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Theses and Dissertations

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Browsing Theses and Dissertations by Subject "additive manufacturing"

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    Additive manufacturing of an intervertebral disc repair patch to treat spinal herniation
    (Colorado State University. Libraries, 2021) Page, Mitchell Ian, author; Puttlitz, Christian, advisor; Heyliger, Paul, committee member; Susan, James, committee member; Kirk, McGilvray, committee member
    Chronic low back pain is ubiquitous throughout society. The consequences of this disease are extensive and lead to physical, mental, and financial suffering in the affected population. Herniation of the intervertebral disc (IVD) is the primary cause of chronic low back pain due to the essential mechanical role of the IVD in the spinal column. Degenerative changes to the IVD tissues, in particular the annulus fibrosus (AF), lead to a pronounced vulnerability to herniation. Although numerous treatments for intervertebral disc herniation currently exist, these treatments are typically palliative and prone to hernia recurrence. Accordingly, there is a distinct need for an IVD hernia therapy that can provide long-term pain relief and recovery of spinal function. One novel strategy to repair the intervertebral disc is to tissue-engineer a construct that facilitates regeneration of the healthy and functional IVD tissue. Advances in additive manufacturing technology offer the fabrication of complex tissue-engineered structures that augment biological content and biocompatible materials. Therefore, this work sought to engineer an additive manufactured repair patch for IVD herniation towards an improved treatment for chronic low back pain. Specifically, the aims of this work were to leverage experimental and computational methods to: (1) to characterize the mechanics of additive manufactured angle-ply scaffolds, (2) evaluate the tissue response of cell-laden scaffolds cultured with dynamic biaxial mechanical stimuli, and (3) to design and implement an annulus fibrosus repair patch. The mechanics of additive manufactured scaffolds for AF repair were experimentally characterized in a physiologically-relevant, biaxial loading modality. To assess sensitivity of the scaffold mechanics to additive manufacturing parameters, a broad scope of scaffold designs were evaluated with a parameterized finite element model. A custom incubator was developed, cell-laden scaffolds were cultured with a prescribed, multi-axial mechanical loading protocol, and ECM production within the scaffold was evaluated. A finite element model was developed to aid in understanding the relationship between global scaffold loading and the local, inhomogeneous cellular micromechanical environment within the scaffold. The developed TE material was translated into an implant and was implemented in a large animal model. The efficacy of the AF repair strategy was also evaluated in finite element model of the human lumbar spine. This work formed a multi-scale approach to consolidate biological and mechanical efficacy of a novel AF repair strategy. Ultimately, this approach may facilitate regeneration of the AF and represent a revolutionary treatment for chronic low back pain.
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    Development of a novel additive manufacturing method: process generation and evaluation of 3D printed parts made with alumina nanopowder
    (Colorado State University. Libraries, 2017) Hensen, Tucker Joseph, author; Williams, John D., advisor; Prawel, David A., advisor; Wang, Qiang, committee member
    Direct coagulation printing (DCP) is a new approach to extrusion-based additive manufacturing, developed during this thesis project using alumina nanopowder. The fabrication of complex ceramic parts, sintered to full density, was achieved and the details of this invention are described. With the use of additive manufacturing, complex features can be generated that are either very difficult or unattainable by conventional subtractive manufacturing methods. Three unique approaches were taken to create a slurry suitable for extrusion 3D-printing. Each represented a different method of suspending alumina nanopowder in a liquid; a bio-polymer gel based on chitosan, a synthetic polymer binder using poly-vinyl acetate (PVA), and electrostatic stabilization with the dispersant tri-ammonium citrate (TAC). It was found that TAC created a slurry with viscosity and coagulation rate that were tuneable through pH adjustment with nitric acid. This approach led to the most promising printing and sintering results, and is the basis of DCP. Taguchi and fractional factorial design of experiments models were used to optimize mixing of the alumina slurry, rheological properties, print quality, and sinterability. DCP was characterized by measuring the mechanical properties and physical characteristics of printed parts. Features as small as ~450 μm in width were produced, in parts with overhangs and enclosed volumes, in both linear and radial geometries. After sintering, these parts exhibited little to no porosity, with flexural modulus and hardness comparing favorably with conventionally manufactured alumina parts. A remarkable aspect of DCP is that it is a completely binderless process, requiring no binder removal step. In addition, DCP can employ nanopowders, allowing for enhanced mechanical properties as observed in nano-grained materials. Perhaps most importantly, any material that acquires a surface charge when in aqueous media has the potential to be used in DCP, making it a method of additive manufacturing using many metals and ceramics other than alumina.
