10,893 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

    The place where curses are manufactured : four poets of the Vietnam War

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    The Vietnam War was unique among American wars. To pinpoint its uniqueness, it was necessary to look for a non-American voice that would enable me to articulate its distinctiveness and explore the American character as observed by an Asian. Takeshi Kaiko proved to be most helpful. From his novel, Into a Black Sun, I was able to establish a working pair of 'bookends' from which to approach the poetry of Walter McDonald, Bruce Weigl, Basil T. Paquet and Steve Mason. Chapter One is devoted to those seemingly mismatched 'bookends,' Walt Whitman and General William C. Westmoreland, and their respective anthropocentric and technocentric visions of progress and the peculiarly American concept of the "open road" as they manifest themselves in Vietnam. In Chapter, Two, I analyze the war poems of Walter McDonald. As a pilot, writing primarily about flying, his poetry manifests General Westmoreland's technocentric vision of the 'road' as determined by and manifest through technology. Chapter Three focuses on the poems of Bruce Weigl. The poems analyzed portray the literal and metaphorical descent from the technocentric, 'numbed' distance of aerial warfare to the world of ground warfare, and the initiation of a 'fucking new guy,' who discovers the contours of the self's interior through a set of experiences that lead from from aerial insertion into the jungle to the degradation of burning human feces. Chapter Four, devoted to the thirteen poems of Basil T. Paquet, focuses on the continuation of the descent begun in Chapter Two. In his capacity as a medic, Paquet's entire body of poems details his quotidian tasks which entail tending the maimed, the mortally wounded and the dead. The final chapter deals with Steve Mason's JohnnY's Song, and his depiction of the plight of Vietnam veterans back in "The World" who are still trapped inside the interior landscape of their individual "ghettoes" of the soul created by their war-time experiences

    FiabilitĂ© de l’underfill et estimation de la durĂ©e de vie d’assemblages microĂ©lectroniques

