TEXT 13. 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 thestress 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 [ә 'biliti] – способность, умение

amount [ә 'maunt] – количество

absorb [ӕ b'zo: b] – поглощать

application – применение

brittle ['britl] – хрупкий, ломкий

car body – кузов автомобиля

constituent [kә n'stitjuә nt] – компонент

crack [krӕ k] – трещина

creep resistance – устойчивость к ползучести

definition [defi'niʃ n] – определение

density ['densiti] – плотность

ductility [dAk'tiliti] – ковкость, эластичность

failure ['feiljә ] – повреждение

gradual – постепенный

permanent – постоянный

rigid – жесткий

to sink [sink] – тонуть

square root ['skweә 'ru: t] – квадратный корень

stiffness ['stifnis] – жесткость

strain [strein] – нагрузка, напряжение, деформация

strength [streŋ Ɵ ] – сила, прочность

stress [stres] – давление, напряжение

tensile strength – прочность на разрыв

toughness ['tAfnis] – прочность, стойкость

yield strength [ji: ld] – прочность текучести

Young modulus – модуль Юнга

 

Exercise 1

Answer the questions for 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.

9. What is toughness?

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

11. Where is aluminium mostly used because of its light weight?

 

Exercise 2

Find the following word combinations in the text:

1. количество массы в единице объема

2. килограмм на кубический метр

3. мера сопротивления деформации

4. жесткая конструкция

5. прочность на сжатие

6. способность материала деформироваться не разрушаясь

7. поглощать энергию путем деформации

8. постепенное изменение формы

9. повышенные температуры

 

Exercise 3

Translate into English:

1. Плотность измеряется в килограммах на кубический метр.

2. Большинство материалов имеют болеевысокую 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 деформирунтся под нагрузкой.

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

7. Свинец, медь, алюминий и золото – самые ковкие материалы.

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

 

UNIT 14

TEXT 14. MACHINE-TOOLS

MILLING, BORING, DRILLING MACHINES. SIMPERS AND PLANERS

Machine-tools are used to shape metals and other materials. The material to be shaped is called the workpiece. Most machine-tools are now electrically driven. Machine-tools with electrical drive are faster and more accurate than hand tools: they were an important element in the development of mass-production processes, as they allowed individual parts to be made in large numbers so as to be interchangeable.

All machine-tools have facilities for holding both the workpiece and the tool, and for accurately controlling the movement of the cutting tool relative to the workpiece. Most machining operations generate large amounts of heat, and use cooling fluids (usually a mixture of water and oils) for cooling and lubrication.

Machine-tools usually work materials mechanically but other machining methods have been developedlately. They include chemical machining, spark erosion to machine very hard materials to any shape by means of a continuous high-voltage spark (discharge) between an electrode and a workpiece. Other machining methods include drilling using ultrasound, and cutting by means of a laser beam. Numerical control of machine-tools and flexible manufacturing systems have made it possible for complete systems of machine-tools to be used flexibly for the manufacture of a range of products.

 

Vocabulary:

machine-tools – станки

electrically driven – с электроприводом

shape [ʃ eip] – форма

workpiece – деталь, заготовка

accurate – точный

development – развитие

to allow [ә 'lau] – позволять, разрешать

interchangeable – взаимозаменяемый

facility [fә 'siliti] – приспособление

relative ['relativ] – относительный

fluid ['flu: id] – жидкость

to lubricate ['lu: brikeit] – смазывать

spark erosion – электроискровая обработка

discharge – разряд

by means of – посредством

beam [bi: m] – луч

drilling – сверление

flexible ['fleksә bl] – гибкий

range [reindʒ ] – ассортимент, диапазон

 

Exercise 1

Read the text again and entitle it. Then divide the text into logical parts, give titles to all of them thus making a plan.

Exercise 2

Write out all the sentences, expressing the main idea(s) of each logical part and translate them into Russian.

Exercise 3

Condense the sentences of the text where possible.

Exercise 4

Write an abstract to the text in your own words using the plan and the sentences you’ve written out and condensed.

LATHE

Lathe is still the most important machine-tool. It produces parts of circular cross-section by turning the workpiece on its axis and cutting its surface with a sharp stationary tool. The tool may be moved sideways to produce a cylindrical part and moved towards the workpiece to control the depth of cut. Nowadays all lathes are power-driven by electric motors. That allows continuous rotation of the workpiece at a variety of speeds.

The modern lathe is driven by means of a headstock supporting a hollow spindle on accurate bearings and carrying either a chuck or a faceplate, to which the workpiece is clamped. The movement of the tool, both along the lathe bed and at right angle to it, can be accurately controlled, so enabling a part to be machined to close tolerances. Modern lathes are often under numerical control.

Vocabulary:

lathe [leiƟ ] – токарный станок

circular cross-section ['sә: kjulә ] – круглое поперечное сечение

surface ['sә: fis] – поверхность

stationary ['steiʃ nә ri] – неподвижный, стационарный

sideways ['saidweiz] – в сторону

variety [vә 'raiә ti] – разнообразие, разновидность

depth [depƟ ] – глубина

headstock ['hedstok] – передняя бабка

spindle [spindl] – шпиндель

chuck – зажим, патрон

faceplate – планшайба

lathe bed – станина станка

to enable [i'neibl] – давать возможность

tolerance – допуск

 

Exercise 1

Answer the questions for general understanding:

1. What are machine-tools used for?

2. How are most machine-tools driven nowadays?

3. What facilities have all machine-tools?

4. How are the cutting tool and the workpiece cooled during machining?

5. What other machining methods have been developed lately?

6. What systems are used now for the manufacture of a range of products without the use of manual labour?

7. What parts can be made with lathes?

8. How can the cutting tool be moved on a lathe?

9. How is the workpiece clamped in a lathe?

10. Can we change the speeds of workpiece rotation in a lathe?

11. What is numerical control of machine tools used for?

 

Exercise 2

Find the following word combinations in the text:

1. обрабатываемый материал

2. электропривод

3. более точный

4. отдельные детали

5. процесс массового производства

6. приспособления для держания резца и детали

7. операции по механической обработке детали

8. высоковольтный разряд

9. сверление ультразвуком

10. резание с помощью лазерного луча

11. гибкие производственные системы

12. детали круглого сечения

13. поворачивать деталь вокруг ее оси

14. двигать в сторону, двигать по направлению к детали

15. глубина резания

16. непрерывное вращение детали

17. движение резца вдоль станины

 

Exercise 3

Translate into English:

1. Токарный станок позволяет производитьт детали круглого сечения.

2. Деталь зажимается в патроне или на на планшайбе токарного станка.

3. Резец может двигаться как вдоль станины, так и под прямым углом к ней.

4. Современные токарные станки часто имеют цифровое управление.

MILLING MACHINE

In a milling machine the cutter is a circular device with a series of cutting edges on its circumference. The workpiece is held on a table that controls the feed against the cutter. The table has three possible movements: longitudinal, horizontal, and vertical; in some cases it can also rotate. Milling machines are the most versatile of all machine tools. Flat or contoured surfaces may be machined with excellent finish and accuracy. Angles, slots, gear teeth and cuts can be made by using various shapes of cutters.

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