14,803 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

    A direct-laser-written heart-on-a-chip platform for generation and stimulation of engineered heart tissues

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    In this dissertation, we first develop a versatile microfluidic heart-on-a-chip model to generate 3D-engineered human cardiac microtissues in highly-controlled microenvironments. The platform, which is enabled by direct laser writing (DLW), has tailor-made attachment sites for cardiac microtissues and comes with integrated strain actuators and force sensors. Application of external pressure waves to the platform results in controllable time-dependent forces on the microtissues. Conversely, oscillatory forces generated by the microtissues are transduced into measurable electrical outputs. After characterization of the responsivity of the transducers, we demonstrate the capabilities of this platform by studying the response of cardiac microtissues to prescribed mechanical loading and pacing. Next, we tune the geometry and mechanical properties of the platform to enable parametric studies on engineered heart tissues. We explore two geometries: a rectangular seeding well with two attachment sites, and a stadium-like seeding well with six attachment sites. The attachment sites are placed symmetrically in the longitudinal direction. The former geometry promotes uniaxial contraction of the tissues; the latter additionally induces diagonal fiber alignment. We systematically increase the length for both configurations and observe a positive correlation between fiber alignment at the center of the microtissues and tissue length. However, progressive thinning and “necking” is also observed, leading to the failure of longer tissues over time. We use the DLW technique to improve the platform, softening the mechanical environment and optimizing the attachment sites for generation of stable microtissues at each length and geometry. Furthermore, electrical pacing is incorporated into the platform to evaluate the functional dynamics of stable microtissues over the entire range of physiological heart rates. Here, we typically observe a decrease in active force and contraction duration as a function of frequency. Lastly, we use a more traditional ?TUG platform to demonstrate the effects of subthreshold electrical pacing on the rhythm of the spontaneously contracting cardiac microtissues. Here, we observe periodic M:N patterns, in which there are ? cycles of stimulation for every ? tissue contractions. Using electric field amplitude, pacing frequency, and homeostatic beating frequencies of the tissues, we provide an empirical map for predicting the emergence of these rhythms

    Moduli Stabilisation and the Statistics of Low-Energy Physics in the String Landscape

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    In this thesis we present a detailed analysis of the statistical properties of the type IIB flux landscape of string theory. We focus primarily on models constructed via the Large Volume Scenario (LVS) and KKLT and study the distribution of various phenomenologically relevant quantities. First, we compare our considerations with previous results and point out the importance of Kähler moduli stabilisation, which has been neglected in this context so far. We perform different moduli stabilisation procedures and compare the resulting distributions. To this end, we derive the expressions for the gravitino mass, various quantities related to axion physics and other phenomenologically interesting quantities in terms of the fundamental flux dependent quantities gsg_s, W0W_0 and n\mathfrak{n}, the parameter which specifies the nature of the non-perturbative effects. Exploiting our knowledge of the distribution of these fundamental parameters, we can derive a distribution for all the quantities we are interested in. For models that are stabilised via LVS we find a logarithmic distribution, whereas for KKLT and perturbatively stabilised models we find a power-law distribution. We continue by investigating the statistical significance of a newly found class of KKLT vacua and present a search algorithm for such constructions. We conclude by presenting an application of our findings. Given the mild preference for higher scale supersymmetry breaking, we present a model of the early universe, which allows for additional periods of early matter domination and ultimately leads to rather sharp predictions for the dark matter mass in this model. We find the dark matter mass to be in the very heavy range mχ10101011 GeVm_{\chi}\sim 10^{10}-10^{11}\text{ GeV}

    Increased lifetime of Organic Photovoltaics (OPVs) and the impact of degradation, efficiency and costs in the LCOE of Emerging PVs

