Sandwich Core Materials Research Articles

Sandwiched Plate Vibration Analysis with Open and Closed Lattice Cell Core

This work attempts to replace the sandwich core's traditional shape and material with a cellular pattern, where the cells have a regular shape, distribution, and size. The contribution of this paper is to design two structures, one open-celled and the other closed, and to evaluate the performance of sandwich plates with lattice cell core as it is used for many industrial applications, particularly in automobile engineering. The new theoretical formulations are constructed for two structures to find the free vibration characteristics. The results of the new design are compared with the traditional shape. Derivation of equations to predict mechanical properties based on relative density with the chosen shapes, specific vibration equation of three-layer sandwich plate, and substitution by equation using excel sheet. Results are promising, and the effectiveness of cellular pattern theoretical analysis estimation. Limitations and error rates for the mechanical properties come through the empirical equations, and their ratio to the relative density values are higher depending on the behavior of the core material. Findings reveal, with open cell decrease in modulus of elasticity by (PLA: -90.4%) and (TPU: -90.4%), increases natural frequency by (PLA: 44.5%) and (TPU: 46.4%), as for closed-cell decreases in the modulus of elasticity by (PLA: -66.9%) and (TPU: -64.4%), increases natural frequency by (PLA: 36%) and (TPU: 37.7%). Converting a solid substance or replacing a foam form with a cellular pattern is one way to better performance and save weight through the selected cell pattern in absorbing the energy of the vibration wave.

Design of double-layered framed plate with equivalent impedance

To get clear the vibration mechanism of sandwith shell using the experiment method, a double-layered plate is designed for the sandwich shell by matching the equivalent impedance. Numerical analysis is conducted, and In 10Hz-4000Hz, the differenceof impedance from double-layered plate and sandwich shell should be less than 10%. A new design is given for the double-layered plate, and which is quite simple for construction. Then, the double-layered plate could be applied to conduct series vibration tests to get clear the influence of sandwich core material on structural vibration, while it could also reduce the costs of money and construction time.

Hydrostatic high strain rate loading response of closed-cell polymeric foams as a function of mass density

Closed cell polymeric foams are widely used in naval structures as sandwich core material owing to their lightweight and high energy absorption capabilities. There has been a great push towards understanding their material response under multiple loading scenarios, however, the loading scenarios have been limited to uniaxial or multiaxial in air loadings at different strain rates. Here, the hydrostatic elastic response and yield behavior of PVC foams with varying mass densities under a high strain rate hydrostatic loading state have been investigated. A novel underwater high strain rate loading facility is utilized to subject these foams to near blast strain rate hydrostatic loading. Using the 3-D Digital Image Correlation (DIC) technique in conjunction with ultra-high-speed photography, full-field volumetric deformation data is obtained. The dynamic loading data is obtained using a piezoelectric pressure sensor. This enables the measurement of material yield strength and bulk modulus for these materials under high strain rate hydrostatic loading conditions. The foams demonstrate increment in bulk modulus and yield strength under high strain rate loading. Further, the increment observed is highly sensitive to the material bulk mass density.

Thermal and Mechanical Characterisation of Sandwich Core Materials for Climatic Chamber Shells Subjected to High Temperatures

Climatic chamber testing conditions are becoming more demanding. A wide range of temperatures is used to check the quality of products and materials, since they are constantly being improved. However, there is no literature on how the components of the climatic chamber panels react under high temperatures. The present work therefore sets out to perform a thermal and mechanical characterisation of four core materials often used in sandwich panels: balsa wood, mineral wool, and polyethylene terephthalate and polyurethane rigid foams. The thermal characterisation focused on thermal conductivity and the specific heat was characterised using an indirect method developed previously by the authors to simulate a real application scenario where one surface of the sandwich panels was subjected to high temperature, while the opposite surface was kept at room temperature. Steady and unsteady conditions were analysed up to 200 °C. Balsa and mineral wool exhibited a nonlinear increase in thermal conductivity with temperature, and the polymeric foams showed linear behaviour. The specific heat results also increased with temperature, and the relation was nonlinear for all the tested materials except for polyethylene terephthalate, which showed linear behaviour. Higher temperatures had the least effect on the specific heat for balsa wood and mineral wool. The polyethylene terephthalate foams were the most affected by temperature. Temperature variation was tested using the impulse excitation technique. The polymeric foams and balsa wood were studied up to 100 °C and 160 °C, respectively. The elastic modulus decreased with temperature. After 24 h of cooling, the tests were repeated and the elastic modulus had regained or even increased its initial value, for all the materials.

