Technical abilities are interrelated personal qualities that manifest themselves independently of each other: to understand technology, to handle technology, to manufacture technical products, to technical invention.
It is believed that these are the abilities that are manifested in working with equipment or its parts. It is taken into account that such work requires special mental abilities, as well as a high level of development of sensorimotor abilities, dexterity, and physical strength. L. Thurston considers technical abilities as general mental ones. It is shown that, along with some general ability, which can be considered as general technical talent or technical experience acquired by a person in working with technology, there are independent factors: spatial concepts and technical understanding. By spatial representations we mean the ability to operate with visual images, for example, when perceiving geometric shapes. Technical understanding is the ability to correctly perceive spatial patterns, compare them with each other, recognize the same ones and find different ones. In accordance with this division into two factors, types of tests are created. The very first creators of technical ability tests required subjects to be able to assemble technical devices from individual parts. Currently, most of these tests are created in the form of blank methods.
Technical ability is viewed as general mental ability. There are independent factors of technical ability:
- spatial representations;
- technical understanding.
Technical Aptitude Tests- diagnostics of abilities manifested in working with equipment or its parts.
Technical ability tests are aimed at identifying the knowledge and experience accumulated by the test taker. They do not allow us to judge how to acquire them, for example:
- The Bennett Test is a test of technical understanding that uses a series of pictures with short questions. To answer questions, you need to understand general, technical principles from everyday situations.
- Spatial Reasoning Test (STT) I.S. Yakimanskaya, V.G. Zarkhin and H.-M.Kh. Kadayasa.
The meaning of technical thinking is to solve problems; in the process of solving them, the necessary qualities of technical thinking are formed.
To solve a technological problem it is necessary:
- have a set goal and strive to obtain a specific answer;
- take into account the conditions and initial data necessary to achieve the goal;
- apply methods of solving problems that correspond to existing conditions.
The development of technical thinking is a complex process, usually proceeds rather slowly and depends on general intelligence, practical skills, a person’s ability to think technically and other factors.
Bennett's psychological test for understanding technology (mechanical understanding) is designed to determine technical abilities in adolescent children (from 12 years old), young adults and adults. Contains 60 tasks that require solving technical problems. In each task, subjects must choose the correct answer from three options. The duration of the test is 27 minutes.
Each correct answer is worth one point. The level of technical ability is determined using a special assessment table. The scale rating has six gradations:
- very tall;
- good;
- above average;
- below average;
- short;
- very low.
To evaluate the performance properties of products and determine the physical and mechanical characteristics of materials, various instructions, GOSTs and other regulatory and advisory documents are used. Methods for testing the destruction of a whole series of products or similar material samples are also recommended. This is not a very economical method, but it is effective.
Definition of characteristics
The main characteristics of the mechanical properties of materials are as follows.
1. Temporary resistance or tensile strength - the stress force that is recorded at the highest load before the destruction of the sample. Mechanical characteristics of strength and plasticity of materials describe the properties of solids to resist irreversible changes in shape and destruction under the influence of external loads.
2. The conditional stress is when the residual deformation reaches 0.2% of the length of the sample. This is the lowest stress while the sample continues to deform without a noticeable increase in loads.
3. The long-term strength limit is the maximum stress that, at a given temperature, causes destruction of the sample over a certain time. Determination of the mechanical characteristics of materials is guided by the ultimate units of long-term strength - destruction occurs at 7,000 degrees Celsius in 100 hours.
4. The conditional creep limit is the stress that causes a given elongation in the sample at a given temperature for a certain time, as well as the creep rate. The limit is considered to be metal deformation in 100 hours at 7,000 degrees Celsius by 0.2%. Creep is a certain rate of deformation of metals under constant loading and high temperature for a long time. Heat resistance is the resistance of a material to fracture and creep.
5. The endurance limit is the highest value of the cycle stress when fatigue failure does not occur. The number of loading cycles can be specified or arbitrary, depending on how the mechanical tests of the materials are planned. Mechanical properties include fatigue and endurance of the material. Under the influence of loads in the cycle, damage accumulates and cracks form, leading to destruction. This is fatigue. And the property of resistance to fatigue is endurance.
