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Titanium honeycomb sandwich structures are gradually used in newly developed aircrafts in China. In this study, low-velocity impact tests on the titanium honeycomb sandwich structures were carried out to obtain the impact dynamic response and investigate the typical impact damage modes and parameters including the depths and diameters of the facesheet indentation and the core crushing region. The test results showed that the maximum contact force, the diameter and depth of the indentation had strong positive correlations to the impact energy. Numerical analysis was also conducted to study the low-velocity impact behaviour of the titanium honeycomb sandwich structures by using parametric finite element models that contained all the geometric and the structural details of the titanium honeycomb cores. The numerical results successfully captured the typical low-velocity impact damage modes of the titanium sandwich structures, similar to those observed in the tests. The predicted impact dynamic response also agreed very well with the test data. By using the validated finite element models, a parameter sensitivity study on the effects of the structural parameters on the low-velocity impact damage behaviour of the titanium sandwich structures was conducted. The parametric analysis results showed that the impactor diameter, the facesheet thickness and the core cell wall thickness had positive effect on the maximum contact force, and negative effect on the indentation depth, while the height of the honeycomb core had positive effect on the contact force, but little influence on the indentation depth. [ABSTRACT FROM AUTHOR]

To study the protection property of aluminum alloy sandwich panels with honeycomb cores under the attack of bullets or debris, quasi-static perforation, and ballistic impact tests were conducted, in which the thicknesses of the face sheet and core were 0.5–2.0 and 12.7 mm, respectively, while projectiles with diameter 7.5 mm and impact velocity 50–220 m/s were employed. Based on the experiments, the influences of impact velocity, face sheet thickness, core density as well as the nose shape of the projectiles were investigated. The results showed that in the impact tests, the sandwich panels dissipated much more energy than those in quasi-static perforation tests, and the energy absorption and ballistic limit of the sandwich panels increased with the increase of impact velocity. The influence of face sheet thickness was more remarkable than the core density, which was due to the relative density of honeycomb is too small. Although the increase of core density could induce the increase of energy absorption, this effect is more effective for thinner face sheet. Moreover, under the same impact velocity about 200 m/s and face sheet thickness 1.0 mm, the ballistic limit for conical-nosed projectile is highest, while it is lowest for flat-nosed projectile. [ABSTRACT FROM AUTHOR]

Auxetic cellular solids in the forms of honeycombs under blast load have great potential in a diverse range of applications, including core material in sandwich plates composite components. Based on Reddy’s first-order shear deformation plate theory, this paper presents an analysis of the nonlinear dynamic response and vibration of sandwich plates with negative Poisson’s ratio in auxetic honeycombes on elastic foundations subjected to blast and mechanical loads. A three-layer sandwich plate is considered discretized in the thickness direction by using analytical methods (stress function method, approximate solution), Galerkin method, and fourth-order Runge-Kutta method. The results show the effects of geometrical parameters, material properties, mechanical and elastic foundations on the nonlinear dynamic response, and vibration of sandwich plates. [ABSTRACT FROM AUTHOR]

Titanium honeycomb sandwich structures are gradually used in several newly developed aircrafts in China. During the manufacturing process and aircraft service life, low-velocity impacts from foreign objects (typically stones, tools and hails, etc.), would quite likely happen and could not be completely avoided. In order to evaluate the influence of low-velocity impact damage on titanium honeycomb sandwich structures, unidirectional in-plane compression tests on both intact and impact damaged sandwich panels were conducted to obtain their failure modes and compressive failure strength. Test results showed that the low-velocity impact damage could cause the change in failure modes and a 9% to 15% decrease in the compressive failure strength. Different impact energy levels showed a limited influence on the compressive failure strength. Numerical analysis was conducted to study the compression after impact behavior of titanium sandwich panels. Parametric finite element models that contained all the geometric and the structural details of honeycomb core cells, as well as the indentation and the crushed core region, were developed in the analysis. The numerical results successfully exhibited the failure process of the intact and impact damaged titanium sandwich panels subjected to unidirectional in-plane compression, similar to what observed in the tests. The predicted compressive failure strength also agreed very well with the test data. [ABSTRACT FROM AUTHOR]