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    Abstract : In order to protect the interconnections in flip-chip packages, an underfill material layer is used to fill the volumes and provide mechanical support between the silicon chip and the substrate. Due to the chip corner geometry and the mismatch of coefficient of thermal expansion (CTE), the underfill suffers from a stress concentration at the chip corners when the temperature is lower than the curing temperature. This stress concentration leads to subsequent mechanical failures in flip-chip packages, such as chip-underfill interfacial delamination and underfill cracking. Local stresses and strains are the most important parameters for understanding the mechanism of underfill failures. As a result, the industry currently relies on the finite element method (FEM) to calculate the stress components, but the FEM may not be accurate enough compared to the actual stresses in underfill. FEM simulations require a careful consideration of important geometrical details and material properties. This thesis proposes a modeling approach that can accurately estimate the underfill delamination areas and crack trajectories, with the following three objectives. The first objective was to develop an experimental technique capable of measuring underfill deformations around the chip corner region. This technique combined confocal microscopy and the digital image correlation (DIC) method to enable tri-dimensional strain measurements at different temperatures, and was named the confocal-DIC technique. This techique was first validated by a theoretical analysis on thermal strains. In a test component similar to a flip-chip package, the strain distribution obtained by the FEM model was in good agreement with the results measured by the confocal-DIC technique, with relative errors less than 20% at chip corners. Then, the second objective was to measure the strain near a crack in underfills. Artificial cracks with lengths of 160 ÎŒm and 640 ÎŒm were fabricated from the chip corner along the 45° diagonal direction. The confocal-DIC-measured maximum hoop strains and first principal strains were located at the crack front area for both the 160 ÎŒm and 640 ÎŒm cracks. A crack model was developed using the extended finite element method (XFEM), and the strain distribution in the simulation had the same trend as the experimental results. The distribution of hoop strains were in good agreement with the measured values, when the model element size was smaller than 22 ÎŒm to capture the strong strain gradient near the crack tip. The third objective was to propose a modeling approach for underfill delamination and cracking with the effects of manufacturing variables. A deep thermal cycling test was performed on 13 test cells to obtain the reference chip-underfill delamination areas and crack profiles. An artificial neural network (ANN) was trained to relate the effects of manufacturing variables and the number of cycles to first delamination of each cell. The predicted numbers of cycles for all 6 cells in the test dataset were located in the intervals of experimental observations. The growth of delamination was carried out on FEM by evaluating the strain energy amplitude at the interface elements between the chip and underfill. For 5 out of 6 cells in validation, the delamination growth model was consistent with the experimental observations. The cracks in bulk underfill were modelled by XFEM without predefined paths. The directions of edge cracks were in good agreement with the experimental observations, with an error of less than 2.5°. This approach met the goal of the thesis of estimating the underfill initial delamination, areas of delamination and crack paths in actual industrial flip-chip assemblies.Afin de protĂ©ger les interconnexions dans les assemblages, une couche de matĂ©riau d’underfill est utilisĂ©e pour remplir le volume et fournir un support mĂ©canique entre la puce de silicium et le substrat. En raison de la gĂ©omĂ©trie du coin de puce et de l’écart du coefficient de dilatation thermique (CTE), l’underfill souffre d’une concentration de contraintes dans les coins lorsque la tempĂ©rature est infĂ©rieure Ă  la tempĂ©rature de cuisson. Cette concentration de contraintes conduit Ă  des dĂ©faillances mĂ©caniques dans les encapsulations de flip-chip, telles que la dĂ©lamination interfaciale puce-underfill et la fissuration d’underfill. Les contraintes et dĂ©formations locales sont les paramĂštres les plus importants pour comprendre le mĂ©canisme des ruptures de l’underfill. En consĂ©quent, l’industrie utilise actuellement la mĂ©thode des Ă©lĂ©ments finis (EF) pour calculer les composantes de la contrainte, qui ne sont pas assez prĂ©cises par rapport aux contraintes actuelles dans l’underfill. Ces simulations nĂ©cessitent un examen minutieux de dĂ©tails gĂ©omĂ©triques importants et des propriĂ©tĂ©s des matĂ©riaux. Cette thĂšse vise Ă  proposer une approche de modĂ©lisation permettant d’estimer avec prĂ©cision les zones de dĂ©lamination et les trajectoires des fissures dans l’underfill, avec les trois objectifs suivants. Le premier objectif est de mettre au point une technique expĂ©rimentale capable de mesurer la dĂ©formation de l’underfill dans la rĂ©gion du coin de puce. Cette technique, combine la microscopie confocale et la mĂ©thode de corrĂ©lation des images numĂ©riques (DIC) pour permettre des mesures tridimensionnelles des dĂ©formations Ă  diffĂ©rentes tempĂ©ratures, et a Ă©tĂ© nommĂ©e le technique confocale-DIC. Cette technique a d’abord Ă©tĂ© validĂ©e par une analyse thĂ©orique en dĂ©formation thermique. Dans un Ă©chantillon similaire Ă  un flip-chip, la distribution de la dĂ©formation obtenues par le modĂšle EF Ă©tait en bon accord avec les rĂ©sultats de la technique confocal-DIC, avec des erreurs relatives infĂ©rieures Ă  20% au coin de puce. Ensuite, le second objectif est de mesurer la dĂ©formation autour d’une fissure dans l’underfill. Des fissures artificielles d’une longueuer de 160 ÎŒm et 640 ÎŒm ont Ă©tĂ© fabriquĂ©es dans l’underfill vers la direction diagonale de 45°. Les dĂ©formations circonfĂ©rentielles maximales et principale maximale Ă©taient situĂ©es aux pointes des fissures correspondantes. Un modĂšle de fissure a Ă©tĂ© dĂ©veloppĂ© en utilisant la mĂ©thode des Ă©lĂ©ments finis Ă©tendue (XFEM), et la distribution des contraintes dans la simuation a montrĂ© la mĂȘme tendance que les rĂ©sultats expĂ©rimentaux. La distribution des dĂ©formations circonfĂ©rentielles maximales Ă©tait en bon accord avec les valeurs mesurĂ©es lorsque la taille des Ă©lĂ©ments Ă©tait plus petite que 22 ÎŒm, assez petit pour capturer le grand gradient de dĂ©formation prĂšs de la pointe de fissure. Le troisiĂšme objectif Ă©tait d’apporter une approche de modĂ©lisation de la dĂ©lamination et de la fissuration de l’underfill avec les effets des variables de fabrication. Un test de cyclage thermique a d’abord Ă©tĂ© effectuĂ© sur 13 cellules pour obtenir les zones dĂ©laminĂ©es entre la puce et l’underfill, et les profils de fissures dans l’underfill, comme rĂ©fĂ©rence. Un rĂ©seau neuronal artificiel (ANN) a Ă©tĂ© formĂ© pour Ă©tablir une liaison entre les effets des variables de fabrication et le nombre de cycles Ă  la dĂ©lamination pour chaque cellule. Les nombres de cycles prĂ©dits pour les 6 cellules de l’ensemble de test Ă©taient situĂ©s dans les intervalles d’observations expĂ©rimentaux. La croissance de la dĂ©lamination a Ă©tĂ© rĂ©alisĂ©e par l’EF en Ă©valuant l’énergie de la dĂ©formation au niveau des Ă©lĂ©ments interfaciaux entre la puce et l’underfill. Pour 5 des 6 cellules de la validation, le modĂšle de croissance du dĂ©laminage Ă©tait conforme aux observations expĂ©rimentales. Les fissures dans l’underfill ont Ă©tĂ© modĂ©lisĂ©es par XFEM sans chemins prĂ©dĂ©finis. Les directions des fissures de bord Ă©taient en bon accord avec les observations expĂ©rimentales, avec une erreur infĂ©rieure Ă  2,5°. Cette approche a rĂ©pondu Ă  la problĂ©matique qui consiste Ă  estimer l’initiation des dĂ©lamination, les zones de dĂ©lamination et les trajectoires de fissures dans l’underfill pour des flip-chips industriels

