Composite Face Sheets Research Articles

Investigation of Mechanical Properties of Honeycomb Sandwich Structure with Kenaf/glass Hybrid Composite Facesheet

ABSTRACT The application of a sandwich structure made with composite facesheet and honeycomb core has increased rapidly due to high stiffness to weight ratio characteristics. There is a growing interest in using natural/synthetic hybrid composite to enhance the mechanical performance of natural fiber and reduce the environmental impact compared to synthetic materials. This study investigates the mechanical properties (tensile, edgewise compression, flexural) of the honeycomb sandwich structure with the hybrid composite facesheet, reinforced with different kenaf and glass fiber ratios. Non-hybrid kenaf and glass fiber composites were fabricated for comparison. The result showed that increasing the glass fiber ratio in a hybrid facesheet significantly improved the mechanical performance of the sandwich structure. Compared to the non-hybrid kenaf composite, the hybrid composite showed improved tensile and compression strength by up to 233% and 79%, respectively. The hybrid composite (glass/kenaf/glass) achieved 76% tensile modulus and 87% compression strength of glass fiber composite when the results were normalized. The specific flexural stiffness of the hybrid facesheet using glass fiber in the outer layer exhibited comparable performance by obtaining 89% flexural stiffness of non-hybrid glass composite sandwich structure.

Buckling of sandwich cylindrical shells with shear deformable core through nondimensional parameters

This paper presents the derivation of nondimensional buckling equations of sandwich cylindrical shells made of composite facesheets with a shear deformable core. The procedure yields an analytical solution in terms of a series of nondimensional parameters for the axial buckling load investigating the influence of the core transverse shear. The developed equations and the nondimensional parameters are used to study the buckling response of different shells, and the calculated buckling loads are compared to the buckling values obtained by neglecting the transverse shear. Graphs and tables are presented to show the effects of the nondimensional parameters on the nondimensional buckling load. The results are verified by finite element analyses using the commercial code Abaqus.

Experimental Damage Evaluation of Honeycomb Sandwich with Composite Face Sheets under Impact Load

Aerospace structures are highly vulnerable to impact loads whose damage tolerance, and its resistance vary over the range of impact velocity. Honeycomb sandwich structures are used in aerospace industries where mass efficient and impact resistant structures are needed. However, the structural integrity of these structures is reduced by impact load due to tool drop, runway debris, hailstones and improper handling of the structure. A thorough investigation of the damage behaviour of honeycomb sandwich under low-velocity impact and the post-impact residual strength determination is required to design a crashworthy lightweight structure. This paper presents the experimental evaluation of specific energy absorption using Charpy impact, residual compressive strength by compression after impact and damage evaluation of honeycomb sandwich structures having composite face sheets. Parametric studies on composites and honeycombs are carried out by varying the cell size, cell thickness, core height, impact velocity, thickness and orientation of lamina. Densely packed thick honeycombs provide higher fracture energy. Under transverse compressive loading, the honeycomb core undergoes cell wall buckling, crushing and densification. Load-displacement history under in-plane compression and compression after impact for different impact energies is observed. The present study contributes for the understanding how various parameters affect the characteristics of face sheet indentation and plastic buckling of honeycomb sandwich structures with composite face sheets, thereby providing useful guidelines for its potential applications in impact engineering.

Optimal design of a lightweight composite sandwich plateused for airplane containers