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    Emerging photovoltaic (PV) technologies such as organic photovoltaics (OPVs) and perovskites (PVKs) have the potential to disrupt the PV market due to their ease of fabrication (compatible with cheap roll-to-roll processing) and installation, as well as their significant efficiency improvements in recent years. However, rapid degradation is still an issue present in many emerging PVs, which must be addressed to enable their commercialisation. This thesis shows an OPV lifetime enhancing technique by adding the insulating polymer PMMA to the active layer, and a novel model for quantifying the impact of degradation (alongside efficiency and cost) upon levelized cost of energy (LCOE) in real world emerging PV installations. The effect of PMMA morphology on the success of a ternary strategy was investigated, leading to device design guidelines. It was found that either increasing the weight percent (wt%) or molecular weight (MW) of PMMA resulted in an increase in the volume of PMMA-rich islands, which provided the OPV protection against water and oxygen ingress. It was also found that adding PMMA can be effective in enhancing the lifetime of different active material combinations, although not to the same extent, and that processing additives can have a negative impact in the devices lifetime. A novel model was developed taking into account realistic degradation profile sourced from a literature review of state-of-the-art OPV and PVK devices. It was found that optimal strategies to improve LCOE depend on the present characteristics of a device, and that panels with a good balance of efficiency and degradation were better than panels with higher efficiency but higher degradation as well. Further, it was found that low-cost locations were more favoured from reductions in the degradation rate and module cost, whilst high-cost locations were more benefited from improvements in initial efficiency, lower discount rates and reductions in install costs

    Translating erasure: Proposing auto-theory as a practice for artistic enquiry and analysis while comprehending personal grief

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    Erasure as an artistic technique has developed in my moving image work after my father's passing. I export videos into sequences of thousands of images and erase outlines of the targeted objects in each frame. The repetitive and low conscious labour is a way to ease the agony and to grieve my father. Hours compressed into thousands of frames, turning into a glimpse of illusion and leaving a ghostly emptiness on the images. Both its visual presentation and making reflect the life events and encounters I've experienced in the UK and Taiwan in the past years. I consider an artwork embodies interconnected relationships between one's personal impulses and artistic training. As an art student, I have found it challenging to describe such a creative process with conventional academic writing. Within a construct that inclines to present thoughts as reasonable and rational arguments, my personal experiences and the intensity of feeling seem out of place. Within an academic framework, how can I make an argument out of how I have developed the erasure in my artwork to perform the grief, fading memories of a loved one, existential crisis and what's in-between? Through auto-theoretical approaches to writing and making of moving image work, this research aims to build a structure that can express both the intimate and intellectual aspects of an art practice. This writing up process interweaves my personal stories that motivate my artistic expression into art theories. The memories about my late father, my relationship with languages, and my lives between the UK and Taiwan meet with different artists' uses of erasure. As the conversations between the introspections and theoretical analysis accumulate, my writing and moving image work unravel an art journey that encompasses the nuances and struggles I've experienced as an international student. Within the search for an ideal model to illustrate an art practice, this research further generates profound understandings of memory, grief, loss, language, conflicted identities and cultural belonging

    Early Neanderthal social and behavioural complexity during the Purfleet Interglacial: handaxes in the latest Lower Palaeolithic.

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    Only a handful of ‘flagship’ sites from the Purfleet Interglacial (Marine Isotope Stage 9, c. 350-290,000 years ago) have been properly examined, but the archaeological succession at the proposed type-site at Purfleet suggests a period of complexity and transition, with three techno-cultural groups represented in Britain. The first was a simple toolkit lacking handaxes (the Clactonian), and the last a more sophisticated technology presaging the coming Middle Palaeolithic (simple prepared core or proto-Levallois technology). Sandwiched between were Acheulean groups, whose handaxes comprise the great majority of the extant archaeological record of the period – these are the focus of this study. It has previously been suggested that some features of the Acheulean in the Purfleet Interglacial were chronologically restricted, particularly the co-occurrence of ficrons and cleavers. These distinctive forms may have exceeded pure functionality and were perhaps imbued with a deeper social and cultural meaning. This study supports both the previously suggested preference for narrow, pointed morphologies, and the chronologically restricted pairing of ficrons and cleavers. By drawing on a wide spatial and temporal range of sites these patterns could be identified beyond the handful of ‘flagship’ sites previously studied. Hypertrophic ‘giants’ have now also been identified as a chronologically restricted form. Greater metrical variability was found than had been anticipated, leading to the creation of two new sub-groups (IA and IB) which are tentatively suggested to represent spatial and perhaps temporal patterning. The picture in the far west of Britain remains unclear, but the possibility of different Acheulean groups operating in the Solent area, and a late survival of the Acheulean, are both suggested. Handaxes with backing and macroscopic asymmetry may represent prehensile or ergonomic considerations not commonly found on handaxes from earlier interglacial periods. It is argued that these forms anticipate similar developments in the Late Middle Palaeolithic in an example of convergent evolution