Tension and compression
Materials used in engineering practice are divided into two groups. The first is plastic, for which significant residual deformations must appear to fail, the second is brittle, which collapses at very small deformations. Naturally, such a division is very arbitrary, because each material, depending on the conditions created, can behave both as brittle and as ductile. This depends on the nature of the stress state, on temperature, on the rate of deformation and other factors.
The mechanical characteristics of materials under tension and compression are eloquent for both ductile and brittle ones. For example, low-carbon steel is tested in tension, and cast iron is tested in compression. Cast iron is brittle, steel is ductile. Brittle materials have greater resistance to compression, but less resistance to tensile deformation. Plastic materials have approximately the same mechanical characteristics under compression and tension. However, their threshold is still determined by stretching. It is in these ways that the mechanical characteristics of materials can be more accurately determined. The tension and compression diagram is presented in the illustrations for this article.
Fragility and ductility
What is ductility and fragility? The first is the ability not to collapse, receiving residual deformations in large quantities. This property is decisive for the most important technological operations. Bending, drawing, drawing, stamping and many other operations depend on the plasticity characteristics. Ductile materials include annealed copper, brass, aluminum, mild steel, gold, and the like. Bronze and duralumin are much less ductile. Almost all alloy steels are very weakly ductile.
The strength characteristics of plastic materials are compared with the yield strength, which will be discussed below. The properties of brittleness and ductility are greatly influenced by temperature and loading rate. Fast tension imparts brittleness to the material, while slow tension imparts ductility. For example, glass is a fragile material, but it can withstand prolonged exposure to load if the temperature is normal, that is, it shows plasticity properties. It is plastic, but under a sharp shock load it appears as a brittle material.
Oscillation method
The physical and mechanical characteristics of materials are determined by the excitation of longitudinal, bending, torsional and other, even more complex ones, depending on the size of the samples, shapes, types of receiver and exciter, methods of fastening and schemes for applying dynamic loads. Large-sized products are also subject to testing using this method, if the application method is significantly changed in the methods of applying load, exciting vibrations and recording them. The same method is used to determine the mechanical characteristics of materials when it is necessary to evaluate the rigidity of large structures. However, when locally determining material characteristics in a product, this method is not used. The practical application of the technique is possible only when the geometric dimensions and density are known, when it is possible to fix the product on supports, and on the product itself - converters, certain temperature conditions are needed, etc.
For example, when temperature conditions change, this or that change occurs, and the mechanical characteristics of materials become different when heated. Almost all bodies expand under these conditions, which affects their structure. Any body has certain mechanical characteristics of the materials from which it consists. If these characteristics do not change in all directions and remain the same, such a body is called isotropic. If the physical and mechanical characteristics of materials change - anisotropic. The latter is a characteristic feature of almost all materials, just to varying degrees. But there are, for example, steels where the anisotropy is very insignificant. It is most clearly expressed in natural materials such as wood. In production conditions, the mechanical characteristics of materials are determined through quality control, where various GOSTs are used. The heterogeneity estimate is obtained from statistical processing when the test results are summed up. Samples must be numerous and cut from a specific structure. This method of obtaining technological characteristics is considered quite labor-intensive.
Acoustic method
There are quite a lot of acoustic methods for determining the mechanical properties of materials and their characteristics, and they all differ in the methods of input, reception and recording of vibrations in sinusoidal and pulsed modes. Acoustic methods are used to study, for example, building materials, their thickness and stress state, and during flaw detection. The mechanical characteristics of structural materials are also determined using acoustic methods. Numerous different electronic acoustic devices are now being developed and mass-produced, which make it possible to record elastic waves and their propagation parameters in both sinusoidal and pulsed modes. On their basis, the mechanical characteristics of the strength of materials are determined. If low-intensity elastic vibrations are used, this method becomes absolutely safe.
