Core Sandwich Research Articles

Low-velocity impact performance of integrally-3D printed continuous carbon fibre composite sandwich panels

Sandwich panels are heavily used in industrial sectors where specific mechanical properties are of utmost importance due to their high weight-stiffness ratio and energy absorption capacity. Lightweight sandwich cores with bio-inspired topologies have been used as an advanced alternative to conventional bulk cores to improve performance. Nowadays, sandwich structures comprised of core and skins are typically manufactured with two different processes and are afterwards adhesively joined together. Here, with a less time-consuming and more efficient assembly procedure in mind, an integrally 3D-printing strategy is proposed. This investigation analyses the low-velocity impact and quasi-static perforation performance of sandwich core designs inspired by the beetle's forewing trabecular structure. It was found that the core material was responsible of the rate dependence observed in the force–displacement curves. More complex finite element models than those employed here are necessary to capture all the fracture modes observed experimentally.

Three-point bending response and energy absorption of novel sandwich beams with combined re-entrant double-arrow auxetic honeycomb cores

The response and energy absorption of novel sandwich beams with combined re-entrant double-arrow auxetic honeycomb (RDAH) cores subjected to three-point bending were studied experimentally and numerically. Two typical sandwich beams loaded at different loading positions were considered. Quasi-static three-point bending experiments were conducted to obtain the failure modes and force–displacement curves. The reliable numerical simulation models were further established based on experimental validations. The results indicate that when the loading roller is located directly above the re-entrant cell, the RDAH core sandwich beam has better load-carrying and energy absorption capacity. Subsequently, the influence of face sheet distribution, cell-wall thickness, impact velocity and cell configuration on the structural response were explored. For the sandwich beams with same total mass, the arrangement where the thickness of the front face sheet is larger than that of the back face sheet is beneficial for improving the load-carrying and energy absorption capacity. In addition, the cell-wall thickness has an influence on the local deformation mode of the sandwich beam, and increasing its value can produce more stable deformation and improve the load-carrying capacity. Increasing impact velocity has a significant influence on the initial deformation but little influence on the final deformation of the sandwich beams. As the impact velocity increases, the total energy absorption of the sandwich beam gradually increases, and the negative Poisson's ratio characteristic of the core still exists. Compared to the traditional re-entrant honeycomb (RH) core sandwich beams, RDAH core sandwich beams have better energy absorption capacity and bending resistance.

Curved grid‐scored foam core sandwich composites under low‐velocity impact loads: Effects of curvature, stacking sequence, and core finishing

AbstractThe objective of this paper is to experimentally investigate how material and geometric parameters, including face sheet layup, curvature, and core finishing, affect the contact force‐displacement behavior, absorbed energy, and damage modes of curved sandwich composites with grid‐scored foam under low‐velocity impact loads. An instrumented drop‐weight testing machine with a hemispherical impactor was used to perform low‐velocity impact tests at different energy levels. Angled‐ply stacking had the maximum contact force and impact resistance for low curvature radii. In the event of penetration, the impact energy absorption of all curved sandwich panels increased as the curvature increased. The core finishing influenced the contact forces of the curved and flat sandwich panels but not the penetration energy. The curvature and face sheet layups affected the delamination size and pattern. Angle‐ply and cross‐ply face sheets with the smallest and maximum radius of curvature, respectively, decreased penetration depth. The resin‐filled channels reduced foam cell impact damage and face‐core debonding. LVI penetration resistance was found to be highest for angle‐ply stacking and knife‐cut foam at small curvature radii. The proposed research will also provide naval designers with valuable insight into the impact response of sandwich structures used in curved parts of the hull and deck.Highlights The effect of curvature on the LVI responses varied due to face sheet layups. Core finishing affected contact forces, but not the absorbed energy. Curvature and face sheet layups affected the damage modes. Resin‐filled channels limited damage area and acted as stoppers for debonding.

