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Discontinuous Recycled And Repurposed Carbon Fiber Reinforced Thermoplastic Organosheet Composites, Philip R. Barnett

Doctoral Dissertations

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 …

Design Of Materials And Processing Methods For High Temperature Composites, Chris Monteleone

Doctoral Dissertations

In this dissertation, various studies are described which seek to improve the properties of high temperature ceramic-matrix and carbon-matrix composites. Processes were developed and optimized to control the compositions, microstructures, and morphologies of ceramic materials. Focus was on the chemical vapor infiltration (CVI) of coatings on continuous fiber substrates. Other processes were involved as well, such as polymer impregnation & pyrolysis (PIP) and silicon melt infiltration (SMI). Both carbon and SiC fibers were used as substrates. Materials deposited using CVI include carbon, SiC, TiC, BN, and B4C. Characterization methods include scanning electron microscopy, mechanical testing, X-ray diffraction, and others.

A …

Silk Fibroin And Hydroxyapatite Composites For Bioresorbable Bone Fixation Devices, Bryant Heimbach

Doctoral Dissertations

Every year there are approximately 9 million bone fractures in the United States, and 30% of these require an internal fixation device to help heal. Currently, the gold standard for fixation devices relies on the use of metals because of their high mechanical properties and bioinertness. However, metal implants often require a second surgery to remove them because they cause stress shielding and metal ion leaching. Current bioresorbable fixation devices on the market have poor mechanical properties and are limited to use in non-load-bearing applications (i.e. maxillofacial fractures). As such, there remains a gap in the fracture fixation devices on …

Freeform Extrusion Fabrication Of Advanced Ceramics And Ceramic-Based Composites, Wenbin Li

Doctoral Dissertations

"Ceramic On-Demand Extrusion (CODE) is a recently developed freeform extrusion fabrication process for producing dense ceramic components from single and multiple constituents. In this process, aqueous paste of ceramic particles with a very low binder content ( < 1 vol%) is extruded through a moving nozzle to print each layer sequentially. Once one layer is printed, it is surrounded by oil to prevent undesirable water evaporation from the perimeters of the part. The oil level is regulated just below the topmost layer of the part being fabricated. Infrared radiation is then applied to uniformly and partially dry the top layer so that the yield stress of the paste increases to avoid part deformation. By repeating the above steps, the part is printed in a layer-wise fashion, followed by post-processing. Paste extrusion precision of different extrusion mechanisms was compared and analyzed, with an auger extruder determined to be the most suitable paste extruder for the CODE system. A novel fabrication system was developed based on a motion gantry, auger extruders, and peripheral devices. Sample specimens were then produced from 3 mol% yttria stabilized zirconia using this fabrication system, and their properties, including density, flexural strength, Young's modulus, Weibull modulus, fracture toughness, and hardness were measured. The results indicated that superior mechanical properties were achieved by the CODE process among all the additive manufacturing processes. Further development was made on the CODE process to fabricate ceramic components that have external/internal features such as overhangs by using fugitive support material. Finally, ceramic composites with functionally graded materials (FGMs) were fabricated by the CODE process using a dynamic mixing device"--Abstract, page iv.

Melt Processability And Post-Processing Treatment Of High Temperature Semi-Crystalline Thermoplastics For Extrusion Deposition Additive Manufacturing, Vidya Kishore

Doctoral Dissertations

Composites of high performance semi-crystalline thermoplastics are emerging as viable candidates for autoclave tooling applications, which typically demand good mechanical and thermal performance, as well as chemical resistance. With the growing demand for tooling, there is a need for versatile customizable tools that can be inexpensively manufactured with low lead times. Extrusion Deposition Additive Manufacturing (EDAM) technique can be extremely beneficial for the manufacture of low-cost composite tools with high degrees of customizability and short lead times. However, EDAM processes for such high temperature polymers can pose several challenges for extrusion and deposition due to their inherent properties as well …

High Temperature Polymer Composites Using Out-Of-Autoclave Processing, Sudharshan Anandan

Doctoral Dissertations

"High performance polymer composites possess high strength-to-weight ratio, corrosion resistance, and have design flexibility. Carbon/epoxy composites are commonly used aerospace materials. Bismaleimide based composites are used as a replacement for epoxy systems at higher service temperatures. Aerospace composites are usually manufactured, under high pressure, in an autoclave which requires high capital investments and operating costs. In contrast, out-of-autoclave manufacturing, specifically vacuum-bag-only prepreg process, is capable of producing low cost and high performance composites. In the current study, out-of-autoclave processing of high temperature carbon/bismaleimide composites was evaluated. The cure and process parameters were optimized. The properties of out-of-autoclave cured laminates compared …

Studies On The Electrical Transport Properties Of Carbon Nanotube Composites, Taylor Warren Tarlton

Doctoral Dissertations

This work presents a probabilistic approach to model the electrical transport properties of carbon nanotube composite materials. A pseudo-random generation method is presented with the ability to generate 3-D samples with a variety of different configurations. Periodic boundary conditions are employed in the directions perpendicular to transport to minimize edge effects. Simulations produce values for drift velocity, carrier mobility, and conductivity in samples that account for geometrical features resembling those found in the lab. All results show an excellent agreement to the well-known power law characteristic of percolation processes, which is used to compare across simulations. The effect of sample …

Performance Evaluation Of Polyurethane Composites Using Vacuum Infusion Process, Mohaned M. Mohamed

Doctoral Dissertations

"Glass fiber-reinforced polymer composites have promising applications in infrastructures due to their low cost, high specific stiffness/strength, and corrosion resistance. The pultrusion process is often used to manufacture glass fiber-reinforced polyurethane (PU) composite. The objective of this study is to use thermoset PU resin to manufacture high quality composites for infrastructure applications using vacuum assisted resin transfer molding (VARTM) process. Using VARTM to fabricate PU resins presents unique challenges. Several modifications however have helped with overcoming these problems. In part I of the research, the mechanical performance of two different PU resin not only evaluated but also compared to one …