In order to investigate the impact resistance of the Nomex honeycomb sandwich structures skinned with thin fibre reinforced woven fabric composites, both drop-weight experimental work and meso-mechanical finite element modelling were conducted and the corresponding output was compared. Drop-weight impact tests with different impact parameters, including impact energy, impactor mass and facesheets, were carried out on Nomex honeycomb-cored sandwich structures. It was found that the impact resistance and the penetration depth of the Nomex honeycomb sandwich structures were significantly influenced by the impact energy. However, for impact energies that cause full perforation, the impact resistance is characterized with almost the same initial stiffness and peak force. The impactor mass has little influence on the impact response and the perforation force is primarily dependent on the thickness of the facesheet, which generally varies linearly with it. In the numerical simulation, a comprehensive finite element model was developed which considers all the constituent materials of the Nomex honeycomb, i.e. aramid paper, phenolic resin, and the micro-structure of the honeycomb wall. The model was validated against the corresponding experimental results and then further applied to study the effects of various impact angles on the response of the honeycomb. It was found that both the impact resistance and the perforation depth are significantly influenced by the impact angle. The former increases with the obliquity, while the latter decreases with it. The orientation of the Nomex core has little effect on the impact response, while the angle between the impact direction and the fibre direction of the facesheets has a great influence on the impact response. [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]

The paper presents the experimental and numerical studies of sandwich panels with a hybrid core. The sandwich panel consists of external steel facings and a core, which is made of polyurethane foam or mineral wool or a combination of those two materials. The polyurethane foam material has a low weight and high thermal insulation properties, while the mineral wool material can provide high acoustic insulation and excellent fire resistance. Various proportions of the core materials are taken into account. It is assumed that a proper combination can provide the benefits of both materials. The structural behavior of a sandwich structure with a hybrid core is observed during laboratory tests. The failure mechanism is investigated in a four-point bending test. The material parameters of the core and facings are determined in standardized tests. The obtained parameters are used for FE simulations of the four-point bending tests. The criteria of damage initiation and propagation are defined in the interface layer of the numerical model. A satisfactory correlation between laboratory tests and numerical results is reported. Additionally, the sensitivity analysis of the numerical model response to the variation of the parameters of the interface is presented. [ABSTRACT FROM AUTHOR]

Honeycomb sandwich structures are increasingly used in the automotive, aerospace and shipbuilding industries where fuel savings, increase in load carrying capacity, vehicle safety and decrease in gas emissions are very important aspects. The aim of this study was to develop the theoretical methods, initially proposed by the authors and by other researchers for the prediction of low-velocity impact responses of sandwich structures. The developed methods were applied to sandwich structures with aluminium honeycomb cores and glass-epoxy facings for the assessment of impact parameters and for the prediction of limit loads. The values of model parameters were compared with data reported in literature and the predictions of the limit loads were validated by means of the experimental data. Good achievement was obtained between the results of the theoretical models and the experimental data. The failure mode and the internal damage of the sandwich panels have been investigated using 3D computed tomography, which allowed the evaluation of parameters of energy balance model, and infrared thermography, which allowed the detection of the temperature evolution of the specimens during the tests. The experimental and theoretical results demonstrated that the use of glass-epoxy reinforcement on aluminium honeycomb sandwiches enhances the energy absorption and load carrying capacities. [ABSTRACT FROM AUTHOR]

Two kinds of innovative re-entrant and hexagonal cell honeycomb sandwich structures filled with reactive powder concrete were proposed, and the ballistic resistance and energy absorption of the sandwich structures were investigated by numerical simulations. The deformation and failure modes of the different structures were analyzed and evaluated in detail. The honeycomb sandwich structures filled with reactive powder concrete prisms improved the capacity of ballistic resistance and energy absorption significantly, compared to the normal reactive powder concrete plates and sandwich structures without reactive powder concrete prisms. The analysis shows that the auxetic re-entrant cell honeycomb sandwich structures have a better ballistic performance than the hexagonal cell honeycomb sandwich structures. The sandwich structures were subjected to impact by three kinds of projectiles: flat, hemispherical and conical nosed. The ballistic limit of the flat nosed projectile is the highest, while the impact performance of the conical and hemispherical nosed projectiles is obviously different from the flat nosed projectile, especially in a relative high velocity range. The sharper nose leads to a higher value of exit velocity and mass loss. In addition, effects of different design parameters on ballistic resistance were also studied by changing the thickness of honeycomb cell and face plates. Results indicate that the thickness of honeycomb walls and face plates have significant effect on the ballistic resistance and energy absorption in a relative low velocity range, while there are no big differences when the initial impact velocity exceeds 400 m/s. [ABSTRACT FROM AUTHOR]

Bridge damage due to over-height vehicle collisions is a major issue throughout the transportation network with damages ranging from minor distortion or spalling in fascia girders to almost complete bridge destruction. Repairing such damages resulted from over-height vehicle collisions is expensive, and it includes costs for bridge repair, and rerouting traffic as well as indirect economic and societal costs. In this study, in correlation with a series of impact tests for reinforced concrete beams with or without Impact-Laminate (I-Lam) panel protection by a wooden projectile, the numerical modeling using the finite element codes ABAQUS is conducted to simulate the over-height vehicle impact, and a good agreement with the available experimental data is obtained. Parametric studies are conducted to further investigate the effects of design parameters (e.g. the velocity of projectile, I-Lam parameters, and core layer sequence, etc.) on the effectiveness of the protection system. The observed phenomena from the parametric studies reflect a design philosophy aiming at improving protection efficiency and providing important information on design of I-Lam honeycomb sandwich collision protection systems for field application. [ABSTRACT FROM AUTHOR]

