Effect of inter laminar toughening layers on the diaphragm forming quality of carbon/epoxy prepreg

Diaphragm forming is a composite forming technique which is suitable for manufacturing composite components with relatively simple geometry such as wing spars and stringers of aircraft and composite structural beams for various applications. Unlike the conventional ply-by-ply lay-up onto a tool surface, this method creates a flat laminate preform first and then deforms it into a desired shape, which significantly reduces the processing time. Due to this advantage, diaphragm forming (also called hot drape forming particularly when the forming is carried out at an elevated temperature) is used in conjunction with automated tape laying (ATL) process to improve production rate. One disadvantage of the diaphragm forming is that bending of the stacked flat preform often creates fibre misalignment and out-of-plane wrinkles around corners [1], which are critical load bearing regions [2]. The misalignment of fibres significantly reduce structural performance of the component[3]. The reduction of tensile [3,4], compressive [5] and bending [6] strengths of the composites including artificially made out-of-plane wrinkles has been reported. The conventional method to avoid such wrinkles is to carry out the forming process at an elevated temperature with a low forming rate. Such conditions promote ply slippage by reducing the resin tackiness and viscosity on the prepreg surface as well as the interply shear stress. However, this approach increases the cycle time and energy cost for heating up the preform material and the mould. Additionally, the forming quality can be decreased significantly even with little increase of geometric complexity of the part. This compromises the main advantage of the diaphragm forming and is the major barrier limiting its wider use in industry. The aim of this research was to investigate the diaphragm forming characteristic of carbon fibre/epoxy preforms with interleaving toughening layers, and the effect of such layers on the wrinkle generation during the preforming process. The interleaving materials used in this research were fine phenoxy powders and non-woven polyester veils, which have shown a positive effect in terms of forming quality and fracture toughness in previous work [7]. This paper focuses on investigating the quality of composite structures with such interleaving materials after autoclave curing. In this work, composite C-section beams were manufactured using unidirectional carbon fibre/epoxy prepregs. A reference specimen for comparison was manufactured by manually laying up ply-by-ply directly on the tool surface, leading to an optimal forming quality without forming-induced defects. The test specimens were manufactured using an in-house diaphragm forming rig to form hand-laid flat laminate preforms into C-section beams. The forming process was carried out at room temperature. The composite stacking sequence was symmetric and quasi-isotropic. During the lay-up of the flat laminates, the interleaving materials were applied at each ply interface to improve the interlaminar fracture toughness as well as to promote ply slippage during the forming process. From the forming test results, it was found that the forming quality of the C-section composite beams with the interleaving materials was comparable with that of the reference specimen made by the ply-by-ply W.T. Wang, H. Yu, K.D. Potter, and B.C. Kim manual lay-up method. The investigation showed that the reduced interply friction by the interleaving materials prevents wrinkle generation during diaphragm forming process. However, it also revealed that the interleaving material can affect the compaction behaviour of the laminate preform, which may result in developing out-of-plane ply wrinkles when the thickness reduction of the preform is significant during autoclave curing [8].