Chrystel D L Remillat

The bending of single layer graphene sheets: the lattice versus continuum approach

The out-of-plane bending behaviour of single layer graphene sheets (SLGSs) is investigated using a special equivalent atomistic-continuum model, where the C-C bonds are represented by deep shear bending and axial stretching beams and the graphene properties by a homogenization approach. SLGS models represented by circular and rectangular plates are subjected to linear and nonlinear geometric point loading, similar to what is induced by an atomic force microscope (AFM) tip. The graphene models are developed using both a lattice and a continuum finite element discretization of the partial differential equations describing the mechanics of the graphene. The minimization of the potential energy allows us to identify the thickness, elastic parameters and force/displacement histories of the plates, in good agreement with other molecular dynamic (MD) and experimental results. We note a substantial equivalence of the linear elastic mechanical properties exhibited by circular and rectangular sheets, while some differences in the nonlinear geometric elastic regime for the two geometrical configurations are observed. Enhanced flexibility of SLGSs is observed by comparing the nondimensional force versus displacement relations derived in this work and the analogous ones related to equivalent plates with conventional isotropic materials.

Evaluation of hexagonal chiral structure for morphing airfoil concept

Abstract In this paper a concept of hexagonal chiral honeycomb is proposed as a truss-like internal structure for adaptive wing box configurations. In contrast with classical centresym-metric cellular structures like rectangular or hexagonal grids, the proposed honeycomb did not present inversion symmetry, and featured an in-plane negative Poissons ratio behaviour. The cellular structure considered exhibited this Poissons ratio behaviour under a large range of strain. A set of numerical (finite element, FE) simulations have been carried out in order to correct the initial theoretical predictions to take into account axial, shear and elastic deformations of all elements composing the unit cell when subjected to uniaxial loading. The homogenized linear elastic mechanical properties were then introduced in an FE wing box model of a racecar wing coupled to a panel code to simulate unidirectional static fluid—structure coupling between the wing box and the flow surrounding the airfoil. The cellular solid proposed as the internal layout of the wing box allowed conforming deformations with the external flow, giving a variation of the camber line and trailing edge displacement, and acting as an aileron

Advances in damping materials and technology

Abstract In the continual search for better damping materials and technologies, significant advances have been made of late. Functionally gradient materials, liquid crystal polymers, magnetostrictive materials and plasma deposited damping coatings are some of the novel materials and technologies being investigated in the Dynamics Research Group at the University of Sheffield. This article presents an overview of the work being carried out in these areas.

SILICOMB PEEK Kirigami cellular structures: mechanical response and energy dissipation through zero and negative stiffness

The work describes the manufacturing, testing and parametric analysis of cellular structures exhibiting zero Poisson?s ratio-type behaviour, together with zero and negative stiffness effects. The cellular structures are produced in flat panels and curved configurations, using a combination of rapid prototyping techniques and Kirigami (Origami and cutting) procedures for PEEK (Polyether Ether Ketone) thermoplastic composites. The curved cellular configurations show remarkable large deformation behaviours, with zero and negative stiffness regimes depending also on the strain rate applied. These unusual stiffness characteristics lead to a large increase of energy absorption during cyclic tests.

Curved Kirigami SILICOMB cellular structures with zero Poisson’s ratio for large deformations and morphing

The work describes the design, manufacturing, and parametric modeling of a curved cellular structure (SILICOMB) with zero Poisson’s ratio produced using Kirigami techniques from polyetheretherketone films. The large deformation behavior of the cellular structure is evaluated using full-scale finite element methods and experimental tests performed on the cellular samples. Good agreement is observed between the mechanical behavior predicted by the finite element modeling and the three-point bending compression tests. Finite element simulations have also been used to perform a parametric analysis of the stiffness against the geometry of the cellular structures, showing a high degree of tailoring that these cellular structures could offer in terms of minimum relative density and maximum stiffness. The experimental results also show high levels of strain-dependent loss factors and low residual deformations after cyclic large deformation loading.

