MECHANICAL PROPERTIES OF MATERIALS

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.

Tension is 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.

Fatigue is 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.

Creep is 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.

Vocabulary

bar — брусок, прут

completely — полностью, совершенно

compression — сжатие

creep — ползучесть

cross-sectional area — площадь поперечного сечения

cyclic stress — циклическое напряжение

decrease — уменьшение

elastic deformation — упругая деформация

elastic limit — предел упругости

exceed — превышать

external forces — внешние силы

fatigue — усталость металла

fracture — перелом, излом

loosen — ослаблять, расшатывать

permanent deformation — постоянная: деформация

remaining — оставшийся

shear — срез

simultaneously —одновременно

to stretch — растягивать

technique —методы

tension — напряженность

to propagate — распространятся)

to bend — гнуть, согнуть

to extend — расширять, продолжаться

to meet the needs — отвечать требованиям

to occur — происходить

to respond — отвечать реагировать

to suffer — страдать

torsion — кручение

twisting — закручивание, изгиб

volume — объем, количество

rupture — разрыв

General understanding:

1.What are the external forces causing 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 result of tension? What happens if the elastic limit of material is exceeded under tension?

5.What do we call fatigue? When does it occur? What are the results of fatigue?

6. What do we call creep? When does this type of permanent deformation take place? What are the results of creep?

Exercise 1. Find the following in the text:

1. отвечать требованиям современной технологии

2. используя лабораторные методы

3. новые способы использования металлов

4. сжатие, растяжение, изгиб, кручение, срез

5. возвращать первоначальный размер и форму

6. внешняя сила

7. постоянная деформация

8. уменьшение объема

9. растягивающие и сжимающие силы

10.превышать предел упругости материала

11.повторяющиеся циклические напряжения

12.разрушение материала

13.развитие и распространение мелких трещин

14.сопротивление материалов ползучести и усталости

Exercise 2. Translate into English the following sentences:

1. Упругая деформация — это реакция всех материалов на внешние силы, такие, как растяжение, сжатие, скручивание, изгиб и срез.

2. Усталость и ползучесть материалов являются результатом внешних сил.

3. Внешние силы вызывают постоянную деформацию и разрушение материала.

4. Растягивающие и сжимающие силы работают одновременно, когда мы изгибаем или скручиваем материал.

5. Растяжение материала выше предела его упругости дает постоянную деформацию или разрушение.

6. Когда деталь работает долгое время под циклическими напряжениями в ней появляются небольшие растущие трещины из-за усталости металла.

7. Ползучесть — это медленное изменение размера детали под напряжением.

Exercise 3.. Find the equivalents

Exercise 4.Look at these examples and translate

A man can easily lift a large roll of glass wool but not a concrete beam.

Glass wool is light but concrete is heavy.

A man can bend a rubber tile but not a concrete tile.

Rubber is flexible but concrete is rigid.

Wood can burn but concrete cannot burn.

Wood is combustible but concrete is non-combustible.

Water vapour can pass through stone but not through bitumen.

Stone is permeable but bitumen is impermeable.

You can see through glass but not through wood.

Glass is transparent but wood is opaque.

Stainless steel can resist corrosion but mild steel cannot.

Stainless steel is corrosion resistant but mild steel is not corrosion resistant.

Heat can be easily transferred through copper but not through wood.

Copper is a good conductor of heat but wood is a poor conductor of heat.

Rubber can be stretched or compressed and will then return to its original shape but clay cannot.

Rubber is elastic but clay is plastic.

Bitumen can be dented or scratched easily but glass cannot.

Bitumen is soft but glass is hard.

Now complete these sentences with properties:

a) The polythene membrane can prevent moisture from rising into the concrete floor. This means that polythene is____.

b) The T-shaped aluminium section can resist chemical action, i.e. aluminium is......

c) The stone block cannot be lifted without using a crane. This means that stone is.____.

d) The corrugated iron roof cannot prevent the sun from heating up the house, i.e. iron is......

e) Glass wool can help to keep a house warm in the winter and cool in the summer, i.e. glass wool is......

f) The ceramic tiles on the floor cannot be scratched easily by people walking on them. This means that ceramic tiles are _ _.

g) Asbestos sheeting can be used to fireproof doors. In other words asbestos is_____.

h) Black cloth blinds can be used to keep the light out of a room, i.e. cloth is_____.

Part 2

«Mechanical 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.

Vocabulary

ability — способность

amount — количество

absorb— поглощать

amount — количество

application — применение

brittle — хрупкий, ломкий

car body — кузов автомобиля

constituent — компонент

crack — трещина

creep resistance — устойчивость к ползучести

definition —определение

density — плотность

ductility — ковкость, эластичность

failure — повреждение

gradual — постепенный

permanent — постоянный

rigid— жесткий

to sink — тонуть

square root — квадратный корень

stiffness — жесткость

strain — нагрузка, напряжение, деформация

strength — прочность

stress — давление, напряжение

tensile strength — прочность на разрыв

toughness — прочность, стойкость

yield strength — прочность текучести

Young modulus — модуль Юнга

General understanding:

1. What is the density of a material?

2. What are the units of density? Where low density is needed?

3. What are the densities of water, aluminium and steel?

4. A measure of what properties is stiffness? When stiffness is important?

5. What is Young modulus?

6. What is strength?

7. What is yield strength? Why fracture strength is always greater than yield strength?

8. What is ductility? Give the examples of ductile materials. Give the examples of brittle materials.

8. What is toughness?

9. What properties of steel are necessary for the manufacturing of: a) springs, b) car body parts, c) bolts and nuts, d) cutting tools?

10. Where is aluminium mostly used because of it slight weight?

Exercise 1. Find the following words and word combinations in the text:

1. количество массы в единице объема

2. килограмм на кубический метр

3. мера сопротивления деформации

4. отношение приложенной силы на единицу площади к частичной упругой деформации

5. жесткая конструкция

6. прочность на сжатие

7. способность материала деформироваться не разрушаясь

8. поглощать энергию путем деформации

9. обратно пропорционально квадрату размера дефекта

10.постепенное изменение формы

11.повышенные температуры

12.высокие растягивающие усилия

Exercise 2. Translate into English the following:

1. Плотность измеряется в килограммах на кубический метр.

2. Большинство материалов имеют более высокую плотность, чем вода и тонут в воде.

3. Плотность материала очень важна, особенно в авиации.

4. Модуль Юнга — отношение приложенной силы к упругой деформации данного материала.

5. Чем более металл жесткий, тем менее он деформируется под нагрузкой.

6. Когда металл растягивают, он сначала течет, то есть пластически деформируется.

7. Свинец, медь, алюминий и золото — самые ковкие металлы.

8. Сопротивление ползучести является очень важным свойством материалов, которые используются в авиационных моторах

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