Honeycomb Core Sandwich Panels Research Articles

Testing and modeling of Nomex™ honeycomb sandwich Panels with bolt insert

Nomex™ honeycomb core sandwich panels with a bolt insert were load tested and modeled. The objective was to predict the honeycomb local buckling load and to identify a Nomex™ honeycomb constituent material model. Sandwich specimens were subjected to bolt pull-out load tests. The same sandwich structure was also tested in flat-wise tension with strain gages installed on the honeycomb walls. Finite element models of the flat-wise tension and bolt pull-out tests were built. The honeycomb geometry and strain gages were modeled with shell elements. An orthotropic honeycomb material model was identified by comparing the two test models to the experimental data. The material parameters identified are in the mid-range of previously published values. The pull-out test model was used to predict honeycomb wall buckling with a nodal rotation vector sum criterion. The buckling loads predictions closely corresponded to the start of the experimental load/displacement slope transition zone.

Embedded FBG Sensor in Aircraft Smart Composite Materials for Structural Monitoring

This paper describes the use of embedded Fiber Brag Grating (FBG) sensor in the honeycomb core carbon fiber sandwich panel in smart composite materials for the application of monitoring the structural integrity of an aircraft. A part of vertical stabilizer was selected and reproduced using carbon fiber honeycomb core sandwich panels. The sandwich panel was fabricated in accordance to the generic sandwich structure and aviation industry standards, including the materials and also the method of construction. Using a carbon fiber from Hexcel as the face-sheet, Nomex honeycomb as the core, the sandwich panel was cured using Hysol EA9330 resin according to a standard curing process in the aviation industry. In order to make the sandwich panel as smart materials, optical sensor which has fiber bragg grating arrays, FBG, were embedded between the carbon fiber plies during the lay-out process. Using an FBG data logger and Instron 8802 Universal Testing Machine, the panel was subjected to flexural load and the FBG sensor signals were read at the load interval of 0.2 kN. From the experiment the results were taken and data was plotted and it shows that the FBG signals responded well to the load applied. In the future, the specimen will be used for further experiment for measuring strains and establishing the existence of damage in the panel.

An Evaluation on Impact Performance of Light-Weight Composite Honeycomb Sandwich Panels

A honeycomb core sandwich panel is superior in impact absorption under whole surface compression, because of the buffer effect of core. However impact properties of honeycomb sandwich panels under local compressions such as drop weight impact are affected by the plates as the face sheets in addition to the core layers. This research describes the drop weight impact properties of honeycomb sandwich panels which consist of various aluminum-alloy honeycomb cores and CFRP composite laminate faces through the spindle falling experiments. In order to confirm the validity of the experiments convictively, analytical approaches based on 3D modeling and ANSYS LS-DYNA software were also carried out. Comparisons of the experimental and analytical results are reported in this study.

Compressive strength of honeycomb-stiffened graphite/epoxy sandwich panels with barely-visible indentation damage

This study examines the parametric effects of core density, core thickness, face-sheet stacking sequence, and indentor diameter on the compressive strength of aluminum honeycomb-core sandwich panels stiffened with eight-ply, quasi-isotropic, graphite/epoxy face sheets. The sandwich panels contained damage at the threshold of visual detectability created through quasi-static indentation with 25.4 mm or 76.2 mm-diameter spherical indentors. During compression-after-indentation testing, failure occurred due to: dent deepening followed by localized, compressive micro-buckling of fibers in the 0° plies; localized buckling of the near-free-surface sub-laminates; or unstable dent growth in the direction lateral to the applied compressive load. Regardless of failure mode or face-sheet type, the compression-after-indentation strength increased with increasing core thickness and with decreasing core density. Additionally, panels containing face sheets with the 0° plies near the mid-plane and 45° angle change between subsequent plies exhibited greater undamaged compressive strength and higher compression-after-indentation strength relative to panels containing 90° angle changes between subsequent plies and 0° plies near the free surface. The compression-after-indentation strength was found to be relatively unaffected by the indentor diameter size and the resulting variations in the face sheet and core damage. These results imply that precise representation of the damage state in models to predict the post-indentation response of sandwich panels may not be necessary in order to make accurate average residual strength predictions.