Development of lignin‐based carbon foam for use as an FRP sandwich core material

AbstractSandwich‐structure materials consist of a high‐strength skin material and a lightweight core material. The advantages of sandwich structures are known to include excellent mechanical properties and low weight, the latter of which is the result of a lightweight core. However, soft‐core members such as plastic foam materials have low rigidity and therefore may not exhibit adequate functionality when used in sandwich structures. This research sought to reduce environmental impact and to facilitate sustainable development by developing a method for fabricating high‐strength carbon foam using alkali lignin for use as an FRP sandwich core and enhancing the mechanical properties of the carbon foam with short glass fiber. PMMA particles decomposed at high temperatures to leave gaps, creating carbon foam with a closed structure. The relationship between PMMA particle size and foam pore size was investigated by observing cross‐sections of the foam.

Mechanical properties of a balsa wood veneer structural sandwich core material

Balsa wood is an appropriate core material for structural sandwich applications due to its high strength- and stiffness-to-weight ratios. However, the mechanical properties vary considerably owing to the inherent scattering of natural wood materials. One approach to reduce this scatter and tailor the mechanical properties according to specific application needs is to recompose the natural material into a veneered material consisting of veneer layers of different grain orientations, which are adhesively bonded together. The mechanical properties of such a veneered balsa wood, composed of alternating 0°/90° grain orientations, were investigated at ambient temperature according to corresponding standards. The properties were significantly influenced by the orthotropy on the material scale within one veneer layer and on the system scale within the assembled veneer layers. Standardized experimental set-ups and specimen geometries may produce artifacts such as buckling or strain hardening which deviate from the material behavior in real structures. The thin adhesive between the veneer layers did not negatively affect the mechanical behavior since failure occurred within the veneer layers and not in the interfaces.

Compression Properties of 3d Printed Honeycomb and Re-Entrant Sandwich Core Materials

In the presented paper, honeycomb (cellular) and re-entrant (auxetic) structures were produced with different thermoplastic filaments on FDM type 3D printer and their compression properties were tested. Samples were prepared from PP, ABS, PLA and PA thermoplastic polymers. The samples were subjected to quasi-static compression in edgewise and flatwise direction and their stress-strain relationships were established. For a proper strain evaluation, digital-image correlation was applied to measure full-field displacements on the sample surfaces and their and their compressive-deformation behaviour in both elastic and plastic regime up to the densification were evaluated. It was shown through compression tests in flatwise and edgewise directions re-entrant type of auxetic cores components exhibit higher compression failure stresses compared to those of cellular honeycomb core components. The stress strain curves of the re-entrant core failure show a higher energy absorbance of the honeycomb cores for the same polymer types

Creative design for sandwich structures: A review

Sandwich structures are important innovative multifunctional structures with the advantages of low density and high performance. Creative design for sandwich structures is a design process based on sandwich core structure evolution mechanisms, material design method, and panel (including core structure and facing sheets) performance prediction model. The review outlines recent research efforts on creative design for sandwich structures with different core constructions such as corrugated core, honeycomb core, foam core, truss core, and folded cores. The topics discussed in this review article include aspects of sandwich core structure design, material design, and mechanical properties, and panel performance and damage. In addition, examples of engineering applications of sandwich structures are discussed. Further research directions and potential applications are summarized.

Investigating the Effect of Dimension Parameters on Sound Transmission Losses in Nomex Honeycomb Sandwich

In this study, an impedance tube test was performed to explore the influence of various dimension parameters of Nomex honeycomb sandwich core material on sound transmission loss (STL). The parameters investigated included the size of the honeycomb cells and thickness of the face sheets and honeycomb cores, and the effects of single- and double-layered sandwich structures were also explored. The boundary element and finite element methods were used to simulate test results. The results show that the size of the honeycomb cells has an insignificant effect on STL. Increasing the thickness of face sheets can move the STL valley point of the material at high frequency (around 5 kHz) in the low-frequency direction and increase the STL in parts of the high frequency band. Increasing the thickness of the honeycomb core can improve STL, on the whole, but the magnitude of the improvement effect becomes weakened after the thickness of the core reaches 30 mm. The STL of double-layered structures was found to be superior to that of the single-layered structures. The simulations reveal that the trends in the STL curves of the honeycomb sandwich panels are influenced by the structural mode of the panels, and are related to the resonance of the materials. The results and relevant conclusions obtained through the above research verify that the law of influence of the structure dimension parameters on the STL measured by the impedance tube is similar to that of the large panel. This can provide a reference for the application of the impedance tube test method in structural noise reduction design.

Low-velocity impact response and compression after impact behavior of tubular composite sandwich structures

This work investigates the dynamic impact response of tubular composite structures with/without honeycomb sandwich core under transverse low-velocity impact (LVI) and compression after impact (CAI) test. Damage mechanisms, such as matrix cracking, delamination and fiber breakage/rupture in face sheets as well as honeycomb crushing and buckling in the core, are characterized by X-ray micro-computed tomography (μCT) to understand failure processes and their relationship with core material and impactor shape. It is found that sandwich core material helps to absorb impact energy and resist localized damage formation. The benefit of core material includes greater energy absorption capability and higher specific CAI strength. The progressive damage events from damage initiation, failure propagation to final collapse of a tubular composite structure during CAI test are also further discussed.

