Flat Sandwich Panels Research Articles

Variable kinematics finite plate elements for the buckling analysis of sandwich composite panels

This paper extends sublaminate-based variable kinematics plate finite elements towards the buckling analysis of composite structures. Robust locking-free 4-node and 8-node interpolations are used to build the finite element matrices in terms of fundamental nuclei, invariant with respect to the employed kinematic model of the composite plate. Geometric nonlinearities are accounted for in the von Kármán sense. The classical linearized stability analysis is mainly conducted for sandwich panels, for which different kinematic assumptions are employed for the skins and the core. Global buckling of the sandwich panel as well as short-wavelength wrinkling of the skins are investigated by referring to a variety of case studies, including homogeneous and laminated skins as well as isotropic and orthotropic cores. Convergence studies are performed to establish the minimum number of elements for the local instabilities to be grasped. The proposed computational framework requires a simple 2D mesh and is capable of providing quasi-3D response patterns. This is as demonstrated by numerous case studies that address the transition from global to local buckling, the onset of wrinkling in flat sandwich panels with anisotropic skins under various loading conditions as well as the local face sheet instability occurring in sandwich panels working in bending. It is concluded that he proposed FEM-based tool is computationally efficient and can be advantageously employed in pre-sizing design phases without resorting to full 3D models.

Curved grid‐scored foam core sandwich composites under low‐velocity impact loads: Effects of curvature, stacking sequence, and core finishing

AbstractThe objective of this paper is to experimentally investigate how material and geometric parameters, including face sheet layup, curvature, and core finishing, affect the contact force‐displacement behavior, absorbed energy, and damage modes of curved sandwich composites with grid‐scored foam under low‐velocity impact loads. An instrumented drop‐weight testing machine with a hemispherical impactor was used to perform low‐velocity impact tests at different energy levels. Angled‐ply stacking had the maximum contact force and impact resistance for low curvature radii. In the event of penetration, the impact energy absorption of all curved sandwich panels increased as the curvature increased. The core finishing influenced the contact forces of the curved and flat sandwich panels but not the penetration energy. The curvature and face sheet layups affected the delamination size and pattern. Angle‐ply and cross‐ply face sheets with the smallest and maximum radius of curvature, respectively, decreased penetration depth. The resin‐filled channels reduced foam cell impact damage and face‐core debonding. LVI penetration resistance was found to be highest for angle‐ply stacking and knife‐cut foam at small curvature radii. The proposed research will also provide naval designers with valuable insight into the impact response of sandwich structures used in curved parts of the hull and deck.Highlights The effect of curvature on the LVI responses varied due to face sheet layups. Core finishing affected contact forces, but not the absorbed energy. Curvature and face sheet layups affected the damage modes. Resin‐filled channels limited damage area and acted as stoppers for debonding.

Design analysis and testing of flat sandwich panels under bird strike

A non-linear energy-based analytical approach to design flat sandwich panels resistant to bird strike is presented. The approach is then complemented by numerical simulation in Abaqus using smooth particle hydrodynamics to model the bird and a non-linear stress-strain model for the core material. Flat sandwich panels were designed to deform and just fail when the maximum deflections are reached for given strike energies. The panels designed with this approach were tested using gelatin birds and two different material combinations with non-toughened and toughened facesheet and core materials. The analytical and numerical approaches were found to be conservative as they predicted failure onset for the bird energies selected while the tests showed no damage. The maximum deflection and maximum strains at different locations of the panels were well predicted by the numerical analysis, but the predictions departed significantly from the tests after the first peak was reached.

Process induced deformations in composite sandwich panels using an in-homogeneous layup design

This work presents an experimental and numerical study of process-induced deformation (PID) in flat sandwich panels with in-homogeneous ply layups. An over-expanded Nomex honeycomb core and aerospace-grade prepreg was used to manufacture sandwich panels using the autoclave technique. Process-induced spring-in and warpage at different locations were measured. A 3D FE analysis was performed for the full cure cycle. The FE model successfully captured the shrinkage due to cure, anisotropic and thermo-physical properties of the sandwich structure and tool-part interactions. The computed maximum spring-in values obtained from the FE model were in good agreement with average spring-in values measured experimentally. An overall error of less than 2% was found in the PID of the sandwich panel. The effect of tooling material and core material on PID was also investigated. The numerical results suggest that an aluminum tool and an aluminum core could potentially reduce warpage in the sandwich panel studied.

