Aluminum Honeycomb Core Research Articles

Failure mode maps of bio-inspired sandwich beams under repeated low-velocity impact

Existing sandwich structures failure maps are confined only to data from the quasi-static bending tests even for describing failure modes due to the impact event. Strengths of the constituent layers, which are not well-described by these maps, can reasonably change especially under the repeated impact load case. Hence, a new series of more realistic failure mode maps have been developed from the experimentally and numerically obtained observations on recently proposed bio-inspired dual-core sandwich beams in the presence of repeated low-velocity impacts of different energy levels. The beams consist of top and bottom carbon fiber reinforced polymer skins sandwiching the rubber and aluminum honeycomb cores. Departing from the modified Gibson model, an actual presentation of skin and core behaviors has been modeled following the trend of strengths variations for the construction of the failure mode maps when subjected to numerous impact numbers and energies. The produced maps offer the flexibility to accommodate the changes in strengths due to deterioration or densification of constituent layers after impact, and hence following more favorably the physical failure description of the sandwich beams. Accompanying these maps, a general set of mathematical expressions have also been produced for practical convenience. It is found that the failures from observations are within the proposed map boundaries with accuracies ranging from 85.7% to 100%.

Residual flexural properties of CFRP sandwich structures with aluminum honeycomb cores after low-velocity impact

This paper aims to evaluate the influences of impact-induced damage on the residual flexural strength of honeycomb core sandwich panels with different structural configurations by combining the experimental, numerical and theoretical methods. Low-velocity impact tests and three-point bending tests after impact are carried out to determine and quantify the effects of structural configuration and impact energy on the impact damage and residual flexural strength of such structures. Subsequently, an integrated FE model with the VUMAT subroutine is developed to further investigate the damage states and failure mechanisms for the impact and bending simulation. The numerical results match well with the experimental ones in terms of impact load, absorbed energy, residual flexural strength and failure mechanisms. Results indicate that increasing cell wall thickness or decreasing side length of honeycomb core has significant effects on peak load, while increasing core height has little effect. Specimens with lower core stiffness fail through core buckling and crushing under the bending load, while specimen with higher core stiffness fails by top face sheet fracture. The residual flexural strength reduces markedly even through the impact damage is barely visible, indicating that it has a strong correlation with impact energy and structural configuration of cores.

A crushable shell for small body landers

Last year, the DLR built Mobil Asteroid Surface Scout (MASCOT) decoupled from the Hayabusa-2 spacecraft and performed surface science at the asteroid (162173) Ryugu. The MASCOT lander itself didn’t need a dedicated damping system for its touchdown since the impact velocity was very low in the range of a few cm/s. This was due to the relatively small size (D = 900m) and hence low gravitational pull of the target asteroid. In a next step, DLR is developing further nano lander technologies for larger celestial bodies with respectively higher surface gravity and therefore higher impact velocities. Previous investigations have shown that for larger bodies, the landing velocity is in the range of 4 m/s which produces high shock loads. A possible low complex and lightweight solution of damping the shocks is the use of a crushable shell around the lander. This crushable shell could be made out of aluminum honeycomb core with a high-performance polyethylene cover sheet. The idea is to convert the kinetic energy into deformation work of the shell and reduce the shock load to the instrument platform. The design is particularly advantageous since no moving parts or other mechanisms are required, thus making the system very robust and fail safe. This paper concentrates on a hardware test campaign recently done at DLR’s landing and mobility test facility (LAMA). It will show the design of the shell, the test setup and the results of the campaign.

The structure response of sandwich beams with metallic auxetic honeycomb cores under localized impulsive loading-experiments and finite element analysis

In recent years, auxetic cellular materials such as auxetic reentrant honeycombs were widely studied due to its attractively negative Poisson's ratio. In this manuscript, the structure response of end-clamped sandwich beam with auxetic reentrant hexagonal aluminum honeycomb core locally subjected to foam projectile with high-velocity impact was experimentally and numerically studied. The reentrant honeycomb cores with different cell-wall thicknesses were fabricated by selective laser melting (SLM). According to experiment results, the failure modes of the sandwich beam were investigated. It was found that sandwich beam with different honeycomb cell's thicknesses could fail in different modes. Meanwhile, the local negative Poisson's ratio deformation was observed in the thin honeycomb core. Based on this, a finite element (FE) model was constructed to analysis the dynamic deformation evolution of two face sheets and auxetic reentrant honeycomb core. The local auxetic and expand deformations of honeycomb core were both found. Finally, the residual deflections of sandwich beam under impulsive loadings were discussed.

