Analytical modeling of local stresses at Balsa/timber core joints of FRP sandwich structures

In order to optimize sandwich structures, core materials of different density and thus different stiffness and strength are often combined. The material discontinuities at joints of dissimilar core materials may lead to abrupt changes of the shear angles and local bending of the face sheets at these locations, both of which may cause significant stress concentrations. Two analytical models have been developed to estimate local stress concentrations at butt and scarf joints of dissimilar sandwich cores. The first model predicts local axial stresses in the face sheets at scarf joints while the second model predicts local shear stresses in cores at both butt and scarf joints. The models are based on an existing closed form model and have been validated by FEM for Balsa-Douglas fir butt and scarf joints, both implemented in a sandwich with glass fiber-reinforced polymer (GFRP) face sheets. The models are applicable for scarf joints with angles of termination between 20 degrees and 60 degrees. They also demonstrate the positive effect of scarf joints compared to butt joints. The axial stress concentrations in the face sheets and shear stress concentrations in the cores almost disappear if a butt joint is changed to a scarf joint configuration. (C) 2014 Elsevier Ltd. All rights reserved.

Biaxial Wrinkling of Thin-Walled GFRP Webs in Cell-Core Sandwiches

Behzad Dehghan Manshadi

Fiber-reinforced polymer (FRP) sandwich structures offer several advantages compared to structures made of traditional materials, such as high specific strength, good corrosion resistance, low thermal conductivity and rapid component installation. In this context, glass fiber-reinforced polymer (GFRP) cell-core sandwiches composed of outer GFRP face sheets, a foam core and a grid of GFRP webs integrated into the core to reinforce shear load capacity are well suited for load-bearing applications in civil engineering i.e. in bridge deck and roof construction. Despite the great potential of these structural concepts, the use of heterogeneous materials in FRP sandwiches results in more complex failure mechanisms compared to conventional structural components and lack of knowledge regarding the prediction of failure modes makes the design of structural components difficult. This is one of the major disadvantages limiting the acceptance of cell-core sandwiches in civil engineering applications. One of the critical failure modes of cell-core sandwich structures is wrinkling in the webs. A great deal of information exists concerning the phenomenon of skin wrinkling failure of sandwich laminates loaded in compression but comparatively little on wrinkling in the webs of sandwich structures where the pure compression loading is complicated by supplementary transverse tension. The purpose of this research is to develop an appropriate model for the prediction of wrinkling in the webs of cell-core sandwich structures. Two new approaches were developed to predict the wrinkling loads of webs. The first approach examines the wrinkling behavior in webs as an in-plane biaxial compression-tension buckling problem according to the rotated stress field theory. In this regard, extensive experimental, numerical and analytical studies were performed to investigate the interaction between the compression and tension stress tensors during the buckling/wrinkling instability phase of GFRP plates and sandwich panels subjected to biaxial compression-tension loading. The investigations demonstrated that the transverse tension in the biaxial compression-tension set-up induced two simultaneous counteracting effects: a stabilizing and a lateral contraction effect. The stabilizing effect tends to push the plate back to the median plane and thereby delays the onset of buckling/wrinkling instability. In contrast, lateral contraction accelerates the bending of the plate, which leads to a significant decrease in buckling/wrinkling loads. In composite plates, the first effect predominates and increases the buckling loads while in sandwich panels the second effect is dominant and decreases the wrinkling loads. Using the second approach, the wrinkling behavior of foam-filled web-core panels was modeled by applying an improved mixed-mode interaction formula in which two approximate models are developed based on the energy method in order to determine the critical loads when the pure shear and bending stresses act independently on the web. The application of both approaches to a real case study, the GFRP cell-core sandwich roof of the Novartis Campus Main Gate Building proved that they are sufficiently accurate to be used as valid tools assisting the optimum design of sandwich structures whereas existing models result in too conservative predictions.

EPFL2011

Total Light Transmittance of Glass Fiber-Reinforced Polymer Laminates for Multifunctional Load-Bearing Structures

Julia De Castro San Roman Fest, Thomas Keller, Carlos Pascual Agullo, Andreas Schueler