Optimized process chain for flexible and automated aircraft interior production

The growing aviation market puts first tier suppliers of cabin interior under great pressure. Historically grown, poorly optimized manual processes limit the production rate of highly individualized interior components, required by the airlines. Different automated approaches offer solutions for high rate production of standardized lightweight structures. However, those solutions can not be adapted to individual sandwich panels. In this paper, first, the manual process chain is analyzed. Necessary components and features are identified and processes are evaluated with respect to automatability, accuracy, as well as flexibility and design freedom. Based on the analysis, a concept for the automated manufacturing of highly individual, flat sandwich panels with a standardized, digital process is developed. A discussion of the results shows that the presented process meets the targeted objectives, allows a simplified panel design and offers great potential for the development of digital assembly processes with low risk to fail aviation certification.

Experimental study of the mechanical behaviour of pin reinforced foam core sandwich materials under shear load

Sandwich structures with a lightweight closed cell hard foam core have the potential to be used in primary structures of commercial aircrafts. Compared to honeycomb core sandwich, the closed cell foam core sandwich overcomes the issue of moisture take up and makes the manufacturing of low priced and highly integrated structures possible. However, lightweight foam core sandwich materials are prone to failure by localised external loads like low velocity impacts. Invisible cracks could grow in the foam core and threaten the integrity of the structure. In order to enhance the out-of-plane properties of foam core sandwich structures and to improve the damage tolerance (DT) dry fibre bundles are inserted in the foam core. The pins are infused with resin and co-cured with the dry fabric face sheets in an out-of-autoclave process. This study presents the results obtained from shear tests following DIN 53294-standard, on flat sandwich panels. All panels were manufactured with pin-reinforcement manufactured with the Tied Foam Core Technology (TFC) developed by Airbus. The effects of pin material (CFRP and GFRP) and pin volume fraction on the shear properties of the sandwich structure and the crack propagation were investigated and compared to a not pinned reference. It has been concluded that the pin volume fraction has a remarkable effect on the shear properties and damage tolerance of the observed structure. Increasing the pin volume fraction makes the effect of crack redirection more obvious and conserves the integrity of the structure after crack occurrence.

On sound transmission through double-walled cylindrical shells lined with poroelastic material: Comparison with Zhou׳s results and further effect of external mean flow

In this discussion, the corrections to the errors found in the derivations and the numerical code of a recent analytical study (Zhou et al. Journal of Sound and Vibration 333 (7) (2014) 1972–1990) on sound transmission through double-walled cylindrical shells lined with poroelastic material are presented and discussed, as well as the further effect of the external mean flow on the transmission loss. After applying the corrections, the locations of the characteristic frequencies of thin shells remain unchanged, as well as the TL results above the ring frequency where BU and UU remain the best configurations in sound insulation performance. In the low-frequency region below the ring frequency, however, the corrections attenuate the TL amplitude significantly for BU and UU, and hence the BB configuration exhibits the best performance which is consistent with previous observations for flat sandwich panels.

Delamination in sandwich panels due to local water slamming loads

We study delamination in a sandwich panel due to transient finite plane strain elastic deformations caused by local water slamming loads and use the boundary element method to analyze motion of water and the finite element method to determine deformations of the panel. The cohesive zone model is used to study delamination initiation and propagation. The fluid is assumed to be incompressible and inviscid, and undergo irrotational motion. A layer-wise third order shear and normal deformable plate/shell theory is employed to simulate deformations of the panel by considering all geometric nonlinearities (i.e., all non-linear terms in strain–displacement relations) and taking the panel material to be St. Venant–Kirchhoff (i.e., the second Piola–Kirchhoff stress tensor is a linear function of the Green–St. Venant strain tensor). The Rayleigh damping is introduced to account for structural damping that reduces oscillations in the pressure acting on the panel/water interface. Results have been computed for water entry of (i) straight and circular sandwich panels made of Hookean materials with and without consideration of delamination failure, and (ii) flat sandwich panels made of the St. Venant–Kirchhoff materials. The face sheets and the core of sandwich panels are made, respectively, of fiber reinforced composites and soft materials. It is found that for the same entry speed (i) the peak pressure for a curved panel is less than that for a straight panel, (ii) the consideration of geometric nonlinearities significantly increases the peak hydrodynamic pressure, (iii) delamination occurs in mode-II, and (iv) the delamination reduces the hydroelastic pressure acting on the panel surface and hence alters deformations of the panel.

An integrated assembly method of sandwich structured ceramic matrix composites

Sandwich structured composites have been widely studied and applied at ambient temperature in aeronautical, automobile and naval applications. For high temperature applications, an integrated ceramic sandwich structure could take advantage of multiple functions such as skin stiffness and core insulation. For thermo-structural applications, skins must be made of ceramic matrix composites (CMC) because of their strength, their resistance to high temperatures (beyond 1000 °C), and their low densities. Concerning foam cores, some carbides (e.g. SiC) are, for their outstanding thermo-mechanical properties, the most appropriate. These foams can withstand long oxidative exposing conditions with low material degradation. This paper presents an assembly method of SiC based sandwich structured CMC. It is performed during sandwich manufacturing in an integrated fashion and allows the production of complex shapes at low costs. Produced flat sandwich panels, characterized by three point bending tests, showed a marked toughening behaviour.

