Curved Sandwich Panels Research Articles

Novel Steel-Concrete-Steel Sandwich Composite Plates Subject to Impact and Blast Load

This paper investigates the structural performance of steel-concrete-steel (SCS) sandwich composite system subject to impact and blast load. Novel J-hook shear connector was invented to prevent the separation of face plates from the concrete core. SCS sandwich specimens subject to 100 kg TNT blast a 5 m standoff distance were tested. The test results are reported and the factors affecting the blast resistance of SCS sandwich structures are discussed. Finite element analysis was carried out and the numerical results are verified against the blast test results. Parametric study of sandwich core strength and plate thickness is presented. Due to the superior impact performance of the proposed SCS sandwich structures, an ice caisson protective offshore structure is proposed based on the curved sandwich panel. The ultimate strength behavior of curved SCS sandwich panels is tested. The effect of arch effect is studied both experimentally and numerically.

Non-linear thermo-mechanical behaviour of delaminated curved sandwich panels with a compliant core

A non-linear analysis of a delaminated curved sandwich panel with a compliant core, and a delamination (debond) at one of the face–core interfaces, and subjected to a thermal field and a mechanical loading or combined is presented. The mathematical formulation outlines the governing equations along with the stress and displacements fields for the cases where the core properties are either temperature independent ( TI) or temperature dependent ( TD). A variational formulation is used following the principles of the high-order sandwich panel theory (HSAPT) to derive the field equations along with the appropriate continuity conditions. The non-linear analysis includes geometrical non-linearities in the face sheets caused by rotation of the face cross sections, and high-order effects that are the result of the radially (transversely) flexible (or compliant) core. The core stress and displacements fields with temperature-dependent ( TD) mechanical properties are determined in closed form using an equivalent polynomial description of the varying properties. The numerical study describes the non-linear response of delaminated curved sandwich panels subjected to mechanical concentrated loads, thermally induced deformations and simultaneous thermal and mechanical loads. In the combined loading case the mechanical loads are below the limit point load level of the mechanical response, and the imposed temperature field is varied. The results are displayed in terms of plots of various structural quantities along the sandwich panel length (circumference), equilibrium curves and strain energy release rate curves. It is shown that the combined thermo-mechanical response shifts the linear or non-linear responses,observed for the separate cases of either temperature induced deformations or mechanical loading, into a strongly non-linear response with limit point behaviour and large stresses in vicinity of supports, loads and tips of delaminated zone.

Response of Curved Sandwich Panels Subjected to Blast Loading

Curved sandwich panels with two aluminium face sheets and an aluminium foam core under air blast loadings were investigated experimentally and numerically. Specimens with two values of radius of curvature and different core/face sheet configurations with the same projected area were tested for three blast intensities. All four edges of the panels were fully clamped. The experiments were carried out by a four-cable ballistic pendulum with corresponding sensors. The impulse acting on the front face of the assembly, the deflection history at the center of the back face sheet, and the strain history at some characteristic points on the back face were obtained. Then the deformation/failure modes of specimens were classified and analyzed systematically. The commercial software LS-DYNA was employed to simulate those physical processes. The finite-element (FE) model was validated by the data from experiments. Detailed deformation and energy dissipation mechanisms were further revealed by the FE models. The valuable experimental data and results from FE models show that the initial curvature of a curved sandwich panel changes the deformation/collapse mode with an extended range for bending-dominated deformation mode, which suggests that the performance of the sandwich shell structures slightly exceeds that of both their equivalent solid counterpart and a flat sandwich plate in certain blast intensity ranges.

Experiments on curved sandwich panels under blast loading

In this paper curved sandwich panels with two aluminium face sheets and an aluminium foam core under air blast loadings were investigated experimentally. Specimens with two values of radius of curvature and different core/face sheet configurations were tested for three blast intensities. All the four edges of the panels were fully clamped. The experiments were carried out by a four-cable ballistic pendulum with corresponding sensors. Impulse acting on the front face of the assembly, deflection history at the centre of back face sheet, and strain history at some characteristic points on the back face were obtained. Then the deformation/failure modes of specimens were classified and analysed systematically. The experimental data show that the initial curvature of a curved sandwich panel may change the deformation/collapse mode with an extended range for bending dominated deformation, which suggests that the performance of the sandwich shell structures may exceed that of both their equivalent solid counterpart and a flat sandwich plate.

