Multiscale modelling of the composite reinforced foam core of a 3D sandwich structure

A key objective dealing with 3D sandwich structures is to maximize the through-thickness stiffness, the strength of the core and the core to faces adhesion. The Napco^® technology was especially designed for improving such material properties and is under investigation in this paper. In particular, the potential of the process is characterized using a micromechanical modelling combined to a parametric probabilistic model. An experimental analysis is further detailed and validates the theoretical estimates of the core-related elastic properties. It is readily shown that the technology is able to produce parts with significantly improved mechanical properties. Finally, thanks to the probabilistic aspect of the modelling, the study allows to establish a link between the randomness of the process and the uncertainties of the final mechanical properties. Thus, the present approach can be used to optimize the technology as well as to properly design structures.     

A novel carbon fiber reinforced lattice truss sandwich cylinder: Fabrication and experiments

To get a strong, stiff and weight efficient cylindrical shell, a novel carbon fiber reinforced corrugated lattice truss-core sandwich cylinder (LTSC) was designed and fabricated. The core is made up of orthogonal corrugated trusses and manufactured by mould pressing method. The LTSC is fabricated by filament winding and co-curing method. The face sheets have layups of [0°/30°/−30°/−30°/30°/0°] to improve the fundamental frequency as it is controlled by the circumferential stiffness. In end-free vibration the fundamental frequency of the LTSC is 112.18 Hz, higher than the referenced quasi-isotropic Isogrid-core sandwich cylinder. Determined by the skin fracture, the compression strength of the LTSC is 328.03 kN, stronger than the referenced Isogrid-core sandwich cylinder failed at rib buckling and the post-failure deformation is ductile. According to the optimization scheme jointly constrained by the strength and the fundamental frequency, an ultra-light and strong cylinder with high fundamental frequency was successfully fabricated.     

The effect of temperature on the bending properties and failure mechanism of composite truss core sandwich structures

The effects of temperature on the bending properties and failure mechanism of carbon fiber reinforced polymer composite sandwich structure with pyramidal truss cores were investigated and presented in this paper. The three-point bending tests of composite sandwich structures were performed at seven different temperatures, and the scanning electron microscope was used to examine the fiber-matrix interface properties in order to understand the deformation and failure mechanism. Then the effects of temperature on deformation modes, failure mechanism and bending failure load were studied and analyzed. The results showed that the temperature has visible impact on the deformation modes, failure mechanism, and bending failure load. The bending failure load decreased as temperature increased, which was caused by the degradation of the matrix properties and fiber-matrix interface properties at high temperature. The analytical formulae were also presented to predict the bending stiffness and failure load of composite sandwich structures at different temperatures.     

Optimisation of sandwich panels with functionally graded core and faces

The distributions of properties across the thickness (core) and in the plane (face sheets) that minimise the interlaminar stresses at the interface with the core are determined solving the Euler–Lagrange equations of an optimisation problem in which the membrane and transverse shear energy contributions are made stationary. The bending stiffness is maximised, while the energy due to interlaminar stresses is minimised. As structural model, a refined zig-zag model with a high-order variation of displacements is employed. Simplified, sub-optimal distributions obtainable with current manufacturing processes appear effective for reducing the critical interfacial stress concentration, as shown by the numerical applications.     

Homogenization of the core layer in stitched sandwich structures

After highlighting the improvement of the mechanical performances involved by transverse reinforcement implementation in previous several studies, the mechanical behavior of stitched sandwich structures is analytically approached in this paper. The final purpose of this work is a modeling of the elastic performances of these structures. To predict the in-plane behavior, the classical theory of sandwiches is adapted and used by treating the foam core strengthened by stitches as a homogenized volume. This approach leads to the creation of an orthotropic equivalent core material. Its elastic properties depend on each component and their volume participation. The comparison between simulated and experimental values is quite good. The main interest of the multi-scale approach concerns a predictive tool. Indeed, it becomes realistic to obtain the elastic properties of stitched sandwich according to the geometrical parameters of the stitches and the mechanical properties of the components.     

