Paper-reinforced biomimetic cellular structures for automotive applications

The aim of this work was to make and test some sandwich structures, made of biomimetic cellular cores of recycled paper. In a preliminary study some paper laminates were investigated. Then two different kinds of cores were made by simple processes, according to two natural structures (i.e. feather and honeycomb). To evaluate the mechanical properties of these new sandwiches, static flexural, flatwise and edgewise compression tests were performed.     

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.     

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.     

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.     

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.     

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.     

Accurate buckling load calculations of a thick orthotropic sandwich panel

This paper is concerned with the buckling of thick sandwich panels with orthotropic elastic face sheets bonded to a linear elastic orthotropic core. When such panels are analyzed for axial load carrying capacity, it is now commonplace to adopt the finite element method to carry out computations. The accuracy of the numerical results will depend not only on roundoff and algorithmic errors, but additionally on the approximations made in computing the incremental (second order) work associated in computing the change of configuration from the unbuckled to the buckled state. Here we show that, particularly for orthotropic thick sandwich structures, large errors can be incurred in computing buckling loads with available commercial software, unless the proper work conjugate measures of stress and strain with their stress-dependent tangential moduli are used in the buckling formulation.     

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.     

Effect of core thickness on wave number and damping properties in sandwich composites

Due to their higher strength-to-weight and stiffness-to-weight ratios compared to metals, fiber reinforced composite materials are a great alternative for use in many structural applications. However these properties lead to poor acoustic performance as composite materials are excellent noise radiators. This is particularly true for sandwich composite structures. Therefore the focus of this study is to investigate the effect of a core thickness change on the vibrational properties of Rohacell foam/carbon-fiber face sheet sandwich composite beams. Four different foam core thicknesses were explored, using a combination of experimental and analytical methods to characterize sound and vibrational properties of the sandwich beams. First, the wave number responses of the beams were obtained, from which coincidence frequencies were identified. Second, from the frequency response functions the structural damping loss factor, η, was determined using the half-power bandwidth method. Experimental and analytical results show that the relationship between core thickness and coincidence frequency is non-linear. A drastic increase in coincidence frequency was observed for the sandwich beam with the thinnest core thickness due to the low bending stiffness. Moreover this low bending stiffness results in low damping values, and consequently high wave number amplitude responses at low frequency ranges (<1000 Hz).     

Examination of the failure of sandwich beams with core junctions subjected to in-plane tensile loading

The paper concerns local effects occurring in the vicinity of junctions between different cores in sandwich beams subjected to tensile in-plane loading. It is known from analytical and numerical modelling that these effects display themselves by an increase of the bending stresses in the faces as well as the core shear and transverse normal stresses at the junction. The local effects have been studied experimentally to assess the influence on the failure behaviour both under quasi-static and fatigue loading conditions. Typical sandwich beam configurations with aluminium and glass-fibre reinforced plastic (GFRP) face sheets and core junctions between polymer foams of different densities and rigid plywood or aluminium were investigated. Depending on the material configuration of the sandwich beam, premature failure accumulating at the core junction was observed for quasi-static and/or fatigue loading conditions. Using Aluminium face sheets, quasi-static loading caused failure at the core junction, whereas no significance of the junction was observed for fatigue loading. Using GFRP faces, a shift of the failure mode from premature core failure in quasi-static tests to face failure at the core junction in fatigue tests was observed. In addition to the failure tests, the sandwich configurations have been analysed using finite element modelling (FEM) to elaborate on the experimental results with respect to failure prediction. Both linear modelling and nonlinear modelling including nonlinear material behaviour (plasticity) was used. Comparing the results from finite element modelling with the failure behaviour observed in the quasi-static tests, it was found that a combination of linear finite element modelling and a point stress criterion to evaluate the stresses at the core junction can be used for brittle core material constituents. However, this is generally not sufficient to predict the failure modes and failure loads properly. Using nonlinear material properties in the modelling and a point strain criterion improves the failure prediction especially for ductile materials, but this has to be examined further along with other failure criteria.     

Enhanced mechanical performance of foam core sandwich composites with through the thickness reinforced core

The purpose of this study is to improve the mechanical performance of the foam core sandwich composites with a rather simpler method of core reinforcement. With this aim; sandwich composite panels are manufactured using only-perforated foam and perforated-stitched foam as the core with multi-axial glass fabrics as the facesheet materials by vacuum infusion method using epoxy resin. Sandwich composites with perforated core, stitched core and plain core have been compared in terms of compressive, bending, shear and impact performances. It was seen that newly proposed perforated core specimens and stitched core specimens with relatively insignificant weight increase have superior mechanical performances than plain core specimens. Thus reinforcing foam core with perforation and stitching is proposed as simpler but very effective method in performance improvement for the sandwich composites.     

