Core Material For Sandwich Structures Research Articles

Lightweight Solid Wood Panels Made of Paulownia Plantation Wood

Light Paulownia seamless-edged glued solid wood panels (SWPs), single-layered and three-layered, were analyzed in this study. Both panel types were calibrated at a thickness of 19 mm, a dimension very often in demand on the SWP market, but produced with other wood species (for example, spruce, pine, larch and fir). The panels were bonded with melamine-urea formaldehyde, polyurethane and polyvinyl acetate resins. The panels were tested for their physical (density) and mechanical (modulus of rupture, modulus of elasticity, compressive shear strength and wood breakage rate) properties. For the single-layered panels, the mechanical and physical properties did not differ significantly and were similar to massive Paulownia wood. For the three-layered panels, the adhesive application of polyurethane influenced positively all SWP properties. Considering the differences in density, these composites failed to achieve the performance of one- and single-layered panels made of spruce. The results of these findings recommend Paulownia SWPs to be used as lightweight and sustainable core materials in sandwich structures for the furniture and packaging industry, sport articles or non-load-bearing constructions.

Flexural Behaviour of GFRP-Softwood Sandwich Panels for Prefabricated Building Construction.

Studies have shown that the proper selection of core materials in sandwich structures improves the overall structural performance in terms of bending stiffness and strength. The core materials used in such systems, such as foam, corrugated, and honeycomb, are frequently applied in aerospace engineering. However, they are a costly option for civil engineering applications. This paper investigates the bending performance of the proposed GFRP softwood sandwich beams assembled using pultruded GFRP with adhesive connection methods for potential applications in prefabricated building construction. The ultimate load capacity, load-deflection responses, failure modes, bending stiffness, load-axial-strain behaviour, and degree of composite action were experimentally evaluated. The effects of varying shear-span-to-depth ratios a/d between 2 and 6.5, as well as different timber fibre directions of the softwood core, on the overall structural performance were clarified. The results showed that changing the timber fibres' orientation from vertical to longitudinal shifted the failure mode from a brittle to progressive process. Moreover, the adhesive bonding was able to provide full composite action until the failure occurred. Finally, numerical modelling was developed to understand failure loads, deformation, failure modes, and strain responses, and to evaluate bending stiffness and composite action. The results showed satisfactory agreement with the experiments.

Compressive behavior of a natural core

Composite sandwich structures with core cellular materials are widely used in diverse industries such as the aerospace, marine, car, and wind industries due to their good mechanical properties on bending. The growing environmental awareness has increased the interest in the use of materials from biodegradable and renewable sources. During recent years, there has been an increasingly growing interest in the use of agglomerated cork as a core material in sandwich structures, replacing synthetic polymeric foams in specific applications. Among its remarkable combination of properties, we can find a hyperelastic behaviour, low thermal conductivity, low permeability, and good energy absorption and vibration damping properties. Additionally agglomerated cork is produced from the waste of the cork industry, being a renewable and biodegradable material that is environmentally sustainable. The aim of this work is to address the quasi-static and dynamic compression testing of two types of agglomerated cork of different densities, using a servo-hydraulic testing machine and drop weight tower. The mechanical behaviour and energy absorption properties at different strain rates are evaluated from the contact force measurements and estimations of impact kinetic energy. Additionally, Digital Image Correlation (DIC) is used to estimate the variability in the local strain through the test specimen.

The Compressive Behavior and Crashworthiness of Cork: A Review.

Cork, a natural material from renewable resources, is currently attracting increasing interest in different industrial fields because of its cellular structure and the presence of the flexible suberin as its main chemical component. In an agglomerated form, it proved to be a compelling product not only as a thermal and acoustic insulator, but also as core material in sandwich structures and as a liner or padding in energy absorbing equipment. From this perspective, the assessment of its compressive response is fundamental to ensure the right out-of-plane stiffness required to a core material and the proper crashworthiness in the safety devices. Considering the complex nature of cork and the resulting peculiar compressive response, the present review article provides an overview of this paramount property, assessing the main parameters (anisotropy, temperature, strain rate, etc.) and the peculiar features (near-zero Poisson’s ratio and unique dimensional recovery) that characterize it in its natural state. Furthermore, considering its massive exploitation in the agglomerated form, the design parameters that allow its compressive behavior to be tailored and the operating parameters that can affect its crashworthiness were assessed, reporting some potential industrial applications.

