The influence of inter-ply friction during double-diaphragm forming of biaxial NCFs

Inter-ply friction plays an important role in the formation of defects in automated preforming of complex components, as relative sliding between plies can generate local compressive stresses in the direction of the primary yarns, leading to out-of-plane wrinkling and other defects such as fibre buckling and bridging. This work presents a novel characterisation method for measuring the coefficient of friction at fabric-fabric interfaces, using a set up that can generate inter-ply slip conditions representative of those experienced during double diaphragm forming. Results for a pillar stitched, biaxial NCF indicate the sensitivity of the coefficient of friction to the level of applied compaction pressure generated by the diaphragm. Friction values are observed to be typically 100% higher than those recorded using a conventional sled test (ASTM D1894). Additionally, this behaviour is dependent on the relative fibre orientation at the inter-ply interface, which affects the nesting/compaction behaviour of the plies. This has been confirmed using optical micrographs and through-thickness compaction tests. The significance of these orientation and pressure dependencies has been studied using a modified hemisphere forming rig to control the degree of inter-ply slip. Results from the test indicate that punch force is dependent on the fibre angles at the inter-ply interface, and that variation in the local friction behaviour can affect the formability of a component and induce unwanted defects.