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Approach for mechanical characteristic experimental analysis of honeycomb core manufactured by fused deposition modelling is suggested. Experimental analyses of shear and tension of the honeycombs specimens fabricated from PLA and ULTEM 9085 materials are performed. The experimental studies are accompanied by the finite element simulations of the shear and the tension experiments of honeycomb core. As follows from experimental and numerical studies, both the geometrical nonlinearity and plasticity behavior of the material must be accounted for the honeycomb tension analysis. The force-displacement response of honeycomb tension has nonlinear behavior. [ABSTRACT FROM AUTHOR]

Disbond damage growth in honeycomb cored sandwich structures due to static and fatigue mixed mode loading is investigated numerically and experimentally. A two dimensional finite element model was generated using core homogenization and the Crack Surface Displacement Extrapolation mode separation method, integrated into a fracture mechanics based analysis sub-routine to predict face/core interface fatigue crack propagation. The Cycle Jump technique was furthermore applied to accelerate fatigue analysis. Mixed mode fatigue characterization testing was conducted using Double Cantilever Beam specimens loaded with Uneven Bending Moments, generating a relationship between crack propagation rates and energy release rate amplitudes as a modified Paris Law, measured at three mode-mixity phase angles. The measured Paris laws were subsequently used as input data for the numerical fatigue model. The numerical model was validated against CFRP/Nomex® Sandwich Tearing Test specimen tests with a propagating face/core interface crack yielding varying mode-mixities. The results from the validation showed good agreement between numerical predictions and experimental measurements. [ABSTRACT FROM AUTHOR]

The hexagonal honeycomb core sandwich panels used in the satellite structure are subjected to severe vibration during launch. Therefore, the amounts of natural frequencies of these panels are of great importance for design engineers. Three-dimensional finite element modeling of the core considering all geometric parameters (i.e., a high-fidelity model) to achieve accurate results is not cost-effective. The honeycomb core is traditionally equivalent to a homogenized continuum core (i.e., a low-fidelity model) using simple analytical relations with ignoring the adhesive layer at the double cell-walls and radius of inclined cell-wall curvature. In this study, analytical formulations are first presented for the prediction of the equivalent elastic properties of a hexagonal aluminum honeycomb with considering all geometric parameters including adhesive layer thickness, cell-wall thickness, inclined cell-wall length, radius of inclined cell-wall curvature at the intersection, internal cell-wall angle, and honeycomb height. Then, two aluminum honeycomb core sandwich beams with free-free boundary conditions are modeled and analyzed in Abaqus finite element software, one with 3D high-fidelity core and the other with 3D low-fidelity core. In order to validate the results of the equivalent model, the modal analysis test was performed and the experimental natural frequencies were compared. The obtained results show a good agreement between the 3D low-fidelity and high-fidelity finite element models and experimental results. In addition, the influence of the above-mentioned geometric parameters has been investigated on the natural frequencies of a sandwich beam. [ABSTRACT FROM AUTHOR]

In this paper, the mechanical properties of Nomex® paper coated with resin and composite Nomex® honeycomb sandwich structures (CNHSS) were obtained by tensile tests and flatwise tests respectively. The fundamental mechanical properties of the Nomex® paper were used as input materials parameters of finite element model generated at meso-scale level for the CNHSS, and the mechanical properties of CNHSSs were used to validate the numerical results. Based on the test and numerical results, the theoretical equations were modified to predict the flatwise compressive buckling strength and modulus of the CNHSS. The numerical and theoretical results clearly revealed that the CNHSS had two flatwise compressive elastic moduli. However, the flatwise test can only capture the second flatwise compressive elastic modulus due to manufacturing geometric defects of the cell walls. The numerical and test results showed that the manufacturing geometric defects of the cell walls showed little influence on the ultimate flatwise compressive strength. And the modified equation can predict the flatwise compressive buckling strength and modulus of the CNHSS with sufficient accuracy. [ABSTRACT FROM AUTHOR]