Response of aluminium honeycomb sandwich panels under combined shock and impact loading: Experimental and numerical investigations

The performance of sandwich structures against individual shock loading and fragment impact loading has been extensively examined, but little is known about their performance in the presence of simultaneous shock and impact loading. Based on a recently developed composite projectile to simulate combined shock and single fragment impact loading, the performance of aluminum honeycomb core sandwich panels (HCSPs) against combined loading is methodically examined. The achieved results reveal that the combined loading exhibits a synergistic effect compared to the single loading, i.e., increased damage of the sandwich panel and enhanced perforation resistance of the sandwich panel. After that, a three-dimensional finite element simulation is performed to explore the underlying mechanism of the synergistic effect. Numerical analysis indicates that the increased damage is a result of perforation-induced reduction of load-carrying capacity of sandwich panel, while the enhanced perforation resistance is the result of deflecting-induced perforation time delay of the perforation process. Finally, the parametric investigation of sandwich geometries such as asymmetric face sheets, core density, and core height is comprehensively conducted. The gained results reveal that both performance and synergistic effect due to combined loading are sensitive to these parameters.

Failure analysis of sandwich beams under three-point bending based on theoretical and numerical models

Abstract This article presents a comprehensive study on the failure behavior of foam core sandwich beams under three-point bending using theoretical analysis and finite element methods. A displacement formula for the foam sandwich beam is derived, considering the shear deformation of the foam core. Based on this formula, the deflection is obtained using energy and Rayleigh–Ritz methods. The failure loads of face yielding, core shearing, and indentation are combined to construct a failure mechanism map. The proposed theoretical model is then compared with existing theoretical analyses, demonstrating higher prediction accuracy. To investigate nonlinear damage and size effects, a series of finite element analyses is conducted. The results suggest that increasing the face sheet thickness has a greater impact on the ultimate load capacity, while the foam core thickness is more effective in enhancing bending stiffness.

Direct-Homogenization-Based Plate Models of Integrated Thermal Protection System for Reusable Launch Vehicles

Integrated thermal protection systems of reusable launch vehicles (RLVs) can have a corrugated core sandwich structure and experience location-dependent thickness-wise temperature gradient. The sandwich structure can be optimized depending on the location on RLV using finite element method simulations of RLV components. However, such analysis can be computationally challenging due to the disparate length scales between the RLV components and features of the sandwich structure. Two different equivalent plate models based on first-order shear and normal deformation theory, and first-order shear and second-order normal deformation theory (FSSNDT) have been utilized in this work to address this drawback. A direct homogenization technique involving unit cell and cantilever beam analysis has been developed to calibrate the equivalent plate properties for different thickness-wise temperature variations. The accuracy of the plate models has been evaluated by comparing their responses with a full-scale model for different thickness-wise temperature gradients and a uniform pressure. The comparisons clearly indicate that the FSSNDT-based plate model calibrated via direct homogenization captures both the displacements and strains in the in-plane and transverse directions accurately, and can be used to perform RLV component analysis efficiently to obtain location-specific optimal design of corrugated core sandwich structures.

Parametric design, simulation, fabrication, and test of an Origami-core based sandwich composite material

Sandwich panel structures are widely used for light-weight applications due to their high strength and stiffness to density ratios. They are composed of two main parts: the skins and the core. This paper investigates a Miura-ori structural sandwich core with open cells that can be used as an alternative to the existing honeycomb structures. A fully parametric design approach, starting from drawing to Finite Element Analysis (FEA), is presented to efficiently analyze the effect of different geometries on the resulting structure. The geometric model was built based on the kinematic model, which can represent the folding process of the foldcore using the Matlab script. The obtained coordinates of the vertices of the folding pattern were exported to ANSYS APDL where the surfaces were automatically created and meshed. The created mesh was imported to LS-Dyna where explicit compression analyses were performed to evaluate the compression stiffness and strength of the studied geometries. The developed parametric model aims to find the best configuration for a given application, based on three sets of geometric parameters that define the shape of the Miura Ori folding pattern. Therefore, in the range of the studied geometries, the best parameter combination for specific compression stiffness and specific compression strength was found. To evaluate the performance of the foldcore and to validate the FE model, experimental quasi-static compression tests were undertaken with two kinds of specimens (‘strip’ and “panel”), and the measured force-displacement curves were obtained. The result of the test agrees with that of the FE simulations, with an error of <6.6% in the elastic region.

