253 research outputs found

    Meshless Mechanics and Point-Based Visualization Methods for Surgical Simulations

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    Computer-based modeling and simulation practices have become an integral part of the medical education field. For surgical simulation applications, realistic constitutive modeling of soft tissue is considered to be one of the most challenging aspects of the problem, because biomechanical soft-tissue models need to reflect the correct elastic response, have to be efficient in order to run at interactive simulation rates, and be able to support operations such as cuts and sutures. Mesh-based solutions, where the connections between the individual degrees of freedom (DoF) are defined explicitly, have been the traditional choice to approach these problems. However, when the problem under investigation contains a discontinuity that disrupts the connectivity between the DoFs, the underlying mesh structure has to be reconfigured in order to handle the newly introduced discontinuity correctly. This reconfiguration for mesh-based techniques is typically called dynamic remeshing, and most of the time it causes the performance bottleneck in the simulation. In this dissertation, the efficiency of point-based meshless methods is investigated for both constitutive modeling of elastic soft tissues and visualization of simulation objects, where arbitrary discontinuities/cuts are applied to the objects in the context of surgical simulation. The point-based deformable object modeling problem is examined in three functional aspects: modeling continuous elastic deformations with, handling discontinuities in, and visualizing a point-based object. Algorithmic and implementation details of the presented techniques are discussed in the dissertation. The presented point-based techniques are implemented as separate components and integrated into the open-source software framework SOFA. The presented meshless continuum mechanics model of elastic tissue were verified by comparing it to the Hertzian non-adhesive frictionless contact theory. Virtual experiments were setup with a point-based deformable block and a rigid indenter, and force-displacement curves obtained from the virtual experiments were compared to the theoretical solutions. The meshless mechanics model of soft tissue and the integrated novel discontinuity treatment technique discussed in this dissertation allows handling cuts of arbitrary shape. The implemented enrichment technique not only modifies the internal mechanics of the soft tissue model, but also updates the point-based visual representation in an efficient way preventing the use of costly dynamic remeshing operations

    Modelling and Visualization of the Surface Resulting from the Milling Process

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    Quasi-Static, Cyclic, and Fracture Characteristics of Plasma Transferred Wire Arc (PTWA) Thermal Sprayed AlSi Cylinder Bores

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    This study examines the microstructure and mechanical properties of plasma transferred wire arc (PTWA) coating of typical alloyed steel, deposited on diecast aluminum alloy cylinder bores. The coating surface and microstructure were characterized in terms of surface roughness, features (i.e., defects, splats formation mechanisms, distribution of oxides, re-solidified particles, and interfacial metallurgical bonding) using laser scanning confocal microscope, scanning and transmission electron microscope (SEM and TEM). Residual stress through the thickness of the coating was measured using x-ray diffraction (XRD) and hole-drilling method. In post-processed samples, compressive residual stress was measured throughout the coating with a value close to 100 MPa at the interface, resulting from the thermal mismatch between coating and substrate materials. In terms of mechanical properties, coating hardness was estimated at both the micro- and nanoscale and examined the influence of microstructure inhomogeneity on the mechanical performance and failure modes. The PTWA coating of typical alloyed steel deposited on diecast aluminum alloy cylinder bores is investigated via experimental pull tests for three types of samples of varying interface pattern and/or substrate material and finite element model (FEM) simulations. FEM simulations account for the portion of adhesion attributed to solely mechanical interlocking, whereas the experimental results rely on the adhesion at the interface due to mechanical interlocking, potential metallurgical bonding, and/or other factors at play. Experimental results show that differences in interface pattern and/or substrate material will result in varying degrees of adhesion failure at the interface and cohesion failure within the sprayed coating itself. The average bonding strength across the three types of samples was found to be 37.59, 27.55, and 33.15 MPa (for D319, W319, and W356 sample types). Monotonic three-point bending tests of curved samples extracted from trial cylinder bores and consequent analysis using the equivalent section method yielded stress-strain properties for both the substrate and coating materials. SEM observation of fracture surfaces showed three modes of failure involving coating delamination and breakage, which is related to the deposition process and the various features within the coating. Further bending tests of flat samples were performed for four combinations of interface pattern orientation and substrate material. Bending tests were conducted at three temperatures within an environmental chamber (room temperature, 100Ā°C, and 250Ā°C) to simulate the relevant thermal conditions within the engine during operation. Results show that differences in interface pattern and/or substrate material will result in differing failure mechanisms and strengths as well as the trends associated with increasing operating temperatures. It was found that the A356 substrate generally performed better than the corresponding A319, and the samples with dovetail rows running along the length were stronger than the opposing orientation. Cyclic three-point bending tests were performed for four combinations of interface pattern orientation and substrate material. The Basquin parameters were obtained for all twelve combinations of substrate, orientation, and temperature. The fatigue strength predicted by Basquin parameters are more conservative compared to what is seen in experimental data. Results show that differences in the interface pattern and/or substrate material will result in differing failure mechanisms and fatigue properties, as well as trends associated with increasing temperatures. It was found that the A356 substrate generally performed better than the corresponding A319 and the dominating failure mechanism was specific to the sample orientation (i.e., delamination and interlock breakage/separation). An increase in temperature is generally associated with reduced fatigue properties and increased delamination/separation due to the thermal coefficient mismatch during expansion between the coating and substrate materials. Overall, it was found that the combination of A356 substrate in the H-orientation with the interfacial wave pattern surface activation is a candidate with high potential towards the application of cylinder bores

