Introduction
The knowledge of material and their properties is of great
importance for a design engineer. A design engineer must be familiar with the
effects which the manufacturing processes and heat treatment have on the
properties of the materials. The engineering materials are mainly classified as
1.
Metals and their alloys, such as iron, steel,
copper, aluminium etc.
2.
Non-Metals, such as glass, rubber, plastic etc.
The metals may further be classified as:
(a)
Ferrous metals; and (b) Non-ferrous metals.
The ferrous metals are those which
have the iron as their main constituent, such as cast iron, wrought iron and
steel.
The non-ferrous metals are those
which have a metal other than iron as their main constituent, such as copper,
aluminium, brass, tin, zinc, etc.
The important mechanical properties of
metals are as follows:
1.
Strength.
It is the ability of material to resist the externally applied forces
without breaking or yielding.
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 as 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 property of a material which
retains the deformation produced under load permanently. This property of
material is necessary for forgings, in stamping images on coins, and in
ornamental work.
5.
Ductility.
It is property of a material enabling it to be drawn into wire with the
application of a tensile force. A ductile material commonly used in engineering
practice (in order of diminishing ductility) are mild steel, copper, aluminium,
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. 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 malleablisty) are lead, soft
steel, wrought iron, copper and aluminium.
8.
Toughness.
It is the property of a material to resist fracture due to high impact
lads like hammer blows. The toughness of a material decreases when it is
heated. This property is desirable in parts subjected to shock and impact
loads.
9.
Resilience. It is 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.
10.
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
11.
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 is usually fine and microscopic
size. This property is considered in designing shafts, connecting rods,
springs, gears etc.
12.
Hardness. It is a very important property of the
metal 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. The hardness is usually
expressed in numbers which are dependent on the method of making the test.