Low strain rate mechanical performance of balsa wood and carbon fibre-epoxy-balsa sandwich structures

The focus of this study is the experimental assessment of the mechanical behaviour of balsa wood and its sandwich structures, where balsa serves as the core, supported by carbon fibre-epoxy skin layers. A comprehensive characterisation is conducted on the mechanical behaviour of balsa wood and carbon fibre-epoxy balsa core sandwich structures subjected to a range of low strain rates. Initially, the study undertakes a consistent procedure for sample preparation. Subsequently, the characterisation of the manufactured composite structures is performed experimentally. A stereo microscope is employed for a detailed visual inspection of the internal structure of the balsa wood and the sandwich structures. Furthermore, the mechanical characterisation is carried out with three-point bending tests at a range of strain rates from 0.1 % to 6 % strain per minute. This research reveals key findings about balsa wood and its sandwich structures, highlighting their performance and their sensitivity even under low strain rates.

High-performance multifunctional energy storage-corrugated lattice core sandwich structure via continuous carbon fiber (CCF)/polyamide 6 (PA6) 3D printing

High-performance multifunctional energy storage-corrugated lattice core sandwich structure via continuous carbon fiber (CCF)/polyamide 6 (PA6) 3D printing

Investigation of the mechanical properties of polyurethane foam-filled FDM-printed honeycomb core sandwich composites for aircraft

AbstractSandwich composites are widely used in aerospace materials thanks to their low weight and high strength properties. The purpose of this study is to observe the effects of polyurethane foam filling on honeycomb core structures produced by additive manufacturing in terms of mechanical strength and moisture absorption properties. Within the scope of the study, honeycomb structures were produced by a 3D printer using polylactic acid (PLA) filament. Then, the honeycomb core was filled with polyurethane foam, which is supplied in liquid form. After the core material was given its final form, it was combined with an epoxy and carbon fibre facesheet material using the vacuum infusion technique. After the sandwich composite production was completed, in-plane compression, three-point bending, shear, and moisture absorption tests were applied. The polyurethane foam filling greatly increased the mechanical strength, but slightly more moisture absorption occurred in this structure compared to a hollow honeycomb structure.

Effect of foam structure on thermo-mechanical buckling of foam core sandwich nanoplates with layered face plates made of functionally graded material (FGM)

Effect of foam structure on thermo-mechanical buckling of foam core sandwich nanoplates with layered face plates made of functionally graded material (FGM)

Links between material pair and energy absorbing capacity of lattice-cored sandwich: A comparison study

Excellent energy-absorbing capacities have been a long-standing requirement for structures in the field of transportation and aerospace, and tension–torsion coupling (TTC) metamaterials have seen promising application opportunities since their emergence. In the current work, we carry out a systematic study on the metamaterial-cored sandwich under impact. Emphasis have been put on revealing the links between the material pair (that of the lattice core and cover sheets) and the energy-absorbing capacity, from both the numerical and experimental points of view. It is disclosed that the mismatch between the stiffness moduli of the lattice core and the cover sheet can alter the deformation mode of sandwiches under impact load, and hence have a significant influence on the energy-absorbing capacity of the sandwich. The change in energy absorption ratio at various impact energy levels is also investigated. Lastly, a graded TTC metamaterial optimization is conducted for the sandwich core by carefully tuning its dimensions and the resulting stiffness, as a means to be compatible with the impact energy and the paring effect of constituents materials.

Fatigue behaviour of PVC foam core sandwich with GFRP faces: Simulation and experimental analysis

A multi-layer structure by sandwiching rigid cellular PVC (polyvinyl chloride) foam between two GFRP (glass-fibre reinforced plastic) faces is studied in order to investigate the sample edge effects on fatigue life of the sandwich structure. Therefore, a new geometry of specimen is developed based on numerical simulation approach. Compared to the results given by the ASTM C393 standard, the experimental study was confirmed that edge effects are leading to early failures as the shear stresses concentration was shifted from the new geometry. An optimal prediction of fatigue life of foam-based sandwich has been established during bending fatigue test. The different failure modes were followed by microstructural analyses to better explain the global behaviour of the structure.