Simple stiffness tailoring of balsa sandwich core material

A concept for improving the shear stiffness properties of balsa core material for sandwich structures is presented. The concept is based on utilization of the strongly orthotropic properties of the balsa wood, applying an appropriate transverse layup sequence. The effective core material shear modulus is modeled using basic laminate theory. This is subsequently validated through sandwich beam bending and lap shear experiments. Compared to the standard balsa core systems, a substantial increase in the shear stiffness is demonstrated, whereas the transverse stiffness is reduced. The concept is suitable for mass production, using standard plywood fabrication technology.     

Experimental investigation of local bending effects in the vicinity of a junction between a straight sandwich beam and a curved sandwich beam

The junction between a curved and a straight sandwich beam is investigated experimentally using electronic speckle pattern interferometry. This technique facilitates a whole field measurement of the displacements through the thickness of the sandwich beam. The experimental results are compared with results obtained using a high order sandwich theory model. The results generally show good agreement within the accuracy of the measurements, thus indicating that the gross response of the model is predicted accurately by the high order sandwich theory, while the localised bending effects in the vicinity of curvature change in sandwich panels have not been verified experimentally.     

A wire-woven cellular metal: Part-I, optimal design for applications as sandwich core

Wire-woven Bulk Kagome (WBK) is a new truss type cellular metal fabricated by systematic assembling of helical wires in six directions. WBK looks promising with respect to morphology, fabrication cost, and raw materials. In this paper, first, the geometry and the effect of the geometry such as the curved shape of the struts, which compose the truss structure of WBK, are elaborated. Then, analytic solutions for the material properties of WBK and the maximum loads withstood by a WBK-cored sandwich panel under bending are derived. Design optimization is carried out in two ways: one is based on the weight of the sandwich panel, and the other is based on the slenderness ratio of the WBK core. The performance of the WBK is evaluated and compared with those of other periodic cellular metals. With designs fully optimized with respect to the first way mentioned, the WBK-cored panel outperformed the octet counter part. With a specified constraint on the core thickness, the WBK truss core panel performed as well as a honeycomb cored panel.     

The effect of pin reinforcement upon the through-thickness compressive strength of foam-cored sandwich panels

Titanium and carbon fibre pins have been inserted into the polymethacrylimide foam core of a sandwich panel (with carbon fibre face sheets) in order to increase the through-thickness strength. The elevation in compressive strength has been measured both quasi-statically and dynamically using a direct Kolsky bar, and the sensitivity of strength to the relative density and thickness of foam have been determined. An X-ray CAT scan machine was used to examine the deformed shape of the pins during interrupted compression testing of the sandwich specimens. It was found that the foam core stabilises the pins against elastic buckling, and the pin-reinforced core has a strength and energy absorption capacity in excess of the individual contributions from the foam and unsupported pins. It is shown that the compressive strength is governed by elastic buckling of the pins, with the foam core behaving as an elastic Winkler foundation in supporting the pins. The peak strength of the pin-reinforced core is increased by a factor of about four when the speed of loading is increased from the quasi-static rate of about 10⁻⁶ ms⁻¹ to the dynamic value of 10 ms⁻¹; it is concluded that the micro-inertia of the pins stabilises them against elastic buckling and leads to the observed elevation in strength.     

Manufacturing and testing of a CFRC sandwich cylinder with Kagome cores

Filament winding and twice co-curing processes were proposed to make a carbon fiber reinforced composite (CFRC) sandwich cylinder with Kagome cores. Axial compression was carried out to reveal the stiffness and load capacity of the fabricated sandwich cylinder. Compared with the stiffened cylinder with similar dimensions and mass, the sandwich cylinder is shown to be stiffer and stronger by several times. Restrained by the double skins, the buckling of lattice ribs is effectively suppressed, leading to the elimination of the dominant failure mode. Skin crippling and strength failure were observed in the testing to be the competing failure mechanisms of the lattice sandwich cylinder. The prediction based on the weakest link of these mechanisms only overestimates the testing load capacity by 30%. The novel sandwich structure shows the promise of lattice composites in upgrading the mechanical properties.     

