Measurement of Anisotropic Elastic Moduli and Comparison with Equivalent Media Theories

Abstract

Anisotropic elastic moduli were determined for a layered glass plate-epoxy composite from oblique angle time-of-flight measurements using an immersion system. The composite consisted of thin glass plates bonded with a UV curing epoxy. The glass plates were 5cm square and 0.04cm thick. The epoxy bonds were very thin, comprising only 1.99% of the composite. Both the glass and the epoxy behave as isotropic materials in bulk form. However, once assembled to produce a heterogeneous structure (composite material) and probed with sufficiently long wavelengths (approximately ten times the thickness of the glass plates at 1 MHz), the stack of thin glass-epoxy layers appeared homogeneous but with anisotropic properties. The anisotropy exhibited by the glass-epoxy stack was transverse isotropy. Equivalent media theories are designed to predict behavior of homogeneous material combined in a heterogeneous structure. Consequently, with the measured properties of the constitutive bulk glass and epoxy and knowledge of their ratio in the layered stack, forward calculations can be made to determine the elastic moduli of the composite. Of the equivalent media theories available for calculating expected behavior in composite media, we consider a thickness-weighted averaging model as well as a theory which models the thin compliant epoxy bonds as a set of fractures. Anisotropic elastic moduli predicted using these two theories will be compared to the measured elastic moduli of the glass-epoxy stack.