Catálogo de publicaciones - libros

The use of sandwich structures continues to increase rapidly for applications ranging from satellites, aircraft, ships, automobiles, rail cars, wind energy systems, and bridge construction to mention only a few. The many advantages of sandwich constructions, the development of new materials, and the need for high performance, low-weight structures insure that sandwich construction will continue to be in demand. The equations describing the behavior of sandwich structures are usually compatible with the equations developed for composite material thin-walled structures, simply by employing the appropriate in-plane, flexural, and transverse shear stiffness quantities. Only if a very flexible core is used, is a higher order theory needed.

This paper gives a brief overview of sandwich application history in general and present composite sandwich structures at Airbus. Current R&D developments for sandwich in primary structures are being outlined followed by a discussion of potentials and challenges for composite sandwich structures.

A review is made of production defects and in-service damage types that arise in sandwich structures having fibre reinforced polymer (FRP) face sheets. A brief overview is given of relevant defect and damage models and how these models can be used in an assessment of criticality with regard to local and global structure, as a basis for deciding on corrective measures, for the case of a naval ship. Challenges resulting from limitations in inspection techniques are discussed. The concept of damage tolerance is discussed in the light of the above. It is argued that the most suitable and economical approach to achieving damage tolerance is dependent on the application.

The paper presents the results of an investigation on the role of localized effects in the geometrically non-linear response of modern sandwich panels made of a “soft” core. The adopted non-linear analysis approach incorporates the effects of the vertical flexibility of the core, and it is based on the approach of the High-Order Sandwich Panel Theory (HSAPT). The non-linear governing equations are solved using the multi-point shooting method along with parametric and arc-length continuation procedures. And the non-linear response is described in terms of deflections and stress resultants in the face sheets, as well as in terms of the interfacial stress components at the upper and lower face-core interfaces and equilibrium path curves of the load versus the extreme absolute values of some structural quantities. The numerical study investigates the localized effects and how do they attenuate with the deformations in the case of a panel with a reinforced core at its mid region and subjected to a uniform distributed load. The results are compared with a panel that has a core with uniform properties. One of the findings of this research is that a typical modern panel exhibits a limit point response as a result of a load that causes bending, with and without localized effects, even when subjected to uniformly distributed loads and the material discontinuity of the core leads to localized effects that significantly affects the non-linear response.

There exist several formulas for the global buckling of sandwich plates, each based on a specific set of assumptions and a specific plate or beam model. It is not easy to determine the accuracy and range of validity of these rather simple formulas unless an elasticity solution exists. In this paper, we present an elasticity solution to the problem of global buckling of wide sandwich panels (equivalent to sandwich columns) subjected to axially compressive loading (along the short side). The emphasis on this study is on the global (single-wave) rather than the wrinkling (multi-wave) mode. The sandwich section is symmetric and all constituent phases, i.e., the facings and the core, are assumed to be orthotropic. The buckling problem is formulated as an eigen-boundary-value problem for differential equations, with the axial load being the eigenvalue. The complication in the sandwich construction arises due to the existence of additional “internal” conditions at the face sheet/core interfaces. Results are produced for a range of geometric configurations and these are compared with the different global buckling formulas in the literature.

In practical sandwich constructions it is often necessary to reinforce the core with high strength inserts to facilitate concentrated loads. The junction between the main core and the core insert is a potential weak spot of the sandwich structure. This section of the sandwich structure is investigated using finite element analysis and the results are compared with experimental data. Furthermore, parameters concerning the shape of the core junction are investigated using finite element analysis.

Local effects occurring near junctions between different cores in sandwich structures subjected to axial and transverse forces and bending moments are considered. These local effects are associated with large stress concentrations in the faces and the core near the core junctions. The local effects are studied for two typical cases representing industrial applications. Finite element analyses show that significant stress concentrations are induced near core junctions subjected to transverse, tension/compression as well as bending loads. Finally, improved designs of core junctions are discussed.

To support the development of sandwich composite failure models, a series of material coupon tests were performed to characterize foam core material response under quasi-static and dynamic tension loading. Material stress-strain response was found to be highly nonlinear and dependent on the loading orientation relative to the axis of the foam sheet. At low to moderate strain rates, less than 150 sec, tension strength and modulus were both increased and ultimate strain at failure decreased.

A study devoted to the dynamic response of sandwich panels to underwater and in-air explosions is presented. The study is carried out in the context of a geometrically nonlinear model of sandwich structures featuring anisotropic laminated face sheets and a transversally compressible orthotropic core. The unsteady pressure generated by the explosion and acting on the face of the sandwich panel includes the effect of the pressure wave transmission through the core. Its implications on the structural time-histories as corresponding to the underwater and in-air explosions are put into evidence. The effects of the transverse core compressibility on dynamic response are highlighted. In this sense, one of its major implications is the possibility to capture interactively the global and local (wrinkling) dynamic response of the panel. It is shown that implementation of the structural tailoring technique in the face sheets can constitute an important mechanism toward enhancing the dynamic load carrying capacity of sandwich panels when exposed to blast pulses. Effects of the core, as well as the ones due to the ply-thickness, combined with that of ply-angle and stacking sequence of face sheets, orthotropy of the material of the core, geometrical nonlinearities, initial geometric imperfections and of the damping ratio are investigated, and their implications upon the dynamic response are highlighted and pertinent conclusions are outlined.

This research examines the effect of design modifications on response of sandwich plates to impulse pressure loads. The objective is to limit damage by delamination of the laminated face sheets and by crushing of the structural foam core that dominates response of conventionally designed sandwich plates. This is achieved by introducing structural elements that store the incident energy and thus reduce damage-related energy dissipation. In particular, ductile interlayers inserted between the outer face sheet and the foam core, can absorb a significant part of the incident energy, and protect the foam core from excessive deformation. These design concepts have been developed in our earlier work on the effect of low and medium velocity impact on sandwich plates, where they enhanced resistance to local deflections of the face sheet, foam crushing and interface delaminations.