4. Mechanical properties of metals

A stress is the force within a body which resists deformation due to an externally applied load. If this load acts upon a surface of unit area, it is called a unit force and the stress resisting it a unit stress. When an external force acts upon an elastic material, the material is deformed and the deformation is in proportion to the load. This distortion or deformation is strain.

Any material subjected to an external load is distorted or strained. Elastically stressed materials return to their original dimensions when the load is released if the load is not too great. The property of regaining the original dimen­sions upon removal of the external load is known as elasti­city. The elasticity of a metallic sub­stance is a resistance of its atoms to separation or compres­sion or rotation about one another, and thus is a fundamental property of the material. So elasticity is demonstrated as a function of atomic forces.

Yield Point is a point on the stress - strain curve at which the stress levels off or actually decreases while strain continues. The term is strictly applicable only to mild steels.

Ductility is the capacity of a metal to be per­manently deformed in tension without breaking.

Toughness has been defined as the property of absorbing considerable energy before fracture. The tough­ness of a metal is indicated by the amount of slip which may occur within the crystals without resulting in rupture of the metal.

Malleability is the property of a metal which permits permanent deformation by compression without rup­ture.

Brittleness implies sudden failure. It is the property of breaking without warning, i.e., without visible permanent deformation. Failure of metals and alloys under repeated or alternating stresses, too small to produce even a permanent deformation when applied statically, is called fatigue failure.

Failure by corrosion fatigue is a fati­gue failure in which corrosion has lowered the endurance limit by the formation of pits that act as centers for the development of fatigue cracks.

The quality of hardness is a combination of a number of physical and mechanical properties.

Task 1 Match the terms with their definitions

1.stress	A. capacity of a metal to be per­manently deformed in tension without breaking.
2.corrosion	B. pressure or tension exerted on a material object
3.ductility	C. ability to return to its original shape, size, and condition after it has been stretched.
4.brittleness	D. ability to be broken or shattered easily
5.elasticity	E. is the damage that is caused when something is corroded.

Task 2 Mark the sentences “True” or “False”

1. Elastically stressed materials return to their original dimensions when the load is released if the load is not too great.

2. Ductility is the property of a metal which permits permanent deformation by compression without rup­ture.

3. The quality of hardness is a combination of a number of physical and mechanical properties.

4. The property of regaining the original dimen­sions upon removal of the external load is known as brittleness.

5. A stress is the force within a body which cannot resist deformation due to an externally applied load.

Task 3 Match the beginnings of sentences to their endings

1. It is the property of breaking	A. is distorted or strained
2. When an external force acts upon an elastic material	B. without warning
3. Any material subjected to an external load	C. the material is deformed and the deformation is in proportion to the load.
4. Malleability is the property of a metal	D. absorbing considerable energy before fracture.
5. Toughness has been defined as the property of	E. which permits permanent deformation by compression without rup­ture.

Task 4 Fill in the gaps

Any material subjected to an external __¹ is distorted or strained. Elastically stressed materials __² totheir original dimensions when the load is released if the load is not too great. The __³ of regaining the original dimen­sions upon removal of the external load is known as elasti­city. The elasticity of a metallic sub­stance is a __⁴ of its atoms to separation or compres­sion or rotation about one another, and thus is a fundamental property of the material. So __⁵is demonstrated as a function of atomic forces.

A. property B. elasticity C. load D. resistance E. return