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1
Ramírez, Anthony. "Ultrasonic grain refinement of magnesium alloys." Thesis, University of Portsmouth, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.494007.
Full textAbstract:
The thesis project investigates ultrasonic grain refinement (UGR) of magnesium alloys. It commences with a brief study of the grain refinement of both aluminium-containing and aluminium-free magnesium alloys, by means of typical inoculating additives. That provides a basis for understanding the effectiveness of ultrasonic grain refinement of magnesium alloys to be presented in the following chapters.
2
Fischer, Walter [Verfasser], and Holm [Akademischer Betreuer] Altenbach. "Modellierung und Berechnung von mechanischen Spannungen und Volumenänderungen bei Reaktionsharzen / Walter Fischer. Betreuer: Holm Altenbach." Magdeburg : Universitätsbibliothek, 2015. http://d-nb.info/1069159778/34.
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3
Konozsy, Laszlo. "Multiphysics CFD modelling of incompressible flows at Low and Moderate Reynolds Numbers." Thesis, Cranfield University, 2012. http://dspace.lib.cranfield.ac.uk/handle/1826/7860.
Full textAbstract:
In this Ph.D. thesis, a novel high-resolution Godunov-type numerical procedure has been
developed for solving the unsteady, incompressible Navier-Stokes equations for constant
and variable density flows. The proposed FSAC-PP approach encompasses both artificial
compressibility (AC) and fractional step (FS) pressure-projection (PP) methods of Chorin
[3, 4] in a unified solution concept. To take advantage of different computational strategies,
the FS and AC methods have been coupled (FSAC formulation), and further a PP
step has been employed at each pseudo-time step. To provide time-accurate solutions,
the dual-time stepping procedure is utilized. Taking the advantage of the hyperbolic nature
of the inviscid part of the AC formulation, high-resolution characteristics-based (CB)
Godunov-type scheme is employed to discretize the non-linear advective fluxes. Highorder
of accuracy is achieved by using from first- up to ninth-order interpolation schemes.
Time integration is obtained from a fourth-order Runge-Kutta scheme. A non-linear fullmultigrid,
full-approximation storage (FMG-FAS) acceleration technique has been further
extended to the FSAC-PP solution method to increase the efficiency and decrease the
computational cost of the developed method and simulations. Cont/d.
4
Grenville, R. K. "Blending of viscous Newtonian and pseudo-plastic fluids." Thesis, Cranfield University, 1992. http://hdl.handle.net/1826/4362.
Full textAbstract:
Blend times and power consumption have been measured for Newtonian and pseudo-
plastic fluids in agitated, baffled vessels with diameters of 0.30,0.61 and 1.83 m. The
viscosities of the fluids were selected to give a range of Reynolds numbers between 200
and 105. Four impellers were tested with diameters between T/3 and T/2 positioned at
T/3 clearance above the lowest point of the vessel base. The impellers were typical of
those found in the chemical industry.
5
Starr, Maxwell. "High-order methods for steady, unsteady and transitional flow over a cylinder." Thesis, Cranfield University, 2012. http://dspace.lib.cranfield.ac.uk/handle/1826/7859.
Full textAbstract:
In this thesis, the flow around a cylinder is chosen as a test case for higher-order
numerical reconstruction techniques. No direct comparison of these higher-order
methods has been carried out for this particular test case. Especially for low Mach
number, incompressible flows with Implicit Large Eddy Simulation method. The
cylinder test case is both a proven test case in literature, as well as a test case
that can be scaled up in terms of flow speed with other parameters remaining
unchanged. The scaling of flow speed around the cylinder allows ease of flow
regime change. Thus the flow was modelled in this thesis from laminar flow to
turbulent flow, going through a transitional regime in between.
The simulations were set up such that numerical reconstruction methods could
be directly compared to one another at the range of flow speeds, and subsequently
in both two-dimensional and three-dimensional flows. The numerical
reconstruction methods for the ILES cases ranged from first order reconstruction
through to higher-order methods as high as ninth-order (in the weighted
essentially non-oscillatory scheme). With the speed of computation for the twodimensional
simulations, it was possible to test all of these schemes directly with
one another. However, three-dimensional simulations require a significantly greater
CPU run-time. Therefore, based on the results of the two-dimensional simulations,
a group of the higher-order methods were chosen for continuing analysis
in the three-dimensional simulations.
In the laminar flow regime, all the numerical schemes agreed very well with
literature data. As the flow speed increased, discrepancies started to appear in
the results, to varying degrees based on the flow speed, the numerical scheme
used, and the dimensionality of the flow. An analysis of the results showed that
two-dimensional simulations were suitable up to Reynolds 300. From this flow speed onwards, three-dimensional simulations are deemed necessary. At lower
Reynolds number flows the two-dimensional simulations provided good predictions
of the flow. At the higher Reynolds numbers, the 3D simulations outperformed
the 2D simulations.
Specific numerical reconstruction schemes were found to perform better at
certain aspects of the flow. For example, the coefficients around the cylinder or
the velocities in the wake varied based solely on the numerical scheme used.
Finally, during the course of the post-processing of the simulations, a spectral
analysis was carried out. The flow field was analysed at specific points in the
wake (ranging from near, medium and far wake). The spectral analysis proved
suitable for examining the fluctuations in the wake of the cylinder, showing the
redistribution of energy in the wake towards higher frequencies. In addition, the
wake showed increased power densities for the fluctuations as the flow moved
away from the cylinder, before then decreasing again as dissipation into the surrounding
flow occurred.
6
Adedigba, A. G. "Two-phase flow of gas-liquid mixtures on horizontal helical pipes." Thesis, Cranfield University, 2007. http://dspace.lib.cranfield.ac.uk/handle/1826/5069.
Full textAbstract:
The aim of this study was to investigate hydrodynamic two -phase (namely
water and air) fluid flow characteristics in helical pipes of low amplitude and
straight pipes of the same internal-diameter and constructional material: the
results for the two pipes have then been compared.
One of the objectives was to measure pressure, pressure drop and liquid
holdup in the two pipes. These are universal dominant parameters in the oiland-
gas industry as they significantly impact on the exploitation and
conveyance of crude oil from wells or reservoirs to the process plant, where the
crude is refined. The second objective was to examine applications of the
helical pipe.
Experiments were performed on three different helical pipes of internal
diameters 25.4 mm, 50 mm and 100 mm and their straight counterparts. The
single-phase preliminary experimental results from the 25.4 mm internaldiameter
for both pipes have shown that both pressure and pressure drop are
higher in the helical pipe than in the straight pipe. The friction factors were also
evaluated for both pipes and found to be higher in the helical pipe than in the
straight pipe. The single-phase and two-phase experimental results for the 50
mm internal-diameter pipes confirmed the conclusions from the preliminary
experimental results. The two-phase results showed that slug flow occurred in
the straight pipe at certain superficial velocities of air and water, whereas at the
same superficial velocities of air and water, slug flow did not ensue in the helical
pipe - instead bubbly flow was observed. Stratified flow occurred in the straight
pipe at very low superficial velocities of air and water but under these same
conditions, bubbly flow ensued in the helical pipe.
A section of 100 mm internal-diameter helical pipe was installed at some
distance from a catenary-shaped riser, with a view to investigating the
effectiveness of the helical pipe in mitigating severe slugging. The results
showed promise as the section of the helical pipe proved to be successful in
reducing the menace of severe slugging. This novel finding is regarded as a
breakthrough for the oil-and-gas industry in this respect. This is because
hydrocarbon proven reserves in the off-shore (i.e. deep sea-water) environment
have been estimated to be close to 60%. All previous research studies over the
past decade to provide solution to the problem posed by severe slugging have
not yielded any appreciable results. This discovery also has the advantages of
reducing the demand on the topside (process) facility and the achievement of
stability of liquid production is resulted from the consequent flow assurance in
the pipeline and riser.
7
Strand, Russell K. "Smoothed particle hydrodynamics modelling for failure in metals." Thesis, Cranfield University, 2010. http://dspace.lib.cranfield.ac.uk/handle/1826/6773.
Full textAbstract:
It is generally regarded to be a difficult task to model multiple fractures leading to
fragmentation in metals subjected to high strain rates using numerical methods.