Damage Identification in Various Types of Composite Plates Using Guided Waves Excited by a Piezoelectric Transducer and Measured by a Laser Vibrometer.

Composite materials are widely used in the industry, and the interest of this material is growing rapidly, due to its light weight, strength and various other desired mechanical properties. However, composite materials are prone to production defects and other defects originated during exploitation, which may jeopardize the safety of such a structure. Thus, non-destructive evaluation methods that are material-independent and suitable for a wide range of defects identification are needed. In this paper, a technique for damage characterization in composite plates is proposed. In the presented non-destructive testing method, guided waves are excited by a piezoelectric transducer, attached to tested specimens, and measured by a scanning laser Doppler vibrometer in a dense grid of points. By means of signal processing, irregularities in wavefield images caused by any material defects are extracted and used for damage characterization. The effectiveness of the proposed technique is validated on four different composite panels: Carbon fiber-reinforced polymer, glass fiber-reinforced polymer, composite reinforced by randomly-oriented short glass fibers and aluminum-honeycomb core sandwich composite. Obtained results confirm its versatility and efficacy in damage characterization in various types of composite plates.

Numerična študija obnašanja tlačno obremenjenega jedra satovja iz zlitine na osnovi aluminija

Numerična študija obnašanja tlačno obremenjenega jedra satovja iz zlitine na osnovi aluminija

Compressive failure of quasi-static indented CFRP/aluminium honeycomb sandwich panels

The present study examines sandwich panels, which are made of carbon fiber facesheets with aluminium honeycomb core and focuses on how varying factors (skin thickness, layup, core density and diameter of hemispherical indenter) are affecting collapse of artificially damaged panels in edgewise compression. Damage in the middle of sandwich specimens was introduced using quasi-static indentation. Hemispherical indenters of two diameters, namely, 20 mm and 150 mm, were applied. The residual strength was estimated through the edgewise compression test of damaged specimens. The study demonstrated that increase of the honeycomb density led to decrease in the safety margin in structure. Thicker skins may also lead to a catastrophic propagation of indent.

Flexural Behavior of Sandwich Structure with AA 8011 Honeycomb Core and Al 1100 Face Skins

This paper details the fabrication and flexural testing of sandwich structure with Aluminium honeycomb core with Aluminium face skins. The material for the face skin is aluminium 1100 and for the core is Aluminium AA8011. The cell size obtained by fabrication is 7mm. The specimen is prepared and tested as per the ASTM standard C393/C393M-11 on a three-point bending test to obtain the ultimate core shear strength and the face skin strength. Finite element analysis is also carried out to validate the experimental test.

Mechanical and electrical properties of hybrid honeycomb sandwich structure for spacecraft structural applications

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.

Experimental Analysis of the Crushing of Honeycomb Cores Under Compression

This paper presents an experimental study of the compressive response of hexagonal honeycomb core from the initial elastic regime to a fully crushed state. To understand the crushing mechanism, aluminum honeycomb core was compressed quasi-statically between rigid platens under displacement control. Honeycomb test specimens made of aluminum alloy with different configurations such as cell size, cell number, core density, and specimen size have been tested. During the crushing, the cells buckle elastically and collapse at a higher stress due to inelastic action, after the cells crush by progressive formation of folds on cell walls. The response densifies when folds consume the whole panel height. The peak load recorded during tests corresponded to the buckling of honeycomb cells. The different specimens showed similar load/displacement curves, and the differences observed were only due to the influence of the core density, number of cells, and specimens size on mechanical behavior of hexagonal honeycomb core which has a very important role in the energy absorption capacity. In addition, this study allowed us to build a database of experimental tests for various configurations and allowed us to know the precise energy absorption capacity.

Experimental and Numerical Studies on Defect Characteristics During Milling of Aluminum Honeycomb Core

The honeycomb sandwich structure has been widely used in the aerospace industry due to its high specific strength and stiffness. However, the machining defects of the aluminum honeycomb core (AHC) have become the key factor that restricts its application. In this paper, the defects' characteristics including the formation mechanism, distribution characteristic, and cutting process of honeycomb cell walls during AHC milling process were experimentally investigated. Furthermore, using normalized Cockcroft and Latham ductile fracture criterion and Johnson–Cook (JC) constitutive model, the numerical simulation of the AHC machining process was conducted concerning the entrance angle. It is indicated that six categories of milling defects are obtained and the quantity as well as distribution regularity of AHC milling defects are determined by the double effects of both the entrance angle and cutting force. Most of the surface defects of honeycomb materials were found concentrated in three regions, named by zones I–III, in which extruding, shear, and tensile deformation was mainly generated, respectively. Besides, the finite element simulation results also agree well with the experimental findings. Finally, a novel optimization method to avoid defects in the aforementioned regions by controlling the entrance angle of all the honeycomb walls during the cutting process was proposed in this paper. Meanwhile, the optimal control equations of the entrance angle for all cell walls were derived. This method was verified by milling experiments at last and the results showed that the optimization effect was obvious since the quality of the machined surface was greatly improved.

