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Development of robust connection models for steel and composite structures in fireLin, Shuyuan January 2014 (has links)
Structural engineers and architects have a responsibility for incorporating fire safety into their building designs in order to minimize loss of life and property. To meet this requirement, extensive research has been carried out, aimed at obtaining better understanding of the performance of steel and composite structures under fire conditions. Recent research indicates that the robustness of steel connections is vitally important to the fire resistance of steel-framed composite buildings. The development of effective connection models is a key issue in this research field. This PhD research is focused on the development of robust connection elements, for modelling steel connections at elevated temperatures. In this work, a robust simplified two-node connection element has been developed, for modelling the behaviour of the bolted end-plate connections between steel beam and column at elevated temperatures. The proposed numerical procedure is based on the model proposed by Huang (2011), incorporating additional developments to more precisely determine the tension, compression, and bending moment capacities of end-plate connections in fire. The proper failure criteria are proposed to calculate the tension capacity for each individual bolt row. In this new model, the connection failure due to bending, axial tension, compression and shear are considered. The influence of the axial force of the connected beam on the connection is also taken into account. This new model has the advantages of both the simple and component-based models. A total of 22 tests are used to validate the model. From these validations, it is evident that this new connection model has ability to accurately predict the behaviour of the end-plate connection at elevated temperatures, and can be used to represent the end-plate connections in supporting performance-based fire resistance design of steel-framed composite buildings. For modelling the behaviour of partial end-plate connections between steel beams and columns under fire conditions, a simplified robust 2-node connection element has also been developed. The rotational response of a partial end-plate connection at elevated temperatures comprises of two stages. These stages are due to the shift of the compression centre of the connection from the end of end-plate, to the centre of the beam bottom flange at large rotation. The model proposed in this research accounts for these two stage behaviours, representing the partial end-plate iv connection as a 2-node non-linear spring element. Characteristics of the spring, such as stiffness, tension, compression, shear strengths and bending moment resistance, are determined based on a component-based approach. This model therefore retains the advantages of both the simple and component-based models. Compared to normal component-based models, this simplified model has very good numerical stability under static solver condition, and is computationally efficient. Fourteen tests are used to validate the model, showing that the model is capable of accurately predicting the behaviour of partial end-plate connections under fire conditions. A series of numerical studies has been conducted on a 2D steel frame, subjected to ISO834 Fire and Natural Fire, in order to investigate the influences of the connections on the behaviour of steel structures. It is clear that the model can be used to represent the partial end-plate connections in performance-based fire resistance design of steel-framed composite buildings. According to full-scale fire tests, tensile membrane action within the concrete floor slabs plays an important role in affecting the fire resistance of composite buildings. It is well known that the development of tensile membrane actions relies on the vertical support along the edges of the slab panel. However, there is at present a lack of research into how vertical supports influence the tensile membrane actions of the slab. In this thesis, the performance of a generic three dimensional 45m x 45m composite floor subjected to ISO834 Fire and Natural Fire are investigated. Different vertical support conditions and three steel meshes are applied, in order to assess the impact of vertical supports on tensile membrane action of floor slabs. Unlike other existing large scale modelling which assumed that the connections behave as pinned or rigid for simplicity, the two robust 2-node connection element models described above are used to model the semi-rigid behaviour of end-plate and partial end-plate connections within the fire compartment. The impact of connections on the 3D behaviour of composite floors is taken into consideration. The load-transfer mechanisms of a composite floor, when connections fail due to axial tension, vertical shear and bending are investigated. Based on the results obtained, some design recommendations are proposed for enhancing the fire resistance of composite buildings.
