VIII.Составьте аннотацию к тексту (2 - 3 предложения).
ВАРИАНТ №18
I.Прочитайте и переведите текст
HOW MATERIALS REACT TO EXTERNAL FORCES
Materials Science and Technology is the study of materials and how they can be fabricated to meet the needs of modern technology. Using the laboratory techniques and knowledge of physics, chemistry, and metallurgy, scientists are finding new ways of using metals, plastics and other materials.
Engineers must know how materials respond to external forces, such as tension, compression, torsion, bending, and shear. All materials respond to these forces by elastic deformation. That is, the materials return their original size and form when the external force disappears. The materials may also have permanent deformation or they may fracture. The results of external forces are creep and fatigue.
Compression is a pressure causing a decrease in volume. When a material is subjected to a bending, shearing, or torsion (twisting) force, both tensile and compressive forces are simultaneously at work. When a metal bar is bent, one side of it is stretched and subjected to a tensional force, and the other side is compressed.
Tensionis a pulling force; for example, the force in a cable holding a weight. Under tension, a material usually stretches, returning to its original length if the force does not exceed the material's elastic limit. Under larger tensions, the material does not return completely to its original condition, and under greater forces the material ruptures.
Fatigueis the growth of cracks under stress. It occurs when a mechanical part is subjected to a repeated or cyclic stress, such as vibration. Even when the maximum stress never exceeds the elastic limit, failure of the material can occur even after a short time. No deformation is seen during fatigue, but small localised cracks develop and propagate through the material until the remaining cross-sectional area cannot support the maximum stress of the cyclic force. Knowledge of tensile stress, elastic limits, and the resistance of materials to creep and fatigue are of basic importance in engineering.
Creepis a slow, permanent deformation that results from a steady force acting on a material. Materials at high temperatures usually suffer from this deformation. The gradual loosening of bolts and the deformation of components of machines and engines are all the examples of creep. In many cases the slow deformation stops because deformation eliminates the force causing the creep. Creep extended over a long time finally leads to the rupture of the material.
PROPERTIES OF MATERIALS
Density (specific weight) is the amount of mass in a unit volume. It is measured in kilograms per cubic metre. The density of water is 1000 kg/m3 but most materials have a higher density and sink in water. Aluminium alloys, with typical densities around 2800 kg/m3 are considerably less dense than steels, which have typical densities around 7800 kg/m3. Density is important in any application where the material must not be heavy.
Stiffness (rigidity) is a measure of the resistance to deformation such as stretching or bending. The Young modulus is a measure of the resistance to simple stretching or compression. It is the ratio of the applied force per unit area (stress) to the fractional elastic deformation (strain). Stiffness is important when a rigid structure is to be made.
Strength is the force per unit area (stress) that a material can support without failing. The units are the same as those of stiffness, MN/m2, but in this case the deformation is irreversible. The yield strength is the stress at which a material first deforms plastically. For a metal the yield strength may be less than the fracture strength, which is the stress at which it breaks. Many materials have a higher strength in compression than in tension.
Ductility is the ability of a material to deform without breaking. One of the great advantages of metals is their ability to be formed into the shape that is needed, such as car body parts. Materials that are not ductile are brittle. Ductile materials can absorb energy by deformation but brittle materials cannot.
Toughness is the resistance of a material to breaking when there is a crack in it. For a material of given toughness, the stress at which it will fail is inversely proportional to the square root of the size of the largest defect present. Toughness is different from strength: the toughest steels, for example, are different from the ones with highest tensile strength. Brittle materials have low toughness: glass can be broken along a chosen line by first scratching it with a diamond. Composites can be designed to have considerably greater toughness than their constituent materials. The example of a very tough composite is fiberglass that is very flexible and strong.
Creep resistance is the resistance to a gradual permanent change of shape, and it becomes especially important at higher temperatures. A successful research has been made in materials for machine parts that operate at high temperatures and under high tensile forces without gradually extending, for example the parts of plane engines.
