Essay on Theory of Impact Test

665 Words Oct 19th, 2013 3 Pages
The impact toughness (AKA Impact strength) of a material can be determined with a Charpy or Izod test. These tests are named after their inventors and were developed in the early 1900’s before fracture mechanics theory was available. Impact properties are not directly used in fracture mechanics calculations, but the economical impact tests continue to be used as a quality control method to assess notch sensitivity and for comparing the relative toughness of engineering materials.
The two tests use different specimens and methods of holding the specimens, but both tests make use of a pendulum-testing machine. For both tests, the specimen is broken by a single overload event due to the impact of the pendulum. A stop pointer is used to
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The striker, having been initially lifted to a specific height h1, and then released, swings against the specimen and breaks it. The striker continues its swing to the other side of the specimen to a height h2. Clearly the difference between the two heights multiplied by the weight of the striker corresponds to the amount of energy that is absorbed in fracture.
In the Izod impact test, the test piece is a cantilever, clamped upright in an anvil, with a V-notch at the level of the top of the clamp. The test piece is hit by a striker carried on a pendulum which is allowed to fall freely from a fixed height, to give a blow of 120 ft lb energy. After fracturing the test piece, the height to which the pendulum rises is recorded by a slave friction pointer mounted on the dial, from which the absorbed energy amount is read.
The principle of the test differs from that of the Izod test in that the test piece is tested as a beam supported at each end; a notch is cut across the middle of one face, and the striker hits the opposite face directly behind the notch.
When the results of a number of tests performed in different temperatures are plotted, ductile-to-brittle transition curves, as in Fig. 3, may be obtained. As the temperature is reduced through the transition range, the fracture surface changes from one having a 'fibrous' or 'silky'

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