Composite Honeycomb Sandwich Research Articles

Characterization of damages in materials by computer-aided tap testing

The tap test is an easy and effective way of characterization of flaws and damages in different materials (especially honeycomb sandwich composites). When the tapping is carried out systematically to cover an area, the measured impact duration can be used for generating a “scan image” of the tapped area. Such an instrumented tap test and imaging system can be used for the detection of flaws and damages, for the evaluation of repaired areas and for the verification of normal reinforced substructures. In this study, the images of intact and damaged aluminum honeycomb sandwich composite, pine board and poplar board samples were obtained by using Computer Aided Tap Tester (CATT) system in the electric and magnetic field and also without any field. As a result of the tests made, it has been figured out that the tests made in the electric field and the magnetic field increased the quality of the images for damages. Also, it has been found out that the CATT system is quite successful in detecting damage of honeycomb sandwich composite materials.

Free vibration analysis of composite sandwich panels with hierarchical honeycomb sandwich core

Abstract The vibration characteristics of sandwich panels with a hierarchical composite honeycomb sandwich core were investigated in this study. An orthotropic constitutive model of the hierarchical composite honeycomb sandwich core was used to propose an equivalent model (two-dimensional model). Two-dimensional (2D) and the three-dimensional (3D) finite element models as well as modal tests were used to predict the natural frequencies and mode shapes of the sandwich panels. The prediction results obtained using the equivalent model were consistent with the experimental and 3D finite element analysis results. Subsequently, a redesigned sandwich panel, with a hierarchical cross-honeycomb sandwich core, was manufactured to implement the multifunctional characteristics such as fluid flow and installation of microelectronic devices. To compare the vibration characteristics of the panel with a hierarchical cross-honeycomb sandwich core with those of a panel with a hierarchical square-honeycomb sandwich core, a dimensionless frequency parameter, ω 1 / ω , was proposed. The frequency parameter had different sensitivities to the geometric parameters, i.e., the face-sheet thickness ratio, wall-sheet thickness ratio, and relative density of the filling foam.

Rainbow metamaterials for broadband multi-frequency vibration attenuation: Numerical analysis and experimental validation

In this study, we propose a ‘rainbow’ metamaterial to achieve broadband multi-frequency vibration attenuation. The rainbow metamaterial is constituted of a Π-shaped beam partitioned into substructures by parallel plates insertions with two attached cantilever-mass acting as local resonators. Both resonators inside each substructure can be non-symmetric such that the metamaterial can have multi-frequency bandgaps. Furthermore, these cantilever-mass resonators have a progressively variant design along the beam, namely rainbow-shaped, for the purpose of achieving broader energy stop bands. Π-shaped beams partitioned by parallel plate insertions can be extended to honeycomb sandwich composites, hence the proposed rainbow metamaterial can serve as a precursor for future honeycomb composites with superior vibration attenuation for more industrial applications. A mathematical model is first developed to estimate the frequency response functions of the metamaterial. Interaction forces between resonators and the backbone structure are calculated by solving the displacement of the cantilever-mass resonators. The plate insertions are modeled as attached masses with both their translational and rotational motion considered. Subsequently, the mathematical model is verified by comparison with experimental results. Metamaterials fabricated through an additive manufacturing technique are tested with a laser doppler receptance measuring system. After the validation of the mathematical model, a numerical study is conducted to explore the influences of the resonator spatial distributions on the frequency response functions of structures. Results show that for metamaterials with both symmetric and non-symmetric resonators, rainbow-shaped resonators can introduce inertial forces inside wider frequency range when compared to the periodic resonators of the same total mass, hence broader bandgaps. Meanwhile, the attenuation inside the bandgaps decreases when the bandgap become broader. Metamaterials with broadband multi-frequency range vibration attenuation can be achieved with non-symmetric sinusoidally varying resonators.