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Finite element simulation of the post-buckled failure mechanics of thin plate structures subjected to in-plane shear displacement loadingHussain, Naveed January 2013 (has links)
The performance of thin-plate structural systems is known to be greatly influenced by the effect of buckling and post-buckling behaviour. The main factors, which affect the buckling, and post-bucking characteristics and consequently the ultimate performance of thin plate structural components, are noted to be structural geometry, material properties, loading and boundary conditions and geometric imperfections. Present day knowledge and understanding of the buckling and post-buckling behaviour of thin plate structures is at a fairly sophisticated level due to the intensive research that has been carried out over the years in this field of study. This is particularly true for the case of compressive loading whereby the collapse and unloading failure mechanics of thin plate structures has been well documented for this case. The same is not true for the case of shear loading and although much work has been carried out there is a lack of knowledge and in-depth understanding of shear post-ultimate conditions which essentially defines the initiation and progressive development of the plastic failure mechanisms of thin plate structural system. This thesis makes a contribution to the area of study by taking advantage of the developments in recent years of computational technology and computing power to develop finite element modelling strategies and solution procedures using the commercially available FE package PATRAN/NASTRAN to describe in detail the post-buckled shear failure of thin plate structural systems. The work of this thesis provides an in-depth understanding of the complex post-buckled failure mechanics associated with thin-plate structures subjected to in-plane shear displacement and combined shear and compression loading. Simply supported in-plane normal stress free and straight edge boundaries are employed to examine the shear performance as well as the failure mechanisms of thin and stocky web plates. Finite element modelling strategies are developed, which are able to describe the complete loading history from the onset of initial buckling through the nonlinear elastic post-buckling to initial material yielding and its further propagation throughout the structure leading to the development of an appropriate failure mechanism that causes final plastic collapse and subsequent load drop-off. The post-buckled failure response of the thin plate structures is determined with due consideration being given to the effects of geometric and material nonlinearities. The effect of stiffeners on structural performance is detailed for single and multiple asymmetrical and symmetrically attached stiffeners. The degrading influence on the structural performance of cut-outs as well as the considerable redeeming effect due to reinforcements attached at the cut-out boundaries is highlighted in this thesis. The work of the thesis covers the in-plane shear displacement loading of thin web plates, thin web plates with transverse stiffeners, web plates with cut-outs, web plates with stiffened cut-outs and the interactive shear and compressive loading of transversely stiffened web structures. The in-plane shear displacement loading of these structures using the multipoint constraint loading strategy in the finite element modelling procedures has shown to be highly successful in being able to provide an in-depth understanding of the failure mechanics of these structures to a level not to be found in the existing literature.
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Micro-mechanical characteristics and dimensional change of Cu-Sn interconnects due to growth of interfacial intermetallic compoundsChen, Zhiwen January 2015 (has links)
Sn-based solder alloys are extensively used in electronic devices to form interconnects between different components to provide mechanical support and electrical path. The formation of a reliable solder interconnects fundamentally relies on the metallurgic reaction between the molten solder and solid pad metallization in reflowing. The resultant IMC layer at the solder/pad metallization interface can grow continuously during service or aging at an elevated temperature, uplifting the proportion of IMCs in the entire solder joint. However, the essential mechanical properties of interfacial IMC (i.e. Cu6Sn5, Cu3Sn) layers, such as Young s modulus and hardness, are drastically different in comparison with Sn-based solder and substrate. Therefore, the increasing fraction of interfacial IMCs in the solder joint can lead to significant deformation incompatibility under exterior load, which becomes an important reliability concern in the uses of solder joints for electronic interconnects. In the past decades, extensive research works were implemented and reported regarding the growth of interfacial IMC layers and its effect on the mechanical integrity of solder joints. But, the following fundamental issues in terms of mechanical and microstructural evolution in the uses of solder joints still remain unclear, demanding