Mechanics of bioinspired flexible composites: experiments, -simulations, and analytical solutions

Motivated by designing bioinspired flexible armor, we study deformable layered materials reminiscent of the structures present on teleost fish species (e.g., zebrafish Danio rerio and Arapaima gigas) [1]. These materials comprise soft matrix and stiff layers. The overlapping stiff scales are embedded in a soft tissue such that the composite material can provide protection while also undergoing large deformations when subjected to a penetrating loading (such as a bullet, knife, or a powerful animal bite). Moreover, the layered materials hold a great potential for a large variety of applications including noise reduction [2] and actuation [3]. Here, we analyze the influence of microstructure parameters on the performance of the composites. We derive an analytical solution for the multilayered -structure accounting for large deformations. The solution predicts the mechanical response of the media as a function of the layer inclination angle, constituent volume fractions and properties [1]. To capture the effects of localized deformation (e.g., in case of penetrating loading), we develop a finite element numerical model of the structure and loading conditions. Physical prototypes of the composites are fabricated by 3D printing. The prototypes are subjected to mechanical loadings and the local deformation mechanics of the layered structure are measured using digital image correlation. The measured mechanical response, macroscopic as well as local, is found to be in good agreement with the simulations as well as with analytical predictions. Moreover, the results provide a detailed picture of the composite deformation mechanisms, which consist of matrix shear, stiff plate rotation and bending, depending on the microstructural parameters and loadings. Understanding the key mechanisms and parameters is an important step towards designing materials with a large variety of functionalities. REFERENCES [1] Rudykh, S., Boyce, M.C. Analysis of elasmoid fish imbricated layered scale-tissue systems and their bioinspired analogues at finite strains and bending. IMA Journal of Applied Mathematics. 2014a (in press). DOI: 10.1093/imamat/hxu005 [2] Rudykh, S., Boyce, M.C. Transforming wave propagation in layered media via instability-induced wrinkling interfacial layer. Physical Review Letters. 2014b, 112, 034301 [3] Rudykh, S., Boyce, M.C. Transforming small localized loading into large rotational motion in soft anisotropically-structured materials. Advanced Engineering Materials. 2014c (in press)

Transforming small localized loading into large rotational motion in soft anisotropically-structured materials

Actuation of rotational motion in machines and robotics is generally achieved through highly engineered mechanical or electromechanical devices. As the field of soft robotics develops, there is an emerging and expanding need for novel actuation mechanisms. Here, we show the ability to transform small localized loading into large rotational motion via the design of soft anisotropically structured composite materials. The transformation mechanism governing the rotational actuation capitalizes on the underlying coupling of shear and normal modes of stress and strain in anisotropic materials together with the ability of the soft material to locally undergo large deformation [1, 2]. The transformation behavior is further shown to be highly tuneable through selection of the microstructure as demonstrated through simulations and through experiments on multimaterial 3D-printed prototypes of soft composite materials with layered microstructures [2]. The study provides guidelines for designing soft anisotropic materials with tailored performance. The mechanisms of large controllable actuation can be used for macro-, micro- and nanoactuators and sensors. The findings can be also used for developing simple techniques for obtaining information on anisotropy, and microstructures of materials at small scales. REFERENCES: [1] Rudykh, S., Boyce, M.C. Analysis of Elasmoid fish imbricated layered scale-tissue systems and their bioinspired analogues at finite strains and bending. IMA Journal of Applied Mathematics. 2014 (in press). DOI: 10.1093/imamat/hxu005 [2] Rudykh, S., Boyce, M.C. Transforming small localized loading into large rotational motion in soft anisotropically-structured materials. Advanced Engineering Materials. 2014 (in press)

Tunable phononic crystals via instability-induced interfacial wrinkling

We present a method to control wave propagation in highly deformable layered media by utilizing elastic instability-induced wrinkling of interfacial layers. The onset of a wrinkling instability in initially straight interfacial layers occurs when a critical compressive strain or stress is achieved [1]. Further compression beyond the critical strain leads to an increase in the wrinkle amplitude of the interfacial layer. This, in turn, gives rise to the formation of a system of periodic scatterers, which reflect and interfere with wave propagation. We demonstrate that the topology of wrinkling interfacial layers can be controlled by deformation and used to produce band-gaps in wave propagation and, hence, to selectively filter frequencies [2]. Remarkably, the mechanism of frequency filtering is effective even for composites with similar or identical densities, such as polymer–polymer composites. Because the microstructure change is reversible, the mechanism can be used for tuning and controlling wave propagation by deformation. REFERENCES [1] Li, Y., Kaynia, N., Rudykh, S., Boyce, M.C. Wrinkling of interfacial layers in stratified composites. Advanced Engineering Materials. 2013, 15(10), 921–926. [2] Rudykh, S., Boyce, M.C. Transforming wave propagation in layered media via instability-induced interfacial wrinkling. Physical Review Letters. 2014, 112, 034301

