The advancement of the structural steel manufacturing industry has led to the recent emergence of steel members with Elliptical Hollow Sections (EHS) and Semi Elliptical Hollow Sections (SEHS). Although these sections are gaining popularity among architects, the lack of design guidelines specifically tailored towards these sections inhibits their efficient structural use. Within this context, this thesis provides several steps towards the development of such guidelines. A review of the manufacturing process of hot-rolled steel sections is conducted with emphasis on hollow structural sections. The main factors affecting the formation of residual stresses during cooling of the sections are discussed. Lower bound plastic interaction relations for EHS subjected to combinations of axial force, bi-axial bending moments and torsion are then derived. The formulation is based on the lower bound theorem of plasticity and the maximum distortional energy density yield criterion. Its applicability for conducting the cross-sectional interaction check in structural steel design problems is illustrated through a practical example. A simplified and conservative interaction equation is then proposed based on curve fitting of the results of the lower bound solution. Upper bound interaction relations are next developed for EHS subjected to combinations of axial force, bi-axial bending moments, torsion and bimoments. The formulation is based on kinematically admissible strain fields within the context of the upper bound theorem of plasticity. The interaction relations derived successfully capture the effect of confining radial strains present at welded end sections, as well as sections that are free to deform in the radial direction away from end welded sections. An iterative solution technique is developed to solve the resulting highly non-linear system of interaction relations. The effects of residual stresses and initial imperfections on axial compressive resistance of hot-rolled EHS are then incorporated into the lower bound interaction relations. Towards that goal, the thermo-mechanical properties of steel were extracted from the literature. A thermo-mechanical finite element model was developed for prediction of residual stresses in rolled sections. The validity of the model was assessed by comparison against residual stress measurements available in the literature. The model is then applied to predict the residual stresses in hot-rolled EHS. A series of geometric and material nonlinear finite element analyses is conducted on columns of EHS sections. The analyses include predicted residual stresses and initial out-of-straightness imperfections in order to determine the inelastic buckling capacity of EHS members and generate column curves for EHS sections. The column curves are subsequently compared to those based on Canadian, American and European design codes. Two column curve equations are proposed in a format similar to that of the Canadian Standards for buckling about major and minor axes. The column curves were subsequently combined with the interaction relations developed to provide design rules for EHS members under combined loads. The last contribution of the thesis provides a formulation of lower bound interaction relations for SEHS subject to combinations of axial force, bi-axial bending moments and torsion. An iterative scheme for solving the parametric form of the interaction relations is developed and a grid of admissible stress resultant combinations is generated. A series of trial functions are fitted to the grid of internal force combinations and two simplified and conservative interaction equations are proposed.

The present work was produced as part of the MUZIC consortium, a collaboration between a multi-university team from the UK and industrial partners working on the field of nuclear energy, fabrication of alloys and nuclear research. The aim of the project is to establish a multidiscipline mechanistic understanding of the corrosion and breakaway processes of zirconium alloys used as fuel cladding materials in the nuclear industry. A better understanding of the corrosion mechanism of zirconium alloys will not only aid the development of better performing alloys, but will also allow more accurate models to be developed to reliably predict the service life of existing alloys. This could lead to higher burn-up, increase of energy production and reduction of nuclear waste produced.This work seeks to provide a better understanding of the role of residual stresses in the oxide, which are produced during oxidation due to high Pilling-Bedworth ratio and their impact on oxide phase transformation and oxidation kinetics by employing high energy synchrotron X-ray diffraction techniques. This is achieved by observing how stresses change as oxide growth approaches and passes through transition of the corrosion kinetics, their evolution across the oxide thickness, in situ characterising stresses and phase growth early in oxidation process and how stress changes can affect corrosion properties.It was found that relatively high compressive stresses in the two oxide crystal structures are present. The stresses relax with time up to moments before transition where a possible threshold stress magnitude is reached to aid an extensive tetragonal to monoclinic phase transformation. This generalised tetragonal to monoclinic transformation is believed to produce highly stressed monoclinic crystal structure grains and cause defects in the oxide. The above observation is further supported by a decrease of the tetragonal zirconia content. This is the moment that the oxide looses its protective character and a transition of the corrosion kinetics occurs. By comparing different materials it was observed that the minimum magnitude of the tetragonal phase is lower in better performing alloys while the tetragonal content is some cases was relatively low. It is suggested that the amount of the tetragonal phase, in the oxide layer, is not as important as the rate of it transforming into monoclinic. The extent of tetragonal to monoclinic transformation, that introduces defects in the oxide, defines how protective an oxide layer is. The present work provides a contribution to the available knowledge of the importance of residual stresses in the oxide layer and metal substrate of zirconium alloys and how they can affect corrosion rates or act as a precursor to the corrosion transition.