Assessment of dynamic properties of hybrid ribbon reinforced multifunctional composite sandwich plates: Numerical and experimental investigation

Abstract This study investigates numerically and experimentally the effect of longitudinal ribbon reinforcement in conventional honeycomb structure on the various dynamic characteristics of a hybrid composite sandwich plate. Longitudinal strips are considered to be embedded along the transverse directions at various locations of a honeycomb core material without and with carbon nanotubes (CNT) reinforcement in a composite sandwich plate. The governing differential equations of motion of the various hybrid honeycomb composite sandwich plate configurations with strips inserted in conventional honeycomb structures as core layer having top and bottom face composite layers are derived using higher order shear deformation theory (HSDT) and solved numerically using a four noded rectangular finite element. Further, experimental investigations are performed by fabricating the various strips embedded hybrid honeycomb core material with and without CNT reinforcement to evaluate the shear and loss moduli using the alternative dynamic approach. The efficacy of the developed finite element formulation is demonstrated by comparing the experimental and numerical results obtained in terms of natural frequencies and loss factors for the various prototypes of honeycomb composite sandwich plates. Also, the effect of variation in CNT content and thickness of longitudinal ribbon reinforcement in conventional honeycomb core material and ply orientation of the face sheets on the various dynamic properties of hybrid composite structures are studied under various end conditions. This study proposes that the ribbon reinforcement location and the addition of CNTs in honeycomb core materials strongly influence the stiffness, damping and transverse vibration displacements of the ribbon reinforced hybrid composite sandwich plates.

Experimental study on the indentation of epoxy resin–aluminum honeycomb composite sandwich panel

The present study develops a new type of epoxy resin–aluminum honeycomb composite sandwich panel. Failure modes and typical force–displacement curves are measured through quasi-static localized indentation tests. The effects of different parameters on the elastic ultimate load and energy absorption are analyzed. The results indicate that the newly developed composite sandwich panel has good integrality, stability, and energy absorption capacity. The typical force–displacement curve exhibits three types and all of them consist of six phases, namely: elastic phase, local damage phase, strengthening phase, global damage phase, compaction phase, and bottom destruction phase. The strength and the energy absorption capacity of the composite sandwich panel are affected by composite layer thickness, additional layer thickness, boundary condition, and indenter type. Compared to those of traditional sandwich panel, the mechanical properties of the integrated composite sandwich panel are significantly improved, as the specimen maintains good stability under indentation without undergoing detachment or cracking between the composite layer and the core.

Mechanical responses of a composite sandwich structure with Nomex honeycomb core

Weights of honeycomb sandwich composite structures are strictly controlled in scenarios such as in contexts of aerospace engineering. However, such structures extensively studied in past investigations were partly or completely made of aluminum or other metallic materials. Sufficient research has not been reported for honeycomb sandwich composites made of lightweight non-metallic materials. This paper presents an investigation into the mechanical responses of the Nomex honeycomb sandwich structure in a three-point bending test. The mathematical relationship between the shear stress distribution in the skin and that in the Nomex honeycomb core under the bending load was deduced. By applying the commercial software ABAQUS/Explicit, numerical simulations were performed with a highly detailed finite element model (micromechanical level) and equivalent elastic parameters. By comparing the experimental and numerical results, the validity of the theoretical derivation was verified. The effects of the thickness of the skin and the direction of the core on the mechanical responses of the honeycomb sandwich structure were studied. The results showed two different failure patterns of the honeycomb sandwich structure in the three-point bending test, which has rarely been reported previously. The improving effect of the increase in the skin thickness on the ultimate bearing capacity was more remarkable when the honeycomb core had a higher shear rigidity. For the same core direction, the honeycomb sandwich structure with the thicker skin had a better crashworthiness. The skin shear force distribution coefficient decreased exponentially with the ratio of core height-to-skin thickness.

Investigating the effects aviation fluids have on the flatwise compressive strength of Nomex® honeycomb core material

Composite honeycomb sandwich structure is widely used for aircraft structures such as control surfaces, radomes, engine cowls, and aircraft interior structure because of its lightweight and high strength characteristics. One of the disadvantages of honeycomb sandwich structure is that they are prone to fluid intrusion. The purpose of this study is to determine if the structural properties of honeycomb core are affected by contact with a fluid. The test specimens were manufactured of fiberglass prepreg for the facesheets and Nomex® honeycomb core for the core material in accordance with ASTM C-365/365M. Test specimens were soaked in several different kinds of fluids, such as aircraft fuel, turbine engine oil, hydraulic fluid, and water for a period of 60 days. Some of specimens were tested right after soaking period ended, and some of them were dried before they were tested. A flatwise compressive test was performed, and the test results were analyzed to determine how the contact with aircraft fluids affected the compressive strength of the Nomex® honeycomb core and how the strength was recovered when the specimens were dry. In addition, the investigation of de-bonding between facesheet and core material after soaking was performed to support the study.

