Ph.D. Thesis Defense by Lei Wu
Tuesday, November 13, 2001

(Dr. Yves Berthelot, advisor)

"The Dynamic Properties of Voided Polymers"

Abstract

The main objective of the research was to develop a new technique to determine the dynamic elastic moduli and loss factors of elastomer composites that are used as damping materials. Accurate determination of these complex moduli is essential to predict the acoustic performance of vibrating structures coated with these damping elastomers. The method combines laser-based (LDV) measurements of the surface velocities of a vibrating sample with finite element analysis (FEA) of the dynamics of the effective material in which the material properties are parameters that must be adjusted to fit the data.  The research presented here focuses on the numerical issues associated with this complex inverse problem. First, the topography of the objective function to minimize was analyzed to characterize the sensitivity of the inverse technique. It was found that the technique is more robust if the Young and shear moduli are chosen as the parameters to be determined.  Second, the sensitivity of the technique to various experimental parameters was assessed. These parameters include the sample shapes, the location of the measurements of surface velocities, and the type of excitation of the sample and the frequency range. The technique was validated by using samples with known material properties and also by comparing the results with those obtained using the established one-dimensional resonant rod technique. Young and shear moduli (and loss factors) of voided elastomers were obtained for temperatures ranging from 7oC to 40oC and from 0 psi to 500 psi (34 atmospheres). However, for proprietary reasons, the actual values of the moduli reported here have been multiplied by an arbitrary factor. Finally, numerical simulations were made to assess the usefulness of the technique to characterize layered samples (two isotropic layers) as well as samples containing a large heterogeneity.