Honeycomb Materials Research Articles

Noise mitigation and wave number characterization in sandwich structures.

Sandwich structures are the ideal materials to use for many engineering applications since they are lightweight, strong, and stiff. While sandwich structures are mechanically advanced, their acoustic properties are not ideal since they are excellent sound radiators. Therefore by studying their wave number characteristics, the acoustic properties of sandwich structures can be improved to help mitigate noise propagation. This study used several different combinations of carbon fiber-epoxy, glass fiber-epoxy, and aluminum face sheets with honeycomb and Rohacell foam core materials. These beams were vibrated with an electro-dynamic shaker, while the vibration response was measured at numerous equidistant points along the beam with a micro-accelerometer. Results showed the frequency response functions, which were transformed to the wave number/frequency domain yielding dispersion plots. From this plot one can calculate the coincidence frequency of the sandwich structure, which determines how and when the structure will radiate noise. Experimental results showed that thin aluminum face sheets with Rohcell 51 WF foam core had the greatest noise mitigation properties with a coincidence frequency of approximately 4500 Hz. Furthermore, it is shown that the experimental wave speed values agree with analytical methods. [Work supported by The Boeing Company.]

Experimental Characterisation and Numerical Modelling of Hexagonal Honeycomb Cellular Solids under Multi‐axial Loading

Abstract: Cellular solids are becoming increasingly popular for sandwich core and energy‐absorbing applications in many automotive and other transportation structures. This paper investigates experimentally and numerically the strength and post‐failure energy absorption of a popular hexagonal aluminium honeycomb material under multi‐axial loading conditions. For the experimental work, an improved Arcan test apparatus is used so that interaction of multi‐axial compression and shear loading on failure and crushing may be studied; optical measuring methods are used to extract deformation data. In addition, experimental work to characterise the material with pre‐deformation in the in‐plane directions has also been conducted. This experimental work provides input for computational modelling of the material and two alternative modelling approaches have been investigated. First, a three‐dimensional anisotropic, elastic–plastic model, with coupling of loading components is used to represent the material at the macro‐level and, second, a meso‐modelling approach using a fine shell representation of the thin‐walled honeycomb cellular structure is applied. For practical analysis of large‐scale structures, the former approach is computationally efficient and can reasonably treat the most important failure and crush characteristics of the material. However, for more accurate analysis, particularly in the case of complex non‐proportional loading, the meso‐shell model may provide a more realistic solution.

Comparing Study of Energy-Absorbing Behavior for Honeycomb Structures

Honeycomb materials，as a type of ultra-light multifunctional material，have been examined extensively in recent years and have been applied in many fields. This study investigated the energy absorption capacity and their mechanisms of honeycomb structures with five different cell geometry (square，triangular，circular, hexagonal，kagome). It has been shown that the honeycomb structure with kagome cells is the best choice under the targets of the energy absorption capacity, peak force and plateau stress, when relative density and cell wall thickness of the five kinds of honeycombs are the same. Besides, honeycomb with hexagonal cells and honeycomb with triangular cells are also ideal structures for energy absorption purpose.

The Dynamic Mechanical Properties Study of Aluminum Alloy Honeycomb Material and its Numerical Simulation

Light weight aluminum alloy honeycomb materials are ideal lightweight structure materials, which have higher specific strength and stiffness, and widely applied in aviation, aerospace, automobile and so on, and also it is an ideal material for damping, buffering, heat insulation and energy absorption. In this paper, the SHPB technique which the aluminum alloy bar diameter is 37mm is used for studying the dynamic mechanics property of the lightweight aluminum alloy honeycomb materials which the diameter is 30mm, and then to study the dynamic mechanical properties of the material, from the results we can see dynamic strain-stress curves can be divided into three phases, elastic phase, stress platform and compaction phase. And the numerical simulation method was used for study the dynamic deformation process of the honeycomb panel material under impact load, and also compare the results with the specimen cell figures which were observed by the microscope.