Composite material-due to low density-causes weight savings, which results in lower fuel consumption of transport vehicles. The aim of the research was to change the existing base-plate of the aluminum airplane container with the composite sandwich plate in order to reduce the weight of the containers of cargo aircrafts. The newly constructed sandwich plate consists of aluminum honeycomb core and composite face-sheets. The face-sheets consist of glass or carbon or hybrid fiber layers. The orientations of the fibers in the face-sheets were 0o, 90o and +-45o. Multi-objective optimization method was elaborated for the newly constructed sandwich plates. Based on the design aim, the importance of the objective functions (weight and cost of sandwich plates) was the same (50%). During the optimization nine design constraints were considered: stiffness, deflection, facing stress, core shear stress, skin stress, plate buckling, shear crimping, skin wrinkling, intracell buckling. The design variables were core thickness and number of layers of the face-sheets. During the optimization both the Weighted Normalized Method of the Excel Solver and the Genetic Algorithm Solver of Matlab software were applied. The mechanical properties of composite face-sheets were calculated by Laminator software according to the Classical Lamination Plate Theory and Tsai-Hill failure criteria. The main added-value of the study is that the multi-objective optimization method was elaborated for the newly constructed sandwich structures. It was confirmed that the optimal new composite sandwich construction-due to weight savings and lower fuel consumption of cargo aircrafts - is more advantageous than conventional all-aluminum container.

Investigation of the repair method alternatives for porous pet structures

Wind turbine systems, which are one of the renewable energy sources, convert wind energy into electrical energy, cover the blade components those are produced as sandwich composite materials for the last half century. TPI Composite, a wind turbine blade manufacturer in the renewable energy sector, has an important position in the field.
 In the sandwich composite structures, the core material providing the force transfer between the face sheets and plays an important role in increasing the bending stiffness of the sandwich structure. This non-structural material component, which is thicker but lightweight, porous and low density, is used in the wind turbine blade subcomponents.
 The main shear web (MSW), which is a subcomponent of the wind turbine composite blade and provides the mechanical strength between the two skins (suction side & pressure side) of the blade, is a composite structure with a parabolic geometric distribution produced by vacuum infusion method. Due to the geometrical shape and placement of various shaped cores of the MSW, there might occur some gaps between the foam cores placed in the MSW during the production due to vacuum. In this study, compression and bending behaviors of the foam core were investigated. Specimens with dimensions of 50x50x30 mm were prepared vacuum infusion method of core structures whose top and bottom skins covered with glass-reinforced epoxy composite face sheets. Then, some specimens were filled with core powder and core strip respectively to represent the repair method. The test specimens were subjected to flatwise compression and three-point bending tests. The performances of different gap filling repair methods were compared, and the changing performance of the damaged core specimens were investigated against specimens without any defect. To evaluate the alternative repair methods that could be applied during the production of MSW, gap filling repair alternatives, repair applicability limits and changing of the mechanical properties compared to virgin specimens were investigated for the sandwich structures with E-glass reinforced/epoxy face sheets and PET foam core.

Free vibration and stability analysis of sandwich pipe by considering porosity and graphene platelet effects on conveying fluid flow

In the present study, free vibration and stability analyses of fluid-conveyed sandwich pipe with porous core and graphene platelet reinforced composite in top and bottom layers of face sheets are investigated based on the Timoshenko beam theory. Using Hamilton’s principle, the governing equations of motion are obtained based on the first variations approach. The differential quadrature method (DQM) is used to solve to solve these equations. The effect of volume fraction of porosity in four different relationships, including nonlinear, and exponential, on the natural frequency, stability, and specific weight of the sandwich pipe is studied. Also, three void shape factors of the porous core, including circle geometry, are assumed. Moreover, five famous and popular core materials covering Steel, Aluminum, Al2O3, Cast Iron, and Polyethylene are considered. The influence of fluid flow velocity inside the pipe as an essential parameter on vibration and stability of the structure is illustrated. Three types of composite face sheets, including graphene platelet (GPL), glass fiber (GF), and three-phase (GPL + GF) as well as polymer as matrix, are investigated. Graphene Platelet as reinforcement is added to Glass Fiber for reinforcing composites factsheet layer. This research can be used to manufacture oil and gas transmission and distribution pipelines and chemical processing plants with high stiffness.

