Read the text and decide whether the statements that follow are true or false

READING

LEAD-IN

MECHANICAL PROPERTIES

UNIT 12

1. Look at the objects in the pictures. What materials do you think they are made of and why?

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2. In pairs, discuss what is meant by mechanical properties. What mechanical properties do you know?

Mechanical Properties

Mechanicalproperties are those associated with how a material reacts when subjected to external forces. Force will cause material to become stressed. Stress relates to how much load is applied to a given area. Strain is another result of the application of force. This is defined as the amount of distortion (extension or compression) that takes place per unit of length.

Strength is the ability of a material to withstand force without breaking or permanently bending.

Elasticity is the ability of a material to bent and flex when subjected to a force and then to return to its previous form when the force is removed. Structures such as bridges and tall building must have a degree of elasticity.

Plasticity is the ability of a material to be changed permanently in shape (deformed) by the application of an external force without breaking or fractures. Most materials become more plastic when heated. The extent to which a material's shape can become changed by hammering, rolling or pressing is called malleability. Another associated term is ductility; this is the ability to undergo cold plastic deformation by bending or stretching. All ductile materials are malleable, but all malleable materials are not necessarily ductile.

Hardness is the ability to resist abrasive wear, indentation and deformation. This is the important property of cutting tools such as turning tools, drills, files and saws.

Toughness is the ability to withstand sudden stress in the form of shocks or blows without fracturing, and also to resist cracking when subjected to bending forces. Repeated bending will cause the toughness of some materials to break down such that they will eventually fracture.

Brittleness is the opposite of toughness. A brittle material will not withstand shocks or bending without fracture.

Durability is the ability to withstand wear, tear and deterioration with time. Durability refers to both the mechanical properties and the appearance of the surface, particularly of metals, is corrosion. Plastic materials are generally less prone to corrosive effects and are therefore durable.

Stability is the ability to resist changes in shape and size over time. Wood is particularly unstable and tends to warp and twist with changes in humidity. Metals and some plastics tend to gradually deform when subjected to stress over long periods; this process is known as creep.

1. All malleable materials are ductile, but all ductile materials are not necessarily malleable.

2. Plastics are generally less prone to corrosive effects and are therefore durable.

3. Structures such as bridges and tall building must have a degree of stability.

4. Stabilityis the ability to resist changes in form and size over time.

5. Chemicalproperties are those associated with how a material reacts when subjected to external forces.

6. Strengthis the ability to resist abrasive wear, indentation and deformation.

7. Strengthis the ability of a material to withstand power without breaking or permanently bending.

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Дата добавления: 2014-11-29