Flatwise buckling optimization of hexachiral and tetrachiral honeycombs

The flatwise compressive behaviour of tetrachiral and hexachiral honeycombs is analysed, using analytical and Finite Element simulations, both with explicit and implicit formulations. The tetrachiral and hexachiral cells are composed by cylinders connected by four and six tangent ligaments respectively. The ligaments act as mixed stiffeners-elastic foundations during flatwise compressive loading, providing different buckling mode shapes during deformation. The models are compared with experimental results obtained using RP-based honeycombs tested according to ASTM C 393-00 and ASTM C365-00.     

A negative Poisson’s ratio carbon fibre composite using a negative Poisson’s ratio yarn reinforcement

The manufacture of negative Poisson’s ratio (auxetic) composites, containing inherently auxetic phases is rare and has been confined to relatively low modulus composite systems with stiffnesses several orders of magnitude below those of structural composites. This paper presents the use of an auxetic double helix yarn that is used to produce a unidirectional fibre composite with both relatively high stiffness (4 GPa) and negative Poisson’s ratio (−6.8), at 30% fibre volume fraction, compared to other auxetic composites. This is the first structural auxetic composite to be produced using carbon fibre and importantly it was produced using standard manufacturing techniques and therefore is potentially applicable in a variety of engineering applications.     

Three-dimensional Finite Element Buckling Analysis of Honeycomb Sandwich Composite Shells with Cutouts

This paper investigates the buckling response of honeycomb sandwich composite shells with cutouts under axial compression. The Wilson's incompatible solid Finite Element (FE) is used around cutouts to obtain the detail stress distribution there. While to reduce the computational expense, a special multilayered relative degrees-of-freedom (DOF) shell FE is used to model the regions far from the cutouts. The efficiency and accuracy of this modeling scheme are illustrated by two benchmarks. Then parametric studies are carried out to reveal how the buckling response is influenced by the area, the shape and the orientation of cutouts.     

The manufacture and characterisation of a novel,low modulus,negative Poisson’s ratio composite

Relatively few negative Poisson’s ratio (auxetic) composites have been manufactured and characterised and none with inherently auxetic phases [Milton G. J. Mech. Phys. Solids 1992;40:1105–37]. This paper presents the use of a novel double-helix yarn that is shown to be auxetic, and an auxetic composite made from this yarn in a woven textile structure. This is the first reported composite to exhibit auxetic behaviour using inherently auxetic yarns. Importantly, both the yarn and the composite are produced using standard manufacturing techniques and are therefore potentially useful in a wide range of engineering applications.     

Measurement of the Poisson’s ratio of flexible polyurethane foam and its influence on a uniaxial compression model

Previous attempts to model the uniaxial compression response of flexible polyurethane foam using a hyperelastic model for rubber-like materials assumed a zero Poisson’s ratio in the model development. The consequences of relaxing this assumption are explored in this investigation. First, measurements are made of the material’s Poisson’s ratio at high compression levels. The measured values range between 0.5 at 5% compression and −0.05 at 66% compression. The experimental results are combined with those of other investigators and incorporated into a nonlinear viscoelastic model for modeling uniaxial compression behavior. It is found that relaxing the zero Poisson’s ratio assumption decreases the accuracy of the model predictions, indicating that an alternative model structure may be required.     

Prebuckling and buckling analysis of variable angle tow plates with general boundary conditions

The concept of variable angle tow (VAT) placement is explored for enhancing the buckling resistance of composite plates subjected to axial compression loading. The problem is relatively difficult to solve because of varying stiffness properties and requires prior prebuckling analysis to determine the non-uniform stress variation followed by the buckling analysis of VAT plates. A stress function formulation for in-plane analysis and displacement formulation for buckling analysis was employed to derive the governing differential equations of VAT plates based on classical laminated plate theory. The Differential Quadrature Method (DQM) is applied to solve the differential equations. The novel aspect of the present work is the use of Airy’s stress function to model the prebuckling analysis of VAT plates which considerably reduces the problem size, computational effort and provides generality to model pure stress and mixed boundary conditions. DQM was applied first to solve the prebuckling problem of VAT plates subjected to cosine distributed compressive load/ uniform end shortening. Then, DQM was applied to solve the buckling problem of rectangular VAT plates subjected to axial compression under different plate boundary conditions. Results were validated with detailed finite element analysis and the relative accuracy and efficiency of the DQM approach is discussed.     

