Showing papers on "Sandwich-structured composite published in 2016"

Finite element analysis of sandwich panels with stepwise graded aluminum honeycomb cores under blast loading

Abstract: This paper presents details and brief results of an experimental investigation on the response of metallic sandwich panels with stepwise graded aluminum honeycomb cores under blast loading. Based on the experiments, corresponding finite element simulations have been undertaken using the LS-DYNA software. It is observed that the core compression stage was coupled with the fluid–structure interaction stage, and the compression of the core layer decreased from the central to the peripheral zone. The blast resistance capability of sandwich panels was moderately sensitive to the core relative density and graded distribution. For the graded panels with relative density descending core arrangement, the core plastic energy dissipation and the transmitted force attenuation were larger than that of the ungraded ones under the same loading condition. The graded sandwich panels, especially for relative density descending core arrangement, would display a better blast resistance than the ungraded ones at a specific loading region.

On compressive properties of composite sandwich structures with grid reinforced honeycomb core

Abstract: In the present study, periodical grids were selected to reinforce soft honeycomb cores of sandwich structures. The grid reinforced honeycomb core can be considered as a combined core with multi-level lattice configuration. In-plane compression tests were carried out to investigate the mechanical properties of carbon fiber sandwich with combined core. Experimental results indicated that the combined core sandwich specimens provided increased stiffness, specific stiffness, energy absorption and critical load, which were higher than the sum of honeycomb core sandwich specimens and grid core sandwich specimens. In addition, a Finite Element Method (FEM) model was proposed to calculate the critical buckling load of the combined core sandwich structures. The effects of core heights, honeycomb-wall thickness and face-sheet thickness on the critical buckling loads of the combined core sandwich structures were examined. The aforementioned experimental and numerical results indicated that the present sandwich structure with grid reinforced honeycomb core could provide improved structural properties for engineering structures.

Composite and Nanocomposite Metal Foams

Abstract: Open-cell and closed-cell metal foams have been reinforced with different kinds of micro- and nano-sized reinforcements to enhance their mechanical properties of the metallic matrix. The idea behind this is that the reinforcement will strengthen the matrix of the cell edges and cell walls and provide high strength and stiffness. This manuscript provides an updated overview of the different manufacturing processes of composite and nanocomposite metal foams.

Structural behaviour of steel–concrete–steel sandwich composite wall subjected to compression and end moment

Abstract: Steel–concrete–steel (SCS) sandwich wall infilled with ultra-lightweight cement composite has been developed and proposed for applications in offshore and building constructions. A new form of J-hook connector is introduced to connect the external plates to improve the composite action between the steel face plates and cement composite core to form an integrated unit which is capable of resisting extreme loads. This research experimentally investigates the structural behaviour of SCS sandwich composite wall based on a series of combined compression and uniaxial bending tests on short SCS sandwich composite wall with interlocking J-hook connectors. From the tests, it is found that the SCS sandwich wall exhibits good ductility behaviour with a bending failure mode. Nonlinear finite element (FE) model is also developed to simulate the mechanical behaviour of sandwich wall in terms of ultimate strength and load-deflection curves. Analytical studies show that the N–M interaction model based on Eurocode 4 may over-predict the combined resistance of the SCS sandwich walls subjected to eccentric compression. Therefore, a new approach is proposed to evaluate the resistance of sandwich wall. The axial force versus moment capacity interaction diagrams of sandwich wall are calculated. The validation against the test and FE results shows a reasonable and conservative estimation on the combined resistance of SCS sandwich wall.

Free-Vibration Analysis of Sandwich Panels with Randomly Irregular Honeycomb Core

Abstract: An analytical framework has been proposed to analyze the effect of random structural irregularity in honeycomb core for natural frequencies of sandwich panels. Closed-form formulas have been developed for the out-of-plane shear moduli of spatially irregular honeycombs following minimum potential energy theorem and minimum complementary energy theorem. Subsequently an analytical approach has been presented for free-vibration analysis of honeycomb core sandwich panels to quantify the effect of such irregularity following a probabilistic paradigm. Representative results have been furnished for natural frequencies corresponding to low vibration modes of a sandwich panel with high length-to-width ratio. The results suggest that spatially random irregularities in honeycomb core have considerable effect on the natural frequencies of sandwich panels.