Shear Strength Optimization of Reinforced Honeycomb Core Materials

This paper studies the performance of novel honeycomb materials with the cell walls made of natural fiber reinforced thermoplastics. It focuses on developing a methodology for the maximization of the specific transverse shear strength of reinforced honeycombs and prediction of the corresponding cell geometry. The load carrying capacity of wood fiber reinforced polypropylene honeycomb shows a substantial improvement as compared to that of a polypropylene honeycomb of the same density. Theoretical predictions agree well with the experimental results. Excellent performance of the reinforced polypropylene honeycombs indicates a promising future for such materials.

Photochromic, Metal-Absorbing Honeycomb Structures

We demonstrate the synthesis and use of a spiropyran functional polymer to form highly ordered honeycomb materials by the breath figure technique, which is based on the self-assembly of water droplets. These materials undergo rapid and intense color changes both in solution and as porous films by irradiation with light (UV or visible). We also demonstrate the metal binding ability of these polymers ultimately to create hybrid organic-inorganic porous structures. Furthermore, by reduction of the metal and calcination of the organic materials, unique palladium microrings can be prepared. The methods described are general techniques that may be applied to a range of heavy metals.

Strain gradient elastic homogenization of bidimensional cellular media

The present paper aims at introducing an homogenization scheme for the determination of strain gradient elastic coefficients. This scheme is based on a quadratic extension of homogeneous boundary condition (HBC). It allows computing strain elastic effective tensors. This easy-to-handle computational procedure will then be used to construct overall behaviors and to verify some theoretical predictions on strain gradient elasticity.

Modelling the fatigue behaviour of composites honeycomb materials (aluminium/aramide fibre core) using four-point bending tests

Composite Sandwich Materials are being increasingly used in high-performance structural applications because of their high stiffness and low weight characteristics. Presently, the long-term performance of such structures, especially under fatigue loading, is not enough studied. The aim of this paper is to address such fatigue behaviour by using a fatigue model verified by experimentation. The fatigue model is based on the fatigue modulus concept (degradation of stiffness) which is proposed for core-dominated behaviour and for two directions cells (L and W). Two non-linear cumulative damage models (L and W) derived from the chosen stiffness degradation equation, are examined in context with the linear Miner’s damage summation and compared with available experimental results.

Crushing Strength of Aluminum Honeycomb with Thinning Cell Wall

To evaluate the crash safety of automobiles, various collision tests are performed by the auto industry. In the offset frontal collision test and the side collision test, the target is an aluminum honeycomb material which has thinning cell walls. In this study, based on the analyses of the shock absorption mechanism, a new crushing strength formula is proposed. First, load-displacement curves obtained from compression tests in quasi-static condition showed an almost linear relation between a thinning rate of cell walls and a crushing strength. Second, based on Wierzbicki's theory, a new formula was proposed, which can estimate a crushing strength of a honeycomb material with thinning wall. In addition, a correcting equation which considered an elastic deformation was also proposed. Third, parametric analyses were carried out with a FE model which can simulate a delamination between cell walls. The results obtained from the theory and FEM almost corresponded to each other for a wide range of the thinning rate. Fourth, impact tests were carried out, in which the weight was dropped freely at the speed used for the automobile tests. Those results almost agreed well with the sum of the theoretical crush strength and the inside air pressure.

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Structures of honeycomb sandwich made of aluminum alloy are light weight and high rigidity, and can be expected to improve fuel economy and environmental improvement. If possible, have the plastic machining in the honeycomb material of the plane can be expected expansion of the scope of application. In this experiment, cell size to double the size of sample using the three-point bending, guided bending and conventional size as compared to results, considering. Three-point bending results that conventional size than sharp local transformation has happened. The result guided bending of conventional than size core materials and surface material detachment and easy-to-be was admitted.