Elastic moduli of solids containing spheroidal pores

We use asymptotic approximations for the elastic compliances (P, Q) of a spheroidal pore as input in the differential effective medium scheme to derive approximate analytical expressions for the effective moduli of an isotropic solid containing randomly oriented spheroids. The approximations are valid for crack-like pores having aspect ratios α as high as 0.3, needle-like pores having aspect ratios as low as 3, and nearly spherical pores (0.7 < α < 1.3). Analytical solutions for the differential scheme have previously only been available for the limiting cases of infinitely thin-cracks (α = 0) and spherical pores (α = 1). The relatively simple approximations found between the limiting cases can account for more realistic pore shapes, and are valid for a wide range of porosities. The behaviour of the effective Poisson’s ratio in the high concentration limit shows that ν is bounded between the Poisson’s ratio of the solid and a fixed point ν_c that only depends on the aspect ratio of the pores. The asymptotic expressions for P and Q can also successfully be used as input in any other effective medium theory, such as the Mori-Tanaka or Kuster-Toksoz schemes. The relatively simple expressions found for the various effective medium schemes, as well as the bounds found for the effective Poisson’s ratio, will be useful to simplify the process of inversion of elastic velocities in porous solids.     

Transverse shear modulus and strength of honeycomb cores

The transverse shear mechanical behavior and failure mechanism of aluminum alloy honeycomb cores are investigated by the single block shear test in this paper. The transverse shear deformation process of honeycomb cores may be approximately categorized into four stages, namely elastic deformation, plastic deformation, fracture of cell walls and debonding of honeycomb cores/facesheets. The elastic deformation of unit cell under transverse shear displacement is also investigated by the finite element method, and the result shows that the bending deformation of the cell walls is similar to that of the cantilever beam. In order to precisely predict the equivalent transverse shear modulus and strength, not only shear deformation but also bending deformation of cell walls should be considered. Therefore, in the present paper, the equivalent transverse shear modulus and strength are predicted by application of the cantilever beam theory and thin plate shear buckling theory in conjunction with simplifying assumption as to the displacement in the cores. It is concluded that the contribution of bending deformation of cell walls to equivalent transverse shear modulus and strength is obvious with the decreasing height of cell walls.     

Three-dimensional thermomechanical buckling analysis for functionally graded composite plates

Three-dimensional thermomechanical buckling analysis is investigated for functionally graded composite structures that composed of ceramic, functionally graded material (FGM), and metal layers. Material properties are assumed to be temperature dependent, and in FGM layer, they are varied continuously in the thickness direction according to a simple power law distribution in terms of the ceramic and metal volume fractions. The finite element model is adopted by using an 18-node solid element to analyze more accurately the variation of material properties and temperature field in the thickness direction. Temperature at each node is obtained by solving the thermomechanical equations. For a time discretization, Crank–Nicolson method is used. In numerical results, the thermal buckling behavior of FGM composite structures due to FGM thickness ratios, volume fraction distributions, and system geometric parameters are analyzed.     

Uniaxial local buckling strength of periodic lattice composites

To reveal the local buckling strength of periodic lattice composites, an important factor in optimal material design, analytical method based on the classical beam-column theory was applied. Buckling modes were decided according to the condition that the curvature of the strut columns is the smallest. Characteristic equations were built according to the equilibrium equations. The buckling strengths and constraint factors of various grids under uniaxial compression and tension were achieved. The strut network supports stronger rotation restrictions than pin-jointed nodes but weaker than the built-in ends. With more stacks of struts and connectivity at nodes, the restriction must be stronger and the buckling load is greater. Commonly, the constraint factors of isogrids and mixed triangle grids are greater than Kagome grids. The regular honeycomb and square grids possesses smaller buckling loads.     

