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Sandwich panels are the fundamental structural element in a wide range of applications, including in satellite primary structures. While sandwich constructions are very efficient, their complex multi-material assembly leaves room for further optimisation of the core volume and improvement in the integration phase. One key technology that can enable the transition to multifunctional sandwich panel cores tailored to certain applications is the additive manufacturing (AM) of satellite primary structure sandwich panel cores. This paper investigates the feasibility of replacing the baseline Aluminium honeycomb core with a core printed out of AlSi10Mg through Powder Bed Fusion. Sandwich panels with carbon fiber-reinforced plastic (CFRP) facesheets and printed honeycomb cores as well as fully printed corrugated panels are produced and tested under three point bending (3PB) and compression as part of the EU funded ReDSHIFT project. The Instron 5560 (3PB) and 4204 (compression) are used to perform the experiments that follow the ASTM C393-11 and C365 standards. When compared against the baseline CFRP-AL panels, the 3D printed honeycomb cores carry up to twice as much load per unit mass in bending and four times as much in compression, while also being stiffer. The fully printed corrugates samples are weaker than the honeycombs, but in conjunction with the honeycomb geometry may present a promising avenue for developing multifunctional cores. While limitations with current metal printing technology prevent AM cores from matching the mass of baseline designs, the superior specific performance and geometrical freedom make printed cores a promising design alternative. [ABSTRACT FROM AUTHOR]

This paper investigates the method for water ingress detection in an aluminum honeycomb sandwich structure by laser ultrasonics. To clarify the propagation of Scholte waves generated at the solid-liquid interface, the theoretical dispersion curves in the water-aluminum layered structure are calculated by the global matrix method. Using a laser ultrasonic visualizing inspector, the guided wave propagation in the wavefields has been visualized, which shows the mode conversion behaviors occurring in the water ingress area. The clarification of mode conversion is then investigated by the finite element method. The Scholte modes at the solid-fluid interface and the Lamb modes in the face sheet are interconverted because of the change in the waveguide structure. Wavenumber filter reconstruction and the resulting energy maps are applied to illustrate the mode conversion in the wavefields and to detect the water ingress area. Results show that the appearance of the water layer contributes to the mode conversion behaviors during the guided wave propagation in the honeycomb sandwich structure, which offers the potential for water ingress detection based on the mode conversion behaviors. [ABSTRACT FROM AUTHOR]

The honeycomb sandwich panel presents a highly promising solution for enhancing ballistic behavior owing to its excellent strength-to-weight ratio and impact resistance. This study focuses on investigating the shock mitigation properties of honeycomb sandwich panels through experimentation and numerical simulation of high-velocity impact tests. Single-scale and double-scale hierarchical honeycomb cores are manufactured using selective laser melting with AlSi10Mg powder, combined with a pair of stainless steel (SS 316) sheets. High-velocity impact experiments were conducted within a velocity ranging from 100 to 270 m/s to examine the effects of different cores and projectile nose shapes on the dynamic response of the panels. Numerical simulations using ABAQUS/Explicit software were performed and validated against the experimental results. The sandwich panel with a double-scale hierarchical honeycomb core exhibited 7.8% and 6.1% more energy absorption compared to the single-scale hierarchical honeycomb core against conical and hemispherical nose projectiles, respectively. Additionally, the ballistic limit for the hemispherical projectile was found to be 8.9% higher than the conical-nose projectile under the same impact velocity and panel thickness. Moreover, an increase in panel thickness from 12 mm to 25 mm prompted a significant improvement of approximately 39% in specific energy absorption and a 44% increase in ballistic limit velocity. These findings highlight the considerable potential of single-scale and double-scale hierarchical honeycomb sandwich panels for the development of threat-resistant structures in critical dynamic loading applications. [ABSTRACT FROM AUTHOR]

The flatwise compression and mechanical response of gradient-tandem Nomex honeycomb sandwich panels were investigated by quasi-static compression tests. Finite element models of the gradient-tandem Nomex honeycomb sandwich panels were established to evaluate the effects of layer sequences, separator materials, and layer heights on the mechanical properties through parametric studies. The different assembly of honeycomb sequence and separator material affected the deformation sequence and peak stress of the sandwich panels. Under quasi-static compression, the response curves are smoother when the appropriate separator material and assembly sequence are chosen. Meanwhile, the peak stresses are effectively reduced. The research results can guide the individualized design of tandem Nomex honeycomb sandwich panels. [ABSTRACT FROM AUTHOR]

The failure behavior and load-carrying capacity of Nomex honeycomb sandwich panels under three-point bending are experimentally and numerically studied. The failure modes observed in experiments are core shear failure and face-sheet/core debonding. A detailed model considering Nomex paper and resin coating is built with Abaqus/explicit simulation. The Hill 1948 quadratic anisotropic yield criteria are adopted to describe the plastic behavior of Nomex paper and the principal stress criteria are used for predicting the failure of resin coating. The effectiveness of the proposed meso-scale model is validated by comparing the experimental and the numerical results. The effects of geometric parameters on the bending behaviors are discussed in detail. It is shown that the ultimate force and bending stiffness of Nomex honeycomb sandwich panels is significantly influenced by the core orientation, resin thickness and cell size. The cutting position has no effect on the ultimate force and bending stiffness. [ABSTRACT FROM AUTHOR]

This study presents experimental results of compression after impact (CAI) testing of aluminum honeycomb core sandwich structure with face sheets made of co-cured T1100/3960 quasi-isotropic carbon/epoxy when tested at +22.5⁰ and −22.5⁰ with respect to the 0⁰ fibers. In a previous study examining the CAI strengths of honeycomb sandwich structure, it was found that specimens had different CAI strengths, based on a [−45/90/+45/0]S layup, depending on whether they were tested in the 0⁰ direction (face sheet layup of [−45/90/+45/0]S) or 90⁰ direction (face sheet layup of [+45/0/-45/90]S). The CAI strength results showed that the specimens tested in the 90⁰ direction had a 19% drop in CAI strength compared to specimens tested in the 0⁰ direction. This was attributed to the 0⁰ load bearing plies in the 0⁰ direction specimens being "tucked in" at the center of the specimen thus providing more stability against microbuckling. This raised the question as to what CAI strength would specimens tested at +22.5⁰ (face sheet layup of [−22.5/−67.5/+67.6/+22.5]S) and −22.5⁰ (face sheet layup of [−67.5/+67.5/+22.5/-22.5]S) have compared to specimens tested in the 0⁰ and 90⁰ direction. Results presented in this study show that the specimens loaded at +22.5⁰ and −22.5⁰ have a similar average CAI strength compared to the specimens loaded in the 0⁰ direction. The specimens loaded in the 90⁰ direction exhibit 16% lower average CAI strength. Additional specimens were tested in the +45⁰ direction to put the 0⁰ load bearing fibers on the outside of the specimen to see if this would decrease the strength as has been documented for undamaged strength. These specimens have average CAI strength values between the 0⁰ direction average CAI strength values and the 90⁰ direction average CAI strength values. [ABSTRACT FROM AUTHOR]