The disadvantage of the acoustic method is the need for acoustic contact, which is not always possible. Therefore, this work is not very productive if there is an urgent need to obtain mechanical characteristics of the strength of materials. The result is greatly influenced by the condition of the surface, the geometric shapes and dimensions of the product being tested, as well as the environment where the tests are carried out. To overcome these difficulties, a specific problem must be solved using a strictly defined acoustic method or, on the contrary, using several of them at once, it depends on the specific situation. For example, fiberglass plastics lend themselves well to such research, since the propagation speed of elastic waves is good, and therefore through sounding is widely used, when the receiver and emitter are located on opposite surfaces of the sample.
Flaw detection
Flaw detection methods are used to control the quality of materials in various fields of industry. There are non-destructive and destructive methods. Non-destructive ones include the following.
1. To determine cracks on surfaces and lack of penetration, it is used magnetic flaw detection. Areas that have such defects are characterized by scattering fields. They can be detected with special devices or simply by applying a layer of magnetic powder to the entire surface. In areas of defects, the location of the powder will change even during application.
2. Flaw detection is also carried out using ultrasound. The directed beam will be reflected (scattered) differently if there are any discontinuities even deep inside the sample.
3. Defects in the material are clearly shown radiation research method, based on the difference in radiation absorption by media of different densities. Gamma flaw detection and X-ray are used.
4. Chemical flaw detection. If the surface is etched with a weak solution of nitric, hydrochloric acid or a mixture of them (regia vodka), then in places where there are defects, a mesh in the form of black stripes appears. You can use a method in which sulfur prints are removed. In places where the material is heterogeneous, the sulfur should change color.
Destructive methods
Destructive methods have already been partially discussed here. Samples are tested for bending, compression, tension, that is, static destructive methods are used. If the product is tested with variable cyclic loads on impact bending, the dynamic properties are determined. Macroscopic methods paint a general picture of the structure of a material in large volumes. For such a study, specially ground samples are needed that are etched. Thus, it is possible to identify the shape and location of grains, for example, in steel, the presence of deformed crystals, fibers, cavities, bubbles, cracks and other inhomogeneities of the alloy.
Microscopic methods are used to study the microstructure and identify the smallest defects. The samples are pre-ground, polished and then etched in the same way. Further testing involves the use of electrical and optical microscopes and X-ray diffraction analysis. The basis of this method is the interference of rays that are scattered by atoms of matter. The characteristics of the material are monitored by X-ray diffraction analysis. The mechanical characteristics of materials determine their strength, which is the main thing for building structures that are reliable and safe to use. Therefore, the material is tested carefully and using different methods in all states that it can accept without losing a high level of mechanical characteristics.
Control methods
To carry out non-destructive testing of the characteristics of materials, the correct choice of effective methods is of great importance. The most accurate and interesting in this regard are flaw detection methods - defect control. Here it is necessary to know and understand the differences between the methods of implementing flaw detection methods and methods for determining physical and mechanical characteristics, since they are fundamentally different from each other. If the latter are based on monitoring physical parameters and their subsequent correlation with the mechanical characteristics of the material, then flaw detection is based on the direct conversion of radiation that is reflected from a defect or passes through a controlled environment.
The best thing, of course, is comprehensive control. Complexity lies in determining the optimal physical parameters, which can be used to identify the strength and other physical and mechanical characteristics of the sample. And also, an optimal set of means for controlling structural defects is simultaneously developed and then implemented. And finally, an integral assessment of this material appears: its performance is determined according to a whole set of parameters that helped determine non-destructive methods.
Mechanical tests
With the help of such tests, the mechanical properties of materials are checked and evaluated. This type of control appeared a long time ago, but has not yet lost its relevance. Even modern high-tech materials are criticized quite often and fiercely by consumers. This suggests that examinations should be carried out more thoroughly. As already mentioned, mechanical tests can be divided into two types: static and dynamic. The former check the product or sample for torsion, tension, compression, bending, and the latter check for hardness and impact strength. Modern equipment helps to perform these not very simple procedures efficiently and identify all the performance properties of a given material.