A novel concept to develop composite structures with isotropic negative Poisson’s ratio: Effects of random inclusions

Materials with negative Poisson’s ratio (NPR) effects have been studied for decades. However, the studies have mainly focused on 2D periodic structures which only have NPR effects in certain in-plane directions. In this paper, a novel concept is proposed to develop composite structures with isotropic NPR effects using NPR random inclusions. The study starts from a finite element analysis of deformation mechanisms of two 2D representative cells which are embedded with a re-entrant square and a re-entrant triangle, respectively. Based on the analysis results, the re-entrant triangles are selected as random inclusions into a matrix to form 2D composite structures. Four such composite structures are built with different numbers of inclusions through a parametric model, and their NPR effects and mechanical behaviors are analyzed using the finite element method. The results show that the isotropic NPR effects of composites can be obtained with high random re-entrant inclusions. Thus, the novel concept proposed is numerically proved by this study.     

The helical auxetic yarn - A novel structure for composites and textiles; geometry, manufacture and mechanical properties

This paper introduces a novel fibre structure known as the helical auxetic yarn (HAY). The geometry of the yarn is defined and the manufacturing process described. A range of HAYs have been manufactured that vary the geometric properties of the structure. A systematic study of the yarns has been completed to evaluate the effect on the auxetic behaviour of the geometry. We also characterise the component fibres and yarns and discuss the influence of geometric and material effects on the observed Poisson’s ratio of the yarns.It can be shown that the starting wrap angle of the yarn has the greatest effect on auxetic behaviour both in terms of magnitude and the strain range over which it may be observed.The maximum negative Poisson’s ratio observed for a yarn manufactured from conventionally available monofilaments with positive Poisson’s ratio is −2.7.     

Mechanical properties of Gd123 bulk superconductors at room temperature

In order to investigate the mechanical properties of Gd123 single-grain bulk superconductors fabricated using a modified quench and melt growth method, tensile tests in the direction parallel and perpendicular to the c-axis have been carried out at 293 K by using the small specimens cut from bulk superconductors. As for the mechanical properties perpendicular to the c-axis, there was no significant difference between those in the crystal growth direction and those perpendicular to it. While the average value of the Young’s modulus of the bulk sample with 33.0 mol%-Gd211 secondary phase particles, 118 GPa, was higher than that of the bulk sample with 28.7 mol%-Gd211, 111 GPa, the average value of the tensile strength of the former, 36 MPa, was lower than that of the latter, 40 MPa. The tensile strength and the Young’s modulus in the c-axis, 10 MPa and 37 GPa, were quite low compared with those mentioned above. Poisson’s ratio based on the transverse strain in the c-axis, 0.15, was significantly smaller than that perpendicular to it, 0.30. In the specimens with higher length, however, the difference was decreased to some extent. With regard to the anisotropy of the Poisson’s ratio, the effect of a pre-existing micro-crack opening in the c-axis direction was discussed coupled with the constraints at the interfaces between the specimen and the sample holder.     

材质对蜂窝纸板缓冲性能的影响

对两种不同材质的蜂窝纸板静态应力应变性能和动态缓冲结果进行了比较分析。讨论了蜂窝纸板力学性能对应变速度的敏感性及两种纸板的缓冲性能差异。缓冲试验结果表明。在不同的冲击应变率下蜂窝纸板的材质影响蜂窝纸板缓冲性能。     

Numerical investigation of the mode III stress intensity factors in FGMs considering the effect of graded Poisson’s ratio

For cracks in functionally graded materials under pure mode three loading, this work makes use of J-integral in domain form to determine K_III. This is done as a post-processing stage in the finite element context. Numerical computations are carried out for penny-shaped and circumferential cracks. For each crack, a sensitivity analysis is done to demonstrate the trend of K_III with respect to the gradation indexes for modulus of elasticity and Poisson’s ratio. It is realized that for a given geometry and boundary condition, K_III depends on gradation of both modulus of elasticity and Poisson’s ratio.     

