Micromechanics of Active Materials
Jiangyu Li
Postdoctoral Scholar
Division of Engineering and Applied Science
California Institute of Technology
Pasadena, CA 91125
Sponsored by the Dept. of Engineering Mechanics
Date: Thursday, April 5, 2001
Time: 2:00 p.m.
Place: W128 Nebraska Hall
Active materials are material systems that are capable of detecting internal and environmental changes, and responding by appropriate measures to adjust the structure, repair the damage, or in an extreme situation, retire the system. Over the last decade the concept of active materials has become widely accepted, and the Scientific American claimed that “They will soon be in everything from computers to concrete bridges”. In this talk, we address the fundamental mechanics and physics issues in active materials from a multiscale point of view of linking the macroscopic properties of active materials and their microstructural details.
The active materials we focus on are the piezoelectric and ferroelectric solids, widely used as sensors and actuators in transducer applications, and the approaches we adopt are micromechanics and energy minimization. The challenges of this class of materials, in terms of mechanics of materials, are their inherent anisotropy and multiple scale microstructure, as well as the non-local electrostatic energy associated with it. To address these difficulties, we first layout the foundation for the micromechanical modeling by (1) establishing a nonlinear homogenization theory of ferroelectric using G-convergence, and (2) solving the auxiliary inclusion and inhomogeneity problem in a magnetoelectroelastic solid where there are full anisotropic couplings between elastic, electric, and magnetic fields. We then investigate the structure-properties relationship of piezoelectric solids using micromechanics, including (1) the estimates of effective properties of piezoelectric polycrystal in terms of its texture, and (2) the establishment of variational bounds for piezoelectric composites. Finally, we study the spontaneous polarization and strain of ferroelectrics using the energy minimization with careful attention to crystallographic symmetry and polycrystal texture, and use the results to identify optimal microstructures for high-strain actuation.
