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The Beetle Elytron Plate (BEP) is a new class of biomimetic sandwich core that features excellent compressive strength, energy absorption capacity and flexural properties. These characteristics make BEPs suitable as potential replacements of classical honeycomb cores in sandwich panels. This work describes the behaviour of the in-plane engineering elastic constants of parametric BEP topologies for the first time. The beetle elytron cores configurations are simulated using Finite Element models, including full-scale models and representative unit cells with periodic boundary conditions for asymptotic homogenization. The models are also benchmarked against experimental results obtained from ASTM tensile tests related to the in-plane Young’s modulus, Poisson’s ratio and shear modulus. The benchmarked models are then used to perform a parametric analysis against the geometry characteristics of the cellular configurations. Results obtained from this work will provide a solid foundation for further research on BEP structures and expand their applications into wider engineering fields.

Two-dimensional metamaterials with patterns of perforations producing auxetic effects can exhibit variable and tailorable deformation mechanisms by varying the distributions of cells with different geometry parameters. The local homogenized Poisson’s ratio can be used as a way to establish as a link between local unit cell parameters and global deformations. We propose here a Poisson’s ratio-based unit cell distribution optimization method to design deformation patterns in a perforated structure. A plate-like structure with centresymmetric perforations is here divided into different regions with dissimilar unit cell topologies that possess different homogenized Poisson’s ratio values. All the unit cells belonging to the same region have equal geometry. A differential evolution (DE) algorithm is used to optimize the permutation and combination of the homogenized local Poisson’s ratios of the unit cells regions. A two-dimensional perforated structure that satisfies the required deformation pattern can be obtained by using the proposed method. Simulations and experiments show that the proposed approach can provide controllable shape changes of 2D perforated mechanical metamaterials under uniaxial tensile loading.

A full factorial design (2 231 ) has been used to investigate the effect of the use of sodium hydroxide fibre treatment, Portland cement and uniaxial pressure on the physical and mechanical properties of hybrid short coir fibre reinforced composites (HSCoirFRCs). The response variables considered in this work were the apparent density, porosity, tensile and flexural strength , the modulus of elasticity and the Charpy impact resistance. The alkali treatment contributed not only to reduce the apparent porosity , but also to increase the mechanical properties of the HSCoirFRCs. A reduction of the impact resistance and an increase of the apparent density was also identified after treatment. Cold pressing significantly affected the physical and mechanical properties of the HSCoirFRCs. Higher pressure levels enhanced the wettability of the fibres and, consequently, the mechanical performance of the composites. The incorporation of cement microparticles as a second reinforcement phase was however not effective, leading to decreased strength and an increased apparent density of the materials. The HSCoirFRC structure can be considered an economical and sustainable alternative for future secondary structural parts in lightweight transport applications.

This paper describes the vibroacoustic behavior of sandwich panels with a novel core topology made from 2-dimensionally gradient auxetic hexagonal honeycombs. The 2D gradient core enables a tailoring of localized mechanical properties of the sandwich structure in different regions of the panel. A homogenized finite element modeling has been used to initially determine the mechanical properties of the structures. The natural frequencies and the radiated sound power level of the sandwich plate with the homogenized properties have been calculated and verified with those obtained from a full-scale detailed model of the sandwich structure. The geometry of the 2-dimensionally gradient auxetic core has been then optimized using two different techniques in order to minimize the radiated sound power level over the frequency range of 0 to 200 Hz. The optimized design of the 2-D gradient core shows a remarkable reduction of the radiated sound power level for the sandwich structure when taking into account the mass of the panels. The results of this study provide new insights about the vibroacoustic behavior of hexagonal auxetic sandwich structures with complex core geometry.

A composite bi-stable structure can deform and maintain its stable states without the need for constant energy inputs. In this paper we describe a class of composite bi-stable shells that makes use of ad-hoc stacking sequences and pre-bending to modify the curvature of the plate. These composite structures possess two types of bi-stability (same direction and opposite direction). The same-direction bi-stable structure is characterized by the presence of two stable states that face the same direction. The same-direction bi-stable configuration can produce a final shape of the shell that effectively minimises the overall volume of the structure and can therefore be used in reconfigurable and stowable antennas. We also describe a procedure to extend the duration of the transition process between the stable states from ~ 1 s to ~ 5 s. This slow transition process can effectively reduce the potential damage to components attached to the bi-stable structure. The slow transition process described here is also relevant to further develop the technology readiness level of engineering applications involving multi-stable composites.

