The author was interested in the piezoelectric solids, as a kind of functional
materials, in the early 1990s. At that time the concept of smart materials and
structures just emerged. It is very important to investigate the coupling effects of
the elastic field and electric field on the material behaviors, especially when the
material contains some inclusions and defects. Based on the research results, one
can predict the effective properties of smart composites. With the progresses of the
research activities related with the piezoelectric, ferroelectric, optoelectronic and
other functional materials, it is getting more and more clear that, the strain or
stress field distributions playa very important role in manipulating the electronic,
magnetic and optical properties of the materials. Definitely at present stage no
systematic theory has been established to deal with these problems.