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Honeycomb cores made of phenol resin impregnated paper are widely used in sandwich structures of aircraft. Owing to the rather weak core material, this kind of sandwich is prone to a range of damages as a result of impact loading which may accidentally occur during operation. When employed in aircraft these structures have to be damage tolerant. This means barely visible damages have to be considered in the structural design process. In the standard design process for damage tolerant aircraft structures especially the effect of impacts has to be considered but is mostly investigated on unloaded sandwich components. However, aircraft structures are not completely stress free, even when the aircraft is in parking position. At least the structural weight will cause a permanent static preload. In the literature there are only few investigations dealing with the impact behaviour of preloaded honeycomb sandwich structures. In general, studies on composite structures have shown that tension preloads lead to higher structural impact resistance, while prestressing in compression reduces mechanical properties such as impact strength and stiffness of the structure. For this reason, a compression preload is considered as the more critical load case. In order to evaluate the effect of static compression preloads an experimental study on five different honeycomb sandwich configurations was conducted. The experimental investigations showed that even low preloads cause a higher structural damage area, which results in a lower residual strength of the structure. The results indicated that it is recommendable to consider structural compression preloads in the aircraft structural design process. [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]

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]

During the autoclave manufacturing of sandwich components, several defects may arise. A typical defect in the use of honeycombs is core crush. The occurrence of this defect depends, among other things, on the angle of the honeycomb chamfer. To design this angle, the friction between honeycomb and prepreg must be known. Due to the low rigidity of Nomex® honeycomb in lateral direction, the determination of the frictional behavior between face sheets and honeycomb is a particular challenge. This paper shows the development of a test device for measuring the friction between honeycomb with film adhesive and carbon/epoxy prepreg for different temperatures and compressions corresponding to realistic manufacturing conditions. The device is also suitable for the evaluation of interlaminar friction between prepreg plies and tool-part-interaction. The requirements for the device are shown in detail. Advantages and disadvantages of various concepts for the individual modules are discussed in order to select the most promising design. As proof of concept, friction measurements between honeycomb and prepreg as well as prepreg to prepreg measurements are carried out. In the case of interlaminar measurements, the friction coefficient determined at 1 mm deformation deviates by 5.3% from already published values obtained with another device and is within the stated scatter range. The measurement results for honeycomb against prepreg are in direct correlation with manufacturing trials performed. Using the lowest coefficient of friction established, a critical angle of the honeycomb chamfer was determined. Subsequently, component samples were manufactured with an inclination angle of 7.5° above and 7.5° below the critical angle. The specimens with larger angle showed a strong core crush, whereas those with smaller angle exhibited no core crush. This demonstrates that with this device the honeycomb chamfer of sandwich components can be dimensioned based on the lowest friction coefficient determined between honeycomb and prepreg. [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]

Fracture testing of aerospace grade honeycomb core sandwich composites is carried out using the Double Cantilever Beam specimen loaded with Uneven Bending Moments, and a Double Cantilever Beam–Uneven Bending Moment test rig capable of applying pure moments is utilized. Specimens with carbon fiber-reinforced plastic face sheets are employed with a range of honeycomb core grades comprising of Nomex® and Kevlar paper. The sandwich specimens are reinforced with steel doublers to reduce excessive rotation of the face sheets. The mode mixity phase angle pertaining to a particular ratio of moments between the two arms of the Double Cantilever Beam specimen is determined using the numerical mode mixity method—Crack Surface Displacement Extrapolation method. For Nomex® honeycomb core sandwich specimens, it is observed that the mode I interface fracture toughness increases with increase in core density. The interface fracture toughnesses for Nomex®-based honeycomb cores are also compared against specimens with Kevlar paper-based honeycomb cores. Crack propagation is observed at the interface just beneath the meniscus layer for the majority of the tested specimen configurations. The Double Cantilever Beam–Uneven Bending Moment test methodology with the concept of direct application of moments on both crack flanks has proven to have a significant potential for mixed mode face/core fracture characterization of aerospace grade sandwich composites. [ABSTRACT FROM AUTHOR]