Meshless methods such as Smoothed Particle Hydrodynamics (SPH) are well suited to
the application of fracture mechanics, since they are not prone to the problems
associated with mesh tangling.
This research demonstrates and validates a numerical inter-particle fracture model for
the initiation, growth and subsequent failure in metals at high strain rate, applicable
within a Total Lagrangian SPH scheme. Total Lagrangian SPH performs calculations in
the reference state of a material and therefore the neighbourhoods remain fixed
throughout the computation; this allows the inter-particle bonds to be stored and tracked
as material history parameters. Swegle (2000) showed that the SPH momentum
equation can be rearranged in terms of a particle-particle interaction area. By reducing
this area to zero via an inter-particle damage parameter, the principles of continuum
damage mechanics can be observed without the need for an effective stress term, held at
the individual particles.
This research makes use of the Cochran-Banner damage growth model which has been
updated for 3D damage and makes the appropriate modifications for inter-particle
damage growth. The fracture model was tested on simulations of a 1D flyer plate impact
test and the results were compared to experimental data. The test showed that the model
can recreate the phenomena associated with uniaxial spall to a high degree of accuracy.
Some limited modelling was also conducted in 2 and 3 dimensions and promising
results were observed.
Research was also performed into the mesh sensitivity of the explosively driven Mock-
Holt experiment. 3D simulations using the Eulerian SPH formulation were conducted
and the best results were observed with a radial packing arrangement.
An in-depth assessment of the Monaghan repulsive force correction was also conducted
in attempt to eliminate the presence of the SPH tensile instability and stabilise the
available Eulerian SPH code. Successful results were observed in 1D, although the
results could not be replicated consistently in 2D. A further study was also conducted
into an approach that makes use of a partition of unity weighting to two different SPH
approximations of the same flow-field; one local and one non-local (or extended).
Unfortunately this approach could not be made to stabilise the code.
8
Antoniadis, Antonios Foivos. "High-order methods on mixed-element unstructured meshes for aeronautical applications." Thesis, Cranfield University, 2012. http://dspace.lib.cranfield.ac.uk/handle/1826/7891.
Full textAbstract:
Higher resolution and reliability are the desiderata for Computational Fluid Dynamics
and main drivers for the development, implementation and validation of highorder
accurate methods. Complex fluid dynamic phenomena such as shock-wave
boundary-layer interactions, turbulent separated flows and fluid problems involving
multiple scales are adequately resolved with high-order schemes. The spatial representation
of the flow field by an unstructured mesh provides flexibility, automation,
fast and effortless grid generation and exceptional load balance on multiple processor
computers. This plethora of advantages is mirrored by the unprecedented popularity
of unstructured-based schemes.
The objective of this PhD project is the implementation of two high-order schemes
for the compressible Navier-Stokes equations in the context of the finite volume “kexact”
framework: the MUSCL-TVD and WENO. The schemes are formulated in
two and three space dimensions for mixed-element unstructured meshes; in addition,
the Spalart-Allmaras turbulence model is implemented into the developed numerical
framework. A wide range of applications are considered spanning from low-speed
flows (M = 0.08) to supersonic conditions (M = 5.0); inviscid and viscous simulations
in a broad spectrum of Reynolds numbers ranging from Re = 500 up to Re = 37×106.
The applications include: the Taylor-Green vortex, the ONERA-M6 wing, flat plate,
the NACA-0012 and the MD 30P-30N aerofoils, and a shock-wave boundary-layer
interaction.
For the examined cases, WENO schemes demonstrate superior accuracy, numerical
dissipation and non-oscillatory behaviour over the MUSCL-TVD. High-order schemes
inherit low numerical dissipation properties while turbulence models induce dissipation,
this disequilibrium has adverse effects on the stability, convergence and accuracy
of the simulation; therefore, turbulence model re-calibration would be required in order
to accommodate high-order discretisation methods.
9
Karantonis, Konstantinos. "Modelling and simulation of turbulence in unsteady separated and suddenly-expanded flows." Thesis, Cranfield University, 2011. http://dspace.lib.cranfield.ac.uk/handle/1826/7900.
Full textAbstract:
The scope of this PhD thesis is the simulation of turbulence in time-dependent, separated
and suddenly-expanded channel flows. High-resolution and very high-order numerical
methods have been employed in the framework of Implicit Large Eddy Simulation (ILES)
to elucidate open questions about the physics in flows with sudden expansion.
It is well known that the planar sudden expansion (PSE), despite its simple and symmetric
geometry it produces a very complex behaviour and a distinctly asymmetric flow
pattern ascribed mainly to the Coanda effect. Such flows are encountered in a wide range
of practical engineering applications, such as combustion, hydraulic and fluidic devices,
air ducts, and mixing equipments. It is of great importance, therefore, to understand the
mechanisms that dominate flows with separation and reattachment of the shear layers, as
well as flows with regions of strong reversed motion.
This thesis has for the first time analysed in detail the turbulent kinetic energy budget
(TKEB) for the PSE. This analysis has been extended to examine the influence of Mach
number on each individual component of the TKEB. The resulting data can be used as
reference for further development of turbulence models capable of accurately resolving
the flow behaviour in suddenly-expanded flows.
Cont/d.
10
Mihaiescu, Adrian N. "Eddy viscosity turbulence models for compressible mixing." Thesis, Cranfield University, 2011. http://dspace.lib.cranfield.ac.uk/handle/1826/7982.
Full textAbstract:
The K - L and K - ϵ turbulence models are used to simulate the turbulent mixing induced
by the Rayleigh-Taylor and Richtmyer-Meshkov instabilities. The models contain
additional source terms for the turbulence kinetic energy which depend on the type of the
instability. A new criterion based on ratio of the averaged flow and turbulence time scales
is introduced for differentiating between the two types of instabilities. The original formulation
of the turbulence kinetic energy source present in the K - ϵ model is modified
in order to accurately capture the evolution of the Richtmyer-Meshkov instability in both
heavy/light and light/heavy configurations. Additional constraints are imposed to the
models in order to prevent non-physical solutions when strong gradients are present in the
flow.
Three test problems are considered and the performance of the turbulence models is assessed
by comparing their solutions with the results obtained by high resolution Implicit
Large Eddy Simulations (ILES). First, the classical Rayleigh-Taylor and Richtmyer-Meshkov
problems are solved. A new approach for initializing the turbulence models in proposed
for the Rayleigh-Taylor problem. It is found that both turbulence models describe successfully
the self similar growth of the Rayleigh-Taylor and Richtmyer-Meshkov instabilities
and can predict accurately the spatial distribution of the fluid concentrations and of the
turbulent kinetic energy. The last problem involves the mixing induced at two planar interfaces
by multiple shock reflections and refractions. The turbulence models estimate correctly
the evolution of the mixing and of the total kinetic energy in the mixing zones.
The transport equations of the turbulence models are solved numerically and the influence
of the numerical schemes on the results is investigated. It is concluded that the numerical
schemes do not have an important influence on the results in the case of the classical
Rayleigh-Taylor problem (provided that grid convergence has been achieved and the turbulence
models have been initialized using the method proposed here). However, in the
presence of shocks (such as in the case of the Richtmyer-Meshkov instability), the HLLC
Riemann solver should be used together with a reconstruction scheme of third or higher
order of accuracy.
11
Reiher, Jörg Christian [Verfasser]. "A thermodynamically consistent framework for finite third gradient elasticity and plasticity / Jörg Christian Reiher." Magdeburg : Universitätsbibliothek, 2017. http://d-nb.info/1133541526/34.
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12
Huck, Juergen. "Emergence in complex systems based on synthetic replicators." Thesis, University of St Andrews, 2011. http://hdl.handle.net/10023/3067.