This study presents the findings of an investigation into the effect of varying the thickness of a carbon/epoxy face sheet of a Nomex honeycomb sandwich panel on its the flexural properties. The thickness (hs) of the face sheet was varied by increasing the number of plies from one 0.25 mm thick layer to eight such layers, giving a total nominal thickness of 2.0 mm, whilst maintaining a constant thickness of the core. The flexural properties of the sandwich panels were investigated through a series of three- and four-point bending tests. A particular focus was on identifying changes in the failure mode with increasing face sheet thickness. The flexural properties of the sandwich panels were predicted using sandwich beam theory, where the deviation from the experimental values was shown to be less than 13%. A two parameter Weibull distribution model was used to predict the maximum flexural load using an analysis of variance (ANOVA) tool and an excellent level of correlation was observed with the experimental values. The difference between the predicted maximum load values and the experimental results was below 5% in all cases. A brittle mode of failure was observed in the thickest panel. The sandwich panel based on 1.5 mm thick face sheet was identified as being the most appropriate design, both in terms of strength and stiffness. [ABSTRACT FROM AUTHOR]

Honeycomb cores are used extensively in different sectors of industry to build advanced lightweight structures which take advantage of high stiffness-to-weight and strength-to-weight ratios. To further increase their mechanical properties, structural hierarchy is considered already from a conceptual point of view. It is proven to be highly effective in enhancing mechanical performance in different loading scenarios but affordable realisation of practical applications remains limited. The present study focuses on the out-of-plane compressive behaviour of a novel hierarchical sandwich honeycomb core with an emphasis on compressive strength, both in virtual and in experimental testing. The finite element model is validated for through-the-thickness compressive loading by physical experiments and it can subsequently be used for further structural optimization. The paper concludes with a comparison between the proposed hierarchical structure and conventional expanded honeycombs. The performance analysis highlights the advantages of the introduction of structural hierarchy to the state-of-the art honeycomb and it shows a high potential for the use in the construction of sandwich panels and parts. [ABSTRACT FROM AUTHOR]

Detecting disbonds in honeycomb sandwich structures using ultrasonics is challenging due to the complex wave propagation. This paper investigates the method for disbond quantification in honeycomb sandwich structures using laser ultrasonics. In the experiments on an aluminum specimen, the detection frequency range was determined by the local defect resonance (LDR) in the disbond regions. The energy maps of bandpass wavefields identified the locations of disbonds. To improve the results, a frequency-wavenumber analysis of the wavefields was performed. The wavenumber spectra were reconstructed by wavenumber filtering to strengthen the outline components for quantifying disbonds. Local cross-correlation (LCC) was proposed to show the outlines of disbonds and honeycomb cells, whose results agreed well with the X-ray imaging. Moreover, LCC could also quantify a disbond in the composite honeycomb sandwich structure. Consequently, combining energy maps and LCC results can effectively detect the locations and sizes of disbonds in honeycomb sandwich structures. [ABSTRACT FROM AUTHOR]

As for high resolution satellite application field, novel C/SiC honeycomb sandwich structure is the first to propose and develop on account of such many attractive properties as excellent ultrahigh dimensional stability in thermal and moisture coupling environment, lightweight and high load-bearing. Three types of C/SiC honeycomb sandwich structures were fabricated for the first time as opto-mechanical structure in satellite. The innovative interlayer-crosslinking method to weave the continuous carbon fiber fabric of honeycomb core was proposed. The compressive properties of novel C/SiC honeycomb sandwich structures were investigated. Meanwhile, the damage mechanism for sandwich structures were analyzed by non-destructive testing. The results showed that the innovative fabrication processing possessed the capability to prepare high quality C/SiC honeycomb, which was shown from the volume and distribution of pores in the X-ray tomography. The compression modulus and strength of the C/SiC honeycomb reached 927 MPa and 10.76 MPa, and showed better rigidity and higher strength compared with classical honeycombs. Two main types of cracks wall were found in the C/SiC honeycomb, i.e., the horizontal ones in the wall with t thickness and the inclined ones in the wall with 2 t thickness. The outstanding compressive performance was attributed to the high mechanical property of ceramic-matrix composite with fiber pull-out mechanism. The exceptional lightweight capacity was inherited from the low bulk density of honeycomb structure ranged from 0.08 g/cm3 to 0.15 g/cm3. This study can help for the development of lightweight materials for opto-mechanical structure in aerospace optical remote sensing technology. [ABSTRACT FROM AUTHOR]

A metamaterial plate with multi bandgaps is proposed for vibration suppression by integrating membrane-mass structures that can be used as locally resonant metastructure into a honeycomb sandwich structure. The finite element method is employed to calculate the band structure and the transmission spectra. The formation mechanisms of the locally resonant bandgaps are investigated via the analysis of the mode shapes of the membrane-mass resonator and face plate in a unit cell. The effects of the location of the attached mass on the bandgap property and an improved multi bandgaps structure is designed and discussed. Results show that the proposed multifunctional metamaterial exhibits an excellent vibration suppression performance, as well as a significant designability. Moreover, a specimen of the proposed metamaterial is fabricated and experimental measurements are performed, and a good agreement between the numerical predictions and the experimental results is obtained. The results of this study will be useful for the design of sandwich structures with multiple vibration bandgaps. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This study proposes a honeycomb with a sine waveform to increase the impact resistance against bird strike on the vertical stabilizer leading edge. A 2-stage comparison was made using the experimental test results in the literature to validate the model in the finite element environment. At first, the change in diameter, velocity, pressure, and scattering of particles when the bird model hits a square plate was compared with experimental data. In the second stage, an impact was made on the vertical stabilizer leading edge made of composite sandwich material. The deformation in the Z-direction occurring in the vertical stabilizer leading edge's center and the ribs' convergence were compared. The Ls-Dyna results show reasonable qualitative agreement with the experimental result. Finally, the straight walls of the honeycomb in the vertical stabilizer leading edge were modeled as a sine wave. The analyses were diversified using design parameters with different values. According to the analysis results, improvements of up to 9.79%, 28.58%, and 5.02% were observed in the deformation, the approach of the ribs, and sum of the reaction forces, respectively. These results contribute to bird strikes where strength per unit weight is essential. [ABSTRACT FROM AUTHOR]