Crack deflection analyses of different peel stopper designs for sandwich structures

This paper relates to a newly developed peel stopper concept for sandwich structures. The proposed concept is a specially designed core insert, which has the ability to confine face sheet debonding/delamination (peeling) by deflecting a delamination crack front away from the face/core interface into the bulk of the sandwich core, and thereby constraining the debonding/delamination to a limited prescribed area. In this paper various peel stopper designs are analysed for their ability to deflect cracks away from propagating along a face–core interface. The crack deflection ability of the studied peel stopper designs leads to design guidelines, which describes the minimum requirements regarding the relation between the two interface toughnesses. The analysis further reveals that compliant peel stopper wedges are preferred because they lead to the lowest interface toughness ratio requirement. This has been confirmed through an experiment with a sandwich beam subjected to three-point bending loading. The experiment has shown that the ability of a peel stopper to deflect cracks is highly dependent on the stiffness of the wedge.     

Multiscale approach for the design of composite sandwich structures for train application

In the present work a multiscale approach is considered for the design of composite sandwich structures for a roof of railway vehicle. The procedure consists in different steps that start from cost/benefit analysis on materials and their manufacturing process and cycle up to analysis of sub-components and entire structures. Each step is characterized by experimental, theoretical and numerical studies. The design activities herein presented count experimental campaigns able to characterize both the properties of novel sandwich material, manufactured expressly for transportation industry, the sandwich and joint behaviors. Analytical and numerical approaches have been used to validate and optimize the structural layout. Finite element analysis has been also performed on a test article to verify the “new” sandwich roof in regard to structural requirements suggested by European Code.     

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.     

On the free vibration of sandwich panels with a transversely flexible and temperature dependent core material – Part II: Numerical study

This paper, the second of two, presents a numerical study of a simply-supported sandwich panel that is based on the mathematical formulation that appears in part I. The solution of the unknowns in the case of a simply-supported panel is based on a trigonometric series solution and it converts the set of PDE’s into an algebraic set of equations that are described by stiffness and mass matrices. In addition, it studies numerically, for a specific sandwich panel construction, the effects of the degradation of the mechanical properties of the core as a result of the thermal field on the free vibration response of the two computational models. The results of the mixed formulation model, denoted by MF, and the displacement formulation, denoted by DF, reveal a significant reduction of the eigenfrequencies as well as a shifting of the eigen-modes from higher modes to lower ones with increasing temperature.     

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.     

Composite structure composed of overlapped fibre bundles,thermoplastic resin and metal plate for secondary forming process

A simple shape of a composite is preferable in mass production, while a curved or stretched shape is sometimes preferable for final products. High formability would enable the composite to deform into a preferable shape by secondary forming. In this paper, a structure for a composite is proposed to enhance formability. The composite is composed of reinforcing fibre bundles, thermoplastic resin as the matrix and metal plates. The reinforcing fibre bundles are discontinuous, and are intentionally overlapped in the longitudinal direction. The resin including fibre bundles is sandwiched between the metal plates. As the thermoplastic resin is melted at an adequate temperature, heating would enhance the mobility of thermoplastic resin resulting in high formability at the secondary forming. If the overlapped length is adequately designed, the composite would still maintain high strength after the secondary formation. The validity of the concept was checked by finite element analyses and experiments.     

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.     

New peel stopper concept for sandwich structures

The paper addresses the damage tolerance of sandwich structures, where the prevention and limitation of delamination failure are highly important design issues. Due to the layered composition of sandwich structures, face–core interface delamination is a commonly observed failure mode, often referred to as peeling failure. Peeling between the sandwich face sheets and the core material drastically diminishes the structural integrity of the structure. This paper presents a new peel stopper concept for sandwich structures. Its purpose is to effectively stop the development of debonding/delamination by rerouting the delamination, and to confine it to a predefined zone in the sandwich structure. The suggested design was experimentally tested for different material compositions of sandwich beams subjected to three-point bending loading. For all the tested sandwich configurations the suggested peel stopper was able to stop face–core delamination and to limit the delamination damage to restricted zones.     

A novel method for producing of steel tubes with Al foam core

In this study, the aluminum foam filling steel tube was produced by powder metallurgy and cold welding process. By this method, Al powder mixed with 0.6 wt.% TiH₂ powder and then pressed into the steel tube. This filled tube was treated in temperature above aluminum melting point for releasing the hydrogen gas by decomposition of TiH₂ particles for providing of foam production conditions. For the first time, the steel tube with Al foam core produced by this method, without using of binder or fitting. Main advantage of these filled tubes is high energy absorption. Energy absorption is very useful in automobile and railway industry. Lightweight is another advantage of these tubes for these applications. It is found that the Al foam filling steel tubes absorb higher energy with respect to the sum of the energy absorptions of the steel tube and aluminum foam alone.