Lightweight Sandwich Structures Research Articles

Sound transmission loss and energy absorbing performance of stiffened doubly-curved shells with corrugated-honeycomb hybrid cores

Compared to traditional lightweight corrugation and cellular cores, the novel cellular cores auxetic metamaterials with negative and zero Poisson's ratios possess distinctive mechanical deformation traits, making them suitable for modeling lightweight sandwich structures. Therefore, the concept of combining auxetic metamaterial cellular with folded corrugation is proposed to construct a new type of corrugated honeycomb hybrid cores for studying the sound insulation and energy absorption characteristics of the stiffened sandwich doubly-curved shells, wherein the honeycomb core possesses a full range of Poisson's ratio characteristic and is composed of cellular cores exhibiting positive, negative, and zero Poisson's ratios (PPR, NPR, and ZPR). The Hamilton's principle is hired to derive the governing equations and considers fluid-structure coupling by applying normal velocities continuity conditions at the fluid-structure interface, which is further analytically solved using Navier’s techniques, and the sound transmission loss (STL) is described analytically. The accuracy of these results is validated through comparisons with both experimental measurements conducted in an impedance tube and simulated outcomes generated by the COMSOL commercial software. The properties of stiffened sandwich doubly-curved shells with corrugated honeycomb hybrid cores are meticulously evaluated and compared, and the results show that the hybrid cellular core type significantly impacts the STL, wherein the average STL of the corrugation ZPR cellular hybrid cores stands at 42.06dB within the broad low-frequency range of 380-2422 Hz, which has increased by 9.11% and 8.31% compared to the values of the corrugation NPR and traditional PPR cellular hybrid cores, the specific energy absorption (SEA) of the corrugation ZPR cellular hybrid cores is 13.06 kJ/m3, which has increased by 16.19% and 18.08% compared with the corrugation NPR and PPR cellular hybrid cores, respectively, indicating that the corrugation ZPR cellular hybrid cores have better sound insulation and energy absorption characteristics than the corrugation NPR and PPR cellular hybrid cores.

Novel triply periodic minimal surfaces sandwich structures: Mechanical performance and failure analysis

AbstractSandwich structures are known for their exceptional mechanical properties and widespread applications. This study integrates additive manufacturing (AM) technology, experiments, theoretical equations, and numerical simulations to investigate a novel lightweight sandwich structure with a core based on Triply Periodic Minimal Surfaces (TPMS). The core incorporates Primitive, Neovius, and Diamond structures and is designed and manufactured using AM. Three‐point bending experiments are conducted to assess bending performance, failure modes, and energy absorption capability. A numerical model is established to analyze behavior under bending loads, validated through comparison with theoretical equations and numerical simulations, focusing on stress concentration areas. A parametric study investigates the effects of skin material, thickness, and TPMS core relative density on bending performance and energy absorption capacity. Furthermore, a functionally graded core layer with a gradient change in relative density is designed to study its influence on bending performance. The parameters of the core have a decisive impact on TPMS sandwich panel performance. This study demonstrates that sandwich structures with TPMS cores can be designed to possess desirable bending performance and energy absorption capacity, providing insights for future engineering applications.Highlights A novel TPMS sandwich structure is fabricated using 3D printing technology. The gradient relative density core is introduced. The bending performance and failure behavior of the structure are evaluated. A parametric study was conducted to investigate the effects on performance.

Impact behavior of periodic, stochastic, and anisotropic minimal surface-lattice sandwich structures