Improved compressive performance of lattice truss core sandwich composites with modified Kagome topologies: An experimental and numerical study

A finite element model (FEM) using a more realistic three-dimensional geometry of lattice truss core was proposed to simulate the flatwise compression test on lattice truss core sandwich composites (LTCSCs). The goal was to reduce the disparity between experimental results and FEM predictions without inclusion of imperfection, as oppose to similar studies. Besides, two modified Kagome topologies were suggested and fabricated by adding vertical composite struts at specific locations. Based on the experimental results, the modified Kagome topologies exhibited superior specific compressive stiffness and strength over the Kagome topology, by about 100%. In addition, facesheet rotation of Kagome topology was found by the FEM, which considerably affect its compressive performance. Facesheet rotation is caused by the arrangement of composite struts of LTCSC. Compressive response of LTCSC was acceptably predicted by the FEM, though there was quite high error for compressive stiffness and strength. Probable sources of discrepancy between experimental and FEM results were discussed.

Development and mechanical characterization of cenosphere-reinforced CFRP and natural rubber core sandwich composite

Driven by the growing concern for environmental sustainability, there is an increasing need to explore innovative approaches for repurposing industrial waste materials. This study focuses on investigating the potential uses and challenges associated with cenosphere, a waste product derived from coal combustion in thermal power plants. Typically regarded as waste, cenosphere offers an opportunity to contribute to sustainability efforts. The objective of this research is to evaluate the influence of cenosphere, a ceramic-rich industrial waste, on the mechanical properties of woven CFRP-Rubber-CFRP (Carbon fibre-reinforced polymers) sandwich composites. The composite specimens were fabricated using the conventional hand lay-up technique, incorporating different weight percentages (5, 10, 15, and 20 wt.%) of cenosphere as a particulate filler. Tensile, flexural, and impact testing were conducted according to ASTM standards to assess the impact of the filler content on the mechanical properties. The results demonstrate that the inclusion of approximately 15% by weight of discarded cenosphere significantly enhances the tensile strength, flexural strength, interlaminar shear strength (ILSS), and impact strength of the sandwich composites, yielding improvements of approximately 1.6, 1.56, 2.06, and 1.85 times, respectively, compared to unfilled composites. Microscopic analysis of the composites reveals a well-dispersed cenosphere distribution within the matrix, contributing to the notable enhancement in overall strength characteristics.

Study on damage failure for a new double-triangular truss core sandwich structure

In order to solve the inherent problems of low bonding strength between face and core, a new double triangular truss core sandwich structure with reinforced frame is proposed, and the face and core are melt connected by thermoplastic composite carbon fiber reinforced polyether ether ketone (CF/PEEK). The response of the new double-triangular truss core sandwich structure under three-point bending load is investigated by both theoretical and finite element methods. Firstly, the theoretical prediction equation of bending stiffness and strength of double-triangular truss core sandwich structure is established, and the failure mechanism diagram is constructed. A valid three-point bending finite element model is established by Abaqus to predict the damage pattern of the structure and obtain the results of damage initiation location, damage process, final failure modes and load-displacement curve. The effect of geometric parameters on the bending performance of the sandwich structure was investigated. The results show that the calculated theoretical deflection curves and the ones obtained by numerical simulation are in good agreement. The average error between the two is 7.94%, verifying the reasonableness of the theoretical prediction model. The effect of geometric parameters on the bending performance of the sandwich structure was investigated. The new sandwich structure has stronger bending resistance and better ultimate load capacity than traditional honeycomb and V-shaped structures. Failure modes such as panel crumpling and crushing, fracture of reinforcement frames, buckling and fracture of core rods occur in sandwich structures under bending loads.

Influence of Profiled Faces on the Overall Shear Performance of Strongly Profiled Sandwich Panel with Mineral Wool Core

AbstractThe total shear force induced on a profiled sandwich panel by an external load is distributed between the profiled face and core. Sandwich panel with a discrete core material such as mineral wool consists of multiple transverse lamella joints across the full width of the panel, affecting the overall shear performance of the panel as these joints have negligible resistance to shear. Additionally, in some cases the core material is partially bonded only to the lower flange of the profiled face which increase the concentration of stresses in the bonded area leading to the crushing of the core on the support. This work studies the distribution of shear force in mineral wool core sandwich panels with one profiled face utilizing a shear beam test on a specimen cut from the panel including the profiled part. Based on the results and failure modes obtained, this work investigates the behavior and role of the profiled face on the shear resistance of profiled sandwich panels. The distributed shear force between the core and profiled part obtained from the shear beam tests are compared with full‐scale tests suggested by the European Standard EN 14509:2013, thus, exploring the applicability of the devised shear beam test method for assessing the shear properties of profiled sandwich panel.