    Hollow Core optical fibre for the mid infrared

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    Fabrication of advanced heat sink materials by powder metallurgy approach

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    To guarantee a safe work environment for high-end chips, electronic packaging materials should have high thermal conductivity, matched coefficient of thermal expansion to semiconductor, low density, good impact toughness, and tensile strength. Diamond/graphite flakes-reinforced composites are considered as the new generation of heat sink material, however, the poor chemical affinity between copper and diamond and between copper and graphite flakes hinder the composites from achieving high thermal conductivity. The interface between copper matrix and diamond reinforcement or graphite flakes reinforcement is the key factor for improving heat transfer of the composites. This study aims to investigate the mechanisms of how the interface improves the thermal conductivity of the diamond/graphite flakes-reinforced composites. The primary research includes: (1) study the effect of vacuum sintering process on the formation of carbide layer; (2) investigate the effect of the volume fraction of diamond particles on relative density and thermal conductivity of the copper/diamond composites, and (3) explore the feasibility to fabricate graphite flakes reinforced copper composites by powder forging and investigate the effect of graphite flakes’ volume fraction on the thermal conductivity of the fabricated composites. Tungsten carbide (WC) can be formed during the vacuum sintering process, however, the formed WC were broken during the subsequent 1050 ℃ hot forging process. The WC can be formed during the vacuum sintering process above 550 ℃ for 30mins, on both diamond -{100}facet and -{111}facet. Cu-55WDia-1050HF has a thermal conductivity of 230.5W/(mK) which is higher than Cu-55WDia-VS800C2h-1050 of 195.1W/(mK). With the volume fraction of diamond from 45vol% to 65vol%, the thermal conductivity reduced from 250.8 to 65.1W/(mK). For copper/graphite flakes composites, the addition of carbide forming element Ti helped to improve the thermal conductivity. Because of the excessive copper fluidity in Cu-Ti-50GFs-1050HF, Cu-Ti-50GFs-1050HF has the lower thermal conductivity of 202.7 W/(mK) than thermal conductivity of Cu-Ti-50GFs-950HF which is 375.0 W/(mK). With the volume fraction of the graphite flakes from 30% to 70%, the relative density changed from 100% to 83.3%

    The processing, production and improvement of hemp-fibre reinforced polypropylene composite materials

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    Increasing worldwide environmental awareness is encouraging scientific research into the development of cheaper, more environmentally friendly and more sustainable construction and packaging materials. Natural fibre reinforced thermoplastic composites are strong, stiff, lightweight and recyclable, and have the potential to meet this need. Industrial hemp fibre is amongst the strongest of the natural fibres, and possesses a similar specific strength and stiffness to Eglass, but with additional benefits such as low cost and low production energy requirements. The favourable mechanical properties of hemp, however, have yet to be transferred successfully to thermoplastic-matrix based composite materials. The main objective of this research was to produce an improved hemp-fibre reinforced polypropylene composite material by optimising the fibre strength, fibreprocessing methods, composite-processing methods and fibre-matrix interfacial bonding. To obtain the strongest and stiffest fibres for use in composites, an investigation was performed on a crop of New Zealand grown hemp to determine the effects of plant growth duration on fibre strength and stiffness. By conducting single fibre tensile tests on retted hemp fibre, it was discovered that the optimum cultivation time was 114 days, producing fibres with an average tensile strength of 671 MPa, and a Young's modulus of 40 GPa. It was also found that the retting process considerably reduced the tensile strength and stiffness of the hemp fibres. Biological fibre-treatments using 3 varieties of fungi were then used to modify the surface morphology of hemp fibres, so that the adhesion between the fibres and a polypropylene matrix could be improved. It was found, however, that these treatments severely degraded the fibres, and reduced their tensile properties to such an extent that they were deemed unsuitable for use as composite reinforcements. An investigation was then conducted to determine a suitable fibre treatment method to remove lignin and other non-structural constituents from the fibre wall, to separate the fibres from their fibre bundles, and to retain the fibre strength. Various alkali treatments were used, and a treatment consisting of fibre digestion in a 10% NaOH solution with a maximum processing temperature of 160°C and a hold time of 45 minutes, was found to provide the best combination of fibre strength retention, lignin removal and fibre separation. Finally, the alkali treated fibres, polypropylene and a maleated polypropylene (MAPP) coupling agent were compounded using a twin-screw extruder, before being injection moulded into composite tensile test specimens. A range of composites with differing fibre and MAPP weight contents were then produced and tensile tested. It was found that the addition of 2% MAPP greatly improved the tensile strength and interfacial bonding of the 30% fibre composites, while only moderate improvements were observed for the 40% fibre composites. An increase in the fibre content from 0% to 40% greatly improved the stiffness of the composites. The most successful composite produced consisted of 40% long alkali treated fibres with 2% MAPP, and had a tensile strength and Young's modulus of 39 MPa and 4.4 GPa respectively

