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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 framework for piezoelectric composites, the generalized Eshelby tensor is obtained for both isotropic and anisotropic matrix. The equivalent inclusion method is employed to treat inhomogeneities. Similar to the classical differential scheme for pure elasticity, the differential equation for the effective electromechanical moduli is derived, from which the overall behavior of cellular space-charge foam can be numerically solved. Detailed analysis is presented with respect to material parameters and geometrical variables of void. Finally, this model is used to simulate the inflation experiments of cellular ferroelectret film published in literature. Results show this model is capable of predicting the extremum of the effective electromechanical moduli. In the meantime, quantitative comparison indicates that theoretical simulation is in good consistency with experiment data.

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The magnetic‐induction field in the vicinity of an elliptical inclusion embedded in an infinite soft ferromagnetic medium is determined based on complex potential theory. By using a constitutive relation of magnetostriction for isotropic materials, the stress field in the vicinity of an elliptical flaw is obtained. Furthermore, the stress field at the tip of a slender elliptical crack is determined for the case in which only an external magnetic field perpendicular to the major axis of the ellipse is applied at infinity. The results indicate that the stress field in the neighbourhood of the tip is governed by the magnetostriction and permeability of the soft ferromagnetic material. The induction magnetostrictive modulus is a key parameter in determining which of the two mechanisms, i.e., magnetostriction and magnetic‐force‐induced deformation, is dominant in determining the stress field in the neighbourhood of the tip of a crack‐like flaw. With regard to the influence of the magnetic field on the apparent toughness of a soft ferromagnetic body with a crack‐like flaw, soft ferromagnetic materials can be roughly divided into two categories: one possesses a large induction magnetostrictive modulus and the other has a small modulus. An approximate criterion for categorizing the materials is presented. For the benefit of engineering design, the expressions of the stress‐intensity factor for these two categories of soft ferromagnetic materials are presented. The results show that the stress‐intensity factor is affected not only by the flaw geometry, but also by the permeability of the medium inside the flaw.

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This article presents an overview of recent progress on magnetomechanical deformation and fracture of functional ferromagnetic materials. Following a brief introduction of the classical magnetoelasticity and the magnetomechanical behavior of traditional ferromagnetics, recent development on the deformation and fracture of soft ferromagnetic materials and the mechanics of ferromagnetic composites is critically reviewed. Also included are the authors' own works both on experimental testing and theoretical modeling of soft ferromagnetics, ferromagnetic composites, and shape memory ferromagnetic alloys. This review article cited 162 references.

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.