For accurately measuring the height of rebound of the hammer in rebound hardness test, the “rebounding time method”, in which the time of rebound is measured and converted into the height of rebound according to the equation of motion of the hammer, has been proposed. This method is adopted in the standard rebound hardness tester which is now being designed by the author, and an electronic device, which is capable of measuring the time interval between two successive impacts with an accuracy of 0.04 ms, has been developed. In order to examine the validity of the height deduced from the rebounding time, the direct measurement of the height is performed by means of a differential transformer device, simultaneously with the measurement of the time of rebound. A ferrite core is attached to the top of the hammer of a D-type Shore hardness tester and transformer coils are set in such a position that the core comes near the null position when the hammer is at the top of rebound. From the output voltage observed by a cathode-ray oscillograph when the hammer is at the top, height of rebound is obtained. The comparison of the height thus obtained with the height deduced from the rebounding time reveals that the latter is distinctly larger than the former, the difference being 0.02 to 0.06mm for the height of 4 to 12mm. The reason of the above discrepancy is discussed and it is found that more than half of the difference is ascribed to the time of impact, which is included in the time of rebound. If the correction of the impact time is applied to the measurement of the time of rebound, therefore, an accuracy of 0.025mm is expected for the rebounding time method.
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