Recent advancements in 3D printing technologies have made it possible to fabricate intricate lattice architectures with high precision. These lattices can now be utilized to design lightweight sandwich structures that serve multiple functions. To enhance the impact loading performance of these structures, it is crucial to understand how the lattice's topological properties, particularly those with minimal surface attributes like periodic or stochastic Primitive and Gyroid triply periodic minimal surfaces (TPMS) and spinodal-like stochastic cellular materials, associate with the mechanical properties of sandwich structures while keeping the skin thickness fixed. Thus, this paper explores the low-velocity impact behavior of various sheet/shell-based minimal surface-latticed cores of sandwich structures with woven composite skins. The elasto-plastic-damage numerical simulations consider lattice core periodicity, randomness, and anisotropy while keeping the relative density constant. Core lattice randomness and anisotropy are designed using the Gaussian Random Field (GRF) method for spinodal-based stochastic cellular materials and stochastic TPMS. The simulation results showed that the periodic Primitive-lattice core exhibits high out-of-plane shearing strength, enabling the sandwich structure to demonstrate the highest perforation limit. GRF spinodal-based core achieved the highest peak load due to its anisotropic mechanical properties. However, the post-yielding bending of the lattice sheet limited its ability to resist perforation, and absorb and dissipated energy. Interestingly, the stochastic Gyroid TPMS topology, with its inherent densely-distributed microstructure, showed high sensitivity to loading rate, resulting in enhanced energy absorption and dissipation of the sandwich structure. These findings offer valuable insights for optimizing multifunctional sandwich structures with superior impact performance and their design for additive manufacturing.

Study on Dynamic Mechanical Properties of Sandwich Beam with Stepwise Gradient Polymethacrylimide (PMI) Foam Core under Low-Velocity Impact.

Different PMI foam materials of 52, 110, and 200 kg/m3 were used to design stepwise gradient cores to improve the impact resistance of the sandwich beam. The stepwise gradient core consists of three layers arranged in positive gradient, negative gradient, and sandwich-core (e.g., 200/52/200). These sandwich beams were subjected to the impact of a steel projectile under impact momentum of 10 to 20 kg·m/s, corresponding to impact energy in the range of 12.5 to 50 J. During the test, the impact force was recorded by an accelerometer, and the different failure modes were also obtained. Subsequently, the influence of the layer arrangement on the energy absorption and load transfer mechanism between the different layers was analyzed. The results showed that the top layer with a large density can improve the impact force, but the middle/bottom layer with a low density promoted specific energy absorption. Thus, based on these two points, the negative gradient core (200/110/52) had an excellent specific energy absorption because it can transfer and expand the area to bear the load layer by layer, which improved the energy absorption in each layer. Combined with the failure modes, the load transfer and deformation mechanisms between the layers were also discussed. The present work provided a valuable method to design an efficient lightweight sandwich structure in the protection field.

Effect of varying core density and material on the quasi‐static behaviors of sandwich structure with 3D‐printed hexagonal honeycomb core

AbstractAdditive manufacturing (AM) involves the development of complex, lightweight sandwich structures for the automotive and aerospace industries. These structures are essential for load bearing and impact resistance. Nevertheless, there is a significant obstacle of failure under compressive loading, e.g. through brittle fractures and crushing. To address this issue, this study evaluates the compressive properties, energy absorption and failure damage in quasi‐static tests (flatwise, in‐plane, and flexural) of sandwich composites with 3D‐printed hexagonal honeycomb cores of different unit cells (6, 8 and 10 mm) and materials (polylactic acid (PLA), PLA‐Carbon and PLA‐Wood). The results show that increasing the core density enhances compressive strength, modulus, and energy absorption. An 8 mm unit cell absorbs energy optimally for lightweight structures. In PLA flatwise testing, the 8 mm unit cell absorbed 419.49 J more energy than the 10 mm unit cell. Additionally, PLA‐Wood has better mechanical performance than PLA‐Carbon due to the better filler with the PLA‐ matrix. In flatwise testing with an 8 mm unit, PLA‐Wood absorbs 214.01 J, while PLA‐Carbon absorbs 122.49 J. The failure modes vary depending on tests performed. The study highlights the potential of 3D‐printed honeycomb core structures for load‐bearing applications in various industries, including aerospace and automotive.Highlights Quasi‐static loading behavior of 3D‐printed hexagonal honeycomb cores. Increased core density improves compressive stress, modulus, and absorbed energy. An optimal unit cell size for lightweight 3D printed core structures is 8 mm. PLA‐Wood performs better in energy absorption due to filler compatibility. The failure modes are related to the type of quasi‐static loads applied.