    Walking with the Earth: Intercultural Perspectives on Ethics of Ecological Caring

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    It is commonly believed that considering nature different from us, human beings (qua rational, cultural, religious and social actors), is detrimental to our engagement for the preservation of nature. An obvious example is animal rights, a deep concern for all living beings, including non-human living creatures, which is understandable only if we approach nature, without fearing it, as something which should remain outside of our true home. “Walking with the earth” aims at questioning any similar preconceptions in the wide sense, including allegoric-poetic contributions. We invited 14 authors from 4 continents to express all sorts of ways of saying why caring is so important, why togetherness, being-with each others, as a spiritual but also embodied ethics is important in a divided world

    The fairy tale therapist

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    Creative Work: The Fairy Tale Therapist:- My original intention was to create a new form of self-help book using fairy tale characters to address psychological problems. The book I wrote does use fairy tale characters to address psychological problems, but it isn’t written as a typical self-help book. Instead, it’s the story of Helen, a therapist whose personal life suddenly and dramatically falls apart. Her solution? Not one she would suggest to her clients. She leaves everyone she knows and sets up a new practice in a small village in Wales, where her troubles start to fade. Until, that is, the fairy tale characters start coming in through her office window. Contextualising Research: The Fragility of Equilibrium in Therapy and Writing:- In this part of the thesis, I explore the methodologies, underpinning literature and the personal journey of discovery that led to the final book. I examine bibliotherapy, the self-help genre and case studies after selecting a canon of well-known self-help books. I then review classical fairy tales and I try using fairy tale characters rather than case studies as examples in a self-help book. I present these attempts and argue why this straight substitution doesn’t work well. Instead, therefore, I decided to use fairy tale characters’ dilemmas to illustrate the process of psychotherapy, so readers can better understand the aims and processes in therapy, and see inside a therapist’s mind. The resulting book ('The Fairy Tale Therapist', the creative part of my thesis), raised two further questions which I answer and explore in the critical component. Is there a concept that is central both to writing fiction and to carrying out/undergoing psychotherapy, and if so how does it achieve this? Story is the bridge that links these disciplines, and I found the key within story is the concept of equilibrium, the desire to return to a stable state an organism is disrupted by a challenge. Equilibrium is, however, elusive: once achieved, disequilibrium begins again. How might changing perspective help to better understand characters in fiction and clients in therapy/self-help? I found that by adhering to the plot but looking at what happens from a different perspective, as an author I gained a deeper understanding of characters other than the central character. I can also apply this technique to help my patients. Instead of trying to change what’s going on outside of themselves, I can challenge them to try changing the way they consider what’s going on, and in so doing help them to learn to accept—perhaps even celebrate—their circumstances

    Elasto-plastic deformations within a material point framework on modern GPU architectures