A tensile test can determine the resistance of a material to the effects of applied constant or increasing tensile stress. The method is old, tried and true, used for a very long time and is still widely used. The sample is stretched along the longitudinal axis by means of a device in the testing machine. The rate of stretching of the sample is constant, the load is measured by a special sensor. At the same time, the elongation is monitored, as well as its compliance with the applied load. The results of such tests are extremely useful if new structures need to be created, since no one yet knows how they will behave under load. Only identifying all the elasticity parameters of the material can give a hint. Maximum stress - yield strength determines the maximum load that a given material can withstand. This will help calculate the safety factor.
Hardness test
The stiffness of a material is calculated by The combination of fluidity and hardness helps determine the elasticity of the material. If the technological process involves operations such as drawing, rolling, pressing, then it is simply necessary to know the magnitude of possible plastic deformation. With high plasticity, the material can take any shape under appropriate load. A compression test can also be used to determine the safety factor. Especially if the material is fragile.
Hardness is tested using an identifier, which is made of a much harder material. Most often it is carried out using the Brinell method (a ball is pressed in), Vickers (a pyramid-shaped identifier) or Rockwell (a cone is used). An identifier is pressed into the surface of the material with a certain force for a certain period of time, and then the imprint remaining on the sample is examined. There are other fairly widely used tests: impact strength, for example, when the resistance of a material is assessed at the moment a load is applied.
Mechanical properties evaluate the ability of a material to resist mechanical loads and characterize the performance of products.
Mechanical are called properties that are determined during tests under the influence of external loads - the result of these tests are quantitative characteristics of mechanical properties. Mechanical properties characterize the behavior of a material under the influence of stresses (leading to deformation and destruction) acting both during the manufacturing process of products (casting, welding, pressure treatment, etc.) and during operation.
Standard characteristics of mechanical properties are determined in laboratory conditions on samples of standard sizes by creating irreversible plastic deformation or destruction of the samples. Tests are carried out under external loads: tension, compression, torsion, impact; under conditions of alternating and wear loads. The values of the obtained characteristics are usually given in reference books.
An example would be the following characteristics:
Fracture resistance, estimated by tensile strength, or tensile strength, is the maximum specific load (stress) that a material can withstand before failure when stretched;
Resistance to plastic deformation, measured by the yield strength, is the stress at which plastic deformation of a material begins under tension;
Resistance to elastic deformations, estimated by the elastic limit, is the stress above which the material acquires residual deformations;
The ability to withstand plastic deformation, assessed by the relative elongation of the sample during tension and the relative narrowing of its cross section;
The ability to resist dynamic loads, assessed by impact strength;
Hardness, estimated by the resistance of a material to penetration of an indenter (reference sample).
The mechanical properties of materials are determined under static and dynamic loading conditions.
Elasticity characterizes the elastic properties of a polymer, the ability of a material to undergo large reversible changes in shape under low loads due to vibration of links and the ability of macromolecules to bend.
Static tests also include tests for compression, torsion, bending and other types of loading.
A common disadvantage of static methods for determining the physical and mechanical properties of materials is the need to destroy the sample, which excludes the possibility of further use of the part for its intended purpose as a result of cutting a test sample from it.
Hardness Determination. This is a method of non-destructive testing of the mechanical properties of a material under static load. Hardness is assessed mainly for metals, since for most non-metallic materials hardness is not a property that determines their performance.
Hardness is assessed by the material’s resistance to penetration into it under a static load of a foreign body of regular geometric shape having a reference hardness (Fig. 14).
Rice. 14 Determination of the hardness of materials: A- loading diagram; b- hardness measurement according to Brinell; V- Vickers hardness measurement
Pressing the reference sample into the test sample is carried out using special instruments, of which Brinnell, Rockwell, and Vickers instruments are most often used.
The Brinell method is the most common - a hardened steel ball is pressed into the sample. Imprint diameter d otp is measured using a magnifying glass with a scale. Next, use the tables to find the hardness of the material. The Vickers test uses a diamond cutter, while the Rockwell test uses a diamond cone.