Sandwich panel which has a design involving acoustic comfort is always denser and larger in size than the design involving mechanical strength. The respective short come can be solved by exploring the impact of core geometry on sound transmission characteristics of sandwich panels. In this aspect, the present work focuses on the study of influence of core geometry on sound transmission characteristics of sandwich panels which are commonly used as aircraft structures. Numerical investigation has been carried out based on a 2D model with equivalent elastic properties. The present study has found that, for a honeycomb core sandwich panel in due consideration to space constraint, better sound transmission characteristics can be achieved with lower core height. It is observed that, for a honeycomb core sandwich panel, one can select cell size as the parameter to reduce the weight with out affecting the sound transmission loss. Triangular core sandwich panel can be used for low frequency application due to its increased transmission loss. In foam core sandwich panel, it is noticed that the effect of face sheet material on sound transmission loss is significant and this can be controlled by varying the density of foam. [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]

This paper presents the results of an experimental and numerical study on aluminium honeycomb sandwich structures under low-velocity impact loadings. In order to investigate the impact behavior of honeycomb sandwich structures, which is consisted of two identical aluminium facesheets and an aluminium honeycomb core, an experimental study was carried out by using a drop-weight impact test system. Using this system, the contact forces and absorbed energies were measured to determine the influence of impact energy for one configuration of the sandwich structure. According to these results, a numerical model by finite element method of sandwich structures was developed, which is in good agreement with experimental results in terms of contact forces and deformations. Later, the effect of the cell size and the height variation of aluminum honeycomb core on the impact response of sandwich structures were investigated using the improved numerical model. The obtained numerical and experimental results were interpreted in detail. [ABSTRACT FROM AUTHOR]

The mechanical properties of paper honeycomb will be different from static situation with the changing of strain rate, which has an influence on the application of paper honeycomb. In this paper, the dynamic mechanical properties of paper honeycomb were discussed and the corresponding mathematic model was built in order to present the stress in different deformation stages and depict the energy absorption properties of honeycomb structure. The strain-rate effect was expressed by Comper-Symonds model. According to the mechanical model, the energy absorption diagram under different strain rates was also built from selecting materials of cushion packaging and optimum design of the packaging. Finally, serious experiments were taken and the result showed a good agreement with theoretical models. The proposed model can be used for practical application of the optimum design and material selection of honeycomb sandwich structure. [ABSTRACT FROM AUTHOR]

In this work, the functionally graded honeycomb structures with defects are investigated. It is shown how the patterns and locations of defects, as well as the density gradients, can affect the in-plane dynamic crushing behavior of honeycombs. Numerical results show that due to missing vertical cell walls, the deformation process for the transverse-vertical mixing defects is more complicated than that for pure vertical defects. For the honeycombs with negative density gradient, the I-shape deformation band is dominant during the dynamic crushing. However, the honeycombs with positive density gradient are crushed to densification with the combination of the first and second deformation bands. It is revealed that the ability of energy absorption could be improved by introducing the functional graded property. Moreover, under a high impact velocity, the gradient displays a high sensitivity on the energy-absorption capability of the honeycomb structures. At the early stage of crushing, the honeycomb structures with the positive density gradient behave the stronger capacity on energy absorption than those with the negative one. The energy-absorption curves for systems with positive and negative densities are symmetric about the homogeneous structures. As the compression proceeds, for the honeycombs with positive and negative density gradients, the trends of energy-absorptive abilities go into reverse. [ABSTRACT FROM PUBLISHER]

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]

The in-plane dynamic crushing behaviors and energy-absorbed characteristics of honeycombs with negative Poisson’s ratio (NPR) have been studied by means of explicit dynamic finite element analysis (DFEA) using ANSYS/LS-DYNA. First, the honeycomb models filled with different reentrant cells by the variation of micro-cell configuration parameters (cell-wall angle and shape ratio) are established. The respective influences of micro-structure and impact velocities on the deformation behaviors, the dynamic plateau stresses and the absorbed energy of reentrant honeycombs are explored in detail. It is shown that owing to the variation of cell micro-structure, reentrant honeycombs display different macro-/micro- deformation properties during the crushing. For the given impact velocity, the dynamic plateau stresses are related to the shape ratio by a power law and to the cell-wall angle by least-square curves. And they are also proportional to the square of impact velocities for a high impact velocity. Based on the finite element simulated results and one-dimensional shock wave theory, an empirical formula for auxetic honeycomb to predict the dynamic plateau stress is derived in terms of relative density and impact velocity. [ABSTRACT FROM AUTHOR]

