Elsevier

Composites Part B: Engineering

Volume 79, 15 September 2015, Pages 262-276
Composites Part B: Engineering

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

https://doi.org/10.1016/j.compositesb.2015.04.023Get rights and content

Abstract

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 low-density 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.

Introduction

The majority of highway bridge decks are constructed with steel-reinforced concrete. The life-span of such materials can be significantly reduced by environmental conditions combined with wear from traffic, de-icing chemicals, and insufficient maintenance. As a result, transportation agencies have been endeavored to find new cost-effective, reliable construction materials. Fiber reinforced polymer (FRP) has shown great promise in eliminating corrosion concerns while also achieving a longer life-span with minimal maintenance [1]. FRP has been used for columns [2], [3], [4], beams [5], [6], and panels [7], [8], [9], [10]. FRP sandwich panels have many advantages, such as high flexural stiffness, strength, and environmental resistance, as well as reduced weight and life cycle cost. Using FRP deck panels should also contribute to accelerated bridge construction. These advantages make FRP sandwich panels an excellent candidate for construction of bridge decks.

Sandwich panels are often composed of two thin facings that are bonded to a much thicker core. The facings are typically made of high strength and stiffness material. The core usually consists of a rigid-foam, which has a low to moderate strength and stiffness [11]. However, the core design is industry-related. The facings are largely responsible for carrying flexural loads while the core provides shear capacity and integrity of the structure [12]. Many alternative forms of sandwich panels can be accomplished by combing different facings and core materials combined with varying geometries. As a result, optimum designs can be produced for specific applications [11].

Researchers and manufacturers have developed many FRP bridge deck designs with honeycomb and cellular cores made of E-glass reinforced polyester or vinyl ester resin. These designs have primarily been manufactured using filament winding, hand lay-up, and pultrusion methods [13]. A honeycomb core is one of the famous cores that being used in sandwich panels, implemented in bridge decks [8], [14], [15], [16], [17], [18], [19], [20]. The honeycomb core consists of sinusoidal wave corrugations and straight components sandwiched between the facings. Testing showed that this type of panels is effective in providing high mechanical performance for minimum unit weight [14], [19].

Researchers have proposed alternative forms for sandwich panels. Potluri et al. [21] proposed a conventional sandwich panel where the top and bottom facings were separated by a foam core. In their study, they introduced FRP stitches to improve the foam core performance. The stitches were used also to prevent core-to-facing debonding. It was found that both static and fatigue structural behavior can be improved by stitching together the top and bottom facings. Hassan et al. and Reis and Rizkalla [22], [23] proposed an alternative system for FRP bridge decks. The proposed panel used three-dimensional fibers (stitches through foam cores) to connect the top and bottom GFRP facings. They observed that the delamination concerns were overcome. In addition, the fiber reinforced stitches increased significantly the core shear modulus. Dawood et al. [24] studied the fatigue behavior of sandwich panels with flexible and stiff cores. They found that the panels with flexible cores exhibit less degradation than those with stiffer cores due to the higher induced shear stresses at the same level of applied shear strain. Zureick [25] used finite element analysis to study different cross-sections of simply supported FRP decks. This study compared four different cross-sections, concluding that the box shaped and V shaped cores behaved much better than the other sections. Although the results from these studies provided a noteworthy understanding of FRP sandwich panel's behavior, most of these results cannot be extrapolated to other products.

The connection between the deck panels to the underlying steel girders is typically made using adhesive glue at the interface, shear studs, bolted connection, or steel clamps in a simply supported condition [26], [27], [28], [29].

Section snippets

Paper scope and objectives

In the present study, small-scale FRP sandwich beams having three different foam core configurations (see Fig. 1) were investigated. The proposed system could reduce the initial production costs and the manufacturing difficulties while improving the system performance. The facings of the proposed three sandwich beams consist of E-glass woven fabric within a compatible polyurethane resin. Each configuration uses polyurethane foam as an infill material for the inner core. The investigated core

Experimental program

This study examined the cross-sections of three different configurations of the closed-cell polyurethane infill-foam beams (see Fig. 1). The facings of the three types consisted of three plies of bidirectional E-Glass woven fabric (WR18/3010) infused with a compatible polyurethane resin. The core of Type 1 was comprised of high-density polyurethane foam that had a mass density of 96 kg/m3. The Type 2 core consists of thin, interconnecting, glass fiber/resin webs that form a bidirectional FRP

Assessment of flexural stiffness (EI)

The flexural stiffness (EI) where E is the equivalent modulus of elasticity and I is the equivalent moment of inertia of the sandwich beam was examined because it is typically the driving factor when designing sandwich panels. The flexural stiffness of each beam was calculated using First-order Shear Deformation Theory (FSDT) [40]. These results were used to compare the flexural stiffness of beams with different core types. The FSDT was also used to estimate the shear stiffness of each sandwich

Polyurethane foam core

Fig. 5a illustrates the average compressive stress–strain curves of the tested low (soft) and high-density (rigid) polyurethane foam cubes. These curves are linear in the elastic region, with a yield region at an average stress of 0.056 MPa for the low-density foam and 1.04 MPa for the high-density foam. The yield behavior can be explained by the buckling of the foam's internal walls. A long flat plateau was followed. Then, a densification (hardening) region was created by a gradual stress

Discussions

The flatwise compressive tests revealed that the Type 2 core was significantly stronger and stiffer than the Type 1 core. These results also revealed excessive deformations under concentrated loads, potentially leading to serviceability issues. The flatwise tensile tests were used to examine the bond quality between the core and the facings. In Type 1, failure occurred in the polyurethane foam itself, as the bond between the foam and the facing was stronger than the foam core. On the other

Numerical study

The low stiffness of the foam materials used in the cores coupled with relatively short spans often lead to complex behavior at the load and support points. As a result, FEM was used to simulate the behavior of the candidate beam to better understand mechanics of the proposed design. As indicated above, Type 3 beam is recommended for real bridge deck applications based on the results of the experimental work. FEM has shown very good accuracy simulating the complex behavior at the loading points

Conclusions

The structural behavior of three different core alternatives for GFRP foam-infill sandwich panels was investigated. The results of our experimental and numerical research demonstrated the engineering and economic feasibility of the proposed design.

All sandwich beams tested in bending exhibited a linear-elastic behavior. This initial response was followed with a stiffness softening prior to failure. The Type 3 construction exhibited better strength as well as flexural and shear stiffness than

Acknowledgments

The authors acknowledge the financial support provided by the Missouri Department of Transportation (TRyy1203) and the National University Transportation Center (NUTC) (DTRT06-G-0014) at Missouri University of Science and Technology.

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