Extensional Viscosity of Thermoplastic TuFF Composites in Stretch Forming Processes

Abstract

Tailored Universal Feedstock for Forming (TuFF) is a novel composite material form consisting of highly aligned short fiber reinforcement. The high degree of alignment allows for fiber volume fractions up to 60% and mechanical performance equivalent to aerospace grade continuous fiber composites. The discontinuous fiber format has demonstrated the ability to allow stretch forming of composite blanks in the fiber direction. This work studies the material behavior during stretch forming to develop constitutive laws for the fiber direction deformation. It is shown that shearing of neighboring fibers through the melt phase thermoplastic matrix imparts a viscous deformation response. Longitudinal fiber direction viscosity was measured as a function of strain, strain-rate, and temperature under various processing conditions. Digital image correlation (DIC) was used to measure the local surface strain data for uniaxial tension tests and derive the extensional viscosity data. A micromechanics model which accounts for polymer properties and microstructure features was formulated to validate the experimentally derived constitutive law.