Time reversal methods in acousto-elastodynamics

The aim of the article is to solve an inverse problem in order to determine the presence and some properties of an elastic “inclusion” (an unknown object, characterized by elastic properties discriminant from the surrounding medium) from partial observa- tions of acoustic waves, scattered by the inclusion. The method will require developing techniques based on Time Reversal methods. A finite element method based on varia- tional acousto-elastodynamics formulation will be derived and used to solve to solve the forward, and then, the time reversed problem. A criterion, derived from the reverse time migration framework, is introduced, to help use to construct images of the inclusions to be determined. Our approach will be applied to configurations modeling breast cancer detection, using simulated ultrasound waves

Source illusion devices for flexural Lamb waves using elastic metasurfaces

Metamaterials with the transformation method has greatly promoted the development in achieving invisibility and illusion for various classical waves. However, the requirement of tailor-made bulk materials and extreme constitutive parameters associated to illusion designs hampers its further progress. Inspired by recent demonstrations of metasurfaces in achieving reduced versions of electromagnetic cloaks, we propose and experimentally demonstrate source illusion devices to manipulate flexural waves using metasurfaces. The approach is particularly useful for elastic waves due to the lack of form-invariance in usual transformation methods. We demonstrate metasurfaces for shifting, transforming and splitting a point source with "space-coiling" structures. The effects are found to be broadband and robust against a change of source position, with agreement from numerical simulations and Huygens-Fresnel theory. The proposed approach provides an avenue to generically manipulate guided elastic waves in solids, and is potentially useful for applications such as non-destructive testing, enhanced sensing and imaging

On the gradient of the Green tensor in two-dimensional elastodynamic problems, and related integrals: Distributional approach and regularization, with application to nonuniformly moving sources

The two-dimensional elastodynamic Green tensor is the primary building block of solutions of linear elasticity problems dealing with nonuniformly moving rectilinear line sources, such as dislocations. Elastodynamic solutions for these problems involve derivatives of this Green tensor, which stand as hypersingular kernels. These objects, well defined as distributions, prove cumbersome to handle in practice. This paper, restricted to isotropic media, examines some of their representations in the framework of distribution theory. A particularly convenient regularization of the Green tensor is introduced, that amounts to considering line sources of finite width. Technically, it is implemented by an analytic continuation of the Green tensor to complex times. It is applied to the computation of regularized forms of certain integrals of tensor character that involve the gradient of the Green tensor. These integrals are fundamental to the computation of the elastodynamic fields in the problem of nonuniformly moving dislocations. The obtained expressions indifferently cover cases of subsonic, transonic, or supersonic motion. We observe that for faster-than-wave motion, one of the two branches of the Mach cone(s) displayed by the Cartesian components of these tensor integrals is extinguished for some particular orientations of source velocity vector.Comment: 25 pages, 6 figure

Parametric study of control of frequency banded behaviour of periodic pressurised composite structures

Periodic structures are very common in engineering, such as airplane fuselages and train rails. This periodicity has been observed to be the cause of banded frequency response after mechanical excitation. This response can be engineered so that noise and vibrations to be isolated or even annihilated. In addition to this, further methods of inducing band-gaps without weight penalty are of interest among the researchers. In this paper a parametric survey was conducted examining the impact of the core geometry and the pressure in the core cells on the suppression of the vibrations. An infinite composite sandwich beam with hollow and pressurised core cells as periodic band gap inducing factors was examined. The periodic theory was used to predict the effect of pressured core cells periodicity on wave propagation and band gaps generation. Three low order finite elements (FE) models were used in this survey, which consisted of a small section of the simple sandwich beam with homogeneous core, with hollow core and with pressurised hollow cor

Mechanics of nonlinear biomembranes: application to ophthalmology

Changes in the mechanics of the lens capsule of the eye arising from alterations of its native configuration can lead to undesirable clinical results. One example is the surgical introduction of a hole into the lens capsule and subsequent removal of the cloudy lens during cataract surgery. The adverse effect is secondary cataract on the posterior lens capsule, brought about by a sudden proliferation of lens epithelial cells in the region. Understanding the biomechanics of the anterior lens capsule is necessary in order to model its behavior under various physiological conditions and predict its response to alterations and perturbations such as those during cataract surgery. Such knowledge will help in the improvement of techniques during cataract surgery, and in the design of artificial intraocular lens. In this study we present, for the first time, results that demonstrate that the anterior lens capsule exhibits non-homogeneity and regionally varying anisotropy. We also compute stresses in the lens capsule due to normal loading conditions and procedures such as a capsulorhexis

Automatic generation of dynamic skin deformation for animated characters

© 2018 by the authors. Since non-automatic rigging requires heavy human involvements, and various automatic rigging algorithms are less efficient in terms of computational efficiency, especially for current curve-based skin deformation methods, identifying the iso-parametric curves and creating the animation skeleton requires tedious and time-consuming manual work. Although several automatic rigging methods have been developed, but they do not aim at curve-based models. To tackle this issue, this paper proposes a new rigging algorithm for automatic generation of dynamic skin deformation to quickly identify iso-parametric curves and create an animation skeleton in a few milliseconds, which can be seamlessly used in curve-based skin deformation methods to make the rigging process fast enough for highly efficient computer animation applications