A multi-scale approach for the simultaneous shape and material optimisation of sandwich panels with cellular core

Abstract: This work deals with the problem of the optimum design of a sandwich panel made of carbon-epoxy skins and a metallic cellular core. The proposed design strategy is a multi-scale numerical optimisation procedure that does not make use of any simplifying hypothesis to obtain a true global optimum configuration of the system. To face the design of the sandwich structure at both meso and macro scales, a two-level optimisation strategy is employed: at the first level the goal is the determination of the optimum shape of the unit cell of the core (meso-scale) together with the material and geometric parameters of the laminated skins (macro-scale), while at the second level the objective is the design of the skins stacking sequence (skin meso-scale) meeting the geometrical and material parameters provided by the first-level problem. The two-level strategy is founded on the polar formalism for the description of the anisotropic behaviour of the laminates, on the NURBS basis functions for representing the shape of the unit cell and on the use of a genetic algorithm as optimisation tool to perform the solution search. To prove its effectiveness, the multi-scale strategy is applied to the least-weight design of a sandwich plate subject to constraints of different nature: on the positive-definiteness of the stiffness tensor of the core, on the admissible material properties of the laminated faces, on the local buckling load of the unit cell, on the global buckling load of the panel and geometrical as well as manufacturability constraints related to the fabrication process of the cellular core.

Acoustic properties of glass fiber assembly-filled honeycomb sandwich panels

Abstract: A new composite structure (glass fiber assembly-filled honeycomb sandwich panel) is prepared in order to improve the acoustic properties. Effect of glass fiber assembly with different filling shapes (random and fiber ball), fiber diameter, fiber content and air-layer on acoustic properties are explored. Sound absorption coefficient (SAC) and sound transmission loss (STL) are determined by a B&K impedance tube. The experiment results indicate that the first resonance frequency of SAC disappears along with the improvement of the second resonance frequency by reducing the fiber diameter or increasing the fiber content. STL can be improved by the increase of the fiber content. Random glass fiber assembly with the fine fibers has the best STL in the all testing samples. The advantage of glass fiber assembly for improving the STL of honeycomb sandwich panel is particularly clear at frequencies below 4.5 kHz. Especially, the STL difference reaches the maximum at around 20 dB at frequencies below 3.0 kHz.

Sandwich Structured Composites for Aeronautics: Methods of Manufacturing Affecting Some Mechanical Properties

Abstract: Sandwich panels are composites which consist of two thin laminate outer skins and lightweight (e.g., honeycomb) thick core structure. Owing to the core structure, such composites are distinguished by stiffness. Despite the thickness of the core, sandwich composites are light and have a relatively high flexural strength. These composites have a spatial structure, which affects good thermal insulator properties. Sandwich panels are used in aeronautics, road vehicles, ships, and civil engineering. The mechanical properties of these composites are directly dependent on the properties of sandwich components and method of manufacturing. The paper presents some aspects of technology and its influence on mechanical properties of sandwich structure polymer composites. The sandwiches described in the paper were made by three different methods: hand lay-up, press method, and autoclave use. The samples of sandwiches were tested for failure caused by impact load. Sandwiches prepared in the same way were used for structural analysis of adhesive layer between panels and core. The results of research showed that the method of manufacturing, more precisely the pressure while forming sandwich panels, influences some mechanical properties of sandwich structured polymer composites such as flexural strength, impact strength, and compressive strength.

Semi-rigid biopolyurethane foams based on palm-oil polyol and reinforced with cellulose nanocrystals

Abstract: In this study, water-blown biopolyurethane (BPU) foams based on palm oil were developed and cellulose nanocrystals (CNC) were incorporated to improve the mechanical properties of the foams. In addition, the foams were compared with petroleum polyurethane (PPU) foam. The foam properties and cellular morphology were characterized. The obtained results revealed that a low-density, semi-rigid BPU foam was prepared using a new formulation. CNC as an additive significantly improved the compressive strength from 54 to 117 kPa. Additionally, cyclic compression tests indicated that the addition of CNC increased the rigidity, leading to decreased deformation resilience. The dimensional stability of BPU foams was increased with increasing CNC concentration for both heating and freezing conditions. Therefore, the developed BPU nanocomposite foams are expected to have great potential as core material in composite sandwich panels as well as in other construction materials.