The stress-strain response and deformation mechanism of a range of Nomex honeycomb cores tested under in-plane compression has been examined experimentally. The cores with a thin wall displayed extensive bending deformation of the cell walls inclined to the horizontal (loading is vertical) and failed in bending. The cores with thicker walls failed by a shear-type instability of the cells indicated by tilting of vertical cell wall segments. The failure strain decreased with increasing core density. The modulus and compressive strength of the core were compared to micromechanical predictions. Normalized modulus and strength values varied between the various cores. The average modulus and strength results allow backing out of the modulus and bending strength of the Nomex paper. The results were in reasonable agreement with published tensile test results and composite micromechanics. [ABSTRACT FROM AUTHOR]

Composite sandwich structures are commonly considered in large-scale aerospace applications due to their performance on a per mass basis. The nature of a large-scale sandwich structure generally necessitates the use of multiple sections of core to fill out the structural form. These core sections must be spliced together to ensure that shear loads are appropriately transmitted throughout the core. Because core installation in a large-scale component is a challenging operation, core splice joint width can be difficult to control in manufacturing. As such, the effects of core splice joint width on sandwich structure performance should be well understood. This study examines the effects of core splice joint width in honeycomb sandwich structures via mechanical testing and post-failure analysis. A threshold core splice joint width is shown to exist with respect to core shear, while the integrity of the facesheet-to-core interface is shown to degrade with increasing core splice joint width. [ABSTRACT FROM AUTHOR]

Based on the differential quadrature method (DQM), the bending of sandwich-curved beams with graphene platelets/aluminum (GPLs/Al) nanocomposite face sheets and aluminum honeycomb core is investigated using a new shear and normal deformations curved beam theory. The present new model is resting on elastic foundation and subjected to a circumferential magnetic field, thermal load, and humid conditions. The face sheets are made of several bonded composite layers with randomly oriented and uniformly distributed graphene platelets in each layer. The mechanical and hygrothermal properties of the faces are assumed to be functionally graded (FG) using a piece-wise law by varying the weight fraction of the GPLs in the face thickness direction. Four governing differential equations are derived based on a novel four-variable curved beam theory taking into account the thickness stretching effect. The governing equations are solved for various boundary conditions on the basis of the DQM. The displacements presented by the DQM are compared with those obtained by Navier solution. Impacts of various parameters such as geometric shape parameters, magnetic parameter, temperature, moisture, elastic foundation parameters, core thickness, boundary conditions, and graphene weight fraction on the displacements and stresses of the functionally graded graphene/aluminum sandwich-curved beams are illustrated. [ABSTRACT FROM AUTHOR]

In the present study, the free and forced vibration responses of the composite sandwich plate with carbon nanotube reinforced honeycomb as the core material and laminated composite plates as the top and bottom face sheets are investigated. The governing equations of motion of hybrid composite honeycomb sandwich plates are derived using higher order shear deformation theory and solved numerically using a four-noded rectangular finite element with nine degrees of freedom at each node. Further, various elastic properties of honeycomb core materials with and without reinforcement of carbon nanotube and face materials are evaluated experimentally using the alternative dynamic approach. The effectiveness of the finite element formulation is demonstrated by performing the results evaluated experimentally on a prototype composite sandwich plate with and without carbon nanotube reinforcement in core material. Various parametric studies are performed numerically to study the effects of carbon nanotube wt% in core material, core thickness, ply orientations, and various boundary conditions on the dynamic properties of composite honeycomb sandwich plate. Further, the transverse vibration responses of hybrid composite sandwich plates under harmonic force excitation are analyzed at various wt% of carbon nanotubes and the results are compared with those obtained without addition of carbon nanotubes to demonstrate the effectiveness of carbon nanotube reinforcement in enhancing the stiffness and damping characteristics of the structures. The study provides the guidelines for the designer on enhancing both the stiffness and damping properties of sandwich structures through carbon nanotube reinforcement in core materials. [ABSTRACT FROM AUTHOR]