Modified electromechanical impedance-based disbond monitoring for honeycomb sandwich composite structure

The increasing use of honeycomb sandwich composites in industries has attracted much interest in its structural health monitoring (SHM) for the localization and quantification of disbond on skin-core interface. The disbond monitoring in sandwich composites is more complicated than that in laminates. In this paper, a comprehensive active monitoring scheme (CAMS) and direct coupling mechanical impedance (DCMI) signatures extraction approach are proposed. It is developed from a modified electromechanical impedance (EMI) analytical model to extract more useful signatures from measured raw signatures (RS). A damage index (DI), root mean square deviation (RMSD), is adopted to locate and quantify the disbond extent inside structures. Experiments are conducted on a honeycomb sandwich composite with a 4 × 4 sensors array mounted on top skin to validate the effectiveness of proposed monitoring scheme. The experimental results show that both disbond location and severity on skin-core interface can be monitored efficaciously.

Defect imaging in layered composite plates and honeycomb sandwich structures using sparse piezoelectric transducers network

Carbon fiber reinforced polymer (CFRP) plates and honeycomb composite sandwich structures (HCSS) are widely used in the aerospace industry as they exhibit excellent strength-to-weight ratio, stiffness, toughness, corrosion resistance, etc. Nevertheless, defects, such as face sheet delamination or core-sheet debonding, may appear due to the impact forces or thermo-mechanical aging and can degrade these properties. Structural health monitoring (SHM) based on the use of guided elastic waves (GW) is regarded as a promising solution to detect such defects, and consequently to reduce maintenance costs and to extend structure service time. GWs propagate over large distances while being sensitive to structural inhomogeneities. Here, a SHM system prototype is proposed. It relies on a sparse grid of piezoelectric transducers distributed over the structure, used for both actuating and sensing GWs. Defect imaging is performed by means of the correlation based algorithm, the so-called Excitelet. It computes correlation coefficients between the theoretical and experimental GW signals for each pixel on the image representing the region of interest of the structure. The theoretical signals for CFRP are computed using a model based on a 2D semi-analytical finite element formulation. Analytical prediction of theoretical signals for the HCSS being intractable, a homogenization model is applied to the honeycomb core to replace it by an equivalent orthotropic plate preserving the same modelling approach. Defect imaging results are presented for both structures, namely a CFRP plate and a HCSS. The resolution of the corresponding images can be related to the wavelength of the inspecting mode.

Estimation of strains in composite honeycomb sandwich panels subjected to low frequency diffused acoustic field

Acceleration responses of panels subjected to diffused acoustic field are often determined using a coupled Finite Element - Boundary Element Method. In many such models the structure is represented by its natural modes and the acceleration responses can be determined. But for the verification of the structure for its survivability, its strain responses are essential to be determined. In practice such coupled models do not have the information on the structural details and hence it is not possible to determine the strains in the structure. A methodology to estimate the strains in the low frequency modes of the structure based on the acceleration responses is developed in this work. An expression relating the spectral density of strain to the spectral density of the acceleration is derived. A composite honeycomb sandwich panel is subjected to diffuse acoustic field in a reverberation chamber and accelerations, strains are measured. The strains are estimated using the methodology developed in this work and it is seen that the strains determined using this technique are very much in agreement with the measured strains. Thus, the present work enables determination of strains in the panel even when the structure is represented by its free vibration characteristics and its physical properties are not known.

Detection and Characterization of Debonding Defects in Aeronautical Honeycomb Sandwich Composites Using Noncontact Air-Coupled Ultrasonic Testing Technique

The finite models of honeycomb sandwich composite with intact and embedded debonding defects are constructed. The sound pressure in fluid domain and the stress strain problem in solid domain are related by acoustic-structure coupling method, which visually shows the propagation process and modal characteristics of the acoustic wave inside the honeycomb sandwich composite. The simulation results show that the transmission longitudinal wave T1 (transmission initial wave) can effectively characterize debonding defects of honeycomb sandwich composite. However, in the actual detection of honeycomb sandwich composite, there are some problems, such as poor Signal-to-noise ratio (SNR) of received signal, incognizable transmission initial wave. In order to solve these problems, this paper proposes to apply polyphase coded pulse compression technique to air-coupled ultrasonic testing system. The actual test results show that the SNR of received signal is effectively improved, the transmission initial wave can be effectively identified, and the compressed signal has a good response to debonding defect. The air-coupled ultrasonic testing C scan result of honeycomb sandwich composite verifies the rationality and correctness of the theoretical simulation and signal processing technique, which promotes industrial application of air-coupled ultrasonic testing technique in the aerospace field.

