Abstract:

A laser-based numerical/experimental technique suitable for laboratory studies has been designed and tested to simultaneously measure the bulk and shear complex dynamic moduli of a viscoelastic sample excited harmonically in the 0.5--3 kHz range by a shaker. The method consists of measuring the dynamic response (amplitude and phase) of the sample at five surface points with noncontact laser Doppler interferometric probes measuring in-plane (tangential) and out-of-plane (normal) surface velocities. To determine the bulk and shear moduli as functions of frequency, the measured dynamic surface response is matched to the predictions obtained from a finite element model of the sample in which the two complex elastic moduli are the adjustable parameters. This inversion procedure is based on a classical multi-dimensional direction set method (Powell's method) of optimization theory. The results are based on measurements made in air under standard pressure and temperature conditions. However, work is under way to build a similar system that would allow for measurements at high static pressure and controlled temperature. [Work supported by ONR, Code 334.]