International audience; The work describes the out-of-plane properties of a curved wall honeycomb structure evaluated using analytical models and finite elements techniques. Out-of-plane properties are calculated using a theoretical approach based on energy theorems and validated using a fullscale

This work describes the out-of-plane bending performance, shape memory effect and variable stiffness of a zero Poisson’s ratio honeycomb structure made from the tessellation of re-entrant hexagons and thin plates. The re-entrant hexagons are fabricated with ABS plastics and the thin plates are made from thermosetting styrene-based shape memory polymers (SMPs). The hexagons and the SMP plates are bonded within the groove joints in the thickness direction of the re-entrant cell units. The re-entrant hexagons generate out-of-plane flatwise compressive stiffness and in-plane compliance, while the SMPs thin plates support out-of-plane flexibility, the shape memory effect and a variable bending stiffness. Because the ABS plastics possesses a significantly higher glass transition temperature than the SMPs, the ZPR honeycomb structure features a higher out-of-plane flexibility when the environmental temperature rises from room temperature to the glass transition temperature of the SMPs. On the contrary, the flatwise compressive stiffness of the ZPR honeycomb remains unchanged. Three-point bending tests have also been performed to determine the out-of-plane bending performance of the ZPR structures at varying temperatures.

Honeycomb structures are essentially constituted of a repetition of regularly-arranged and loaded sub-structures. The present study carries out a parametrically investigation of the behavior of a multi re-entrant honeycomb structure with variable stiffness and Poisson’s ratio effects. A refined analytical model is specifically developed and compared to full-scale numerical simulations. The analytical model developed is based on energy theorems and takes into full consideration bending, shearing and membrane effects. The influence of the cell walls thickness on the elastic homogenized constants is investigated. The results obtained show a good agreement between the refined analytical approach developed and the numerical computations carried out.

Referring to the basic principle of bi-stable structures and the design of continuous multi-stable composite surfaces, a new method for designing multi-stable structures is proposed in this paper. The multi-stable plate is divided into deformation and transition elements. By introducing transition elements of which the stiffness is appropriately tailored, the geometric compatibility problem of multi-stable composite structure can be reduced. New multi-stable structures with different sizes, layers, one-direction splicing and two-direction splicing are manufactured. The experimental results demonstrate that these new multi-stable structures not only reduce the geometric compatibility but also have better designability of layers and sizes. The principle of generating geometric compatibility and influence of transition element stiffness on multi-stable structures are analyzed, as well as the relationship between the number of deformed elements and the number of deformation states. The shapes of the new multi-stable structure can be any value in theory. In the experiments, the maximum number of shapes reached 10. The deformability is also enhanced in comparison with other multi-stable structures.

The work describes a concept of shape memory polymer (SMP) mandrel to manufacture a complex shaped filament wound carbon composite air duct. The mandrel is designed, fabricated and tested to verify the shape recovery and extraction characteristics for the manufacturing of the composite structure. A finite element model is developed to predict the shape recovery and overall deformation behavior due to the combination of pressure and temperature profiles used during the molding. The agreement between the deformations predicted by the finite element model and the ones measured on the SMP demonstrator are good, and show the feasibility of the smart material mandrel to be used in the production of composite structures with complex and ducted forms. A demonstrator of a PAN-carbon/epoxy filament wound composite air duct is manufactured, and the capability of the SMP mandrel verified through a hot gun extraction process.

The work describes a structural composite damper concept based on a chiral auxetic configuration. Chiral structures couple uniaxial and rotational deformations to provide a negative Poisson’s ratio behaviour and high dissipation through shear strain energy, and this feature is exploited by up-scaling the deformation mechanism of the chiral cell to design a damper that dissipates energy in the edgewise/shear modes, like the ones occurring in wind turbine blades. The damper concept and its configuration are evaluated through a series of Finite Element parametric and probabilistic models. A small-scale demonstrator is manufactured and subjected to compressive cyclic loading at increasing maximum displacements. Good agreement between the numerical and experimental force–displacement and energy dissipated-displacement curves is observed, showing the feasibility of the chiral composite damper concept for vibration damping-related applications at low frequencies.