further research to elaborate: (1) The protrusion of IMCs: Though the growth of interfacial IMC layers along the diffusion direction in solder joints were studied extensively, the growth of IMCs perpendicular to the diffusion direction were reported in only a few papers without any further detailed investigation. This phenomena can crucially govern the long-term reliability of solder interconnects, in particular, in the applications that require a robust microstructural integrity from a solder joint. (2) Fracture behaviour of interfacial IMC layers: The fracture behaviour of interfacial IMC layers is a vital factor in determining the failure mechanism of solder joints, but this was scarcely investigated due to numerous challenges to enable a potential in-situ micro-scale tests. It is therefore highly imperative to carry out such study in order to reveal the fracture behaviour of interfacial IMC layers which can eventually provide better understanding of the influence of interfacial IMC layers on the mechanical integrity of solder joints. (3) Volume shrinkage: The volume shrinkage (or solder joint collapse) induced by the growth of interfacial IMC layers was frequently ascribed as one of the main causes of the degradation of mechanical reliability during aging due to the potentially resulted voids and residual stress at the solder/substrate interface. However, very few experimental works on the characterisation of such type of volume shrinkage can be found in literatures, primarily due to the difficulties of observing the small dimensional changes that can be encountered in the course of IMCs growth. (4) Residual stress: The residual stress within solder joints is another key factor that contributes to the failure of solder joints under external loads. However, the stress evolution in solder joints as aging progresses and the potential correlation between the residual stress and the growth of interfacial IMC layers is yet to be fully understood, as stress/strain status can fundamentally alter the course of total failure of a solder joint. (5) Crack initiation and propagation in solder joints: Modelling on the mechanical behaviour of solder joints is often undertaken primarily on the stress distribution within solder joints, for instance, under a given external loading. But there is lack of utilising numerical analysis to simulate the crack initiation and propagation within solder joints, thus the effect of interfacial IMC layers on the fracture behaviour of the solder joints can be elaborated in further details. In this thesis, the growth of interfacial IMCs in parallel and perpendicular to the interdiffusion direction in the Sn99Cu1/Cu solder joints after aging was investigated and followed by observation with SEM, with an intention of correlating the growth of IMCs along these two directions with aging durations based on the measured thickness of IMC layer and height of perpendicular IMCs. The mechanism of the protrusion of IMCs and the mutual effect between the growth of IMCs along these two directions was also discussed. The tensile fracture behaviour of interfacial Cu6Sn5 and Cu3Sn layers at the Sn99Cu1/Cu interface was characterised by implementing cantilever bending tests on micro Cu6Sn5 and Cu3Sn pillars prepared by focused ion beam (FIB). The fracture stress and strain were evaluated by finite element modelling using Abaqus. The tensile fracture mechanism of both Cu6Sn5 and Cu3Sn can then be proposed and discussed based on the observed fracture surface of the micro IMC pillars. The volume shrinkage of solder joints induced by the growth of interfacial IMC layers in parallel to the interdiffusion direction in solder joint was also studied by specifically designed specimens, to enable the collapse of the solder joint to be estimated by surface profiling with Zygo Newview after increased durations of aging. Finite element modelling was also carried out to understand the residual stress potentially induced due to the volume shrinkage. The volume shrinkage in solder joints is likely to be subjected to the constraint from both the attached solder and substrate, which can lead to the build-up of residual stress at the solder/Cu interface. Depth-controlled nanoindentation tests were therefore carried out in the Sn99Cu1 solder, interfacial Cu6Sn5 layer, Cu3Sn layer and Cu with Vickers indenter after aging. The residual stress was then evaluated in the correlation with aging durations, different interlayers and the locations in the solder joint. Finally, finite element models incorporated with factors that may contribute to the failure of solder joints, including microstructure of solder joints, residual stress and the fracture of interfacial IMC, were built using Abaqus to reveal the effect of these factors on the fracture behaviour of solder joints under applied load. The effect of growth of IMC layer during aging on the fracture behaviour was then discussed to provide a better understanding of the degradation of mechanical integrity of solder joints due to aging. The results from this thesis can facilitate the understanding of the influence of interfacial IMC layers on the mechanical behaviour of solder joints due to long-term exposure to high temperatures.
