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MECH PROPERITY ((Machinability. It is the property of a, material which…
MECH PROPERITY
Machinability. It is the property of a
material which refers to a relative case with
which a material can be cut. The machinability
of a material can be measured in a number of
ways such as comparing the tool life for cutting
different materials or thrust required to remove
the material at some given rate or the energy
required to remove a unit volume of the
material. It may be noted that brass can be
easily machined than steel.
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 are usually
fine and of microscopic size. This property is
considered in designing shafts, connecting rods, springs, gears, etc.
Resilience. It is the 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.
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.
expressed in numbers which are dependent on the method of making the test. The hardness of a metal
may be determined by the following tests :
(a) Brinell hardness test,
(b) Rockwell hardness test,
(c) Vickers hardness (also called Diamond Pyramid) test, and
(d) Shore scleroscope.
in parts subjected to shock and impact loads.
the point of fracture. This property is desirable
material has absorbed after being stressed upto
amount of energy that a unit volume of the
hammer blows. The toughness of the material decreases when it is heated. It is measured by the
Toughness. It is the property of a material to resist fracture due to high impact loads like
Ductility. It is the property of a material enabling it to be drawn into wire with the application
of a tensile force. A ductile material must be both strong and plastic. The ductility is usually
measured by the terms, percentage elongation and percentage reduction in area. The ductile material
commonly used in engineering practice (in order of diminishing ductility) are mild steel, copper,
aluminium, nickel, zinc, tin and lead.
The mechanical properties of the metals are those which are associated with the ability of the
material to resist mechanical forces and load. These mechanical properties of the metal include strength,
stiffness, elasticity, plasticity, ductility, brittleness, malleability, toughness, resilience, creep and
hardness. We shall now discuss these properties as follows:
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 malleability) are
lead, soft steel, wrought iron, copper and aluminium.
Plasticity. It is property of a material which retains the deformation produced under load
permanently. This property of the material is necessary for forgings, in stamping images on coins and
in ornamental work.
Elasticity. It is the property of a material to regain its original shape after deformation when
the external forces are removed. This property is desirable for materials used in tools and machines.
It may be noted that steel is more elastic than rubber.
Brittleness. It is the property of a material opposite to ductility. It is the property of breaking
of a material with little permanent distortion. Brittle materials when subjected to tensile loads, snap
off without giving any sensible elongation. Cast iron is a brittle material.
Hardness. It is a very important property of the metals 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
Strength. It is the ability of a material to resist the externally applied forces without breaking
or yielding. The internal resistance offered by a part to an externally applied force is called *stress.
Stiffness. It is the ability of a material to resist deformation under stress. The modulus of
elasticity is the measure of stiffness.