Nonlinear behavior of thermally loaded curved sandwich panels with a transvesely flexible core

The nonlinear analysis of a curved sandwich panel with a compliant core subjected to a thermal field and mechanical load is presented. The mathematical formulation is developed first, along with the solution of the stress and displacement fields for the case of a sandwich core with mechanical properties that are independent of the temperature. The nonlinear analysis includes geometrical nonlinearities in the face sheets caused by rotation of the face cross sections and high-order effects due to the transversely flexible core. The mathematical formulation uses the variational principle of minimum energy along with HSAPT (high-order sandwich panel theory) to derive the nonlinear field equations and the boundary conditions. The full displacement and stress fields of the core with uniform temperature-independent mechanical properties and the appropriate governing equations of the sandwich panel are given. This is followed by the general solution of the core stress and displacement fields when the mechanical core properties are dependent on the radial (through-the-thickness) coordinate. The displacement fields of a core with temperature-dependent mechanical properties are determined explicitly using an equivalent polynomial description of the varying properties. A numerical study then describes the nonlinear response of curved sandwich panels subjected to concentrated and distributed mechanical loads, thermally induced deformations, and simultaneous thermal and mechanical loads where the mechanical load is below the limit load level of the mechanical response and the imposed temperature field is made to vary. The results reveal that the thermomechanical response is linear when the sandwich panel is heated, but becomes nonlinear with limit point behavior when the panel is cooled down.

Doubly-Curved Anisotropic Sandwich Panels: Modeling and Free Vibration

Problems related with the modeling and behavior in free vibration of doubly curved sandwich panels with laminated face sheets are developed in this paper. Implications of anisotropy and stacking sequence of face sheets as well as of transverse orthotropy of the core on the free vibration behavior are highlighted. Other parameters, mainly geometrical ones, are also considered in the numerical simulations, and their implications on the vibrational behavior are put into evidence. Because of the absence of similar results in the specialized literature, this paper is likely to fill a gap in the state of the art of this problem and provide pertinent results that are instrumental in the design of advanced sandwich shells.

Heating and Cryogenic Cooling Stress Analysis of Curved Sandwich Panels

The honeycomb sandwich panel can experience open-mode debonding failure due to temperature difference and curvature variation. In order to better understand the thermostuctural behavior and these debonding failures, the thermal stress analysis is performed for the honeycomb core sandwiches, with different radii of curvature. The panels are simply supported or clamped and are subjected to uniform heating on the convex side and cryogenic cooling on the concave portion. The panels are subjected to varied temperature distribution through the thickness and therefore developed the open-mode stress. Nonlinear finite element analysis code ABAQUS was used to analyze the effects of panel curvature and the boundary conditions on the open-mode stress in the curved sandwich panel with the designated temperatures. Based on analysis results, the critical stress points are found at the tangential centerline of the sandwich segment. The chance of open-mode debonding failure increases with the increase in curvature variation and large difference in temperature applied to each panel. The open-mode stress is also greater in the panels with simply supported edges rather than the clamped ends.

Auxetic materials

The current status of research into auxetic (negative Poisson's ratio) materials is reviewed, with particular focus on those aspects of relevance to aerospace engineering. Developments in the modelling, design, manufacturing, testing, and potential applications of auxetic cellular solids, polymers, composites, and sensor/actuator devices are presented. Auxetic cellular solids in the forms of honeycombs and foams are reviewed in terms of their potential in a diverse range of applications, including as core materials in curved sandwich panel composite components, radome applications, directional pass band filters, adaptive and deployable structures, MEMS devices, filters and sieves, seat cushion material, energy absorption components, viscoelastic damping materials, and fastening devices. The review of auxetic polymers includes the fabrication and characterization of microporous polymer solid rods, fibres, and films, as well as progress towards the first synthetic molecular-level auxetic polymer. Potential auxetic polymer applications include self-locking reinforcing fibres in composites, controlled release media, and self-healing films. Auxetic composite laminates and composites containing auxetic constituents are reviewed and enhancements in fracture toughness, and static and low velocity impact performance are presented to demonstrate potential in energy absorber components. Finally, the potential of auxetics as strain amplifiers, piezoelectric devices, and structural health monitoring components is presented.

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.

Dynamic buckling of flat and curved sandwich panels with transversely compressible core

In the present study, the effect of the transverse compressibility of the core on the transient dynamic response of structural sandwich panels under rapid loading conditions is investigated. The analysis is based on a higher-order sandwich shell theory in an effective multilayer formulation. The model is based on the standard Kirchhoff–Love hypothesis for the face sheets whereas a first/second order power series expansion is employed for the core. Consistent equations of motion and boundary conditions are derived by means of Hamilton’s principle. An analytical solution is obtained by an extended Galerkin procedure. The theory is applied to the dynamic buckling and postbuckling analyses of plane and curved sandwich panels subjected to rapidly applied tangential and transverse loads. It is observed that the transverse compressibility of the core can have distinct effects on both, the frequency and the amplitudes of the resulting free oscillations. Due to interactions between the overall oscillation and a local oscillation in the presence of a face wrinkling instability mode, the dynamic response can be chaotic, resulting in oscillating face wrinkling instability modes with unpredictable amplitudes.