Homogenized estimates for soft fiber-composites and tissues with two families of fibers

The macroscopic response of hyperelastic fiber composites is characterized in terms of the behaviors of their constituting phases. To this end, we make use of a unique representation of the deformation gradient in terms of a set of transversely isotropic invariants. Respectively, these invariants correspond to extension along the fibers, transverse dilatation, out-of-plane shear along the fibers, in-plane shear in the transverse plane, and the coupling between the shear modes. With the aid of this representation, it is demonstrated that under a combination of out-of-plane shear and extension along the fibers there is a class of nonlinear materials for which the exact expression for the macroscopic behavior of a composite cylinder assemblage can be determined. The macroscopic response of the composite to shear in the transverse plane is approximated with the aid of an exact result for sequentially laminated composites. Assuming no coupling between the shear modes, these results allow to construct a closed-form homogenized model for the macroscopic response of a fiber composite with neo-Hookean phases. A new variational estimate allows to extend these results to more general classes of materials. The resulting explicit estimates for the macroscopic stresses developing in composites and connective tissues with one and two families of fibers are compared with corresponding finite element simulations of periodic composites and with experimental results. Estimates for the critical stretch ratios at which the composites loose stability at the macroscopic level are compared with the corresponding numerical results too. It is demonstrated that both the primary stress–strain curves and the critical stretch ratios are in fine agreement with the corresponding numerical results

Mechanical Behavior of Bio-Inspired Nacre-Like Composites: A Hybrid Multiscale Modeling Approach

Abstract In this paper, the mechanical behavior of bio-inspired nacre-like staggered composites is studied. The bio-inspired materials, combining stiff and soft constituents, exhibit superior mechanical properties. Here, the attention is focused on the competing properties: penetration resistance and flexibility of the composites. To this end, a novel hybrid multiscale method is developed, combining a hierarchical multiscale approach with a concurrent approach. The method allows to perform accurate parametric nonlinear analyses at a low computational cost. The influence of the microstructural parameters (i.e., platelet aspect ratio and volume fraction) on the macroscopic mechanical behavior is thus analyzed. Finally, the potential of achieving tailored protective properties and flexibility through microstructural design of the bio-inspired composites is illustrated

Soft Magnetoactive Laminates:Large Deformations, Transverse Elastic Waves and Band Gaps Tunability by a Magnetic Field

We investigate the behavior of soft magnetoactive periodic laminates under remotely applied magnetic field. We derive explicit formulae for the induced deformation due to magnetic excitation of the laminates with hyperelastic magnetoactive phases. Next, we obtain the closed-form formulas for the velocities of long transverse waves. We show the dependence of the wave velocity on the applied magnetic intensity and induced strains, as well as on the wave propagation direction. Based on the long wave analysis, we derive closed form formulae for the critical magnetic field corresponding to loss of macroscopic stability. Finally, we analyze the transverse wave band gaps appearing in magnetoactive laminates in direction normal to the layers. We illustrate the band gap tunability – width and position – by magnetically induced deformation

Emerging topics in nanophononics and elastic, acoustic, and mechanical metamaterials:An overview

This broad review summarizes recent advances and “hot” research topics in nanophononics and elastic, acoustic, and mechanical metamaterials based on results presented by the authors at the EUROMECH 610 Colloquium held on April 25–27, 2022 in Benicássim, Spain. The key goal of the colloquium was to highlight important developments in these areas, particularly new results that emerged during the last two years. This work thus presents a “snapshot” of the state-of-the-art of different nanophononics- and metamaterial-related topics rather than a historical view on these subjects, in contrast to a conventional review article. The introduction of basic definitions for each topic is followed by an outline of design strategies for the media under consideration, recently developed analysis and implementation techniques, and discussions of current challenges and promising applications. This review, while not comprehensive, will be helpful especially for early-career researchers, among others, as it offers a broad view of the current state-of-the-art and highlights some unique and flourishing research in the mentioned fields, providing insight into multiple exciting research directions