Flow forming is a fairly new technique used for the production of dimensionally accurate near net shaped hollow components. The process has many advantages such as cost effectiveness and eliminating further operations like welding, machining, etc. This study focuses on the characterization of flow formed components to understand the process. Flow formed components are composed of different reductions and characterization techniques are applied to reveal the resulting microstructural differences. Effect of number of passes on the material is also investigated. Metallographic analysis was conducted by optical microscope, electron micro probe analyser (EPMA) and the electron back scatter diffraction technique (EBSD) in a scanning electron microscope (SEM). Texture evolution of the samples was examined either by laboratory X-ray diffraction or EBSD technique. Furthermore, residual stresses were measured by neutron diffraction (at StrainAnalyzer for Large and Small Scale Engineering Applications (SALSA) and PulseOverlap Diffractometer (POLDI) instruments), laboratory X-ray diffraction and hole drilling. Stress relief heat treatments were carried out at 500°C for either 4 or 16 hours to mitigate residual stresses without losing much of the strength. The experiments conducted show that flow forming is a process resulting in heterogeneous microstructure with grains elongated along the deformation direction. Texture evolution is different from the typical rolling of steels with body centred cubic crystal structure. Any significant effect of the number of passes was not observed. Due to the nature of the process, residual stresses in the axial and hoop directions are critical. Therefore, stress distributions through thickness of the samples are plotted. It is observed that in the thick section, the stresses are higher. Heat treatments applied at 500°C for 4 or 16 hours are effective in diminishing the stresses.

Heat treatment processes are used to develop the desired mechanical properties for steels. Unfortunately, heat treatment, especially quenching, can cause distortion. Failure to meet geometry specifications can result in extensive rework or rejection of the parts. A series of quenching simulations, using DANTE, have been conducted on an AISI 4140 steel Navy C-ring distortion coupon and a WPI designed plate with a hole to determine the effects of quenching process parameters including part geometry, agitation during quenching, and quench start temperatures on distortion. The heat transfer coefficients (HTC) of the quenchant with selected pump speeds were measured by CHTE quench probe system, which is the key input for heat treatment simulation. The maximum HTC of the quenching oil was increased from 2350 W/m2K to 2666 W/m2K with higher pump speed. Quenching experiments were also conducted. It was found that the experimental measured gap opening of the standard Navy C-rings increased from 0.307mm without agitation to 0.536mm at a high agitation. Quench start temperature does not have a significant effect on the gap opening. The experimental results showed good agreement with simulation results. The important processing parameter identification was conducted using design of experiments (DoE) coupled with analysis of variance (ANOVA). The effect of processing parameters in decreasing order of importance were determined to be: quenchant type, part geometry, agitation speed, quenching orientation, quenchant temperature, immersion rates, and quench starts temperature. Based on the simulation and experimental results, it was found that the two most import parameters are: 1. The part geometry and size (product design) 2. The temperature dependent heat transfer coefficients between the part and the quenchant (process design) The coupling of these product and process parameters is necessary to apply the systems analysis that must be accomplished to understand the interaction between the part design and process design parameters. This coupling can be accomplished by locally applying the well-known Biot number. Bi (T) = h(T) * L / k(T) Where h(T) = film coefficient or convective heat transfer coefficient [W/m2*K]. LC = characteristic length, which is generally described as the volume of the body divided by the surface area of the body [m]. k(T) = thermal conductivity of the body [W/m*k] The concept of a local Biot number is introduced to quantify the local variations of part size, geometry and heat transfer coefficient. First, a large Bi indicates large temperature gradients within the part. Second, large local (geometry dependent) variations in Bi number will lead to large lateral temperature gradients. Therefore, variations in local Bi can lead to large temperature gradients and therefore high stress during quenching and finally distortion. This local Bi concept can be used in a systems approach to designing a part and the quenching system. This systems approach can be designated as design for heat treating.