A novel arc-type auxetic cellular doubly-curved shells with negative Poisson's ratio for broadband low-frequency sound insulation

The novel cellular cores auxetic metamaterials with negative Poisson's ratio (NPR) have unique mechanical deformation characteristics compared to conventional cellular cores, which can be further applied to modeling lightweight sandwich structures. The Arc-type cellular core is based on the traditional re-entrant cellular core and incorporates a bending configuration, facilitating a smooth transition of the cell elements. Based on that, the sound transmission characteristics of sandwich doubly-curved shells with NPR arc-type auxetic cellular core layer is studied in this article. The Hamilton's principle is applied to derive the governing equations and considers fluid-structure coupling by applying normal velocities continuity conditions at the fluid-structure interface, which is further analytically solved using Navier's method, and the sound transmission loss (STL) is described analytically. The correctness of the proposed formula is verified by comparing the theoretical solution with the calculation results of the commercial software COMSOL and the experimental results of impedance tube sound insulation. Based on the theoretical model, it is calculated that the average STL of the arc-type cellular is 6.61% higher than that of the traditional re-entrant cellular within the broad low-frequency range of 100–2448Hz. Then the effect of key parameters on the STL of sandwich structure is systematically analyzed, and the results show that the first frequency of the sandwich shell decreases with the increase of the rib thickness, but increases with the increase of the arc radius. Notably, a smaller arc radius and a larger rib thickness exhibit superior sound insulation properties across a broad frequency range. In addition, when the core layer thickness increases from 0.007m to 0.009m, the average STL of sandwich structures is reduced by 7.38% in a broad low-frequency range of 100–2448Hz.

Broadband low-frequency sound insulation of stiffened sandwich PFGM doubly-curved shells with positive, negative and zero Poisson's ratio cellular cores

Compared to the conventional cellular cores, the cellular cores auxetic metamaterials with negative and zero Poisson's ratios have unique mechanical deformation characteristics, which can be further applied to modeling lightweight sandwich structures. Based on that, the sound transmission characteristics of a novel stiffened sandwich porous functionally graded materials (PFGM) doubly-curved shells with full Poisson's ratio characteristic range cellular cores are investigated in this study. The adjustment of material properties of PFGM face sheets in the thickness direction is achieved through power law based on component volume fraction, and the core layer consists of cellular cores with positive, negative, and zero Poisson's ratios (PPR, NPR, and ZPR). Two common types of stiffened sandwich shell types, namely, the shell with PFGM face sheets and metal-rich cellular cores (type-A), and the ceramic-rich cellular cores (type-B), are discussed, and the Hamilton's principle is used to derive the governing equations. By applying normal velocities continuity conditions at the fluid-structure interface to consider fluid-structure coupling, which is further analytically solved using Navier's technology, and the average sound transmission loss (STL) with broadband low-frequency is described. The effectiveness of the established theoretical model is confirmed by comparing it with previously published results, experimental and simulated results obtained by impedance tube and COMSOL commercial software. A systematic comparison is conducted on the performance of stiffened sandwich PFGM doubly-curved shells with full Poisson's ratio characteristic range cellular cores, and the results show superior performance in the sound insulation for the sandwich shell with ZPR cellular cores, considerably at a broad low-frequency range of 220–2040 Hz, in comparison with the sandwich shell with NPR cellular cores and conventional PPR cellular cores types. This work's findings can serve as a benchmark for future studies and help to design and create engineering sandwich structures with improved sound insulation properties.