Luminescence (fluorescence and phosphorescence) - glow effects when absorbing energy from incident light, mechanical action, chemical reactions or heat.
The optical properties of substances are of great practical importance. Refraction of light is used to make lenses for optical instruments, reflection is used for thermal insulation: by selecting appropriate coatings, it is possible to influence the properties of materials in order to absorb or reflect thermal radiation, but transmit visible light. Window glass has a characteristic color for air conditioning.
Self-tinting chameleon glasses, fluorescent lights and oscilloscope screens are widely used. Metal coatings (anodized aluminum) are used for decorative purposes (the reflectivity of the material is important), and precision mirrors of metallized surfaces are used.
Decorative properties materials are determined by their appearance and depend on their external pattern, design, texture, structure, surface treatment method, the presence of coatings and reliefs.
Biological properties materials are determined:
Their impact on the environment, the degree of their toxicity to living organisms;
Their suitability for the existence and development of any organisms (fungi, insects, mold, etc.).
Mechanical properties characterize the ability of metals and alloys to resist the action of loads applied to them, and mechanical characteristics express these properties quantitatively. The main properties of metallic materials are; strength, ductility (or toughness), hardness, impact strength, wear resistance, creep, etc.
The mechanical characteristics of materials are determined during mechanical tests, which, depending on the nature of the load over time, are divided into static, dynamic and re-variable.
Depending on the method of applying external forces (loads), tensile, compression, bending, torsion, impact bending, etc. tests are distinguished.
Basic mechanical characteristics of metals and alloys.
Tensile strength (ultimate strength, tensile strength - conditional stress corresponding to the greatest load preceding the destruction of the sample.
True tensile strength (true stress) is the stress determined by the ratio of the load at the moment of rupture to the cross-sectional area of the sample at the point of rupture.
Yield strength (physical) is the lowest stress at which the sample is deformed without a noticeable increase in tensile load.
Yield strength (conditional) - the stress at which the residual elongation reaches 0.2% of the length of the sample section, the elongation of which is taken into account when determining the specified characteristic. Limit of proportionality (conditional) - stress at which the deviation from the linear relationship between load and elongation reaches such a value that the tangent of the angle of inclination formed by the tangent to the deformation curve (at the point under consideration) with the axis of the load increases by 50% of its value on the linear elastic plot. It is allowed to increase the tangent of the angle of inclination by 10 or 25%.
The elastic limit is the conditional stress corresponding to the appearance of residual deformation. It is possible to determine the elastic limit with tolerances of up to 0.005%, then it will be designated accordingly.
Relative elongation after rupture is the ratio of the increment in the length of the sample after rupture to its original calculated length. There are relative elongations obtained when testing on samples with a five-fold and ten-fold length-to-diameter ratio. Other ratios are also allowed, for example 2.5, when testing castings.
Relative contraction after rupture is the ratio of the cross-sectional area of the sample at the rupture site to the initial cross-sectional area.
The specified characteristics of mechanical properties are determined by testing materials for tension according to the methods set out in GOST 1497-61, on cylindrical and flat samples, the shapes and dimensions of which are established by the same standard. Tensile tests at elevated temperatures (up to 1200°C) are established by GOST 9651-73, for long-term strength - GOST 10145-62.
Modulus of normal elasticity is the ratio of stress to its corresponding relative elongation in tension (compression) within the limits of elastic deformation (Hooke’s law).
Impact toughness, a mechanical characteristic of the toughness of a metal, is determined by the work expended on an impact fracture on a pendulum impact driver of a sample of a given type and related to the working cross-sectional area of the sample at the point of the cut. Tests at normal temperatures are carried out according to GOST 9454-60, at low temperatures - according to GOST 9455-60 and at elevated temperatures - according to GOST 9656-61.
Endurance (fatigue) limit is the maximum stress at which the sample materials can withstand a given number of symmetrical cycles (from +P to -P) without destruction, taken as the base. The number of cycles is specified by technical specifications and represents a large number. Methods for testing metals for endurance are regulated by GOST 2860-65.