The effects of impactor shape on the compression after impact strength of carbon/epoxy face sheet foam core sandwich structure

This study presents experimental results of compression after impact (CAI) strength testing of foam core sandwich structure with carbon/epoxy face sheets impacted at three different energies with either a blunt or sharp tip impactor. The impact energies used were chosen to span the barely visible impact damage (BVID) thresholds for the sharp (4.1 J) and blunt (12.9 J) impactors with an impact energy approximately halfway between these (8.1 J) also used. While most impact testing on composites utilize a hemispherical (blunt) impactor, actual damage to a part may be due to an object impacting the part that is not blunt, but sharp and the impact response and resulting CAI strength values may be different for a given impact energy level. In this study, with regards to the impact response, the sandwich specimens showed larger transverse displacements (by about a factor of two) during the impact event when impacted by a sharp impactor versus a blunt impactor. The maximum load of impact was larger for the blunt impactor by about a factor of three. The barely visible impact damage (BVID) threshold energy was lower by about a factor of three for the sharp impactor. The absorbed energy of impact was higher for the sharp impactor. The CAI strength results showed that the sharp impactor gave lower average CAI strength values at the lowest impact energy level used, slightly lower average CAI strength values for the medium impact energy used and about the same average CAI strength values for the highest impact energy used.

Geometrical imperfection sensitivity on in-plane compressive modulus and strength of honeycomb core and deviations from isotropy

Honeycomb core sandwich may suffer from face/core debonding. Candidate debond fracture test specimens such as End-Notch Flexure (ENF) and Mixed Mode Bending (MMB) contain unbonded regions of the sandwich where the core is required to carry in-plane com-pressive load. In-plane modulus and compressive strength of honeycomb core are not pro-vided by core producers and must be estimated from unit cell models assuming perfect hexagonal cells. In this study it is shown that geometrical imperfections, such as deviations in the angle between cell walls and curved cell walls have a strong influence on modulus and strength because of deviations from transverse isotropy. A range of Nomex honeycomb cores were tested under in-plane compression. Modulus and strength ratios ( L/W) were found to vary from 0.3 to 3.24 and 0.96 to 1.8, respectively. Analysis based on unit cell models verified that deviations of the inclined cell wall angle, θ, from 30 ◦ is a major cause for orthotropy. Rounded corners and curved walls were analyzed using finite element analysis. Both principal elastic moduli of the core ( E L and E W) were reduced by the curvature while the Poisson’s ratios remained largely unaffected. Implications of the results for design of MMB and ENF fracture test specimens are discussed.

Impact Properties of Hemp Natural – Glass Fibers Hybrid Polypropylene Sandwich Composites

One way to improve the mechanical properties of composite structures is by hybridizing natural and synthetic fibers. Besides that, combined with sandwich structure composites consists of two relatively strong, thin, and stiff faces separated by a core, for example, balsa, foam, and honeycomb, a relatively thick lightweight. This research develops sandwich composites for structures that have able to withstand high loads and modulus-to-weight ratios but can absorb impacts through impact tests by utilizing the raw material of jute natural fiber, which is abundant in Indonesia so that this research study can predict the effect of variations in the hybridization of hemp natural fiber and the combination of hemp natural fiber with e-glass using polypropylene core sandwich composites by using hand lay-up and vacuum bagging methods. The current impact test results show that the hemp natural-e-glass fibers hybrid sandwich composites get a higher impact strength with a value of 0,019 J/mm² than the hemp-PP honeycomb hybrid sandwich composite with a value of 0,013 J/mm². It shows that by combining e-glass fiber in the composite, it can increase its impact strength and can be a lightweight structural material as being a new alternative material of jute and e-glass natural fiber hybrid sandwich composites with polypropylene cores to substitute conventional materials such as metals which is potential for applications in the automotive, building, and unmanned aerial vehicle industries.