Material Stiffness and Cutting Parameters for Honeycomb Aluminum Sandwich Panel: a Comparison with Bulk Material

Abstract Aluminum sandwich panel provides a high-strength, light-weight structural material for use in aircraft and aerospace applications such as cabin bulkheads, rotor blades, and luggage and cargo unit load devices. This unique material also has application in many other industries such as machine tool enclosures, museum exhibits, marine craft bulkheads, and hurricane panels for storm survivability. This material, consisting of aluminum face sheets surrounding an aluminum honeycomb core, offers very high rigidity in a low density material. Machining this material is quite challenging due to variable cutting conditions in the low density, low lateral stiffness honeycomb core. Machining often requires a significant amount of post-processing in the form of manual removal of the partially released core walls (flags) along machined edges. The purpose of this work is to reduce the flagging created in the machining of this material. The first step to improving the cutting conditions is to better understand the causes that impact them. In this pursuit, a series of experiments was conducted to measure, quantify, and study the cutting forces during the machining of aluminum sandwich panel. A force dynamometer was used to measure forces during slot milling in bulk aluminum generating cutting coefficients for the force model. Cutting results in bulk aluminum showed generally good agreement with the model with over-predictions ranging from 5 to 20% depending on the feed rate. A series of cutting tests was then conducted on the aluminum sandwich panel in order to decouple the machining forces for the face sheets, the honeycomb core, and combinations of face sheets and core. The data revealed vibration in the honeycomb material that were significantly worse for shallower depths of cut involving the top face sheet and the upper portion of the honeycomb structure despite efforts at stiffening the fixture. The data showed force spikes that correlate with specific engagement conditions in the honeycomb structure. Peak forces were measured as high as 400N though it is not entirely clear whether these peak values are representative of actual peak forces or a combination of peak force with harmonic vibration. The resulting cut walls showed signs of tearing, rubbing, and remaining cell walls indicating that for much of the cut, ideal shearing of the material did not occur. The research highlights the need for further study of the actual mechanism of cell wall removal in this complex cutting environment.

Numerical modeling of high velocity impact in sandwich panels with honeycomb core and composite skin including composite progressive damage model

In this paper, a numerical model is developed to simulate the ballistic impact of a projectile on a sandwich panel with honeycomb core and composite skin. To this end, a suitable material model for the aluminum honeycomb core is used taking the strain-rate dependent properties into account. To validate the ballistic impact of the projectile on the honeycomb core, numerical results are compared with the experimental results available in literature and ballistic limit velocities are predicted with good accuracy. Moreover, to achieve composite skin material model, a VUMAT subroutine including damage initiation based on Hashin’s seven failure criteria and damage evolution based on MLT approach modulus degradation is used. To validate the composite material model VUMAT subroutine, the ballistic limit velocities of the projectile impact on the composite laminates are predicted similar to the numerical results presented by other researchers. Next, the numerical model of the sandwich panel ballistic impact at different velocities is compared with the available experimental results in literature, and energy absorption capacity of the sandwich panel is predicted accurately. In addition, the numerical model simulated the sandwich panel damage mechanisms in different stages similar to empirical observations. Also, the composite skin damages are investigated based on different criteria damage contours.

Development of proposals for the synthesis of polymer composite materials capable of combustion after the mission

One of the ways of solving the problem of reducing the man-made impact of rocket and space activities on the environment, based on the reduction of the impact areas of separating launch vehicle component (LVC) in the process of launching payloads into orbit are considered. Known particular solutions to this problem, for example, the return of spent LV stage accelerators to the point of launch, a multiple reduction in the impact areas of spent LV stage accelerators due to their controlled descent, however, there is open the question of how to deal with such LVC as payload fairings (PF) and aft-interstage (AIS) having significant areas of the impact areas. The possibility of burning LVC, such as PF and AIS, made of a composite material based on carbon fiber is considered. The thermal conditions of LVC during their movement on the descent path are investigated, the necessary amount of heat for heating the LVC to the ignition temperature is determined and various options of energy materials (EM) are considered. On the example of a typical PF, having a design of a polymer composite material (carbon fiber) with an aluminum honeycomb core (AHC), another structure is proposed, where a filler based on a mixture of polymer films such as ABS structure and energy material is used instead of AHC. The proposed design of the polymer composite material allows to provide the necessary operation parameters of the PF (strength, thermal protection, etc.) at the stage of preparation of the PF for a launch, LV ascent in the active section of the trajectory. And the possibility of burning after the mission with minimal energy costs allows to provide.