An analytical model is developed to predict the loading and unloading response, as well as the residual dent diameter and dent depth, of carbon/epoxy-aluminum honeycomb core composite sandwich structures undergoing quasi-static indentation loading. The model considers damage created using spherical indenters and is valid up to the barely visible external damage threshold. The initial low load regime (until the onset of core crushing) is modeled using a combination of local Hertzian indentation of an elastic half-space and small deflection plate theory of a circular plate on an elastic foundation. For loads above those required to cause core crushing, the model uses the Rayleigh-Ritz method of energy minimization with the total system energy determined using a combination of face sheet bending energy, face sheet membrane energy and work done to the core during both elastic deformation and crushing. Degraded face sheet properties are used in the model beyond the onset of face sheet delamination, which is predicted using Griffith's energy criterion. The model is validated using experimental results for sandwich structures consisting of quasi-isotropic 8- (thin) and 16- (thick) ply carbon/epoxy face sheets and aluminum honeycomb cores. The results show that the overall mechanics of the model are fundamentally correct and reflective of physical behavior. Thus, in its present form the model shows promise as a preliminary design tool. [ABSTRACT FROM AUTHOR]

The behavior of a simple and innovative multi-layer sandwich panels having a polypropylene honeycomb core has been investigated carefully, theoretically and experimentally A four-point bending test was performed to detect the mechanical characteristics of the multi-layer core. The experimental results emphasize a better rigidity of the multi-layer structure compared to the weakness displayed by the single-layer configuration. In fact, a small increase in the final weight of the component leads to a significant increase of the mechanical properties. In the second part of this study, analytical and numerical homogenization approaches were developed to compute the effective properties of the single polypropylene honeycomb core. The numerical model complies with the experimental protocol, and the simulation conducted is aiming to reproduce a typical four-point bending test on a polypropylene honeycomb multi-layer sandwich panel. Both numerical and experimental results are presented in details and a good correlation between them is highlighted. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation on impact response of sandwich composite panels with different face-sheet thicknesses. A number of low velocity impact tests were performed under various impact energies. The damage process of the sandwich composites consisted of glass/epoxy face-sheets, and foam cores are analyzed from cross-examining some graphs such as load–deflection curves and damaged specimens. The primary damage modes observed are fiber fractures at upper and lower skins, delaminations between adjacent glass-epoxy layers, and core shear fractures. [ABSTRACT FROM AUTHOR]

Sandwich structures of honeycomb cores and carbon laminate face sheets are ideal to meet low-weight and high-stiffness specifications. However, these structures are susceptible to noise problems. They tend to radiate structure-borne sound very efficiently or be transparent for air-borne sound. The problem’s root is the coincidence frequency, at which the wavelengths of beam and air coincide. In the vicinity of the coincidence frequency, observing wave fields with slanted incidence, the air’s trace wavelength can match the beam’s wavelength. Unlike homogenous structures, where wave trace matching only occurs in a narrow frequency band, sandwich structures can display this effect in a more than two decade wide frequency region. This facilitates noise transmission in a wide frequency band. The first focus of this paper is how to avoid this problem in the first place by designing the sandwich adequately. To check the region of wave trace matching, dispersion curves are used. A simplified but sufficiently accurate dispersion curve is proposed. The second focus lies in the interpretation of the dispersion curve, i.e. what is physically happening in the beam structure. The third focus is placed on the experimental identification of the sandwich beam structure’s shell parameters including its damping. Special attention is placed on which measures are necessary and which not in the experimental setup and experimental procedure to achieve sufficiently accurate results. [ABSTRACT FROM AUTHOR]

Honeycomb composite structures are used in airplanes, railway cars and vehicles. The sandwich panels consist of two stiff face sheets of aluminium, which are bonded to a very lightweight honeycomb core of aluminium. Compared to normal plates, sandwich panels have a very high stiffness and simultaneously a low weight. The core of these structures is mainly subjected to shear stresses. The shear stresses depend strongly on the angle of the load application. The distribution and the level of the shear stresses are investigated using analytical calculations. The load direction which induces highest stresses in the honeycomb core is derived. This direction is not the W-direction, which is the most compliant one. When doing finite element simulations of honeycomb cores, often the core is homogenized in order to reduce the calculation time. In this article, some equations are derived in order to calculate the real shear stresses from the shear stresses of the homogeneous core. The equations are validated by finite element simulations and partially by tests. Three-point bending tests and additionally some Food Cart Roller Tests were conducted in order to test the panels in different angles. [ABSTRACT FROM AUTHOR]