COMPOSITE MATERIALS
The combinations of two or more different materials are called composite materials. They usually have unique mechanical and physical properties because they combine the best properties of different materials. For example, a fibre-glass reinforced plastic combines the high strength of thin glass fibres with the ductility and chemical resistance of plastic. Nowadays composites are being used for structures such as bridges, boat-building etc.
Composite materials usually consist of synthetic fibres within a matrix, a material that surrounds and is tightly bound to the fibres. The most widely used type of composite material is polymer matrix composites (PMCs). PMCs consist of fibres made of a ceramic material such as carbon or glass embedded in a plastic matrix. Usually the fibres make up about 60 per cent by volume. Composites with metal matrices or ceramic matrices are called metal matrix composites (MMCs) and ceramic matrix composites (CMCs), respectively.
Continuous-fibre composites are generally required for structural applications. The specific strength (strength-to-density ratio) and specific stiffness (elastic modulus-to-density ratio) of continuous carbon fibre PMCs, for example, can be better than metal alloys have. Composites can also have other attractive properties, such as high thermal or electrical conductivity and a low coefficient of thermal expansion.
Although composite materials have certain advantages over conventional materials, composites also have some disadvantages. For example, PMCs and other composite materials tend to be highly anisotropic-that is, their strength, stiffness, and other engineering properties are different depending on the orientation of the composite material. For example, if a PMC is fabricated so that all the fibres are lined up parallel to one another, then the PMC will be very stiff in the direction parallel to the fibres, but not stiff in the perpendicular direction. The designer who uses composite materials in structures subjected to multidirectional forces, must take these anisotropic properties into account. Also, forming strong connections between separate composite material components is difficult.
The advanced composites have high manufacturing costs. Fabricating composite materials is a complex process. However, new manufacturing techniques are developed. It will become possible to produce composite materials at higher volumes and at a lower cost than is now possible, accelerating the wider exploitation of these materials.
II.Переведите на русский язык следующие английские словосочетания:
1) pulling force 6) resistance of materials
2) maximum stress 7) steady force acting on a material
3) small localised cracks 8) deformation of components
4) cross-sectional area 9) rupture of the material
5)cyclic force 10) resistance to deformation
III.Найдите в тексте английские эквиваленты следующих словосочетаний:
1) отвечать требованиям современной технологии
2) используя лабораторные методы
3) новые способы использования металлов
4) тонна на кубический метр
5) мера сопротивления деформации
6) сжатие, растяжение, изгиб, кручение, срез
7) возвращать первоначальный размер и форму
8) постоянная деформация
9) внешняя сила
10) уменьшение объема
IV.Найдите в тексте слова, имеющие общий корень с данными словами. Определите, к какой части речи они относятся, и переведите их на русский язык:
Appear, compress, great, success, force, day, build, stiff, advantage, direction.
V.Задайте к выделенному в тексте предложению все типы вопросов: общий, альтернативный, разделительный, два специальных: а) к подлежащему, б) к любому члену предложения.
VI.Выполните анализ данных предложений, обратив внимание на следующие грамматические явления: формы глаголовto be, to have; оборот there is/are; степени сравнения прилагательных и наречий; множественное число существительных; Present, Past, Future Simple Active/Passive; модальные глаголы.
1.Engineers must know how materials respond to external forces, such as tension, compression, torsion, bending, and shear.
2. Continuous-fibre composites are generally required for structural applications.
3. Ductile materials can absorb energy by deformation but brittle materials cannot.
4. For example, if a PMC is fabricated so that all the fibres are lined up parallel to one another, then the PMC will be very stiff in the direction parallel to the fibres, but not stiff in the perpendicular direction.
5. Many materials have a higher strength in compression than in tension.
VII.Ответьте на вопросы по текстам:
1. What are the external forces that cause the elastic deformation of materials? Describe those forces that change the form and size of materials.
2. What are the results of external forces?
3. What kinds of deformation are the combinations of tension and compression?
4. What is the density of a material?
5. What are the units of density? Where low density is needed?
6. What is called «composite materials»?
7 What are the best properties of fiberglass?
VIII.Составьте аннотацию к тексту (2 - 3 предложения).