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Nicolas Krumenacker and Pascal Hubert quality and performance as their autoclave counterparts, while significantly lowering equipment and operating costs, improving energy efficiency, and streamlining production [2-4]. High-quality, flat composite laminates can already be readily achieved via VBO processes thanks to advances in dedicated prepregs that feature improved air-evacuation and optimized resin chemistry [5-9]. More recently, such processes have been used to cure a select few aircraft parts of increasing size, complexity, and structural importance [4, 9, 10]. Despite this recent progress, the key challenge for OOA/VBO technologies remains the inherent variability observed in cured complex-shape regions (e.g. corners) that primarily exhibit significant local thickness deviation and porosity, as well as in- and out-of-plane fibre waviness. The accelerated insertion of these technologies in high-end industries hinges on a deeper understanding of the influence of processing parameters on laminate consolidation and mechanical performance in complex-shape laminates. In addition, the development of robust predictive tools that can accurately predict thickness deviation porosity and their combined effect on mechanical performance will further help to bolster industry confidence in and acceptance of these developing technologies. 1.2 Literature survey A decent understanding of the compaction of complex-shape laminates exists in the literature stemming from lab-scale studies that have been mostly conducted on autoclave-cured laminates [11- 13], and to a lesser extent on VBO-cured laminates [14-19]. All studies irrespective of the chosen process have observed local thickness deviation in complex-shape regions: thickening is prevalent in most cases over both convex and concave tool features; in turn, thinning occurs only over convex tool features and given specific fibre orientation (i.e. normal to the gradient) and resin bleeding conditions. Hubert and Poursartip first offered a fundamental explanation for this local thickness deviation [20]. In order to satisfy force equilibrium in complex-shape regions, the tool-side reaction and bag- side applied pressures must differ given that the local tool and vacuum bag surface areas likewise differ. The resulting consolidation pressure differential in complex-shape regions induces a local thickness deviation from that induced in flat regions. Brilliant and Hubert proposed a model based on this explanation to predict the thickness deviation in VBO-cured right-angle corner laminates [14]. The model assumes inter-ply slippage during compaction and resin impregnation and accounts for the prepreg bulk factor, though it fails to account for inter-ply friction and other phenomena that may impede compaction. Levy et al. proposed a subsequent model that incorporates inter-ply friction and highlighted two overarching mechanisms that drive local thickness deviation: a) reduction in consolidation pressure due to inter-ply slippage as originally proposed by Hubert and Poursartip, and b) geometric constraint when ply slippage does not occur [18]. Further work is required to better understand and capture additional effects such as operator-induced variability, which may have a large impact on lab-scale experimental data as VBO-cured laminates are typically manually laid-up and vacuum-bagged. Besides thickness deviation, it should also be emphasized that local reductions in consolidation pressure will result in higher porosity and thus poorer matrix-dominated properties. Whereas both autoclave- and VBO-cured complex-shape laminates exhibit local thickness variation, OOA prepregs are prone to greater deviations due to their generally much larger bulk factors. They are only partially impregnated and contain a dry fibre mid-plane region in the case of unidirectional tape and dry intra-tow channels in the case of bidirectional woven architectures. These dry regions are designed to maximize air-evacuation during laminate compaction and resin impregnation [5, 6, 9]. A large thickness reduction thus occurs during the laminate compaction and resin impregnation phases of the cure. This “de-bulking” can have a drastic impact on local thickness deviation in the common occurrence that inter-ply slippage is restricted, in which case fibre bridging will ensue over concave tool features and fibre compression and waviness will ensue over convex tool regions [14]. In addition, the limited consolidation pressure in VBO processes (≤ 1 atm) amplifies the effect that other processing parameters can have [16]. The high consolidation pressure present in the case of autoclave curing would otherwise suppress such effects. Hence, higher prepreg bulk factors and limited consolidation pressure are expected to increase local thickness deviation in VBO-cured complex-shape laminates compared to similar autoclave-cured laminates.PDF Image | VACUUM-BAG-ONLY COMPLEX-SHAPE PREPREG LAMINATE
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