Heat treatment processes are used to develop the desired mechanical properties for steels. Unfortunately, heat treatment, especially quenching, can cause distortion. Failure to meet geometry specifications can result in extensive rework or rejection of the parts. A series of quenching simulations, using DANTE, have been conducted on an AISI 4140 steel Navy C-ring distortion coupon and a WPI designed plate with a hole to determine the effects of quenching process parameters including part geometry, agitation during quenching, and quench start temperatures on distortion. The heat transfer coefficients (HTC) of the quenchant with selected pump speeds were measured by CHTE quench probe system, which is the key input for heat treatment simulation. The maximum HTC of the quenching oil was increased from 2350 W/m2K to 2666 W/m2K with higher pump speed. Quenching experiments were also conducted. It was found that the experimental measured gap opening of the standard Navy C-rings increased from 0.307mm without agitation to 0.536mm at a high agitation. Quench start temperature does not have a significant effect on the gap opening. The experimental results showed good agreement with simulation results. The important processing parameter identification was conducted using design of experiments (DoE) coupled with analysis of variance (ANOVA). The effect of processing parameters in decreasing order of importance were determined to be: quenchant type, part geometry, agitation speed, quenching orientation, quenchant temperature, immersion rates, and quench starts temperature. Based on the simulation and experimental results, it was found that the two most import parameters are: 1. The part geometry and size (product design) 2. The temperature dependent heat transfer coefficients between the part and the quenchant (process design) The coupling of these product and process parameters is necessary to apply the systems analysis that must be accomplished to understand the interaction between the part design and process design parameters. This coupling can be accomplished by locally applying the well-known Biot number. Bi (T) = h(T) * L / k(T) Where h(T) = film coefficient or convective heat transfer coefficient [W/m2*K]. LC = characteristic length, which is generally described as the volume of the body divided by the surface area of the body [m]. k(T) = thermal conductivity of the body [W/m*k] The concept of a local Biot number is introduced to quantify the local variations of part size, geometry and heat transfer coefficient. First, a large Bi indicates large temperature gradients within the part. Second, large local (geometry dependent) variations in Bi number will lead to large lateral temperature gradients. Therefore, variations in local Bi can lead to large temperature gradients and therefore high stress during quenching and finally distortion. This local Bi concept can be used in a systems approach to designing a part and the quenching system. This systems approach can be designated as design for heat treating.

No description available.

<p>Damaged components and a lack of spare components are issues which are currently affecting military aircraft capability. Laser Cladding is an additive manufacturing technique which shows promise in repairing damaged aviation components. However, there are considerable certification requirements for critical components which stand to gain the most benefits from laser clad repair methodologies. These requirements involve establishing crack growth rate data for the laser clad material to gain confidence in the reliability of the repair's performance on in-service aircraft. This research seeks to understand the fatigue behavior of Ti-6Al-4V that has undergone a simulated laser clad repair, with unrepaired specimens also tested to allow for comparison. </p>