Effect of impactor shapes on the low velocity impact damage of sandwich composite plate: Experimental study and modelling

Abstract: An experimental and numerical analysis of the influence of impactor shapes on the low velocity impact performance of aluminium sandwich composite plates has been carried out. The aluminium composite panels were manufactured by using two aluminium sheets and a low density polyethylene core under heat and pressure, which shows the outstanding properties of low weight, good rigidity and impact resistance. Experimental tests were performed using drop weight test machine, samples were impacted using steel conical, ogival, hemispherical and flat impactors, all 12 mm in diameter, for different initial impact energies of 29.43 J and 44.15 J and specimen thickness of 4 mm containing three different parts (0.5 + 3.0 + 0.5). A three dimensional non-linear finite element model is developed for simulating the impact behaviour of sandwich composite plate and the ABAQUS/Explicit commercial program was used. The face sheet material aluminium alloy 3003-O of the plate was modelled as isotropic with elastic–plastic characteristics. The description of the material characteristic of the attenuator was made by means of the Johnson–Cook elastic–plastic law. The material constitutive law of the Al 3003 plates has been implemented in a user-defined subroutine UMAT. The foam core was modelled as a crushable foam material. The finite element results showed a good correlation to the experimental data in terms of contact-force histories, energy histories, absorbed energy, and failure of the sandwich composite was observed between the experimental data.

Response of precast foamed concrete sandwich panels to flexural loading

Abstract: This paper presents the results of an experimental and analytical investigation of a total of six precast foamed concrete sandwich panels (PFCSPs) as one-way acting slabs tested under flexural loads. Foamed concrete of 25.73 MPa was used to produce the PFCSP concrete wythes. The results obtained from the tests have been discussed in terms of ultimate flexural strength capacity, moment-vertical deflection profile, load–strain relationship, strain variation across the slab depth, influence of aspect ratio, cracking patterns, and ultimate flexural load at failure. An analytical study of finite element analysis (FEA) as a one-way slab model was then conducted. The increase in aspect ratio (L/d) from 18.33 to 26.67 shows a reduction of 50% and 69.6% on the ultimate flexural strength capacity as obtained experimentally and in FEA models, respectively. Theoretical analyses on the extremes of fully composite and non-composite actions were also determined. The experimental results showed that cracking patterns were observed in one direction only, similar to those reported on a reinforced concrete solid slab, as well as precast concrete sandwich panels, when both concrete wythes act in a single composite manner. The experimental results were compared with FEA model data, and a significant degree of accuracy was obtained. Therefore, the PFCSP slab can serve as an alternative to the normal concrete slab system in buildings.

Experimental study on the dynamic response of foam-filled corrugated core sandwich panels subjected to air blast loading

Abstract: Effective approaches to enhance the blast resistance of sandwich structures with corrugated cores were developed by adopting three different strategies to fill the spaces within cores with polymeric foam. The baseline unfilled panels and foam-filled panels were designed and fabricated, and finally subjected to air blast loading generated by detonating cylindrical explosive. Deformation modes and failure mechanisms of tested panels were investigated. Experimental results demonstrated that the panels with back side filling strategy did not show better blast performance compared with the unfilled panels, even though extra weight was expended due to the addition of foam fillers. The panels with front side filling and fully filling strategies encouragingly appeared to possess desirable blast resistance to prevent severe fracture under high intensity blast loading. This benefit should be attributed to the sufficient crushing deformation of foam fillers and the enhanced buckling resistance of core webs. In addition, a preliminary study has been conducted to investigate the effects of front face thickness on the blast performance of foam-filled panel. Attempts of allocating component mass and filling different material have been made to explore the potential of performance improvement.