Titanium lattice structures manufactured by EBM process: Effect of skin material on bending characteristics

Lattice structures are very interesting for applications in the aerospace field since they combine high stiffness with lightweight. Moreover, they can constitute the core material for sandwich structures. In this work, the bending properties of two kinds of sandwich structures were compared through experimental activity: both of them had a lattice core of titanium alloy manufactured through Electron Beam Melting process, but one presented composite material skin and the other titanium skin. For the same geometrical parameter, the bending stiffness of the all-titanium structure was found higher, but the hybrid one was deemed more performant if the weight is equal.

Experimental and multiscale numerical investigations on low-velocity impact responses of syntactic foam composites reinforced with modified MWCNTs

Abstract This study focused on experimental and numerical investigations into the low-velocity impact behavior of epoxy resin matrix syntactic composites with embedded hollow glass microspheres (HGMs) and multiwalled carbon nanotubes (MWCNTs). The synergistic effects of HGMs and MWCNTs on the mechanical properties of epoxy resin composites were improved by applying amine and acid treatments to HGMs and MWCNTs, respectively. The influence of the MWCNT content and the applied impact energy on the impact responses and compression strength after the impact of these syntactic foam panel samples were discussed. The results indicated that modifying HGMs and MWCNTs contributed to improving the energy absorption and the strength retention factor (SRF) of these panels and the SRF increased with increased MWCNT content. Moreover, multiscale finite-element (FE) models were developed to simulate panel impact behavior, and modeling results were compared with experimental data. Then, the verified FE model was used to analyze the influence of CNT types (helical CNTs vs MWCNTs) and the diameter-to-thickness ratios of HGMs. This study provided a theoretical basis and design reference for a novel lightweight composite material subjected to low-velocity impact, which could be applied as a core material for sandwich structures in aerospace, marine engineering, transportation, and civil infrastructures.

Bending properties of textile reinforced concrete sandwich beams with gypsum and calcium silicate core

Sandwich systems are gaining prominence because they offer thermal insulation in many building structures. There is a growing interest in better understanding the behavior of sandwich structures, as well as there is a need to monitor and predict the consequences of the limitations and weaknesses inherent in their design. The aim of this study is to evaluate the effect of two types of core materials on the bending properties of textile reinforced concrete (TRC) sandwich beams. In TRC, bi-directional glass textiles were used as reinforcement along with fine grained cementitious binder. TRC sandwich beam consisted of gypsum or calcium silicate panels with different density as core and TRC as skins. The stress transfer between skin and core was attained using adhesive tension without the use of mechanical anchors or adhesives. The bending properties of TRC sandwich beams including the effect of number of layers of textile in the TRC skins and independent behavior of core materials were investigated using three-point bending test. The properties such as flexural strength, toughness and residual strength have been evaluated for all specimens. Feasibility of using ASTM guidelines has been explored in estimating the toughness of various TRC systems. The general outcomes of this investigation promise a good influence for the application of gypsum and calcium silicate as core material for sandwich structures.

On the Filler Materials of Metal Matrix Syntactic Foams.