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Hybrid composite wires for tensile armour in flexible risersGautam, Mayank January 2001 (has links)
Flexible risers that carry hydrocarbon fuels from the subsea facilities to the floatation units above the sea surface are composed of multiple metallic and polymeric layers (in their wall). Among these layers, the tensile armour layer consists of several helically wound metallic wires; these tensile armour layers carry the weight of the riser, provide tensile stiffness & strength and maintain the structural integrity of the riser structure during harsh underwater currents. However, as the oil & gas fields in shallow waters are receding, the oil & gas industry is being forced to move towards deeper offshore waters, where the metallic tensile armour wires pose limitations (fatigue, corrosion, weight, etc.). In this thesis an alternative to metallic tensile armour wires will be presented in form of a flexible hybrid composite formed by stacking seven pultruded composite (carbon and vinyl-ester) circular rods in form of hexagonal pack, held together by an over-braid (Dyneema fibres) sleeve. The manufacturing process for hybrid composite tensile armour wires will be studied and their mechanical properties will be presented. A multi-scale finite element model developed for hybrid composite wires will be presented in this thesis to help further understand the mechanical properties of hybrid composite wires.
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Fundamental characterisation of thermal influence in hot mix asphalt repairByzyka, Juliana January 2018 (has links)
The study focuses on the issue of hot mix asphalt pothole repairs, the performance of which is greatly reduced by repair edge disintegration. This is caused by low interface temperatures which result in low density interfaces and poor repair bonding. The study examined heat flow in shallow and deep pothole excavations under controlled pre-heating done in heating-cooling cycles, referred as "dynamic heating", and the effect of asphalt thermal properties on this. Dynamic heating was applied with an experimental infrared heater operating between 6.6 kW and 7.7 kW heat power and with the heater being stationary or moving above simulated potholes at offsets of 130 mm and 230 mm. The study also examined heat flow in traditional non-heated shallow repairs, referred as "static repairs", and dynamic shallow repairs and the effect of pothole pre-heating in repair adhesion. Finite Element modelling was also used to enhance understanding of heat flow in the executed repairs. Then, the bonding properties and rutting resistance of the repairs were assessed using shear bond tests (SBT's) and wheel track tests (WTT's) respectively. The results showed that irrespective of excavation depth, heating power and heater offset, temperature distribution in the pothole excavation and inside the slabs under dynamic heat was non-uniform. Dynamically heating pothole excavations for approximately 10 minutes yields better heat distribution than 20 minutes heating time while minimising the possibility of asphalt overheating. The temperature profile at the interface of the dynamically heated repair is improved when compared to static repair suggesting better interface adhesion. A significant role in this profile is played by thermal contact conductance which determines the resistance to pavement-repair thermal conduction per unit area at the repair interface. This was reflected in the assessment and simulation of the repairs with the latter generating reasonable transient temperature profiles within the dynamically heated pothole excavation, at the interface of the repairs, and inside the host pavement. Further, the shear strength at the bottom and side interfaces of dynamically heated repairs was 78.2% and 68.4% higher respectively than that of static repairs. On average, static and dynamic repairs showed repair interface rutting depths of 14.82 mm and 10.36 mm respectively. It was concluded that dynamically heating a pothole excavation increases repair interface adhesion and repair durability.
