Composite Honeycomb Sandwich Structures Research Articles

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.

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.

Nanocrystalline Aluminum Truss Cores for Lightweight Sandwich Structures

Substitution of conventional honeycomb composite sandwich structures with lighter alternatives has the potential to reduce the mass of future vehicles. Here we demonstrate nanocrystalline aluminum-manganese truss cores that achieve 2–4 times higher strength than aluminum alloy 5056 honeycombs of the same density. The scalable fabrication approach starts with additive manufacturing of polymer templates, followed by electrodeposition of nanocrystalline Al-Mn alloy, removal of the polymer, and facesheet integration. This facilitates curved and net-shaped sandwich structures, as well as co-curing of the facesheets, which eliminates the need for extra adhesive. The nanocrystalline Al-Mn alloy thin-film material exhibits high strength and ductility and can be converted into a three-dimensional hollow truss structure with this approach. Ultra-lightweight sandwich structures are of interest for a range of applications in aerospace, such as fairings, wings, and flaps, as well as for the automotive and sports industries.

Fatigue fracture of fiber reinforced polymer honeycomb composite sandwich structures for gas turbine engines

Fiber reinforced polymer honeycomb composite sandwich structures are commonly used in different industries. In particular, they are used in the manufacture of gas turbine engines. However, fiber reinforced polymer honeycomb composite sandwich structures often have a manufacturing flaw. In theory, such flaws due to their rapid propagation reduce the durability of fiber reinforced polymer honeycomb composite sandwich structures. In this paper, bending fatigue tests of fiber reinforced polymer honeycomb composite sandwich structures with manufacturing flaws were conducted. Comparative analysis of fatigue fracture of fiber reinforced polymer honeycomb composite sandwich specimens was conducted before and after their bending fatigue tests. The analysis was based on the internal damage X-ray observation of fiber reinforced polymer honeycomb composite sandwich specimens.

Ultrasonic guided wave propagation and disbond identification in a honeycomb composite sandwich structure using bonded piezoelectric wafer transducers

A coordinated theoretical, numerical, and experimental study is carried out in an effort to understand ultrasonic guided wave propagation and interaction with disbond, as well as, to identify disbond in a honeycomb composite sandwich structure using surface-bonded piezoelectric wafer transducers. In contrast to most of the work done previously, a fast and efficient two-dimensional semi-analytical model based on global matrix method is used to study dispersion characteristics as well as transient response of the healthy honeycomb composite sandwich structure subjected to relatively high-frequency piezoelectric wafer transducer excitations. Numerical simulations are then conducted using commercially available finite element package, ABAQUS, in order to explore guided wave propagation mechanisms due to the presence of disbond. Numerical simulations are further broadened to investigate the effect of disbond size on the amplitudes and group velocities of propagating guided wave modes. A good agreement is observed between the theoretical, numerical and experimental results in all cases studied. It is noticed that the presence of disbond, in particular, amplifies the first anti-symmetric (A0) mode and increases its group velocity. Finally, based on these modal behaviors, the location of an unknown disbond, within the piezoelectric wafer transducer array is experimentally determined by applying a probability-based damage detection algorithm.

Guided wave propagation in a honeycomb composite sandwich structure in presence of a high density core

A coordinated theoretical, numerical and experimental study is carried out in an effort to interpret the characteristics of propagating guided Lamb wave modes in presence of a high-density (HD) core region in a honeycomb composite sandwich structure (HCSS). Initially, a two-dimensional (2D) semi-analytical model based on the global matrix method is used to study the response and dispersion characteristics of the HCSS with a soft core. Due to the complex structural characteristics, the study of guided wave (GW) propagation in HCSS with HD-core region inherently poses many challenges. Therefore, a numerical simulation of GW propagation in the HCSS with and without the HD-core region is carried out, using surface-bonded piezoelectric wafer transducer (PWT) network. From the numerical results, it is observed that the presence of HD-core significantly decreases both the group velocity and the amplitude of the received GW signal. Laboratory experiments are then conducted in order to verify the theoretical and numerical results. A good agreement between the theoretical, numerical and experimental results is observed in all the cases studied. An extensive parametric study is also carried out for a range of HD-core sizes and densities in order to study the effect due to the change in size and density of the HD zone on the characteristics of propagating GW modes. It is found that the amplitudes and group velocities of the GW modes decrease with the increase in HD-core width and density.

