The review outlines modern trends in theoretical developments, novel designs and modern applications of sandwich structures. The most recent work published at the time of writing of this review is considered, older sources are listed only on as-needed basis. The review begins with the discussion on the analytical models and methods of analysis of sandwich structures as well as representative problems utilizing or comparing these models. Novel designs of sandwich structures is further elucidated concentrating on miscellaneous cores, introduction of nanotubes and smart materials in the elements of a sandwich structure as well as using functionally graded designs. Examples of problems experienced by developers and designers of sandwich structures, including typical damage, response under miscellaneous loads, environmental effects and fire are considered. Sample applications of sandwich structures included in the review concentrate on aerospace, civil and marine engineering, electronics and biomedical areas. Finally, the authors suggest a list of areas where they envision a pressing need in further research.

Review of current trends in research and applications of sandwich structures

The use of polyurethane for structural and infrastructural engineering applications: A state-of-the-art review

The objective of this study was to evaluate three potential core alternatives for glass fiber reinforced polymer (GFRP) foam-core sandwich panels. The proposed system could reduce the initial production costs and the manufacturing difficulties while improving the system performance. Three different polyurethane foam configurations were considered for the inner core, and the most suitable system was recommended for further prototyping. These configurations consisted of high-density polyurethane foam (Type 1), a bidirectional gridwork of thin, interconnecting, GFRP webs that is in-filled with lowdensity polyurethane foam (Type 2), and trapezoidal-shaped, low-density polyurethane foam utilizing GFRP web layers (Type 3). The facings of the three cores consisted of three plies of bidirectional E-glass woven fabric within a compatible polyurethane resin. Several types of small-scale experimental investigations were conducted. The results from this study indicated that the Types 1 and 2 cores were very weak and flexible making their implementation in bridge deck panels less practical. The Type 3 core possessed a higher strength and stiffness than the other two types. Therefore, this type is recommended for the proposed sandwich system to serve as a candidate for further development. Additionally, a finite element model (FEM) was developed using software package ABAQUS for the Type 3 system to further investigate its structural behavior. This model was successfully compared to experimental data indicating its suitability for parametric analysis of panels and their design.

Evaluation of sandwich panels with various polyurethane foam-cores and ribs

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.

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

Foam-filled thin-walled structures signify a class of a promising energy absorber for improving the crashworthiness and safety of vehicles. Although the conventional deterministic optimization has been extensively applied to crashworthiness design of foam-filled thin-walled structures, the optimal solution could become infeasible when uncertainties of design variables and noise factors present in real world. To address this issue, a reliability based design optimization (RBDO) is adopted to consider the uncertainties of design variables and noise factors in crashworthiness optimization for the foam-filled bitubal tapered structure in this paper. Moreover, to comprehensively investigate the differences between deterministic and reliability based design optimization, single objective and multiple objective RBDO are established by integrating Kriging approximation with Monte Carlo Simulation (MCS). Since the optimal results of deterministic design usually converge at the constraint boundary, the solutions of RBDO often need to compromise some objective performance to satisfy the predetermined reliability levels. Furthermore, a comparative study on different Pareto fronts yielded from the deterministic optimization and RBDO under different reliability levels is conducted here. Besides, a grey relational analysis is carried out to determine the most satisfactory solution from the Pareto-set. The results demonstrate that the optimized foam-filled bitubal tapered columns are capable to considerably improve capacity of energy absorption with an increased reliability, potentially being a structural configuration for energy absorber.

Nondeterministic optimization of tapered sandwich column for crashworthiness

Polyurethane foam core has been considered for sandwich design due to its high stiffness and toughness. One of the methods of potential improvement of the foam is impregnating it with stiff nanoparticles, increasing its stiffness and collapse strength. In this paper we use the Mori–Tanaka and the self-consistent methods to estimate an increase in the stiffness of closed-cell and open-cell foams impregnated by stiff nanoparticles that do not affect foam geometry. Subsequently, the effect of the impregnation of the foam core on the response of a sandwich beam is demonstrated. The response characteristics considered in the paper include bending deflections in a three-point test, global buckling and wrinkling loads, and the effect of the foam impregnation on its compressive collapse stress associated with local instability of the elements of the foam. Recognizing that the introduction of nanoparticles in the foam material creates local stress concentrations around the inclusions we also examine the local strength of the impregnated foam using the classical Goodier solution for dilute particles and the Mori–Tanaka based stress analysis for a finite particle concentration. In addition, we demonstrate the approach to the evaluation of the resilience of impregnated foam utilizing the dilute approach. While numerical examples concentrate on polyurethane foam, the methodologies and conclusions can be extrapolated to any polymeric foam material reinforced by nanoparticles.