This paper presents an analytical study of the in-plane dynamic crushing and energy absorption of hexagonal honeycombs with density gradients under different impact loading. Explicit dynamic finite element method simulations are carried out by using ANSYS/LS-DYNA. Firstly, under the assumption that the cell wall length is the same, a density-graded honeycomb mode is established by the variation of the cell wall thicknesses along the crushing direction. The effects of density gradient and impact velocity on the crushing deformation modes, plateau stresses and energy absorption characteristics of the specimens are explored in detail. Numerical results show that except for the impact velocity, the dynamic crushing performance and energy absorption abilities of honeycombs also rely on the density/strength gradients. The weakest layer is suggested to be placed at the impact end or the output end, and the strongest layer at the intermediate stage to achieve higher energy absorbing efficiency. According to the one-dimensional shock wave theory, the simple empirical formulae for graded honeycombs to predict the plateau stress are given under high-impact velocities. These results will provide some useful guides in the multi-objective optimization dynamic design and shock energy absorbing control of sandwich structures. [ABSTRACT FROM AUTHOR]

The amount of impact energy used to damage a composite laminate face sheet of a sandwich structure is a critical parameter when assessing residual compression strength. The compression after impact strength of impacted laminates used as face sheets on honeycomb core sandwich structure is dependent upon how thick the face sheet laminate is and this has traditionally been accounted for by normalizing (dividing) the impact energy by the laminate’s thickness. However when comparing compression after impact strength values for a given lay-up sequence and fiber/resin system, dividing the impact energy by the specimen thickness has been noted by the author to give higher compression after impact strength values for thicker face sheet laminates. A study was thus undertaken to assess the comparability of compression after impact strength data of sandwich structure by normalizing the impact energy by the face sheet thickness raised to a power to account for the higher strength of thicker laminates. Two data sets generated in this study were analyzed by dividing the impact energy by the face sheet thickness to the 1.0, 1.5, 2.0 and 2.5 powers. Results show that raising the face sheet thickness to a power of approximately 2.5 and dividing the impact energy by this quantity yields more comparable compression after impact strength data for comparing 8- and 16-ply face sheet laminates. For comparison of 24-ply face sheet laminates to 8- or 16-ply face sheet laminates, a value closer to 2 was found to give more comparable compression after impact strength data. [ABSTRACT FROM PUBLISHER]

Damaged glass fibre reinforced plastic–honeycomb core sandwich beams are repaired using uncured glass fibre reinforced plastic fabrics and a handheld iron. The effect of iron temperature, application time and pressure on the effectiveness of repair is investigated by measuring the failure load and flexural stiffness of the repaired beams using third span four-point bending tests. Repairs are tested in compression and tension. A repair process is suggested which consistently recovers 95% of the compressive strength and 77% of the tensile strength of the damaged beam. The repair is shown to have little effect on beam flexural stiffness. [ABSTRACT FROM PUBLISHER]

Buckling-induced skin–core debond growth in honeycomb sandwich cantilever beam is demonstrated using a cohesive zone model. The input parameters for the analysis are interfacial bond strength, mode I and mode II interfacial fracture toughness values, obtained from flatwise tension tests, drum-peel tests and three-point end notch flexure tests, respectively. Debonded honeycomb specimens are tested and the acoustic emission technique was used to observe the initiation of the debond growth. The load-displacement response from the cohesive zone model model shows a good agreement with the experimental results. The conventional analysis without cohesive zone model overestimates failure load by 56%. Cohesive zone model is able to predict the coupled debond growth and buckling failure in honeycomb sandwich structures. [ABSTRACT FROM PUBLISHER]

This paper presents a crashworthiness robust optimization design of aluminum honeycomb sandwich panel with regular hexagonal core cell based on explicit finite element analysis. First, the crashworthiness of honeycomb sandwich panel and the bare honeycomb core are compared with each other in the aspects of specific energy absorption and peak crushing force. The comparative results show that honeycomb sandwich panel can absorb more energy than the bare honeycomb core, though the peak crushing force of honeycomb sandwich panel is larger. Then the factor screening experiments are carried out to find out variables that have significant effects on the crashworthiness of honeycomb sandwich panel. The ultimate purpose of factor screening experiments is to decrease the computational expense by reducing the number of design variables. Finally, the dual response surface method and the crossed array design are employed to formulate the complex robust optimization design problem. The regression expressions of specific energy absorption and peak crushing force are defined as the objective and constraint function respectively in the robust design of crashworthiness optimization. [ABSTRACT FROM PUBLISHER]