Buckling Load Design of Sandwich Panels Made with Hybrid Laminated Faces and Transversely Flexible Core

This article presents an analytical approach for evaluating the buckling load of sandwich panels made of hybrid laminated faces and a transversely flexible core. Useful design indications are obtained once the influence of main parameters influencing the structural response is examined. The proposed model is based on a priori assumption of the displacement field through the thickness, which is composed of two sets of modal configurations, one symmetric and one antisymmetric, besides a pure compressive mode. An equivalent `single layer approach', considering all displacement components (u, v, w) that linearly vary across the panel thickness, is used. For simply supported flat sandwich panels subjected to in-plane biaxial and shear loads, the Rayleigh—Ritz method is used to evaluate the buckling loads. Predictions of the proposed model are compared with those obtained by other available theoretical models and with experimental results. Finally, the best arrangement of sandwich panels, corresponding to the maximum buckling loads, is investigated by varying the parameters of both the faces and the core material.

Shear behavior of aluminum lattice truss sandwich panel structures

Age hardenable 6061 aluminum tetrahedral lattice truss core sandwich panels have been fabricated by folding perforated sheets to form highly flexible cellular cores. Flat or curved sandwich panels can be fabricated by furnace brazing the cores to facesheets. Flat sandwich panels with core relative densities between 2 and 10% have been fabricated and tested in the σ ±13 shear orientation (minimum shear strength orientation for a tetrahedral lattice) in the fully annealed (O) and aged (T6) conditions. The shear strength of the lattices increased with relative density, parent alloy yield strength and work hardening rate. Analytical stiffness and strength predictions agree well with measured values for all relative densities and parent alloy heat treatments investigated. The stiffness and strength of 6061-T6 aluminum tetrahedral lattice structures are shown to be comparable to those of conventional 5052-H38 aluminum closed cell hexagonal honeycombs and more than 40% stiffer and stronger than flexible honeycombs used for the cores of curved sandwich panels.

Influence of the Foam Core Material on the Indentation Behavior of Sandwich Composite Panels

Foam core sandwich composite materials have been used more and more frequently in various engineering applications in recent decades. Due to the increased usage of these materials, the concern about their strength and reliability has gained momentum. One of the basic disadvantages of the sandwich structures is their sensitivity to failure by the application of localized lateral loads. The main aim of this work is to investigate the influence of the foam core material on the indentation response of flat sandwich panels. For this purpose, quasi-static indentation test are conducted on sandwich panel specimens that are manufactured with glass fiber reinforced plastic (GFRP) face sheets and three types of Divinycell foam core material (H60, H100 and HCP100). The load is applied through a steel spherical indentor with diameter of 25 mm. The load-displacement diagram for both loading and unloading steps is obtained from the experiments. A numerical modeling of the elastic-plastic indentation response of sandwich structures is performed using the ABAQUS finite element program. In order to predict the residual dent magnitude, both indentation and unloading phases are analyzed. The finite element results demonstrate good agreement with the experimental data. It is found that the load (at a given indentation) is the highest for sandwich panels with the HCP100 foam core, and is the lowest for sandwich panels with the H60 foam core. The load (at the same indentation) for panels with the H100 foam core has intermediate magnitude.

Flexural free vibration of sandwich flat panels with laminated anisotropic face sheets

A study of the effects of anisotropy of face sheets, of core layer, and as of other mechanical/geometrical parameters of flat sandwich panels on their eigenfrequency characteristics is presented. The study is carried out in the context of an advanced model of sandwich structures featuring monoclinic anisotropy-type laminated face sheets and an orthotropic light core. A detailed analysis of the influence of a number of important parameters associated with panel geometry, fiber orientation and stacking sequence in the face sheets, and material properties of the core is carried out, and pertinent conclusions are outlined. In spite of the intricacy of the investigated problem, the adopted solution methodology of the problem enables one to obtain a closed-form solution of eigenfrequency predictions. Numerical simulations highlighting the implications of a number of parameters on eigenfrequencies, as well as validations against the few ones available in the specialized literature, are presented. Pertinent conclusions regarding their dynamic optimization via the use of directional properties of fiber-composite materials in the face sheets and of orthotropicity in the core layer material are supplied.

