PREFORMING OF MULTI-PLY NON-CRIMP FABRIC LAMINATES USING DOUBLE DIAPHRAGM FORMING

Double diaphragm forming (DDF) is a cost effective, semi-automated method of producing non-crimp fabric (NCF) preforms for use in liquid composite moulding processes. It relies on vacuum-only generated forming forces and single-sided tooling to produce complex double-curved fabric preforms. However, the formation of wrinkles presents a significant barrier to the wider adoption of DDF, as they reduce the efficacy of the mechanical properties of the finished composite component. The fabric shear and bending deformation mechanisms that are responsible for generating these wrinkles are well understood for thin laminates constructed of only one or two plies, but the influence of inter-ply friction complicates matters, particularly for thicker preforms with higher ply counts. This paper aims to characterise the bending and shear behaviours of multi-ply laminates using modified cantilever bend and bias extension tests respectively. It is demonstrated that the inter-ply friction increases both the bending and shear stiffnesses of a dry fibre preform, reducing its formability. A DDF case study is presented for an industrially relevant geometry, demonstrating the combined effect of the number of plies and the layup sequence on the formability of the preform.