Response of sandwich composites with nanophased cores under flexural loading

It is well known that in a sandwich structure, the core plays an important role in enhancing the flexural rigidity and by controlling the failure mechanisms. If the core is made from foam, the strength of the core material and the debond strength at the core–skin interface almost entirely dictate the performance of structural sandwich composites especially under flexure. In this investigation attempts have been made to improve the performance of the sandwich by strengthening the core but partially sacrificing the debond fracture toughness of the sandwich construction. Strengthening of the core has been accomplished by infusing nanoparticles into the parent polymer of the core material when it was in the liquid stage. The core material is polyurethane foam made from polymeric isocyanate (Part A) and reacting with polyol (Part B). Spherical nanoparticles such as TiO₂ of about 29 nm in diameters were dispersed in Part A of liquid polyurethane through an ultrasonic cavitation process. The amount of nanoparticles infused into liquid foam varied from 1 to 3% by weight. Once Part A was doped with nanoparticles, it was mixed with Part B, and was cast in a rectangular mold to produce the nanophased polyurethane foam. The nanophased foam was then used with regular S-2 Glass fiber preforms and SC-15 epoxy to manufacture sandwich composites in a VARTM set up. Test coupons were then extracted from foam as well as from sandwich panels to conduct flexural and various other chemical tests. A parallel set of control panels were also made with neat polyurethane core materials. Thermogravimetric and SEM analyses have indicated that the decomposition temperature of the nanophased foams increases by about 27 °C and the cell size almost doubles with nanoparticle infusion. A significant improvement in flexural strength and stiffness has also been observed with 3% loading of TiO₂ nanoparticles. Debond fracture toughness parameters (G_c) were also determined for both categories of sandwich constructions, and it was seen that nanoparticle infusion reduces the value of G_c by almost a factor of three. Despite this reduction, strength of nanophased sandwich increased by about 53% over the neat system. Details of manufacturing and analyses of test results are included in the paper.     

Transverse shear modulus of SILICOMB cellular structures

This work describes the transverse shear stiffness properties of a novel honeycomb with zero Poisson’s ratio. The cellular configuration is simulated using a series of finite element models representing full-scale and representative unit cells of the honeycomb topology. The models are benchmarked against experimental results from pure shear and 3-point bending ASTM tests. The benchmarked models are used to perform a parametric study of the shear moduli (G₁₃ and G₂₃) against the geometry of the unit cell and the gauge thickness of the honeycomb panels. The shear stiffness maps obtained allow comparison of the SILICOMB configuration against classical centresymmetric and rectangular honeycomb topologies.     

Hybrid processing of thick skins for honeycomb sandwich structures

Carbon-epoxy prepregs are generally used to form the skins of honeycomb sandwich structures used in aerospace or racing yachts. For some applications, it is desirable to increase the thickness of the skins. In order to achieve an ideal core pressure level during cure for maximal skin-core bonding, the issue of air extraction from the honeycomb cells through the skin during processing thus becomes critical, in particular if vacuum only processing is used. In the present work, partially impregnated prepregs, called semipregs, having high initial transverse permeability to air, are combined with traditional prepregs to form a hybrid skin. Results are presented on the pressure change inside the honeycomb cells and the skin permeability to air during cure, as well as on skin-core adhesion. The final sandwich quality is assessed and found to be comparable to that obtained with prepreg skins.     

Enhancement of through-thickness thermal conductivity of sandwich construction using carbon foam

As a thermal management system, a sandwich construction was developed to have both superior thermal conductivity and structural integrity. The sandwich construction consists of a carbon foam core and unidirectional graphite/epoxy composite facesheets. An emphasis was put on enhancing the thermal conductivity of each phase of sandwich construction as well as interface between the phases. A commercially-available carbon foam was characterized mechanically and thermally. Property variation and anisotropy were observed with the highly conductive graphitic carbon foam. Co-curing of the composite facesheets with the carbon foam core was demonstrated to minimize the thickness of the adhesive layer between the facesheets and the core to produce the best construction of those tested. Comparison made with an adhesively bonded specimen shows that the co-curing is a more efficient method to enhance the through-thickness conductivity. Parametric studies with an analytic model indicate that degree of enhancement in the overall through-thickness conductivity of the sandwich construction from the enhancement of each component including the foam core, facesheet and the bonding methods.     