Glass fiber-reinforced polymer (GFRP) materials are increasingly used in building construction for the design of multifunctional structures. These composite materials allow the integration of structural functions, building physics functions (mainly thermal insulation) and architectural functions (complex forms and color) in single large-scale building components. GFRP materials also allow the fabrication of translucent structural components with high degree of transparency when optically aligned resins and glass fibers are used, i.e. their refractive indices are identical. In this study the total light transmittance of hand lay-up GFRP laminates for building construction was investigated with a view to two architectural applications: translucent load-bearing structures and the encapsulation of photovoltaic (PV) cells into GFRP building skins of sandwich structures. Spectrophotometric experiments using an integrating sphere set-up were performed on unidirectional and cross-ply GFRP specimens in the range from 20% to 35% fiber volume fraction. Results were compared with the shortcircuit currents generated by amorphous silicon (a-Si) PV cells encapsulated in GFRP laminates exposed to artificial sunlight radiation. The total amount of fibers in the laminates was the major parameter influencing light transmittance, with fiber architecture having little effect. 83% of solar irradiance in the band of 300-800 nm reached the surface of a-Si PV cells encapsulated below structural GFRP laminates with a fiber reinforcement weight of 820 g/m2, demonstrating the feasibility of conceiving multifunctional GFRP structures.

EPFL Solar Energy and Building Physics Laboratory (LESO-PB)2013

Structural performance of complex core systems for FRP-balsa composite sandwich bridge decks

Michael Osei-Antwi

Based on current fiber-reinforced polymer (FRP) composite construction principles, FRP decks fall into two categories: pultruded decks and sandwich decks. Sandwich decks comprise face sheets and either honeycombs or foams reinforced with internal FRP webs for shear resistance. The honeycomb structure and the webs cause debonding between the upper face sheets and the core due to the uneven support of the former. An alternative material that has high shear capacity and can provide uniform support for the upper face sheet is balsa. Balsa panels have therefore been proposed as the core material for sandwich decks in this research work. Balsa panels are produced by adhesively bonding dissimilar balsa blocks, resulting in a non-homogenous and anisotropic material. These inherent characteristics are not taken into account in the current shear behavior of balsa, thus making it unreliable. Balsa also exhibits high ductility when subjected to compressive loads, however, the shear ductility required by engineers to design safe sandwich structures is lacking. Furthermore, currently existing GFRP-balsa sandwich bridge concepts can only be applied to short-span bridges due to high cost and manufacturing challenges in the case of sandwich slab bridges. In hybrid sandwich deck-steel girder bridges, low bending stiffness in the bridge direction and low composite action in the deck have been the drawbacks. The purpose of this research is to develop novel concepts for lightweight, stiffer and stronger sandwich decks, using balsa cores, which can be fabricated with fewer manufacturing challenges and offer longer spans than existing decks. Balsa panels were experimentally investigated to establish their shear properties and shear ductility at the three orthotropic shear planes. The influence of shear plane, balsa density and adhesive joints on the shear properties was quantified. Two new GFRP-balsa sandwich bridge concepts (complex core systems) have been proposed for long-span bridges. In the first concept, the sandwich core comprises high- and low-density balsa cores and an FRP arch reinforced at the core interface. Sandwich beams based on this concept were experimentally investigated to evaluate their structural performance. The beams demonstrated high bending stiffness and strength and were lightweight. Crack initiation and propagation in the balsa blocks of the complex balsa core could finally be explained. A new analytical model to predict the bending behavior of the new sandwich beams was developed. The second bridge concept involves integrating timber inserts into the balsa core of a sandwich deck. GFRP-balsa sandwich beams, with timber inserts, were numerically investigated to evaluate their structural performance. High stress concentrations occurred in the face sheets and cores at the balsa/timber core joints which were eliminated by changing the core joints from butt to scarf. An optimum angle of termination of scarf joints, based on low stress concentrations at the joints, low costs and manufacturing challenges, was recommended in the design of GFRP-balsa sandwich decks. An existing analytical model for predicting axial stress concentrations in face sheets at butt core joints was extended to scarf joints and a new analytical model was developed to predict shear stress concentrations in the cores at both butt and scarf joints. Finally, the results obtained from the experimental work, the new models and the two proposed sandwich bridge concepts were implemented in the design of new GFRP-balsa sandwich slab bridges and decks. The structural limits of the new bridges were established and the potential of the new GFRP-balsa sandwich deck to replace a reinforced concrete deck was explored. Taking manufacturing limits into account (800-mm slab thickness) and using the proposed complex core system, sandwich bridge slabs of up to approximately 19 m can be constructed. Furthermore, GFRP-balsa sandwich decks bonded on steel girders can reach spans of up to 30 m. The findings of this research work therefore establish that deteriorated reinforced concrete decks with long spans of even up to 30 m can be replaced by GFRP-balsa sandwich decks. Also, based on this work, in the future, existing reinforced concrete decks can be widened using the proposed sandwich decks due to their lightweight, which has been extensively demonstrated. This will lead to cost saving since the bridge substructure will no longer need to be reconstructed.