Mechanical properties and energy absorption of heterogeneous and functionally graded cellular structures

AbstractThe crushing behaviour and energy absorption of honeycombs made of a linear elastic-perfectly plastic material with constant and functionally graded density were studied up to large crushing strains using finite element simulation. Our numerical simulations showed three distinct crushing modes for honeycombs with a constant relative density: quasi-static, transition and dynamic. Moreover, irregular cellular structures showed to have energy absorption similar to their counterpart regular honeycombs of same relative density and mass. To study the dynamic crushing of functionally graded cellular structures, a relative density gradient in the direction of crushing was introduced in the computational models by a gradual change of the cell wall thickness. Decreasing the relative density in the direction of crushing was shown to enhance the energy absorption of honeycombs at early stages of crushing. We also developed detailed finite element models of a three-dimensional closed-cell rhombic dodecahedron structure subjected to dynamic crushing. We specifically quantified the distribution of plastic strain and energy absorption of the cellular structure and provided a comparison with the results obtained in analysis of 2-D cellular structures. The results provide new insight into the behavior of engineered and biological cellular materials, and could be used in development of a new class of energy absorbent cellular structures

Harnessing instability to control wave propagation in phononic crystals and acoustic metamaterials

Artificially structured composite materials have the ability to manipulate the propagation of elastic waves due to the existence of band gaps, i.e., frequency ranges of strong wave attenuation. However, most configurations proposed to date cannot be tuned after the manufacturing process. We propose new strategies using elastic buckling mechanisms to design novel devices with in-situ adaptive properties that can be reversibly tuned. Buckling and large deformations can be effectively exploited to reversibly tune not only the width and location of band gaps, but also the directional preferences of the wave propagation, even for low-frequency elastic waves. Our proof-of-concept demonstrations also indicate that the proposed mechanisms work robustly over a wide range of length scales, opening avenues for the design of smart systems for applications, such as vibration/noise reduction, wave guiding, frequency modulation, and acoustic imaging

Non-Linear Mechanics of Three-Dimensional Architected Materials; Design of Soft and Functional Systems and Structures

In the search for materials with new properties, there have been significant advances in recent years aimed at the construction of architected materials whose behavior is governed by structure, rather than composition. Through careful design of the material's architecture, new mechanical properties have been demonstrated, including negative Poisson's ratio, high stiffness to weight ratio and mechanical cloaking. However, most of the proposed architected materials (also known as mechanical metamaterials) have a unique structure that cannot be recon figured after fabrication, making them suitable only for a specific task. This thesis focuses on the design of architected materials that take advantage of the applied large deformation to enhance their functionality. Mechanical instabilities, which have been traditionally viewed as a failure mode with research focusing on how to avoid them, are exploited to achieve novel and tunable functionalities. In particular I demonstrate the design of mechanical metamaterials with tunable negative Poisson ratio, adaptive phononic band gaps, acoustic switches, and reconfigurable origami-inspired waveguides. Remarkably, due to large deformation capability and full reversibility of soft materials, the responses of the proposed designs are reversible, repeatable, and scale independent. The results presented here pave the way for the design of a new class of soft, active, adaptive, programmable and tunable structures and systems with unprecedented performance and improved functionalitiesEngineering and Applied Sciences - Engineering Science

Wave propagation in random fiberous networks

Random fiberous networks are ubiquitous in different length scales with a broad range of applications including biological tissues, paper, polymer transistors, protective clothing and packaging materials. Given the importance of fiber networks, their static behavior has been extensively studied and it has been shown that network deformation is nonaffine for compliant, low-density networks and affine for stiff, high-density networks. However, little is known about the dynamic response of fibrous systems. In this study, we investigated numerically the propagation of small-amplitude elastic waves in these random networks and characterize their dynamic response as a function of network parameters. Interestingly, our numerical analysis revealed that the low-frequency response of these fiberous networks is highly affected not only by the network parameters, but also by the wavelength of the propagating waves

Reconfigurable origami-inspired acoustic waveguides

International audienceWe combine numerical simulations and experiments to design a new class of reconfigurable waveguides based on three-dimensional origami-inspired metamaterials. Our strategy builds on the fact that the rigid plates and hinges forming these structures define networks of tubes that can be easily reconfigured. As such, they provide an ideal platform to actively control and redirect the propagation of sound. We design reconfigurable systems that, depending on the externally applied deformation, can act as networks of waveguides oriented along one, two, or three preferential directions. Moreover, we demonstrate that the capability of the structure to guide and radiate acoustic energy along predefined directions can be easily switched on and off, as the networks of tubes are reversibly formed and disrupted. The proposed designs expand the ability of existing acoustic metamate-rials and exploit complex waveguiding to enhance control over propagation and radiation of acoustic energy, opening avenues for the design of a new class of tunable acoustic functional systems

Wave propagation in cross-linked random fiber networks

We numerically investigate the propagation of small-amplitude elastic waves in random fiber networks. Our analysis reveals that the dynamic response of the system is not only controlled by its overall elasticity, but also by the local microstructure. In fact, we find that the longest fibersegment plays a key role in dynamics when the network is excited with waves of short wavelength. In this case, the Bloch modes are highly non-affine as the longest segments oscillate close to their resonances. Based on this observation, we predict the low frequency dispersion curves of random fiber networks.Engineering and Applied Science

Bioinspired kirigami metasurfaces as assistive shoe grips

© 2020, The Author(s), under exclusive licence to Springer Nature Limited. Falls and subsequent complications are major contributors to morbidity and mortality, especially in older adults. Here, by taking inspiration from claws and scales found in nature, we show that buckling kirigami structures applied to footwear outsoles generate higher friction forces in the forefoot and transversally to the direction of movement. We identified optimal kirigami designs capable of modulating friction for a range of surfaces, including ice, by evaluating the performance of the dynamic kirigami outsoles through numerical simulations and in vitro friction testing, as well as via human-gait force-plate measurements. We anticipate that lightweight kirigami metasurfaces applied to footwear outsoles could help mitigate the risk of slips and falls in a range of environments

Honeycomb phononic crystals with self-similar hierarchy

We highlight the effect of structural hierarchy and deformation on band structure and wave-propagation behavior of two-dimensional phononic crystals. Our results show that the topological hierarchical architecture and instability-induced pattern transformations of the structure under compression can be effectively used to tune the band gaps and directionality of phononic crystals. The work provides insights into the role of structural organization and hierarchy in regulating the dynamic behavior of phononic crystals, and opportunities for developing tunable phononic devices.Engineering and Applied Science

Reconfigurable origami-inspired acoustic waveguides

We combine numerical simulations and experiments to design a new class of reconfigurable waveguides based on three-dimensional origami-inspired metamaterials. Our strategy builds on the fact that the rigid plates and hinges forming these structures define networks of tubes that can be easily reconfigured. As such, they provide an ideal platform to actively control and redirect the propagation of sound. We design reconfigurable systems that, depending on the externally applied deformation, can act as networks of waveguides oriented along one, two, or three preferential directions. Moreover, we demonstrate that the capability of the structure to guide and radiate acoustic energy along predefined directions can be easily switched on and off, as the networks of tubes are reversibly formed and disrupted. The proposed designs expand the ability of existing acoustic metamaterials and exploit complex waveguiding to enhance control over propagation and radiation of acoustic energy, opening avenues for the design of a new class of tunable acoustic functional systems.Engineering and Applied Science