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]

A response surface analysis of data from factorial experiments is used to determine the effect of different design factors on the low-velocity impact response of the honeycomb sandwich structures. A low-cost, out-of-autoclave manufacturing process is utilized to manufacture aerospace quality honeycomb sandwich panels. CYCOM® 5320 out-of-autoclave prepreg and FM® 300-2M are used as facesheet and adhesive, respectively. The out-of-autoclave process uses the vacuum bag pressure, thus, avoiding costly tooling and making the process more economical. A completely randomized design is used while manufacturing and testing the samples. Three design factors: angle difference between successive prepreg layers, number of prepreg layers, and number of adhesive layers, are selected as experimental variables. Each variable is considered at three levels, yielding a 33 factorial design. To investigate the effect of the experimental variables on the three response variables: energy absorbed (X); peak contact force (Y) and maximum deflection (Z), a low-velocity impact test is conducted. Analysis of variance and response surface techniques are used to analyze the data. The significant design factors for each response variable are identified using analysis of variance. Response surface analysis is carried out and the resulting regression models are employed to quantify the behavior of each of the response variables in response to changes in the design factors. The regression models are verified with the confirmation test results and both are in close agreement. [ABSTRACT FROM AUTHOR]

Sandwich panels have been developed to either produce lighter structures capable of carrying prescribed loads or to increase the load-carrying capacity subject to limitations on the weight. The major load-carrying elements of a sandwich structure are its facings, while the core primarily serves to resist transverse shear loads, enhance local strength and stability of the facings, and combine two facings into a single structural system. The facings being subject to in-plane tensile/compressive loads and to in-plane shear, their strength and stiffness are paramount to the sandwich structure. In this article we elucidate potential advantages of so-called ‘stepped’ facings with geometry modified to locally enhance the strength and stiffness at strategically important locations with a minimum effect on the weight. Numerous examples presented in the article validate our suggestion that a combination of a relatively simple manufacturing process and improved structural response of sandwich panels with stepped facings may present a designer with an attractive alternative to conventional sandwich structures. [ABSTRACT FROM AUTHOR]

An investigation on the structural performance of inserts within honeycomb sandwich panels is presented. The investigation considers metallic inserts in all aluminum sandwich panels and emphasis is placed on the structural performance difference between hot bonded and cold bonded inserts. The former are introduced during panel manufacture while the latter are potted into existing panels. The investigation focuses on the static performance of the two insert systems subject to loads in the normal direction to the facing plane. The experimental part of the work presented involved carrying out pullout tests on hot bonded and cold bonded reference samples by loading them at a centrally located insert. The experimental results were compared with results from an analytical model and results from a finite element model. Contrary to what was expected it was found from the experiments that the cold bonded inserts outperformed the hot bonded inserts in terms of load carrying capability. From the finite element study it was found that this was mainly due to the difference in stiffness of the different filler materials used in the two insert systems. [ABSTRACT FROM AUTHOR]

Although X-radiography has been widely used in studies of damage in composite materials, micro-tomography has received less attention, due to the high cost and limited availability of equipment. However, micro-tomographic analysis is a powerful tool, allowing 3D images of sections through sandwich materials to be obtained without specimen preparation. Four examples from a recent study are presented here, which show how micro-tomography can provide unique information on the quality of honeycomb sandwich structures, information on damage mechanisms and internal details of through thickness reinforcement in pinned foam cores. [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]

Analytical solutions were derived for the transient response and damage initiation of a foam-core composite sandwich panel subjected to blast loading. The panel response was modeled in two consecutive phases: (1) a throughthickness wave propagation phase leading to permanent core crushing deformations and (2) a transverse shear wave propagation phase resulting in global panel deflections. The predicted transient deformation of a sandwich panel consisting of E-glass vinyl ester facesheets and H100 PVC foam core compared well with ABAQUS predictions. Analytical predictions of the critical impulse for damage initiation in several foam sandwich panels also compared well with ABAQUS predictions. [ABSTRACT FROM AUTHOR]

An experimental study of the in-plane compressive behavior of both aluminium and nomex composite sandwich panels with 8 ply carbon/epoxy skins was conducted. All sandwich panels were impact-damaged with a range of impact energies from 1 to 55 J. Dominant damage mechanisms were found to be core crushing, skin delamination, and fracture with the former two absorbing most of the impact energy. While the intact panels failed in region close to one loaded end, all the impact-damaged nomex panels failed around the mid-section region. Two thirds of the aluminium panels also failed in the mid-section region and one third failed in the loaded end region. The presence of the core played a unique role in in-plane compression with a substantial stabilizing support to the skins, which counteracted the deleterious effect of impact damage. The in-plane compressive behavior has shown the combined effects of impact damage and the core in a complex manner. [ABSTRACT FROM AUTHOR]

