Generation of Sound Bullets with a Nonlinear Acoustic Lens

Acoustic lenses are employed in a variety of applications, from biomedical imaging and surgery, to defense systems, but their performance is limited by their linear operational envelope and complexity. Here we show a dramatic focusing effect and the generation of large amplitude, compact acoustic pulses (sound bullets) in solid and fluid media, enabled by a tunable, highly nonlinear acoustic lens. The lens consists of ordered arrays of granular chains. The amplitude, size and location of the sound bullets can be controlled by varying static pre-compression on the chains. We support our findings with theory, numerical simulations, and corroborate the results experimentally with photoelasticity measurements. Our nonlinear lens makes possible a qualitatively new way of generating high-energy acoustic pulses, enabling, for example, surgical control of acoustic energy.Comment: 19 pages, 7 figures, includes supplementary informatio

An Isotropic Auxetic Structural Network with Limited Shear Stiffness

The chiral lattice is a unique structural network not symmetric to its mirror image, and with a negative Poisson’s ratio. Previous investigations have considered this structural network for the design of superior structural components with sandwich construction, but these were limited by the in-plane Poisson’s ratio predicted to be exactly -1. This paper presents estimates of the mechanical properties of the chiral lattice obtained from a multi-cell finite-element model. It is shown that the chiral lattice has a shear stiffness bound by that of the triangular lattice and it is very compliant to direct stresses. The minimum in-plane poisson’s ratio is estimated to be approximately -0.94

Numerical and Experimental Analysis of the Static and Dynamic Morphing Characteristics of a Chiral Core Airfoil

Aeroelastic tailoring requires structural compliance and thus often conflicts with stiffness requirements to carry prescribed aerodynamic loads. Recently however, the application of cellular structural concepts has suggested the potential to achieve compliance while conserving required load-carrying capacity. Among the proposed concepts, a chiral geometry in particular is a novel configuration which features an in-plane negative Poisson’s ratio which leads to a very high shear modulus, while maintaining some degree of compliance. In particular, the chiral geometry allows large continuous deformations of the airfoil assembly, with the constitutive material remaining in the linear region of its stress-strain curve. The ability to sustain large deformations without exceeding yield conditions is required to recover the original shape and to provide smooth deformations as required by aerodynamic considerations. In previous work, a coupled-physics model, comprising of simultaneous CFD and elastic analyses, is developed to investigate the influence of the chiral core geometry on the behavior of a given airfoil. The modification of geometric parameters defining the considered layout leads to significant variations in mechanical properties, which can be exploited to achieve various levels of compliance. The morphing capabilities of the proposed airfoil, quantified as camber changes, are evaluated for various design configurations of the internal core structure. Specifically, three such airfoils have been constructed to study the influence of core geometric parameters on the elastic behavior observed in numerical simulations. Experiments on the aforementioned airfoil samples are characterized by imposing large camber-wise deflections, via static loading, and measuring the resulting strain, both in the honeycomb core and in the airfoil profile. The experimental results confirm the ability of the airfoils to sustain large deflections while not exceeding yield strain limits, in addition to producing continuous deformations, which are critical for the implementation of aeroelastic tailoring

Static aeroelastic response of chiral-core airfoils

Extensive research is being devoted to the analysis and application of cellular solids for the design of innovative structural components. The chiral geometry in particular features a unique mechanical behavior which is here exploited for the design of 2D airfoils with morphing capabilities. A coupled-physics model, comprising computational fluid dynamics and structural analyses, investigates the influence of the chiral core on the aerodynamic behavior of the airfoil. Specifically, the model predicts the static deflection of the airfoil as a result of given flow conditions. The morphing capabilities of the airfoil, here quantified as camber changes, are evaluated for various design configurations of the core

Vibration isolation via linear and nonlinear periodic devices

The current manuscript deals with the design of passive mechanical filters for vibration attenuation a low frequencies. Traditionally, this has been addressed employing dissipation as the attenuation mechanism. While such strategy provides broadfrequency effectiveness, attenuation at any given frequency is modest. Mass and stiffness-modulated periodic systems, on the other hand, exploit dispersion as the attenuation mechanism and represent an alternative to dissipation-based devices. Attenuation due to dispersion may be significantly higher than what is afforded by dissipation-based systems within a design frequency rage. The proposed assemblies, however, are not easily tailored to filter lowe-frequency vibrations. To this end, embedding such periodic systems into an elastic matrix yields a high-pass mechanical filter with tunable stop bands were waves are not allowed to propagate. Significant improvements in performance moreover may be obtained if intrinsically nonlinear devices are adopted. Specifically, a strongly nonlinear medium such as ordered granular media supports a limited number of waveforms, resulting in an efficient mechanical filter. Results reported here, in fact, suggest matrix-embedded sphere chains as highly tunable mechanical filters for vibration attenuation

