Air permeability of balsa core, and its influence on defect formation in resin infused sandwich laminates
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Many large composite structures are manufactured using sandwich laminates to achieve high specific bending strength and stiffness. For wind turbine blades, the self-weight becomes increasingly important as blade size increases. Resin infusion of three-dimensional sandwich laminates can result in complex flow paths, and subsequent defect formation is difficult to predict. The core material used for sandwich construction and its interaction with liquid resins may also influence the formation of defects. In the case of balsa, this effect can be used to reduce defect severity. This paper considers the effect of cored sandwich laminate construction on the formation of defects. The primary focus is the characterisation of commonly used core materials and their interaction with liquid resin under high vacuum conditions. For balsa core, experiments indicate that the available pore space can act as a sink for trapped air, which can aid the reduction of defects when multiple flow fronts converge due to the complex flow paths in sandwich laminates. Empirical data for air absorption and desorption rates in balsa core were obtained using a custom-designed experiment. Using these data, a theoretical model was developed that can indicate available pore space, which in turn informs optimum processing conditions, such as time under vacuum. The diffusion coefficients obtained for air absorption and desorption in balsa are very similar, and lie in the middle of published ranges for hard woods at around 2 × 10-7 m2/s. The methodology developed represents actual behaviour of air absorption/desorption during resin infusion, whilst other techniques do not, merely measuring diffusion of air through a sample while not allowing for finite pore space. In consequence, infusion strategies can be planned more precisely because the core/resin interaction is better understood. Knit line defect formation could be predicted with greater accuracy with suitably modified flow-modelling programs.
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