Thermo-elastic buckling of honeycomb micro plates integrated with FG-GNPs reinforced Epoxy skins with stretching effect

Halpin-Tsai and extended rule of mixture (ERM) schemes are used to evaluate the effective material properties of composite face sheets of a micro-scaled sandwich rectangular plate, to assess its thermo-elastic buckling behavior, in the current paper. For more details, it should be added that under evaluation model, which is resting on a Pasternak elastic foundation, is a honeycomb micro structure which is made of Nomex or Glass Phenolic and, to integrity, is patched with two same composite layers including graphene nanoplatelets (GNPs) dispersed through Epoxy resin. Great novelty of quasi-3D hyperbolic shear deformation theory (QHSDT) which accounts for stretching effect, virtual displacement principle, and modified couple stress theory (MCST) produced a good motivation to hire it to extract equations related to the current sandwich micro plate. Finally, variations of critical loads of thermal and mechanical buckling considered in figures and tables to examine the effect of different parameters’ changes on them. As an example, based on acquired results, the sensitivity of thermo-mechanical critical buckling loads to the internal aspect ratio of honeycomb core ϕ0 and force intensity distributions is proved where force intensity enhancement is caused critical buckling load reduction.

Finite-element analysis of EPP foam core/self-reinforced PP sandwich structures

In this study, fully recyclable and elastic–plastic deformable sandwich panels were developed by considering the mass production requirements, as well as the lightweighting needs of the automotive industry. Expanded polypropylene (EPP) foam core and self-reinforced polypropylene (PP) thermoplastic composite face sheets were selected to produce sandwich panels. Three-point bending experiments were performed to obtain the flexural behavior of the sandwich panels. Finite-element analysis (FEA) of the sandwich beams was carried out and confirmed by experimental results. Three different EPP foam cores with densities of 65.7, 36.5 and 27.0 kg/m3 were used. The FEA models were built in the Abaqus commercial software in three-dimensional (3D) space, and analyses were performed using 3D elements. The crushable foam material model was selected for FEA from the Abaqus material model database to obtain the best correlation with the foam compression test results. The force–displacement results of the three-point bending FEA results excellently correlated with the experiments. In three-point bending experiments of the sandwich panels, elastic–plastic deformation of the core with indentation failure was observed. The lowest-density foam core sandwich beam showed the highest deformation and energy absorption performance. Also, deflection recovery was observed after load removal for all sandwiches.

Experimental and numerical study of buckling behavior of foam-filled honeycomb core sandwich panels considering viscoelastic effects

Honeycomb sandwich panels are widely used in marine, aerospace, automotive and shipbuilding industries. High strength to weight and excellent energy absorption are features that make these structures unique. Foam filling the honeycomb core enhances the mechanical properties of sandwich panels considerably. In the present study, the buckling behavior of Nomex honeycomb core/glass-epoxy face sheet sandwich panel for both bare and foam-filled honeycomb core is investigated numerically and experimentally, considering the viscoelastic properties of the sandwich panel. Indeed, the viscoelastic properties of the composite face sheet and foam are determined by relaxation test and are implemented in ABAQUS using VUmat code. The finite element method is also performed using ABAQUS to model the buckling behavior of the sandwich panel incorporating both elastic and viscoelastic material behaviour. The effects of composite face sheet lay-up, core thickness, core cell size, and foam filling are also evaluated. The experimental and numerical results show that the foam increases the critical buckling load and energy absorption.

A Three-Layer Quasi-3D Finite Element Analysis for Smart Actuation on Sandwich Plates

The quasi-3D finite element model includes the smart actuation on a three-layer sandwich plate with laminated composite face-sheets. In the model, the face-sheets are represented as Reissner-Mindlin plates and the core is modeled as a three-dimensional continuum. This representation allows accurate modeling for a wide range of core types. In this model, the electrical constitutive relations of piezoelectric layers are included in the formulation of the face-sheets. In previous publications, this quasi-3D finite element formulation has demonstrated some advantages in comparison with solid finite element models. The aspect ratio of three-dimensional elements can make it rather inconvenient to use on very thin faces-sheets, which makes the number of degrees of freedom very high. Analytical through-thickness integration of the energy expressions is used to reduce the three-dimensional problem to two dimensions for the evaluation of mass and stiffness matrices. In the same way, the analytical integration of the electrical voltages work applied to the piezoelectric layers produces the piezoelectric actuation force vector. This research assesses the accuracy of the proposed model for dynamic responses of sandwich plates using a broad range of core-to-face-sheet stiffness ratio. The numerical results show that deflections promoted by the voltage applied to piezoelectric layers of the sandwich plate are very small, even if the core is very flexible. The results also indicate that the core flexibility strongly affects the natural frequencies of the higher bending modes.