Honeycomb sandwich panels of various skin thicknesses and core densities have been investigated under quasi-static loading in bending and indentation with both hemispherical (HS) and flat-ended (FE) indenters. Core crushing, top skin delamination, and top skin fracture are identified as major damage mechanisms. Their characteristics and energy-absorbing capabilities are established using load-displacement and load-strain curves and inspections of cross-sectioned specimens. The effects of varying skin thickness, core density and type, indenter nose shape, and boundary conditions on the damage and energy-absorbing characteristics are examined. The variation of the indenter nose shape is shown to induce a change in the damage mechanisms and have the most significant effect on energy absorption, especially for panels with relatively thicker skins. Increasing the skin thickness significantly increases not only the initial threshold and ultimate loads but also the absorbed energy (AE) of the panels. Increasing the core density has a very small effect on either the ultimate loads or the energy-absorbing capacity, while the effect of the support conditions on the damage and energy-absorbing characteristics is small. The larger 220 mm diameter panels absorb significantly more energy than the 100 mm diameter panels because of the much greater ultimate displacement. Different core materials with a similar density show little difference in either the daniage or the energy-absorbing characteristics due to the limited contribution of transverse shear resistance. Panels with a delaminated top skin have lower threshold loads under both indenters and lower ultimate load for the HS indenter. [ABSTRACT FROM AUTHOR]

ABSTRACT: Low order and high order shear deformation models are applied to investigate the effect of honeycomb core (transversely shear deformable) on flexural vibration of thick rectangular sandwich panel with isotropic facesheets. Strain and kinetic energy of sandwich panel are expressed in terms of material properties and structural parameters. Partial differential equations are derived based upon a variational principle. Solutions in Navier form are obtained for a simply supported rectangular panel. The effect of honeycomb core parameters, such as characteristic angle, cell wall thickness, and cell size is studied. Comparison between low order model, high order model, and finite element method is provided. It is shown that in most cases results from a high order model without facesheet shear effect are close to those from finite element analysis. [ABSTRACT FROM AUTHOR]

This article is intended to be a tutorial on the subject of fatigue of foam and honeycomb core composite sandwich structures. First, several different analytical models for predicting the fatigue life of sandwich composites are presented. Then representative publications which have reported on the major failure modes in sandwich beams under dynamic fatigue loading are summarized, along with several related publications dealing with static and impact loading. Papers dealing with the effects of loading frequency, environmental factors, and block loading on the fatigue life of sandwich composites are discussed. Finally, recent research on different types of non-destruction evaluation (NDE) techniques employed for failure investigations during fatigue testing of sandwich structures is reviewed. Conclusions and generalizations that can be drawn from the literature are presented along with discussions of areas in which further research is needed. [ABSTRACT FROM AUTHOR]

Damage characteristics of composite-skinned honeycomb sandwich panels in bending are investigated with both hemispherical (HS) and fiat-ended (FE) indenters. The thickness of the cross-ply skins varies from 8 to 16 plies, whereas the density of the 12.7-mm thick aluminum honeycomb core varies from 50 to 70 kg/m³. Clamped panels with a 100-mm testing area are loaded quasi-statically either in bending or on a rigid base. The effects of varying these parameters on damage mechanisms are examined through response curves as well as cross sections of selected specimens. Special emphasis is placed on their potential change induced by the variation of skin thickness and core density with a specific indenter. Damage mechanisms are identified as core crush, top-skin delamination, and fracture or shear-out. The threshold and ultimate loads as well as the initial slope increase significantly either on increase of skin thickness or change of the nose shape of indenter from a hemisphere to a flat-end. The increase in the post-initial-damage slope is small and can be attributed to membrane stretching of the damaged top skin. Increasing the core density affects substantially not only the threshold load, but also the initial slope associated with the FE indenter. Changing the nose shape of the indenter has an overriding effect on the nature of damage mechanisms. In particular, top-skin delaminations occur after core crush. The panel deflection contributes to 20–53% sandwich deformation. The bottom skin in all the tests remains intact. [ABSTRACT FROM AUTHOR]