Tension is a pulling force; for example, the force in a cable holding a weight. Under tension, a material usuallystretches, returning to its original length if the force does notexceed the material'selastic limit. Under larger tensions, the material does

Tension is a pulling force; for example, the force in a cable holding a weight. Under tension, a material usuallystretches, returning to its original length if the force does notexceed the material'selastic limit. Under larger tensions, the material does not returncompletely to its original condition, and under greater forces the materialruptures.

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 localized 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 gradualloosening 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 deformationeliminates the force causing the creep. Creepextended over a long time finally leads to the rupture of the material.

Mechanical properties of materials (part 2)

Vocabulary

absorb — поглощать

application — применение

brittle — хрупкий, ломкий

car body — кузов автомобиля

constituent — компонент

crack — трещина

creep resistance — устойчивость к ползучести

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

Density (specific weight) is theamount of mass in a unitvolume. It is measured in kilograms per cubic metre. The density of water is 1000 kg/ m³ but most materials have a higher density andsink 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. Theyield 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 theirabilityto be formed into the shape that is needed, such ascar body parts. Materials that are not ductile are brittle. Ductile materials canabsorb 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 agradual 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:

fibreglass — стекловолокно

fibre — волокно, нить

reinforced — упрочненный

expansion — расширение

matrix — матрица

ceramic — керамический

specific strength — удельная прочность

specific stiffness — удельная жесткость

anisotropic — анизотропный

The combinations of two or more different materials are called composite materials. They usually have unique mechanical and physical properties because they combine the best properties of different materials. For example, afibre-glass reinforced plastic combines the high strength of thin glassfibres with the ductility and chemical resistance of plastic. Nowadays composites are being used for structures such as bridges, boat-building etc.

Composite materials usually consist of synthetic fibres within a matrix, a material that surrounds and is tightly bound to the fibres. The most widely used type of composite material ispolymer matrix composites(PMCs). PMCs consist of fibres made of a ceramic material such as carbon or glass embedded in a plastic matrix. Usually the fibres make up about 60 per cent by volume. Composites with metal matrices or ceramic matrices are calledmetal matrix composites (MMCs) andceramic matrix composites (CMCs), respectively.

Continuous-fibre composites are generally required for structural applications. Thespecific strength(strength-to-density ratio) andspecific stiffness (elastic modulus-to-density ratio) of continuous carbon fibre PMCs, for example, can be better than metal alloys have. Composites can also have other attractive properties, such as high thermal or electrical conductivity and a low coefficient of thermalexpansion.

Although composite materials have certain advantages over conventional materials, composites also have some disadvantages. For example, PMCs and other composite materials tend to be highlyanisotropic— that is, their strength, stiffness, and other engineering properties are different depending on the orientation of the composite material. For example, if a PMC is fabricated so that all the fibres are lined up parallel to one another, then the PMC will be very stiff in the direction parallel to the fibres, but not stiff in the perpendicular direction. The designer who uses composite materials in structures subjected to multidirectional forces, must take these anisotropic properties into account. Also, forming strong connections between separate composite material components is difficult.

The advanced composites have high manufacturing costs. Fabricating composite materials is a complex process. However, new manufacturing techniques are developed. It will become possible to produce composite materials at higher volumes and at a lower cost than is now possible, accelerating the wider exploitation of these materials.

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Tension is a pulling force; for example, the force in a cable holding a weight. Under tension, a material usu­allystretches, returning to its original length if the force does notexceed the material'selastic limit. Under larger tensions, the material does

Tension is a pulling force; for example, the force in a cable holding a weight. Under tension, a material usuallystretches, returning to its original length if the force does notexceed the material'selastic limit. Under larger tensions, the material does