Compression After Impact Analysis on Thin Face Sheet Honeycomb Core Sandwich Panels

A research study has been conducted on compression after impact of thin facesheet honeycomb core sandwich panels. This paper is focused on the modeling and analysis used to simulate the tests described in a companion paper. Of interest are composite sandwich panels with aerospace applications, which contain either a 3 or honeycomb core. It was found that compression after impact testing of these coupons resulted in either an indentation propagation failure for the lower density core or a crack propagation failure mode for the higher density core. An analysis model is developed to account for both modes through the inclusion of progressive failure analysis of the facesheets and a homogenized, nonlinear material model for the honeycomb core. In addition, significant impact damage detail is included in the model based on experimental observation. Analysis results are compared with the experimental results for each of the 24 sandwich panel specimens tested. Good agreement of failure mode predictions with test results demonstrates the importance of considering both facesheet and core failure, as well as the initial damage severity. Finally, a parametric study highlights the strength benefits compared with mass penalty for various core densities.

Compression After Impact Experiments on Thin Face Sheet Honeycomb Core Sandwich Panels

Experiments have been used to study the compression after impact of thin face-sheet composite honeycomb core sandwich panels. The sandwich panels consist of very thin, woven S2-fiberglass face sheets adhered to a Nomex honeycomb core. Two sets of coupons were tested; they were identical, except one set contained a density core, and the other contained a heavier density core. Static indentation and low-velocity impact using a drop tower were used to study damage formation in these materials with energy levels up to 9 ft-lb (foot-pounds). A series of highly instrumented compression after impact tests was then completed. New techniques for studying the response and failure include high-speed video photography as well as digital image correlation for the full-field deformation measurement. Two failure modes were observed. It is concluded that the failure mode of these materials depends solely on the honeycomb core density of the coupon, with the lighter density core experiencing an indentation propagation failure, while the heavier specimens experienced a crack propagation failure mode.

A Study on the Bending Analysis of the Al Honeycomb Core Sandwich Composite Panel Bearing Large Bending Load

Al honeycomb core sandwich composite panels have different core and plate materials. The core is the Al honeycomb core, and the thin plate is GFRP sheets with fibers laminated in the 0°/90° symmetric structure. The Al honeycomb core sandwich composite panel is used for structures, which involve relatively high bending load. Before designing the structures, their stability is evaluated via the finite element analysis. In this study, an analysis method that is closest to the reality was proposed for designing the structures with Al honeycomb core sandwich composite panels. For that purpose, the modulus was reviewed. In the finite element analysis, the tensile modulus is generally used. In the results of this study, however, the tensile modulus led to significant deviations from the test results, whereas the bending modulus led to a closer value to the test results.

곡면형상의 알루미늄 하니콤/CFRP 샌드위치 패널에 관한 실험적 연구

본 논문에서는 탄소섬유강화플라스틱 면재와 알루미늄 샌드위치 심재를 가지는 복합재 샌드위치의 제조와 3절점 굽힘 실험에 대해 연구하였다. 시편은 3가지 하니컴 종류(3.18 mm, 4.76 mm, 6.35 mm의 셀 크기)와 3가지 곡률 반지름(평판, r = 1.6 m, r = 1.3 m)을 가지도록 제작하였다. 샌드위치 곡률의 기준은 W-방향을 기준으로 제작 하였다. CFRP <TEX>$2{\times}2$</TEX> 트윌의 인장에서 기계적 물성치(탄성계수, 강도, 푸아송 비)를 측정하여 그 값들을 다른 CFRP 섬유 적층판의 값과 비교하였다. 실험결과 평판 샌드위치 패널의 3절점 굽힘 실험에서 심재의 전단강도는 공개된 데이터에 비해 11-30% 낮게 나왔다. 제한된 시편 크기에서 1.3미터 곡률을 가지는 패널은 평판 패널에 비해 심재의 극한 전단강도가 0.8-3.8% 감소한 것으로 나타났다. This paper presents the fabrication and 3-point flexion testing of carbon fiber reinforced polymer (CFRP) composite face/aluminum honeycomb core sandwich panels. Specimen sandwich panels were fabricated with three honeycomb types (3.18 mm, 4.76 mm, and 6.35 mm cell size) and three panel radii (flat, r = 1.6 m, r = 1.3 m). The curved sandwiches were fabricated normally with the core in the W-direction. The tensile mechanical properties of the CFRP <TEX>$2{\times}2$</TEX> twill fabric face laminate were evaluated (modulus, strength, Poisson's ratio). The measured values are comparable to other CFRP fabric laminates. The flat sandwich 3-point flexion test core shear strength results were 11-30% lower than the manufacturer published data; the test set-up used may be the cause. With a limited sample size, the 1.3 meter panel curvature appeared to cause a 0.8-3.8% reduction in ultimate core shear strength compared to a flat panel.