Ultimate compressive strength is the ratio of the breaking load to the cross-sectional area of the sample before testing.
Conditional creep limit is the stress that causes a given elongation of a sample (total or residual) over a specified period of time at a given temperature.
Brinell hardness - determined on a TSh hardness tester by pressing a hardened steel ball p. test metal or alloy.
Rockwell hardness HRA, HRB and HRC is determined by pressing a steel ball with a diameter of ~ 1.6 mm or a cone (diamond or carbide) into the metal with a corner at the apex of 120° on a TK hardness tester. Depending on the determination conditions, which are standardized by GOST 9013-68, three HR values are distinguished: HRA - for very hard materials (scale A) - the test is carried out by indenting a diamond cone; HRB - for mild steel (scale B) - steel ball; HRC - for hardened steel (scale C) - carbide or diamond cone.
The penetration depth of the diamond cone when testing in metal is small, which makes it possible to test thinner products than when determining Brinell hardness. Rockwell hardness is a conditional characteristic, the value of which is measured on the scale of the device.
Vickers hardness HV is determined by indentation of a diamond standard regular tetrahedral pyramid. The hardness number is determined by measuring the length of the diagonals (the arithmetic mean of the sum of two diagonals) and recalculating using the formula
Standard loads, depending on the thickness of the sample, are 5, 10, 20, 30, 50 and 100 kgf. The time delay under load is taken for ferrous metals 10-15 seconds, for non-ferrous metals - 28-32. Accordingly, the symbol HV 10/30-500 means: 500 - hardness number; 10 - load and 30 - holding time.
The Vickers method is used to measure the hardness of small cross-section parts and hard thin surface layers of cemented, nitrided or cyanidated products.
49.Secondary crystallization of metals Secondary crystallization is of great practical importance and serves as the basis for a number of processes of heat treatment, aging, etc., which significantly change and improve the properties of alloys. Most secondary crystallization processes involve diffusion. Diffusion in hard alloys is possible for a number of reasons. In particular, in substitutional solutions it occurs due to the presence of unfilled sites (vacancies) in the lattices. Both solvent atoms and solute atoms can move. During the formation of interstitial solutions, the movement of dissolved atoms occurs through the interstices of the lattices. Diffusion proceeds the faster, the greater the difference in concentration; the higher the temperature. I (coagulation refers to the growth of large crystals at the expense of small ones; under spheroidization - the transformation of elongated crystals into rounded ones. Both processes occur due to the desire of the system to reduce free energy. In this case, THIS is achieved because the sum ratio.
The surfaces of grains become smaller in relation to their volumes. Coagulation and spheroidization proceed more easily the higher the temperature. In Fig. 41 shows a diagram of the state of the alloy in which the solubility of the second component in the solid solution decreases. On this diagram (unlike the diagram in Fig. 39), the EQ line appears, characterizing the selection of excess crystals of component B, which are called secondary (B2), in contrast to the primary crystals (B\), which are distinguished along the CD line. As an example, let us consider the process of formation of secondary crystals during cooling of solid solutions a with concentration K. At temperature t\, the structure is single-phase, when the EQ line is reached, the solution becomes saturated and as further cooling occurs, excess phase B2 is released from it, the latter can be released along the boundaries of crystals a and take the form of a grid. Here, too, the formation of nuclei first occurs and then their growth. However, the place where the nuclei appear and their growth is predetermined by the surfaces of the primary grains. Sometimes the arrangement of the secondary phase in the form of a network is undesirable, then it is either prevented from forming or eliminated. The network is removed in different ways, for example, by spheroidizing annealing. Crystallization according to the diagram (Fig. 41) makes it possible to significantly change the properties of the alloy by quenching and tempering or by aging.
50.DS alloys with unlimited solubility of components Both component unlimited soluble in liquid and solid states do not form chemical compounds.
Components: A, B.
Phases: L, α.