Low-velocity impact failure of foam-filled GFRP trapezoidal corrugated sandwich structures

In this paper, a new lightweight sandwich structure with polyurethane foam filled glass fiber reinforced polymer (GFRP) trapezoidal corrugated sandwich structure is fabricated. Based on the low-velocity impact tests, the impact failure performance and energy absorption efficiency of polyurethane foam filled sandwich structures are investigated. Low-velocity impact tests were carried out on the node and base positions of the foam filled trapezoidal corrugated sandwich structure by using a drop hammer test machine, and the set impact energy range was 25 J – 150 J. Experimental results show that when the impact energy is the same, the maximum impact load and maximum energy absorption generated by the node impact are greater than the base impact, which proves that the node position has better impact resistance and energy absorption effect than the base position. In addition, by comparing the displacement-time curves of foam filled and unfilled sandwich structures, it is found that when the impact energy is the same, the slope of the displacement-time curve of unfilled specimen is greater than that of foam filled specimen. Therefore, within the same time, the deformation of unfilled specimen is larger than foam filled specimen during the impact process, which demonstrates the effectiveness of polyurethane foam filling on the gain of sandwich structure.

Bending behavior of 3D printed sandwich structures with different core geometries and thermal aging durations

In recent years, the Fused Deposition Modeling (FDM) method has been employed in the production of small-scale structural elements with moderate loads, such as small-unmanned aerial vehicles, sports equipment, dental implant molds, and similar applications. In these cases, the FDM technique can utilize polymers like Polyamide (PA), Polylactic Acid (PLA), and Acrylonitrile Butadiene Styrene (ABS) to manufacture structural components. Lightweight sandwich structures are utilized as structural elements in various industries due to their unique characteristics, high stiffness-to-weight ratio, and energy absorption capabilities. Although the number of studies on the mechanical properties of sandwich structures manufactured with FDM has increased in recent years, experimental data on the mechanical characteristics of sandwich structures manufactured with FDM under different thermal aging durations are still insufficient. Driven by this motivation, the energy absorption capabilities of sandwich structures with different four core geometries (i.e., circular, hexagonal, square, and triangular) were experimentally investigated under different thermal aging durations. The sandwich structures were manufactured from PLA material by the FDM method. Four different thermal aging durations, 0, 15, 30 and 45 days, were considered for environmental conditions. Quasi-static three-point bending experiments were conducted to assess the energy absorption capability of lightweight sandwich structures featuring diverse core topologies. The bending test results demonstrate that the core topology significantly affects the energy absorption abilities of sandwich structures. Moreover, fractographic analysis using scanning electron microscopy (SEM) was conducted to gain deeper insights into the impact of thermal aging on aged specimens. In addition, it was concluded that the energy absorption performances of all sandwich structures with different core structure topologies were adversely affected by increasing the thermal aging time.

Fabrication and mechanical properties of three-dimensional enhanced TiAl/GH3536 hetero-honeycomb sandwich structure

The mechanical properties of honeycomb sandwich structures are improved using topological-reinforcement and material-strengthening. This study introduces a novel type of TiAl/GH3536 hetero-honeycomb sandwich structure, called the enhanced honeycomb sandwich structure, which boasts a relative density of core range of 3.2%–21.2%. The enhanced honeycomb sandwich structure is designed and fabricated through a vacuum-brazing method using BNi-2 brazing filler metal. The microstructure and out-of-plane compression behavior of both the as-brazed and original honeycomb cores are experimentally and analytically investigated. The study establishes good correlations between analytical predictions and experimental results. The results highlight the sensitivity of the mechanical properties of the enhanced honeycomb cores to the unit-cell size and parent–alloy properties, specifically inelastic buckling, and elastic modulus. The compressive characteristics of the enhanced honeycomb sandwich structures were thoroughly examined, and it was determined that the efficient filling of gaps between the honeycomb core walls with liquid BNi-2 brazing filler metal, coupled with the optimal core structure, resulted in the precipitation of mesh-like Mo-boride compounds within the GH3536 parent material. This resulted in a significant increase in the compressive strength and elastic modulus, by up to 57.4% and 26.3%, respectively. The combination of topological-reinforcement and material-strengthening provided a way to achieve lightweight sandwich structures with high specific strengths and low densities, which is important on the fabrication of engineering bioinspired materials and devices designed based on honeycomb structures.

Revealing the Sound Transmission Loss Capacities of Sandwich Metamaterials with Re-Entrant Negative Poisson's Ratio Configuration.