Metal matrix syntactic foams (MMSFs) are becoming increasingly relevant from the lightweight structural materials point of view. They are also used as energy absorbers and as core materials for sandwich structures. The mechanical properties of MMSFs are extensively influenced by the properties of their filler materials which are used to create and ensure the porosity inside the metal matrix. As the properties of fillers are of such importance in the case of MMSFs, in this paper three different filler materials: (i) ceramic hollow spheres (CHSs), (ii) metallic hollow spheres (MHSs) and (iii) lightweight expanded clay particles (LECAPs), have been investigated in numerous aspects. The investigations cover the microstructural features of the fillers and the basic mechanical properties of the fillers and the produced MMSFs as well. The microstructure was studied by optical and electron microscopy extended by energy-dispersive X-ray spectrometry, while the basic mechanical properties were mapped by standardized compression tests. It was found that in the terms of cost-awareness the LECAPs are the best fillers, because they are ~100 times cheaper than the CHSs or MHSs, but their mechanical properties can be compared to the aforementioned, relatively expensive filler materials and still exceed the properties of the most ‘conventional’ metallic foams.

Lattice materials with pyramidal hierarchy: Systematic analysis and three dimensional failure mechanism maps

Sandwich core materials that offer superior mechanical properties at minimum weight are essential in designing high-performance sandwich structures. Hierarchical materials are ideal templates for this purpose. In this paper, we investigate the mechanical performance of a pyramidal–pyramidal hierarchical lattice material to highlight its potential as the core material for sandwich structures. Three-dimensional failure mechanism maps for the pyramidal–pyramidal hierarchical lattice material are developed under different loading conditions and the results are compared to finite element simulations. Next, we study the mechanical response and failure modes of a sandwich panel with self-similar pyramidal lattice core construction subjected to in-plane compression and three-point bending. The current study indicates that the pyramidal–pyramidal hierarchical configuration can improve the load bearing capacity and core buckling resistance of the sandwich structures at low density. The study provides insights into the role of structural hierarchy in tuning the mechanical response of the lattice materials and expands the application envelope of lightweight sandwich structures by effectively increasing the structural buckling resistance.

Failure In metal honeycombs manufactured by selective laser melting of 304 L stainless steel under compression

ABSTRACTCellular structures, specifically honeycombs, are commonly used as core materials in sandwich structures. This is especially true in aerospace applications where high bending and out-of-plane compressive stiffness coupled with low component weight is required. Additive manufacturing techniques are well suited for the manufacture of such cellular structures in a cost-effective manner. The current work focuses on honeycombs using selective laser melting of 304 L stainless steel. The mechanical behaviour of honeycombs was evaluated using out-of-plane compression tests. A numerical model was built to describe failure of the additively manufactured honeycombs. Compression tests were performed, on cylindrical samples to build the nonlinear material model. The material behaviour was found to be dependent on the build direction. Results of experiments and simulation show that failure occurs through a plastic buckling mechanism.

Effects of the core density on the quasi-static flexural and ballistic performance of fibre-composite skin/foam-core sandwich structures

Polymeric foams are extensively used as the core materials in sandwich structures and the core material is typically bonded between relatively thin fibre-composite skins. Such sandwich structures are widely used in the aerospace, marine and wind-energy industries. In the present work, various sandwich structures have been manufactured using glass-fibre-reinforced polymer (GFRP) skins with three layers of poly(vinyl chloride) foam to form the core, with the densities of the foam layers ranging from 60 to 100 kg/m3. This study has investigated the effects on the quasi-static flexural and high-velocity impact properties of the sandwich structures of: (a) the density of the polymeric-foam core used and (b) grading the density of the foam core through its thickness. The digital image correlation technique has been employed to quantitatively measure the values of the deformation, strain and onset of damage. Under quasi-static three-point and four-point bend flexural loading, the use of a low-density layer in a graded-density configuration reduced the likelihood of failure of the sandwich structure by a sudden force drop, when compared with the core configuration using a uniform (i.e. homogenous) density layer. The high-velocity impact tests were performed on the sandwich structures using a gas-gun facility with a compliant, high-density polyethylene projectile. From these impact experiments, the graded-density foam core with the relatively low-density layer located immediately behind the front (i.e. impacted) GFRP skin was found to absorb more impact energy and possess an increased penetration resistance than a homogeneous core structure.