Mode II interfacial fracture characterization of foam core sandwich materials at elevated temperatures: the effects of frictional stresses between the crack edges

In recent years, composite sandwich material has been widely used in civil engineering due to its high strength to weight ratio, high stiffness to weight ratio and anti-corrosion ability. However, the existing research studies indicated that interfacial delamination between skins and core was a common failure mode for this kind of material, which seriously threatens the durability and safety of structures. Furthermore, up to now, there are no research studies investigating the effects of elevated temperature on the interfacial fracture and damage behavior of the sandwich material. To address the above questions, this paper investigated the effects of elevated temperature on mode II interfacial fracture response of foam core sandwich materials by adopting the end-notched flexure method. Test results showed that the loads and the strain energy release rate decreased under the same crack length as the temperature increased. In the meantime, the corresponding analytical model, considering the effect of frictional stresses between the crack edges, has been proposed to predict the interfacial strain energy release rate. The comparison of the analytical and experimental results revealed that the analytical model could accurately predict the strain energy release rate of mode II interfacial fracture of foam core sandwich materials at elevated temperatures.

Thermomechanical characterization of a balsa-wood-veneer structural sandwich core material at elevated temperatures

The tensile, compressive and shear behavior of standardized specimens cut from a veneered balsa wood used as structural sandwich core was investigated at ambient temperatures of up to 250 °C. The average moisture content was 9.1%. The specimens’ responses were strongly affected by the configuration of the veneer layers. Specimens with a higher number of 0° veneer layers in the loading direction exhibited higher strength and stiffness at each temperature. Independent of the loading type, the specimens gradually lost strength and stiffness up to the wood-burning temperature of 250 °C due to the softening of the hemicellulose and lignin. Smaller cross sections with more surfaces cut perpendicular to the grain dried faster and thus delayed the degradation of properties. The dominant failure modes did not change with increasing temperature if the behavior remained either fiber- or matrix-dominated. The failure mode changed if the behavior shifted from fiber-dominated at lower temperatures to matrix-dominated at higher temperatures. The degradation of the properties of the adhesive between the veneer layers affected specimen behavior only at the highest temperature.

Fiber-Reinforced Composite Sandwich Structures by Co-Curing with Additive Manufactured Epoxy Lattices

In this paper, a new process of joining additive manufactured (AM) lattice structures and carbon fiber-reinforced plastics (CFRPs) to manufacture hybrid lattice sandwich structures without secondary bonding is investigated. Multiple variations of lattice structures are designed and 3D printed using Digital Light Synthesis (DLS) and a two-stage (B-stage) epoxy resin system. The resulting lattice structures are only partially cured and subsequently thermally co-cured with pre-impregnated carbon fiber reinforcement. The mechanical properties of the additive manufactured lattice structures are characterized by compressive tests. Furthermore, the mechanical properties of hybrid lattice sandwich structures are assessed by flexural beam testing. From compressive testing of the additive manufactured lattice structures, high specific strength can be ascertained. The mechanical behavior shows these lattice structures to be suitable for use as sandwich core materials. Flexural beam testing of hybrid lattice sandwich structures shows high strength and stiffness. Furthermore, the strength of the co-cured bond interface is high enough to surpass the strength of the lattice core.

Core failure in sandwich structures subjected to water slamming loads

Sandwich composite materials are widely used within the marine industry, particularly as hull panels. Water impact loads, known as slamming, can be very significant for these structures, particularly for high-speed craft. These loadings generate local regions of high transverse shear forces near panel boundaries, which can result in transverse shear failures of core materials. The transient nature of slamming loads can cause stress rates that are high enough to affect the strength of the core material, particularly for polymeric foams. Despite the significant body of work on the constitutive behaviour and failure mechanics of sandwich core materials, there is a lack of understanding of how core materials fail in transverse shear during slamming events. There is also only very limited knowledge of how the core shear strengths measured using standardised, often quasi-static material coupon testing relate to their behaviour in a panel-slamming situation. This paper contributes in two novel areas; controlled experimental characterisation of the failure mechanics of sandwich panels subjected to water slamming to understand and quantify the strength of different polymeric core materials, comparison of the failure modes and transverse shear strength of slam-loaded sandwich panels to predictions from material coupon properties. Core types include low, medium and high elongation polymeric foams. The results demonstrate that the more ductile foams perform better as panel structures under slamming relative to their quasi-static properties compared with the more brittle cores. Prediction of the strength of a panel is shown to be highly dependent on the load distribution and whether the static or dynamic core strength is considered. The results support empirical experience that ductile foams perform well under slamming loads, and that high-elongation materials can perform better in slamming situations than predicted by their quasi-static strengths.