Effects of nanoparticle impregnation of polyurethane foam core on the performance of sandwich beams

Impregnating polymeric matrix with stiff particles may significantly improve structural response of a composite material. Such improvements have to be weighed against the effects of the stress concentration at the particle–matrix interface that influence local strength and toughness. In the present paper we elaborate on the issue of local stresses and strength in particulate polymer matrix composites considering polyurethane matrix impregnated with alumina particles in numerical examples. The parametric analysis presented in the paper is concerned with the effects of the particle volume fraction and the particle-to-matrix stiffness ratio on the local stresses and initial damage. We also discuss the resilience of the impregnated polyurethane, i.e. the density of energy necessary to produce initial damage. The approach to the analysis of fracture in the composite with initial damage is discussed accounting for available experimental observations. Three scales of fracture corresponding to different phases of the development and propagation of the crack are identified, including microfracture at the particle–matrix interface and mesofracture limited to the matrix surrounding the particle. While these scales of fracture should be analyzed by numerical methods, macrofracture that occurs after the crack “emerged” from the representative unit cell where it originated can be considered using available analytical techniques. The methodology of the stress analysis of a particulate material consisting of an incompressible hyperelastic matrix and much stiffer elastic particles is also proposed in the paper.

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Strength analysis of particulate polymers

Although still in their infancy, fiber-reinforced polymer (FRP) bridges have shown great promise in eliminating corrosion concerns and meeting (or exceeding) the Federal Highway Administration's (FHWA’s) goal of 100-year life spans for bridges. While FRP bridges are cost-effective in terms of life cycle analyses, the combination of higher first costs and limited state department of transportation (DOT) budgets has restricted their use. One area that has shown some headway is the use of FRP for bridge decks, focusing on the location where the majority of corrosion-related damage normally occurs. However, first costs still hamper widespread use of this approach. FRP bridge deck panels offer superior corrosion resistance, at one-fifth the weight of reinforced concrete. However, current FRP bridge deck panels typically rely on an intricate geometric honeycomb system between the top and bottom layers of the sandwich panel. This labor-intensive honeycomb construction more than doubles the cost of FRP panels compared to reinforced concrete. Although cost-effective in terms of longevity of the bridge and overall reductions in weight, the lower first cost of reinforced concrete precludes the use of FRP bridge decks in the majority of situations. Closed-cell, high-density polyurethane foams lower first cost, offering a cost-effective alternative to the complex honeycomb construction. Structural sandwich panels with a polyurethane foam infill are well established in other commercial applications, such as automobiles, aircraft, and prefabricated buildings. Several recent advances in polyurethane foam formulations have resulted in a material that can resist the localized compressive stresses and fatigue loading beneath a truck wheel, making this type of sandwich panel construction a viable alternative for bridge decks. Once these panels can compete against reinforced concrete on a first-cost basis, their significantly longer life expectancies will save considerable money for Missouri Department of Transportation (MoDOT) and the residents of Missouri. The objective of the research was to develop, test, and evaluate fiber-reinforced, polyurethane (PU) foams to replace the costly honeycomb construction currently used to manufacture FRP bridge deck panels. Initially, the effort focused on developing an FRP sandwich panel to replace the precast, stay-in-place forms currently used to construct reinforced concrete bridge decks. However, during the course of the project, the research effort expanded to include full-depth bridge deck panels as well. This report documents the results of this study.

https://rosap.ntl.bts.gov/view/dot/27483/dot_27483_DS1.pdf

Matthew Hopkins

Papers

Evaluation of sandwich panels with various polyurethane foam-cores and ribs

Effects of nanoparticle impregnation of polyurethane foam core on the performance of sandwich beams

Strength analysis of particulate polymers

Polyurethane Foam Infill for Fiber-Reinforced Polymer (FRP) Bridge Deck Panels