High-Order Bending Analysis of Unidirectional Curved ‘‘Soft’’ Sandwich Panels with Disbonds and Slipping Layers

The bending behavior of unidirectional singly curved sandwich panels with a ‘‘soft’’ core and delaminated (debonded) zones or slipping layers is described herein. The delaminated zone is assumed to exist through the width of the panels, and is in the form of an interfacial debonded zone at one of the face–core interfaces, thus subdividing the length of the panel into three zones, i.e. two fully bonded zones and one that is debonded (delaminated). The formulation uses an interfacial spring description, i.e. distributed shear and radial normal springs at the face–core interfaces, to model the effects of the slipping layers and the various delaminated (debonded) zones. The formulation is based on a closed-form high-order approach (HSAPT) that incorporates the high-order effects of the ‘‘soft’’ core. The variational approach is used to derive the governing equations along with the general explicit boundary and continuity conditions. A typical curved sandwich panel with an inner or an edge delamination, with or without contact, subjected to a distributed load and pinned supports is discussed. A comparison between the various delamination types and the fully bonded case is presented in terms of the displacements, stresses and internal stress resultants of the faces and in the vicinity of the edges of the delaminated zones. Finally, the effect of slipping interfacial layers, with or without contact when subjected to a distributed load, is considered and compared with the behavior of a fully bonded sandwich panel. From the numerical studies it is found that the membrane action of the face sheets plays a major role in the overall structural resistance of the sandwich panel compared to that of flat panels. However, in the near vicinity of the edges of the debonded zone, the membrane action does not prevent the interfacial stresses to reach very large values in the case of delamination without contact. For all of the delamination types, contact exerts a favorable effect, in the sense that the induced interfacial stresses are much smaller when compared to cases of delamination without contact.

A review of analytical methods for aircraft structures subjected to high-intensity random acoustic loads

A review of the acoustic fatigue design process for aircraft structures is presented in this paper, together with the current design guides, which are used to predict the stresses that an acousticallly loaded aircraft structure may experience in service. These methods are based on linear theory and use the single-degree-of-freedom approximation method. A recent programme of research which uses this method together with the finite element method to predict the root mean square strains experienced by acoustically excited, doubly curved sandwich panels is briefly discussed. Recent developments in prediction methods based on the non-linear dynamic response of thermoacoustic loaded structures are reviewed, and suggestions are made as to possible future directions in the area of acoustic fatigue research

Dynamic response of doubly curved honeycomb sandwich panels to random acoustic excitation. Part 2: Theoretical study

In this paper a single-degree-of-freedom model is developed to predict the dynamic response of an acoustially excited doubly curved sandwich panel. Three variants of the model are investigated, based on differing assumptions regarding the spatial distribution of the applied loading. When the loading is assumed to be uniform then the model reduces to the Miles approach, and when the loading is assumed to conform to the structural mode shape then the method is very similar to the Blevins approach. The third variant involves a more detailed consideration of the travelling wave characteristics of the applied loading, and this is found to give much improved agreement with experimental results obtained in a progressive wave tube facility. In addition, an approach using the finite element method is presented in which the response to grazing incidence excitation is computed, and this is also found to yield good agreement with the experimental results.

A nonlinear theory for doubly curved anisotropic sandwich shells with transversely compressible core

In the present paper, an advanced geometrically nonlinear shell theory of doubly curved structural sandwich panels with transversely compressible core is presented. The model is based on the adoption of the Kirchhoff theory for the face sheets and a second/third order power series expansion for the core displacements. The theory accounts for dynamic effects as well as for initial geometric imperfections. In the v. Kármán sense, large displacement theory is employed with respect to the transverse direction while the displacement gradients with respect to the tangential directions are assumed to be small. The equations of motion are derived by means of Hamilton’s principle and hold valid for all types of elastic and elastic–plastic material models. The theory is illustrated by an analysis of the elastic buckling and postbuckling behavior of flat and curved sandwich panels using an extended Galerkin scheme. Owing to the assumed transverse flexibility of the core, both the global and the local (face wrinkling) instability modes can be addressed.

Conceptual Design Principles for Non-Circular Pressurized Sandwich Fuselage Sections – a Design Study Based on a High-Order Sandwich Theory Formulation

Results obtained as part of a design study regarding a non-circular pressurized sandwich fuselage section are presented. The originating problem is associated with preliminary studies for the “Global Range Transport” envisaged by the “New World Vistas” program of the United States Air Force. The modeling and analysis is conducted usinga high-order sandwich theory formulation in which the elastic response of each face laminate is accounted for, includingbending-stretching couplingand transverse shear deformations, and where the transverse flexibility of the core is included. The sandwich fuselage envisaged may contain flat, tapered and curved sandwich elements, and a high-order sandwich theory formulation is developed to analyze each of these configurations. The paper includes a presentation of the adopted high-order sandwich theory with special emphasis on the application for the analysis of sandwich panels with variable core thickness, and for the analysis of curved sandwich panels. Numerical results obtained for two different mid-plane asymmetric fuselage sections are presented and compared; one with constant core thickness, and one with varyingcore thickness.