Honeycomb materials are commonly used as cores for sandwich panels. While empirical evidence exists showing that the transverse shear strength of a honeycomb core is affected by its thickness, the accuracy of empirical guidelines and the reasons for the changes in strength are not well understood. Four-point flexural testing of sandwich beams and a detailed cellular finite element model are used to investigate transverse shear failure of Nomex™ honeycomb core material. Transverse shear strengths at varying core thicknesses were compared to manufacturer specifications and used to determine normalised strength correction factors for thickness. These factors correlated well with trends shown in previous literature, where core shear strength decreases with increasing core thickness. The failure mode investigation showed a transition between modes as core thickness increased. The thinner cores experience shear fracturing of individual cell walls followed by local cell wall buckling, intermediate thickness cores initially failed by cell wall local buckling, then fractured and larger thickness cores appeared to fail by overall cell column buckling. [ABSTRACT FROM AUTHOR]

The trapped air in unvented honeycomb core and the surrounding environment can induce a pressure differential across the facesheet during launch ascent. Combined with the presence of a defect, the pressure differential across a facesheet in combination with external loads can cause facesheet failure or unstable facesheet debond growth. While honeycomb core venting is the recommended solution, there might be practical limitations in implementing this solution or the possibility that a structure intended to be vented is partially unvented due to manufacturing issues. A methodology for accepting composite sandwich structures for space applications is presented, which heavily relies on the building block approach to characterize the facesheet strength and interfacial fracture toughness. To demonstrate the approach, an allowable flaw size is defined for a hypothetical flight condition using analyses anchored to subscale tests. [ABSTRACT FROM AUTHOR]

Disbond damage growth in honeycomb cored sandwich structures due to static and fatigue mixed mode loading is investigated numerically and experimentally. A two dimensional finite element model was generated using core homogenization and the Crack Surface Displacement Extrapolation mode separation method, integrated into a fracture mechanics based analysis sub-routine to predict face/core interface fatigue crack propagation. The Cycle Jump technique was furthermore applied to accelerate fatigue analysis. Mixed mode fatigue characterization testing was conducted using Double Cantilever Beam specimens loaded with Uneven Bending Moments, generating a relationship between crack propagation rates and energy release rate amplitudes as a modified Paris Law, measured at three mode-mixity phase angles. The measured Paris laws were subsequently used as input data for the numerical fatigue model. The numerical model was validated against CFRP/Nomex® Sandwich Tearing Test specimen tests with a propagating face/core interface crack yielding varying mode-mixities. The results from the validation showed good agreement between numerical predictions and experimental measurements. [ABSTRACT FROM AUTHOR]

As an aircraft or spacecraft ascends, the atmospheric pressure drops. If the internal pressure is not released, a pressure differential develops on the facesheet. With an in-plane load from vehicle acceleration as well, the pressure and in-plane load may cause failure. Vented sandwich structures are strongly preferred since they release the internal pressure; however, special cases still require an unvented design. This paper provides an analysis and test methodology for unvented sandwich structures. Parametric studies of facesheet-core debonding and buckling were conducted via finite element analysis, considering geometric, material, and load parameters. Design curves based on the key nondimensional parameters were tuned to the data from the parametric study, providing a tool to predict facesheet-core debond failure. Examples of how to use the design curves in practical applications are provided. [ABSTRACT FROM AUTHOR]

In this study, dynamic explicit analysis was performed to examine the air-blast performance of various hybrid sandwich designs in terms of face plate deflections and energy dissipation capacity under the conventional weapons effects program (CONWEP) air-blast loads ranging from 3 kg to 8 kg trinitrotoluene for stand-off distance ranges from 150 mm to 200 mm. The blast resistance of honeycomb sandwich configurations was evaluated using steel honeycomb with different core topologies, crushable Al foam-filled steel honeycomb, and steel or steel with 3D Kevlar/polypropylene laminate employing fiber metal laminate (FML) front face. For an accurate prediction of the deformation mechanism of all steel parts, the Johnson-Cook (J-C) model was used. The composite failure criteria of Hashin, Puck, and Matzenmiller were implemented to accurately examine the fiber and matrix damage behavior. The novel hybrid design of the honeycomb sandwich structure's blast resistance is improved by the employment of foam-filled honeycomb, an FML front face, and a circular honeycomb core. In comparison to other sandwich configurations, a novel designed hybrid sandwich construction composed of foam filled circular honeycomb with FML front facing and steel back facing (FCH-1KP0.5) achieved the highest blast resistance due to its lowest face deflection with the smallest plastic dissipation energy. [ABSTRACT FROM AUTHOR]

This study presents experimental results of compression after impact (CAI) testing of aluminum honeycomb core sandwich structure with face sheets made of quasi-isotropic carbon/epoxy with two orthogonal directions of testing. In a previous study examining the CAI strength of honeycomb sandwich structure,1 it was found that that specimens had different CAI strengths depending on whether the core was oriented in the "L" or "W" direction. Since the face sheets were quasi-isotropic and the core should not (theoretically) affect the CAI strength for a given amount of damage (if the specimens fail by face sheet failure), this result was puzzling. In the study presented in this paper, further CAI tests, along with open hole compression testing were used in an attempt to ferret out the cause of the differing CAI strengths in the aforementioned study. The results showed that the lower CAI strength values were not due to the core orientation, but to the change in the quasi-isotropic face sheet ply sequence due to the 90° rotation. [ABSTRACT FROM AUTHOR]