    Plasma spray deposition of polymer coatings

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    This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This work investigates the feasibility of the use of plasma spray deposition as a method of producing high performance polymer coatings. The work concentrates on the understanding of the processing of the plasma spraying of polymers, the behaviour of polymeric materials during deposition, and the study of process-structure-properties relationships. Processing modelling for the three stages of the evolution of a polymer deposit (droplet-splat-coating) has been carried out using heat transfer theory. A theoretical model is proposed which consists of three parts: the first part predicts the temperature profile of in-flight particles within plasma jet, the second part predicts the cooling of isolated splats impacting on a substrate and the third part, the heat transfer through the coating thickness. The heat transfer analysis predicts that the development of large temperature gradients within the particle is a general characteristics of polymers during plasma spraying. This causes difficulties for polymer particles to be effectively molten within the plasma jet without decomposition. The theoretical calculations have predicted the effect of processing parameters on the temperature, the degree of melting and decomposition of in-flight polymer particles. With the aid of the model, the conditions for the preparation of high integrity thermoplastic deposits have been established by the control of the plasma arc power, plasma spraying distance, feedstock powder injection, torch traverse speed and feedstock particle size. The optimal deposition conditions are designed to produce effective particle melting in the plasma, extensive flow on impact, and minimal thermal degradation. The experimental work on optimizing processing parameters has confirmed the theoretical predictions. Examination of polymer coating structures reveals that the major defects are unmelted particles, cracks and pores. Five major categories of pores have been classified. It also revealed a significant loss in crystallinity and the presence of a minor metastable phase in the plasma deposited polyamide coatings due to rapid solidification. The study has indicated that the molecular weight of a polymer plays an important role on the splat flow and coating structure. Under non-optimal deposition condition, substantial thermal degradation occurred for which a chain scission mechanism is proposed for plasma deposited polyamide coatings. There are difficulties in achieving cross-linking during plasma spray deposition of thermosets. The theoretical calculations predict that adequate cross-linking is unlikely in a coating deposited under normal conditions, but preheating the substrate to above the cross-linking temperature improves the degree of cross-linking of the coatings substantially. In addition, the coating thickness has a major effect on the degree of cross-linking of thermosets. The calculations also predict that lowering the thermal conductivity by applying a thermal barrier undercoat and using a faster curing agent to reduce time required for the cross-linking reaction can improve the degree of cross-linking of thermoset deposits. The experimental results for the degree of cross-linking and wear resistance confirmed these predictions.EPSRC and Mo