Numerical and experimental analysis of the static compliance of chiral truss-core airfoils

This paper presents an innovative wing profile featuring an internal truss-like structure of chiral topology. The chiral design is selected because of its unique deformation characteristics, which produce a theoretical, in-plane Poisson's ratio of -1. Such a Poisson's ratio yields a very high shear modulus, which in principle does not require the wing profile to be defined by a closed section or stressed-skin configuration. In addition, the peculiar deformation mechanism of the chiral configuration allows large decambering deflections to occur, with all the members of assembly behaving within the linear range of the material. Hence the proposed design combines large chordwise compliance and large in-plane shear stiffness. Such conflicting mechanical properties can be achieved through the proper selection of a limited number of geometric parameters defining the core configuration. The objective of the paper is to investigate the compliance characteristics of the airfoil. Two-dimensional profiles, designed according to results from previous investigations, are manufactured and tested to assess compliance and evaluate decambering deflection limits. The experimental analysis is guided by numerical models that account for deviations from the ideal configuration due to manufacturing limitations. Numerical and experimental results demonstrate the influence of core geometry on the compliance and confirm the ability of chiral-core airfoils to sustain large deflections while not exceeding yield strain limits

Global and local linear buckling behavior of a chiral cellular structure

This paper investigates the flat-wise compression behavior of an innovative cellular structure configuration. The considered layout has a hexagonal chiral geometry featuring cylinders, or nodes, joined by ligaments, or ribs. The resulting assembly is characterized by a number of interesting properties that can be exploited for the design of alternative honeycombs or cellular topologies to be used in sandwich construction. The flat-wise strength of the chiral geometry is investigated through classical analytical formulas for the linear buckling of thin plates and shells and a bifurcation analysis performed on a Finite Element model. The analytical expressions predict the global buckling behavior and the resulting critical loads, and can be directly compared with the results obtained from the Finite Element analysis. In addition, the Finite Element model predicts local buckling modes, which should be considered to evaluate the possible development of localized plasticity. A sensitivity study is performed to evaluate the influence of the geometry of the chiral structure on its buckling strength. The study shows that the considered topology can offer great design flexibility, whereby several parameters can be selected and modified to improve the flat-wise performance. The comparison with traditional, hexagonal centro-symmetric structural configurations concludes the paper and demonstrates the enhanced performance and the potentials of chiral noncentrosymmetric designs

Chiral hexagonal cellular sandwich structures: dynamic response

Periodic cellular configurations with negative Poisson's ratio have attracted the attention of several researchers because of their superior dynamic characteristics. Among the geometries featuring a negative Poisson's ratio, the chiral topology possesses a geometric complexity that guarantees unique deformed configurations when excited at one of its natural frequencies. Specifically, localized deformations have been observed even at relatively low excitation frequencies. This is of particular importance as resonance can be exploited to minimize the power required for the appearance of localized deformations, thus giving practicality to the concept. The particular nature of these deformed configurations and the authority provided by the chiral geometry, suggest the application of the proposed structural configuration for the design of innovative lifting bodies, such as helicopter rotor blades or airplane wings. The dynamic characteristics of chiral structures are here investigated through a numerical model and experimental investigations. The numerical formulation uses dynamic shape functions to accurately describe the behavior of the considered structural assembly over a wide frequency range. The model is used to predict frequency response functions, and to investigate the occurrence of localized deformations. Experimental tests are also performed to demonstrate the accuracy of the model and to illustrate the peculiarities of the behavior of the considered chiral structures

Dynamic response of chiral truss-core assemblies

Periodic cellular configurations with negative Poisson's ratio have attracted the attention of several researchers because of their superior dynamic characteristics. Among the geometries with a negative Poisson's ratio, the chiral topology features localized deformed configurations when excited at one of its natural frequencies. This is of particular importance as resonance can be exploited to minimize the power required for the appearance of localized deformations, thus giving practicality to the concept. The particular nature of these deformed configurations and the authority provided by the chiral geometry suggest the application of the proposed structural configuration for the design of innovative lifting devices, such as helicopter rotor blades or airplane wings. The dynamic characteristics of chiral structures are here investigated through a numerical model and experimental investigations. The numerical formulation uses dynamic shape functions to accurately describe the behavior of the considered structural assembly over a wide frequency range. The model is used to predict frequency response functions, and to investigate the occurrence of localized deformations. Experimental tests are also performed to demonstrate the accuracy of the model and to illustrate the peculiarities of the behavior of the considered chiral structures