If two component dissolve indefinitely in liquid and solid states, then the existence of only two phases is possible - liquid solution Land solid solutionα. Therefore, there cannot be three phases, crystallization at constant temperature there is no horizontal lines on diagram No.
The diagram shown in Fig. 1, consists of three areas: liquid, liquid + solid solution and solid solution.
The AmB line is line liquidus, and lineАnВ - line solidus. Crystallization process represented by a curve cooling alloy(Fig. 2).
Point 1 corresponds to the beginning crystallization, dot 2 - end. Between dots 1 and 2 (i.e. between linesliquidus and solidus) alloy is in a two-phase state. At two components and two phases system monovariant (c = k-f+1 = 2 - 2 + 1 = 1), i.e. if the temperature changes, then so does concentration of components in phases; each temperature correspond strictly to certain compositions phases concentration and the number of phases alloy, lying between linessolidus and liquidus are determined rule segments. So, alloy K in point a consists of liquid and solid phases. Compound liquid phase will be determined by the projection points b lying on lines liquidus, and Compound solid phase - projection points with lying on lines solidus. The amount of liquid and solid phases is determined from the following ratios: amount of liquid phase ac/bc, amount of solid phase ba/bc.
In everything crystallization interval(from points 1to points 2) from liquid alloy,
having the original concentration K, crystals that are richer in the refractory component stand out. Compound first crystals will be determined by the projection s. End crystallization of the alloy K must in point 2, when the last drop of liquid having Compound l, will harden. The segment showing the amount of solid phase was equal to zero in point/ when it just started crystallization, and the amount of everything alloy V point 2 when crystallization ended. Compound liquid changes along the curve 1 - l, and Composition of crystals- along the curve s- 2, and in moment graduation crystallizationComposition of crystals same as Compound original liquid.
51. Temperature properties of materials For materials, several characteristic temperature points are introduced, indicating the performance and behavior of the materials when temperature changes. Heat resistance - the maximum temperature at which the service life of the material does not decrease. According to this parameter, all materials are divided into heat resistance classes.
Heat resistance
- temperature at which deterioration of characteristics occurs when it is reached for a short time.
Heat resistance
- the temperature at which chemical changes in a material occur.
Frost resistance
- ability to work at low temperatures (this parameter is important for rubbers).
Flammability
- the ability to ignite, maintain fire, self-ignite. These are different degrees of flammability. All these concepts define characteristic temperatures at which any property of a material changes. There are some temperatures that are characteristic of all materials, and there are temperatures that are specific to some electrical materials. in which any characteristics change dramatically. Most materials have melting and boiling points. Melting point is the temperature at which the transition from solid to liquid occurs. Liquid helium does not have a melting point; it remains liquid even at zero Kelvin. The most refractory are tungsten - 3387 °C, molybdenum 2622 °C, rhenium - 3180 °C, tantalum - 3000 °C. There are refractory substances among ceramics: hafnium carbide HfC and tantalum carbide TaC have melting points of 2880 °C, titanium nitride and carbide - more than 3000 °C. There are materials, mainly thermoplastic polymers, that have a softening point, but it does not reach melting, because... the destruction of polymer molecules begins at elevated temperatures. With thermosetting polymers, it doesn’t even reach the point of softening; the material begins to decompose earlier. There are alloys and other complex substances that have a complex melting process: at a certain temperature, called “solidus,” partial melting occurs, i.e. the transition of part of a substance to a liquid state. The rest of the substance is in a solid state. It turns out something like a mush. As the temperature rises, more and more of it turns into a liquid state, and finally, at a certain temperature called “liquidus”, complete melting of the substance will occur. For example, an alloy of tin and lead for soldering, simply called “solder,” begins to melt at approximately 180 °C (solidus point) and melts at approximately 230 °C (liquidus point).