Full textAbstract:
Biopolymers with defined recognition pattern were used to generate the first artificial replicating systems. Stripping down these systems to their most fundamental properties allowed to move away from the biological origins to construct replicators consisting of simple organic molecules. These systems have proven highly instructive for the in-depth understanding of the main requirements for the targeted development of efficient replicators. With this knowledge at hand, it is now possible to combine several replicators for the formation of molecular networks, and to use the unique properties of replication to manipulate these networks by external stimuli. In the thesis presented, the investigation of a family of self-replicators culminated in the successful construction of several examples of a multicyclic system in which four building blocks are able to react via two autocatalytic and two reciprocal pathways. Owing to the connectivity in this reaction system, it was demonstrated that its outcome can be influenced in a programmable manner by the addition of informational template. Some of the responses can be deduced directly from the functioning of the individual systems, others however are to be classified as emergent properties of the network. Upon elucidation of the multicyclic systems, it became apparent that working in closed reaction systems puts intrinsic boundaries on the possibility to bias the outcome of the reaction network. This limitation prevented the extinction of the inferior type of replicators even under highly unfavourable conditions and instead always led to coexistence for all species.
13
Afolabi, Eyitayo Amos. "Experimental investigation and CFD simulation of multiphase flow in a three phase pipe separator." Thesis, University of Newcastle Upon Tyne, 2012. http://hdl.handle.net/10443/1756.
Full textAbstract:
This thesis describes an experimental study and CFD simulation of the hydrodynamic behaviour of multiphase flow in a 30mm pipe separator. Stereoscopic particle image velocimetry (SPIV) technique was used for three-dimensional velocity measurements of water and air water flows in a pipe separator. The instantaneous and time averaged whole field velocities at three dimensional axial positions were recorded and velocity profiles across the diameter of the pipe separator extracted. Three-phase flow experiments were then conducted to determine the separation efficiency of the air-water-oil flow in a pipe separator. Numerical simulation of the single and multiphase flow behaviour within the separator was then performed through the commercially available CFD software ANSYS FLUENT.
14
Simm, Anthony. "Quantitative interpretation of magnetic field measurements in eddy current defect detection." Thesis, University of Newcastle Upon Tyne, 2013. http://hdl.handle.net/10443/1809.
Full textAbstract:
For many years, the theoretical and experimental study of eddy current non-destructive evaluation (NDE) has been conducted separately, as most models were not suited to practical industrial applications. The aim of this work is to bridge this gap by investigating the relationship between magnetic fields and defects using both modelling and experimental study and to link these results to quantitative NDE. In this work, 3D FEM numerical simulations are used to predict the response of an eddy current probe being scanned over the area of a defect and understand the underlying change in magnetic field due to the presence of the defect. Experimental investigations are performed to study the feasibility of the proposed magnetic field measurement techniques for defect detection. This experimental work investigates the inspection of both surface and subsurface defects, the use of rectangular (directional) probes and the measurement of complex magnetic field values, as the response in these cases has been found to have a greater correlation with the shape of the defect being studied. As well as the detection of defects, both frequency spectrum and transient information from pulsed eddy current responses are used to reconstruct the profile (depth and width) of a slot shaped defect. The work concludes that the use of magnetic field measurements provides useful information for defect detection and quantification. This will have applications in both industrial and research areas, including visualisation of defects from magnetic field measurements, which can be applied to the monitoring of safety critical components.
15
Edirisinghe, Ajith A. G. "Stress relaxation testing with a novel machine using rapid application of load." Thesis, Loughborough University, 1998. https://dspace.lboro.ac.uk/2134/10324.
Full textAbstract:
It has long been desirable to conduct Stress Relaxation measurements over shorter test times. The problem has been to achieve a short well-characterised loading phase and to obtain the first stress relaxation reading soon after the completion of loading. Although it is possible to programme a standard computer-controlled mechanical tester to acquire readings down to very short time periods, uncertainty of achieving the correct deformation at very high loading rates restricts its use. The Hounsfield Test Equipment company in collaboration with the Loughborough University has produced a compression tester intended to overcome the problem. This new machine is capable of giving compressive strain rates from 0.1 nun/min upto 250 mm/min (SLOW MODE) and one extremely high compressive strain rate of 2 m/sec (120,000 mm/min - FAST MODE) The machine was employed initially to produce sets of stress relaxation curves to investigate possible machine errors. The main error found was incorrect compression produced by the tester; it varied from 20% to 27% instead of the desired 25%. After this and some smaller errors were identified, several precautionary actions were taken to compensate for them. The compensation techniques permitted progress to a study of three different Natural Rubber compounds at three different temperatures using several different compressive strains in both slow and fast modes. A clear difference in the residual forces between fast mode test curves and slow mode test curves has been identified and the possible reasons for differences are discussed. It has been shown that the relaxation curves in slow mode can be represented by two linear portions representing two different relaxation processes. On the each relaxation process the effect of the rate of compressive strain and the temperature of the material is discussed.
16
Read, Daniel Edward. "Electrical and magnetic properties of n-Cd(_1-x)Mn(_x) Te close to the metal-insulator transition." Thesis, Durham University, 2001. http://etheses.dur.ac.uk/3783/.
Full textAbstract:
Electrical transport and magnetic measurements have been made on n-Cd(_1-x)Mn(_x) Te (0.047 < X < 0.197) for samples doped with hi or In,Al. Results are presented for measurements made as a function of temperature (40 mK < T < 300 K), applied magnetic field (0 T < B < 6 T) and photogenerated carrier density. Low field magnetic susceptibility measurements have identified a transition from a paramagnetic phase to a spin glass phase at low temperatures. The measured temperature dependence of the magnetic susceptibility is consistent with a cluster glass model. Measurements of the spin glass freezing temperature have been carried out on four samples having different Mn fractions The results obtained are in agreement with the limited number of previous measurements on nominally undoped samples. At low temperatures photo-induced changes in magnetism have been measured in both the paramagnetic and the spin glass phase. The change in magnetisation on illumination is due to an increased number of bound magnetic polarons formed around quasi localised s-spins. High temperature susceptibility measurements have been used to examine the parameters characterising the paramagnetic phase, and their variation with Mn fraction. Electrical transport measurements at very low temperatures (T < 800 mK) have shown a strong temperature dependent electron localisation. This has resulted in the confirmation of insulating behaviour in a sample (x = 0.047) having n. At low temperatures and in zero field an activated form of the conductivity is observed. In applied magnetic fields (B > 50 mT) Efros-Shklovskii variable range hopping is observed in the insulating phase. These results are attributed to the formation of a hard gap in the density of states, having a magnetic origin. At higher fields an insulator-metal phase transition occurs. In the metallic phase the conductivity can be described by a quantum correction to the zero temperature conductivity due to the effect of electron-electron interactions. Results obtained before and after illumination are consistent with scaling theory of electron localisation, having a critical exponent close to unity, indicative of the importance of electron-electron interactions. A reduction in the value of the critical field is seen after increasing the carrier density (B(_c) = 2.0 and 1.3 T for n = 3.3 xl0(^17)cm(^-3) and 3.8 x10(^17) cm(^-3) respectively).At low temperatures an anisotropy in the resistivity has been measured for samples in the spin glass phase. Magnetoresistance measurements have shown results consistent with previous measurements, in addition to a large, low field component that is attributed to the effect of magnetic field on electrons in the variable range hopping regime.
17
Smith, Jodie Melissa. "A study of atomic structure of glassy materials." Thesis, University of Kent, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.633701.
Full textAbstract:
Glassy materials have applications in a variety of fields. The composition of these materials can be varied easily and so tailored to particular applications. To improve the design of materials an understanding of the atomic structure is required. Herein, several materials have been studied each with a different rationale. The technique of neutron diffraction has been employed to elucidate the atomic structure; with other techniques used to further the understanding of the structure. Amorphous calcium gallate was studied primarily because it contains no traditional glass formers. The gallium environment is of interest as it acts as a networker former within this material. The material was synthesised using a novel method of laser heating with aerodynamic levitation. The system was studied using neutron diffraction, molecular dynamic modelling and high field magic-angle-spinning NMR (MAS-NMR). A calcium phosphate based glass has been studied to determine the effect of increasing aluminium content on the atomic structure, using neutron diffraction and MASNMR. This allowed the interesting trend in the material properties, with increasing aluminium concentration, to be linked to the local environment of the aluminium. Bioglass ®, a commercial product to regenerate tissue, has been doped with transition metals, the local environment of which is critical to determine the suitability of this addition. Neutron diffraction with isotopic or isomorphic substitution enabled the study of the local environment of these elements. The effect of substituting strontium for calcium was studied in a material which has a similar composition to several commercial biomaterials. The -materials were studied using neutron diffraction to reveal information regarding changes in crystallisation as a result of the strontium substitution. Finally the potential of the unique NIMROD instrument is explored, using data collected from several silica based sol gel materials including a titanium silicate series and a calcium silicate series.