Research on dynamic response characteristics of CFRP/Al HC SPs subjected to high-velocity impact

Abstract CFRP/Al HC SPs (Carbon Fiber Reinforced Plastic/Aluminum Honeycomb Sandwich Panels)composite structures are widely used in the aerospace and many other fields due to their excellent performances. In order to reveal profoundly the dynamic response characteristics of CFRP/Al HC SPs composite structure under high-velocity impact loading, the experiments about aluminum honeycomb core and CFRP/Al HC SPs composite structure impacted by high-velocity projectiles have been performed by using a one-stage light gas gun loading and a high-speed camera systems, respectively. The whole physical process of collision is analyzed by using TEMA software which is provided by high-speed camera's manufacturer, and the pictures and related physical parameters of the projectile passing through the different layers at the key moments are extracted based on the set frame rate of the video. Meanwhile, the ABAQUS/Explicit module was used to conduct numerical simulation under the same conditions as the experiments, and extracting the pictures and related physical parameters during the projectile penetrating through aluminum honeycomb core and the sandwich panels at the different key moments. The results show that the durations of action for the projectile impacting the Al HC core and the CFRP/Al HC SPs at the velocity about 280 m/s were 86 μs and 240 μs in experiments, respectively, and the durations of action were 90.7 μs and 236.2 μs at the same experimental conditions in numerical simulation, which were basically consistent comparing the experimental to simulated results. Moreover, more than 70% for the kinetic energy of the projectile consumed by the sandwich panels was attributed to the front panel and the aluminum honeycomb core during the entire impact process, and all these have verified the reliability of the numerical simulation.

Investigate on milling force of cryogenic cooling processing aluminum honeycomb treated by ice fixation

Aerospace metal honeycomb materials with low stiffness had often the deformation, burr, collapse, and other defects in the mechanical processing. They were attributed to poor fixation method and inapposite cutting force. This paper presented the improvement of fixation way. The hexagonal aluminum honeycomb core material was treated by ice fixation, and the NC milling machine was used for a series of cryogenic machining. Considering the similar structure of fiber-reinforced composite materials, the milling force prediction model of ice fixation aluminum honeycomb was established, considering tool geometry parameters and cutting parameters. Meanwhile, the influence rule on milling force was deduced. The results show that compared with the conventional fixation milling method, the honeycomb processing effect is improved greatly. The machining parameters affect order on milling forces: the cutting depth is the most important, followed by the cutting width, then the spindle speed and the feed. Moreover, too small cutting depth (ap = 0.5 mm) will cause insufficient cutting force, while ap > 2 mm with higher force will reduce the processing quality of honeycomb. Simultaneously, the honeycomb orientation (θ) has a great influence on processing quality. Using the model, the predicted and measured error values of the feed and main cutting force are all small in θ 90°, respectively, while they all exhibit the smallest error in θ = 60°. This bigger error mainly is due to unstable cutting force with obtuse angle. In addition, the tool rake angle has little influence on cutting quality in θ 90°. Furthermore, the calculation model successfully conforms to the main deformation mechanism and influences parameters of the cutting force in the milling process, and it can accurately predict the cutting force in θ < 90° and guide the milling process.

Validity check of an analytical dimensioning approach for potted insert load introductions in honeycomb sandwich panels

