Mechancanical Test Lec3
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Transcript of Mechancanical Test Lec3
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Impact test
• Many machine parts are subjected to suddenly applied
load called impact blows/loads.
• An impact test signifies toughness of material, that is
ability of a material to withstand impact blow
• The capacity of a metal to withstand such blows without
fracture is known as an impact resistance or impact
strength
One must determine:
the impact energies the part can be expected to see in its
lifetime,
the type of impact that will deliver that energy, and then
select a material that will resist such impact energy` over
the projected life span.
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• There are many types of impact testing machines available
in the market. Following two types are important from the
subject point of view.
1. Charpy testing machine, and
2. Izod testing machine.
Charpy test
The charpy test is carried out on a specimen, which is
55mm x 10mm x 10mm in size and has a 2 mm deep notch
at its centre making an angle of 45°.
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• The specimen is placed horizontally as a simply support
beam between two anvils 40 mm apart in such a way that
the striking hammer strikes the specimen on the face
which is opposite to the notch.
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Izod test
• The Izod test is carried out on a specimen, which is
75mm x10mm x 10mm in size and has a 2mm deep
notch making an angle of 45°.
• The specimen is held vertically as a cantilever
between two jaws, in such a way that the striking
hammer strikes the specimen on the same face as that
of notch.
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Izod test
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• The energy used in rupturing the specimen in both Charpy
and Izod tests is calculated as follows:
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= 𝑾𝒉 = 𝑾(𝑹 – 𝑹𝒄𝒐𝒔𝜶) = 𝑾𝑹 (𝟏 – 𝒄𝒐𝒔𝜶)
= 𝑾𝒉’ = 𝑾 (𝑹 – 𝑹𝒄𝒐𝒔𝜷) = 𝑾𝑹 (𝟏 – 𝒄𝒐𝒔𝜷)
= 𝑾𝒉 – 𝑾𝒉’ = 𝑾𝑹 [(𝟏 − 𝒄𝒐𝒔𝜶) – (𝟏 – 𝒄𝒐𝒔𝜷)] = 𝑾𝑹 (𝒄𝒐𝒔𝜷 – 𝒄𝒐𝒔𝜶)
Initial energy
Energy after rupture
Energy used to rupture
specimen
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Factors Affecting Impact Resistance
• Dimension of the notch of the test specimen.
• Impact velocity.
• Temperature of specimen.
• Angle and shape of notch.
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Fatigue test
• The failure of a material, under repeatedly applied
stress, is called fatigue.
• Some of the machine parts such as axles, shafts
crankshafts, connecting rods springs, pinion teeth etc.
are subjected to varying stresses.
• It includes the variation in the intensity of the same
type of stress as well as different types of stresses (i.e.,
change of stress from tensile to compressive and vice
versa).
• The varying stresses may be broadly classified into
following four types:
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1. The stress varying between two limits of equal value, but
of opposite sign.
2. The stress varying between two limits of unequal values,
but of opposite sign.
3. The stress varying between zero and a definite value.
4. The stress varying between two limits of unequal values,
but of same sign.
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Creep test
• The continuous deformation of a metal, under a
steady load, is known as creep.
• This test is very essential to predict the working life
of some members or machine components which are
subjected to creep.
• The creep test is generally performed by applying a
static load to one end of the lever system. The other
end is attached to the specimen, under test in the
furnace, and held at constant temperature.
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• The axial deformation is read, periodically, throughout the
test. And the curve is plotted between extension (i.e.,
strain) along the vertical axis and time along the
horizontal axis.
• This procedure is repeated for different loads at the same
temperatures. The maximum permissible strain and
working life can be estimated from these curves.
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Factors affecting creep resistance
• Effect of grain size.
• Effect of strain hardening.
• Effect of heat treatment.
• Effect of alloying addition.
• Effect of manufacturing process.
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