Real-time reversible tunable elasticity in cellular solids via electromagnetic actuation

The ability for reversible, real-time control of elastic moduli in solids can find significant application in advanced mechanical components, protective structures, and biomedical devices. Here, we propose a novel concept for controlling the linear and nonlinear elastic properties of cellular structures via electromagnetically triggered mechanisms in the cellular solid. Three structural systems with orthotropic material properties were proposed and studied numerically, experimentally, and analytically. Using the proposed concept, the elastic modulus can be controlled over two to four orders of magnitude. The Poisson ratio of the isotropic structure can be varied from 0 to 0.5 continuously. The adjustments over nonlinear elastic (i.e., buckling) behavior of the structure are achieved by activation of supplementary cell walls in the lattice through electromagnetic actuation. Magnetic actuation will hamper the first symmetrical buckling pattern of the structure and force the structure to buckle according to a higher buckling pattern with smaller sinusoidal wavelength in the cell walls. The uniaxial buckling strength of the structure was tuned over two orders of magnitude

SpiderWeb honeycombs

A new class of hierarchical fractal-like honeycombs inspired by the topology of the “spiderweb” were introduced and their small and large deformations were investigated analytically, numerically, and experimentally. Small deformation elasticity results show that the isotropic in-plane elastic moduli (Young’s modulus and Poisson’s ratio) of the structures can be controlled over several orders of magnitude by tuning dimension ratios in the hierarchical pattern of spiderweb, and the response can vary from bending to stretching dominated. In large deformations, spiderweb hierarchy postpones the onset of instability compared to stretching dominated triangular honeycomb (which is indeed a special case of the proposed spiderweb honeycomb) and exhibits hardening behavior due to geometrical nonlinearity. Furthermore, simple geometrical arguments were obtained for large deformation effective Poisson’s ratio of first-order spiderweb honeycombs, which show good agreement with numerical and experimental results. Spiderweb honeycombs exhibit auxetic behavior depending on the nondimensional geometrical ratio of spiderweb

Multi-step self-guided pathways for shape-changing metamaterials

Multi-step pathways, constituted of a sequence of reconfigurations, are central to a wide variety of natural and man-made systems. Such pathways autonomously execute in self-guided processes such as protein folding and self-assembly, but require external control in macroscopic mechanical systems, provided by, e.g., actuators in robotics or manual folding in origami. Here we introduce shape-changing mechanical metamaterials, that exhibit self-guided multi-step pathways in response to global uniform compression. Their design combines strongly nonlinear mechanical elements with a multimodal architecture that allows for a sequence of topological reconfigurations, i.e., modifications of the topology caused by the formation of internal self-contacts. We realized such metamaterials by digital manufacturing, and show that the pathway and final configuration can be controlled by rational design of the nonlinear mechanical elements. We furthermore demonstrate that self-contacts suppress pathway errors. Finally, we demonstrate how hierarchical architectures allow to extend the number of distinct reconfiguration steps. Our work establishes general principles for designing mechanical pathways, opening new avenues for self-folding media, pluripotent materials, and pliable devices in, e.g., stretchable electronics and soft robotics.Comment: 16 pages, 3 main figures, 10 extended data figures. See https://youtu.be/8m1QfkMFL0I for an explanatory vide

Ultra-light hierarchical meta-materials on a body-centred cubic lattice

Modern fabrication techniques offer the freedom to design and manufacture structures with complex geometry on many lengthscales, offering many potential advantages. For example, fractal/hierarchical struts have been shown to be exceptionally strong and yet light (Rayneau-Kirkhope D. et al., Phys. Rev. Lett., 109 (2012) 204301). In this letter, we propose a new class of meta-material, constructed from fractal or hierarchical struts linking a specific set of lattice points. We present a mechanical analysis of this meta-material resulting from a body-centred cubic (BCC) lattice. We show that, through the use of hierarchy, the material usage follows an enhanced scaling relation, and both material property and overall efficiency can be optimised for a specific applied stress. Such a design has the potential of providing the next generation of lightweight, buckling-resistant meta-materials

A mechanical property evaluation of graded density Al-Si10-Mg lattice structures manufactured by selective laser melting

