Impact toughness testing (Part 1).

From the moment of emergence of interest in viscosity of metals numerous and various test methods have been developed for measurement of this major property. Speaking about ability of metals to absorb energy, it is necessary to understand that metals absorb energy gradually. First of all, a certain amount of energy is spent for origin of a crack. Then additional energy for increase or distribution of such crack is required.

Some impact tests with a notch allow the crack propagation energy to be measured separately from the nucleation energy, while other methods simply provide information on the amount of total energy needed to initiate and propagate the crack. The engineer himself chooses the kind of tests that can give him the necessary information.

Though the set of test methods on the impact toughness testing is known, in the USA test of samples with V – figurative cut across Charpy is most widely applied. In this test the standard sample in the form of whetstone of metal 55 mm long and with the square section of 10 mm x 10 mm is used. On one of long sides of a sample with a machine way it is carefully put to V – figurative cut of 2 mm in depth. The radius has to be equal in top of a cut precisely 0.25 mm. The careful machining on this radius has crucial importance as small deviations can cause considerable dispersion of results of tests.

In those cases where the test sample of the metal is too small compared to a standard sample, samples having a smaller side in the cross section, usually three quarters, one half and one quarter of the standard value, 7.5 mm, 5.0 mm, 2.5 mm, respectively, are used.

After carrying out tests using samples of reduced cross-section, to calculate the impact strength it is necessary to use the coefficients specified in the standards.

After careful machining of the sample, it should be cooled to the specified test temperature, if it should be below room temperature. This can be done in a liquid or gaseous medium. For moderately low temperatures, ice water is often used, and if it is necessary to obtain a very low temperature, dry ice and alcohol are used. After stabilizing the temperature of the sample, it is removed from the low-temperature bath and quickly transferred to the support of the test machine (maximum 5 seconds).

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