An easy to handle mechanical-analytical dimensioning approach for insert load introductions on sandwich panels with honeycomb cores is of great value, since remarkable weight reductions can be achieved if the diameters of all inserts in a structure are reduced to their inevitable minimum. This is of special interest for mass critical structures of e.g. satellites, airplanes or race cars. For these structures, a combination of core connected (i.e. potted), through-the-thickness inserts in sandwich panels with fiber reinforced face sheets and aluminum honeycomb core material is frequently in use, since it offers an outstand lightweight performance. Unfortunately, a straight forward, mechanical-analytical dimensioning approach, especially for this particular insert-sandwich combination, is still missing.An analytical-mechanical model, basing on the higher order sandwich theory, was developed by Ericksen in 1953 for inserts without a connection to the core (i.e. clamped between the face sheets). In 1981, Hertel modified Ericksen’s model in order to achieve more convenient to use solution equations. However, Hertel used his modified model also for strength predictions of core connected inserts, although the core shear stress progression, decisive for the strength of an insert, differs highly from clamped inserts. However, since Hertel did not validate his extension of the modified Ericksen model onto core-connected inserts in any way, the authors suspect a misunderstanding. Unfortunately, the model was used frequently for core connected inserts afterwards and is even cited in a standard reference published by ESA and ECSS.To address this uncertainty, the authors carried out a comparison of experimental results to the predictions of the modified Ericksen model for core connected inserts within this work. Therefore, initially a cumulated overview of the Ericksen model and Hertel’s modifications is provided, since this is not available in literature. Included are current approaches for the homogenization of the anisotropic FRP- and honeycomb material properties in order to receive moat precise results.The comparison of experimental and analytical results offers mostly a poor correspondence in terms of the strength of an core connected insert. This, on one hand, reveals that the modified Ericksen formulation is not applicable on core connected inserts without significant restrictions in contrast to Hertel’s assumption. Moreover, it also becomes apparent that the various state-of-the-art test data interpretations deliver widely diverging results for the strength of an insert load introduction. Consequently, the analytical dimensioning of core connected inserts remains unreliable. Yet, this investigation derives useful improvement measures on both, theoretical and practical side.

Experimental and computational studies on honeycomb sandwich structures under static and fatigue bending load

Sandwich structures with glass fiber face sheets and aluminum honeycomb core are investigated computationally and experimentally. A three point bending load arrangement is conducted to examine the static and fatigue performance of honeycomb sandwich panel. Under static loading, the load and displacement response is indicated in five phases. The decrease in fatigue life with load level was observed in approximately linear manner. The visual and Scanning Electron Microscopic (SEM) analysis were carried out to analyze the failure modes. For static and high amplitude fatigue load, the failure initiates due to face yielding, while for low fatigue load failure initiates as a result of delamination at core and skin interface. However, in all cases principle failure mode is indentation. The honeycomb sandwich structure was also modeled with commercially available finite element packages ANSYS and the fatigue analyses were carried out to determine the life of specimens under load-displacement response. The experimental results were in good agreement with the Finite Element Analysis (FEA) results in both static and fatigue loads, and fracture modes prediction.

Bio-inspired vs. conventional sandwich beams: A low-velocity repeated impact behavior exploration

A new bio-inspired honeycomb sandwich beam (BHSB) based on the woodpecker’s head configuration was developed for repeated low-velocity impact resistance improvement. The new design contains four main layers: carbon fiber reinforced plastic top and bottom skins sandwiching rubber and aluminum honeycomb cores. Impact loadings were performed numerically and experimentally in a repeated manner with a hemispherical steel impactor for three energy levels: 7.28 J, 9.74 J, and 12.63 J. In all cases, stresses developed at the bottom skin of BHSB were significantly smaller with substantially lower damage area while sustaining more number of impact than those of conventional honeycomb sandwich beam (HSB). Also, the impact resistance efficiency indices, Ie, which assess overall performance, of both beam designs were compared. Ie of BHSB were 1.65–16.22 times higher, conclusively exhibiting its superiority to HSB in repeated impact resisting performance.

The Bending Responses of Sandwich Panels with Aluminium Honeycomb Core and CFRP Skins Used in Electric Vehicle Body

The aim of this paper was to investigate bending responses of sandwich panels with aluminium honeycomb core and carbon fibre‐reinforced plastic (CFRP) skins used in electric vehicle body subjected to quasistatic bending. The typical load‐displacement curves, failure modes, and energy absorption are studied. The effects of fibre direction, stacking sequence, layer thickness, and loading velocity on the crashworthiness characteristics are discussed. The finite element analysis (FEA) results are compared with experimental measurements. It is observed that there are good agreements between the FEA and experimental results. Numerical simulations and experiment predict that the honeycomb sandwich panels with ±30° and ±45° fibre direction, asymmetrical stacking sequence (45°/−45°/45°/−45°), thicker panels (0.2 mm∼0.4 mm), and smaller loading velocity (5 mm/min∼30 mm/min) have better crashworthiness performance. The FEA prediction is also helpful in understanding the initiation and propagation of cracks within the honeycomb sandwich panels.