Experimental investigations on the sandwich composite beams and panels with elastomeric foam core

In this paper, the load-carrying capacity and failure mechanisms of sandwich beams and panels with elastomeric foam core and composite laminate face sheets are investigated. For this purpose, the flexural behavior of laminated composite beams and panels (applied as face sheets) is firstly investigated under three-point bending and central concentrated loads, respectively. Then, the same examination is conducted for the sandwich beams and panels, in which the proposed elastomeric foam is utilized as the core material. It is shown that the failure mechanisms which are associated to the core in the sandwich structures with crushable foams are not considered in the examined sandwich structures. The collapse of the sandwich specimens, examined here, is observed due to the failure of the skins in some steps. By multi-step collapse of these specimens via separately failure of the top and bottom skins, a considerable amount of energy is absorbed between these steps. Due to non-brittle behavior of the core material under loading, a large compression resistance is observed after failure of the top skin which led to the recovery of the load-carrying capacity in the sandwich beams. A similar behavior for the sandwich panels led to the increase of the ultimate strength after appearance of the failure lines on the top skin. The general outcomes of this investigation promise a good influence for the application of elastomeric foam as core material for sandwich structures.

Dynamic Response of Green Sandwich Structures

Synthetic fibre-synthetic foam core sandwich composites are widely used for many structural applications due to their superior mechanical performance and low weight but the limited end of life disposal options and environmentally friendly character are currently envisaged as barriers to their continued development. The objective of this article is to analyse the suitability of using agglomerated cork as core material in sandwich structures to be used in applications where energy absorption due to low velocity impacts can be of importance. Green sandwich specimens with flax/epoxy face sheets and agglomerated cork as core have been manufactured and their response to low velocity impacts has been compared to the results obtained with similar specimens using traditional synthetic core. This study shows that the peculiar deformation mechanisms of cork can allow to tailor the damage extension through-the-thickness thus providing in principle a better damage tolerance after impact.

Entangled Cross-Linked Fibres for an Application as Core Material for Sandwich Structures - Part I: Experimental Investigation

Entangled cross-linked fibres were studied for an application as core material for sandwich structures. Specimens were produced from carbon, aramid and glass fibres, and cross-links were achieved using epoxy spraying. It was observed that this type of entangled cross-linked fibres could be fabricated without any major technical difficulties. The scope of this paper is to study the effect of some different parameters on the mechanical properties of these materials. Different effects were investigated: effect of fibres length, of fibres nature, of mixing fibres, of carbon skins and of the resin. The first part of this paper deals with the production of these entangled cross-linked fibres. The compression, tension and three point bending tests are detailed in the second part and the results are compared with usual core material currently used in industries.

Entangled Cross-Linked Fibres for an Application as Core Material for Sandwich Structures - Part II: Analytical Model

Entangled cross-linked carbon, aramid and glass fibres were recently produced by epoxy spraying for an application as core material for sandwich panel. The Young’s moduli in compression and tension have been previously measured and briefly summarized in this paper. To optimize the core structure, modelling of these properties has been achieved in the present paper. The cross-link fibres have a random orientation and the stiffness of the epoxy joint is modelled by a torsion spring. A parallel model is chosen for homogenisation. It was found that the experimentally estimated stiffness of these materials fits fairly well with the modelled ones.

Investigation of dynamic properties of natural material-based sandwich composites: Experimental test and numerical simulation

With the development of technology, the use of glass fibres and sandwich structures in a wide variety of engineering applications has had important growth in recent years because of their low self-weight and high strength when compared to conventional materials. Moreover, natural materials can be used as the core material in sandwich structures instead of conventional materials. Since synthetic materials cause a reduction of carbon release during material formation, these kinds of materials are renewable and recyclable. Therefore, in sandwich structures, renewability and recycling can be provided. In this study, dynamic properties of natural material-based sandwich composites were investigated experimentally and numerically. The effects of the thickness of the core and fibre orientation and number of layers on frequency and damping were analysed. It was observed that if the core thickness of the structure is properly optimised, this sandwich structure demonstrates better dynamic properties. Thus, sandwich materials from natural origin may offer more environmental friendly solutions compared to other materials.