Sandwich structures are frequently used in structural areas where lightness and strength are essential. These structures are indispensable for sailing boats, and ground and air vehicles. The base purpose of this study is to investigate the effect of wave parameters on the sandwich structure. The data obtained from the bending tests of the model created using Ls-Dyna was compared with the experimental data of the literature. There is a 3.05% difference between the peak force in experimental and Ls-Dyna. The force-deformation plots are coherent, and the progressive images of the sandwich structure during bending are similar. In addition, using theoretical approaches, the highest force and the amount of collapse during bending were determined. There is a difference of 3.1% between the theoretical approach and Ls-Dyna values. Thus, the Ls-Dyna model was validated. The flat cell walls of the honeycomb were modeled as a sine wave. Four wave numbers and wave amplitudes were used. In this way, 16 different analysis files were created. The results show that the new sandwich structure's specific peak force and specific energy absorption (SEA) increased by 7–110% compared to the ordinary flat walled sandwich structure. This research will assist in the design of new sandwich structures. [ABSTRACT FROM AUTHOR]

A novel hybrid composite honeycomb sandwich structure (HCHSS) and metallic honeycomb sandwich structure (MHSS) were developed to obtain realistic and blast resistance parameters for the consideration of blast loading. Hybrid composite square honeycomb sandwich structure (HCSHSS) and metallic honeycomb sandwich structure with different cores such as square, hexagonal, octagonal, and circular were modelled to obtain the target protection level (lowest deflection) for the vehicle armour structure. The Johnson-Cook and Donadon failure criteria were incorporated into finite element analysis (FEA) for MHSS and HCSHSS, respectively. For the progressive damage modelling of the HCSHSS, the Donadon criterion was implemented in ABAQUS/Explicit software as a user-defined subroutine. The HCSHSS showed the fibre failure, matrix failure and delamination modes of failure in composite laminates and the plastic deformation failure of the core and metal plates. The HCSHSS was shown to have excellent blast worthiness properties in comparison to the MHSS. The metallic circular honeycomb sandwich structure (MCHSS) was shown to have the minimum deflection of the top and bottom face plates as compared to the square, hexagonal and octagonal honeycomb sandwich structures. A novel HCSHSS has obtained the minimum deflection for both the top and the bottom face plates in comparison to the other MHSS. [ABSTRACT FROM AUTHOR]

The hexagonal honeycomb core sandwich panels used in the satellite structure are subjected to severe vibration during launch. Therefore, the amounts of natural frequencies of these panels are of great importance for design engineers. Three-dimensional finite element modeling of the core considering all geometric parameters (i.e., a high-fidelity model) to achieve accurate results is not cost-effective. The honeycomb core is traditionally equivalent to a homogenized continuum core (i.e., a low-fidelity model) using simple analytical relations with ignoring the adhesive layer at the double cell-walls and radius of inclined cell-wall curvature. In this study, analytical formulations are first presented for the prediction of the equivalent elastic properties of a hexagonal aluminum honeycomb with considering all geometric parameters including adhesive layer thickness, cell-wall thickness, inclined cell-wall length, radius of inclined cell-wall curvature at the intersection, internal cell-wall angle, and honeycomb height. Then, two aluminum honeycomb core sandwich beams with free-free boundary conditions are modeled and analyzed in Abaqus finite element software, one with 3D high-fidelity core and the other with 3D low-fidelity core. In order to validate the results of the equivalent model, the modal analysis test was performed and the experimental natural frequencies were compared. The obtained results show a good agreement between the 3D low-fidelity and high-fidelity finite element models and experimental results. In addition, the influence of the above-mentioned geometric parameters has been investigated on the natural frequencies of a sandwich beam. [ABSTRACT FROM AUTHOR]

The use of 3D printing to produce acoustic panels with good mechanical and acoustic properties was investigated in this paper. Various fiber layups of the fiberglass face sheet and core designs were fabricated and tested for their indentation resistance and acoustic absorption performance. It was found that the bidirectional face sheet layup exhibited the best indentation energy absorption recording 4.2 J, which is 37% more than the 45-degree layout and 66% more than the quasi-isotropic layup. The specific energy absorption of the hybrid honeycomb core is the best among the three core designs recording 404 J/kg, which is 56% higher than the corrugated triangle with horizontal beam core (359 J/kg) and 20% higher than double ellipse core (335 J/kg). Computed-Tomography (CT) scan was used to study the fracture behavior of the sandwich structures. It was found that the bidirectional layup exhibited a different failure mode as compared to the 45-degree and quasi-isotropic layup. In terms of the acoustic properties, the face sheets with various layup patterns have a low acoustic absorption coefficient with minimal differences from each other at low frequencies (500 Hz–3000 Hz) and have higher absorption coefficients with greater differences from each other at frequencies between 3000 Hz–6500 Hz. The absorption curve was significantly affected by the design of the core. The orientation of the core also comes into play if the core is asymmetrical. The hybrid honeycomb sandwich structure was the optimal structure among the three designs for balanced indentation resistance and acoustic insulation. [ABSTRACT FROM AUTHOR]

A novel angle graded auxetic honeycomb (AGAH) core is designed for sandwich structures in the present study. The angle of the cells is varied through the thickness of the AGAH core using linear functions. Therefore, the thickness of the cell walls is kept constant along the gradation of the cell angle, and the length of the cell walls is changed through the core thickness as the result of angle variation. New analytical relations are proposed to predict the equivalent elastic properties of the AGAH core. The performance of the new proposed core is analytically assessed for the vibrational behavior of a sandwich plate. The governing equations are deduced adopting Hamilton's principle under the assumption of quasi-3D exponential plate theory. Three-dimensional finite element (3D-FE) simulation is accomplished to verify the analytical results of the vibrational response of the sandwich structure. The influence of variation of the cell wall, the cell angle and cell aspect ratio of AGAH core, and geometric parameters of the sandwich structure are investigated on the vibration response of the sandwich panel. The present graded design of the auxetic honeycomb enhances the specific stiffness (i.e., stiffness to density ratio) and consequently increases the natural frequencies of sandwich structures with this type of core. [ABSTRACT FROM AUTHOR]