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Contact deformation of carbon coatings: mechanisms and coating design.Singh, Rajnish Kumar, Materials Science & Engineering, Faculty of Science, UNSW January 2008 (has links)
This thesis presents the results of a study focussed on the elucidation of the mechanisms responsible for determining the structural integrity of carbon coatings on ductile substrates. Through elucidation of these mechanisms, two different coating systems are designed; a multilayered coating and a functionally graded coating. While concentrating upon carbon coatings, the findings of this study are applicable to a broad range of hard coatings on ductile substrates. The thesis concludes with a chapter outlining a brief study of the effects of gold coatings on silicon under contact load at moderate temperatures to complement the major part of the thesis. Carbon coatings with differing mechanical properties were deposited using plasma enhanced chemical vapour deposition (PECVD), filtered assisted deposition (FAD) and magnetron sputtering deposition methods. Combinations of these techniques plus variation of deposition parameters enabled composite multilayered and functionally-graded coatings to also be deposited. Substrates were ductile metals; stainless steel and aluminium. Characterisation of the coating mechanical properties was undertaken using nanoindentation and nano-scratch tests. The same techniques were used to induce fracture within the coatings to allow subsequent analysis of the fracture mechanism. These were ascertained with the assistance of cross-sectional imaging of indents prepared using a focussed ion beam (FIB) mill and transmission electron microscopy (TEM) using specimen preparation techniques also utilising the focussed ion beam mill. A two dimensional axisymmetric finite element model (FEM) was built of the coating systems using the commercial software package, ANSYS. Substrate elastic-plastic properties were ascertained by calibrating load-displacement curves on substrate materials with the finite element model results. Utilising the simulation of spherical indentation, the distribution of stresses and the locations for fracture initiation were ascertained using finite element models. This enabled determination of the influence such factors as substrate mechanical properties, residual stresses in the coatings and importantly the variation of elastic properties of the different coating materials. Based upon the studies of monolithic coatings, simulations were undertaken on multilayer and functionally-graded coatings to optimise design of these coating types. Based on the results of the modelling, multilayered and functionally graded coatings were then deposited and mechanical testing undertaken to confirm the models. Three major crack types were observed to occur as the result of the spherical nanoindentation on the coatings; ring, radial and lateral cracks. Ring cracks were found to initiate from the top surface of the film, usually at some distance from the edge of the spherical contact. Radial cracks usually initiated from the interface between the coating and the substrate directly under the symmetry axis of indentation and propagated outwards in a non symmetrical star-like fashion. Lateral cracks formed either between layers in the multilayer coatings or at the interface with substrate. Ring and radial cracks were found to form upon loading whereas lateral cracks formed upon both loading and unloading depending upon the crack driving mechanism. Pop-in events in the load displacement indentation curve were found to be indicative of the formation of ring cracks, while the formation of the other types of cracks was not signified by pop-ins but rather by variations in the slope of the curve. The substrate yield strength was found to influence the initiation of all crack systems while compressive stresses in the coating were seen to only influence the formation of ring and radial cracks. However, it was also noted that the initiation of one form of crack tended to then hinder the subsequent formation of others. In multilayer coatings, the lateral cracks were suppressed, as opposed to the monolayer coating system, but a ring crack was observed. This drawback in the multilayer system was successfully addressed by the design of a graded coating having the highest Young??s modulus at the middle of the film thickness. In this coating, due to the graded nature of the elastic modulus, the stresses at the deleterious locations (top surface and interface) were guided toward the middle of coating and hence increased the load bearing capabilities. The effect of substrate roughness upon the subsequent surface roughness of the coating and also upon the fracture process of the coating during indentation was also investigated. For the coatings deposited on rough substrates, the radial cracks were observed to form initially and this eventfully delayed the initiation of ring cracks. Also the number of radial cracks observed at the interface was found to be proportional to the distribution of the interfacial asperities. In summary, the study elucidated the fracture mechanisms of monolayer, multilayer and graded carbon coatings on ductile substrates under uniaxial and sliding contact loading. The effects of the yield strength, surface roughness of the substrate, along with the residual stress and elastic modulus of the coatings on the fracture of coatings were investigated. The study utilised finite element modelling to explain the experiments observations and to design coating systems.
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Characterisation of the flexural behaviour of Aluminium Foam Sandwich StructuresStyles, Millicent, milli.styles@anu.edu.au January 2008 (has links)
Aluminium foam has a range of properties that are desirable in many applications. These properties include good stiffness and strength to weight ratios, impact energy absorption, sound damping, thermal insulation and non combustibility. Many of these characteristics are particularly attractive for core materials within sandwich structures. The combination of aluminium foam cores with thermoplastic composite skins is easily manufactured and has good potential as a multifunctional sandwich structure useful in a range of applications. This thesis has investigated the flexural behaviour of such structures using a combination of experimental and modelling techniques. The development of these structures towards commercial use requires a thorough understanding of the deformation and strain mechanisms of the structure, and this will, in turn, allow predictions of their structural behaviour in a variety of loading conditions.