Metallic sandwich panels subjected to multiple intense shocks

The mechanical response and fracture of metal sandwich panels subjected to multiple impulsive pressure loads (shocks) were investigated for panels with honeycomb and folded plate core constructions. The structural performance of panels with specific core configurations under multiple impulsive pressure loads is quantified by the maximum transverse deflection of the face sheets and the core crushing strain at mid-span of the panels. A limited set of simulations was carried out to find the optimum core density of a square honeycomb core sandwich panels under two shocks. The panels with a relative core density of 4%–5% are shown to have minimum face sheet deflection for the loading conditions considered here. This was consistent with the findings related to the sandwich panel response subjected to a single intense shock. Comparison of these results showed that optimized sandwich panels outperform solid plates under shock loading. An empirical method for prediction of the deflection and fracture of sandwich panels under two consecutive shocks – based on finding an effective peak over-pressure – was provided. Moreover, a limited number of simulations related to response and fracture of sandwich panels under multiple shocks with different material properties were performed to highlight the role of metal strength and ductility. In this set of simulations, square honeycomb sandwich panels made of four steels representing a relatively wide range of strength, strain hardening and ductility values were studied. For panels clamped at their edge, the observed failure mechanisms are core failure, top face failure and tearing at or close to the clamped edge. Failure diagrams for sandwich panels were constructed which reveal the fracture and failure mechanisms under various shock intensities for panels subjected to up to three consecutive shocks. The results complement previous studies on the behavior and fracture of these panels under high intensity dynamic loading and further highlights the potential of these panels for development of threat-resistant structural systems.

J046032 高面精度フォームコアCFRPサンドイッチパネルの作製および表面形状の評価(〔J046-03〕高分子基複合材料の加工と評価(3))

As a new material suitable for the primary mirror of space telescopes, we focused on foam core CFRP sandwich panels which satisfy lower surface roughness than the conventional honeycomb core sandwich panels. However, large deformations appeared at edge of the ROHACELL (polymethacrylimide based foam) core sandwich panels surface. In order to improve the surface accuracy, we analytically investigated the influence of core thickness and of using the GRAFOAM core (carbon-based foam) having a low coefficient of thermal expansion. Then, ROHACELL core sandwich panels with two different core thickness and GRAFOAM core sandwich panels were manufactured and the surface accuracy of those panels were experimentally measured. Analytical and experimental results showed that edge effect decreased on ROHACELL core sandwich panels with thinner core thickness, and that the edge effect was suppressed by using the GRAFOAM core. In addition, because of lower CME(coefficient of moisture expansion), deformation due to moisture absorption is small.

Process for design optimization of honeycomb core sandwich panels for blast load mitigation

A general process for optimization of a sandwich panel to minimize the effects of air blast loading is presented here. The panel geometry consists of two metal face plates with a crushable honeycomb or other type of core. Optimization is necessary as there is strong coupling between the several variables and the physics, which makes parametric studies relatively ineffective. Virtual testing is used to develop a homogenized model for the stress---strain curve of the honeycomb core, which can be readily applied to other types of cellular core. The homogenized model has been validated by comparison to existing results as well as to results from detailed finite element (FE) models. A design of experiments (DOE) based response surface optimization method in combination with LS-DYNA is used to minimize dynamic deflection or acceleration of the back face plate. Constraints on total mass and on plastic strain in the face plates are imposed. The mechanism of lowering the backface deflection is by increasing front face plate thickness which effectively distributes the blast load to a larger area of the core and avoids local concave deformation of the front face plate. Further, core depth is increased which increases panel stiffness. For acceleration minimization, results again produce a stiffer front face plate, but accompanied by a sufficiently soft core. The mechanism of lowering the backface acceleration is by absorbing energy with low transmitted stress. A clear cut comparison between monolithic metal plates and sandwich plates, for the same loading and failure criteria, is presented here.