Following paper Part I, this article investigates the last step of ThermHex process namely fusion bonding. The critical point of the fusion bonding step is to prevent catastrophic deformation (collapsing) of cell walls under heat and pressure. This demands a rapid heating rate and short heating time to keep sufficient stiffness for the inner part of cell walls against the pressure applied on the honeycomb. Hence, in this work the transient heat transfer behavior during the fusion bonding is investigated by using a finite element model for the detailed hexagonal unit cell. Furthermore, a mathematical model is developed to analyze the deformation of molten cell walls and their gradual contact to the skin strips. The experiments based on the talc-filled polypropylene material have been performed to validate the analytical model. Consequently, the optimal fusion bonding conditions can be determined to ensure good quality for the final products. [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 study shows that a simple approach to stiffness tailoring of composite face sheets can improve the performance of sandwich panels under compressive loading when delaminations are present. The simple stiffness tailoring concept used here is to reposition of all 0° material (aligned with the loading direction) into regions of certain width near the edges of the sandwich panel. This concept has been shown to improve the buckling and postbuckling performance of solid plates and to offer control of delamination growth. To evaluate this tailoring design concept in sandwich panels, numerical simulations of onset and propagation of the delamination growth were conducted with uniform face sheets and tailored face sheets. The delamination front tracing method previously developed by the authors was used to perform the delamination growth analysis. The interfacial elements, which allow the growth of the delamination front to be traced, are placed at the interface between the top face sheet and the core in the undelaminated region. They enable calculation of the strain energy release rates and application of a fracture mechanics delamination growth criterion. Gap elements were used to avoid interfacial overlap. Based on the numerical study, the improvement in the ultimate load provided by the simple stiffness tailoring concept ranges from 80% to 100%, depending on the extent of tailoring. Substantial improvements in the residual strength and stiffness, after significant delamination growth, can also be observed. Therefore, this article provides a potential design concept that can improve the damage tolerance of sandwich panels without adding weight. [ABSTRACT FROM AUTHOR]

A continuous process of thermoplastic honeycomb core, ThermHex, has been patented and is under development at K. U. Leuven. This new concept of thermoplastic honeycomb provides more affordable choices for structural applications than the previous expensive honeycombs thanks to the automated and continuous processing technology. The ThermHex process starts from one continuous thermoplastic sheet and consists of three main processing steps: thermoforming of half-hexagonal webs, folding of webs to honeycomb geometry, and internal fusion bonding. In this article, the polymeric material behavior during the thermoforming step is investigated in detail. First the material is characterized by a series of polymer tests to obtain basic data for the next numerical calculation. Then a 1D viscoelastic model and a 2D finite element (FE) model are used to analyze the polymer behavior during the thermoforming step, to acquire the correct understanding and hence to minimize the effect of the anelastic strain recovery (ASR) during the production. It is found that the material stress relaxation is one of the key features of the final process quality. [ABSTRACT FROM AUTHOR]

Critical facesheet wrinkling stresses are determined for thin gauge sandwich structure constructed from thick, highly orthotropic, honeycomb-cored materials containing sub-surface damage due to soft-body impact. A closed-form, analytical expression is extended to account for regions of crushed core extending a uniform depth into the core material. The results show that sub-surface core damage can significantly decrease facesheet support, further reducing the critical wrinkling stress computed from classical formulae. The model accounts for shear traction at the core/facesheet interface and provides a means of assessing the fundamental parameters governing the initiation of facesheet wrinkling in initially flat panels constructed from thick orthotropic cores containing sub-surface damage. The results are discussed in terms of the damage tolerance of panels used in aerospace applications. [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]

Fiber-reinforced polymer (FRP) composite sandwich panels with a core of sinusoidal geometry have been recently used in highway bridge and aquaculture tank applications. The selection of geometric shape and size of sinusoidal cores to meet given design requirements is critical in efficient applications of sandwich structures, and their effective transverse shear stiffness properties are important material properties in analysis and design. Based on a combined homogenization and multi-objective optimization technique, the effective transverse shear stiffness properties of sinusoidal cores are studied and optimized. An analytical approach using two-scale homogenization technique is used to predict the effective transverse shear stiffness properties of thin-walled sinusoidal honeycomb cores. The optimization problem is then solved using a sequential quadratic programming algorithm. Two types of optimization problems are presented: the maximization of effective transverse shear stiffness of honeycomb core with a given composite solid volume fraction and the minimization of composite solid volume with prescribed effective transverse shear stiffness. The geometric effects and weighting factors on the optimal design of sinusoidal cores are discussed. An optimization procedure using a response surface method and finite element analysis is also conducted to verify the proposed approach. The present combined homogenization and multi-objective optimization approach can be used to obtain favorable material performance behaviors and meet desired design requirements for honeycomb core structures. [ABSTRACT FROM AUTHOR]