18
Algahtani, Ali Muhammad A. "Optimising aluminium alloy surfaces for wear and corrosion controlled by plasma electrolytic oxidation (PEO)." Thesis, University of Leeds, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.634748.
Full textAbstract:
Aluminium and its alloys are broadly used in a wide variety of applications. Aluminium's main advantages include: lightness, high specific strength, high thermal and electrical conductivities, good formability, excellent machinability, diversity of aluminium alloys, extensive range of forms and options (e.g. rolling, extrusions, stampings, forgings and castings), and suitability for a diverse range of joining techniques, surface treatment and recyclability. A number of surface treatment technologies are available. They produce thicker oxide coating layers that can be used to combat corrosion and wear of aluminium alloys under aggressive environments, such as petroleum extraction environments. Coating processes to surface modify e.g. with alumina include Plasma Electrolytic Oxidation (PEO), Plasma Spray Ceramic (PSC) and Hard Anodizing (HA). In this thesis, aqueous slurry erosive wear characteristics of oxide coatings produced using the aforesaid three processes have been compared and benchmarked against the uncoated aluminium substrate. This research investigates the extent of erosion resistance, in particular, the effect of sand concentration and temperature variations to the aqueous slurry impingement against material properties such as adhesion, ductility and roughness. Also, a series of electrochemistry tests has been conducted. Coatings properties have been characterised using various tools including optical microscopy, scanning electron microscopy (SEM), x-ray diffraction (XRD), surface profilometry and hardness indentations. The results indicate that PEO coating can offer the most effective barrier to the aluminium substrate as it has the lowest weight loss under erosion tests and the highest enhancement factor under electrochemistry tests compared to the other materials. Therefore, PEO surface treatment should be considered as one of generation materials and innovative problem-solving technologies to be introduced in oil and gas industry.
19
Lin, Lianshan. "Characterization of material properties using inverse method." Thesis, University of Manchester, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.505378.
Full textAbstract:
The inverse analysis introduced in this dissertation involves detennining an object's properties. In engineering problems, the most important object properties are material or constitutive constants (elastic/plastic). In direct problems, engineers can conveniently look up for the material constants in standard materials handbooks, and use them to pred'ict the structure's responses under the given loads. However, in the cases where the material constants are not available from standard handbooks, or the material properties need to be detennined precisely since a small difference from those listed may have a great influence on the results, inverse methods will need to be used. So far the characterization inverse methods focus mostly on homogeneous isotropic or orthotropic materials. Primarily the characterization concentrated on two important material parameters: Young's Modulus and Poisson's ratio. An experimental-numerical solution of inverse method is brought up, which is verified to be more efficient than previous methods, in the characterization of homogeneous isotropic materials and orthotropic materials. Another important part of this thesis is the inverse method for the characterization of heterogeneous material properties. Applications of this inverse method show its capability in predicting inhomogeneous distribution of material constants. Furthermore, this heterogeneous characterization method can also provide detail localized material information for studies such as trivial initial crack failure evolution, etc.
20
Singh, Jetinder. "Additive manufacturing of functional materials for polymer micro reactors." Thesis, University of Liverpool, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.569535.
Full textAbstract:
This work investigates the development of functional materials by using additive manufacturing techniques; specifically digital light processing (DLP) based Stereolithography to manufacture micro reactors containing functional materials for sensing and actuating devices in a single build. The envisionTEC Perfactory is a DLP based Stereolithography machine (addressed as the DLP system) and has the ability to manufacture feature size in the micron range with high precision. This ability to manufacture small features with high precision is used for the manufacture of polymer micro reactors. Using the DLP system for processing functional composites is a novel approach. The DLP system offers high resolution and accurate manufacture of parts. This ability to process functional materials will be used for developing devices for sensing and actuating applications. In order to identify the feasibility of manufacturing composite of the DLP system, barium titanate (BaTi03) ceramic was added to the photopolymer. The composites containing up to 19 vol% BaTi03 as functional material were manufactured by the DLP system. Poling these specimens showed a maximum piezoelectric coefficient of 5 pC/N, measured at 30 MV /m and 90°C. Measuring ferroelectric properties of these composite specimens showed a maximum remnant and saturation polarisation of ±0.041 IlC/cm2 and ±0.359 IlC/cm2, measured at electric field of 249.97 kV/cm at room temperature. As the parts were manufactured in layers; an investigation was undertaken to identify the effect of build layer orientation on the mechanical properties. The polymer specimens for the XY (horizontal) and the Z (vertical) build direction showed strength of 39 MPa and 29 MPa, being achieved in the respective specimen.
21
Cresswell, Philip Thomas. "Multi-component stimuli-responsive polymer brushes grafted from flat surfaces." Thesis, University of Bristol, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.633147.
Full textAbstract:
Grafting polymer brushes from surfaces is an effective method of surface modification. Importantly, such coatings are robust due to covalent bonding, establishing a chemically and mechanically stable interface not relying solely on weak physical bonding (e . g. van del' Waals forces) as alternative coating methods do. Polymer brushes have been shown to mediate effective lubrication, among other applications. In this work, stainless steel is the substrate of interest and a novel system of reagents specifically designed for this material has been applied to produce various polymer brush structures by surface initiated atom transfer radical polymerisation (SI-ATRP). For friction reduction in an aqueous environment, a multi-component hierarchical brush system, incorporating an underlying hydrophobic layer and a hydrophilic block on the outer periphery was proposed. The first layer of brushes, closest to the metals surface, was exploited as a barrier to drive water away from the solid interface, hence minimising corrosion of the metal, with the facile wetting of the second block giving a hydration layer at the interface to promote lubrication in the presence of water. Additionally, the hydrocarbon brush chosen as the outer layer in this system is known to be thermoresponsive, as chains collapse and become more hydrophobic at elevated temperatures. A co-polymer structure was employed and allowed the critical temperature of the brush to be tuned over a range of temperatures. This tuneable thermal response was intended to give switchable surface friction. Furthermore, use of an industrially relevant material in these investigations should potentially lead to a more straightforward utilisation in real world applications.
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Childs, Laura Helen. "Low Reynolds number flows of generalized non-Newtonian fluids." Thesis, University of Bristol, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.633090.
Full textAbstract:
A Newtonian fluid is one for which the stress induced by flow is proportional to the strain rate, with the constant of proportionality being the fluid's viscosity. Fluids not adhering to this linear relationship are described as non-Newtonian. This thesis considers several problems involving flows of generalized non-Newtonian fluids - where the effective viscosity is a nonlinear function of the strain rate - experiencing negligible inertial effects. First, we consider the flow of a power-law fluid through a rectangular duct. We propose a numerical method of solution, reliant on expressing the governing equations in terms of the stress tensor components, rather than the velocity field. This results in an effective method of determining the flux of fluid through a cross-section of a duct. The calculation is extended to duct flows of regularized Herschel-Bulkley fluids, and the determination of the critical yield stress below which there is no flow. We then consider free-surface dam-break flows of power-law fluids. By utilizing a similarity scaling for the front position of the flow with time, the flux calculation of the preceding work, and laboratory experiments, we propose a method of determining the rheological parameters of a power-law fluid. The settling velocity of a spherical particle through a power-law fluid is next examined. For particles settling within a background shear flow, the settling velocity is found to have two distinct dependencies on the dimensionless flow parameters, corresponding to regimes of dominant background shear or gravitational settling. Finally, we consider the effect that a modified sedimentation law, based upon the results of the settling calculation, has on shear flows of dilute particulate suspensions. The method of characteristics is applied to investigate the sedimentation law's impact on properties such as the concentration profile, and run-out length, of a sustained intrusion of particles suspended in a power-law fluid.
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Kelly, David. "Computational mechanics in practice : mathematical adaptions and experimental applications." Thesis, University of Bristol, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.570852.
