Ligia Munteanu

A genetic algorithm for determination of the elastic constants of a monoclinic crystal

This paper describes a nonlinear inverse method which allows the determination of the second- and third-order elastic constants for a caesium dihydrogen phosphate lattice via ultrasonic velocity measurements. This analysis is based on a genetic algorithm. The efficiency and accuracy of the method and the influence of measurement errors are discussed.

On the sonic composites without/with defects

The sound attenuation in a sonic composite without/with point defects is studied using a new method that combines the features of the cnoidal method and the genetic algorithm. Acoustic scatterers are composed by piezoceramic hollow spheres of functionally graded materials—the Reddy and cosine graded hollow spheres. This method enables to obtain the dispersion relation for defect modes, and the prediction of the evanescent nature of the modes inside the band-gaps.

On three-dimensional spherical acoustic cloaking

Transformation acoustics opens a new avenue towards the design of acoustic metamaterials, which are materials engineered at the subwavelength scale in order to mimic the parameters in wave equations. The design of the acoustic cloaking is based on the property of equations being invariant under a coordinate transformation, i.e. a specific spatial compression is equivalent to a variation of the material parameters in the original space. In this paper, the sound invisibility performance is discussed for spherical cloaks. The original domain consists of alternating concentric layers made from piezoelectric ceramics and epoxy resin, following a triadic Cantor sequence. The spatial compression, obtained by applying the concave-down transformation, leads to an equivalent domain with an inhomogeneous and anisotropic distribution of the material parameters.

Determination of the second- and third-order elastic constants in Al from the natural frequencies

The Hamiltonian approach to the calculation of vibrational modes of elastic cubic objects is exploited, including the effect of higher-order elastic constants. A technique is presented for solving the inverse problem, i.e., the determination of second- and third-order elastic constants for face cubic centered crystals by inversion of natural frequencies data. An unconstrained minimization algorithm, using gradient methods, is used to numerically solve the inverse problem. The method is applied to the case of Al alloys, since they are particularly suitable for applications of the acoustoelastic effect, which requires a knowledge of the third-order elastic constants. The efficiency and the accuracy of the method and the influence of measurement errors are discussed.

On the Complexity of the Auxetic Systems

Two major levels of complexity are discussed in a way of understanding the structure and processes that define an auxetic system. The auxeticity and structural complexity is interpreted in the light of Cosserat elasticity which admits degrees of freedom not present in classical elasticity, i.e. the rotation of points in the material, and a couple per unit area or the couple stress. The Young modulus evaluation for a laminated periodic system made up of alternating aluminum and an auxetic material is an example of computing complexity.

A new class of sonic composites

Transformation acoustics opens a new avenue towards the architecture, modeling and simulation of a new class of sonic composites with scatterers made of various materials and having various shapes embedded in an epoxy matrix. The design of acoustic scatterers is based on the property of Helmholtz equations to be invariant under a coordinate transformation, i.e., a specific spatial compression is equivalent to a new material in a new space. In this paper, the noise suppression for a wide full band-gap of frequencies is discussed for spherical shell scatterers made of auxetic materials (materials with negative Poissons ratio). The original domain consists of spheres made from conventional foams with positive Poissons ratio. The spatial compression is controlled by the coordinate transformation, and leads to an equivalent domain filled with an auxetic material. The coordinate transformation is strongly supported by the manufacturing of auxetics which is based on the pore size reduction through radial compression molds.Transformation acoustics opens a new avenue towards the architecture, modeling and simulation of a new class of sonic composites with scatterers made of various materials and having various shapes embedded in an epoxy matrix. The design of acoustic scatterers is based on the property of Helmholtz equations to be invariant under a coordinate transformation, i.e., a specific spatial compression is equivalent to a new material in a new space. In this paper, the noise suppression for a wide full band-gap of frequencies is discussed for spherical shell scatterers made of auxetic materials (materials with negative Poissons ratio). The original domain consists of spheres made from conventional foams with positive Poissons ratio. The spatial compression is controlled by the coordinate transformation, and leads to an equivalent domain filled with an auxetic material. The coordinate transformation is strongly supported by the manufacturing of auxetics which is based on the pore size reduction through radial compr...

Nonlinear dynamics of the left ventricle.

The cnoidal method is applied to solve the set of nonlinear dynamic equations of the left ventricle. By using the theta-function representation of the solutions and a genetic algorithm, the ventricular motion can be described as a linear superposition of cnoidal pulses and additional terms, which include nonlinear interactions among them.

An inverse problem for the motion of blood in small vessels

In this paper, the blood motion in vessels with small radius is analyzed. The blood is modeled as a micromorphic fluid containing deformable material particles with 12 degrees of freedom: three translations, three rotations and six stretch and shears. Seven micromorphic viscosity coefficients are introduced as a function of the initial particle concentration and are reconstructed by a genetic algorithm based on experimental data.

On intrinsic time measure in the modeling of cyclic behavior of a Nitinol cubic block

In this paper, the cyclic behavior of a superelastic-plastic nitinol cubic block is described by using the Bouc?Wen model coupled to an intrinsic time measure other than clock time, which governs the behavior of the materials. As a consequence, the thermodynamic admissibility of the Bouc?Wen model is provided by the endochronic theory of plasticity. The role of the intrinsic time measure is described by capturing the stiffness and strength degradation and the opposite phenomena. Such behavior is due to the permanent-strain addition of residual martensite and alterations in the properties of the texture during phase transformation.

On the Beams with External Auxetic Patches

The paper discusses the behavior of beams with external damping patches made of auxetic materials. The damping force is modeled by using the nonlocal theory. Unlike the local models, the nonlocal damping force is modeled as a weighted average of the velocity field over the spatial domain, determined by a kernel function based on distance measures. The performance with respect to the eigenvalues is discussed next, in order to avoid resonance. The optimization is performed by determining the location of patches from maximizing the eigenvalues, or the gap between them.