The behavior of regular hexagonal honeycomb core with and without a facesheet is studied using 3D finite element-based micromechanics. The core without a facesheet is also studied using several versions of beam analysis in order to assess the accuracy of the simplified analyses. The effect of the facesheet on the core behavior is evaluated in terms of how much the distortion of the core and the stress distribution were changed by the presence of the facesheet. In situ effective properties of the core with a facesheet are estimated using periodic analysis of a representative volume element. The effects of various geometric parameters on in situ effective properties are determined. Finally, linear buckling analysis is conducted to study the effect of facesheet stiffness and core geometry on the core buckling behavior under out-of-plane shear loading. [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]

Sandwich structures are used extensively in marine structures and other applications. Cellular materials such as foams, honeycomb, and balsa wood used as core materials in sandwich structures are subjected to multiaxial loadings. Concentrated loads, impacts, and explosions can result in large deformations and damage, and yield or failure criteria are needed to predict the end of the elastic phase. Yielding or failure of isotropic foams is generally dependent on the hydrostatic stress, a feature that these materials share with many polymers, concrete, and soils. Appropriate criteria which are different from those traditionally used for metals have been classified in four major categories. Similarities between seemingly different criteria and between foams and other materials are brought out. Foams can also be anisotropic as honeycomb and balsa wood, and the anisotropy has to be accounted for in addition to the differences in strength in tension and compression in various directions. Suitable anisotropic criteria are shown to be the extensions of criteria previously used for isotropic materials and methods to present these criteria in a unified manner or to systematically extend isotropic criteria are discussed. Finally the strain rate effects and size effects are discussed. [ABSTRACT FROM AUTHOR]

A review is made of production defects and in-service damage types that arise in sandwich structures having fiber reinforced plastic (FRP) face sheets. A brief overview is given of relevant defect and damage models and how these models can be used in an assessment of criticality with regard to local and global structure, as a basis for deciding on corrective measures, for the case of a naval ship. Challenges resulting from limitations in inspection techniques are discussed. The concept of damage tolerance is discussed in the light of the above. It is argued that the most suitable and economical approach to achieving damage tolerance is dependent on the application. [ABSTRACT FROM AUTHOR]

Wrinkle defects may reduce the compressive strength of a face laminate for in-plane loading applied perpendicularly to the line of the wrinkle. To be able to decide whether a repair is needed, it is necessary to know the magnitude of the strength reduction for a given wrinkle geometry. In the studies reported here, the influence of wrinkle defects on the in-plane compressive strength of quasi-isotropic carbon fiber reinforced plastic (CFRP) laminates used in PVC foam-cored sandwich panels has been investigated by three approaches: testing of sandwich beam specimens in four-point bending, testing of sandwich panels with in- plane compression, and finite element simulation. Wrinkles involving different numbers of plies have been considered. Two different sandwich lay-ups typical of deck and hull bottom panels in naval ships have been included. [ABSTRACT FROM AUTHOR]

This article presents a technique for the production of graded sandwich cores, and a study of the effect of lengthwise grading of core properties for the facilitation of transverse loading. A significant failure mode of sandwich panels is caused by indentation of the foam core in regions at which local forces are introduced into the sandwich panel. The aim of this work is to synthesize a microstructure that can diminish this failure mode. By applying a rapid prototyping technique a periodic, open beam structure is synthesized. Additionally, the structure has been graded in the longitudinal direction by alternating over the beam diameter. The structure obtained is joined to aluminium face sheets to form a sandwich beam. The panel has been subjected to a three point bending test and the results have been compared to a finite element model, and further indentation tests show an increased transverse stiffness in the strengthened part of the beam. [ABSTRACT FROM AUTHOR]