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The experimental research involved the use of an advanced 3D optical measuring technique that produces realtime, full-field strain evolution during loading. This experimental characterisation of strain evolution in this class of sandwich structure under flexural loading is the first of its kind in the world. The experimental work studied the sandwich structure undergoing four-point bend testing. Initial studies compared the behaviour of the aluminium foam structure with a more traditional polymer foam sandwich structure. The aluminium foam structure was found to have equivalent or improved mechanical properties including more ductile deformation and an enhanced energy absorption. An investigation was conducted on the effect of core and skin thickness on the metal structure and a range of flexural behaviours were observed. Analysis of the strain distribution showed a complex development including localised effects from the non-uniform cellular structure of the material. An understanding of the variation with size is important in establishing design methods for utilising these structures. In particular, it is desirable that finite element simulations can be used to predict behaviour of these structures in a diverse range of loading conditions. This aspect was considered in the second half of this study. An existing constitutive model for aluminium foam, developed for use in compression energy absorption studies, was used to investigate finite element simulations of the flexural behaviour of the sandwich structure. The FE model was able to predict the general deformation behaviour of the thinner skinned structures although the magnitude of the load-displacement response was underestimated. It is suggested this may be related to the size effect on the input parameter characterisation. The strain distribution corresponded well with the experimental strain measurements. It was found a simple increase in the material model input parameters was able to more closely match the magnitude of the load-displacement response while maintaining the appropriate strain distribution. The general deformation shape of the model with the thicker skin corresponded reasonably well with the experimental observations. However, further work is necessary on the element failure criterion to capture the shear cracking observed. The strain distributions of the model predicted this failure with high strain concentrations matching those of the experimental contours. The last part of the thesis describes a parametric study on the effect of the foam material model input parameters on the flexural behaviour of the sandwich structure model. An important conclusion of this work is that this material model for aluminium foam can, with some development, be utilized to provide a viable method for simulating aluminium foam composite sandwich structures in flexural loading situations.
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Studies on Wheel/Rail Contact Impact Forces at Insulated Rail JointsPang, Tao, tony_pang@hotmail.com January 2008 (has links)
To investigate the wheel/rail contact impact forces at insulated rail joints (IRJs), a three-dimensional finite element model and strain gauged experiments are employed and reported in this thesis. The 3D wheel/rail contact-impact FE model adopts a two-stage analysis strategy in which the wheel-IRJ railhead contact is first established in the static analysis and the results transferred to dynamic analysis for impact simulations. The explicit FE method was employed in the dynamic analysis. The Lagrange Multiplier method and the Penalty method for contact constraint enforcement were adopted for the static and dynamic analyses respectively.
The wheel/rail contact-impact in the vicinity of the end post is exhibited via numerical examples from the FE modelling. The wheel/rail contact impact mechanism is investigated. The strain gauged experiments which consist of a lab test and a field test are reported. The signature of the strain time series from the field test demonstrates a plausible record of the dynamic responses due to the wheel/rail contact impact. By using the experimental data, both the static and the dynamic FE models are validated.
It is found that the stiffness discontinuity of the IRJ structure causes a running surface geometry discontinuity during the wheel passages which then causes the impact in the vicinity of the end post. Through a series of sensitivity studies of several IRJ design parameters, it is shown that the IRJ performance can be effectively improved with optimised design parameters.