Full textAbstract:
The definition and quantification of complexity is a source of debate. A promising answer, from Crutch field, Shalizi and co-workers, identifies complexity with the amount of information required to optimally predict the future of a process. Computational mechanics computes this quantity for discrete time series; quantifying the complexity and generating minimal, optimally predictive models. Here we adapt and apply these methods to two very different problems. First, we extend computational mechanics to continuous data which cluster around discrete values. This is applied to the analysis of single molecule experimental data; allowing us to infer hidden Markov models without the necessity of assuming model architecture and allowing for the inference of degenerate states, giving advantages over previous analysis methods. The new analysis methods are demonstrated to perform well on both simulated data, in high noise and sparse data conditions; and experimental data, namely fluorescence resonance energy transfer spectra of Holliday junction conformational dynamics. Secondly, we apply computational mechanics to investigations of the HP model of protein folding. Computational mechanics was used to investigate the properties of the sequence sets folding to the highly designable structures. A hypothesised correlation between structures' designability and the statistical complexity of its sequence set was unsupported. However, methods were developed to succinctly encapsulate the non-local statistical regularities of sequence sets and used to accurately predict the structure of designing and randomly generated sequences. Finally, limitations of the standard algorithm for reconstructing predictive models are addressed. The algorithm can fail due to pair-wise comparisons of conditional distributions. A clustering method, considering all distributions simultaneously has been developed. This also makes clear when the algorithm may be effectively employed. A second issue concerns a class of processes for which computational mechanics cannot infer the correct, optimally predictive models. Adaptions to allow the inference of these processes have been devised.
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Mirzaei, Mahsanam. "Dynamic effects on two-phase flow in porous media." Thesis, University of Oxford, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.640167.
Full textAbstract:
A correct description of multiphase flow behaviour in a porous media involves the determination of various fluid and porous media parameters and, a constitutive relationship among capillary pressure (Pc), saturation (S) and relative permeability (kr). The determination of a Pc-S relationship is difficult because of two effects: (i) presence of heterogeneities in the domain and (ii) the dynamic effects in Pc-S relationships. While the significance of the individual factors has been studied at various scales using different approaches, the combination of the two effects on Pc-S relationships is not well characterised.
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Abraham, Meg. "Incorporation of laser ablation into a PIXE system in order to study metallurgy and corrosion products on archaeological objects." Thesis, University of Oxford, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.645189.
Full textAbstract:
This work demonstrates the set up and testing of a laser ablation system incorporated into a Particle Induced X-ray Emission Beam line in order to expose bare metal for analysis. The setup allows for X-ray analysis of the composition of the objects without large scale or visible damage to the patina on the object. In some instances the process also allows for some analysis of the corrosion products that make up the patina on the objects. The process is developed as a method for handling a specific class of archaeological objects which are not suited to sampling for various reasons. The aim is to minimize invasive testing of these objects while gaining information about their composition. Using this technique it is possible to infer something about the bulk makeup of the corrosion products on the surface of the objects. It also allows for a good quantitative understanding of the trace elements in the underlying metal substrate that remains below the corroded surface in most cases. This process is not intended to produce exact descriptions of the mineralogical makeup of the corrosion products that form the patina on the object, such as those which can be obtained through metallurgical cross-sectioning. Further it will not yield good quantitative analysis of the trace elements in corrosion products like those obtained using synchrotron radiation. Finally this technique cannot give detailed information regarding the metallurgical structure of the base metal composition of the object again like the data obtained using cross-sections.
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Buggakupta, Wantanee. "Microstructure and thermal expansion behaviour of magnesia-magnesium aluminate composites." Thesis, University of Surrey, 2008. http://epubs.surrey.ac.uk/713/.
Full textAbstract:
Knowledge of the coefficient of thermal expansion (CTE) of a ceramic material is important in many application areas. Whilst the CTE can be measured, it would be useful to be able to predict the expansion behaviour of multiphase materials. There are several models for the CTE, however, most require a knowledge of the elastic properties of the constituent phases and do not take account of the microstructural features ·of the material. If the CTE could be predicted on the basis ofmicrostructural information, this would then lead to the ability to engineer the microstructure of multiphase ceramic materials to produce acceptable thermal expansion behaviour. To investigate this possibility, magnesia-magnesium aluminate spinel (MMAS) composites, consisting of a magnesia matrix and magnesium aluminate spinel (MAS) particles, were studied. Having determined a procedure to produce MAS from alumina and magnesia, via solid state sintering, magnesia-rich compositions with various magnesia contents were prepared to make the MMAS composites. Further, the MMAS composites prepared from different powders (i.e. from an alumina-magnesia mixture and from a magnesia-spinel powder) were compared. Com starch was added into the powder mixtures before sintering to make porous microstructures. Microstructural development and thermal expansion behaviour of the MMAS composites were investigated. Microstructures of the MAS and the MMAS composites as well as their porous bodies were quantified from backscattered electron micrographs in terms of the connectivity of solids i.e. solid contiguity by means of linear intercept counting. Solid contiguity decreased with increasing pore content and varied with pore size, pore shape and pore distribution whereas the phase contiguity depended strongly on the chemical composition and was less influenced by porosity. The thermal expansion behaviour of the MAS and the MMAS composites between 100 and 1000 °C was determined experimentally. Variation in the CTE of the MAS relates to the degree of spinel formation while the thermal expansion of the MMAS composites depends strongly on phase content. However, the MMAS composites with similar phase compositions but made from different manufacturing processes showed differences in microstructural features and thermal expansion behaviour. Predictions of the CTE values for composites based on a simple rule-of-mixtures (ROM) using volume fraction were compared with the measured data. A conventional ROM accurately predicted the effective CTE of a range of dense alumina-silicon carbide particulate composites but was not very accurate for porous multiphase structures. It provided an upper bound prediction as all experimental values were lower. Hence, the conventional ROM was modified to take account of quantitative microstructural parameters obtained from solid contiguity. The modified ROM predicted lower values and gave a good agreement with the experimental data. Thus, it has been shown that quantitative microstructural information can be used to predict the CTE of multiphase ceramic materials with complex microstructures.
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Mulyadi. "Hot compression behaviour of two-phase Ti-6Al-4V : experiments and state-variable modelling." Thesis, Open University, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.486503.
Full textAbstract:
The flow stress behaviour of two-phase Ti-6AI-4V and its individual α and β phases has been characterised during isothermal forging at temperatures of 850-1050°C and strain rates of 0.3-0.003/s. The influence of initial pre-form microstructure has also been investigated by heat-treating the as-received globular microstructure to produce an acicular β-transformed microstructure. It is found that flow stress behaviour exhibits a strong dependence on working temperature and imposed strain rate for the two-phase globular, two-phase acicular and single a-phase materials. However, for the single β-phase, steady state stress is found to be relatively constant with increasing temperature for the range of 925-975°C and strain rates of 0.3-0.003/s.
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Al-Hazmi, Farag Saeed. "High strain rate behaviour of carbon fibre composites." Thesis, Loughborough University, 1995. https://dspace.lboro.ac.uk/2134/14116.
Full textAbstract:
This thesis describes a study of the dynamic mechanical properties of two types of carbon fibre composite: IM7/977-2 with 977-2 toughened thermoset resin, and IM7/APC2 with thermoplastic PEEK resin, both using Hercules IM7 carbon fibres with a volume fraction of 62%. The Split Hopkinson Pressure Bar (SHPB) is used to produce dynamic strain rates ranging from 10³ to 5x10³s⁻¹, while Instron and Hounsfield screw driven machines are used to give a constant strain rate of about 2 x 10⁻³s⁻¹ for comparative quasi-static loading. The Split Hopkinson Pressure Bar (SHPB) technique uses specimens in the form of a cylinder, sandwiched between two maraging steel bars. One of the maraging steel bars is subjected to an impact produced by a projectile. The Instron and Hounsfield machine used the same form of specimen. A strain gauge method has also been used to record the strain directly by attaching a strain gauge on to the specimen under test. Three different fibre orientations have been investigated: unidirectional fibre with loading parallel (0°) to the axis of the specimen; transverse loading (90°) where the fibres are perpendicular to the axis of specimen; and quasi-isotropic with transverse loading . The dynamic stress-strain properties of the carbon fib~e composites have been investigated experimentally at room temperature under quasistatic condition and at the high strain rates produced in the SHPB method using loading pulses of about 100 μS duration and rise time of 12 μS. The effects of strain rate on stiffness, yield stress and strength for the composites have been determined and compared with those obtained from a mathematical model (Rule of Mixtures). The longitudinal carbon fibre composite had the highest stiffness, followed by the quasi-isotropic composite, and then the transverse composite. For compressive strength, the quasi-isotropic had the highest strength followed by the longitudinal and then the transverse. The fracture energy has also been obtained indicating that for the quasi-isotropic composites, IM7/977-2 has a considerably high fracture energy than IM7/APC2. The bulk temperature rise has also been predicted up to the fracture stress.