Conceptual Design Principles for Non-Circular Pressurized Sandwich Fuselage Sections -- A Design Study Based on a High-Order Sandwich Theory Formulation

Results obtained as part of a design study regarding a non-circular pressurized sandwich fuselage section are presented. The originating problem is associated with preliminary studies for the “Global Range Transport” envisaged by the “New World Vistas” program of the United States Air Force. The modeling and analysis is conducted usinga high-order sandwich theory formulation in which the elastic response of each face laminate is accounted for, includingbending-stretching couplingand transverse shear deformations, and where the transverse flexibility of the core is included. The sandwich fuselage envisaged may contain flat, tapered and curved sandwich elements, and a high-order sandwich theory formulation is developed to analyze each of these configurations. The paper includes a presentation of the adopted high-order sandwich theory with special emphasis on the application for the analysis of sandwich panels with variable core thickness, and for the analysis of curved sandwich panels. Numerical results obtained for two different mid-plane asymmetric fuselage sections are presented and compared; one with constant core thickness, and one with varyingcore thickness.

Analysis and Parametric Study of Non-Circular Pressurized Sandwich Fuselage Cross Section Using a High-Order Sandwich Theory Formulation

Results obtained as part of a design study regarding a non-circular pressurized sandwich fuselage cross section are presented. The originating problem is associated with preliminary studies for the “Global Range Transport” envisaged by the “New World Vistas” program of the United States Air Force. The modeling and analysis is conducted using a high-order sandwich theory formulation in which the elastic response of each face laminate is accounted for, including bending-stretching coupling, and including the transverse flexibility of the core material. The paper includes a presentation of the high-order sandwich theory approach adopted for the analysis of the fuselage section, including the formulations for flat and curved sandwich panels. The paper is concluded by presentation of the results of a parametric study, which includes the effects on the structural response of varying the sandwich panel mid-plane asymmetry, the core properties, and the radius of curvature of the fuselage cross section corners.

Buckling and Nonlinear Response of Sandwich Curved Panels to Combined Mechanical Loads-Implications of Face-Sheet Elastic Tailoring

Results concerning the linear and nonlinear static response of curved sandwich panels to combined mechanical loads are addressed, and in this context, the implications of elastically tailored face-sheets upon the enhancement of their load carrying capacity are emphasized. The analysis is based on a geometrically nonlinear theory of shallow sandwich shells restricted to the case of small strains and moderately small rotations. In addition to the implications of ply-angle, ply-sequence and ply-thickness of composite face-sheets, those related with the panel curvature and aspect ratio on their buckling and nonlinear static response are analyzed, and pertinent conclusions are outlined.

Free vibration analysis of deep doubly curved sandwich panels

This paper presents the free vibration analysis of doubly curved open deep sandwich shells made of thin outer layers and a relatively thick core. The outer layers are assumed to be made of high strength and high density material. The core is a low strength low density material. The Rayleigh–Ritz method, in which the displacement fields are defined by Bezier surface patches, is used to obtain the natural frequencies. The formulation procedure is developed for the analysis of open panels circumscribed by four curvilinear edges. Numerical results are obtained for circular cylindrical and spherical sandwich panels with the boundary conditions given by: one curved edge clamped and the remaining three edges free as well as two opposite edges clamped and the other two completely free.

Analysis of Curved Sandwich Panels with Cutouts Subjected to Combined Temperature Gradient and Mechanical Loads

The results of detailed study of the effect of the cutout on the response of curved sandwich panels are presented. The panels have honeycomb core composite face sheets. The loading consists of a temperature gradient through-the-thickness combined with pressure loading and edge shortening or edge shear. The analysis is based on a first-order shear-deformation Sanders-Budiansky type theory with the effects of large displacements, moderate rotations, transverse shear deformation and laminated anisotropic material behavior included. A mixed formulation is used with the fundamental unknowns consisting of the generalized displacements and the stress resultants of the panel. The nonlinear displacements, strain energy, principal strains, transverse shear stresses, transverse shear strain energy density, and their hierarchical sensitivity coefficients are evaluated. The hierarchical sensitivity coefficients measure the sensitivity of the nonlinear response to variations in the panel parameters, the effective properties of the face sheet layers and the core, and the micromechanical parameters. Numerical results are presented for cylindrical sandwich panels and show the effects of variations in the loading and the size of the cutout on the global and local response quantities and their sensitivity to changes in the various panel, effective layer and micromechanical parameters.