Combustion kinetics of two core materials for sandwich structures

The thermal behavior of a polymeric foam (Divinycell H100) and balsa wood (ProBalsa LD7), which are extensively applied as core materials for sandwich panels, is investigated. Chars, produced from pyrolysis of thick samples at 950 K, reflect significant shrinkage of the sample and, for the polymeric foam, the complete loss of the cellular structure following softening. Thermogravimetric curves, measured in air at different heating rates, show that combustion takes place over a temperature range of 650–940 K (polymeric foam) and 475–740 K (balsa). The curves are well predicted by a four-step and a six-step reaction model, respectively, with kinetic parameters that are invariant with the heating rate.

An investigation on the flexural properties of balsa and polymer foam core sandwich structures: Influence of core type and contour finishing options

Polymer foams are frequently used as core materials in sandwich structures for applications such as aerospace, naval, and wind industry. It is known that the core material contributes to the overall mechanical properties of these sandwich structures up to a remarkable extent. In addition, due to the curvature and geometrical complexities of several applications, these cores are available with special cuts/grooves (finishing options) to provide bendability and better resin infusion during processing. The goal of this study is to investigate the mechanical performance of synthetic polymer foams as core materials for sandwich structures. Balsa wood, which is the most common traditional structural core, was used as reference material. Glass fiber reinforced epoxy was employed as face sheets. End-grain Balsa wood, commercially available polyvinyl chloride and polyethylene terephthalate, and experimental grades of polyurethane foams were chosen as alternative core materials. Quasi-static flexural tests were carried out using a four-point bending setup according to ASTM C 393. Sandwich properties such as stiffness, core shear strength, and energy absorption were determined and compared. The contour cuts filled with resin show a reinforcing effect against transverse shear stresses leading to higher shear strengths. Digital image correlation was used to study the yielding and permanent strain of the foam core sandwich beams.

In situ mechanical characterization during deformation of PVC polymeric foams using ultrasonics and digital image correlation

Cellular solids such as polymeric foams are finding increasing applications including its use as a core material for sandwich structures. In this study, a novel method is used to measure both the longitudinal and shear wave speeds of a material simultaneously while applying a compressive load in four different densities of polymeric foams made from the same base polymer, polyvinyl chloride (PVC). The study showed that there was a significant difference in evolution of the wave speeds and hence in the apparent modulus during deformation of the lower density foams in comparison to the higher density foams. The non-contact full-field method of digital image correlation (DIC) is used to gain insights into the failure modes during deformation. The lower density foams undergo heterogeneous deformation and failed due to buckling of cell walls. In contrast, the higher density foams undergo nominally homogeneous deformation due to plastic collapse. The failure mode transition is shown to be governed by the relative density of the foams and the mechanical properties of the polymer.

Microvascular based self-healing polymeric foam

A self-healing microvascular polymeric foam has been developed to improve the resilience of rigid foam core materials for sandwich structures. We investigated the healing of brittle polyisocyanurate (PIR) foam after mode-I crack separation in a 3-point single edge notch bend (SENB) specimen. A two-part healing chemistry based on a commercially available polyurethane (PUR) foam formulation is employed to rebond the interface. Both components are initially sequestered in separate channels in a vascularized SENB geometry. Upon loading and subsequent crack propagation through the network, the healing agents are released and polymerize on contact to create new foam material in the crack plane. An attractive feature of this system is the volumetric expansion of the healing chemistry, demonstrating the ability to repair macro-scale damage. The foaming reaction occurs on the order of minutes at room temperature, enabling rapid in-situ healing. Furthermore, by using a vascular delivery technique, multiple damage–recovery cycles are achieved at consistently high healing efficiencies. Through repeated mechanical testing, we have demonstrated over 100% recovery in fracture toughness for this new class of bioinspired, self-healing cellular materials.