25,563 research outputs found

    Meso-scale FDM material layout design strategies under manufacturability constraints and fracture conditions

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    In the manufacturability-driven design (MDD) perspective, manufacturability of the product or system is the most important of the design requirements. In addition to being able to ensure that complex designs (e.g., topology optimization) are manufacturable with a given process or process family, MDD also helps mechanical designers to take advantage of unique process-material effects generated during manufacturing. One of the most recognizable examples of this comes from the scanning-type family of additive manufacturing (AM) processes; the most notable and familiar member of this family is the fused deposition modeling (FDM) or fused filament fabrication (FFF) process. This process works by selectively depositing uniform, approximately isotropic beads or elements of molten thermoplastic material (typically structural engineering plastics) in a series of pre-specified traces to build each layer of the part. There are many interesting 2-D and 3-D mechanical design problems that can be explored by designing the layout of these elements. The resulting structured, hierarchical material (which is both manufacturable and customized layer-by-layer within the limits of the process and material) can be defined as a manufacturing process-driven structured material (MPDSM). This dissertation explores several practical methods for designing these element layouts for 2-D and 3-D meso-scale mechanical problems, focusing ultimately on design-for-fracture. Three different fracture conditions are explored: (1) cases where a crack must be prevented or stopped, (2) cases where the crack must be encouraged or accelerated, and (3) cases where cracks must grow in a simple pre-determined pattern. Several new design tools, including a mapping method for the FDM manufacturability constraints, three major literature reviews, the collection, organization, and analysis of several large (qualitative and quantitative) multi-scale datasets on the fracture behavior of FDM-processed materials, some new experimental equipment, and the refinement of a fast and simple g-code generator based on commercially-available software, were developed and refined to support the design of MPDSMs under fracture conditions. The refined design method and rules were experimentally validated using a series of case studies (involving both design and physical testing of the designs) at the end of the dissertation. Finally, a simple design guide for practicing engineers who are not experts in advanced solid mechanics nor process-tailored materials was developed from the results of this project.U of I OnlyAuthor's request

    Design and manufacturing of novel hybrid metal-polymer heat exchangers

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    Nearly 60% of waste heat from industrial processes in the U.S. lies in low-temperature regime (<200°C). Current conventional metal heat exchangers for waste heat recovery (WHR) of low-grade heat are too expensive and subject to corrosion from condensates below <150°C. Polymers, although an attractive alternative to metals, have low thermal conductivity which typically makes them unsuitable for thermal applications. Combining thermally conductive metals with polymers can improve the thermal conductivity of the hybrid to be sufficient for WHR applications. Prior work has explored helically wound metal-polymer hybrid tubes as a means to combine the best aspects of both these materials for suitable heat exchanger performance. Here we show that compared to joining metal strips to polymer strips using laser welding, ultrasonic welding, or resistance seam welding, using adhesives to join the strips is easier for manufacturing and provides adequate joining strength. This work explores the improvement of a roll-to-roll setup to incorporate an adhesive dispensing system to manufacture heat exchanger tubes. To better understand conformal rolling, we develop a theoretical mechanics model to study important forces/factors responsible. Finally, we report data from a real-world demonstration for such heat exchangers within a campus building to analyze the performance and gauge potential under wider operating conditions. This work paves the way for realizing the cost-effective manufacturing of heat exchangers for low-grade WHR.LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste

    Advanced manufacturing of sustainable materials

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    Since Bakelite and Rayon were first introduced in the early twentieth century, global plastic production has grown to over 300 million metric tons per annum and is expected to double in the next twenty years. Unfortunately, plastics were integrated into our daily lives and virtually every industrial sector without first considering the lifecycle of these materials. Despite their ease of processing, polymeric materials require significant external energy inputs during manufacturing, and subsequently, plastics have an embodied energy that greatly exceeds most material classes. Further, the covalent chemical networks to which plastic materials owe their exceptional performance also prevents their degradation. Polymeric materials are, therefore, accumulating in our landfills and oceans at alarming rates. To prevent further damage to our ecosystem, both the initial manufacture and the end of life of plastic materials must be engineered for sustainability. We first created a model system capable of cycled polymerization and depolymerization utilizing an engineering thermoplastic with a low ceiling temperature, cyclic poly(phthalaldehyde) (cPPA). cPPA depolymerization proceeded under mild thermal conditions, and quantitative recovery of the monomer was possible. Direct repolymerization of the recovered monomer yielded high-quality materials with chemical and mechanical properties equivalent to or greater than the original material. Closed-loop recycling of cPPA was also extended to fiber-reinforced polymer composites. cPPA depolymerization proceeded without damage to the fibers, and both the fiber reinforcements and the composite materials retained 100 % of their mechanical performance through multiple generations. To improve the performance of cPPA-based materials, we explored the thermal depolymerization mechanism and discovered that single electron transfer (SET) and cleavage of the resulting radical cation served as the primary thermal trigger for cPPA depolymerization. These findings were extended to other modes of SET triggering, including photoredox catalysis. Incorporation of acridinium salts into cPPA blends yielded photodegradable monolithic solids that rapidly deteriorated in the presence of UV-light. The discovery of a SET triggering mechanism provides a route towards novel environmental triggers including electrochemical oxidation and enzymatic catalysis. We next sought to address the initial manufacture of polymeric materials, specifically patterned materials. In synthetic systems, patterns are often generated through lengthy multistep processes and access to spatially varying properties is limited. Patterns in natural systems, however, arise through non-deterministic symmetry-breaking events. Impressively, natural patterns are replicated throughout species with high fidelity of both function and form. Inspired by natural phenomena, we leveraged the competition between reaction and thermal transport processes of frontal polymerization to create tunable thermal instabilities and spontaneous breaks from symmetry. The undulations in reaction temperature were harnessed to drive orthogonal chemistries and pattern the morphological, optical, chemical, and mechanical properties of engineering polymers. The complex dynamics of frontal polymerization represent a path towards the non-deterministic, developmental manufacturing of synthetic materials. Finally, the concepts of frontal polymerization and degradable materials were combined by copolymerizing cleavable cyclic olefins during the frontal ring-opening metathesis polymerization (FROMP) of dicyclopentadiene (DCPD). FROMP copolymers possessed material properties that were comparable with native pDCPD, but they were easily degraded into high-value fragments by hydrolysis. The degradation fragments, which were rich in hydroxyl functionality, were recycled into polyurethane networks by reaction with commercially available diisocyanates. The recycled materials displayed thermomechanical properties that greatly exceeded those of the original copolymer, a true embodiment of upcycling. We foresee many opportunities for the energy-efficient manufacture of multifunctional materials by frontal copolymerization.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste

    Impact of polyester and cotton microfibers on growth and sublethal biomarkers in juvenile mussels

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    Anthropogenic microfibres are a prevalent, persistent and globally distributed form of marine debris. Evidence of microfibre ingestion has been demonstrated in a range of organisms, including Mytilus spp. (mussels), but the extent of any impacts on these organisms are poorly understood. This study investigates, for the first time, the effect of exposing juvenile mussels to polyester and cotton microfibres at environmentally relevant concentrations (both current and predicted future scenarios) over a chronic timescale (94 days). Sublethal biomarkers included growth rate, respiration rate and clearance rate. Mussels were exposed to polyester (median length 149 µm) and cotton (median length 132 µm) microfibres in three treatments: polyester (~ 8 fibres L−1), polyester (~ 80 fibres L−1) and cotton (~ 80 fibres L−1). Mussels exposed to 80 polyester or cotton microfibres L−1 exhibited a decrease in growth rate of 35.6% (polyester) and 18.7% (cotton), with mussels exposed to ~ 80 polyester microfibres L−1 having a significantly lower growth rate than the control population (P < 0.05). This study demonstrates that polyester microfibres have the potential to adversely impact upon mussel growth rates in realistic future scenarios, which may have compounding effects throughout the marine ecosystem and implications for commercial viability

    Adding functionality to powder bed fusion materials: Creating magnetic polymers using hybridized hollow carbon nanofibres

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    A method is presented, using Fe3O4 nanoparticles hybridized with hollow carbon nanofibers (Fe3O4NP@CNF) as an example, to add functionality to polymer powders for powder bed fusion with laser beam (PBF/LB-P). There are currently only a small number of polymers that can be processed successfully and reliably by PBF/LB-P. It was proposed that coating PA12 powder, a material that has a good track record in laser sintering, with a small amount (0.1 wt%) of Fe3O4@CNF would provide a new material with additional functionality without affecting the processability of the PA12 powder, since the Fe3O4 is contained within the CNF. Commercial PBF/LB-P PA12 particles were coated with Fe3O4@CNF without altering the morphology of the powder particles. No significant reduction in the PBF/LB-P processing window was observed when processing the resulting polymer nanocomposites, and parts were produced with comparable mechanical properties to the base polymer. Interestingly, magnetic investigations of PBF/LB-P cylinders built in three different orientations, with alignment of the long symmetry axis along the X, Y or Z axes of the build chamber, showed a preferential orientation of the hybridized magnetic fibers along the Z-axes in the composite. This suggests the appealing possibility of tailoring pieces with preferential magnetic orientation. Moreover, no agglomeration or nanoparticle growth was observed after PBF/LB-P. It is proposed that the low-cost method used in this work could be easily applied to other nanoparticles, without creating restrictive processing windows and the time-consuming process to determine them. Thus, a range of powders with additional functionality could be easily created for use in a variety of applications and industries

    Microplastics in European sea salts – An example of exposure through consumer choice and of interstudy methodological discrepancies