Due to the influence of mass law, traditional lightweight sandwich structures have struggled to surpass solid structures in sound insulation performance. To this end, we propose an acoustic metamaterial structure with a sandwich configuration based on the re-entrant negative Poisson's ratio (NPR) structure and systematically investigate its sound transmission loss (STL) performance under incident plane wave conditions. We used the acoustic impedance tube method to experimentally study the sound insulation performance of the re-entrant NPR sandwich structure under free boundary conditions, and then established an acoustic analysis simulation model based on COMSOL Multiphysics software, which verified that the results obtained by the experiment and the numerical simulation were in good agreement. The results show that the sandwich structure exhibits excellent sound transmission loss performance in the studied frequency range (250-4000 Hz), and the overall sound insulation performance exceeds the curve of the mass theorem, basically achieving more than 20 dB when the sandwich thickness is 2 cm. Finally, we conduct parametric studies to establish a correlation between the geometric design of NPR sandwich structures and their sound transmission loss performance. The research shows that the changes of the length of the ribs, the distance from the ribs to the center of the unit, and the length of the upper wall and the lower wall have a significant impact on the sound insulation performance of the re-entrant NPR sandwich structure, while the change of the wall thickness basically will not affect the sound insulation performance of the sandwich structure. This research can provide practical ideas for the engineering application of noise suppression designs of lightweight structures.

Experimental investigation on 3D printed lightweight sandwich structures for energy absorption aerospace applications

One of the ongoing challenges in aeronautics is the development of increasingly lighter structures. This has become a crucial goal because structural lightening means improving aircraft performance in terms of speed, manoeuvrability, fuel efficiency, and at the same time reducing manufacturing and service costs. The aerospace industry continues to focus more and more on this goal and every improvement in technology and materials can bring significant advantages in performance, efficiency, and safety of aircraft. The present work is part of this framework, as it analyses the feasibility of achieving very high weight reduction in sandwich panels using a new manufacturing approach. It introduces an experimental limited sensitivity analysis on the mechanical responses of composite sandwich structures for shock-absorption applications lightened by appropriately setting a specific additive manufacturing process parameter: the infill value. The investigated sandwich structures are characterised by a core Designed for Additive Manufacturing (DfAM) in order to maximise their performance in terms of energy-to-weight ratio and damping of impact loads. The material chosen for the inner part of the sandwich structure (core and internal face sheets) is polypropylene (PP) while the external face sheets are in Carbon Fibre Reinforced Polymers (CFRP).By comparing the post-impact responses at 20 J impact of three sandwich configurations with the same external shape but different material layer densities related to different setting of the infill parameter in the frame of the printing process, the work proves that this approach leads to lightening of this specific sandwich structures by up to 28% and at the same time improves their structural effectiveness in terms of energy absorption characteristics.The comparison was made by relating specific absorption indices, force-time and force-displacement graphs and CT scans.

Low-Velocity Impact Resistance of 3D Re-Entrant Honeycomb Sandwich Structures with CFRP Face Sheets

Lightweight sandwich structures have been receiving significant attention. By studying and imitating the structure of biomaterials, its application in the design of sandwich structures has also been found to be feasible. With inspiration from the arrangement of fish scales, a 3D re-entrant honeycomb was designed. In addition, a honeycomb stacking method is proposed. The resultant novel re-entrant honeycomb was utilized as the core of the sandwich structure in order to increase the impact resistance of the sandwich structure under impact loads. The honeycomb core is created using 3D printing. By using low-velocity impact experiments, the mechanical properties of the sandwich structure with Carbon-Fiber-Reinforced Polymer (CFRP) face sheets under different impact energies were studied. To further investigate the effect of the structural parameters on the structural, mechanical properties, a simulation model was developed. Simulation methods examined the effect of structural variables on peak contact force, contact time, and energy absorption. Compared to traditional re-entrant honeycomb, the impact resistance of the improved structure is more significant. Under the same impact energy, the upper face sheet of the re-entrant honeycomb sandwich structure sustains less damage and deformation. The improved structure reduces the upper face sheet damage depth by an average of 12% compared to the traditional structure. In addition, increasing the thickness of the face sheet will enhance the impact resistance of the sandwich panel, but an excessively thick face sheet may decrease the structure's energy absorption properties. Increasing the concave angle can effectively increase the energy absorption properties of the sandwich structure while preserving its original impact resistance. The research results show the advantages of the re-entrant honeycomb sandwich structure, which has certain significance for the study of the sandwich structure.