NDE Detection and Characterization of Damage in Honeycomb Sandwich Composites

This study deals with a non-destructive evaluation technique for detecting defects in composite panels. In this non-destructive evaluation technique, various low-frequency techniques are applied such as a tapping technology. Another terahertz wave application will be utilized. In particular, the tapping technique is utilized in order to evaluate the characteristics of the honeycomb member based on the hysteresis effect analysis. Here, the area surrounded by the hysteresis loop in the force-displacement curve is related to the increased internal friction loss that is the cause of the energy absorption absorbed by the composite member after the load is applied. The loop area where the composite member was damaged was well agreed with the damage level. Artificial defects were manufactured on the surface of honeycomb sandwich composite panels. An NDE technique was proposed in order to detect defects by using terahertz waves and was discussed for tuning the practical use.

Vibration analysis of a multifunctional hybrid composite honeycomb sandwich plate

In the present study, the free and forced vibration responses of the composite sandwich plate with carbon nanotube reinforced honeycomb as the core material and laminated composite plates as the top and bottom face sheets are investigated. The governing equations of motion of hybrid composite honeycomb sandwich plates are derived using higher order shear deformation theory and solved numerically using a four-noded rectangular finite element with nine degrees of freedom at each node. Further, various elastic properties of honeycomb core materials with and without reinforcement of carbon nanotube and face materials are evaluated experimentally using the alternative dynamic approach. The effectiveness of the finite element formulation is demonstrated by performing the results evaluated experimentally on a prototype composite sandwich plate with and without carbon nanotube reinforcement in core material. Various parametric studies are performed numerically to study the effects of carbon nanotube wt% in core material, core thickness, ply orientations, and various boundary conditions on the dynamic properties of composite honeycomb sandwich plate. Further, the transverse vibration responses of hybrid composite sandwich plates under harmonic force excitation are analyzed at various wt% of carbon nanotubes and the results are compared with those obtained without addition of carbon nanotubes to demonstrate the effectiveness of carbon nanotube reinforcement in enhancing the stiffness and damping characteristics of the structures. The study provides the guidelines for the designer on enhancing both the stiffness and damping properties of sandwich structures through carbon nanotube reinforcement in core materials.

Fast-curing halogen-free flame-retardant epoxy resins and their application in glass fiber-reinforced composites

A series of novel fast-curing halogen-free flame-retardant epoxy resins were formulated and used to prepare glass fiber-reinforced composites. Dynamic mechanical analysis showed that the optimized epoxy system could be completely cured in 0.5 h at 150℃ and had a glass transition temperature ( Tg) of above 130℃. The optimized epoxy system was also used as matrix resin to make glass fiber prepregs and composite panels. The flame-retardant properties of the glass fiber-reinforced composites were investigated, including the limiting oxygen index (LOI) and flaming, smoke and toxicity properties. The glass fiber-reinforced composite had good flame retardancy with a UL-94 V-1 rating and high LOI of ∼36%. More significantly, the composite based on the flame-retardant epoxy resin showed lower smoke density compared with those based on phenolic resins. Finally, the glass fiber prepregs were used to fabricate honeycomb sandwich composites. The peel strength of the epoxy-based composites was almost twice that of the composites based on phenolic resin.

Fatigue Life of Al-Honeycomb Core Composites Construction

Polymer composite honeycomb sandwich panels (PCHSPs) are used in many sectors of industry, such as in aerospace, defense and public transport factory. Sandwich panels consist of two facing skins and the core. Products of all industry sectors have to meet requirements on their dependability. This research paper deals with a proposal of the most appropriate methodology for dependability testing of PCHSPs products used in transport industry. Dependability is a global concept that includes terms such as availability, reliability, durability, maintainability, supportability, etc. The experiments have shown S-N curves with damages and without damages of PCHSPs. An important part of dependability is to be fonded the limit states of the studied object, which are for PCHSPs are characterized by fiber cracking and (core) delamination. Dependability evaluation consists of analysis and tests. For every test, it is necessary to be developed its test plan. Facing skin components of the sandwich panels may particularly be damaged. There are several methods of non-destructive testing, which can be used to determine the damage of facing skins of sandwich panels. Infrared thermography (IRNDT) is one of them. IRNDT utilizes thermo-physical properties of the materials, including thermal diffusivity.

