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Study of the attenuation of elastic waves in metals. Hasegawa, Henry S.

Abstract

The primary purpose of this thesis is to determine experimentally if the attenuation of small -amplitude elastic waves in metals is governed principally by a linear mechanism (i.e. one for which the principle of superposition is valid). The secondary purpose is to interpret the attenuation measurements in terms of existing theories on acoustic dissipation in solids. Attenuation measurements of the Fourier components of a Rayleigh pulse were compared with those of sinusoidal Rayleigh waves of the same frequency. One copper, one aluminum and two α-brass circular cylindrical shells were used, and Rayleigh waves propagating along the truncated edges of these tubes were studied. Rayleigh pulses were detected at strain levels of approximately four and forty microstrain in order to test for any amplitude-dependent effects accompanying the attenuation. The sinusoidal Rayleigh waves were detected at strain amplitudes between three and ten microstrain. For three out of the four tubes the results indicated that the dominant attenuation mechanism is a linear process in the frequency range from 100 to 500 kc/s and in the strain region from four to forty microstrain. For the copper tube, however, the scatter in the results is such that no definite conclusion could be drawn. For all four tubes the internal friction, 1/Q, increases with frequency. For some of them there is evidence of a relaxation peak, probably as a result of the Zener effect, superimposed on the general trend. Dislocation damping, as proposed by Koehler (1952) and later generalized by Granato and Lucke (1956), is suggested as a dissipative mechanism which could account for this general trend. Most of the internal friction values are found to be between 20 X 10¯⁵ and 100 X 10¯⁵.

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