Co-authors​ :

 Yuan WU (FRANCE), Pascal CASARI (FRANCE), Jamal FAJOUI (FRANCE), Sylvain FRÉOUR , Marianne PERRIN (FRANCE), Marie-Laetitia PASTOR , Xiaojing GONG  

Due to the increasing concern about climate change on our planet, bio-based composite materials, such as wood or eco-foam core sandwich structures, have attracted more attentions in recent years. A common sandwich structure is composed of a lighter core and two thinner laminated skins [1-2], so it is designed to ensure higher specific strength and stiffness. If the core is made from bio-based natural plant fibers, then the carbon footprint of a sandwich structure can be reduced during its entire life cycle. However, natural plant fibers often exhibit high hydrophilic property, which could affect their mechanical performance. The moisture diffusion mechanism is highly dependent on the physical and chemical properties of the material, dimensions of the structure, and the ambient temperature etc. For example, moisture diffusion coefficients and saturated Moisture Content (MC) of balsa wood, which is a preferred core material, could vary with the density and thickness. When balsa serves as the core, owing to the protection from the stiffer skin, moisture absorption behavior in the sandwich can be different from that in the pure balsa. Therefore, this work investigates the moisture diffusion behavior of bio-based balsa wood material and sandwich structure [1-2]. The skins of the sandwich specimens were made of 3-ply Glass-Fiber-Reinforced-Polymer (GFRP) laminates. All the balsa wood and sandwich specimens were immersed in water at room temperature until the final mass equilibrium, and the Moisture Content (MC) absorption kinetics were identified analytically and numerically. The final saturated MC in the GFRP-balsa sandwich was only 18 % of that in the pure balsa wood. Considering the sustainability issue of reducing the carbon footprint in composites, it verifies that a composition of the eco-friendly balsa wood and conventional GFRP laminates can be a good alternative to pure hydrophilic wood materials [3] or high-carbon-footprint laminates. Fickian behavior of moisture diffusion was found in the pure balsa wood, while Non-Fickian behavior [4] was found in the sandwich. Thus, two different moisture diffusion models, Fickian model and Dual Fickian model, were considered to calculate the moisture diffusion coefficients and the saturated moisture absorption capacity of the balsa wood core and GFRP skin, respectively. These parameters were then input into Abaqus models to predict the MC absorption and distribution in the balsa wood core and GFRP skin. It has been found that the mesh size of the balsa wood core has a great influence on the prediction accuracy of the numerical models. In the initial stage of Dual Fickian behavior, it is the moisture diffusion coefficient in the thickness direction of the balsa wood core that contributes more to the moisture absorption rate of the sandwich.