Honeycomb sandwich panels of various skin thicknesses and core densities have been investigated under quasi-static loading in bending and indentation with both hemispherical (HS) and flat-ended (FE) indenters. Core crushing, top skin delamination, and top skin fracture are identified as major damage mechanisms. Their characteristics and energy-absorbing capabilities are established using load-displacement and load-strain curves and inspections of cross-sectioned specimens. The effects of varying skin thickness, core density and type, indenter nose shape, and boundary conditions on the damage and energy-absorbing characteristics are examined. The variation of the indenter nose shape is shown to induce a change in the damage mechanisms and have the most significant effect on energy absorption, especially for panels with relatively thicker skins. Increasing the skin thickness significantly increases not only the initial threshold and ultimate loads but also the absorbed energy (AE) of the panels. Increasing the core density has a very small effect on either the ultimate loads or the energy-absorbing capacity, while the effect of the support conditions on the damage and energy-absorbing characteristics is small. The larger 220 mm diameter panels absorb significantly more energy than the 100 mm diameter panels because of the much greater ultimate displacement. Different core materials with a similar density show little difference in either the daniage or the energy-absorbing characteristics due to the limited contribution of transverse shear resistance. Panels with a delaminated top skin have lower threshold loads under both indenters and lower ultimate load for the HS indenter. [ABSTRACT FROM AUTHOR]

The continuous production of thermoplastic honeycomb cores is a recent development at Katholieke Universiteit Leuven. The production equipment has been improved to deliver high quality material. This equipment is used for the determination of optimum process parameters. This article describes the production equipment, its development, and the experiments that have been carried out. [ABSTRACT FROM AUTHOR]

ABSTRACT: Low order and high order shear deformation models are applied to investigate the effect of honeycomb core (transversely shear deformable) on flexural vibration of thick rectangular sandwich panel with isotropic facesheets. Strain and kinetic energy of sandwich panel are expressed in terms of material properties and structural parameters. Partial differential equations are derived based upon a variational principle. Solutions in Navier form are obtained for a simply supported rectangular panel. The effect of honeycomb core parameters, such as characteristic angle, cell wall thickness, and cell size is studied. Comparison between low order model, high order model, and finite element method is provided. It is shown that in most cases results from a high order model without facesheet shear effect are close to those from finite element analysis. [ABSTRACT FROM AUTHOR]

The authors have developed a novel technique to detect debonding in honeycomb sandwich structures using small-diameter fiber Bragg grating (FBG) sensors. A small-diameter FBG sensor is embedded in the adhesive layer between the core and the facesheet during the curing process of the adhesive layer. After the curing process, the reflection spectrum from the small-diameter FBG sensor is distorted because the formation of fillets induces a non-uniform strain distribution in the adhesive layer. However, after debonding, the core with the fillets peels off the facesheet, and, as a result, the reflection spectrum recovers its original shape due to the release of the non-uniform strain. Debonding can thus be detected with high sensitivity in real time from the recovery in the shape of the reflection spectrum. In addition, a more reliable debonding detection technique using a small-diameter chirped FBG sensor is proposed, in which two indicators in the reflection spectrum have been successfully used for a more reliable judgment of debonding. [ABSTRACT FROM AUTHOR]

This article is intended to be a tutorial on the subject of fatigue of foam and honeycomb core composite sandwich structures. First, several different analytical models for predicting the fatigue life of sandwich composites are presented. Then representative publications which have reported on the major failure modes in sandwich beams under dynamic fatigue loading are summarized, along with several related publications dealing with static and impact loading. Papers dealing with the effects of loading frequency, environmental factors, and block loading on the fatigue life of sandwich composites are discussed. Finally, recent research on different types of non-destruction evaluation (NDE) techniques employed for failure investigations during fatigue testing of sandwich structures is reviewed. Conclusions and generalizations that can be drawn from the literature are presented along with discussions of areas in which further research is needed. [ABSTRACT FROM AUTHOR]

Damage characteristics of composite-skinned honeycomb sandwich panels in bending are investigated with both hemispherical (HS) and fiat-ended (FE) indenters. The thickness of the cross-ply skins varies from 8 to 16 plies, whereas the density of the 12.7-mm thick aluminum honeycomb core varies from 50 to 70 kg/m³. Clamped panels with a 100-mm testing area are loaded quasi-statically either in bending or on a rigid base. The effects of varying these parameters on damage mechanisms are examined through response curves as well as cross sections of selected specimens. Special emphasis is placed on their potential change induced by the variation of skin thickness and core density with a specific indenter. Damage mechanisms are identified as core crush, top-skin delamination, and fracture or shear-out. The threshold and ultimate loads as well as the initial slope increase significantly either on increase of skin thickness or change of the nose shape of indenter from a hemisphere to a flat-end. The increase in the post-initial-damage slope is small and can be attributed to membrane stretching of the damaged top skin. Increasing the core density affects substantially not only the threshold load, but also the initial slope associated with the FE indenter. Changing the nose shape of the indenter has an overriding effect on the nature of damage mechanisms. In particular, top-skin delaminations occur after core crush. The panel deflection contributes to 20–53% sandwich deformation. The bottom skin in all the tests remains intact. [ABSTRACT FROM AUTHOR]