Thermoelastic damping of the low density metals AZ91 and DISPAL

Abstract

Light metals have high thermoelastic internal friction due to high coefficients of thermal expansion which has to be considered for various structural damping investigations. The thermoelastic effect of a powder metallurgically produced aluminium composite DISPAL™ and a magnesium casting alloy AZ91 (9 wt.% Al, 1 wt.% Zn) was investigated by measurements of the logarithmic decrement of free vibrations of bending beams. Because of inhomogenous stress distribution, a temperature gradient occurs between the compressed and the extended surface of the specimen, which leads to a transverse heat flow. This causes damping that is sensitive to the frequency of vibration. When the time of stress reversal equals the time necessary for heat flow from the compressed to the extended regions, a maximum of the logarithmic decrement occurs. The damping was measured as a function of the specimen thickness and the frequency of vibration ranging from 10 to 130 Hz. The frequency range was realised by both, a variation of the thickness and the mass at the end of the bending beams. By drawing the logarithmic decrement versus the product of the frequency and the quadratic thickness, a Debye peak is obtained which overlays all other damping mechanisms. The thermal diffusivity was calculated by determining the position of the maximum of the peak. The height of the peak, Young's modulus and the specific heat of each sample were used to calculate the coefficient of thermal expansion of the specimens. The values agree, within the limits of scatter, with values measured by a dilatometer and taken from the literature, respectively.