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 specime n by a light mechanical impact (impulse excitation) and performs a sof tware-based analysis of the resulting vibration. The resonant frequenc ies f(r) of the test object are determined and, in the case of isotrop ic and regular shaped specimens, the elastic moduli are calculated. Th e internal friction value (Q(-1)) is determined for each f(r) as Q(-1) =k/(pi f(r)) with k the exponential decay parameter of the vibration c omponent of frequency f(r). A furnace was designed and equipped with a utomated impulse excitation and vibration detection devices, thus allo wing computer-controlled measurements at temperatures up to 1750 degre es C. The precision of the measured f(r) depends on the size and stiff ness 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 acc urate whenever the relation Q(-1)=k/(pi f(r)) holds. The precision of the Q(-1) measurement depends on the suspension or support of the spec imen, and on the specimen size. Since external energy losses are relat ively smaller for larger specimens, the lower limit of measurable Q(-1 ) extends from 10(-3) for small specimens (for example <1 g) down to 1 0(-5) with increasing specimen size. High temperature tests have shown that Q(-1) can be monitored up to values of about 0.1. (C) 1997 Ameri can Institute of Physics.