In any melting process, reaching a certain point is a necessary but not sufficient condition for melting. In order to melt a substance, you need to impart energy to it, which is called the heat of fusion. It is calculated per gram (or per molecule). Boiling point is the temperature at which the transition from liquid to vapor occurs. Almost all simple substances boil, complex organic compounds do not boil; they decompose at lower temperatures, without reaching a boil. The boiling point is significantly influenced by pressure. So, for example, for water, you can shift the boiling point from 100 ° C to 373 ° C by applying a pressure of 225 atm. Boiling of solutions, i.e. The process of mutually soluble substances in each other occurs in a complex way; two components boil at once, only in the vapor there is more of one substance than the other. For example, a weak solution of alcohol in water boils away so that there is more alcohol in the vapor than in water. Due to this, distillation works and after condensation of the steam, alcohol is obtained, but enriched with water. There are mixtures that boil away at the same time, for example 96% alcohol. Here, during boiling, the composition of the liquid and the composition of the vapor are the same. After condensation of the steam, alcohol of exactly the same composition is obtained. Such mixtures are called azeotropic. There are temperatures specific to electrical materials. For example, for ferroelectrics the so-called Curie point. It turns out that the ferroelectric state of matter arises only at low temperatures. There is a temperature for each ferroelectric above which domains cannot exist and it turns into a paraelectric. This temperature is called the Curie point. The dielectric constant below the Curie point is high; it increases slightly as it approaches the Curie point. After reaching this point, the dielectric constant drops sharply. For example, for the most common ferroelectric: barium titanate, the Curie point is 120 °C, for lead zirconate titanate 270 °C, for some organic ferroelectrics the Curie temperature is negative. A similar temperature (also called the Curie point) exists for ferromagnets. The behavior of magnetic permeability is similar to the behavior of dielectric constant as the temperature increases and approaches the Curie point. The only difference is that the decrease in magnetic permeability with increasing temperature occurs more sharply after reaching the Curie point. Curie point values for some materials: iron 770 °C, cobalt 1330 °C, erbium and holmium (-253 °C), ceramics - in a wide temperature range. For antiferromagnets, a similar point is called Néel point.
Related information.
All people are very different from birth. Smart adults in different countries have been asking such questions for a long time. They realized long ago that all children differ from each other genetically, psychologically, and in their physical development. And no amount of moralizing, training, various scientific methods of education, and even a belt will make them the same. Different children need to be raised differently. When children grow up, professions will choose them themselves. But we cannot escape the abilities that manifest themselves from early childhood. Abilities can be technical, organizational, artistic and aesthetic. Almost all of them somehow influence the choice of our professions. It often happens that our abilities guide us when choosing professions. Let's take a closer look at technical abilities and their impact on our lives.
Imagine that you took a customs clearance course, and subsequently a large number of vehicles will pass through your hands. What will happen if you don’t learn to understand everything? You simply will not be able to live up to your chosen profession. What does technical ability mean?
An indispensable attribute of technical abilities is an interest in technology, a desire to work on machines, with tools and with equipment.
Components of technical abilities:
a) ability to understand drawings, diagrams, graphs; b) the ability to read drawings, graphs, and vividly imagine the real objects behind them is very important for technical professions;
c) abilities in physics, mathematics, chemistry. Technology is closely related to these sciences. You are required not only to have a good grasp of mathematical material and memory, but also to be able to work with numbers and formulas;
d) the ability to understand and reason, analyze and generalize - logical thinking;
e) developed spatial imagination is a very significant component of technical abilities.
Such abilities are ideal for a person with a mathematical mindset who knows how to think. That is, if your choice fell on customs declaration courses, and you consider yourself to be one of the people who have technical character traits, then you have chosen the right profession.
Diagnosing your own abilities is a very delicate matter. It is likely that you have not found the above technical abilities. Don't be alarmed. This is fine. Firstly, people with a full set of qualities for only one profession are rare - one in thirty. This is called a calling. The rest, as a rule, have a set of qualities that are equally suitable for several professions, and they either have to develop the missing abilities through constant training, or compensate for them with something else. You should be wary if your abilities are too clearly inconsistent with the requirements of the profession you want to choose. Listen to yourself, and everything will definitely work out, and you will become a master of your craft.