Experimental and numerical study of composite sandwich panels for lightweight structural design

Composite sandwich panels are commonly considered as lightweight materials for structural design in many applications including automotive, aerospace and defence systems. These panels must be developed and validated for its mechanical strength characteristics under different loading conditions before deploying them for the practical application. Hence, the present paper deals with the experimental and numerical studies on composite sandwich panels subjected to quasi-static edgewise and flatwise compression loads. The composite face sheet was developed using 2 × 2 bi-directional 200GSM twill carbon fibre with hot cure benzoxazine resin. Sandwich panels were made using the developed composite face sheets with aluminium honeycomb core. The developed composite face sheets were tested for tensile and flexural conditions and the sandwich panels for edgewise and flatwise compression loads. Three samples for each of these tests were conducted in Shimadzu UTM with 50 kN capacity machine. A consistent load-displacement profile and the failure pattern were observed for all the samples tested in each test category. It is noted from these tests that the composite face sheets exhibited an ultimate tensile strength of 444 MPa and a flexural strength of 842 MPa which is pretty to close to the epoxy based composites used in the aerospace applications; sandwich panels showed an edgewise compressive strength of 610 MPa and flatwise compressive strength of 3.5 MPa. Tensile test coupons initially had undergone certain delamination before failure and failed close to the specimen holding end. Flexural test specimens were shown delamination of only one layer at the loading face. However, the specimen was well intact together even after failure. Bending mode of failure was noted during edgewise compression test of the sandwich; severe delamination was seen in the tension side of the sandwich whereas a line-folding crack with failure was exhibited at the mid-plane of the specimen. In case of flatwise compression, no evident of damage was observed in the sandwich structure. The second part of the paper describes the numerical simulations of these test cases including tensile, flexural, edgewise compression and flatwise compression using an explicit finite element solver viz. LS-DYNA. Material type 22 and 26 was selected to define the material behaviour of composites and honeycomb respectively. The delamination was modelled using *CONTACT_AUTOMATIC_SURFACE_SURFACE_TIEBREAK card with option 9 that works under B-K linear fracture mechanics law to define the normal and shear failure of cracks. The numerical results were found to match well with experimental observations for the individual coupons as well as the sandwich samples. Therefore, it is concluded from this study that the developed LS-DYNA simulation methodology can be used as a tool for composite sandwich panels based lightweight structural design.

Theoretical and Numerical Analysis of an Aluminum Foam Sandwich Structure

The aim of the research was to develop a new lightweight sandwich structure, which can be used for elements of air containers. The structure consists of aluminum foam core with fiber reinforced composite face-sheets. Nine different laminated glass or/and carbon fiber reinforced plastic face-sheet combinations were investigated. Finite element analysis of the sandwich structures was introduced. Single-objective optimization of the new sandwich structure was achieved for minimal weight. Five design constraints were considered: stiffness of the structure, face-sheet failure, core shear, face-sheet wrinkling, size constraints for design variables. The elaborated composite structure results significant weight savings due to low density.

Quasi-3D Hyperbolic Shear Deformation Theory for the Free Vibration Study of Honeycomb Microplates with Graphene Nanoplatelets-Reinforced Epoxy Skins.