Metal components with applications across a range of industrial sectors can be manufactured by selective laser melting (SLM). A particular strength of SLM is its ability to manufacture components incorporating periodic lattice structures not realisable by conventional manufacturing processes. This enables the production of advanced, functionally graded, components. However, for these designs to be successful, the relationships between lattice geometry and performance must be established. We do so here by examining the mechanical behaviour of uniform and graded density SLM Al-Si10-Mg lattices under quasistatic loading. As-built lattices underwent brittle collapse and non-ideal deformation behaviour. The application of a microstructure-altering thermal treatment drastically improved their behaviour and their capability for energy absorption. Heat-treated graded lattices exhibited progressive layer collapse and incremental strengthening. Graded and uniform structures absorbed almost the same amount of energy prior to densification, 6.3±0.26.3±0.2 MJ/m3 and 5.7±0.25.7±0.2 MJ/m3, respectively, but densification occurred at around 7% lower strain for the graded structures. Several characteristic properties of SLM aluminium lattices, including their effective elastic modulus and Gibson-Ashby coefficients, C1 and α, were determined; these can form the basis of new design methodologies for superior components in the future

Mechanical metamaterials at the theoretical limit of isotropic elastic stiffness

Acknowledgements H.N.G.W. is grateful for support for this work by the ONR (grant number N00014-15-1-2933), managed by D. Shifler, and the DARPA MCMA programme (grant number W91CRB-10-1-005), managed by J. Goldwasser.Peer reviewedPostprintPostprintPostprintPostprin

The development of TiNi-based negative poisson's ratio structure using selective laser melting

AbstractThere is a growing interest in using additive manufacturing to produce smart structures, which have the capability to respond to thermal and mechanical stimuli. In this report, Selective Laser Melting (SLM) is used to build a Negative Poisson's Ratio (NPR) TiNi-based Shape Memory Alloy (SMA) structure, creating a multi-functional structure that could be used as reusable armour. The study assesses the influence of SLM process parameters (laser power, scan speed, and track spacing) on the microstructural and structural integrity development in a Ti-rich TiNi alloy, as well as the impact of the post-process homogenisation treatment on the microstructure and phase transformations. The builds generally shows stress-induced cracks and residual porosity, which could be minimised through process optimisation. Nonetheless, the homogenisation treatment is essential to reduce the fraction of Ti2Ni intermetallics, which are known to disturb the B19′-chemistry, and hence the required phase transformation temperatures. The optimum process parameters are finally used to fabricate NPR structures, which were mechanically tested to validate the Poisson's ratio predictions. A higher ductility was observed in the structures that have undergone the homogenisation treatment

3D printed hierarchical honeycombs with shape integrity under large compressive deformations

We describe the in-plane compressive performance of a new type of hierarchical cellular structure created by replacing cell walls in regular honeycombs with triangular lattice configurations. The fabrication of this relatively complex material architecture with size features spanning from micrometer to centimeter is facilitated by the availability of commercial 3D printers. We apply to these hierarchical honeycombs a thermal treatment that facilitates the shape preservation and structural integrity of the structures under large compressive loading. The proposed hierarchical honeycombs exhibit a progressive failure mode, along with improved stiffness and energy absorption under uniaxial compression. High energy dissipation and shape integrity at large imposed strains (up to 60%) have also been observed in these hierarchical honeycombs under cyclic loading. Experimental and numerical studies suggest that these anomalous mechanical behaviors are attributed to the introduction of a structural hierarchy, intrinsically controlled by the cell wall slenderness of the triangular lattice and by the shape memory effect induced by the thermal and mechanical compressive treatment

Phononic dispersion in anisotropic pseudo-fractal hyper-lattices

Fractal and pseudo-fractal microstructures have proved promising in increasing the range of detectable frequen-cies for devices used in the realm of electromagnetism. Due to mechanical-electrical duality it is conjectured theymay provideflexible solutions capable of closing/widening bandgaps and increasing tailorability in phononic lat-tices. Pseudo-fractal hyper-lattices have been considered in this work and different aspects of dispersion surfacemorphology and frequency band structure are studied. It has been observed that higher frequencies that can beexcited in the simple square lattice are almost the same as those in the pseudo-fractal structures, however;through introduction of higher levels the pseudo-fractal hyper-lattice presents new features not observable inthe ordinary lattice. By increasing the order of pseudo-fractal structure the number of degrees-of-freedom in-creases and dispersion surfaces morphologies change thus frequency gaps are eliminated. This phenomenoncan be of advantage for acoustic/phononic visibility/detectability e.g. in designing sensors. In the classical analogyto quantum level repulsion surfacesflatten which sufficiently decreases the sound group velocity in the pseudo-fractal structure, and can be used for numerous practical applications