In this paper, the mechanical properties of Nomex® paper coated with resin and composite Nomex® honeycomb sandwich structures (CNHSS) were obtained by tensile tests and flatwise tests respectively. The fundamental mechanical properties of the Nomex® paper were used as input materials parameters of finite element model generated at meso-scale level for the CNHSS, and the mechanical properties of CNHSSs were used to validate the numerical results. Based on the test and numerical results, the theoretical equations were modified to predict the flatwise compressive buckling strength and modulus of the CNHSS. The numerical and theoretical results clearly revealed that the CNHSS had two flatwise compressive elastic moduli. However, the flatwise test can only capture the second flatwise compressive elastic modulus due to manufacturing geometric defects of the cell walls. The numerical and test results showed that the manufacturing geometric defects of the cell walls showed little influence on the ultimate flatwise compressive strength. And the modified equation can predict the flatwise compressive buckling strength and modulus of the CNHSS with sufficient accuracy. [ABSTRACT FROM AUTHOR]

The paper composite sandwich structure with corrugation and honeycomb cores has been widely used in civil and national defense industries, and the cushioning energy absorption characteristic is a key indicator to evaluate the performance of this composite structure. Therefore, this paper is focused on the influences of honeycomb thickness on the shock acceleration response and deformation characteristics to analyze cushioning energy absorption performance of the composite structure by various experimental tests. The experimental result shows that the paper corrugation layer firstly comes into crushed, and then the paper honeycomb layer is crushed. Additionally, the large honeycomb thickness may cause the secondary collapse of paper honeycomb layer. Under the same impact energy or impact mass, the cushioning energy absorption of the single-sided composite sandwich structure is better than that of the double-sided structure with the same honeycomb thickness. However, the impact resistance of the double-sided composite structure is better than that of the single-sided structure. For the paper composite sandwich structures with the honeycomb thicknesses 10, 15, 20, and 25 mm, the increase of honeycomb thickness would decrease the cushioning energy absorption of the whole structure under the drop impact with low energy. However, under the drop impact with high energy, the influence of honeycomb thickness on cushioning energy absorption is contrary. For the paper composite sandwich structure, the specific energy absorption, unit volume energy absorption, and stroke efficiency for the honeycomb thicknesses 10, 15, 20, and 25 mm are higher than those for the honeycomb thickness 70 mm. Therefore, the low honeycomb thickness is more advantageous for the cushioning energy absorption of paper composite sandwich structure. [ABSTRACT FROM AUTHOR]

In this study, the compression mechanical response of tandem honeycomb cores was investigated to determine the effect of dislocation length and layer height on the compression resistance of tandem honeycomb. A series of flatwise compressive tests were carried out on single-layer and double-layer honeycombs, which showed that tandem honeycomb obtains higher equivalent elastic modulus and collapse stress, and the core assembled with dislocation can achieve almost the same collapse stress with the aligned assembled honeycombs. In addition, a mesoscale finite element modeling method was developed and used to evaluate the effect of dislocation length and layer height on the mechanical response of tandem honeycomb through parametric simulation. Overall, our results suggest that dislocation length is proportional to the collapse stress, and the layer height decides the equivalent modulus and collapse stress of tandem honeycomb, specifically, dislocation length helps achieve higher collapse stress in the tandem honeycomb of varying layer heights. [ABSTRACT FROM AUTHOR]

This research presents a numerical study on vibro-acoustic and sound transmission loss behavior of an aluminum honeycomb core sandwich panel with fabric-reinforced graphite (FRG) composite face sheets. The sandwich theory, which assumes the honeycomb core as an orthotropic structural layer, is applied to investigate the free and forced vibration behavior of the panel. The radiated sound power from the panel is quantified by Rayleigh integral method, and the random diffuse field as an incident sound source is derived based on finite element method with the employment of ACTRAN. A validation between the simulated results and the experimental data published is carried out to demonstrate the accuracy and reliability of the present approach. The comparison between different materials of honeycomb sandwich structures illustrates the advantages of the fabric-reinforced graphite honeycomb sandwich structure over the other types of sandwich structures considered. The effects of different boundary conditions and honeycomb structural geometry properties on the acoustical performance of the stiffness of the FRG panel are also investigated. The approach of the present research provides useful guidance for evaluating and selecting the other honeycomb sandwich panels when the vibratory and acoustic behaviors of the panels are considered. [ABSTRACT FROM AUTHOR]

Hybridization of sandwich panels and their different components have drawn huge attention due to the significant improvement in their attributes. Hybrid core of 'Polyester Pin-reinforced Foam filled Honeycomb Sandwich panels' (PFHS) were fabricated and compared with unreinforced 'Foam filled Honeycomb Sandwich panels' (FHS) in terms of low velocity impact and Compression After Impact (CAI) performance. The impact damage area was calculated by employing MATLAB image processing technique. Incorporating through thickness pins for connecting faces and core is an effectual way to improve interfacial bonding, specific bending stiffness and also imparts out of plane properties for sandwich panels. The low velocity impact tests performed on the sandwich panels revealed that the polyester pin reinforcement in foam filled honeycomb sandwich panel improved the load bearing capacity, total absorbed energy and reduced the impact damage area significantly. In CAI test, debond, wrinkling of face sheet, and buckling of face sheet and core are the major modes of failure. The addition of the pins enhanced the compressive strength for all the impact energy levels. [ABSTRACT FROM AUTHOR]

A theoretical model is developed to study the superior sound absorption performance of ultralight mirco-perforated sandwich panels with double-layer hierarchical honeycomb core. Numerical simulations are performed to validate theoretical model predictions and explore physical mechanisms underlying the sound absorption. Systematic parametric study is implemented to investigate the influence of specific structural parameters on sound absorption. To maximize sound absorption, optimal structural parameters of the hierarchical sandwich are obtained using the method of simulated annealing. It is demonstrated that viscous dissipation of the air inside micro-perforations and around inlet/outlet regions dominates sound absorption. Compared to micro-perforated sandwich panels with regular honeycomb core, not only the proposed hierarchical construction has much improved load-bearing capacity, but also significantly enhanced sound absorption covers a wide range of frequency. [ABSTRACT FROM AUTHOR]

