Date of Award
Doctor of Philosophy
Energy Science and Engineering
Uday Vaidya, David P. Harper, Brett G. Compton
There is a significant need for low cost, high volume composites in the automotive industry to aid in vehicle lightweighting and safety. The current state-of-the-art severely compromises the mechanical properties of composites to achieve cost and cycle time goals. In this dissertation, a novel composite format, termed discontinuous carbon fiber organosheets, using recycled and repurposed carbon fibers in a thermoplastic matrix is developed and studied. Unlike traditional composites, the long fiber length and rapid processing time yield mechanical properties and cycle times competitive with automotive metals.
Several studies were performed to characterize this new material format. First, samples were manufactured from carded recycled carbon fiber and wet deposited virgin carbon fiber preforms infiltrated with a polyphenylene sulfide (PPS) matrix. The orientation-dependent tensile and shear properties of these composites were characterized. It was found that classical laminate theory in combination with statistical methods could accurately describe their mechanical behavior. Next, the effects of varying processing time and atmosphere, as well as post-process annealing, were studied using dynamic mechanical analysis, differential scanning calorimetry, tensile testing, and nano-indentation. Due to the stochastic nature of discontinuous fiber composites, statistical methods were applied to evaluate changes. Then, a model based on the stochastic microstructure of the composites was developed to predict their strength and modulus. Composites with an acrylonitrile butadiene styrene and a common structural epoxy matrix were also produced and tested to validate the applicability of the model to a range of composites. An ensuing study of the infiltration behavior of the preforms was used to optimize the processing conditions for rapid production. Here, fluid permeability and preform compression were studied to evaluate an existing model of composite infiltration, which was validated by in-situ compression molding experiments.
Finally, the composites were used to produce complex geometry samples for a study on their crashworthiness. Sinusoidal specimens were produced from PPS, ABS, and epoxy composites and tested at a range of temperatures and loading rates relevant to automotive applications. The results showed that discontinuous fiber organosheet composites not only achieve competitive mechanical properties but also exceed the crashworthiness of traditional composite materials.
Barnett, Philip R., "Discontinuous Recycled and Repurposed Carbon Fiber Reinforced Thermoplastic Organosheet Composites. " PhD diss., University of Tennessee, 2020.