To perform Impact Test on Metal Specimens

To perform Impact Test on Metal Specimens

Objective:

To study the impact resistance of metals using Impact testing machine of the Charpy type, to study the behavior of brittle and ductile materials.

Apparatus:

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  1. Impact testing machine.
  2. Scale.
  3. Standard specimens.

Related Theory:

It gives the measure of toughness of a material. Impact strength is defined as the resistance of the materials to shock. The impact testing is to find out the energy absorbed by a specimen when brought to fracture by hammer blow and gives a quality of the material, particularly its brittleness. Highly brittle materials have low impact strength. Heat treatment of metals has found to lower impact resistance considerably.

Forms of test

There are two forms of this test

  1. Charpy test
  2. Izod Test

Procedure and concept behind the two tests is same, the only difference is the type of machine used. The students will be given practice only on Charpy Machine.

Types of Load

Static Load

A load which does not change its position, magnitude and direction is termed as static load. For example self weight of any body.

Dynamic Load

A load which changes its position, magnitude and direction is termed as dynamic load. For example load in elevators, cranes etc.

Impact Load

Sudden application of a large magnitude of load for a very short period of time is termed as impact load. For example Landing of Aero planes, impact of moving vehicle on expansion joints, movement of railways, removal of form work.

Procedure:

The test piece is a square bar of material, 10 mm x 10 mm x 55 mm, containing a notch cut in the middle of one face. The notch is a 45 vee, 2 mm deep, with a root radius of 0.25 mm. the test piece is simply supported at each end on anvils 40 mm apart. A heavy pendulum is supported at one end in a bearing on the frame of the machine, and a striker is situated at the other end. The pendulum in its initial raised position has energy of 150 joule for small hammer and 300 joule for large hammer and on release swings down to strike the specimen immediately behind the notch, bending and fracturing it between the supports. A scale and pointer indicate the energy absorbed during fracture.

The following procedure has been followed:

  1. Note down the dimensions of the specimen and find the working area of the specimen at the place of notch.
  2. With no specimen on the anvil raise the pendulum to an initial reading R1 in the dial and release it.
  3. Note the reading R2 of the pointer on the dial. The difference is the energy loss due to friction.
  4. Now place the specimen accurately in position on the anvil.
  5. Raise the pendulum to the same initial height and release. The pendulum swings to the other side rupturing the specimen.
  6. Note the reading R3 on the pointer on the dial.
  7. Tabulate the readings.
  8. The absorbed energy required to produce two fresh fracture surfaces will be recorded in the unit of Joule.

As the pendulum is raised to a specific position, the potential energy (mgh) equal to approximately 300J is stored. The potential energy is converted into the kinetic energy after releasing the pendulum. During specimen impact, some of the kinetic energy is absorbed during specimen fracture and the rest of the energy is used to swing the pendulum to the other side of the machine. The greater the height of the pendulum swings to the other side of the machine, the less energy absorbed during the fracture surface. This means the material fractures in a brittle manner. On the other hand, if the absorbed energy is high, ductile fracture will result and the specimen has high toughness.

Observations:

Dimensions of material:

Area of the specimen =

Length of side without considering notch =

Size of the Notch =

Aluminum test piece =

M.S test pieces =

Calculations:

Material Temperature Frictional Loss (Joules) Area of Specimen (mm-sq) Energy of Rupture (Joules) Net energy of rupture
Brass
M.S. (Mild Steel)

Precautions:

  1. Keep away from the machine during the impact test.
  2. The test piece has to be carefully kept so that area of impact is minimized.
  3. When the pendulum hammer is loaded at maximum position make sure that it is latched.

 

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