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Shaw, Alexander. "Vibration isolation utilising the nonlinear deformation of anisotropic plates." Thesis, University of Bristol, 2015. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.649368.
Full textAbstract:
This thesis has two motivations; vibration isolation and the desire to yield additional functional benefits from composite materials. Vibration isolation is a vital requirement throughout much of engineering, particularly when there is a strong source of vibration such as a motor. It is frequently required to prevent the transmission of these vibrations to other elements of the system, for reasons such as passenger comfort, or the protection of delicate electronic equipment. Carbon fibre based composites are seeing widespread application wherever highly weight efficient strength or stiffness is required. However, their high cost and complex processing requirements motivates a desire to achieve additional benefits from their use, and many such 'benefits have been identified in the study of multifunctional composites. In particular, much work has shown that composite plates may be created to exhibit highly nonlinear behaviour, including bistability. In this work, we consider exploiting bistability and related nonlinear behaviours for the function of vibration isolation. Specifically, we wish to engineer the directional stiffness and residual stresses in composite laminates to achieve a lightweight and effective vibration isolation mount. Anti-vibration mounts typically exploit damping and low stiffness to reduce vibration. Damping is effective in limiting the peak response of the mount at resonance, but is detrimental to isolation at higher frequencies. Reducing the stiffness of the mount is beneficial to isolation, but can conflict with the usual requirement that the mount must support a static load. The approach adopted in this work is that of a High Static Low Dynamic Stiffness (HSLDS) mount, whereby the mount has a nonlinear force displacement response, that features a region of low stiffness near equilibrium for vibration isolation, whilst increasing stiffness elsewhere. This work presents new results for HSLDS mounts, providing insight into many features of their performance including high parameter sensitivity and possible unbounded response. The transverse force-displacement response of CFRP plates with asymmetric stacking sequences is investigated both numerically and experimentally. It is shown that a bistable plate may be connected in parallel with conventional springs to provide a HSLDS response, providing that bistable curvature is limited to exclude undesirable secondary buckling patterns. It is then shown that a mono-stable plate may be created that eliminates the requirement for additional springs, and therefore implements a HSLDS mount in a single lightweight shell.
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Nixon-Pearson, Oliver James. "An investigation into the fatigue damage development in open- and bolted-hole composite laminates." Thesis, University of Bristol, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.652047.
Full textAbstract:
There is a significant amount of work in the open literature regarding final failure of notched composite laminates, however there has been rather less characterisation of sub-critical damage prior to final failure, particularly under fatigue loading conditions_ A previous investigation into the static tensile strengths of scaled open-hole specimens showed three distinct failure modes which were either matrix- or fibre-dominated. Three open-hole specimen configurations which exhibited clear examples of these failure modes were chosen for the fatigue investigation. The primary objective of this thesis was to introduce a detailed experimental investigation into the sub-critical damage prior to specimen failure by interrupting fatigue tests part way through the decrease in stiffness with increasing number of cycles. X-ray computed tomography (CT) was carried out for the interrupted test specimens in order to establish the damage development. In some cases a different failure mode is observed under fatigue loading to the quasi-static cases. It is shown how the role of delamination is critical in fatigue even when it may not dominate in quasi-static tests. Cohesive element models in finite element analysis have been shown to be effective at predicting delamination failure, including fatigue formulations. These have largely been applied to simple configurations such as the Double Cantilever Beam (DCB) specimen. The University of Bristol fatigue cohesive formulation has been applied to models of the open-hole tests to predict the damage seen in interrupted fatigue test specimens. This is the first such application of fatigue cohesive element models to complex cases involving matrix cracks, delaminations and their interactions.
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Costin, William James. "Radial basis function interpolation applied to discontinuous mesh interfaces." Thesis, University of Bristol, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.653069.
Full textAbstract:
For large-scale Computational Fluid Dynamics (CFD) simulations it is often necessary to divide the domain into a mesh of discrete points or volumes. However, the domain can also be split into a set of zones separated by interfaces, allowing the zones to be meshed individually. Discontinuous or nonmatching mesh spacing across the zonal interfaces offers many advantages, particularly in terms of easing the mesh generation process, reduction of required mesh densities, and relative motion between mesh zones. However, accurate data transfer across the interfaces is required for global solution accuracy to be maintained. A more versatile data transfer method for discontinuous interfaces has the potential to reduce both the complexity of, and constraints place on, the process of discretising complex domains. This research was motivated by work on large-scale parallelised CFD simulations using high quality structured multiblock meshes. For such tasks the accuracy of the final result has a significant dependence on the quality of the mesh used to discretise the domain. The mesh is split into separate zones for parallel computation but must be generated as a topographically consistent whole, rather than individually for each zone. The objective therefore was to create a new method of discontinuous interface data transfer that does not place topographic limitations on the mesh, simplifying the mesh generation process. To this end, the fields of flow solution, high accuracy interpolation and mesh generation have been investigated with the aim of formulating and assessing a new data transfer approach. A new data transfer method based on Radial Basis Function (RBF) interpolation is developed and presented. Strategies for the construction of the interpolation data set are compared and a new more advanced approach developed. A series of analytical tests are used to assess the properties of the new method: Both the order of accuracy of the method and the ability to accurately model the full range of frequency content are considered. The method is applied to both finite difference and finite volume numerical solutions. For the latter both multi-grid discretisation and parallelised solution have been ,j ABSTRACT implemented as part of integration into an existing parallel, multiblock, multi grid compressible flow solver. Additionally, the relative merits and cost of localised and global forms of the method are assessed. The method has been applied to a series of aeronautical test cases including a subsonic and transonic aerofoil. The results show that the method is both effective and robust, providing accurate transmission for well and poorly conceived interface geometries. One great success is that in generic form the method remains applicable to all iterative solution methods with similar domain discretisation requirements; as a result there are many opportunities for further work.
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Robinson, Elizabeth Clare. "Towards broadband optical spectroscopy of single nanostructures." Thesis, University of Bristol, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.653073.
Full textAbstract:
Optical properties of nanostructures strongly depend on their size, shape, material, and local environment. To fully characterise these properties, measurements of the full spectral response are required . The initial challenge is due to the small response of a nanoparticle in an optical field, the extinction. This can be overcome by using interferometry where the field scattered from the nanoparticle is multiplied by a larger reference field. This thesis aims to investigate the use of a broad band supercontinuum source for interferometric detection of single nanostructures.
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Gemkow, Karla Simone. "Modelling of Damage in Orthotropic Materials: Including Strain-Softening Effects in Dynamic Problems." Thesis, Cranfield University, 2013. http://dspace.lib.cranfield.ac.uk/handle/1826/9302.
Full textAbstract:
Damage models are developed within the continuum damage mechanics framework
which allows the description of material degeneration with general constitutive
equations. The difficulty in the description of damage behaviour increases with
increasing complexity of the material behaviour. This is especially true when it comes
to composite materials which have an orthotropic material behaviour. The conventional
description of damage, i.e. the local continuum damage mechanics description, leads
to strain-softening behaviour which is characterised by a decline in stress with
simultaneously increasing strain.
Due to strain-softening the tangent stiffness becomes negative which forces the wave
speed to become imaginary in dynamic problems. Consequently the partial differential
equations governing the dynamic problem change from hyperbolic to elliptic and,
therefore, the initial boundary value problem no longer has a unique solution. Due to
this the physical meaning becomes unrealistic. Strain-softening is limited to an infinitely
small area in which waves are not able to propagate in a process called wave trapping.