Aramid/epoxy composites sandwich structures for low-observable radomes

Low-observable radomes are usually made of E-glass/epoxy composite due to its low dielectric constant which is necessary not to interfere electromagnetic (EM) wave transmission characteristics. Since aramid fibers have lower dielectric constant and higher strength than those of E-glass fiber, aramid fiber radome structures may have better the EM transmission and mechanical characteristics than those of E-glass/epoxy radomes. In this work, the low-observable radome was constructed with a sandwich construction composed of aramid/epoxy composites faces, foam core and Frequency Selective Surface (FSS) which had the abilities of transmitting EM waves selectively in the X-band range. The EM wave transmission characteristics of the low-observable radome were simulated by a 3-dimensional electromagnetic analysis software and also measured by the free space measurement method with respect to the pattern size of FSS and foam cores. The mechanical properties of the low-observable radome made of aramid/epoxy composite were measured by the 3-point bending test and compared to those of the conventional low-observable radome made of E-glass/epoxy composite.     

Experimental validation of modal strain energies based damage identification method for a composite sandwich beam

A vibration-based damage identification method, based on changes in modal strain energies before and after occurrence of damage, is presented for a composite sandwich beam. Experiments were performed to obtain the natural frequencies and mode shapes for validation of the presented method. The observed changes in modal strain energies were used for the prediction of existence and location of damage in a composite sandwich beam. Subsequently, these changes were also used to predict damage extents in the two stages. In the first stage, the proposed method is used to approximate the damage extents in the face and core. These were updated to obtain more accurate values of damage extent at the second stage by using the model updating theory. Experimental and numerical results were presented to demonstrate and verify the effectiveness of this method for several single and multiple damage cases. These damage cases also include interactive damage modes. Results indicate that the proposed method is capable of identifying the location and extent of damage in the faces and core of a composite sandwich beam.     

Non-linear finite element analysis of inserts in composite sandwich structures

In aeronautics, sandwich structures are widely used for secondary structures like flaps, landing gear doors or commercial equipment. The technologies used to join these kinds of structures are numerous: direct bonding or joining, tapered areas, T-joints, etc. The most common is certainly the use of local reinforcement called an insert. The insert technologies are numerous and this study focuses on high load bearing capacity inserts. They were made with a resin moulded in the Nomex™ sandwich core. Such structures are still designed mainly empirically and the lack of efficient numerical models remains a problem. In this study, pull-out tests were conducted on a representative sample and the non-linearities and the types of failure were analysed. Core shear bucking, failures of the potting and perforation of the composites skins are the main modes of failure. For each mode, local experimental and numerical analysis was carried out that led to the identification of the independent non-linear behaviour of each component. Including the results in a global non-linear finite element model gave good prediction of the failure scenario and an acceptable correlation with the tests.     

Sensitivity of structurally loaded sandwich panels to localized ballistic penetration

This paper describes a series of tests focused on the combination of structural loading (bending, shear) and simultaneous penetrating impact on sandwich panels with thin GFRP face-sheets, with emphasis on the specific damage morphologies and developments depending on the type and magnitude of structural loading. The test specimens were sandwich panels, length 250 mm and width 150 mm, with carbon fibre prepreg face-sheets ([0°/90°], thickness t_f ≅ 0.5 mm) bonded to the faces of a foam core (density 80 kg/m³, thickness H = 10 mm). The impact velocity was approximately 420 m/s, using a spherical steel impactor, diameter 10 mm, with a mass of 4.1 g. A high-speed camera was used for registration of panel response. It was demonstrated, that, at preload levels above a specific limit, the impact would cause catastrophic failure, i.e., complete or near-complete loss of structural load carrying capacity. Developments of failure morphology, consistent with the observed evidence, were derived and outlined.     

Quasi-static and dynamic fracture behavior of composite sandwich beams with a viscoelastic interface crack

Fracture analysis of sandwich beams with a viscoelastic interface crack under quasi-static and dynamic loading has been studied. Firstly, a three-parameter standard solid material model was employed to describe the viscoelasticity of the adhesive layer. And a novel interfacial fracture analysis model called three material media model was established, in which an interface crack was inserted in the viscoelastic layer. Secondly, a finite element procedure based on Rice J-integral and Kishimoto J-integral theories was used to analyze quasi-static and dynamic interface fracture behavior of the sandwich beam, respectively. Finally, the influence of viscoelastic adhesive layer on the quasi-static J-integral was discussed. In addition, comparison of quasi-static Rice J-integral with Kishimoto J-integral under various loading rates was carried out. The numerical results show that the oscillating characteristic of dynamic J-integral is more evident with shorter loading rise time.