Damage characterization of quasi-statically indented composite sandwich structures

The nature of quasi-static indentation damage is studied in aluminum honeycomb core sandwich panels with eight ply, quasi-isotropic, graphite/epoxy face sheets. Parameters that are varied include the core thickness, core density, face sheet layup, and indentor diameter. The majority of induced damage is in the vicinity of the barely visible threshold. The permanent dent in the panel is found to be always larger than the contact area of the indentor, and specimens with denser cores exhibit smaller dent diameters for a given dent depth. Regardless of specimen layup, delaminations occur essentially only at the 3rd, 5th, 6th, and 7th interfaces. Stiffer cores, either in terms of a higher density or, for those cores considered, a smaller thickness, result in lower dent depths, smaller dent diameters, and more face sheet delamination for a given indentation event. The manner in which core orthotropy influences the size and pattern of the delaminations is shown to depend on face sheet layup. Regardless of the core, larger delaminations occur in face sheets that contain 90° angle changes between the adjacent plies in comparison to those that contain only 45° angle changes. Results are compared to those previously reported in the literature, and mechanisms that are related to plate boundary conditions are described that reconcile what would appear to be conflicting results obtained by different studies. Findings are then discussed in the context of choosing the most damage-resistant structural configuration and how this translates to damage tolerance.

Light-weight honeycomb core sandwich panels containing biofiber-reinforced thermoset polymer composite skins: Fabrication and evaluation

The application of biofiber based paper-reinforced polymer (PRP) composites as skin materials for light-weight sandwich panel constructions was explored. Various sandwich panels with PRP composite skins and a commercial resin-impregnated aramid paper honeycomb core of different cell sizes and core heights were fabricated in the laboratory. The effects of honeycomb core height and cell size on the flexural properties of the lab-made sandwich panels were evaluated. The flexural moduli and strengths of the lab-made panels were compared to the reported values for three existing commercial products used for automotive load floor applications. The lab-made PRP composite/honeycomb core sandwich panels had comparable bending rigidity and flexural load bearing capability but lower areal weights when compared to the commercial products suggesting that PRP composites have the potential to be used as an alternative to glass fiber-reinforced polymer composites as skin materials in sandwich panel fabrication.

Investigation of elastic moduli of Kraft paper honeycomb core sandwich panels

In order to understand the influence of the thicknesses of Kraft paper honeycomb core and medium density fiberboard skins on the stiffness of the sandwich panel, the corresponding finite element models for the resulting sandwich panels were developed. The material properties for the core and skin components of these finite element models were determined using the published data and specifications. It was found that a decrease in the thickness ratio of the core to skin layer (shelling ratio) resulted in an increase in the modulus of elasticity and shear modulus of the sandwich panels. The increase was significant when the shelling ratio was smaller than six. Cell size only affected the modulus of elasticity of the sandwich panels under the flat-wise compression and panel’s inter-laminar shear modulus. Regression equations relating the stiffness of the sandwich panels to the shelling ratio and core cell size were obtained using the finite element model simulated results and were found to compare well with the existing models for layered wood composites.

Compression Behavior of Adhesive Butt Joints of Aluminum Hexagonal Core Sandwich Panels with Different Edging Configurations

Aluminum honeycomb core sandwich panels are now commercially available and are widely used as light weight materials in aircraft, naval and other applications because of their high strength to weight ratio, high bending stiffness, fire retardant resistance, insulation properties and sound attenuation properties. In all these applications extensive joining is encountered in fabricating the panels to complex shapes. This paper has investigated various edging and configuration details to secure butt joints with adequate soundness through adhesive joining methods. The compression strength of the joints has been assessed experimentally. The configuration of each butt joint is discussed and best combinations of edging and configuration are recommended for successful fabrication of complex assembles with aluminum honey core sandwich panels.

