TOPIC 7. MATERIAL PROPERTIES

Text 1

 

1.Обратите внимание на перевод следующих слов и словосочетаний:

tensile strength - растягивающее усилие, предел прочности при отрыве;

tension - напряжение, напряжённость, натянутость;

compression - сжатие;

stretching forces - сила растяжения, сила натяжения;

crushing forces - разрушающие нагрузки;

extension -     вытягивание;

elongation - вытягивание; удлинение;

resist - сопротивляться, противиться;

compressive strength - компрессионная прочность, прочность на сжатие.

 

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TENSILE STRENGTH AND DEFORMATION

When materials are exposed to forces, such as tension (stretching forces ~0~) and

compression (crushing forces ~0~), they deform- that is, they change shape. The type of deformation depends on the type of force that is applied.

When a material is subjected to tension, its length will increase by a certain amount. This is called extension or elongation. It is especially important to understand the performance of materials in tension, as their tensile strength (ability to resist tension) is usually lower than their compressive strength (ability to resist compression).

 

Text 2

 

1.Обратите внимание на перевод следующих слов и словосочетаний:

elasticity - эластичность; упругость;

elastic material - упругий материал;

elastically deformed - упруго деформированный;

stiff - жёсткий;

brittle - хрупкий, ломкий;

significantly - значительно;

hammering - ковка;

rolling - прокатка;

malleable -     ковкий;

ductile – пластичный.

 

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ELASTICITY AND PLASTICITY

Some materials can extend significantly, but still return to their original shape. A material's ability to do this is called elasticity. Rubber is an example of a very elastic material- it can be elastically deformed to a considerable extent.

If a material has very low elasticity, and is strong, engineers say it is stiff. If a material has low elasticity and is weak, it is described as brittle- that is, it fractures (breaks, due to tension) very easily. Glass is an example of a brittle material.

Some materials can change shape significantly, but do not return to their original shape. We say these materials are plastic. Often, plasticity is described in specific terms. A material that can be plastically deformed by hammering or rolling- for example, lead (Pb)- is malleable. A material that can be drawn out (stretched) into a long length- for example, copper (Cu)- is ductile.

 

Text 3

 

1.Обратите внимание на перевод следующих слов и словосочетаний:

tensile testing - испытание на растяжение;

sample - образец;

bar - брусок, болванка (металла);

tensile force - растягивающая сила, сила растяжения;

limit of proportionality - предел упругости;

elastic limit - предел упругости;

yield point - предел вынужденной эластичности, точка текучести;

ultimate tensile strength - предельная прочность при растяжении;

fracture point - излом;

 

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STAGES IN ELASTIC AND PLASTIC DEFORMATION

The graph below shows the typical extension behaviour of ductile materials in tensile testing - where a sample bar is subjected to a progressively increasing tensile force.

Points 0-1. The extension of the bar is proportional to the increase in tension. For example, when tension increases by 10%, length increases by 10%.

Point 1. The bar reaches the limit of proportionality. Beyond this point, length begins to increase at a slightly greater rate than tension.

Point 2. The elastic limit is reached. Beyond this point, the bar will no longer return to its original length. In many materials, the elastic limit occurs almost immediately after the limit of proportionality.

Point 3. The bar reaches its yield point. Once it yields, it continues to increase in   length, even without a further increase in tension.

Point 4. This is the ultimate tensile strength (UTS) of the material. Beyond this point, a waist (a narrower section) appears at a point along the length of the bar, signalling that it is about to fracture.

Point 5 . This is the fracture point, where the bar breaks in two.

 

4. Дополните предложения словами: compression, deformation, elongation, extension, tension

1 A stretching force is called ................................ .

2 A crushing force is called ................................ .

3 Extension is also called ................................ .

4 Tension causes ................................ or ................................ .

5 Tension or compression cause ................................ .

 

Text 4

 

1.Обратите внимание на перевод следующих слов и словосочетаний:

hardness - прочность;

durability - выносливость, прочность;

scratch hardness -    твёрдость по царапанью;

abrasion resistance - сопротивление истиранию;

indentation hardness - сопротивление вдавливанию;

impact - удар.

 

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HARDNESS

The hardness of a material affects its durability- that is, how long it will last. Generally, hard materials are more durable than soft materials, because they are better at resisting wear progressively worsening damage - to their surfaces. Hardness can be defined in two main ways:

• Scratch hardness describes a material's ability to resist being scratched. Materials with a high degree of scratch hardness are said to have good abrasion resistance - they are good at resisting damage due to abrasion (the action of two surfaces being rubbed together).

• Indentation hardness describes a material's ability to resist indentations - that is, compressions in the surface of a material caused by impacts.

             

Text 5

 

1.Обратите внимание на перевод следующих слов и словосочетаний:

metal fatigue - усталость металла;

cyclic loads - циклическая нагрузка;

micro-cracking - образование микротрещин;

fatigue cracking - появление трещин от усталости;

fracture toughness - изломостойкость;

creep - ползучесть (металла).

 

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FATIGUE, FRACTURE TOUHNESS AND CREEP

  In aircraft construction, special attention must be paid to two materials problems that are well understood by mechanical and structural engineers. One is fatigue, often called metal fatigue in metals.

 This problem is caused by cyclic loads- forces that continually vary. In aircraft, the wings are affected by cyclic loading as they frequently flex, continually bending up and down due to air turbulence. The consequence of fatigue is micro-cracking – the formation of cracks too small to see with the eye, and which worsen over time. The speed at which fatigue cracking progresses depends on the material's fracture toughness. This is a measure of how easily cracks that have already formed continue to open up and increase in length.

Another problem is creep - where components become permanently deformed (stretched, for example), due to loads. Creep increases over time. The problem is made worse by heat, so is a major issue in engines, where both loads and temperatures are high.

Text 6

 

1.Обратите внимание на перевод следующих слов и словосочетаний:

thermal conductor - материал повышенной теплопроводности;

thermal insulator - теплоизоляционный материал;

thermal expansion - тепловое расширение;

linear expansion -    линейное расширение.

 

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BASIC THERMAL PROPERTIES

Some materials conduct (carry or transmit) heat better than others. Therefore, thermal conductivity varies, depending on the material. Copper, for example, is an excellent thermal conductor. Polystyrene, on the other hand, is an excellent thermal insulator (and so a very poor thermal conductor).

  As temperature increases, most materials expand (increase in size due to heating), and as temperature falls, they contract (decrease in size due to cooling). The extent to which expansion and contraction occur is measured by a material's coefficient of thermal expansion - that is, its change in size for a given change in temperature. The coefficient for aluminium, for example, is 0.000023. This means that for an increase in temperature of one degree Celsius, a one-metre length of aluminium will increase in length by 0.000023 metres. This figure can also be referred to as the coefficient of linear expansion, since it describes change in length (a linear measurement).

 

4. Дополните предложения словами: abrasion, durability, durable, hard, indentation, scratch, soft

The cutting wheel will be surrounded by transparent guards. These will allow the operator to see the cutting wheel at all times, and will shield the operator from flying metal fragments. The guards must therefore be constructed from material with a high degree of (1) ........... hardness, to protect it from impacts. As the guards will require regular cleaning, the action of wiping away metal fragments will result in (2) ..........  The guards must, therefore, have sufficient (3) .............. hardness in order to retain their transparency and ensure adequate (4) ............. .