This paper reports on the production and evaluation of a new class of "Z-axis" composite sandwich panel where the core consists of a dense array of vertically-aligned, 3 mm long E-glass fibre composite "beams". The E-glass fibre bundles were aligned using electrostatic charging. A procedure was developed to retain the orientation of the short-fibre bundles whilst they were impregnated and cured with an epoxy/amine resin system. The skins were manufactured from 4-ply carbon/epoxy prepregs with a layup sequence of (0,90)s. The out-of-plane compressive strength of these Z-axis composites was found to be 25.2 and 15.2 times greater than equivalent sandwich panels made with Nomex® and aluminium honeycomb cores respectively. Their compressive strength was found to increase in proportion to the density of the core. Buckling and fracture of the vertically-aligned Z-axis composite were the predominant failure modes observed. The shear and flexural properties of the Z-axis composites were comparable to equivalent honeycomb sandwich panels manufactured from Nomex® and aluminium honeycomb cores. [ABSTRACT FROM AUTHOR]

The effect of extreme cold temperatures on the quasi-static indentation and the low velocity impact behavior of woven carbon/epoxy composite sandwich panels with Nomex honeycomb core was investigated. Impact tests were performed at room temperature, –70°C, and –150°C. Two sizes of hemispherical impactor were used combined to three different impactor masses. All the impact tests were performed at the same initial impact velocity. The effect of temperature on the impact behavior is investigated by studying the load history, load-displacement curves and transmitted energy as a function of time curves. Impact damage induced at various temperatures was studied using different non-destructive and destructive techniques. Globally, more damages are induced with impact temperature decreasing. The results also show that the effect of temperature on the impact behavior is function of the impactor size. [ABSTRACT FROM AUTHOR]

This research presents the adaptation of an anisotropic hyperelastic constitutive model for predicting the experimentally observed in-plane, orthotropic, bi-modular and nonlinear-elastic responses of a commercial adhesively bonded HRP-C fiberglass/phenolic hexagonal honeycomb core. The hyperelastic constitutive model is evaluated under simple states of loading using single-element finite element analysis. The predictions of the model, including stress-strain behavior, Poisson effects, and strain energy densities, are compared with test data for the in-plane uniaxial tension/compression responses of the honeycomb core as well as the single-element model with a linear orthotropic constitutive model to highlight the effectiveness of the hyperelastic model at high strain levels. Good agreement is observed between the model predictions and test data. Tension tests in ribbon and transverse directions with the full-scale honeycomb core are also simulated to ensure the suitability of the single element model for simulations of the simple loading cases and preliminary validation process. [ABSTRACT FROM AUTHOR]

Based on the hyperbolic tangent shear deformation theory, free vibration and sound insulation of two different types of functionally graded (FG) honeycomb sandwich plates with negative Poisson's ratio are studied in this paper. Using Hamilton's principle, the vibration and vibro-acoustic coupling dynamic equations for FG honeycomb sandwich plates with simply supported edges are established. By applying the Navier's method and fluid–solid interface conditions, the derived governing dynamic equations are solved. The natural frequencies and the sound insulation of FG honeycomb sandwich plates obtained in this work are compared with the numerical results by the finite element simulation. It is proven that the theoretical models for the free vibration and the sound insulation are accurate and efficient. Moreover, FG sandwich plates with different honeycomb cores are investigated and compared. The corresponding results show that the FG honeycomb core with negative Poisson's ratio can yield much lower frequencies. Then, the influences of various geometrical and material parameters on the vibration and sound insulation performance are systematically analyzed. [ABSTRACT FROM AUTHOR]

Composite sandwich structures are commonly considered in large-scale aerospace applications due to their performance on a per mass basis. The nature of a large-scale sandwich structure generally necessitates the use of multiple sections of core to fill out the structural form. These core sections must be spliced together to ensure that shear loads are appropriately transmitted throughout the core. Because core installation in a large-scale component is a challenging operation, core splice joint width can be difficult to control in manufacturing. As such, the effects of core splice joint width on sandwich structure performance should be well understood. This study examines the effects of core splice joint width in honeycomb sandwich structures via mechanical testing and post-failure analysis. A threshold core splice joint width is shown to exist with respect to core shear, while the integrity of the facesheet-to-core interface is shown to degrade with increasing core splice joint width. [ABSTRACT FROM AUTHOR]

In recent years, the synthetic cores of sandwich panels have experienced an increase in demand to be replaced by environmentally friendly materials. Furthermore, with the stringent fire protocols introduced in the building codes due to recent fire incidents around the world, it is imperative to conduct fire performance studies for all structural materials. The mechanical performances of the different core structures in sandwich panels have been extensively studied and documented in the literature, although the influence of those core structures on the fire reaction properties has not yet been fully understood. The aim of this work is to experimentally investigate, for the first time, the effects of the core structures, namely, corrugated and honeycomb cores manufactured from flax reinforced polymeric composites and radiata pine plywood, on their flammability. A bench-scale cone calorimeter has been employed to measure the fire reaction properties of the two types of materials along with the subsequent effects of the core structures. The orientations of the cores were observed to significantly impact the performances of the samples under fire. The honeycomb cores, with the open cells exposed to the heat flux, generally had better fire performance compared to those of the corrugated cores with higher time to ignition (10 s or more) and time to peak heat release (65 s or more), having almost similar initial masses and peak heat release rates. Furthermore, among the two material systems, the plywood cores outperformed the flax-FRPP cores, specifically in ignition time, smoke production, total heat release and peak heat release rate. The results helped in confirming that the honeycomb cores have overall better fire performance and the use of plywood as the core material is viable even when fire is involved. [ABSTRACT FROM AUTHOR]

Honeycomb sandwich panels are widely used in marine, aerospace, automotive and shipbuilding industries. High strength to weight and excellent energy absorption are features that make these structures unique. Foam filling the honeycomb core enhances the mechanical properties of sandwich panels considerably. In the present study, the buckling behavior of Nomex honeycomb core/glass-epoxy face sheet sandwich panel for both bare and foam-filled honeycomb core is investigated numerically and experimentally, considering the viscoelastic properties of the sandwich panel. Indeed, the viscoelastic properties of the composite face sheet and foam are determined by relaxation test and are implemented in ABAQUS using VUmat code. The finite element method is also performed using ABAQUS to model the buckling behavior of the sandwich panel incorporating both elastic and viscoelastic material behaviour. The effects of composite face sheet lay-up, core thickness, core cell size, and foam filling are also evaluated. The experimental and numerical results show that the foam increases the critical buckling load and energy absorption. [ABSTRACT FROM AUTHOR]