Effect of Core Height Ratio on Drop Weight Impact Properties of Double Layer Honeycomb Core Sandwich Panels

A honeycomb core sandwich panel (hereafter, HSP for brevity) is superior in impact absorption under whole surface compression, because of the buffer effect of core. However impact properties of HSP under local compression such as drop weight impact are affected by the plate as the face sheet in addition to the core layer. This research describes drop weight impact properties of double layer HSP which consists of two core layers and three plates. In order to build up the experimental database for design of double layer HSP, a series of drop weight tests was carried out by using double layer HSPs with the several ratios of the core heights. From obtained results, the follows were summarized: 1) Two or three peaks of the load appeared in the load—displacement curve. The peak loads were larger in sequence of their appearances. 2) In the load—displacement curve until the last peak of the load, there are load increase regions by deep drawing of the plates and load equilibrium regions similar to plateau region under whole surface compression. 3) When the height of the upper layer core was lower, the absorption energy was larger in total. 4) An energy absorption model has been developed and verified for design of double layer HSP.

G030043 二層積層ハニカムコアサンドイッチパネルの落錘衝撃特性に及ぼす積層コア高さの影響

G030043 二層積層ハニカムコアサンドイッチパネルの落錘衝撃特性に及ぼす積層コア高さの影響

Using Embedded Fiber Bragg Grating (FBG) Sensors in Smart Aircraft Structure Materials

This paper describes the process of developing a smart material with monitoring application to the aircraft structures. A part of vertical stabilizer was selected and reproduced using carbon fiber honeycomb core sandwich panels. The sandwich panels reproduced were fabricated in accordance to the generic sandwich structure and aviation industry standards, including the materials and also the method of construction. Using a carbon fiber from Hexcel as the face-sheet, Nomex honeycomb as the core, the sandwich panel was cured using Hysol EA9330 resin according to aviation industry standard curing process. In order to make the sandwich panel as smart materials, optical sensor which has fiber bragg grating arrays, FBG, were embedded between the carbon fiber plies during the lay-out process. Using an FBG data logger, the FBG sensor signals were read before and after the FBG arrays were installed in the sandwich panels. From the initial measurement, the experiment was a success since the FBG sensors were readable and the difference in the signals was less than 1nm. In the future, the specimen will be used for further experiment for measuring strains and establishing the existence of damage in the panel.

Effect of Cell Wall Thickness on Local Compression Fatigue Property of Honeycomb Core Sandwich Panel

Local compression fatigue properties of peripherally clamped HSP have been studied in consideration of usage condition as a floor panel. In this study, the effect of cell wall thickness on local compression fatigue of HSP was clarified. For that purpose, a series of fatigue test of HSP was carried out by changing the cell wall thickness, the thickness of face sheet contacted the indenter and the maximum load value. When the cell wall thickness was thin, the fracture pattern of face sheet contacted the indenter changed from the tensile type to shearing type with the increase in the thickness of face sheet contacted the indenter. In the transitional region between these two fracture types, the only dent without crack was observed on the face sheet contacted the indenter. On the other hand, when the cell wall thickness was thick, the shearing fracture around dent was observed instead of the dent type in the transitional region. This was because of the increase of buckling strength of cell wall which constrained the formation of dent. This situation resulted in the increase in the shearing stress around the dent.

Influence of Cell Size on the Core Shear Properties of FRP Honeycomb Sandwich Panels

This article deals with the influence of cell size on the core shear modulus and shear strength of fiber-reinforced plastic (FRP) honeycomb core sandwich panels. A relationship between the cell size and core density has been established for the honeycomb core made by matching the mold method. The cell size ranges from 8 to 25 mm, with the cell height kept constant at 8 mm. The sandwich panels are prepared with facings made of Bi-woven glass “E” fabric, using the vacuum bagging technique. The core shear modulus “G” and flexural stiffness “D” of the sandwich panels have been experimentally determined as per the ASTM C393. The shear strength of the sandwich panel has been determined using a short beam bend test. A correlation has been established between the core density and the core shear modulus as well as the core density and the core shear strength. The results have been compared with those of sandwich panels made with polyurethane (PUR) foam as the core material of different densities. It has been found that for the given core density the core shear modulus of the FRP honeycomb core is far higher than that of PUR foam. However, the shear strength of the FRP honeycomb sandwich panels is only marginally higher when compared to that of PUR foam.