Low-velocity impact response of sandwich composites with different foam core configurations

Abstract: In this study, the low-velocity impact response of sandwich composites consisting of different foam core configurations is investigated experimentally. Polyvinyl chloride foam core and glass fibers were used as core material and face sheets, respectively. A number of tests under various impact energy levels for three different sandwich composite configurations were conducted with Ceast 9350 Fractovis Plus impact testing machine in order to improve the energy absorption capacity of sandwich composites panels. Absorbed energies, maximum loads and the maximum deflection of sandwich panels were obtained for each impact energy level. As the impact energy was increased, fiber fractures at face sheets, delaminations between glass-epoxy layers, core fractures, and indentations failures were observed by visual inspection. According to the obtained results, the sandwich composite with proposed new foam core design with two internal face sheets exhibits high energy absorption capacity compared to sandwich panels for...

Finite element analysis of foam-filled honeycomb structures under impact loading and crashworthiness design

Abstract: The aim of this research is the investigation of foam-filled honeycomb sandwich panels under in-plane impact loading and the analysis of their crashworthiness. This paper presents a finite element analysis of foam-filled honeycomb sandwiches under in-plane impact loading. Three different aluminium honeycombs filled with three different polyurethane foams were considered in the numerical simulation, and results were compared with those obtained for bare honeycomb panels. For what concerns the crashworthiness analysis, the response of the foam-filled honeycomb panels under out-of-plane impacts was compared with those of unfilled honeycomb panels and circular tubes.

Non-destructive evaluation of damage modes in nanocomposite foam-core sandwich panel subjected to low-velocity impact

Abstract: This research studied the effect of multi-walled carbon nanotubes (MWCNTs) on the internal and external damages of foam-core sandwich panels with kevlar fiber reinforced epoxy face sheets subjected to a low-velocity impact. The sandwich panels were subjected to six levels of energy. Energy profile diagrams (EPDs) were plotted to determine the rebounding, penetration and perforation thresholds of the sandwich panels. Non-destructive evaluation methods have been employed for detecting and measuring damage size of the sandwich panels using X-ray radiography and active infrared thermography. The results show that MWCNTs can improve the absorbed energy and penetration threshold of the foam-core sandwich panels.

The dynamic response of sandwich panels with cellular metal cores to localized impulsive loading

Abstract: The deformation/failure modes and dynamic response of peripherally clamped square monolithic and sandwich panels of localized impulsive loading were investigated experimentally by metallic foam projectile impact. The sandwich panels comprise three different types of cellular metallic cores, i.e., closed-cell aluminum foam core, open-cell aluminum foam core and aluminum honeycomb core. Experimental results show that all the sandwich panels present mainly large global inelastic deformation with obvious local compressive failure in the central area, except for those open-cell foam core sandwich panels. The dynamic response of sandwich panels is sensitive to the applied impulse and their geometrical configurations. Based on the experimental investigation, a theoretical analysis was developed to predict the dynamic response of sandwich panels by employing a comprehensive yield locus and a modified classic monolithic panel theory. A comparison of experimental results and theoretic predictions was made, and a good agreement was then found. These findings are very useful to guide the engineering applications of metallic sandwich structures for the protection purpose.

Mechanical performance of modular FRP-steel composite beams for building construction

Abstract: This paper presents an experimental and modelling investigation into modular web-flange fibre-reinforced polymer (FRP)-steel composite systems for use in building floor construction. The modular FRP slabs are formed from adhesively bonding pultruded box profiles sandwiched between two flat panels. They are then connected via adhesive or novel bolted connections to steel beams to form a composite system. Two different fibre (pultrusion) configurations are investigated in this paper: flat panel pultrusion with direction either parallel or perpendicular to the box profiles. Composite beams were tested under four-point bending and evaluated for bending stiffness, load-carrying capacity, and the degree of composite action within the FRP web-flange sandwich slab and that provided by the shear connections. All the composite beams showed ductile load–deflection responses, with yielding of the composite beam commencing prior to failure of the FRP slabs. Furthermore, adhesive bonding provided full composite action, but the novel bolted connections with a certain spacing provided either full or partial composite action, dependent on the pultrusion configuration of the FRP slab. Finally, an analytical procedure is presented to evaluate the bending stiffness and load-carrying capacity of the composite beams. Finite element analysis was also employed in this study, showing good comparisons to the experimental results.