Experimental investigation and simulation of 3D printed sandwich structures with novel core topologies under bending loads

Abstract In a range of applications, such as the automotive, aerospace, and shipbuilding sectors, where weight reduction is essential, sandwich structures are getting more popular. The performance of sandwich structures in bending can be enhanced by using lightweight core topologies. In this study, six different novel and new core topologies were designed with CATIA V5. Polylactic acid (PLA) sandwich structures with new core designs were produced using the fused deposition modeling (FDM) additive manufacturing method. In order to determine the mechanical characteristics of these six designed core topologies, three-point bending tests on sandwich structures were performed. The influence of core topology on the flexural characteristics of lightweight sandwich structures was investigated to appropriately choose and design the core topology of the sandwich structures to meet desired structural requirements. To evaluate the flexural behavior of sandwich structures, finite element simulation using ANSYS Workbench 2021 R2 was also performed. Both the experimental data and simulation were in good agreement and clearly showed that the sandwich structure with the triple bow core exhibited the highest mechanical properties. These results provide new perspectives on the investigation of the mechanical response of sandwich structures, which can be beneficial for many other industries and applications.

Modal analysis of Free Vibration of an Extremely Lightweight Panel Model for Bridge Bearing Applications

This paper reports a novel extremely lightweight panel model under free vibration. This novel model is likely to be used for railway/highway bridge bearing applications due to its high performance to weight ratio, which offers superior mechanical properties, such as sound and vibration attenuation, rigidity, and energy absorption. The structure of the model is based on triply periodic minimal surfaces (TPMS) conceived by observing the scales of butterflies' wings. The vibration behaviours of this novel panel model used as bridge bearings are not well-known and have never been fully investigated under free vibration. Therefore, it is important to comprehend the free vibration behaviours of the model and to identify its dynamic modal parameters. In term of modelling, a TPMS sandwich panel finite element (FE) model and a typical bridge bearing FE model under free vibration for bridge bearing applications are designed and examined with a computational method. In general, FEA predictions of the free vibration behaviours of the novel panel model compared to a conventional bridge bearing model provide very good results. These results can be implemented to better generate design standards of extremely lightweight sandwich structures under different vibrations for bridge bearing applications in the near future.

Investigation of quasi-static indentation on sandwich structure with GFRP face sheets and PLA bio-inspired core: Numerical and experimental study

Composite materials incorporated in naval, aerospace, and automobile structures are exposed to unforeseen damages, which seriously damage their mechanical performance. In particular, the existence of hardly visual impairment on the in-service composite structures is a critical issue that highlights the severity of structural safety in an aircraft. The ability of these materials to sustain multiple impacts requires an understanding of the impact energy absorption to improve the material’s damage tolerance. The present study investigates the energy absorption, load response, failure modes, and damage mechanics under the quasi-static indentation of a sandwich structure. The structure comprises bio-inspired Poly-lactic Acid (PLA) core and Glass Fibre Reinforced Polymer (GFRP) laminates, used in real naval and aeronautical structures, under repeated low-velocity impact with reduced energies and quasi-static indentation. In addition, a lightweight sandwich structure’s damage performance is greatly influenced by its core geometry. Therefore, choosing an appropriate core structure is critical. A Numerical model is developed for the composite structure under quasi-static indentation loads, considering suitable material properties and damage models for the core and face sheets. These results are then compared with the experimental investigation. The load–displacement response and the energy absorption characteristics of the sandwich structure showed good agreement for the experimental and numerical analysis results.

