Modeling the instability of CNT tweezers using a continuum model

Carbon nanotube (CNT) tweezers are composed of two parallel cantilever CNTs with a distance in between. In this paper, the static response and instability of CNT-made nano-tweezers is theoretically investigated considering the effects of Coulomb electrostatic force and van der Waals molecular attraction. For this purpose, a nano-scale continuum model is employed to obtain the nonlinear constitutive equation of the nano-tweezers. The Euler-Bernoulli beam theory is applied to model the elastic response of the CNT. The van der Waals attraction is computed from the simplified Lennard-Jones potential. In order to solve the nonlinear constitutive equation of the system, three approaches, e.g. the hemotopy perturbation method (HPM), the Adomian decomposition (AD) and the finite difference method (FDM) are employed. The obtained results are in good agreement with the experimental measurements. As a case study, freestanding CNT tweezers has been investigated and the detachment length and minimum initial gap of the tweezers are determined. Moreover, the effective operation range of the van der Waals attraction that affects the instability behavior of the CNT tweezers is discussed

Design and characterization of an orthotropic accordion cellular honeycomb as one-dimensional morphing structures with enhanced properties

This study develops the governing equations and characterizes the mechanical properties of a new orthotropic accordion morphing honeycomb structure containing periodic arrays of U-type beams reinforced with glass fibers. Castigliano’s second theorem is modified to develop the analytical equations to predict the deformation behavior of a single orthotropic ply under a combined axial, bending, and shear loadings. Accordingly, the elastic properties of the orthotropic structure including elastic stiffness, shear stiffness, and in-plane Poisson’s ratios are calculated by the developed equations. The honeycomb structure is manufactured by 3D printing, and the samples are subjected to tensile tests to experimentally validate the analytical solutions. Multiple finite element simulations are also used to validate the results. A good agreement is observed between the analytical solution, the experiments, and simulations, confirming the robustness of the analytical solution to predict the full elastic properties of the composite cellular. The results show that the periodic arrays of U-type and vertical beams can generate low in-plane stiffness in the morphing direction and high in-plane stiffness in the transverse direction, respectively. A zero Poisson’s ratio feature is achieved by employing straight beams which result in a high stiffness in the perpendicular direction. The proposed accordion cellular honeycomb structure exhibits the flexible response along the accordion shape direction, with a significant stiffness in its transverse direction. Moreover, the new orthotropic structure has considerably greater strain to failure in the morphing direction compared with a conventional isotropic configuration. These features prove that this type of structure can be applied for the aerospace morphing structures such as wings

Modeling the static response and pull-in instability of CNT nanotweezers under the Coulomb and van der Waals attractions

In this paper, the static response and pull-in instability of nanotweezers fabricated from carbon nanotubes (CNT) are theoretically investigated considering the effects of the Coulomb electrostatic and van der Waals molecular attractions. For this purpose, a nanoscale continuum model is employed to obtain the nonlinear constitutive equation of this nano-device. The van der Waals attraction is computed from the simplified Lennard-Jones potential. In order to solve the nonlinear constitutive equation of the nanotweezers, three different approaches, e.g. developing a lumped parameter model, applying the analytical modified Adomian decomposition (MAD) and using a commercial numerical integration routine, are employed. The obtained results are in good agreement with experimental measurements as reported in the literature. As a case study, we have investigated a freestanding nanotweezer and have determined the detachment length and minimum initial gap. Furthermore, range of dominancy of the molecular attraction has been discussed. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved

Investigating the Effect of Interface Angle and Ply Thickness on Mode II Delamination Behaviour of Carbon/Epoxy Laminated Composites

The effect of angle and thickness of plies are investigated on the fracture energy release rate (ERR) in mode II of carbon/epoxy prepreg laminated composites using finite element modelling. The ply’s angle of = 0o, 22.5o, 30o, 45o, 60o, and 90o with 4 different thicknesses are investigated. The finite element modelling is done using the virtual crack closure technique (VCCT) to evaluate the asymmetric effect. The overall stiffness of the laminates is designed to have the least difference from the Mode II standard ASTM test. The results show that by increasing the thickness of the plies, the asymmetric effect and consequently the mode I/Mode II loading contribution is increased. The results show that ERR is decreased by increasing both angle and thickness of the plies. This understanding can help in designing composite materials with better delamination resistant properties under different loadings such as low-velocity impact

Investigation of the equivalent mechanical properties of the bone-inspired composite cellular structure: Analytical, numerical and experimental approaches

This study investigated the development of an analytical model using the energy method and Castigliano's second theory to evaluate the equivalent mechanical properties for a bone-inspired cellular structure of a glass fiber-reinforced Polylactic Acid (PLA) composite material. The bone-inspired cellular structure was fabricated using a Fused Deposition Modeling (FDM) 3D printing technique. The fabricated specimens were subjected to compression testing. The digital image correlation technique was employed for obtaining stain and displacement contours in experimental tests. Furthermore, a finite element numerical model was developed to evaluate the mechanical properties of the bone-inspired composite cellular structure. The comparison of the experimental and numerical results with the outcomes of analytical model revealed that the proposed analytical model can correctly determine the mechanical properties of the composite bio-inspired cellular structure. The results showed that by reinforcing the cellular structure with continuous fiber, significantly higher mechanical properties can be obtained. A comprehensive parametric study has also been performed to investigate the effect of geometric parameters on equivalent mechanical properties

EFFECT of SURFACE LAYER on ELECTROMECHANICAL STABILITY of TWEEZERS and CANTILEVERS FABRICATED from CONDUCTIVE CYLINDRICAL NANOWIRES

Herein, the impact of surface layer on the stability of nanoscale tweezers and cantilevers fabricated from nanowires with cylindrical cross section is studied. A modified continuum based on the Gurtin-Murdoch surface elasticity is applied for incorporating the presence of surface layer. Considering the cylindrical geometry of the nanowire, the presence of the Coulomb attraction and dispersion forces are incorporated in the derived formulations. Three different approaches, i.e. numerical differential quadrature method (DQM), an approximated homotopy perturbation method (HPM) and developing lumped parameter model (LPM) have been employed to solve the governing equations. The impact of surface layer on the instability of the system is demonstrated. © 2016 World Scientific Publishing Company

Investigation of energy absorption performances of a 3D printed fiber-reinforced bio-inspired cellular structure under in-plane compression loading

This article proposes glass-fiber-reinforced bone-inspired cellular structures to enhance energy absorption capability. The elastic modulus of the bone-inspired unit cell is obtained analytically based on the energy method and then employed in Particle Swarm Optimization algorithm to get optimized cellular structures. In the optimized cellular structure, the stiffness is optimized and the energy absorption capacity is investigated. A Fused Filament Fabrication 3D printing process is used to fabricate the cellular structures with continuous glass fiber-reinforced polylactic acid (PLA). In-plane compression tests are performed to investigate the mechanical performance of cellular structures. Finite Element Modeling (FEM) is conducted to analyzed the mechanical performance of the structures. In FEM, the failure criterion is determined using the maximum stress and VUSDFLD subroutine, and the damage growth is modeled by decreasing the mechanical properties. A good agreement between numerical and experimental results was observed. Results demonstrated that the energy absorption in glass-fiber-reinforced PLA is ∼250% higher than in the un-reinforced structure. The optimized cellular structure exhibits a stable prolonged plateau stress region and very high specific energy absorption parameters