Yongping Wan

Time dependence of piezoelectric d33 coefficient of cellular ferroelectret polypropylene film

Due to the inherent viscosity of polymer, piezoelectric response in the thickness direction (d33) of cellular ferroelectret films usually depends on the time of measurement. In this letter, the micromechanical theory of viscoelastic composite was extended to predict the time dependence of the overall piezoelectric d33 coefficient of voided charged polymer foam. Experiments were carried out to find the time spectra of piezoelectric d33 coefficient of voided charged polypropylene film. Theoretical simulation agrees well with experiment data.

Micromechanical prediction of the effective electromechanical properties of cellular ferroelectrets

Cellular space-charge foams, which have very sensitive transducer properties and are called cellular ferroelectrets, emerged as a kind of novel electromechanical transducer materials. In recent years, the understanding of charging is advanced significantly. However, some fundamental theoretical aspects, as to the relation between the electromechanical behavior and their mesostructures, are still far from clear. As is well known, the transducer coefficients in the thickness direction strongly depend on the relative density of foam, which is essentially relevant to the mesostructure, such as void shape, void fraction, etc. In this article, a theoretical model based on micromechanics is proposed to estimate the overall electromechanical moduli of cellular ferroelectrets. From the viewpoint of composites, in this model, the voids with surplus charge are deemed as heterogeneous inclusions, which show piezoelectriclike effect under deformation. The matrix is the nonpolar polymer. Within the micromechanical fram...

Permeability dependence of the effective magnetostriction of magnetostrictive composites

A model is presented in this article to investigate the dependence of effective magnetostriction of the magnetostrictive composites on the parameters of components including the elastic modulus, permeability, and volume fraction, etc. Concentrating on the two-component magnetostrictive composites and choosing the two components to be general magnetostrictive materials, this model yields, analytically, the effective magnetostriction of composites by means of the method of complex potential. In terms of the analysis of this model, the magnetostrictive composites can be roughly divided into two kinds. One kind is that the matrix material is nonmagnetic or a material with very low magnetostriction, in which the effective magnetostriction is independent of the permeability of components. Another kind is the case in which the two components have close magnetostriction, and the effective magnetostriction of the composites generally depends on the elastic and the magnetic parameters of both components and the vol...

Electro-elastic analysis of bonded piezoelectric finite wedges with two collinear interface cracks under anti-plane deformation

This paper studies two collinear impermeable interface cracks in bonded piezoelectric finite wedges. The radial edges are subjected to anti-plane concentrated forces and in-plane electric surface charges. The circular boundary is considered as traction free and electrically insulated. The finite complex transform developed in the literature for elastic wedges is extended to electro-elasticity for piezoelectric wedges. Based on the finite Mellin transforms and series expansions, the crack surface conditions are expressed analytically in the form of a set of simultaneous standard singular integral equations, which can be solved numerically by the Lobatto–Chebyshev method. The Normalized Intensity Factors (NIFs) around crack tips are obtained for the elastic and electric field in explicit forms. Dependence of the NIFs on the crack length and crack interval, as well as the wedge angle, is discussed in detail. Crack interaction is graphically demonstrated and analyzed. Results show that extension of one crack length generally enhances the NIFs for both tips of the other crack. Crack interval significantly affects the NIFs of the inner crack tips of the collinear cracks. The formulation and results in this paper include, as special cases, some wedge problems presented in the literature. These analyses are expected to provide some guidance for the design of piezoelectric composite wedges.

Anti-plane time-harmonic Green’s functions for a coated circular inhomogeneity in a piezoelectric medium with spring- or membrane-type imperfect interfaces:

Two-dimensional anti-plane time-harmonic dynamic Green’s functions for a coated circular inhomogeneity in an infinitely extended matrix with spring- or membrane-type imperfect interfaces are presented. The inhomogeneity, coating and matrix are all assumed to be piezoelectric and transversely isotropic. By using the Bessel function expansions, explicit solutions for the electromechanical fields induced by a time-harmonic anti-plane line force and line charge located in the unbounded matrix, the annular coating and the circular inhomogeneity are derived. The present solutions can recover the anti-plane Green’s functions for some special cases, such as the dynamic or quasi-static Green’s functions of piezoelectricity with perfect interfaces, as well as the dynamic or quasi-static Green’s functions for a two-phase composite with perfect or imperfect interfaces. By means of detailed discussions, selected calculated results are graphically shown to demonstrate the dependence of the electromechanical fields on the circular frequency and the interface properties as well as the coating and size of the inclusion.

Micromechanical models for the effective electromechanical properties of cellular piezoelectret polymer film

Cellular piezoelectret film is actually the voided charged polymer film. They show very large piezoelectric effect in the film thickness direction, and have great potential in the application of flexible electromechanical sensor. In this paper, cellular piezoelectret film is considered as a composite where the voids containing charges of opposite polarities are viewed as inclusions, while the polymer as matrix. We develop a micromechanical model to find the effective electromechanical moduli of cellular piezoelectret film. Sensitivity analysis is presented for effective electromechanical moduli with respect to void parameters. Furthermore, this micromechanical formulation is extended to include the viscoelastic deformation of polymer matrix. Temporal evolution of the effective properties, including elastic modulus and piezoelectric d33 coefficient in the film thickness, of cellular piezoelectret film can be obtained.