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    Plastic strain localization is an important process on Earth. It strongly influ- ences the mechanical behaviour of natural processes, such as fault mechanics, earthquakes or orogeny. At a smaller scale, a landslide is a fantastic example of elasto-plastic deformations. Such behaviour spans from pre-failure mech- anisms to post-failure propagation of the unstable material. To fully resolve the landslide mechanics, the selected numerical methods should be able to efficiently address a wide range of deformation magnitudes. Accurate and performant numerical modelling requires important compu- tational resources. Mesh-free numerical methods such as the material point method (MPM) or the smoothed-particle hydrodynamics (SPH) are particu- larly computationally expensive, when compared with mesh-based methods, such as the finite element method (FEM) or the finite difference method (FDM). Still, mesh-free methods are particularly well-suited to numerical problems involving large elasto-plastic deformations. But, the computational efficiency of these methods should be first improved in order to tackle complex three-dimensional problems, i.e., landslides. As such, this research work attempts to alleviate the computational cost of the material point method by using the most recent graphics processing unit (GPU) architectures available. GPUs are many-core processors originally designed to refresh screen pixels (e.g., for computer games) independently. This allows GPUs to delivers a massive parallelism when compared to central processing units (CPUs). To do so, this research work first investigates code prototyping in a high- level language, e.g., MATLAB. This allows to implement vectorized algorithms and benchmark numerical results of two-dimensional analysis with analytical solutions and/or experimental results in an affordable amount of time. After- wards, low-level language such as CUDA C is used to efficiently implement a GPU-based solver, i.e., ep2-3De v1.0, can resolve three-dimensional prob- lems in a decent amount of time. This part takes advantages of the massive parallelism of modern GPU architectures. In addition, a first attempt of GPU parallel computing, i.e., multi-GPU codes, is performed to increase even more the performance and to address the on-chip memory limitation. Finally, this GPU-based solver is used to investigate three-dimensional granular collapses and is compared with experimental evidences obtained in the laboratory. This research work demonstrates that the material point method is well suited to resolve small to large elasto-plastic deformations. Moreover, the computational efficiency of the method can be dramatically increased using modern GPU architectures. These allow fast, performant and accurate three- dimensional modelling of landslides, provided that the on-chip memory limi- tation is alleviated with an appropriate parallel strategy

    Application of wearable sensors in actuation and control of powered ankle exoskeletons: a Comprehensive Review

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    Powered ankle exoskeletons (PAEs) are robotic devices developed for gait assistance, rehabilitation, and augmentation. To fulfil their purposes, PAEs vastly rely heavily on their sensor systems. Human–machine interface sensors collect the biomechanical signals from the human user to inform the higher level of the control hierarchy about the user’s locomotion intention and requirement, whereas machine–machine interface sensors monitor the output of the actuation unit to ensure precise tracking of the high-level control commands via the low-level control scheme. The current article aims to provide a comprehensive review of how wearable sensor technology has contributed to the actuation and control of the PAEs developed over the past two decades. The control schemes and actuation principles employed in the reviewed PAEs, as well as their interaction with the integrated sensor systems, are investigated in this review. Further, the role of wearable sensors in overcoming the main challenges in developing fully autonomous portable PAEs is discussed. Finally, a brief discussion on how the recent technology advancements in wearable sensors, including environment—machine interface sensors, could promote the future generation of fully autonomous portable PAEs is provided

    Investigating the Effects of SiC Abrasive Particles on Friction Element Welding

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    The growing demands on reducing the harmful emissions from automobiles have forced automakers to reduce the weight of the vehicle. The increasing demands on improving the fuel economy also has challenged automotive manufacturers to make the vehicle as lightweight as possible. However, the challenge is also to ensure that the vehicle meets safety standards. For the vehicle to meet these standards, it needs to be of adequate strength as well. Automotive manufacturers have adopted a strategy of using multi-material construction to achieve the target. But with multi-material construction comes the requirement of advanced joining techniques that are capable of joining dissimilar materials. The requirement of the advanced techniques is due to the difference in physical and chemical properties of the dissimilar materials to be joined. The conventional methods are either unable to join the dissimilar material or form a joint with defects and of poor quality. Friction Element Welding (FEW) is one of the advanced joining techniques capable of joining dissimilar materials effectively. The process is based on the concepts of friction welding technique where the materials to be joined are heated to the temperature below their melting temperatures. In FEW, a friction element is used to form a friction weld. It has been found that the FEW process although has a low processing time, it is still higher than a few of its competitors. Most of the processing time of the FEW process is taken by the second step of the process, i.e., the cleaning step. Cleaning step parameters are the dominating factors that affect the processing time of the process. The cleaning step involves removing the coatings/impurities present on the bottom sheet of the materials to be joined while also pre- iii heating the friction element. The removal of coatings/impurities, however, can be accelerated with the use of abrasive particles. This study focuses on the effect of abrasive particles on the cleaning time and processing time of FEW. Silicon carbide abrasive particles have a high hardness and provide higher wear rates. The higher wear rates promote the wearing off of coatings from the surface of the materials. Silicon carbide abrasive particles were placed in a pre-drilled pocket in an aluminum top sheet. Design of Experiments (DOE) involved two levels of pocket size, pocket depth, abrasive particle size, and volume fraction of abrasives. The results show that abrasive particle size and volume fraction of abrasive particles were the dominating factors in determining the cleaning step time and overall processing time. Lower particle size and volume fraction of abrasives resulted in a reduction of cleaning time and processing time. Cross-tension strength (CTS) tests were performed, followed by microscopy analysis and hardness testing to study the effect of abrasives on the joint quality. The best case was observed for 6 mm pocket size, 0.2 mm pocket depth, 5 ÎŒm abrasive particle size, and 50% volume fraction of abrasives. The best case with abrasives was compared with the FEW sample which does not involve pocket and abrasives. The comparison showed that the inclusion of abrasives results in a reduction in cleaning time by 39.93% and processing time by 14.28%. The CTS of the joints formed with abrasives was slightly higher than the case without abrasives. Both the cases showed a button pull-out failure when subjected to CTS loading conditions. Microstructural analysis showed a presence of hard SiC and wider martensite phase, which is a probable reason for an increase in the joint strength for the joints that involved iv abrasives. The Microhardness tests further supported the CTS results. For the joints involving abrasives, a marginally higher hardness was observed along the cross-section. The significance of this study lies in the opportunities to reduce the processing time of the joining process using abrasive particles

