PROPERTIES OF METALS

The properties of metals can be grouped into certain categories. The major categories to be considered are:

a) chemical properties that are material characteristics which relate to the structure of a material and its formation out of the elements; these properties are usually measured in a chemical laboratory and they cannot be determined by visual observations.

b) physical properties such as colour, density, crystal structure, electrical and heat conductivity, etc. Physical properties are characteristics of materials that pertain to the interaction of these materials with various forms of energy and with other forms of matter.

c) mechanical properties that are of foremost importance in selecting materials for structural machine components.

The mechanical properties of the metals are those which are associated with the ability of the material to resist mechanical forces and load. These mechanical properties of the metal include strength, stiffness, elasticity, plasticity, ductility, brittleness, malleability, toughness, resilience, creep and hardness.

1. Strength. It is the ability of a material to resist the externally applied forces without breaking or yielding. The internal resistance offered by a part to an externally applied force is called stress.

2. Stiffness. It is the ability of a material to resist deformation under stress. The modulus of elasticity is the measure of stiffness.

3. Elasticity. It is the property of a material to regain its original shape after deformation when the external forces are removed. This property is desirable for materials used in tools and machines. It may be noted that steel is more elastic than rubber.

4. Plasticity. It is the property of a material which retains the deformation produced under load permanently. This property of the material is necessary for forgings, in stamping images on coins and in ornamental work.

5. Ductility. It is the property of a material enabling it to be drawn into wire with the application of a tensile force. A ductile material must be both strong and plastic. The ductility is usually measured by the terms, percentage elongation and percentage reduction in area. The ductile materials commonly used in engineering practice (in order of diminishing ductility) are mild steel, copper, aluminum, nickel, zinc, tin and lead.

6. Brittleness. It is the property of a material opposite to ductility. It is the property of breaking of a material with little permanent distortion. Brittle materials when subjected to tensile loads, snap off without giving any sensible elongation. Cast iron is a brittle material.

7. Malleability. It is a special case of ductility which permits materials to be rolled or hammered into thin sheets. A malleable material should be plastic but it is not essential to be so strong. The malleable materials commonly used in engineering practice (in order of diminishing malleability) are lead, soft steel, wrought iron, copper and aluminum.

8. Toughness. It is the property of a material to resist fracture due to high impact loads like hammer blows. The toughness of the material decreases when it is heated. It is measured by the amount of energy that a unit volume of the material has absorbed after being stressed up to the point of fracture. This property is desirable in parts subjected to shock and impact loads.

9. Machinability. It is the property of a material which refers to a relative case with which a material can be cut. The machinability of a material can be measured in a number of ways such as comparing the tool life for cutting different materials or thrust required to remove the material at some given rate or the energy required to remove a unit volume of the material. It may be noted that brass can be more easily machined than steel.

10. Resilience. It is the property of a material to absorb energy and to resist shock and impact loads. It is measured by the amount of energy absorbed per unit volume within elastic limit. This property is essential for spring materials.

11. Creep. When a part is subjected to a constant stress at high temperature for a long period of time, it will undergo a slow and permanent deformation called creep. This property is considered in designing internal combustion engines, boilers and turbines.

12. Fatigue. When a material is subjected to repeated stresses, it fails at stresses below the yield point stresses. Such type of failure of a material is known as * fatigue. The failure is caused by means of a progressive crack formation which are usually fine and of microscopic size. This property is considered in designing shafts, connecting rods, springs, gears, etc.

13. Hardness. It is a very important property of the metals and has a wide variety of meanings. It embraces many different properties such as resistance to wear, scratching, deformation and machinability etc. It also means the ability of a metal to cut another metal.

3. Запомните необходимый минимум профессиональной лексики:

Задание II

1. Определите по формальным признакам, какой частью речи являются следующие слова, и переведите их:

Observation, interaction, externally, elastic, permanent, sensible, commonly, essential, ability

2. Заполните следующую таблицу:

Задание III

1. Подберите эквиваленты к глаголам, обозначенным цифрами:

2. Подберите эквиваленты к словосочетаниям, обозначенным цифрами:

3. Составьте синонимичные пары:

4. Соотнесите термины с их определениями:

Задание IV

1. Найдите в тексте сказуемые в пассивном залоге.
2. Найдите в тексте инфинитивы и определите их функции.
3. Найдите в тексте примеры Причастия I в функции определения.

Задание V

1. Завершите предложение в соответствии с содержанием текста:

1. The internal resistance offered by a part to an externally applied force is called …

a) stress b) load c) impact

2. The toughness of the material decreases when it is …

a) rolled b) measured c) heated

3. When a part is subjected to a constant stress at high temperature for a long period of time, it will undergo …

a) fatigue b) creep c) fracture

2. Определите, какие из данных утверждений истинны, а какие ложны:

1. Steel is less elastic than rubber.

2. Copper and zinc are widely used ductile materials.

3. Toughness is desirable in parts subjected to shock and impact loads.

4. Steel is more easily machined than brass.

5. Creep is considered in designing shafts and gears.

6. Hardness embraces many other properties such as deformation and machinability.

3. Ответьте на вопросы к тексту:

1. How can the properties of metals be grouped?

2. What are the mechanical properties of metals associated with?

3. What is elasticity?

4. What are the commonly used malleable materials?

5. How is resilience measured?

Задание VI

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