    Investigation into laser re-melting of inconel 625 HVOF coating blended with WC

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    High velocity oxy-fuel (HVOF) spraying of Diamalloy 1005 powders mixed with WC particles onto steel (304) is considered and laser re-melting of the resulting coatings is examined. Laser re-melting process is modeled to determine the melt layer thickness while temperature increase is formulated using the Fourier heating law. The morphological and metallurgical analyses prior and post laser re-melting process are carried out using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) technique is used to determine the residual stress developed in the coating while the analytical formulation is adopted to predict the residual stress levels at the coating base material interface. The indentation tests are carried out to determine the Youngā€™s modulus and fracture toughness of the coating prior to laser re-melting. Corrosion resistance of coating is measured using potentiodynamic polarization technique prior and post laser treatment process. The predictions of the melt layer thickness are in good agreement with experimental results. The presence of WC particles modifies temperature rise and its gradient in the coating while affecting the Youngā€™s modulus, residual stress levels, and fracture toughness of the coating. The differences in the thermal properties of Inconel 625 powders and WC particles result in formation of small size cellular structure through polyphase solidification. WC dissolution in the central region of the large polycrystalline cells is observed due to the loss of carbon through carbonic gas formation. The results of corrosion tests prevail that significant improvement of corrosion resistance can be achieved after laser treatment process

    Investigation of wear and scuffing behaviour of ferrous thermal spray coatings for aluminum engines.

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    The development of lightweight internal combustion engines using materials such as cast aluminum alloys represents one of the most significant technological developments in the automotive industry. These engines reduce weight, which in turn reduce fuel consumption and emission. However, poor wear resistance and low seizure load of unprotected Al-Si alloys are a major drawback for applications involving sliding contact in automotive engine blocks. The wear resistance of cast aluminum parts can be improved by depositing coatings on the sliding surfaces. In this respect, iron based coatings deposited through a thermal spray process may play an important role in improving wear resistances of aluminium parts used in the automotive industry. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis2004 .E37. Source: Dissertation Abstracts International, Volume: 65-10, Section: B, page: 5340. Advisers: A. T. Alpas; T. Perry. Thesis (Ph.D.)--University of Windsor (Canada), 2004

    Microstructural characterization of high-velocity oxy-fuel (HVOF) sprayed nickel coatings

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    The high-velocity oxy-fuel (HVOF) thermal spray process is a form of surface engineering technology in which high kinetic energies are used at relatively low temperatures to deposit high quality coatings. In this thesis, a detailed microstructural analysis was carried out to investigate the effect of substrate material properties and surface conditions, including surface chemistry and roughness, on splat formation behaviour. Nickel powder was sprayed onto five different substrates using the HVOF process. A range of characterisation techniques, including scanning electron microscopy, focused ion beam microscopy, transmission electron microscopy, together with scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy were employed to analyse the samples. The particle solidification behaviour and mechanisms of splat formation were analysed, with particular attention to the changes occurring at the splat-substrate interface, the subsequent bonding behaviour due to the impact of high thermal and kinetic energy particles on the substrates of different compositions and with variable surface conditions. It was shown that most particles reached the substrate surface in a partially melted form owing to the high velocity and low temperature typical in the HVOF process. It was observed that splat morphologies, their frequency of occurrence and splat-substrate bond quality are all greatly affected by the surface condition and thermo-physical properties of the substrate. Relatively higher surface roughness led to a reduction in the flattening velocity and the splashing of the splats. The presence of adsorbates, revealed through X-ray photoemission spectroscopy (XPS), promoted the formation of doughnut-shaped splats. It was observed that the interface temperature and the thermophysical properties of the substrate contributed significantly toward splat-substrate elemental interdiffusion at the interface. Both particle size and the location of impact on a grit-blasted rough surface played important roles in defining the melting state, flattening degree and, ultimately, adhesion of the particles with this substrate. It was shown that the degree of substrate melting and the associated time for re-solidification are critical for particle bonding on a softer, low melting point substrate. The microstructural findings, aided by theoretical models, revealed that a mixture of mechanical and metallurgical bonds can exist between the splats and substrates
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