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    Microplastics are contaminants of emerging concern, not least due to their global presence in marine surface waters. Unsurprisingly, microplastics have been reported in salts harvested from numerous locations. We extracted microplastics from 13 European sea salts through 30% H2O2 digestion and filtration over 5-µm filters. Filters were visually inspected at magnifications to x100. A subsample of potential microplastics was subjected to Raman spectroscopy. Particle mass was estimated, and human dose exposure calculated. After blank corrections, median concentrations were 466 ± 152 microplastics kg-1 ranging from 74 to 1155 items kg-1. Traditionally harvested salts contained fewer microplastics than most industrially harvested ones (t-test, p < 0.01). Approximately 14 µg of microplastics (< 12 particles) may be absorbed by the human body annually, of which a quarter may derive from a consumer choosing sea salt. We reviewed existing studies, showing that targeting different particle sizes and incomplete filtrations hinder interstudy comparison, indicating the importance of method harmonisation for future studies. Excess salt consumption is detrimental to human health; the hazardousness of ingesting microplastics on the other hand has yet to be shown. A portion of microplastics may enter sea salts through production processes rather than source materials

    A critical review of the production of hydroxyaromatic carboxylic acids as a sustainable method for chemical utilisation and fixation of CO2

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    Hydroxyaromatic carboxylic acids (HACAs) such as salicylic acids, hydroxynaphthoic acids and their halogenated derivatives are essential feedstocks for the pharmaceutical, dye, fragrance, cosmetic and food industries. Large-scale production of HACAs is currently based on the Kolbe–Schmitt reaction between CO2 and petroleum-based phenolic compounds. This batch reaction is carried out at ∼125 °C, ∼85 bar and reaction times of up to 18 hours to achieve high conversions (≈99%). The long reaction times and dependence on fossil-derived phenols have negative sustainability implications. However, as a CO2-based process, HACA production has the potential for large-volume anthropogenic CO2 sequestration and contributes to net zero. A big challenge is that the current global production capacity of HACAs uses only about 41 450 tonnes per year of CO2 which is just ≈0.00012% of the annual anthropogenic emissions. Therefore, significant efforts are needed to increase both the sustainable production and demand for such CO2-based products to enhance their economic and environmental sustainability. This review covers the basic kinetic and thermodynamic stability of CO2. Thereafter, a comprehensive coverage of early and current developments to improve the carboxylation of phenols to make HACAs is given, while discussing their industrial potential. Moreover, it covers new propositions to use biomass-derived phenolic compounds for sustainable production of HACAs. There is also a need to expand the uses and applications of HACAs and recent reports on the production of HACA-based recyclable vinyl polymers point in the right direction

    Epoxy as Filler or Matrix for Polymer Composites

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    Epoxy is a widely used polymer because of its ease of processing, high adhesiveness, and high chemical resistance. Epoxy-based composites are commonly used in aerospace, automotive, and marine applications. The epoxy type, function, curing agent, and curing process are discussed in this chapter. Epoxy is used as either a filler or polymer matrix in composite applications. As a filler, the epoxy modification on the fiber is discussed. As a polymer matrix, the epoxy is reinforced by natural and synthetic fibers. The manufacturing process and the fabricated epoxy-based composites’ performance (e.g., mechanical and thermal properties) are investigated. The advantages and disadvantages of epoxy’s function are discussed and summarized. Epoxy modification is an effective approach to improve the composites’ performance

    Interview with Wolfgang Knauss

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    An oral history in four sessions (September 2019–January 2020) with Wolfgang Knauss, von Kármán Professor of Aeronautics and Applied Mechanics, Emeritus. Born in Germany in 1933, he speaks about his early life and experiences under the Nazi regime, his teenage years in Siegen and Heidelberg during the Allied occupation, and his move to Pasadena, California, in 1954 under the sponsorship of a local minister and his family. He enrolled in Caltech as an undergraduate in 1957, commencing a more than half-century affiliation with the Institute and GALCIT (today the Graduate Aerospace Laboratories of Caltech). He recalls the roots of his interest in aeronautics, his PhD solid mechanics studies with his advisor, M. Williams, and the GALCIT environment in the late 1950s and 1960s at the dawn of the Space Age, including the impact of Sputnik and classes with NASA astronauts. He discusses his experimental and theoretical work on materials deformation, dynamic fracture, and crack propagation, including his solid-propellant fuels research for NASA and the US Army, wide-ranging programs with the US Navy, and his pioneering micromechanics investigations and work on the time-dependent fracture of polymers in the 1990s. He offers his perspective on GALCIT’s academic culture, its solid mechanics and fluid mechanics programs, and its evolving administrative directions over the course of five decades, as well as its impact and reputation both within and beyond Caltech. He describes his work with Caltech’s undergraduate admissions committee and his scientific collaborations with numerous graduate students and postdocs and shares his recollections of GALCIT and other Caltech colleagues, including C. Babcock, D. Coles, R.P. Feynman, Y.C. Fung, G. Neugebauer, G. Housner, D. Hudson, H. Liepmann, A. Klein, G. Ravichandran, A. Rosakis, A. Roshko, and E. Sechler. Six appendices contributed by Dr. Knauss, offering further insight into his life and career, also form part of this oral history and are cross-referenced in the main text
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