Identification of disbond and high density core region in a honeycomb composite sandwich structure using ultrasonic guided waves

The aim of this study is to identify disbond and high-density (HD) core region in a honeycomb composite sandwich structure (HCSS) using ultrasonic guided waves (GWs) and surface-bonded piezoelectric wafer transducers (PWTs). Towards this, an organized theoretical, numerical and experimental study has been carried out in order to understand the characteristics of GW propagation in a HCSS. A global matrix method based efficient and fast two dimensional (2D) semi-analytical model is used to study transient response and dispersion characteristics of the healthy HCSS under PWT excitations. Numerical simulation of GW propagation in HCSS with disbond and HD-core is then carried out using finite element package, ABAQUS. Laboratory experiments are then carried out to validate the theoretical and numerical results. A good agreement is observed between the theoretical, numerical and experimental results in all cases studied. It is found that the presence of HD-core region leads to decrease in amplitude of the propagating GW modes and the presence of disbond leads to substantial amplification of the primary anti-symmetric mode. Finally, based on these changes in modal behaviors, the location and size of unknown disbond and HD-core region within the PWT array are experimentally determined using a probability based damage detection algorithm.

Stress Distribution on Composite Honeycomb Sandwich Structure Suffered from Bending Load

In this paper, we prepared a kind of carbon fibre/matrix composite honeycomb using carbon fibre and epoxy resin with compression molding technique. Composite Honeycomb Sandwich was prepared by gluing composite honeycomb with composite panels. Mechanical performance of the Composite Honeycomb Sandwich was characterized using finite element analysis (FEA) and three point bending performance. Results indicate that when suffering from bending loads, stress concentration is located at the loading zone as well as supporting zone. When bending load increases to 7200N, cracks occur on the interface between honeycomb and composite panel, the failure mode is interfacial de-bonding between honeycomb core and composite panel. Result of three-point bending performance indicates that composite honeycomb Sandwich breaks down when bending load increases to 6800N, which agrees with FEA results. Compared with traditional aluminum and Nomex honeycomb Sandwich, carbon fibre/epoxy Honeycomb Sandwich has higher bending strength. It can be used in aviation and aerospace industries.

Theoretical modeling of guided wave propagation in a sandwich plate subjected to transient surface excitations

A fast and efficient two-dimensional (2D) semi-analytical model based on global matrix method is developed to study the general characteristics of guided wave propagation in a honeycomb composite sandwich structure (HCSS) subjected to time-dependent transient surface excitations. The HCSS used in this study has an extremely lightweight and thick nomex honeycomb core, which is sandwiched between two thin graphite woven composite skins. The homogenized material properties of the skin and the core are considered to be elastic and quasi-isotropic in nature. Far-field time history of surface displacements are calculated for vertical and horizontal tone-burst surface excitations that are representative of thickness and radial mode of vibrations of piezoelectric transducers, respectively. Results are compared with those obtained from Finite element modeling (FEM) in LS-DYNA showing good agreement. Wavelet transform is then performed on the time-domain signal to obtain the group velocities of the propagating modes for their accurate identification on the basis of the theoretical dispersion curve. It is found that the response signal is dominated by the first anti-symmetric mode for a vertical excitation, whereas, the signal characteristics are multimodal in nature with dominating higher order symmetric and anti-symmetric modes for a horizontal excitation. The model is expected to be helpful for appropriate guided wave mode tuning and rapid analysis of data for experimental detection of disbonds in these novel structures.

Manufacturing and mechanical performance evaluation of resin-infused honeycomb composites

In spite of numerous advantages of open-cell core sandwich composites, the applications have been limited due to the problems involved in manufacturing using low cost processes. Resin accumulation in the core is a major challenge in the fabrication of honeycomb sandwich panels using resin infusion techniques. Foam-filled cores and polymer film barriers are some of the methods used in the literature to address this issue. However, these techniques will increase the weight of the sandwich composites. In this study, honeycomb sandwich panels were manufactured using commercially available film adhesive and modified vacuum-assisted resin transfer molding process. The resin incursion into the core openings was investigated. No accumulation of resin was observed in the core. Flatwise tension, flatwise/edgewise compression, and three-point bending tests were conducted to evaluate the mechanical performance of the sandwich composites. The performance of sandwich panels during a low-velocity impact event was also evaluated. Results indicate that the vacuum-assisted resin transfer molding process can be successfully used to manufacture honeycomb composite sandwich structures using currently available barrier adhesive films.

