Piezoceramic hollow fiber active composites

While active fiber composites (AFC) based upon solid cross-section piezoelectric fibers are very useful for anisotropic activation of composites, they require high voltages and are constrained to non-conductive matrix materials. AFC’s based upon hollow cross-section piezoelectric fibers lower operating voltages and broaden the choice of possible matrix materials. This paper presents an investigation of the key design parameters for hollow piezoelectric fibers (matrix/fiber Young’s modulus ratio, aspect ratio of the individual fibers, and the overall active composite volume fraction) and their effect on the performance, manufacturing and reliability of active fiber composites. Because the ultimate objective in utilizing active composites centers on their ability to deform, optimal parameter values were identified employing analytical fiber and lamina strain/electric field models and limitations were determined utilizing an analytical embedment stress model. These models assume perfect fibers; as such, practical fabrication of the fibers will have a clear impact on these parameters. To assess this, standard machinery inspection criteria were used to evaluate fibers, manufactured through microfabrication by coextrusion (MFCX), with respect to their geometric (cross-sectional ovality, eccentricity, straightness) and material (density, porosity, piezoelectric properties, Young’s modulus) properties. These studies indicate that there are circumstances under which low aspect ratio (thin walled) fibers will be optimal, but clearly, reliability issues will arise. Unfortunately, little data exist on reliability of either hollow or solid piezoelectric fibers. To identify the primary failure mechanism of both solid and hollow fibers, ultimate strength, strain-to-failure and interfacial shear strength were examined using three types of experimental tests: (1) tensile strain-to-failure, (2) single fiber fragmentation, and (3) single fiber indentation. From this series of work, it is apparent there will always be design trade-offs present and it is important to consider performance, fabrication, and reliability issues simultaneously in the design of hollow piezoelectric fiber active composites.