    Physical phenomena controlling quiescent flame spread in porous wildland fuel beds

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    Despite well-developed solid surface flame spread theories, we still lack a coherent theory to describe flame spread through porous wildland fuel beds. This porosity results in additional complexity, reducing the thermal conductivity of the fuel bed, but allowing in-bed radiative and convective heat transfer to occur. While previous studies have explored the effect of fuel bed structure on the overall fire behaviour, there remains a need for further investigation of the effect of fuel structure on the underlying physical phenomena controlling flame spread. Through an extensive series of laboratory-based experiments, this thesis provides detailed, physics-based insights for quiescent flame spread through natural porous beds, across a range of structural conditions. Measurements are presented for fuel beds representative of natural field conditions within an area of the fire-prone New Jersey Pinelands National Reserve, which compliment a related series of field experiments conducted as part of a wider research project. Additional systematic investigation across a wider range of fuel conditions identified independent effects of fuel loading and bulk density on the spread rate, flame height and heat release rate. However, neither fuel loading nor bulk density alone provided adequate prediction of the resulting fire behaviour. Drawing on existing structural descriptors (for both natural and engineered fuel beds) an alternative parameter ασδ was proposed. This parameter (incorporating the fuel bed porosity (α), fuel element surface-to-volume ratio (σ), and the fuel bed height (δ)) was strongly correlated with the spread rate. One effect of the fuel bed structure is to influence the heat transfer mechanisms both above and within the porous fuel bed. Existing descriptions of radiation transport through porous fuel beds are often predicated on the assumption of an isotropic fuel bed. However, given their preferential angle of inclination, the pine needle beds in this study may not exhibit isotropic behaviour. Regardless, for the structural conditions investigated, horizontal heat transfer through the fuel bed was identified as the dominant heating mechanism within this quiescent flame spread scenario. However, the significance of heat transfer contributions from the above-bed flame generally increased with increasing ασδ value of the fuel bed. Using direct measurements of the heat flux magnitude and effective heating distance, close agreement was observed between experimentally observed spread rates and a simple thermal model considering only radiative heat transfer through the fuel bed, particularly at lower values of ασδ. Over-predictions occurred at higher ασδ values, or where other heat transfer terms were incorporated, which may highlight the need to include additional heat loss terms. A significant effect of fuel structure on the primary flow regimes, both within and above these porous fuel beds, was also observed, with important implications for the heat transfer and oxygen supply within the fuel bed. Independent effects of fuel loading and bulk density on both the buoyant and buoyancy-driven entrainment flow were observed, with a complex feedback cycle occurring between Heat Release Rate (HRR) and combustion behaviour. Generally, increases in fuel loading resulted in increased HRR, and therefore increased buoyant flow velocity, along with an increase in the velocity of flow entrained towards the combustion region. The complex effects of fuel structure in both the flaming and smouldering combustion phases may necessitate modifications to other common modelling approaches. The widely used Rothermel model under-predicted spread rate for higher bulk density and lower ασδ fuel beds. As previously suggested, an over-sensitivity to fuel bed height was observed, with experimental comparison indicating an under-prediction of reaction intensity at lower fuel heights. These findings have important implications particularly given the continuing widespread use of the Rothermel model, which continues to underpin elements of the BehavePlus fire modelling system and the US National Fire Danger Rating System. The physical insights, and modelling approaches, developed for this low-intensity, quiescent flame spread scenario, are applicable to common prescribed fire activities. It is hoped that this work (alongside complimentary laboratory and field experiments conducted by various authors as part of a wider multi-agency project (SERDP-RC2641)) will contribute to the emerging field of prescribed fire science, and help to address the pressing need for further development of fire prediction and modelling tools
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