Mechanical properties balance in novel Z-pinned sandwich panels: Out-of-plane properties

The paper presents the first complete set of test results obtained from quasi-static measurements of the out-of-plane tension, shear and compression properties of novel X-Cor™ and K-Cor™ flat sandwich panels. The cured panels were composed of 0.75-mm-thick quasi-isotropic IM7/8552 skins, separated by 12.7-mm-thick Rohacell® foam core containing 0.51 mm diameter carbon fibre pins (Z-Fiber®), arranged in a truss pattern at pin angle of either 22 or 30° to the vertical. To obtain a suitable baseline comparison, the equivalent set of properties was measured for Nomex honeycomb core panels sandwiched by the same composite skins. The novel Z-pinned cores are found to exhibit higher specific stiffness than conventional sandwich cores, but lower strength.

Dynamic response of anisotropic sandwich flat panels to explosive pressure pulses

This paper addresses the problem of the dynamic response in bending of flat sandwich panels exposed to time-dependent external blast pulses. The study is carried out in the context of an advanced model of sandwich structures that is characterized by anisotropic laminated face sheets and an orthotropic core layer, and a closed form solution of the dynamic response to a variety of blast pulses is provided. A detailed analysis of the influence of a large number of parameters associated with the particular type of pressure pulses, panel geometry, fiber orientation in the face sheets and, presence of tensile uni/biaxial edge loads on dynamic response is carried out, and pertinent conclusions are outlined.

Engineering Method for Prediction of Impact Response and Damage in Sandwich Panels

An engineering method is suggested for prediction of impact response and damage of flat sandwich panels. The approach accounts for local core crushing, delamination and large face sheet deflections and does not rely on empirical indentation laws. Different models are suggested depending upon the impactor mass being either larger or significantly smaller than the mass of the impacted panel. The solution for large mass impact is based on closed form expressions. The solution for small mass impact is obtained from a dimensionless two-parameter integral equation. The validity of the approach is demonstrated on a number of static indentation experiments and impacts on sandwich panels.

Non-linear response of geometrically imperfect sandwich curved panels under thermomechanical loading

This paper deals with the non-linear response of sandwich curved panels exposed to thermomechanical loadings. The mechanical loads consist of compressive/tensile edge loads, and a lateral pressure while the temperature field is assumed to exhibit a linear variation through the thickness of the panel. Towards obtaining the equations governing the postbuckling response, the Extended Galerkin’s Method is used. The numerical illustrations concern doubly curved, circular cylindrical and as a special case, flat panels, all the edges being simply supported. Moveable and immoveable tangential boundary conditions in the directions normal to the edges are considered and their implications upon the thermomechanical load-carrying capacity are emphasized. Effects of the radii of curvature and of initial geometric imperfections on the load-carrying capacity of sandwich panels are also considered and their influence upon the intensity of the snap-through buckling are discussed. It is shown that in special cases involving the thermomechanical loading and initial geometric imperfection, the snap-through phenomenon can occur also in the case of flat sandwich panels.

A refined multilayered finite-element model applied to linear and non-linear analysis of sandwich plates

This paper presents a computationally efficient and mechanically accurate finite-element formulation for linear and non-linear analysis of flat sandwich panels. Computational efficiency is guaranteed by preserving C 0-continuity as in the Reissner–Mindlin model. Mechanical accuracy is acquired by allowing a zig-zag in-plane displacement field in the thickness direction and by fulfilling interlaminar equilibrium at the interface between the core and skins for the transverse shear stress components. The resulting mixed model is reduced to the standard displacement formulation by writing a weak form of Hooke’s Law. The model is denoted by the acronym RMZC, Reissner–Mindlin, zig-zag, interlaminar continuity. The principle of virtual displacement is employed to write finite element equations governing the linear and non-linear static response, free vibration, buckling and postbuckling behaviour. Non-linear equations are solved iteratively by Newton–Raphson linearization accounting arc-length type strategies. The numerical investigation assesses the proposed model by comparing it to classical lamination theory (CLT), First-order shear deformation theory (FSDT) and other available results. The efficiency and accuracy of the RMZC model is then demonstrated by solving several problems dealing with linear and non-linear bending, natural frequencies, in-plane buckling and postbuckling. Appendices quote details of the most interesting points of the models presented.

Sandwich Construction

IN Part I of this paper, the basic equations were established for the various modes of instability of flat sandwich panels subjected to lengthwise compression. Whilst these equations are valuable in determining the strength of a given panel they do not give a direct indication of the cross‐sectional dimensions of a panel required to carry a given load, especially if the panel is to be the optimum, i.e. of minimum weight.

An Exact Solution for the Buckling Load of Flat Sandwich Panels with Loaded Edges Clamped

The solution is based on small deflection theory, and it is assumed that the stresses are always below the proportional limit. The material for both the faces and the core is considered isotropic. Three equations of equilibrium, derived by means of the principle of virtual displacements, form the starting point of the theory. The equilibrium equations are partial linear differential equations with constant coefficients—two are of the second order, and the third is of the fourth order. Both the equations of equilibrium and 16 boundary conditions are satisfied identically.