Tokutaro Hirone and Kazuo Kamigaki
Attenuation of longitudinal ultrasonic waves in polycrystalline pure aluminium rods (99.9%) was measured with impulse method at several frequencies between 2 and 25 megacycles per second with various heat treatments of rods. Due to the successive heat treatment the mean grain diameter of the aluminium rods made a growth from about 0.2 to 3 mm, as shown in photo 1. Measured values of attenuation are plotted in Fig. 3 as a function of frequency. As it shows, the value of attenuation changes greatly with the grain sizes of the specimens. When grain size is small, attenuation is nearly propotional to frequency and the proportional constant increases with grain diameter. For large grain size, however the attenuation curves seem to saturate at high frequency range. If the grain diameter becomes very large, attenuation reaches a constant value independent of frequency. Attenuation constant of the present experiment is considerably smaller than the previous results by Mason and McSkimin and Roth. Those changes of attenuation can be explained as follows. First, the attenuation of ultrasound in proportional range is due to elastic hysteresis loss. If the wave length becomes shorter, a considerable part of the attenuation is caused by the Rayleigh scattering and is proportional to the fourth power of frequency. If wave length becomes nearly equal to grain size, attenuation is proportional to the second power of frequency due to the change in phase of traveling sound at the grain boundaries. Last stage of constant attenuation will be explained as a result of reflection and refraction of the sound at the grain boundaries.
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