Coupled thermomechanics of single-wall carbon nanotubes

The temperature-dependent transverse mechanical properties of single-walled nanotubes are studied using a molecular mechanics approach. The stretching and bond angle force constants describing the mechanical behavior of the sp2 bonds are resolved in the temperature range between 0 and 1600 K, allowing to identify a temperature dependence of the nanotubes wall thickness. We observe a decrease of the stiffness properties (axial and shear Young’s modulus) with increasing temperatures, and an augmentation of the transverse Poisson’s ratio, with magnitudes depending on the chirality of the nanotube. Our closed-form predictions compare well with existing molecular dynamics simulations.

Vibro-acoustic properties of auxetic open Cell foam: Model and experimental results

The combined mechanical and acoustic properties of auxetic (negative Poisson’s ratio) foams are described both from a numerical and experimental point of view. Samples of open cell PU-PE foams with negative Poisson’s ratio are produced using a dedicated manufacturing process, and subjected to tensile quasi static and cyclic loading, as well as sound absorption measurements based on ISO 10-534-2 Standard. A homogenization model based on the Biot’s theory is also derived to calculate the poroelastic parameters of the foam. The experimental and numerical results are compared and commented to provide explanations regarding the unusual acoustic absorption of these porous materials.

Lamb wave propagation in negative Poisson's ratio composites

Lamb wave propagation is evaluated for cross-ply laminate composites exhibiting through-the-thickness negative Poissons ratio. The laminates are mechanically modeled using the Classical Laminate Theory, while the propagation of Lamb waves is investigated using a combination of semi analytical models and Finite Element time-stepping techniques. The auxetic laminates exhibit well spaced bending, shear and symmetric fundamental modes, while featuring normal stresses for A0 mode 3 times lower than composite laminates with positive Poissons ratio.

Morphing nacelle inlet lip with pneumatic actuators and a flexible nano composite sandwich panel

We present a hybrid pneumatic/flexible sandwich structure with thermoplastic (TP) nanocomposite skins to enable the morphing of a nacelle inlet lip. The design consists of pneumatic inflatables as actuators and a flexible sandwich panel that morphs under variable pressure combinations to adapt different flight conditions and save fuel. The sandwich panel forms the outer layer of the nacelle inlet lip. It is lightweight, compliant and impact resistant with no discontinuities, and consists of graphene-doped thermoplastic polyurethane (G/TPU) skins that are supported by an aluminium Flex-core honeycomb in the middle, with near zero in-plane Poissons ratio behaviour. A test rig for a reduced-scale demonstrator was designed and built to test the prototype of morphing nacelle with custom-made pneumatic actuators. The output force and the deflections of the experimental demonstrator are verified with the internal pressures of the actuators varying from 0 to 0.41 MPa. The results show the feasibility and promise of the hybrid inflatable/nanocomposite sandwich panel for morphing nacelle airframes.

Microstructural modelization of viscoelastic auxetic polymers

In this paper a theoretical and numerical study on the viscoelastic behavior of auxetic polymers and cellular materials is presented. Negative Poissons ratio materials ((alpha) (upsilon) (eta) (xi) (epsilon) (omicron) (sigma) in Greek) expand in all directions when pulled in only one, and contract when compressed in one direction. This behavior is due to the special geometrical layout of their unit cells. A theoretical model including viscoelastic and inertia effects on the unit cell has been prepared in order to compute the equivalent in- plane dynamic storage modulus and loss factor of the cellular material. The calculations show how inertia effects and geometric layout of the unit cell affect the viscoelastic behavior of the material over the frequency domain. The results show a very good agreement with the ones from analogous FEM models. Auxetic honeycombs are a good example of cellular materials with negative Poissons ratio behavior. A Finite Element model has been elaborated to model also the viscoelastic response of the transverse shear modulus of this kind of honeycombs and compared with analytical results.