Lightweight vitreous carbon material: approaches to making open-pore cellular structure

Cellular vitreous carbons with the open-pore structure are attractive materials in some aerospace applications, such as the super-lightweight structural materials or sandwich core materials in the thermal protection systems. Cellular vitreous carbon (or VC foam) with the open-pore structure can be synthesized by pyrolysis of cellular precursor structures made from some types of resins, such as epoxy or phenolic resins. Several approaches elaborated at RPMI, Belarus can be applied to create the precursor resin foams. The replication approach ensures perfect open-pore structure with the porosity of >90% and high hydraulic permeability. The sacrificial template approach ensures lower porosity and hydraulic permeability, and higher specific strength. In the approach with the deformable space-holder granules in the sacrificial template, the foam porosity can be controlled in a wider range. The paper compares different methods of making the precursor resin foams and properties of VC structures synthesized by pyrolysis of these precursor foams.

Review on Sandwich Composite and their Applications

Abstract Sandwich composite material structures arevery important factors because the advantages of light weight and high strength and stiffness of the sandwich core composite structure. Honeycomb sandwich composite materials and corrugated cores are mostly used in the aerospace, marine structure, automobile, structural and construction etc. To withstand high temperature and low temperature applications are also on important factor to manufacture a new material sandwich structure. To find a new method and to adopt a new technology for a new material sandwich structures and skin materials are the main objective of this paper. The prepared sandwich core to work in high temperatures and low temperatures applications and withstand heavy loads is important factor. This review paper aims to find a new method and materials for sandwich core materials for all suitable applications of composite structures with cost effectiveness suitable for the next advancement.

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.

Analysis of syntactic foam – GFRP sandwich composites for flexural loads

The use of glass microballoon (GMB) — epoxy syntactic foams as a sandwich core material is studied. The skins and foam core are fabricated and joined instantaneously unlike the procedures followed in the previous studies. Each successive layer of the sandwich is fabricated when the previous layer is in a semi-gelled state. These sandwich samples are characterized for their properties under flexural loading. The failure modes and mechanical properties are carefully investigated. The change in fabrication technique results in a significant increase in the load bearing pattern of the sandwich. In earlier studies, debonding was found to occur prematurely since the bonding between the skins and core is the weakest plane. Using the current technique, core cracking occurs first, followed by skin fiber breaking and debonding happens at the end. This ensures that the load carrying phase of the structure is extended considerably. The sandwich is also analytically studied using Reddy’s higher order shear deformation theory. A higher order theory is selected as the sandwich can no longer be considered as a thin beam and thus shear effects also need to be considered in addition to bending effects.

Sandwich Core Material Development for Ship Deck Structure

Development of sandwich materials for ship structure is aimed mainly to reduce the ship light weight so that the ship payload increases. This paper addresses development of sandwich core material suitable for ship structure. The core materials were composed of synthetic resin and talc; percentages of talc by total weight investigated were 10%, 20%, 30%, and 40%. Laboratory tests stipulated by the Lloyd’s Register (LR) and Det Norske Veritas (DNV) were performed on the core materials. Moreover, sandwich plates consisted of steel faceplates and the core materials were tested using ASTM standards including tension, shear, and flexure tests. The sandwich plates were applied for designing car deck of a ro-ro ship. Analysis of the sandwich deck under design load utilizing finite element approach was carried out to select the best sandwich core materials developed in this study. It was observed that the sandwich panel deck composed of synthetic resin and core material with 40% talc was better than the others.

Low velocity impact resistance and energy absorption of environmentally friendly expanded cork core-carbon fiber sandwich composites

This study investigates the use of expanded cork (a 100% natural lightweight agglomerate material) as a core material in composite sandwich structures with carbon fiber face sheets. The characterization performed focused particularly on energy absorption capability. Rohacell® 110 IG, a synthetic foam commonly used as a core material in high performance aerospace applications, was also used and characterized to compare the energy absorbing characteristics of the sandwich composites. Bending tests were done to examine the stiffness of the sandwich beams. Wavenumber and damping tests were performed to characterize the acoustic and vibrational damping properties of the composites. Both full penetration and partial penetration low velocity impact tests were completed to determine the damage resistance of the composites. It was seen that when compared to the synthetic foam, expanded cork showed a decrease in bending stiffness, but a large improvement in acoustic, damping, and impact damage resistance characteristics. It seems that expanded cork could be an attractive lightweight sandwich core material that will allow for increased energy absorption and will also reduce the carbon footprint of the structures.