Characterization of Material Response of Highly Aligned Discontinuous Fiber Composites During Loading

• Charles Whealton, Mechanical Engineering, University of Delaware

• Thomas Cender, Center for Composite Materials, University of Delaware

Highly aligned discontinuous fiber (ADF) composites are a material class which is stretch formable in the fiber direction when the polymer matrix is viscous, which has several manufacturing process advantages over continuous fiber composites. This formability is a subject of particular interest. Previous work has focused on understanding the viscous material deformation under constant temperature and strain rate. The extensional viscosity of an ADF composite is a function of the polymer viscosity and the microstructural properties of the composite (fiber volume fraction, aspect ratio of the fibers). The models correlating the effect of the microstructure have underpredicted viscosity at low strain rates, as well as the relaxation time corresponding to the onset of shear thinning. To understand why these issues occur, the material was analyzed under a viscoelastic (viscous and elastic) framework using the Maxwell material model to understand loading (constant strain rate). With the framework established, a series of stress relaxation tests above the glass transition temperature of the thermoplastic polymer matrix (250-350C). The viscosity and relaxation time were found by fitting the Maxwell model to the data. Finally, elasticity was calculated using a constitutive (observed) relationship between viscosity and modulus. This data allowed observations to be made about how each changed with each processing condition. Relaxation time and viscosity varied with strain rate by a power law, and nearly linearly with temperature, though this may be due to a shortened range of temperature data. Elasticity remained generally constant, though in the case of temperature this (again) could be the rest of not a large range of data. Future work will examine the stress relaxation behavior in order to determine multiple relaxation modes as well as time-temperature superposition shift factors.