Impact analysis of fiber reinforced polymer honeycomb composite sandwich beams

Large scale fiber reinforced polymer (FRP) composite structures have been used in highway bridge and building construction. Recent applications have demonstrated that FRP honeycomb sandwich panels can be effectively and economically applied for both new construction and rehabilitation and replacement of existing structures. This paper is concerned with impact analysis of an as-manufactured FRP honeycomb sandwich system with sinusoidal core geometry in the plane and extending vertically between face laminates. The analyses of the honeycomb structure and components including: (1) constituent materials and ply properties, (2) face laminates and core wall engineering properties, and (3) equivalent core material properties, are first introduced, and these properties for the face laminates and equivalent core are later used in dynamic analysis of sandwich beams. A higher-order impact sandwich beam theory by the authors [Yang MJ, Qiao P. Higher-order impact modeling of sandwich beams with flexible core. Int J Solids Struct 2005;42(20):5460–90] is adopted to carry out the free vibration and impact analyses of the FRP honeycomb sandwich system, from which the full elastic field (e.g., deformation and stress) under impact is predicted. The higher order vibration analysis of the FRP sandwich beams is conducted, and its accuracy is validated with the finite element Eigenvalue analysis using ABAQUS; while the predicted impact responses (e.g., contact force and central deflection) are compared with the finite element simulations by LS-DYNA. A parametric study with respect to projectile mass and velocity is performed, and the similar prediction trends with the linear solution are observed. Furthermore, the predicted stress fields are compared with the available strength data to predict the impact damage in the FRP sandwich system. The present impact analysis demonstrates the accuracy and capability of the higher order impact sandwich beam theory, and it can be used effectively in analysis, design applications and optimization of efficient FRP honeycomb composite sandwich structures for impact protection and mitigation.

Dynamic Characteristics and Effective Stiffness Properties of Honeycomb Composite Sandwich Structures for Highway Bridge Applications

In this paper, a combined analytical and experimental study of dynamic characteristics of honeycomb composite sandwich structures in bridge systems is presented, and a relatively simple and reliable dynamic experimental procedure to estimate the beam bending and transverse shear stiffness is proposed. This procedure is especially practicable for estimating the beam transverse shear stiffness, which is primarily contributed by the core and is usually difficult to measure. The composite sandwich beams are made of E-glass fiber and polyester resins, and the core consists of the corrugated cells in a sinusoidal configuration. Based on the modeling of equivalent properties for the face laminates and core elements, analytical predictions of effective flexural and transverse shear stiffness properties of sandwich beams along the longitudinal and transverse to the sinusoidal core wave directions are first obtained. Using piezoelectric sensors, the dynamic response data are collected, and the dynamic characteristics of the sandwich structures are analyzed, from which the flexural and transverse shear stiffness properties are reduced. The experimental stiffness results are then compared to the analytical stiffness properties, and relatively good correlations are obtained. The proposed dynamic tests using piezoelectric sensors can be used effectively to evaluate the dynamic characteristics and stiffness properties of large sandwich structures suitable for highway bridge applications.

Core Crush Problem in the Manufacturing of Composite Sandwich Structures: Mechanisms and Solutions

Manufacturing of composite honeycomb sandwich structures is significantly impacted by poor production yields caused by the core crush problem that occurs during the autoclave curing process. It is a major manufacturing defect that leads to costly part rejects because the defects are nonrepairable. This failure mechanism also constrains aircraft engineers, limiting the design range of core density and core thickness in attempts to mitigate core crush. Recent studies that have led to basic understanding of core crush mechanism are discussed. It was found that the prepreg frictional resistance is the key factor in controlling core crush. Research in the scientific community has mainly focused on resin effects in core crush. However, studies conducted show that core crush can also be significantly reduced by controlling construction of the fiber tow shape and the fabric architecture. Rounder fiber tow or more open fabric produces rougher prepreg surface, which results in a higher prepreg frictional resistance, reducing the effects of core crush. Experimental results indicate that a developed core crush resistant prepreg increases the prepreg frictional resistance and effectively reduces core crush, without changing the resin system.

