Mechanical properties of materials. Part II.

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.

Strengthis 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.

After you read

Questions

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, aluminum 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 its light weight?

Tasks

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

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.

Before you start

Work in groups and discuss the question.

What is machine tool?

TEXT 13. Machine tools.

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 developed lately. 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.

After you read

Questions

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 labor?

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?

Tasks

1.Find English equivalents 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

2. Translate into English:

1. Токарный станок позволяет производить детали круглого сечения.

2. Деталь зажимается в патроне или 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.

5.Станки классифицируются по множеству признаков.

6. Металлорежущий станок — станок, предназначенный для размерной обработки металлических заготовок путем снятия материала механическим способом.

7. Для осуществления процесса резания на металлорежущих станках необходимо обеспечить взаимосвязь формообразующих движений.

8. Станок — машина, используемая (как правило, в промышленности) для обработки различных материалов, либо приспособление для выполнения чего-либо.

 

Before you start

Work in groups and discuss the questions.

Why did you choose mechanical engineering? What is the mechanical engineering concerned with?