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In an effort to better understand the nature and the limitations of impact testing, the Charpy impact test was studied in detail via instrumentation of both the tester and the specimen. It was found that the energy associated with the impact testing of cemented carbide consists mainly of elastic bending energy and absorption of energy by the testing machine. The vibrational and plastic components are insignificant. The toss energy and energy necessary to create new surfaces can be fairly well calculated from other considerations. The tests have shown no rate effects in cemented carbides, and the state of stress in the beam is uniaxial tension. Thus the test does not describe the material's propensity for cleavage or brittle failure, but rather describes mainly the modulus and strength of cemented carbides (which are routinely and more easily measured in other ways), and the stiffness of the testing machine—all of which have very little bearing on the toughness of the cemented carbide. The components which describe this are the energy of plastic work and the energy necessary to create two new surfaces, and these consist of only a few percent of the total measured energy. The amount of energy which the machine absorbs will vary with the modulus and strength of the material being tested. In the case of a hard low-strength grade like Carboloy Grade 320, the anvil sees very little load before failure and thus the energy partitioning will be significantly different than that for a stronger grade.
The Charpy test conducted with the conditions in this study has little value in ascertaining the toughness of cemented carbide; in fact, results from it may be misleading. When cemented carbide is evaluated for toughness by impact testing of any kind, careful attention must be given to the aforementioned factors because they are encountered to a large extent in most, if not all, types of impact tests. Charpy impact-type tests on cemented carbide are essentially fast transverse-rupture strength tests on a “soft” test machine.
impact tests, toughness, cobalt containing alloys, evaluation, test equipment
Research metallurgist, General Electric Company, Detroit,