Mechanics of Composite Sinusoidal Honeycomb Cores

Lightweight and heavy-duty fiber-reinforced polymer (FRP) composite honeycomb sandwich structures have been increasingly used in civil infrastructure. Unique cellular core configurations, such as sinusoidal core, have been applied in sandwich construction. Due to specific core geometry, the solutions for core effective stiffness properties are not readily available. This paper presents a mechanics of materials approach to evaluate the effective stiffness properties of sinusoidal cores. In particular, the internal forces of a curved wall in a unit cell are expressed in terms of resultant forces, and based on the energy method and principle of equivalence analysis, the in-plane stiffness properties of sinusoidal cores are derived. Both finite-element modeling and experimental testing are carried out to verify the accuracy of the proposed analytical formulation. To illustrate the present analytical approach as an efficient tool in optimal analysis and size selection of sinusoidal cores, several design plots are provided and discussed. The simplified analysis and formulation presented for sinusoidal cores can be used in design application of FRP honeycomb sandwich and optimization of efficient cellular core structures.

Evaluation of the In-Service Performance Behavior of Honeycomb Composite Sandwich Structures

When honeycomb composite structures are fabricated for the aerospace industry, they are designed to be closed to their operating environment for the life of the composite structure. However, once in service, this design can break down. Damage can set in motion a chain reaction of events that will ultimately degrade the mechanical integrity of the composite structure. Through thermographic analysis, the tendency of honeycomb composite structures to absorb and retain water was investigated, and an attempt was made to quantify the extent of water ingression in the Boeing 767 aircraft. Through thermographic analysis, the exterior honeycomb composite structures were found to contain less than 50 kg of water per plane. On average, over 90% of the water found on an aircraft was contained in five problematic parts, which included the outboard flap wedge, the nose landing gear doors, the main landing gear doors, the fixed upper wing panels, and the escape slide door. Kevlar lamina induced microcracking, skin porosity problems, and cracked potting compound were the root causes of water ingression and migration in these structures. Ultimately, this research will aid in the fundamental understanding and design of future honeycomb composite sandwich structures.

Composite robot end effector for manipulating large LCD glass panels

Recently, the design and the manufacture of light robot end effectors with high stiffness have become important in order to reduce the deflection due to the self-weight and weight of glass panel, a part of LCD, as the size of glass panels as well as robot end effectors increases. The best way to reduce the deflection and vibration of end effectors without sacrificing the stiffness of end effectors is to employ fiber reinforced composite materials for main structural materials because composite materials have high specific stiffness and high damping. In this work, the end effector for loading and unloading large glass panels were designed and manufactured using carbon fiber epoxy composite honeycomb sandwich structures. Finite element analysis was used along with an optimization routine to design the composite end effector. A box type sandwich structure was employed to reduce the shear effect arising from the low modulus of honeycomb structure. The carbon fiber epoxy prepreg was hand-laid up on the honeycomb structure and cured in an autoclave. A special process was used to reinforce the two sidewalls of the box type sandwich structure. The weight reduction of the composite end effector was more than 50% compared to the weight of a comparable aluminum end effector. From the experiments, it was also found that the static and dynamic characteristics of the composite end effector were much improved compared to those of the aluminum end effector.

Water Intrusion in Thin‐Skinned Composite Honeycomb Sandwich Structures

Thin-skinned composite honeycomb sandwich structures from the trailing edge of the U.S. Army's Apache and Chinook helicopters have been tested to ascertain their susceptibility to water intrusion as well as such intrusions' effects on impact damage and cyclic loading. Minimum-impact and fatigue conditions were determined which would create microcracks sufficiently large to allow the passage of water through the skins; damage sufficient for this to occur was for some skins undetectable under a 40X-magnification optical microscope. Flow rate was a function of moisture content, damage, applied strain, and pressure differences.