A displacement discontinuity in an area of width zero (localisation zone) develops. The
strain becomes infinite in this zone and is accompanied with a zero stress. Areas
outside the softening zone are not able to interact with the strain-softening domain. As
a consequence the strain-softening domain acts similar to a free boundary at which
waves reflect. The implementation of local continua with strain-softening behaviour in
finite element codes leads to additional numerical problems. Strain-softening behaviour
manifests itself in the smallest area possible which is a single point in analytical
considerations. This area is defined by the element discretisation in finite element
codes. Therefore, strain-softening leads to a pronounced mesh sensitivity of results in
addition to mathematical and physical issues.
This work aims to find a solution which removes problems associated to strain-
softening. Its aim is to represent material behaviour due to damage realistically and
enable numerical results to convergence to a unique solution.
The strain-softening problem is the focus of this work. It was investigated using a 1D
wave propagation problem described by Bažant and Belytschko [1]. This simple
experiment allows for an easy comparison of analytical and numerical results and
therefore gives an insight into the problems connected to strain-softening.
Furthermore, regularisation methods, specifically nonlocal and viscous methods, were
investigated. Regularisation methods add additional terms to constitutive equations
which keep the initial boundary value problem well-posed and enable a unique solution
independent of the element discretisation. It was found that these methods are indeed
capable of regulating the softening problem; however, they add additional difficulties in
the description of material behaviour.
A new approach to the strain-softening issues, unique at this point of time, was
developed in this work which implements damage as an equivalent damage force. This
approach is able to keep the initial boundary value problem stable and converge to a
unique solution without adding additional terms in the constitutive equations, such as
regularisation methods. This new approach to strain-softening was implemented for an
isotropic material with scalar damage variable in DYNA3D successfully. Numerical
results converged to a unique solution and were physically reasonable. The concept of
an equivalent damage force was further developed to orthotropic material behaviour.
This made an advanced representation, using an 8th rank damage tensor, necessary.
The 8th rank damage tensor is able to represent anisotropic damage and it is also the
most general damage representation possible.
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Hejda, Marek. "Deformation micromechanics of single glass fibre reinforced composites." Thesis, University of Manchester, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.491333.
Full textAbstract:
The current work presents for the first time the application of luminescence spectroscopy in following the micromechanical deformation of glass fibre reinforced composites; this has been achieved using luminescence-active glass fibres prepared from glass doped with small amounts of Sm3+. Glass prepared in this way exhibited several relatively sharp and intense luminescence peaks observed in the range 550 nm to 700 nm. The luminescence band located at 648 nm was used for the calibration of the local strain state of the fibre due to its distinctive linear shift towards lower wavelengths with increasing strain and the factors affecting this shift were studied in detail. The fragmentation of both untreated and silane-treated Sm3+ doped glass fibre has been followed in detail and the behaviour analysed using a classical shear-lag analysis. Silane treatment slightly enhanced adhesion between glass fibre and epoxy resin, which was confirmed by a supplementary fragmentation study, which employed carbon nanotubes dispersed in the silane agent as an additional strain sensor. This work has demonstrated luminescence spectroscopy as a new significant development in the ability to follow local mechanics of the interface between glass fibres and transparent resins.
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Al-Gayem, Qaid K. Omran. "Test and condition monitoring technologies for bio-fluidic microsystems." Thesis, Lancaster University, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.654975.
Full textAbstract:
Lab-on-chip devices and bio-fluidic microsystems are emerging technologies that exhibit the required fluidic transport, bio chemical detection and control functions to enhance the detection resolution and accuracy within a single miniaturised portable instrument for bio-diagnostic and bio-synthesis applications. These devices impact a number of markets, including medical diagnostics, pharmacology, environmental monitoring and industrial control. Example applications are DNA extraction from blood and detection of pathogens or genetically modified organisms. Electrode technology is fundamental to numerous actuation and sensing functions within these microsystems. Reliability is a key component of these microsystems and most applications require extremely low probabilities of false positives or negatives. New methods of validating the functionality and integrity of measurements are hence required. Embedded test and condition monitoring are crucial technologies for delivering these capabilities. This thesis makes significant contributions to knowledge in these areas to further the development and viability of accurate test equipment. The work in this thesis focuses on delivering novel solutions and methodologies that can be used to increase the reliability of electrode-based bio-fluidic mi'crosystems and cellbased biosensors. Three self-test solutions have been developed to address surface degradation within the sensing interface of a cell-based biosensor system and microfluidic chips. The first solution, a mid-frequency oscillation test approach, is based on the sensitivity of the interface capacitance to degradation, contamination and fouling. The second solution proposes a new fault diagnosis approach using an Artificial Neural III F -=---';~I_---- Abstract Network for detecting degradation in electrodes that interface to fluidic or biological systems. The third solution studies the feasibility of scanning the strength of a test signal over an array of electrodes to monitor degradation. The design of the monitoring structures has been validated through physical characterisation involving data extracted for both a single electrode and an array of electrodes where a micro controller, an analogue multiplexer, a test circuit, and an LeD have been used to achieve a real-time condition monitoring system. The novel test techniques provide analogue values directly related to the degradation of the electrode, so have the ability to provide accurate on-line degradation detection for a range of conditions across the electrode surface. Optimising the design of the microfluidic chips to maximise flow rate and the investigation into new methods for validating functionality and integrity of the readings have also been studied numerically. In addition, this thesis concretely examines potential research directions for future bio-fluidic microsystems.
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Abdelatif, Mohammed Amged Osman. "Assessment of bonded post-tensioned concrete with ruptured tendons." Thesis, University of Nottingham, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.662211.
Full textAbstract:
Prestressed concrete was introduced in the early part of the last century and has been used in a wide range of bridge structures. It is classified into two types; pre-tensioned and post-tensioned concrete (i .e. bonded and un-bonded post tensioned concrete). Both types have proved to be a durable and economical form of construction. However, many bonded post-tensioned concrete bridges have exhibited rupture of their tendons due to corrosion. In a few extreme cases this has led to structural collapse. While much of the effort has been focused on developing corrosion detection techniques, little attention has been paid to assessing the structural behaviour of bonded post-tensioned concrete bridges with ruptured tendons. A ruptured tendon is able to re-anchor into the surrounding materials and to contribute to the structural behaviour of the corroded structure. The tendon re-anchorage has been approximated in a number of structural assessment studies using pre-tensioned concrete models or empirical bond-slip relations. In some studies, reanchoring of the ruptured tendon is completely neglected. This is attributed to the lack of models regarding the tendon re-anchorage in post-tensioned concrete. The wrong estimation of the re-anchoring phenomenon of the ruptured tendon leads to an inaccurate prediction of the structural capacity of post-tensioned concrete structures after the rupture. The aim of this research is to predict the residual structural capacity of post-tensioned concrete beams with a ruptured tendon developing a re-anchorage model of the ruptured tendon.
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Tajik, Nosratollah. "The early age thermal cracking in concrete structures." Thesis, University of Westminster, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.502220.
Full textAbstract:
Investigations on the assumptions, the limitations and the factors influencing the development of early age thermal cracking in concrete have been carried out for many decades, but there is a better understanding of these phenomena in the recent years. Review of the literature on the early age thermal cracking (EATC) of concrete structures has shown that this phenomenon is very important and has significant influence on the durability, serviceability and aesthetically aspects of concrete structures, however despite this its mechanisms are still not well understood. Furthermore, most efforts to control this type of cracking have been by specifically designing distribution reinforcement, but this is proving uneconomic and is not a failsafe technique. The experimental technique carried out in this research investigation is an improvement on the past research on early age cracking. The experiments were carried out successfully and the effects of internal and external restraints were studied. Further methods of assessing the risk of early age thermal cracking in mass concrete under natural environmental conditions developed and the results were compared with those recorded under laboratory conditions. It has been shown that the early age· thermal cracking in concrete resulted from either internal restraint caused by differential expansion within concrete member during heating due to larger temperatures gradient between the core, and that the adjacent media or by external restraint to contract on cooling from a temperature peak of the concrete specimen are the critical factors in developing early age thermal cracking. It has been found that the temperature differential that causes early age thermal cracking . in concrete is greater than that figure found previous research. Other factors influencing the development of the thermal cracking were investigated and recommendations on controlling the early age thermal cracking were given.