In cardiac biomechanics, there is an apparent knowledge gap in 3D cardiac elastin structure and its biomechanical roles. In this study, we fill this knowledge gap via novel biomedical imaging and bioengineering means. In Aim 1, we created an overall mapping of 3D microstructures of the epicardial elastin fibers on porcine left ventricles (LV) using a laser scanning confocal microscope. We demonstrated the location- and depth-dependencies of the epicardial elastin network. Histological staining was also applied to reveal the patterns of endocardial and interstitial elastin fibers, as well as elastin fibers associated with the Purkinje fibers. In Aim 2, a novel algorithm was developed to better reconstruct the elastin fiber network and extract topological fiber metrics. We created a “fiberness” mask via fiber segmentation and fiber skeletonization to obtain the one-voxel-thick centerline skeleton and remove spurious fiber branches, thus generating topological and geometrical descriptors and bringing the study of cardiac elastin to a new level. In Aim 3, we successfully developed a semi-quantitative approach to characterize the residual stress in the epicardial layer by calculating the total angular change due to curling. Our novel curling angle characterization clearly reveals the existence of residual stress as well as the direction (circumferential vs. longitudinal) and location-dependency of the residual stress. In Aim 4, for the first time we estimated the regional residual stress of the epicardial layer on the intact LV via a four-step methodology: (i) quantify regional residual strains by comparing in situ and stressree marker dimensions; (ii) obtain regional tension-stretch/stress-stretch curves along the circumferential and longitudinal directions; (iii) adjust the biaxial curves to the 0g load reference; (iv) estimate the circumferential and longitudinal residual stresses via residual strains. This method accurately estimates the residual stress in the epicardial layer in various LV anatomical locations. We found that the location-dependency of circumferential and longitudinal residual stresses correlates with the curvature of heart surfaces. Our studies show that the epicardial layer, with its rich elastin content, might function as a balloon that wraps around the heart, and the residual stress sets up a boundary condition that assists with LV contraction.

Any machining operation induces significant deformation and associated stress states within the component being machined. Once the component has been finished and is removed from the machining tool, a portion of these stresses remain within the finished component, and are termed residual stresses. These stresses have a significant effect upon the performance of the final component. However, despite their importance there is no accurate and cost effective method for measuring residual stresses. For this reason predicting these stresses without the need for measurement is highly desirable. The focus of this thesis is on advancing the development and implementation of finite element models aimed at predicting residual stresses induced by metal cutting operations. There are three main focus areas within this research, the first of which is concerned with predicting residual stresses when small feed rates are used. It is shown that in the existing cutting models residual stress prediction accuracy suffers when feed rates are small. A sequential cut module is developed, which greatly increases the accuracy of the predicted residual stress depth profiles. A second area of focus concerns the influence of friction models on predicted residual stresses. A detailed set of simulations is used to elucidate the effect of friction not only for sharp tools, but also for tools which have accrued wear. It is shown that whilst friction is not of critical importance for new tools, as tools continue to wear the choice of friction model becomes significantly more important. The third area of focus is on phase transformations, induced by the cutting process. A decoupled phase transformation module is developed in order to predict the depth, if any, of a phase transformed layer beneath the newly machined surface. Furthermore, the effect of this layer on the residual stress depth profile was also studied. All three focus areas present new and novel contributions to the field of metal cutting simulations, and serve to significantly increase the capabilities of predictive models for machining. / Thesis / Doctor of Philosophy (PhD)

<p> This thesis deals with the analytical and experimental study of the influence of residual stress due to cold bending on the behaviour of thin-walled open sections. The residual stress distribution caused by cold forming the sections is predicted theoretically. The influence of this residual stress on the load-displacement characteristic, and load carrying capacity of similarly curved tension and compression specimens is then analyzed. A local buckling analysis based on the virtual work and incremental theories is performed to predict the collapse load of compression specimens containing residual stresses.</p> <p> The experimental work consisted of tests to confirm theoretical elastic springback strains due to cold bending of steel sheet to various radii. Tension and compression tests were then performed on various cold formed sections to observe the effects of residual stress and to confirm analytical predictions. </p> <p> Conclusions have been deduced from the theory and from these tests, and suggestions made for further research.</p> / Thesis / Master of Engineering (MEngr)