Investigating In-plane Shear Behaviour of Uncured Unidirectional Prepreg Tapes

Sammanfattning: Steering of prepreg tows in an automated fibre placement (AFP) process allows geodesic layup trajectory over a doubly-curved surface, as well as the potential to improve the efficiency of composite structures by tailoring their stiffness. However, defects (such as out-of-plane buckling and tow pull off) are commonly generated when the steering radius exceeds a critical limit, which impact the mechanical properties of the finished product. The in- and out-of-plane material properties of the prepreg tows have been shown to significantly influence the quality of the layup. In this thesis, in-plane shear behaviour of uncured IMA-M21 unidirectional (UD) prepreg was characterised using an off-axis tensile test to derive material parameters for process models to predict steering limits and defects. Test parameters, such as shear strain rates and temperatures, that were investigated were consistent to the actual AFP process. The results demonstrates the expected strain-rate and temperature dependencies related to the resins viscoelastic behaviour. Subsequently, a novel micro-mechanical finite element (FE) simulation of a 3-dimensional UD prepreg unit cell in pure shear was conducted to gain qualitative insights into the complex rheological behaviour at play. It effectively demonstrates how fibre friction, resin viscosity and shear strain rates influence the load transfer between fibres and melt, as well as the movement of fibres during the shearing process. These were reflected in the shear stress-strain curves generated in the simulation and elastic micro-buckling observed in the fibre elements. The results pave a way for future development of a robust material model for predicting the critical process parameters to achieve quality layups from AFP steering process.