Ultrasonic Attenuation in Steels at Low Temperatures

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  • 低温における鋼材内の超音波減衰
  • テイオン ニ オケル コウザイ ナイ ノ チョウオンパ ゲンスイ

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Abstract

Ultrasonic attenuation in solid is very sensitive to conditions under which the material is placed. Therefore the attenuation may be considered as a measuring factor of the properties of solid under the given condition. In the present paper, the result of measurement of the ultrasonic attenuation in normalized steel of 0.15, 0.4 and 0.55°C at low temperatures down to -180°C is reported and some conclusions are derived as to possible relation of the properties with the attenuation. The result shows that the attenuation decreases with the lowering of temperature and at around -60° to -80°C, the attenuation drops discontinuously by an appreciable amount, corresponding to the drop of Charpy's impact value of similar steels.<br>Extension of Maxwell-type model to continuous body leads to the wave equation<br>∂2u/∂t2=a2∂2u/∂x2+c∂3u/∂x2∂t, u=u(x, t)<br>where c is the coefficient of internal friction.<br>The solution of this equation with the boundary conditions<br>u(∞, t)=0<br>ux(0, t)=Acoswt<br>is given in the form of<br>u(x, t)=A0e-αxsin(wt-ηx)<br>where, A0=A√a2+c2ω2 and as the first approximation, α is proportional to w2 and c. The quadratic change of α with w agrees with the result of experiment within a certain range of frequency but, with specimens of lower carbon content, additional attenuation appears to superpose, resulting a maximum attenuation at a frequency. Since α decreases with the temperature fall, the coefficient of viscous friction c in steels decreases with the lowering of temperature, contrary to the viscous friction of fluid which generally increases with the temperature fall. This seems to indicate that the drop of absorbed energy of a shock at low temperatures cannot be explained by the decrease of internal viscous friction of Maxwell model.

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