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 fr 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 (Q−1) is determined for each fr as Q−1=k/(πfr) with k the exponential decay parameter of the vibration component of frequency fr. 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 fr depends on the size and stiffness of the specimen, and varies from the order of 10−3 (that is ±1 Hz at 1 kHz) in soft, high damping materials or light specimens, to values as precise as 10−5 (that is ±0.1 Hz at 10 kHz) in larger or stiffer specimens. The highly reproducible Q−1 measurements are accurate whenever the relation Q−1=k/(πfr) holds. The precision of the Q−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−1 extends from 10−3 for small specimens (for example <1 g) down to 10−5 with increasing specimen size. High temperature tests have shown that Q−1 can be monitored up to values of about 0.1.