Vibration behavior of a carbon fiber‐reinforced polymer composite sandwich panel: Rhombus core versus elliptical core

AbstractExploring lightweight sandwich structures with excellent load‐bearing and vibration damping performances is one of the important topics in structural and functional applications. The aim of the present research is to design, fabricate carbon fiber‐reinforced polymer (CFRP) sandwich panels with rhombus cores and investigate their modal characteristics by comparing with the traditional elliptical sandwich structure experimentally and numerically. In order to obtain modal properties including natural frequencies, damping ratio, mode shapes, and frequency responses of the fabricated structure, modal experiment is performed using a modal hammer with an acceleration sensor. Furthermore, a finite element method is developed to validate the precision of the experimental data. It is recognized that the finite element results are in remarkably great agreement with those obtained by modal experiment. The experimental results showed that the natural frequencies of the sandwich structures with rhombus are much higher than that of the elliptical sandwich structures under free–free boundary conditions and equal relative density of the truss cores.

Mechanical response of galvanised steel sandwich structure with different numbers of web core and different spacing distance of web plate under bending load

Sandwich structures are widely used in a variety of industrial applications due to their ability to provide high bending stiffness while remaining lightweight. The deformation of this structure and its relation to the stiffness of the galvanized steel is investigated. A series of three-point bending response and subsequent failure modes in web-core laser-welded sandwich structure based on galvanised steel is also investigated. The web-core sandwich structure was manufactured using fibre laser welding technique to joint face and web plates perpendicularly to produce a range of lightweight sandwich structure. The role of the number of cores and spacing distance were purposed to determine the overall deformation of global deflection behaviour of the sandwich structure. The results were compared, and it is showed that the acted load produced bending on faceplate and caused debonding at weld joint (between faceplate and web plate). The continued bending was also caused debonding between PVC foam and adjacent plate. Subsequently, load-displacement trace was used as evidence of the comparison, where seven cores with 20 mm spacing distance exhibited higher force, approximately 1.091 kN. The three-point bending test results indicated that the higher number of cores possessed better performance in bending strength. The effect of the spacing distance of web plates in sandwich structure was also examined. In five cores specimen, it is showed that as the spacing distance decreased, the bending strength increased, where bending stiffness value of 18 mm (0.313 kN/mm) is higher than 19 mm (0.288 kN/mm) and 20 mm (0.281 kN/mm). The effectiveness of the sandwich structure depended on the optimal design as to achieve lightweight and its bending strength.

Experimental Study on the Bending and Shear Behaviors of Chinese Paulownia Wood at Elevated Temperatures.

Chinese Paulownia wood has been extensively used in the construction of timber buildings and lightweight sandwich structures. However, the bending and shear behaviors at elevated temperatures were not well understood. A total of 162 specimens were tested to investigate the bending, tangential shear, and radial shear performances of Chinese Paulownia wood under temperatures from 20 to 220 °C. It was found that the bending specimens exhibited ductile failure due to the progressive damage after reaching the peak load, while the tangential and radial shear specimens exhibited brittle shear failure along the shear plane. The elevated temperatures had limited effects on the failure modes. Under the same temperature, the retention rate of the modulus of elasticity is significantly higher than that of the modulus of rupture. Moreover, the bending strength, tangential shear strength, and radial shear strength generally and nonlinearly decreased with the increasing temperature. The EN 1995-1-2 design curve for the shear strength of wood at elevated temperatures is conservative for both the tangential and radial shear specimens. However, the design curve may not be adopted to estimate the tangential shear strength at temperatures higher than 220 °C.

The compressive mechanical properties of honeycomb plates and beetle elytron plates with different foam densities and height-to-thickness ratios

To investigate a new type of bionic energy-saving sandwich plate—the beetle elytron plate (BEP), the compressive mechanical properties of short basalt fiber reinforced epoxy resin composite BEPs and honeycomb plates with different polyvinyl chloride foam densities and height-to-thickness ratios were investigated. The mechanism of the coupling effect of the core structure, foam density and height-to-thickness ratio was revealed by observing the failure mode of the outer and inner ring honeycomb walls. This study can provide useful instruction for designing lightweight sandwich structures and accelerate the application of BEPs in engineering.