is measured in kilograms of force per square centimeter (kgf / cm 2), and is also indicated in megapascals (MPa).

Distinguish:

• tensile strength,
• compressive strength,
• bending strength,
• tensile strength.

Short-term strength limit (MPa) determined by tensile tests, deformation is carried out until failure. Tensile tests are used to determine tensile strength, elongation, elastic limit, etc. Long-term strength tests are intended mainly to assess the possibility of using materials at high temperatures (long-term strength, creep); as a result, σ B / Zeit is determined - the limit of limited long-term strength for a given service life.

ICM (www.site)

Strength of metals

Physics of strength founded by Galileo: summarizing his experiments, he discovered (1638) that under tension or compression, the load of destruction P for a given material depends only on the cross-sectional area F... This is how a new physical quantity appeared - tension. σ = P/F- and the physical constant of the material: fracture stress.

Destruction physics like fundamental science of the strength of metals emerged in the late 40s of the XX century; this was dictated by the urgent need to develop scientifically based measures to prevent the frequent catastrophic destruction of machines and structures. Previously, in the field of strength and fracture of products, only classical mechanics was taken into account, based on the postulates of a homogeneous elastic-plastic solid, without taking into account the internal structure of the metal. Fracture physics also takes into account the atomic-crystalline structure of the lattice of metals, the presence of defects in the metal lattice and the laws of interaction of these defects with elements of the internal structure of the metal: grain boundaries, the second phase, non-metallic inclusions, etc.

Great influence on material strength has the presence of surfactants in the environment, capable of being strongly adsorbed (moisture, impurities); there is a decrease in the ultimate strength.

Purposeful changes in the metal structure, including modification of the alloy, lead to an increase in the strength of the metal.

Educational film about the strength of metals (USSR, year of issue: ~ 1980):

Metal tensile strength

Tensile strength of copper... At room temperature, the ultimate strength of annealed technical copper σ B = 23 kgf / mm 2. With an increase in the test temperature, the tensile strength of copper decreases. Alloying elements and impurities affect the tensile strength of copper in different ways, both increasing and decreasing it.

Tensile strength of aluminum... Annealed technical grade aluminum at room temperature has a tensile strength σ B = 8 kgf / mm 2. As the purity increases, the strength of aluminum decreases and the ductility increases. For example, aluminum cast into the ground with a purity of 99.996% has a tensile strength of 5 kgf / mm 2. The tensile strength of aluminum decreases naturally as the test temperature rises. When the temperature drops from +27 to -269 ° C, the temporary resistance of aluminum increases - 4 times for technical aluminum and 7 times for high-purity aluminum. Alloying increases the strength of aluminum.

ICM (www.site)

Tensile strength of steels

As an example, the values ​​of the tensile strength of some steels are presented. These values ​​are taken from government standards and are recommended (required). The real values ​​of the tensile strength of steels, as well as cast irons, as well as other metal alloys, depend on many factors and should be determined, if necessary, in each specific case.

For steel castings made of unalloyed structural steels provided by the standard (steel casting, GOST 977-88), the tensile strength of steel is approximately 40-60 kg / mm 2 or 392-569 MPa (normalization or normalization with tempering), category strength K20-K30. For the same steels, after quenching and tempering, the regulated strength categories KT30-KT40, the values ​​of the ultimate strength are no less than 491-736 MPa.

For structural carbon high-quality steels (GOST 1050-88, rolled products up to 80 mm in size, after normalization):

• Tensile strength of steel 10: steel 10 has a short-term strength limit of 330 MPa.
• Tensile strength of steel 20: steel 20 has a short-term strength limit of 410 MPa.
• Tensile strength of steel 45: steel 45 has a short-term strength limit of 600 MPa.

Strength categories of steels

The strength categories of steels (GOST 977-88) are conventionally designated by the indices "K" and "KT", after the index is a number that represents the value of the required yield strength. Index "K" is assigned to steels in annealed, normalized or tempered condition. The KT index is assigned to steels after quenching and tempering.

Cast iron tensile strength

The method for determining the tensile strength of cast iron is regulated by the standard GOST 27208-87 (Castings from cast iron. Tensile tests, determination of ultimate strength).

Tensile strength of gray cast iron... Gray cast iron (GOST 1412-85) is marked with the letters СЧ, after the letters are numbers that indicate the minimum value of the ultimate strength of cast iron - tensile strength (MPa * 10 -1). GOST 1412-85 applies to cast irons with lamellar graphite for castings of grades SCH10-SCH35; from here it can be seen that the minimum values tensile strength of gray cast iron in a cast state or after heat treatment, they vary from 10 to 35 kgf / mm 2 (or from 100 to 350 MPa). Exceeding the minimum value of the tensile strength of gray cast iron is allowed by no more than 100 MPa, unless otherwise specified separately.

Tensile Strength of Ductile Iron... The marking of ductile iron also includes numbers indicating the ultimate tensile strength of cast iron (tensile strength), GOST 7293-85. The tensile strength of ductile iron is 35-100 kg / mm 2 (or 350 to 1000 MPa).

From the above, it can be seen that nodular cast iron can compete successfully with steel.

Prepared by A.E. Kornienko (ICM)

Lit .:

1. Zimmerman R., Gunther K. Metallurgy and Materials Science. Ref. ed. Per. with him. - M .: Metallurgy, 1982 .-- 480 p.
2. Ivanov V.N. Dictionary-reference book on foundry production. - M .: Mechanical Engineering, 1990 .-- 384 p .: ill. - ISBN 5-217-00241-1
3. Zhukovets I.I. Mechanical testing of metals: Textbook. for environments. Vocational school. - 2nd ed., Rev. and add. - M .: Higher school, 1986 .-- 199 p .: ill. - (Vocational education). - BBK 34.2 / Zh 86 / UDZh 620.1
4. Shtremel M.A. Strength of alloys. Part II. Deformation: Textbook for universities. - M .: * MISIS *, 1997 .-- 527 p.
5. Meshkov Yu.Ya. Physics of steel fracture and topical issues of structural strength // Structure of real metals: Collection of articles. scientific. tr. - Kiev: Nauk. dumka, 1988. - pp. 235-254.
6. Frenkel Ya.I. Introduction to the theory of metals. Fourth edition. - L .: "Science", Leningrad. otd., 1972.424 p.
7. Receiving and properties of nodular cast iron. Edited by N.G. Girshovich - M., L .: Leningrad branch of Mashgiz, 1962, - 351 p.
8. Bobylev A.V. Mechanical and technological properties of metals. Directory. - M .: Metallurgy, 1980.296 p.