Extensional Viscosity Of Highly Aligned Discontinuous Fiber Composites In Stretch Forming Processes

Abstract

The demand for high quality materials in the aerospace industry has led to continuous aligned fiber composites to be a popular choice in many applications. However, these materials suffer from high costs and manufacturing restrictions. In addressing these problems, aligned discontinuous fiber composites (ADFCs) are being evaluated as an alternative. The relatively recent development of these composites has resulted in a gap in literature regarding fiber direction deformation. The work herein develops a model to predict the extensional deformation of ADFCs for a given microstructure. From a selected matrix material, known fiber geometries, and a desired fiber volume fraction, extensional viscosity is predicted as a function of temperature and strain rate. Model validation was conducted with uniaxial stretch tests on a single ply of thermoplastic matrix AFDC in the fiber direction. These tests implemented digital image correlation techniques to accurately measure strain and inform microstructural nonuniformity. Using this method, experimental extensional viscosity was compared to the model for known input parameters. The model is comprised of a piecewise Newtonian and shear thinning viscosity prediction. Relaxing constituent material assumptions and improving the Newtonian and shear thinning regimes for the polymer were considered to align observations with the model. Implementing these methods was shown to successful bound extensional viscosity data. Nevertheless, the start of the shear thinning regime was not accurately predicted by the model. Further investigation of porosity and fiber reorientation is suggested to explore possible explanations for early onset shear thinning.