Dimpling in the composite face sheets of honeycomb sandwich structures due to mismatch in the thermal expansion coefficients of the constituent materials was studied with emphasis on its monitoring and prediction. Strain distributions along optical fibers embedded in the face sheet were monitored during manufacturing. Dimple formation and in-plane strain distributions in the face sheets were studied using finite element analysis, and an analytical model based on the beam theory was constructed to predict the dimple depths from the strain data. A system using twin optical fiber sensors was proposed to accurately measure the dimpling-induced strains. The usability and performance of the system was evaluated using small scale specimens and finally on a more realistic large-scale specimen. The system could measure the strain changes due to dimpling of the face sheets and provided decent prediction of the dimple depth distribution along the sandwich panels. [ABSTRACT FROM AUTHOR]

In the present study, a 2D-based large-scale metallic auxetic double arrowhead honeycomb core sandwich panel (DAHSP) was proposed and its deformation response, energy dissipation characteristics and associated mechanisms under air blasts were investigated using a validated numerical model. It aims at the performance improvement of DAHSPs through the design of core relative density with respect to different strategies. The DAHSPs considered mainly experienced a local dome superimposed upon global deformation of front face and global deformation of back face, while the core webs were heavily buckled and progressively collapsed. The results confirmed the material concentration effect of DAH cores induced by the negative Poisson's ratio (NPR). It was found that the panel deformation response was highly related to their deformation/failure mechanisms. Relative to core web thickness, the increase of number of core layers led to a more remarkable decrease in permanent deflections. However, the decline of inclined angles not always reduced the back face deflection due to the competition between enhanced bending stiffness and deteriorated local contact force. An ideal means to decrease the panel deformation is to enlarge the inclined angles at low relative density but to decrease the horizontal distance when the relative density increases to a high level. The panel with thinner core webs at low relative density and the panel with narrowed inclined angles at high relative density is more beneficial to plastic energy absorption. In addition, a core configuration with a thinner tendon but a thicker stuffer promoted the exploitation of NPR and further improved the panel energy absorption. [ABSTRACT FROM AUTHOR]

A failure criterion for predicting the damage of honeycomb structures under combined compression and transverse shearing is put forward in this paper. An analytical method using the Bloch wave function and von Kármán plate theory is employed to predict the onset of damage based on the fact that failure of honeycomb structures is closely related to the buckling behavior of its core. After obtaining a bunch of data points of different stress conditions where damage occurs, a failure envelop for honeycomb cores under out-of-plane loads is formed and fitted by a nonlinear function. In order to validate the proposed failure criterion, experimental studies are performed on honeycomb sandwich structures consisting of Nomex honeycomb core and CFRP (carbon-fiber reinforced plastics) facesheets including in-plane compression, three-point bending and combined-compression-shearing. The accuracy of the proposed criterion is evaluated by comparing experiments with results predicted by progressive damage model incorporating the failure envelope of honeycomb core. [ABSTRACT FROM AUTHOR]

Effective thermal conductivity is an essential parameter to investigate thermal properties of metallic honeycomb sandwich structures. And it cannot be measured by traditional methods due to sandwich structure imbedded with air. A practical experimental equipment was designed to evaluate the value under different temperature from 100°C to 400°C. And it was found that the value of effective thermal conductivity can also be calculated by knowing the thermal conductivity of the reference, thickness of the reference and the slope and intercept of temperature in different layers. Meanwhile, numerical simulation was conducted and the results agreed well with that achieved by experiment. Also, the value of effective thermal conductivity calculated by experiment is close to the value calculated by Swann-Pittman empirical equation. And the method is not limited in metallic honeycomb sandwich structure while it can be applied in most structures with amounts of air. On that basis, heat transfer process of the structure is discussed including heat conduction, heat convection and heat radiation. [ABSTRACT FROM AUTHOR]

In the present work, nonlinear dynamics of symmetric rectangular honeycomb sandwich thin panel is investigated. The nonlinear governing equations of the thin panel are derived by using Hamilton's principle and Reddy's third-order shear deformation theory. After that, the Galerkin method is used to discretize the equation of motion, resulting in a set of ordinary differential equations with respect to time. The ordinary differential equations are solved analytically by utilizing the homotopy analysis method. The influence of structural parameters to the nonlinear frequencies of the symmetric rectangular honeycomb sandwich panel with simply supported boundaries along all four edges is discussed by using the analytic approximation method. Our findings demonstrate that the nonlinear frequency ratio decreases first and then increases with the increase of the width-to-length ratio and thickness-to-length ratio. When the width-to-length ratio is greater than 10, the nonlinear frequency ratio remains almost unchanged with the further increase of width-to-length ratio. [ABSTRACT FROM AUTHOR]

The honeycomb sandwich structures with carbon composite face sheets combined with aluminium or Nomex® honeycomb core are extensively used in the aircraft airframe structure. They provide some key benefits over conventional structures such as increased bending strength and stiffness combined with low weight. However, they show high sensitivity to a certain type of damage, such as object impact, mishandling, etc. Non-destructive inspection methods are needed for a structural health examination. Active infrared thermography is one of them. The quality of the inspection and defect detection depends heavily on the proper application of the external thermal excitation. This paper presents a concept of the sandwich structure with internal excitation by means of an integrated thin-film resistive heating element. [ABSTRACT FROM AUTHOR]

Fracture testing and analysis of aerospace grade honeycomb core sandwich constructions using a single cantilever beam test methodology is presented here. Influence of various parameters such as facesheet thickness, core density, honeycomb cell-size, and core thickness were studied. A Winkler-based foundation model was used to calculate compliance and energy-release rate, and further compare with finite element model and experiments. A cohesive zone model was developed to predict the disbond initiation and simulate the interface crack propagation in the single cantilever beam sandwich specimen. The mode I interface fracture toughness obtained from the translating base single cantilever beam setup was provided as input in this cohesive zone model. It is shown that the presented cohesive zone approach is robust, and is able to capture the debonding phenomenon for majority of the honeycomb core specimens. [ABSTRACT FROM AUTHOR]