Interlayer tuning of electronic and magnetic properties in honeycomb ordered Ag3LiRu2O6

We report a switching of electronic, magnetic and lattice properties in honeycomb ruthenates by interlayer cation exchange. The new material Ag3LiRu2O6 was made by ion-exchange of the ordered Li/Ru honeycomb material Li2RuO3 in an AgNO3 melt at 200 °C. Neutron powder diffraction and electron microscopy show that the Li/Ru order is preserved in the honeycomb layers, however, significant stacking disorder is found between layers. In contrast to Li2RuO3, which is insulating, dimerised and diamagnetic, Ag3LiRu2O6 has low electrical resistivity (0.01 ohm cm−1) and a large magnetic susceptibility at room temperature. This is attributed to the electronic influence of the highly polarisable interlayer Ag+ cation. The combination of two dimensionality, good conductivity and stacking disorder means this family of materials have potential for thermoelectric applications.

Monolayer honeycomb structures of group-IV elements and III-V binary compounds: First-principles calculations

Using first-principles plane wave calculations, we investigate two dimensional honeycomb structure of Group IV elements and their binary compounds, as well as the compounds of Group III-V elements. Based on structure optimization and phonon mode calculations, we determine that 22 different honeycomb materials are stable and correspond to local minima on the Born-Oppenheimer surface. We also find that all the binary compounds containing one of the first row elements, B, C or N have planar stable structures. On the other hand, in the honeycomb structures of Si, Ge and other binary compounds the alternating atoms of hexagons are buckled, since the stability is maintained by puckering. For those honeycomb materials which were found stable, we calculated optimized structures, cohesive energies, phonon modes, electronic band structures, effective cation and anion charges, and some elastic constants. The band gaps calculated within Density Functional Theory using Local Density Approximation are corrected by GW0 method. Si and Ge in honeycomb structure are semimetal and have linear band crossing at the Fermi level which attributes massless Fermion character to charge carriers as in graphene. However, all binary compounds are found to be semiconductor with band gaps depending on the constituent atoms. We present a method to reveal elastic constants of 2D honeycomb structures from the strain energy and calculate the Poisson's ratio as well as in-plane stiffness values. Preliminary results show that the nearly lattice matched heterostructures of ...

Photocatalytic air purification: Comparative efficacy and pressure drop of a TiO 2-coated thin mesh and a honeycomb monolith at high air velocities using a 0.4 m 3 close-loop reactor

Photocatalytic purifiers, which are in development for treating gaseous effluents at flow rates generally higher than several tens of m 3 h −1, usually employ TiO 2-coated materials either as planar or folded fibrous filters, or as honeycomb monoliths. Our primary objective was to compare the photocatalytic efficacy of two types of these materials using a ca. 0.4 m 3 close-loop, air tight, photocatalytic reactor we built at EDF. This loop – through which air can be flowed at rates from 16 to 1800 m 3 h −1, i.e. 0.11–12.25 m s −1 – includes a paralleliped (560 mm × 295 mm × 200 mm) into which alternate banks of lamps and TiO 2 coated-materials (entrance area = 408 cm 2) can be accommodated. The materials tested were (i) an Ahlstrom-supplied, thin, non-woven tissue dried at room temperature after impregnation with both Degussa TiO 2 P-25 and colloidal SiO 2 used as a binder and (ii) an aluminum honeycomb-shaped material we coated with Degussa TiO 2 P-25 and dried at room temperature. U-shaped lamps emitting at 254 nm and having an electrical power of 18 or 35 W were employed. The geometries of the materials and the spatial arrangements of the lamps and materials provided an irradiance of the materials as high and homogeneous as possible according to our modeling [Catal. Today 122 (2007) 66–77]. Methanol was chosen as the test pollutant because it is easily mineralized, which minimized the inhibition of the photocatalytic activity by intermediate products. In the recirculation regime, the initial rates of methanol removal observed for the folded tissue were multiplied by at least four – depending on the numbers of materials and banks of three 35 W lamps – when the honeycomb material was utilized. Several factors can add to cause this high increase in efficacy: distinct shapes and TiO 2 supports, differences in photons scattering, and the reduced accessibility of reactants to TiO 2 because of SiO 2. Regarding energy consumption, the use of two photocatalytic materials sandwiching one bank of three 35 W lamps instead of one material in-between two banks of three lamps led to about the same efficacy. The honeycomb material was also tested with toluene at an initial concentration corresponding to the same amount of carbon as when methanol was the pollutant. The removal rate and CO 2 formation rate were lower for toluene than for methanol, which can be easily explained by a lower reactivity with respect to oxidation, a smaller adsorbed amount and a higher competition with the more numerous degradation intermediate products. That comparison illustrates the need of trials for every effluent to be treated. Considerable differences in pressure drop between the two materials clearly demonstrated another interest of using honeycomb shapes, at least for the materials and configurations investigated. Furthermore, a deactivation, together with a yellowing, was noticed at high irradiance for the Ahlstrom tissue, which included cellulose fibers. Folding the material enables one to employ lower irradiances for minimizing this effect, while maintaining the efficacy because of the increase in the material area in the reactor, as well as slightly lowering the pressure drop.