A novel quasi-3D hyperbolic shear deformation theory (QHSDT) with five unknowns is here employed, together with the Hamilton’s principle and the modified couple stress theory (MCST) to analyze the vibrational behavior of rectangular micro-scale sandwich plates resting on a visco-Pasternak foundation. The sandwich structure features a Nomex or Glass phenolic honeycomb core, and two composite face sheets reinforced with graphene nanoplatelets (GPLs). The effective properties of both face sheets are evaluated by means of the Halpin-Tsai and extended rule of mixture (ERM) micromechanical schemes. The governing equations of the problem are derived by applying the Hamilton’s principle, whose solutions are determined theoretically according to a classical Navier-type procedure. A parametric study checks for the effect of different material properties, length-scale parameters, foundation parameters and geometrical properties of the honeycomb cells, and the reinforcing GPLs, on the vibration response of the layered structure, which can be of great interest for many modern engineering applications and their optimization design.

Fabrication and mechanical characterization of CFRP X-core sandwich panels

A hot-press molding technique is developed to design and manufacture carbon fiber reinforced polymer (CFRP) X-core sandwich panels that allowed both the composite face sheets and its X-core to be simultaneously molded together. Compression tests were performed on the sandwich panels to assess the effects of relative density on its compressive properties and the failure mode that it develops. Analytical models were developed to predict the panel stiffness and strength; and a failure map of its deformation mode is generated based on the geometric parameters of the unit cell. Linear and nonlinear finite element analyses were performed to further investigate the effects of initial geometric imperfection and progressive damage on the response of the sandwich panels with different cell wall slenderness. Predictions from analytical and numerical models will be shown to be in good agreement with the experiments. The CFRP X-core sandwich panels will be shown to attain good compressive properties that are comparable to typical high strength cellular materials, which will be potentially useful for lightweighting applications.

Semi-numerical simulation for vibrational responses of the viscoelastic imperfect annular system with honeycomb core under residual pressure

This composition investigates the frequency analysis of sandwich imperfect viscoelastic disks with graphene nano-platelets (GPLs)-reinforced viscoelastic composite (GPLRVC) face sheets and honeycomb core. The honeycomb core is made of aluminum because of its high stiffness and low weight. The modified Halpin-Tsai model and rule of the mixture have been utilized to provide the effective material constant of the composite layers. Through employing Hamilton's principle, the governing equations of the structure are accordingly discerned and resolved by utilizing the Generalized Differential Quadrature Method (GDQM). Throughout this investigation, viscoelastic properties have been modeled in accordance with Kelvin-Voigt viscoelasticity. The deflection as the function of time is capable of being resolved through employing the fourth-order Runge-Kutta numerical method. Afterwards, a parametric study is conducted to discern the effects of the FG patterns, outer to inner radius ratio, hexagonal core angle, thickness to length ratio of the GPLs, the weight fraction of GPLs, FG face sheet thickness ratio, the thickness of honeycomb core to inner radius ratio, tensile, imperfect coefficient, and in-plane force on the frequency of the sandwich viscoelastic disk with honeycomb core and FG-GPLRVC face sheet.

Buckling and free vibration analyses of a sandwich beam made of a soft core with FG-GNPs reinforced composite face-sheets using Ritz Method

The present paper studies buckling and free vibration analyses of sandwich beam. The sandwich beam is composed of a soft core integrated with functionally graded graphene nanoplatelets reinforced composite face sheets. Kinematic relations are developed based on Extended Higher-Order Sandwich Beam Theory (EHSPT). The governing equations are derived using Ritz-Lagrange formulation. The effective mechanical properties of epoxy/GPLs composites are obtained from the Halpin-Tsai micro-mechanical model and rule of mixture. The numerical results are obtained using the Ritz Method. The natural frequencies and buckling loads are obtained in terms of weight fraction and distribution of graphene nanoplatelets, length to thickness ratio, core to surface ratio, and different boundary conditions. Before presentation of complete numerical results, a comparative study is presented to check trueness and correctness of the results. It is concluded that maximum and minimum natural frequencies and critical buckling loads of sandwich nanobeam are obtained for FG-X and FG-O distributions, respectively.