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    Dynamic mechanical analysis for quality evaluation of additively manufactured continuous fiber reinforced thermoplastic matrix composites subject to manufacturing defects
    (Colorado State University. Libraries, 2019) Rodriguez, Patrick A., author; Radford, Donald W., advisor; Ma, Kaka, committee member; Heyliger, Paul, committee member
    Continuous fiber reinforced polymers (CFRP) have become integral to modern mechanical design as value-added alternatives to metallic, ceramic and neat polymeric engineering materials. Despite the advantages of CFRP, current methods of preparing laminated continuous fiber reinforced polymers are fundamentally limiting in that reinforcement is typically applied only in the plane of the mold or tool. Additionally, key operations inherent to all CFRP processing approaches require a variety of skilled labor as well as costly net-shape, hard tooling. As such, additive manufacturing has risen to the forefront of manufacturing and processing research and development in the CFRP arena. Additive manufacture of continuous fiber reinforced thermoplastics (CFRTP) exhibits the potential to relieve many of the constraints placed on the current design and manufacturing of continuous fiber reinforced structures. At present, the additive manufacture of CFRTP has been demonstrated successfully to varying extents; however, comprehensive dialogue regarding manufacturing defects and quality of the processed continuous fiber reinforced thermoplastics has been missing from the field. Considering the preliminary nature of additive manufacture of CFRTP, exemplary processed composites are typically subject to various manufacturing defects, namely excessive void content in the thermoplastic matrix. Generally, quality evaluation of processed composites in the literature is limited to test methods that are largely influenced by the properties of the continuous fiber reinforcement, and as such, defects in the thermoplastic matrix are usually less-impactful on the results and overlooked. Hardware to facilitate additive manufacturing of CFRTP was developed and continuous fiber reinforced specimens, with high fiber volume fractions (~ 50 %), were successfully processed. Early efforts at evaluating the processed specimens using defect-sensitive Short-Beam Strength (SBS) analysis exhibited limited sensitivity to void content, coupled with destructive, inelastic failure modes. As a path forward, an expanded study of the effects of void content on the processed specimens was conducted by means of Dynamic Mechanical Analysis (DMA). Utilization of DMA allows for thermomechanical (i.e. highly matrix sensitive) evaluation of the composite specimens, specifically in terms of the measured elastic storage modulus (E'), viscous loss modulus (E"), damping factor (tan δ) and the glass transition temperature (Tg) of the processed composite specimens. The results of this work have shown that DMA exhibits increased sensitivity, as compared to SBS, to the presence of void content in the additively manufactured CFRTP specimens. Within the relevant range of void content, non-destructive specimen evaluation by DMA resulted in a measured, frequency dependent, 5.5 – 5.8 % decrease in elastic storage modulus per 1 % increase in void content by volume. Additionally, quality evaluation by DMA realized a marked decrease in the maximum measured loss modulus in the additively manufactured composites, ranging from 7.0 – 8.2 % per 1 % increase in void content by volume. Effects of void content were also measured in both the damping factor and glass transition temperature, where an approximate 1.6 °C drop in Tg was recorded over the relevant range of void content. The results of this work indicate, firstly, that DMA is a superior evaluation method, as compared to SBS, in terms of sensitivity to void content in additively manufactured CFRTP. Additionally, the results of this work provide a clear expansion of the current state of the literature regarding additive manufacture of CFRTP materials in that the effects of prominent manufacturing defects have been assessed with regard to thermomechanical material performance. Furthermore, and finally, the results of this work establish a direct path forward to characterize long-term effects of manufacturing defects, by means of DMA, on the creep-recovery and stress relaxation behavior of the relevant composite material system.
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    Low work function, long lifetime filament for electron beam-based, wire-fed metal additive manufacturing
    (Colorado State University. Libraries, 2018) Nguyen, Bao Gia, author; Bradley, Thomas, advisor; Williams, John, advisor; de la Venta Granda, Jose, committee member
    Tantalum filaments are used in electron beam additive manufacturing to thermionically emit electrons that are used to build near-net shape, metal parts. High operating temperatures are required to emit electrons which consequently limits the lifetime of these filaments. This thesis presents the thermionic emission characteristics of drop-in filament replacements that incorporate barium calcium aluminate cermets. Barium calcium aluminate is a low work function material used with hollow cathodes in electric propulsion devices to provide very long service lifetimes by acting as a moderate temperature, electron source. A marriage of these two technologies may limit downtime and increase the productivity and output of electron beam additive manufacturing. Results of extended runtime tests are presented from configurations that immerse the modified filament in plasma and operate it as a vacuum emitter. The effect of contamination by air and fabrication methods are examined and evaluated based on effective work function and current density measurements. The latter includes formation methods for barium diffusion orifices as well as surface preparation methods for cermets. The experimental data collected were used to validate a predictive model that evaluates emission current densities, in both temperature and space-charge limited conditions, and effective work functions based on the fractional surface coverage of barium over a tantalum substrate.
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    Material validation and part authentication process using hardness indentations with robotic arm implementation
    (Colorado State University. Libraries, 2021) Weinmann, Katrina J., author; Simske, Steve, advisor; Chen, Thomas, committee member; Ma, Kaka, committee member; Zhao, Jianguo, committee member
    In today's global economy, there are many levels of validation and authentication which must occur during manufacturing and distribution processes to ensure sufficient cyber-physical security of parts. This includes material inspection and validation during manufacturing, a method of track-and-trace for the entire supply chain, and individual forensic authentication of parts to prevent counterfeiting at any point in the manufacturing or distribution process. Traditionally, each level of validation or authentication is achieved through a separate step in the manufacturing or distribution process. In this work, a process is presented that uses hardness testing and the resulting indentations to simultaneously provide three critical functions for part validation and authentication: (i) material property validation and material property mapping achieved by administering multiple hardness tests over a given area on the part, (ii) part serialization that can be used for track-and-trace through administering hardness tests in a specific 'barcode' pattern, and (iii) the opportunity for forensic-level authentication through use of high-resolution images of the indents. Additionally, a fourth manufacturing advantage is gained in the provision of improved bonding potential for adhesive joints provided by the increase in surface area and surface roughness resulting from the addition of indents to the adherend surface. A methodology for implementing this process using a robotic arm with an end-effector-mounted portable hardness tester is presented. Implementation using a robotic arm allows a high degree of customizability of the process without changes in setup, making this process ideal for additive manufactured parts, which are often custom or low-batch and require a higher level of material validation. As a whole, this work presents a highly-customizable, single-step process that provides multi-level quality control, validation, authentication, and cyber-physical security of parts throughout the manufacturing and distribution processes