Evaluation of Compressive Densification Strain of Paper Honeycombs

The significance of the compressive densification strain to the evaluation of energy absorption properties is analyzed. The evaluation equation of compressive densification strain of paper honeycombs is modeled by introducing the instantaneous and relative density of paper honeycombs. The results of many experiments of paper honeycombs with different structure factors show how geometry aspects of hexagonal paper honeycombs, e.g. the thickness and length of honeycomb cell-wall and the drawing ratio of hexagonal honeycomb, affect the compressive densification strain of the paper honeycombs. With the increase of cell-wall thickness, the densification will reduce; with the increase of cell-wall length, the densification will increase; with the increase of thickness-length ratio of cell-walls, the densification will reduce; with the increase of drawing ratios, the densification will reduce; when the drawing ratio is 1, the compression densification is up to the maximum. The experiment results agree with the evaluation equation, and when the instantaneous and relative density is almost 0.39, the densification appears. The evaluation equation applied to the five-layer corrugated paperboard shows it also can be used for other structure honeycomb materials. The equation is universal.

Quasi-Static Indentation Behavior of Honeycomb Sandwich Materials and Its Application in Impact Simulations

Both the crushing and indentation behaviors of sandwich materials are important aspects in failure analysis and energy absorption. In this paper, the core crushing strength of honeycomb materials from the experiment is briefly introduced, and the indentation of sandwich structures is studied in detail. Using the beam on elastic-plastic foundation models, theoretical formulations for predicting indentation behavior of sandwich materials are proposed, from which the mechanical response of elastic-plastic sandwich beams are obtained. Three stages of failure are clearly depicted by the global stiffness changes in the load versus displacement curve of elastic-plastic beams. The models are compared with the available experimental data and numerical simulation, and relatively close agreements are achieved. The compliance and compliance gradient derived form the indentation models are then incorporated with the equation of motion of the projectile to study impact response of elastic-plastic beams, and the impact ...

Prediction of the adhesive fillet size for skin to honeycomb core bonding in ultra-light sandwich structures

The formation of resin fillet between honeycomb core cell walls and skin in light sandwich structures was studied to gain a better understanding of the bonding process. A method was developed for tailoring the amount of adhesive between 8 and 80 g/m 2 . The size of the adhesive menisci and the contact angles between the adhesive and the skin and the core materials were measured. A model was developed to predict the size of the menisci. Their shape was driven by the surface energy of skin and honeycomb materials. When adhesive films were used for bonding, up to 50% of the adhesive did not form the menisci whereas 100% did when the newly developed adhesive deposition method on honeycomb was used, which allowed better bonding with lower weight.

Impact behavior and energy absorption of paper honeycomb sandwich panels

Abstract Dynamic cushioning tests were conducted by free drop and shock absorption principle. The effect of paper honeycomb structure factors on the impact behavior was analyzed. Results of many experiments show that the dynamic impact curve of paper honeycomb sandwich panel is concave and upward; the thickness and length of honeycomb cell-wall have a great effect on its cushioning properties; increasing the relative density of paper honeycomb can improve the energy absorption ability of the sandwich panels; the thickness of paper honeycomb core has an up and down fluctuant effect on the cushioning properties; with the increase of the thickness of paper honeycomb core, the effect dies down; flexible corrugated paperboard as liners can improve the compression resistance and cushioning properties of paper honeycombs. The research results can be used to optimize the structure design of paper honeycomb sandwich panel and material selection for packaging design.