Composite honeycomb metasurface panel for broadband sound absorption.

Composite honeycomb sandwich panels have been adopted in a wide range of applications owing to their excellent mechanical properties. This paper demonstrates a design of a composite honeycomb metasurface panel that can achieve 90% sound absorption from 600 to 1000 Hz with a thickness less than 30 mm. The panel is comprised of periodically and horizontally arranged honeycomb "supercells" which consist of unit cells of different geometric parameters (pore size). Two different analytical models (Helmholtz resonator model and micro-perforated panel model) are used to calculate the sound absorption of the panel, and they are further validated by a numerical model. The relatively broadband sound absorption is found to be attributed to the coupling between unit cells, which is illustrated by both the complex frequency plane theory and the calculated sound intensity field.

Thermally induced microcracks and mechanical property of composite honeycomb sandwich structure: Experiment and finite element analysis

Microcracking in composite honeycomb sandwich structure and its effect on mechanical properties are studied in this paper. A methodology is presented to study the extent of mechanical strength degradation of composite sandwich structure, subjected to thermal fatigue. The material under study is used for spacecraft structural applications. The test coupons were exposed to thermal cycling at elevated temperature as high as +150°C inside the oven and cryogenic temperature of −190°C by dipping in liquid nitrogen, which is comparable to the thermal environment experienced by spacecraft structures. After each thermal cycle, coupons were inspected for microcracks under an optical microscope at the cross section. The microcracks were then quantified using parameters like crack length and crack density with increase in the number of cycles. Flatwise tensile test was conducted on the coupons after every 10 thermal cycles, up to 60 cycles, to make a correlation between crack density and mechanical strength. It was observed that by increasing the number of thermal cycles, the crack density increases and the flatwise tensile strength decreases up to a specific number of cycles. Finite element analysis was performed to predict the possible location of microcracks formation and compared with experimental observation. Good correlation was observed.

Investigation of low velocity impact behaviors of honeycomb sandwich composites

Honeycomb sandwich composites are used as significant structural members in advanced engineering applications. Thus, it is critical to determine how they behave under impact loading, in addition to other loads. In this study, low velocity impact loading behaviors of honeycomb sandwich composites were experimentally investigated. Almost all of the design parameters of honeycomb sandwich composites were investigated. The results indicated that the core thickness of honeycomb had no effect on the strength of the composite, and the parameter influencing the impact behavior of the specimen the most was the face sheet thickness. When the face sheet thickness of the specimen was increased, the most apparent strength increase was observed in the models using carbon fiber-reinforced composite face sheets. For all face sheet types subject to impact energy of 10 Joules, the upper face sheets of 0.5 mm-thick specimens were perforated.

Characterization of sandwich composite T-joints under different ageing conditions

High-tech demands of the aviation industry require a detailed knowledge of the mechanical and structural response of composite structures. This contribution presents experimental investigations on composite honeycomb sandwich T-joint connections. To understand the effect of loading and ageing on the adhesively bonded joints, quasistatic tension experiments have been performed with the original T-joints and with two sets of artificially aged specimen. The accelerated ageing was enforced by thermal cycling with temperatures in the range of -35°C to 70°C.The failure behavior of the original and the aged specimens are compared to provide primary data for analyzing the behavior of honeycomb sandwich T-joints in limit conditions. A fractographic analysis is performed to determine the failure modes. Additionally the provided data are verified with three-dimensional finite element analyses using a cohesive element technique. The achieved results provide not only new data for designers but can also be used for future computational models of sandwich structures.

Vibration analysis of honeycomb sandwich composites filled with polyurethane foam by Taguchi Method

In this study, the effect of polyurethane foam filler, in addition to surface layer thickness and core material thickness, on vibration characteristics of sandwich structures was investigated. The manufacturing process was carried out according to the Taguchi method. The natural frequencies and damping ratios of the produced samples were determined experimentally for fixedfree boundary conditions. In addition, solid models were developed for test samples and their finite element analyses were performed with ANSYS® to obtain their natural frequencies and mode shapes. An acceptably good agreement was found with the comparison of experimental results with the numerically obtained ones. The most effective parameters on the vibration characteristics of the sandwich structure were determined by the Taguchi method.