PSI - Issue 52

Wouter De Corte et al. / Procedia Structural Integrity 52 (2024) 99–104 W. De Corte, J. Uyttersprot & W. Van Paepegem / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction and background During the last decade, web-core sandwich glass fibre reinforced polymer (GFRP) composite panels have gained substantial attention for small footbridges (Canning and Luke 2010; De Corte and Uyttersprot 2022; Smits 2016). In this bridge type, a flexural rigid structural element is created through a sandwich construction comprising upper and lower GFRP laminate flanges separated by a core material (e.g. PUR foam). However, due to the nature of the bridge application, the core material cannot resist all shear forces and concentrated local pressure forces and therefore requires to be complemented with longitudinal and in some cases transverse GFRP webs, hence the denomination web-core sandwich panels (De Corte et al. 2017, 2018). From previous research (Uyttersprot, De Corte, and Ingelbinck 2021) it is demonstrated that only the serviceability limit state (SLS) requirements (i.e. deflection, first natural flexural frequency and vertical accelerations) are relevant for the design of lightweight GFRP footbridges. As a result, the stiffness of the upper and lower flange of the bridge deck is are most important, while the strength will be used to a limited extent (i.e. maximum 20%) in a conventional footbridge design. In the search for ever longer and slimmer footbridges with limited maintenance requirements during their lifetime, GFRP materials can play an important role. However, it will not always be possible to achieve larger spans by installing intermediate support points (FiberCore n.d.), because of a free passage under the bridge deck or clamping the bridge deck on both sides due to the high investment costs and/or the limited strength of the soil. In this case, the stiffness of the footbridge itself has to be increased, for example by integrating steel profiles or plates in the structure of the GFRP footbridge. An innovative solution is to integrate cold-formed steel plates between the fibre layers of the top and bottom flange composite (Veltkamp and Kroon n.d.). Due to the integration of the plates in the composite by means of vacuum infusion (Creasy 2012; Liu 2012), the steel plates will also be protected against environmental conditions (i.e. moisture, UV, impact) by the matrix. In this paper hybrid steel / glass fibre composite laminates are explored, with the aim of increasing the longitudinal stiffness of the composite, while maintaining the vacuum infusion based manufacturing process. The results in this paper are obtained using tensile testing together with DIC processing. In the following sections, the test pieces and test method are discussed before proceeding to a presentation of the obtained results, including a short discussion. Finally, a summary of the results and conclusions is given in the last section. 2. Materials and methods The hybrid composite studied in this paper consists of a glass fibre reinforced polymer (GFRP) combined with a single steel plate with a certain permeability. The GFRP on itself consists of unidirectional (UD) multiaxial E-Glass fibres and an unsaturated polyester resin, Distitron 3501 LS1. 2% MEPK Butanox M-50 is used as a hardener and 0.1% inhibitor NLC-10 is added to improve the pot life and processability of the resin for infusion. A decoupling of the strains and curvatures in the laminate is achieved by obtaining a quasi-isotropic balanced symmetrical layer structure with respect to the central plane. The final structure of the GFRP is [[0/90/±45/0] S ] S , and the steel plates are integrated in the middle of the layer structure, on the one hand to maintain the symmetry, on the other hand so that sufficient resin encloses the steel plate. The VARI method is used for the production of the hybrid steel-GFRP laminates, in which the polyester resin is drawn into the mould using a vacuum. In order to obtain a good flow of the resin in the plate during the VARI method and to prevent internal dry spots in the laminate, two perforated steel plates with a different permeability and one steel wire mesh were used in the different hybrid laminate specimens. The perforated steel plates, further referred to in the text as Steel A and Steel B, consist of cold-formed DC01 steel with round perforations in a square pattern. Due to the square grid of the round perforations, the occurring stress concentrations decrease compared to non-circular perforations or a triangular pattern (i.e. staggered pattern). Further, the cold-formed steel has the advantage that it is available in much smaller thicknesses compared to hot-rolled steel and is easier to process (i.e. bending, cutting). The perforated steel plates are sandblasted and degreased with isopropanol before they are infused into the layer structure in order to obtain an improved bonding between the polyester resin and the steel. The geometric properties of Steel A and Steel B are shown in Table 1.