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Tuesday, 22 September 2015

CHARPY TEST

               This test is more common than Izod test and it uses simply supported test piece of 10 mm X 10 mm section. The specimen is placed on supports or anvil so that the blow of striker is opposite to the notch.




The energy used in rupture the specimen in both Charpy and Izod tests is calculated as follows:

Initial energy = WH = W(R-R cos α) = WR (1- cos α )
Energy after rupture = WH1  = W(R-R cos β) = WR(1- cos β)
Energy used to rupture specimen = WH- WH1
                                                       WR (1-cos α) - WR (1-cos β)
                                                       = WR [(1-cos α) - (1-cos β) ]
                                                       = WR [cos β - cos α ]

Where, W = Weight of pendulum/strike
               H = Height of fall of center of gravity of pendulum/strike
               H1 = Height of rise of center of gravity of pendulum/strike
               α  = Angle of fall
               β = Angle of rise, and
               R = Distance from C.G of pendulum/striker to axis of rotation O.

Effect of important variables on impact strength:
  1. Angle of notch. There is no appreciable effect of notch angle until its value exceeds 60°
  2. Shape of the notch. As the sharpness of the notch increases the energy required to rupture the specimen deceases.
  3. Dimensions of the specimen. By decreasing the dimensions of the specimen the energy of rupture decreases.
  4. Velocity of Impact. The important resistance decreases above certain critical velocity, this varies from metal to metal.
  5. Specimen Temperature. The temperature of specimen for a particular metal, determines whether the failure will be brittle, ductile or mixed character.


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