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A study on the biomechanics of axonal injuryAnderson, Robert William Gerard January 2000 (has links)
The current focus of research efforts in the area of the biomechanics of traumatic brain injury is the development of numerical (finite element) models of the human head. A validated numerical model of the human head may lead to better head injury criteria than those used currently in crashworthiness studies. A critical step in constructing a validated finite element model of the head is determining the mechanical threshold, should it exist, for various types of injury to brain tissue. This thesis describes a biomechanical study of axonal injury in the anaesthetised sheep. The study used the measurements of the mechanics of an impact to the living sheep, and a finite element model of the sheep skull and brain, to investigate the mechanics of the resulting axonal injury. Sheep were subjected to an impact to the left lateral region of the skull and were allowed to survive for four hours after the impact. The experiments were designed specifically with the numerical model in mind; sufficient data were collected to allow the mechanics of the impact to be faithfully reproduced in the numerical model. The axonal injury was identified using immunohistological methods and the injury was mapped and quantified. Axonal injury was produced consistently in all animals. Commonly injured regions included the sub-cortical and deep white matter, the hippocampi and the margins of the lateral ventricles. The degree of injury was closely related to the peak impact force and to kinematic measurements, particularly the peak change in linear and angular velocity. There was significantly more injury in animals receiving fractures. A three-dimensional finite element model of the sheep skull and brain was constructed to simulate the dynamics of the brain during the impact. The model was used to investigate different regimes of material properties and boundary conditions, in an effort to produce a realistic model of the skull and brain. Model validation was attempted by comparing pressure measurements in the experiment with those calculated by the model. The distribution of axonal injury was then compared with the output of the finite element model. The finite element model was able to account for approximately thirty per cent of the variation in the distribution and extent of axonal injury, using von Mises stress as the predictive variable. Logistic regression techniques were used to construct sets of curves which related the extent of injury, to the predictions of the finite element model, on a regional basis. The amount of observable axonal injury in the brains of the sheep was clearly related to the severity of the impact, and was related to the predictions of a finite element model of the impact. Future improvements to the fidelity of the finite element model may improve the degree to which the model can explain the variation in injury throughout the brain of the animal and variations between animals. This thesis presents results, and a methodological framework, that may be used to further our understanding of the limits of human endurance, in the tolerance of the brain to head impact. All experiments reported herein conformed with the Australian Code of Practice for the Care and Use of Animals for Scientific Purposes. / Thesis (Ph.D.)--Mechanical Engineering, 2000.
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Biomechanics of the Human Eye and Intraocular Pressure MeasurementsLjubimova, Darja January 2009 (has links)
This thesis addresses the reliability of Goldmann-type applanation tonometers (GAT). It deals with the investigation of the relation between predicted intraocular pressure, IOPG and true pressure, IOPT. The problem of the accuracy of GAT readings has acquired special importance over the last two decades as new types of surgical procedures to correct vision disorders are being explored and gain universal acceptance. The overall aim of the present study is to assess the effects of individual variations in the corneal central thickness (CCT), material properties of the involved tissues and paracentral applanation on the accuracy of IOPG. Two finite element models have been constructed: a two-dimensional axisymmetric model of the cornea and a three-dimensional model of the whole corneoscleral envelope. Various material descriptions were adopted for the cornea in 2D, whereas the 3D model accounted for collagen microstructure and represented a hyperelastic ber reinforced material. Nonlinear analyses were carried out using the commercial general-purpose finite element software ABAQUS. An extensive literature survey and consultations with ophthalmologists and clinicians were the platform for establishing relevant modelling procedures. The results reveal a clear association between all considered parameters and measured IOPG. The effect of assumed CCT is highly dependent on the corneal material properties. Material model alone has a profound effect on predicted IOPG. Variations in tonometer tip application produce clinically signi cant errors to IOPG measurements. Potential effects of corneal stiffness and paracentral applanation on GAT readings are larger than the impact of CCT. The behaviour of the models is broadly in agreement with published observations. The proposed procedures can be a useful tools for suggesting the magnitudes of corrections for corneal biomechanics and possible human errors. The present modelling exercise has an ability to reproduce the behaviour of human cornea and trace it under IOP and GAT, providing potentially useful information on the distribution of stresses and strains. Some recommendations can be drawn in pursuit of the clinical imperatives of ophthalmologists. / QC 20100729
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