Geometry effects and scaling in thermoacoustics

Current work at TU Eindhoven on thermoacoustics will be discussed. The end effects of the geometry of a stack on the performance of thermoacoustic machines will be shown. End effects and the formation of vortices is an issue that contributes to enhanced convective losses at the stack ends. Influence of Reynolds and Strouhal numbers on the oscillatory flow field in the stack are studied. It is possible to plot Sr and Re number diagrams in which various zones can be identified that display characteristic flow patterns. Furthermore the influence of the type of regenerator material is studied in traveling wave engines. Performance of honeycomb material of high-density pores is compared with wire screen regenerators in a thermoacoustic motor. As a last point the effects of downscaling to miniature systems is discussed. Limits of how far the size of a thermoacoustic cooler can be downscaled before conduction effects limit the performance are relevant.

Quasi-static Crushing Behavior of Aluminum Honeycomb Materials

The quasi-static crushing behavior of aluminum honeycomb materials is thoroughly evaluated using a combined experimental, analytical, and numerical approach. Based on experimental characterization, the constitutive properties of the honeycomb cores under flat-wise compression are approximated by an elastic perfectly plastic material with inclusion of hardening after densification. Two different cell size materials are tested and compared, and the effect of strain rate on the crushing stress is studied. The experimental results show that the crushing platen stress is directly related to the relative density of core materials, and it can be associated with the strain rate, even though the effect of strain rates is not so dominant based on the conducted quasi-static tests. A simple physical model for predicting the crushing wave length and stress is proposed and compared with the experimental data and available formulas in the literature. It is observed that the crushing wave length is close to the cell size and related to the geometric dimension and strain rate. The folding mechanism is also measured by the ARAMIS system (a photogrammetry technique), and the measured von Mises strains are compared with the numerical results from LS-DYNA, demonstrating that the folding mechanism is initiated by two plastic hinge lines formed at the cell corners. Multilayer effect is also investigated, and it indicates that including the second layer slightly decreases the maximal crushing stress, but the simple superposition is still applicable for crushing multilayer honeycomb cores with different density. Partial crushing due to small size cylindrical indenter is further studied, and the experiment shows that the partial crushing process can be described by an elastic—plastic hardening material. Side impact process of honeycomb materials is also investigated, and the collapse band and its propagation process are captured. The thorough characterization of core crushing behavior conducted in this study provides better understanding of the failure process of honeycomb materials and can be further employed to study energy absorption and impact response of sandwich structures.

Multifunctional Topology Design of Cellular Material Structures

Abstract Prismatic cellular or honeycomb materials exhibit favorable properties for multifunctional applications such as ultralight load bearing combined with active cooling. Since these properties are strongly dependent on the underlying cellular structure, design methods are needed for tailoring cellular topologies with customized multifunctional properties. Topology optimization methods are available for synthesizing the form of a cellular structure—including the size, shape, and connectivity of cell walls and openings—rather than specifying these features a priori. To date, the application of these methods for cellular materials design has been limited primarily to elastic and thermoelastic properties, and limitations of classic topology optimization methods prevent a direct application to many other phenomena such as conjugate heat transfer with internal convection. In this paper, a practical, two-stage topology design approach is introduced for applications that require customized multifunctional properties. In the first stage, robust topology design methods are used to design flexible cellular topology with customized structural properties. Dimensional and topological flexibility is embodied in the form of robust ranges of cell wall dimensions and robust permutations of a nominal cellular topology. In the second design stage, the flexibility is used to improve the heat transfer characteristics of the design via addition/removal of cell walls and adjustment of cellular dimensions without degrading structural performance. The method is applied to design stiff, actively cooled prismatic cellular materials for the combustor liners of next-generation gas turbine engines.