By combining the high-dielectric copper phthalocyanine oligomer ͑PolyCuPc͒ and conductive polyanline ͑PANI͒ within polyurethane ͑PU͒ matrix an all-organic three-component dielectric-percolative composite with high dielectric constant is demonstrated. In this three-component composite system, the high-dielectric-constant PolyCuPc particulates enhance the dielectric constant of the PU matrix and this combined two-component dielectric matrix in turn serves as the high-dielectric-constant host for the PANI to realize percolative phenomenon and further enhance the dielectric response. As a result, an electromechanical strain of 9.3% and elastic energy density of 0. Electroactive polymers ͑EAPs͒ are capable of converting energy in the form of electric charge and voltage to mechanical force and movement and vice versa. In recent years, several electroactive polymer actuator materials, whose responses are controlled by external electric fields, have generated considerable interest.1-4 However, one severe drawback of these EAPs is the very high operation electric field ͑Ͼ100 V/m͒ required to generate a high strain with elastic energy density much higher than those in the current available piezoelectric materials ͑elastic energy density Ͼ0.1 J/cm 3 ). 5,6 As has been pointed out in earlier publications, 7,8 one fundamental cause for such high fields is the low dielectric constant of these EAPs. From the energy conservation, the elastic energy density generated in an electroactive polymer cannot exceed the input electric energy density, which is K⑀ 0 E 2 /2 for a linear dielectric, where K is the dielectric constant, ⑀ 0 ϭ8.85ϫ10 Ϫ12 F/m, and E is the applied field. This simple fact points out the importance of raising the dielectric constant of the EAPs to achieve high electromechanical response with low applied fields.Recently high dielectric and electromechanical responses have been demonstrated in two-component composite actuator materials.7-9 Especially, a two-component percolative composite, fabricated by a combination of conductive polymer particulates ͓polyaniline ͑PANI͔͒ within the relaxor ferroelectric polymer matrix, raises the dielectric constant ͑Ͼ1000͒ substantially higher than the polymer matrix ͑ϳ50͒. 8,10,11 In composites of conductive fillers embedded in an insulation matrix, the variation of the dielectric constant of the composite with the concentration f of the conductive filler has been predicted to follow a critical behavior, 12,13

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A stress-driven model for the relation between the collagen morphology and the loading conditions in arterial walls is proposed. We assume that the two families of collagen fibers in arterial walls are aligned along preferred directions, located between the directions of the two maximal principal stresses. For the determination of these directions an iterative finite element based procedure is developed. As an example the remodeling of a section of a human common carotid artery is simulated. We find that the predicted fiber morphology correlates well with experimental observations. Interesting outcomes of the model including local shear minimization and the possibility of axial compressions due to high blood pressure are revealed and discussed.

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A variational procedure is developed for estimating the effective constitutive behaviour of polycrystalline materials undergoing high-temperature creep. The procedure is based on a new variational principle allowing the determination of the effective potential function of a given nonlinear polycrystal in terms of the corre­sponding potential for a linear comparison polycrystal with an identical geometric arrangements of its constituent single-crystal grains. As such, it constitutes an extension, to locally anisotropic behaviour, of the variational procedure devel­oped by Ponte Castañeda (1991) for nonlinear heterogeneous media with locally isotropic behaviour. By way of an example, the procedure is applied to the de­termination of bounds of the Hashin-Shtrikman type for the effective potentials of statistically isotropic nonlinear polycrystals. The bounds are computed for the special class of untextured FCC polycrystals with isotropic pure power-law viscous behaviour, first considered by Hutchinson (1976), in the context of a calculation of the self-consistent type. The new bounds are found to be more restrictive than the corresponding classical Taylor-Bishop-Hill bounds, and also more re­strictive, if only slightly so, than related bounds of the Hashin-Shtrikman type by Dendievel et al . (1991). The new procedure has the advantage over the self-consistent procedure of Hutchinson (1976) that it may be applied, without any essential complications, to aggregates of crystals with slip systems exhibiting dif­ferent creep rules - with, for example, different power exponents - and to general loading conditions. However, the distinctive feature of the new variational proce­dure is that it may be used in conjunction with other types of known bounds and estimates for linear polycrystals to generate corresponding bounds and estimates for nonlinear polycrystals.

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Wave propagation in hollow dielectric elastomer cylinders is studied. The quasi-static deformation of the tube owing to a combination of radial electric field and mechanical loading is determined first. Two combinations are accounted for, one at which the tube is free to expand in the axial direction, and another at which the tube is axially pre-stretched and restricted from elongating. Subsequently, longitudinal axisymmetric incremental motions are superposed on the underlying state. The governing equations in the tube and in the surrounding space are formulated and a numerical procedure is used in order to solve the resulting set of equations. The fundamental mode in the frequency spectrum is determined for thin, intermediate and thick wall tubes. The influences of the tube geometry, the mechanical pre-stretch and particularly the electric bias field are examined. An important observation is the ability to manipulate the propagation of the waves by adjusting the electromechanical bias field. This infers the use of dielectric elastomers in tubular configurations as active waveguides or isolators by a proper tuning of the electrostatic stimuli.