Stiffness characterization in Non-Crimp Fabric Composites

Abstract: Lightweight materials with high stiffness and damage tolerance are requested for aerospace, marine and automotive industries. Many types of composite materials are today used in various types of load carrying structures, due to their excellent strength and stiffness to weight ratio. Simplicity, reliability and low cost of the material processing are important factors affecting the final selection. In the last years new types of composites; Non-crimp-fabric (NCF) reinforced composites, where the cost-efficiency is reached by using dry preforms which are impregnated by resin infusion, resin transfer molding etc.; have made a break-through and have been widely used. As its names indicates, NCF composites consist of layers with ideally straight fiber bundles oriented in different directions, knitted by secondary yarn and separated by resin. This technique of dry preforms impregnated by resin infusion or RTM combine a perfect placement of reinforcement with easy, cheap and automated manufacturing. It produces a composite that can be formed easily in complex shapes, with improvement in damage tolerance as well as the out-of-plane fracture toughness. However, the stitching distorts and crimps the fiber bundles, which leads to large out-of-plane waviness. This deviation affects the mechanical properties of NCF composites. The bundle crimps reduces the stiffness and causes incorrect predictions of the laminate elastic properties employing assumption of the classical laminate theory (CLT). In the present study, the fiber tow waviness is assumed as sinusoidal and the undulation effect on the stiffness reduction is analyzed using Finite Element Method (FEM). The waviness parameters i.e. wavelength and amplitude as well as geometrical parameters like bundle thickness are used in modeling the elastic properties of the representative volume element of the waved structure using meso-scale FEM analysis. The possibility of applying CLT for cross-ply NCF composite stiffness determination is approved, by replacing the curved structure by idealized straight one using effective stiffness for the 0⁰- and the 90⁰- layers. The cross-ply NCF stiffness reduction is dominated by the stiffness reduction of the 0⁰-layer. The 0⁰-layer effective stiffness can be determined either by modeling a single curved tow subjected to distributed load, to reproduce its interaction with the neighboring layers, together with symmetry boundary conditions, or using a master curve approach, where a knock down factor is introduced to characterize the stiffness reduction and analytical expression is suggested. This expressions allows for determination of knock down factor for any given wavelength and amplitude of the waviness.