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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