Free vibration of rotating pretwisted FG-GRC sandwich conical shells in thermal environment using HSDT

The free vibration behavior of rotating pretwisted sandwich conical shell panels with functionally graded graphene-reinforced composite (FG-GRC) face sheets and homogenous core is investigated in uniform thermal environment using finite element method in conjunction with a higher-order shear deformation theory (HSDT). The volume fraction distribution of the graphene in the face sheets is considered to be uniform or layer-wise functionally graded across its thickness. The temperature-dependent effective elastic moduli of the FG-GRC face sheets are computed employing the refined Halpin-Tsai model, while the effective Poison’s ratio, density, and thermal expansion coefficients are obtained using the mixture-rule. The effects of non-linearity arising out of the initial stresses due to thermal and centrifugal loads are incorporated via geometric stiffness based on Green-Lagrange’s strains. The governing equations for sandwich conical shell panels are derived using Lagrange’s equation of motion at moderate rotational speeds. The influence of graphene distribution patterns on the fundamental frequencies of FG-GRC sandwich conical shells under different thermal conditions is studied with emphasis on triggering parameters like pretwist angle, cone length-to-thickness ratio, core-to-face sheets thickness ratio, and dimensionless rotational speed.

Effects of stitching parameters on the mechanical properties of stitched sandwich panel structures

This paper studied the mechanical performances of a novel stitched sandwich panel structure (SSPS), which contains two Al2O3/Al2O3 woven composite face-sheets, an aerogel heat-insulation core, and several mullite z-pin fiber yarns. Four types of experiments were conducted, including out-of-plane compression and tension, interlaminar shear, and three-point bending, to reveal the effects of stitching density, stitching yarn twist, and strands on the mechanical performances of the SSPS. Experimental results indicated that the stitching parameters have significant effects on the mechanical properties of the SSPS, in particular, the strengths and failure features. This study indicated that perforation and stitching are convenient and efficient approaches to promote the performances of panel-core-panel sandwich structures.

Thermal response and edgewise compression failure of thermally degraded sandwich composite structures

Manufacturing and repair tasks using additive and subtractive laser processing (e.g., paint stripping and drilling) are becoming widely used in ships, aircrafts, and bridge structures. As with fire and exhaust impingement, such low level heat fluxes (<10 W) could negatively impact mechanical properties of composite structures. In this work, thermal excursions were simulated via continuous-wave laser exposure of the center region of a composite sandwich construction (i.e., E-glass fiber-reinforced vinyl ester matrix composite facesheets with a PVC foam core). Sandwich specimens were investigated with different slenderness ratios, facesheet thicknesses, foam core thicknesses and type of core materials. The 3-D temperature history response was measured with the spatially calibrated IR camera technology to map out the resulting damage state across the thickness of the specimens. The morphology of the foam core damage and interfacial debonding were postulated with the temperature profiles and evidenced by the X-ray CT images. Depending on the slenderness ratio and the volume of material heated above Tg, the thermally-induced damage significantly affects the edgewise compression strengths and failure modes.

Computational analysis of failure mechanisms in composite sandwich space structures subject to cyclic thermal loading

Accurate prediction of the thermal stability and durability of composite sandwich materials used in space structures remains a challenge due to the variability of thermal related material coefficients at different temperatures and the extensive use of adhesively bonded joint fittings. This paper presents an investigation into the thermomechanical performance of complex honeycomb core composite sandwich structures with bonded fittings exposed to extreme temperature ranges. First, detailed analyses are conducted to investigate the failure and predict the intrinsic nonlinear material properties of a specific aerospace-grade aluminum honeycomb core and carbon-epoxy composite face sheet. Sandwich panels with two types of bonded fittings are chosen to investigate nonlinear effects such as shear buckling and plastic deformation in the honeycomb core, and the progressive damage response in composite skins under different loading conditions. The nonlinear properties and modeling strategies are then incorporated in the subsequent analysis of a spacecraft’s thrust tube sandwich structure subject to cyclic thermal loading. The predicted damage and failure in the thrust tube section with an adhesively bonded cup fitting correlated with experimental observations. The current methodology offers a consistent technique for investigating the thermomechanical performance of sandwich structures with other configurations of bonded joints and fittings.