Facile approach for temperature-responsive polymeric nanocapsules with movable magnetic cores

A novel facile approach was developed to prepare the well-defined temperature-responsive polymeric nanocapsules with movable magnetic cores (TPNMCs) based on the organic/inorganic sandwich-structured composite nanoparticles with temperature-responsive crosslinked shells via the one-pot approach surface-initiated atom transfer radical copolymerization (SI-ATRP) of N-isopropyl acrylamide (NIPAM) and N,N′-methylenebisacrylamide (MBA) from the surfaces of the Fe₃O₄@SiO₂ core-shell nanoparticles, followed by the selective removal of the silica layer. Based on the temperature-induced volume phase transition and impressive magnetic response, the TPNMCs are expected to be used for the targeted controlled release of sensitive molecules, such as enzymes, proteins or DNA, by responding to the changing the environmental temperature.     

Circumferential stiffness tailoring of general cross section cylinders for maximum buckling load with strength constraints

Stiffness tailoring of laminated composite structures using steered fibre tows is a design method that maximally uses the directional properties of composite materials. Cylindrical structures usually have circular cross sections while some application, geometric or aerodynamic requirements can necessitate other cross sections, e.g. elliptical. Circumferential tailoring can increase the buckling load of thin cylinders by compensating for non-uniform sectional loading such as bending and/or varying radius of curvature in general cylinders. Here, strength constraints are considered in maximum buckling load design, to ensure that the failure load is greater than the buckling load. A two-step optimisation framework is used to separate the theoretical and manufacturing issues in design. A computationally cheap semi-analytical finite difference method is used to solve the linear static and buckling problems. Conservative failure envelopes based on Tsai-Wu failure criterion are used for strength evaluation. To avoid repetitive analyses, successive convex approximation method is used. For demonstration, circumferential tailoring framework is applied to a circular cylinder under bending and an elliptical cylinder under axial compression. The improvements in buckling capacity of variable over constant stiffness designs are shown and verified using nonlinear buckling analysis in the commercial FEM software Abaqus^TM, and the mechanisms of improvements are investigated.     

Delamination buckling and propagation analysis of honeycomb panels using a cohesive element approach

The cohesive element approach is proposed as a tool for simulating delamination propagation between a facesheet and a core in a honeycomb core composite panel. To determine the critical energy release rate (G _c) of the cohesive model, Double Cantilever Beam (DCB) fracture tests were performed. The peak strength (_c) of the cohesive model was determined from Flatwise Tension (FWT) tests. The DCB coupon test was simulated using the measured fracture parameters, and sensitivity studies on the parameters for the cohesive model of the interface element were performed. The cohesive model determined from DCB tests was then applied to a full-scale, 914×914 mm (36×36 in.) debond panel under edge compression loading, and results were compared with an experiment. It is concluded that the cohesive element approach can predict delamination propagation of a honeycomb panel with reasonable accuracy.     

Bending and creep buckling response of viscoelastic functionally graded beam-columns

The viscoelastic creep response of flexural beams and beam-columns made with functionally graded materials is numerically investigated. The paper highlights the challenges associated with the modeling and analysis of such structures, and presents a nonlinear theoretical model for their bending and creep buckling analysis. The model accounts for the viscoelasticity of the materials using differential-type constitutive relations that are based on the linear Boltzmann’s principle of superposition. The model is general in terms of its ability to deal with any material volume faction distribution through the depth of the beam, and with different linear viscoelastic laws, boundary conditions, and loading schemes. The governing equations are solved through time stepping numerical integration, which yields an exponential algorithm following the expansion of the relaxation function into a Dirichlet series. A numerical study that examines the capabilities of the model and quantifies the creep response of functionally graded beam-columns is presented, with special focus on the stresses and strains redistribution over time and on the creep buckling response. The results show that the creep response of such structures can be strongly nonlinear due to the variation of the viscoelastic properties through the depth, along with unique phenomena that are not observed in homogenous structures.     

Simplified buckling and ultimate strength analysis of composite plates in compression

The work presented here concerns the ultimate strength predictions of simply supported, square plates of laminated composite material subjected to uniaxial in-plane compressive load. Plates having a range of thicknesses and initial geometric imperfections have been investigated. Several models are established, each based on first order shear deformation theory and assumption of small deflections. The approaches give reasonable but somewhat conservative estimates for the thicker plates considered, while for the thinner plates, neglect of the post-buckling behaviour makes the results very conservative. It will be necessary to use a large deflection plate theory for some of the models to realise their full potential.