Previous works [1] used nonlinear finite element models to investigate the effects of barely visible impact damage (BVID) in sandwich structures. Through a comprehensive numerical study it was found that sandwich honeycomb panels with BVID fail due to wrinkling or core crushing at significantly lower loads than undamaged panels. The critical load is controlled by the depth and size of damage, both of which control the foundation support and the resistance to wrinkles forming. In this article a simplified, closed-form analytical model is developed to predict critical stresses through a complete range of damage sizes and depths. This model extends previous work [1] by finding an expression for the critical wrinkling wavelength in a closed-form solution. A further expression is developed to account for rotational support at the edge of the damage site, as the wrinkle approaches the critical wrinkling wavelength. This article concludes by comparing numerical and analytical models against experimental values. Very good agreement is seen in all cases. [ABSTRACT FROM AUTHOR]

Bonded inserts are commonly used to introduce localized loads into sandwich panels. This article investigates sources of variability in the pull-out strength of metallic inserts in aramid honeycomb sandwich panels. Finite element modeling is used to investigate the sensitivity of core stresses to potting depth in partially potted inserts, which is found to be significant. Experimental specimens showed large variations in potting radii and cell wall dimensions, both critical parameters since the failure mode is predominantly core shear, adjacent to the potting. Foam core specimens were also tested and showed significantly lower variability in pull-out strength compared with honeycomb specimens. [ABSTRACT FROM AUTHOR]

This article examines the effect of impact damage on minimum gauge sandwich structures. This type of damage can cause panels to fail at significantly lower loads than an equivalent undamaged panel. For this reason, impact damage in sandwich structures has been a focus for numerous studies, including those that examine damage tolerance of structures and also their post-impact residual strength. This study specifically examines thin-gauge Nomex honeycomb panels and is concerned with barely visible impact damage (BVID), the type of damage that can be found on aircraft components. An overview of BVID, including an in-depth examination of the failure modes and failure loads of damaged panels is presented in this article. The study uses nonlinear finite element models to accurately predict failure loads, and understand how this type of damage causes panels to fail at reduced loads. A simplified, closed-form analytical model is developed to predict failure stresses through a complete range of damage sizes and depths. This study finishes by comparing numerical and analytical models against experimental values. Good agreement is seen in all cases for a wide variety of damage sizes. [ABSTRACT FROM AUTHOR]

This paper describes a method to predict failure loads accurately in undamaged sandwich panels subject to bending. The current method of predicting failure loads using linear wrinkling models is limited. A general lack of correlation exists between experimental and analytical/numerical wrinkling models. In some cases, the linear wrinkling models over-estimate wrinkling failure loads in undamaged panels by 100%. Various authors have tried to account for this discrepancy by modifying their linear models to take into account imperfections or irregularities in the structure or in some cases by using correction factors to get them to agree. This research uses these Finite Element models to find failure stresses in undamaged panels and to track the failure mode and determine the failure mechanism. Based on the failure mechanism found from the Finite Element analyses, a revised analytical expression is developed. These numerical and analytical models are verified against four point bend tests for a range of panel configurations. The results show that the panels collapse due to localised core crushing as a consequence of face sheet wrinkling. By using correct core failure criteria an almost perfect correlation is found between experimental, numerical and analytical results. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Knowledge of edge effects in composite with honeycomb cores is essential for the design of sandwich structures. This paper deals with the measurement of strain by the application of reflection photoelasticity. Because of the geometry of honeycomb cores a classical method by strain gages is inaccurate and unsuitable. The measurements by photoelasticity is compared to an analytical and numerical results. The final aim of this work is to determine the behavior of such structures by the exact knowledge of its mechanical properties deduced in part from edge effect results. [ABSTRACT FROM AUTHOR]