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Closed form solutions for all three modes of fracture for an infinite piezoelectric medium containing a center crack subjected to a combined mechanical and electrical loading were obtained. The explicit mechanical and electrical fields near the crack tip were derived, from which the strain energy release rate and the total potential energy release rate were obtained by using the crack closure integral. The suitability in using the stress intensity factor, the total energy release rate, or the mechanical strain energy release rate as the fracture criterion was discussed.

A new adaptive sandwich structure is constructed using the shear mode of piezoelectric materials. Governing equations for the proposed beam and its surface-mounted counterpart are derived based on the variational principle. Static solutions of a cantilever sandwich beam and its corresponding surface-mounted beam are obtained based on the derived general formulations. The theoretical formulations are verified by finite element analysis. Furthermore, stress distributions of the two types of adaptive beams are also theoretically investigated. It is shown that the sandwich construction offers many advantages over the conventional actuation structure.

The impact response behavior of initially stressed composite laminates is investigated using the finite element method. An experimentally established contact law is incorporated into the finite element program. The Newmark time integration algorithm is used for solving the time dependent equations of the plate and the impactor. Numerical results, including the contact force history, deflection, and strain in the plate, are presented. Effects of impact velocity, initial stress, and the mass and size of the impactor are discussed.

Static indentation tests are described for glass/epoxy and graphite/epoxy composite laminates with steel balls as the indentor. Beam specimens clamped at various spans were used for the tests. Loading, unluading, and reloading data were obtained and fitted into power laws. Results show that (1) contact behavior is not appreciably affected by the span; (2) loading and reloading curves seem to follow the 1.5 power law; and (3) unloading curves are described quite well by a 2.5 power law. In addition values were determined for the critical indentation, a cr ' which can be used to predict permanent indentations in unloading. Since a cr only depends on composite material properties, only the loading and an unloading curve are needed to establish the complete loading-unloading-reloading behavior., 17.