This work describes the manufacture and characterisation of a PEEK-based zero Poisson’s ratio honeycomb (SILICOMB) produced using Kirigami-inspired cutting and folding techniques. The flatwise compression and transverse shear properties of the structure are determined through ASTM mechanical testing, and the results compared against commercially available honeycombs, including several other zero Poisson’s ratio cellular structures. An analytical model to predict the shear strength is compared to the results. SILICOMB specimens are found to have lower stiffness compared to other honeycomb configurations, but comparable strength. Factors influencing the results and variations to the manufacturing process are discussed.

The paper describes the equivalent thermal conductivities of iso-volume periodic honeycomb concepts with 3 and 4-connectivities along the in-plane and out-of-plane principal directions. A centre-symmetric 3-connectivity hexagonal layout and a centre-symmetric 4-connectivity cross-chiral configuration are considered. Analytical closed-form solutions of the equivalent thermal conductivities are derived and compared against Finite Element simulations, providing an excellent match. Parametric analyses show the influence of several geometry parameters of the unit cells over the homogenised thermal conductivities along the three principal directions. The analysis shows the capability of tailoring the core geometrical parameters of these iso-volume honeycomb concepts to achieve a desired thermal response in structural sandwich panels or core fillers, for which the coupled thermo-mechanical performance is paramount.

The work describes the numerical and experimental assessment of using biphase composite structures with non-classical shape inclusions. Star-shaped biphase cells have been designed, modeled and tested to evaluate the complex engineering constants corresponding to various deformation modes. A Finite Element homogenisation method using the complex modulus approach has been used to evaluate the variation of the storage moduli, loss factors and amounts of strain energy dissipated in the matrix versus the unit cell geometry parameters. Experimental results have been obtained on aluminium/cast epoxy sample using a shear dynamic test rig and a dynamic mechanical analyser. The results have been benchmarked against unit biphase composite configurations with cylindrical inclusions having the same contact surface between inclusion and matrix than the star-shaped reinforcements. The composite cells are intended for a possible use as structural damping units to be located in maximum nodal strain positions corresponding to specific wind turbine blade modes.

The introduction of material into the void of honeycomb-like structures, such as foam, viscoelastic or particulate filling, has been credited with improving the damping properties of the honeycombs. Optimisation of such damping inserts has been investigated, and indicates that partial occupation of the void could be more efficient, on a density basis, than full filling. The main goal of this study is to explore fully damping in honeycomb cells with inserts from the point of view of minimal increase in density and location of inserts. In this paper, damping of vibrations in the plane is investigated using analytical, finite element and topological optimisation methods to find the best locations of a damping insert within the cell.

This work describes a truss-core structure made of hemp/epoxy biocomposite based on a topology with auxetic (negative Poisson’s ratio) characteristics. Epoxy-based composites with 25 wt.% of hemp yarns have been manufactured and characterized with tensile and flexural tests. Truss-core hexagonal chiral panels with the biocomposite core have been produced using a Resin Transfer Molding technique. The panels have been subjected to standard transverse shear tests, and their properties predicted with a Finite Element nonlinear modeling. The results show that the hexachiral biocomposite truss core exhibits specific shear modulus and strength significantly higher compared to the ones observed in previous demonstrators made of polymeric core.

The paper describes the transverse shear properties of a novel centresymmetric honeycomb structure evaluated using analytical and finite element models. The cellular structure features a representative volume element (RVE) geometry allowing in-plane auxetic (negative Poisson’s ratio) deformations, and multiple topologies to design the honeycomb for multifunctional applications. The out-of-plane properties are calculated using a theoretical approach based on Voigt and Reuss bounds. The analytical models are validated using a full scale Finite Element technique to simulate transverse shear tests, a quarter FE of the RVE with periodic shear conditions and an FE homogenisation method for periodic structures. The comparison between the analytical and numerical models shows good convergence between the different set of results, and highlights the specific deformation mechanism of the multi re-entrant honeycomb cell.

In this work, the mechanical in-plane and thermal conductivity properties of a novel cellular configuration for multifunctional applications are described using analytical and finite element models. The hexagonal–rectangular honeycomb proposed allows one more geometric parameter in the unit cell compared to the classical hexagonal honeycomb configurations. The added geometric entity provides enhanced in-plane flexibility and tailoring of properties, such as the in-plane Poisson’s ratio, which can become negative under specific geometry configurations. The thermal conductivities are modeled using the electric-thermal analogy, allowing closed-form solutions for the thermal properties of the honeycomb. Comparison between analytical models and finite element simulations provides good convergence of results.