Defect determination is important for guaranteeing the quality of sandwich structure. The present study, a geometric recognition methodology based on dynamic window was constructed for honeycomb structure, by identifying the vertices from given images captured in the manufacturing process. The combination of traversing window, annihilation window, and tracking window was developed to reconstruct the contoured maximums, locate the vertices positions, and determine the cell–node relationships, respectively. Moore boundary tracking was practically used to deal with the problem caused by the edge effect. Based on these, all relevant vertices as well as their relationship of each honeycomb cell can be determined. Afterward, quality assessment of the honeycomb product was carried out in terms of the maximum, the average, and the maximum average degree deviation. Illustrations validate the present methodology well. All these achievements shed a light on design of the consistency, reliability, and homogeneity of high-standard sandwich structure. [ABSTRACT FROM AUTHOR]

Failure mechanism of sandwich panels is very complicated. The objective of this study is to demonstrate the usefulness of high-speed observation to elucidate the failure mechanism of sandwich panels. Failure processes of carbon fiber reinforced plastic composite/Nomex™ honeycomb sandwich panels were observed using two synchronized high-speed cameras with an 8 µs frame rate. The sandwich panel was subjected to a four-point bending load in accordance with ASTM-D7249. Digital image correlation was also adopted for measuring the surface strain. Results show fracture initiation: (1) local (intra-cell or wrinkling) buckling of the face sheet at the edge (Type I and III), or (2) local buckling (intra-cell or wrinkling) of the face sheet at the middle of the specimen (Type II). Following local buckling of the face sheet, debonding between the face sheet and the honeycomb core occurred within 10–20 µs. Subsequently, crack propagation of the face sheet occurred, leading to final failure of the sandwich panel. It was about 150–200 µs from the local buckling of face sheet to the final failure of sandwich panel. Results demonstrated that high-speed observations are effective to elucidate failure mechanisms of the sandwich panel subjected to a bending load. [ABSTRACT FROM AUTHOR]

Fracture mechanics analysis of a face/core interface crack in a honeycomb core sandwich under mode I-dominated loading conditions is considered. The main objective of the study is to examine how to represent the homogenized in-plane extensional properties of hexagonal honeycomb core under deformation constraints due to bonding of the core to the face sheets. 2D parametric finite element analysis is conducted over a wide range of homogenized in-plane extensional properties of the core. The results show that plane stress assumption may lead to crack closure effects and highly distorted core elements for core with low in-plane modulus, while plane strain stabilizes the core deformation. Experimental results from SCB testing of a honeycomb core sandwich support the plane strain constrained modulus approach to face/core debond analysis. [ABSTRACT FROM AUTHOR]

In view of cushioning energy absorption of paper composite sandwich structures with sinusoidal corrugation and hexagonal honeycomb cores, the static yield strength and compression deformation were firstly analyzed by experiments, then the unit volume energy absorption, unit area energy absorption, and specific energy absorption were evaluated. Further, the influences of static compression rate, corrugation types, and honeycomb thickness on the cushioning energy absorption were respectively studied. The experimental observations show that the paper composite sandwich structures are crushed layer by layer due to the difference in the yield strength of sandwiches, and the honeycomb core is crushed after the collapsing of corrugation core. The three evaluation indices of cushioning energy absorption have similar change rule with the compression rate, corrugation types, and honeycomb thickness. The influence of static compression rate on the yield strength and cushioning energy absorption of the paper composite sandwich structure is not obvious. The composite sandwich structures consisting of paper corrugation sandwich with large inertia moment and honeycomb sandwich with large thickness have more excellent cushioning energy absorption, the B-corrugation can increase the unit volume energy absorption of paper composite sandwich structures by up to 18% than C-corrugation, and the 3-honeycomb can increase the unit volume energy absorption of paper composite sandwich structures by up to 32% than 2-honeycomb. [ABSTRACT FROM AUTHOR]

Composite honeycomb sandwich structure is widely used for aircraft structures such as control surfaces, radomes, engine cowls, and aircraft interior structure because of its lightweight and high strength characteristics. One of the disadvantages of honeycomb sandwich structure is that they are prone to fluid intrusion. The purpose of this study is to determine if the structural properties of honeycomb core are affected by contact with a fluid. The test specimens were manufactured of fiberglass prepreg for the facesheets and Nomex® honeycomb core for the core material in accordance with ASTM C-365/365M. Test specimens were soaked in several different kinds of fluids, such as aircraft fuel, turbine engine oil, hydraulic fluid, and water for a period of 60 days. Some of specimens were tested right after soaking period ended, and some of them were dried before they were tested. A flatwise compressive test was performed, and the test results were analyzed to determine how the contact with aircraft fluids affected the compressive strength of the Nomex® honeycomb core and how the strength was recovered when the specimens were dry. In addition, the investigation of de-bonding between facesheet and core material after soaking was performed to support the study. [ABSTRACT FROM AUTHOR]

Multiphase composite sheets containing carbon fiber and functionalized multiwall carbon nanotubes in epoxy matrix were manufactured through compression molding technique. The loading of f-MWCNTs was varied (0.0, 0.1, 0.25 and 0.5 wt.% of epoxy matrix), while the quantity of carbon fiber was kept constant, i.e. ∼60 wt.% of the final composite in all samples. Prepared multiphase composite sheets were subjected to flexural test and the sheet offering maximum flexural strength was selected for subsequent hybrid sandwich structure manufacturing. Hybrid sandwich structure was manufactured via vacuum bagging process by bonding composite face sheet to the aluminum honeycomb core through an adhesive. Mechanical performance of hybrid sandwich structure was evaluated by performing flexural and quasi-static indentation tests, while electrical performance of hybrid sandwich structure was investigated by performing electromagnetic interference shielding effective and electrostatic dissipation tests. Optical and scanning electron microscopy was carried out for microstructural investigations. Fractography was also performed on composite face sheet after flexural test to study the underlying high flexural strength mechanism. Neat sandwich structure without functionalized multiwall carbon nanotubes addition was also manufactured and tested for comparison. Results show that hybrid sandwich structure outperformed the neat sandwich structure in all tests. [ABSTRACT FROM AUTHOR]