Impulse excitation apparatus to measure resonant frequencies, elastic moduli, and internal friction at room and high temperature

This paper presents a new apparatus to measure elastic properties and internal friction of materials. The apparatus excites the test specimen by a light mechanical impact (impulse excitation) and performs a software-based analysis of the resulting vibration. The resonant frequencies $f_r$ of the test object are determined and, in the case of isotropic and regular shaped specimens, the elastic moduli are calculated. The internal friction value ${\mathrm{(Q}}^{\text{−1}})$ is determined for each $f_r$ as $Q^{\text{−1}}{\mathrm{=k/(\pi f}}_r)$ with $k$ the exponential decay parameter of the vibration component of frequency $f_r.$ A furnace was designed and equipped with automated impulse excitation and vibration detection devices, thus allowing computer-controlled measurements at temperatures up to 1750 °C. The precision of the measured $f_r$ depends on the size and stiffness of the specimen, and varies from the order of 10${}^{\text{−3}}$ (that is $\text{±1}$ Hz at 1 kHz) in soft, high damping materials or light specimens, to values as precise as 10${}^{\text{−5}}$ (that is $\text{±0.1}$ Hz at 10 kHz) in larger or stiffer specimens. The highly reproducible $Q^{\text{−1}}$ measurements are accurate whenever the relation $Q^{\text{−1}}{\mathrm{=k/(\pi f}}_r)$ holds. The precision of the $Q^{\text{−1}}$ measurement depends on the suspension or support of the specimen, and on the specimen size. Since external energy losses are relatively smaller for larger specimens, the lower limit of measurable $Q^{\text{−1}}$ extends from 10${}^{\text{−3}}$ for small specimens (for example $\text{<1}$ g) down to 10${}^{\text{−5}}$ with increasing specimen size. High temperature tests have shown that $Q^{\text{−1}}$ can be monitored up to values of about 0.1.