    Tribochemical investigation of ZDDP tribofilm

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    The current trend for using lower-viscosity lubricants with the aim of improving fuel efficiency of mechanical systems means that machine components are required to operate for longer periods in thin oil film, mixed and boundary lubrication conditions, where the risk of surface damage is increased. For this reason, the role of tribofilms generated from the antiwear additive zinc dialkyldithiophosphate (ZDDP) in providing surface protection has become increasingly important. However, the properties, performance and the mechanisms of tribofilm formation are not fully understood. Therefore, this thesis aims to further understand the tribochemical behaviour of ZDDPs. Several inter-connected areas of research are described in this thesis. These all investigate the formation of tribofilms by ZDDP and the impact of tribofilm formation on wear. Taken together they contribute to our understanding of the mechanisms of tribological behaviour of ZDDPs and should assist in the design of lubricants and rubbing components. Firstly, the evolution of ZDDP tribofilm properties, in particular, tribofilm durability and the origins of this durability, are examined. It is found that ZDDP tribofilms undergo a structural transformation during rubbing from a predominantly amorphous structure to one that is nanocrystalline, resulting in the tribofilm becoming much stronger and more durable. Secondly, the reaction mechanisms of tribofilm formation on various non-ferrous metal and non-metallic materials are studied, both by ion-implanting various alloying elements into steel surfaces, and by using non-metallic rubbing materials. It is found that a potentially important factor in the formation of ZDDP tribofilms is the presence and concentration of ferrous and/or non-ferrous metal atoms at the surface. Non-ferrous metals may act as adsorption sites for ZDDP in a similar manner to Fe in steel, enabling formation of ZDDP tribofilms. Thirdly, to further understand the antiwear performance of ZDDP, its impact on the wear of a-C:H DLC in the presence of molybdenum dialkyldithiocarbamate (MoDTC), which is known to be problematic, is investigated. It is well known that MoDTC can produce very high wear of DLC and that this can be mitigated by the presence of ZDDP in the lubricant. From this research it is shown that ZDDPs reduce DLC wear in the presence of MoDTC mainly by forming thick antiwear tribofilms and reducing the ratio of MoO3/MoS2 in the tribofilm. Fourthly, although ZDDP is very effective for reducing most types of wear, it is known that ZDDP can promote one particular form of wear, micropitting wear. In this research a new method of studying micropitting that enables both micropitting and tribofilm formation to be studied in parallel is developed which is key to assessing the impact of lubricant chemistry on micropitting. This is used to show that the influence of ZDDP on micropitting originates from its tendency to prevent running-in of the contacting surfaces. The mechanism by which a widely-used black oxide coating limits micropitting is explored and it is shown that this relatively soft coating provides adequate running-in even in the presence of ZDDP-containing oils. Finally, the impact of friction on micropitting is studied by isolating its effects from those of running-in, by controlling separately the formation of ZDDP and MoDTC tribofilms. Results show that friction has a very significant impact on micropitting.Open Acces
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