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Tsaousoglou, E. "Characterisation of corrosion processes and cementitious." Thesis, University of Manchester, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.527486.
Full textAbstract:
Different formulations of Blast Furnace Slag blended with Ordinary Portland Cements are used in encapsulation of Intermediate Level Waste containing magnesium and aluminium alloys. A set of different candidate formulations are investigated to determine the corrosion rates of encapsulated materials, and the structural and chemical properties of the matrices which influence their suitability as encapsulant materials. The microstructural and mechanical properties of the cements tested by performing compressive strength, porosity and permeability measurements. The cements were found to display increased porosity with increasing initial water content, with a consequent decrease in the compressive strength. Pore Water Expression was used to extract the electrolyte present in the porous structure of the cements. The pH of the samples was found to be decreased by increasing BFS addition and increasing water content. The CSA sample was found to have similar pore water pH to the blended cements. Electrochemical Impedance characterisation of samples of the different Mg and Al containing samples in the different cement systems were performed. The EIS results for aluminium showed two time constant present in the system, either of which could possible have been related to the charge transfer process, but both of which evolved in the same proportion with time. The EIS results for magnesium showed only one time constant, allowing the corrosion rates to be calculated unambiguously. Tomographic analysis of the cement/metal interface, clearly shows the effects of hydrogen bubble formation of the surface area of the samples. This allows a correction factor to be applied to the corrosion rates found by EIS. Conclusions are presented in which the fitness of BFS: OPC blends for use as encapsulants is confirmed, and the merits of CSA as an encapsulant for aluminium, but not for Magnox are described.
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Ajour, Mohammed Nasser. "The effect of high voltage fields on epoxy laminates." Thesis, University of Leicester, 2003. http://hdl.handle.net/2381/858.
Full textAbstract:
This thesis describes the characterisation of epoxy/glass fibre composite material before and progressively through electrical and thermal ageing. Glass fibre reinforced epoxy (GFRE) material is used in pressboard transformers for optical telecommunication systems, typically at voltages between 1 and 2kV. The material was characterised by means of space charge measurements using the Pulsed Electroacoustic (PEA) technique, Dielectric response, Dynamic Mechanical Analysis (DMA), Differential Scanning Calorimetry (DSC), and Scanning Electronic Microscopy (SEM). An ageing programme was set up to follow the thermal and electrical ageing (at DC fields) of the GFRE by the same means. The results show a q-dc transport process with an activation energy of 1.1eV. The qdc process is associated with a charge transport process on the surface of the fibres. The results for the aged samples show delamination and debonding between the epoxy and the glass fibre at the glass epoxy interface. The delamination creates free volumes and voids which lead to partial discharge and hence failure. Electrical ageing can be characterised in term of dielectric, PEA, and DSC responses. Thermal ageing does not produce the same effect as electrical ageing. The samples that were only thermally aged behave in the same way as un-aged samples.
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Yi, Guanyu. "A front-tracking solidification model and its application in modelling alloy solidification." Thesis, University of Leicester, 2010. http://hdl.handle.net/2381/8934.
Full textAbstract:
A front-tracking solidification model has been developed to simulate the dendritic structure evolution during alloy solidification. In the model the growth of dendrites is governed by heat and mass transport and a finite difference technique is employed to solve heat and solute diffusion during solidification. The model incorporates front-tracking technique to calculate and track the exact position of the Solid/Liquid (S/L) interface as a part of solution process and a new capture rule was designed and implemented in the model to efficiently track the growing S/L interface. The model has been evaluated and verified using simulated data from Al-Cu 4 wt. % alloy solidification. The effect of curvature undercooling on crystal growth was investigated. The simulated results reveal that solute redistribution, curvature of the S/L interface and anisotropy of interface tension are important factors in determining the dendritic morphology. The calculation of the S/L interface curvature and anisotropy of surface tension was found to be particularly important in determining the dendritic growth direction. Based on the above observations and simulated data, the parameters in the developed model have been optimised for predicting the solidification structure in binary alloys. Simulations of Al-Cu alloy solidification were then performed using the optimised model for single-grain and multi-grain solidification. The simulated results of single-grain growth were compared with the results from the Lipton-Glicksman-Kurz (LGK) model (Lipton et al. 1984). Solute profile ahead of the S/L interface was examined using different techniques for approximating solute profile in the growing cell. The solidification segregation in the multi-grain growth was investigated; and the dendritic evolution and solute interaction during multi-grain growth were investigated.
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Klerck, Paul Alexander. "The finite element modelling of discrete fracture in quasi-brittle materials." Thesis, Swansea University, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.539299.
Full textAbstract:
An effective methodology for discrete fracture in quasi-brittle material is presented within an explicit finite discrete element framework. Simple pragmatic models are envisaged that reflect the data deficiency of the quasi-brittle material and recover the observed physical response within engineering accuracy. Phenomenological strain-softening constitutive models are adopted for the modelling of micromechanical processes in an average sense. An extensional basis for fracture is assumed in both tensile and compressive stress fields, with only the mechanism with which inelastic strain is realised differing between the two stress states. To overcome the mesh dependence introduced by local softening constitutive relationships, the socalled localisation limiters are adopted in the form of the tensile crack band, nonlocal and viscous smeared crack models. Effective localisation lengthscales introduced by these regularisation methods ensure mesh objective failure localisation a priori to discrete crack insertion. A nonlocal map of failure indicators initiates fracture, with discrete cracks inserted into the finite element continuum by the splitting of the discretisation. An isotropic, non-associative Mohr-Coulomb model is derived in principal stress space as a first order approximation to the quasi-brittle response in compression. A model for discrete fracture in tensile and compressive stress fields is proposed, defined by a composite yield surface consisting of the fully anisotropic rotating crack band model coupled with the isotropic, non-associative Mohr-Coulomb model. The novel inclusion of an explicit coupling between the extensional inelastic dilation strain accrued during compressive failure and tensile strength degradation in the dilation directions permits the realisation of discrete fracturing in purely compressive stress fields. The so-called continuum-discrete transition introduces additional degrees of freedom into quasi-brittle systems and permits large deformation to be realised through the process of cataclastic flow. This advancement is considered significant and necessary in the recovery of the observed quasi-brittle response. The effectiveness of the proposed constitutive fracture models is verified by application to a number of physical quasi-brittle fracture systems, including borehole breakout, fracturing around excavations, strip punch tests, dynamic spalling and anchor pullout tests, amongst others.
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Hutchings, D. "Hydrogen embrittlement of duplex stainless steel." Thesis, University of Manchester, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.631722.
Full textAbstract:
Duplex stainless steels (DSS's) are frequently used in oil and gas production and are subsequently subjected to cathodic protection. There is now growing concern about the cathodic evolution of hydrogen produced from this protection system, which may diffuse into the alloy and cause an embrittled condition. DSS's have a microstructure that is a mixture of austenite and ferrite and combines the advantages of these grades, whilst minimising their deficiences. In this research, Zeron 100 DSS was studied in six conditions to investigate the effects of hydrogen embrittlement (HE) on the various strengths and microstructures. The six conditions wer~ as follows: as-received, cold worked, age-hardened (475°C embrittlement), high temperature heat treated, rod and powder. To simulate service environments, 3.5% wt NaCI solution at ambient temperature with an applied potential of -1.1 V (SCE) was used. The effect of pre-charging for up to 550 hours at 80°C was also investigated. Test methods included slow strain rate testing (SSRT), monitoring of transient crack propagation (TCP) using circumferentially notched tensile specimens using a DC potential drop method, acoustic emission CAE) and some conventional bolt loaded fracture mechanics specimens. Test results were correlated with the varying microstructures and environmental conditions and consisted of mechanical properties, threshold crack growth including transient effects and AE data. In this work transgranular cleavage cracks were obtained in the susceptible ferrite phase as a direct result of HE; the depth of these cracks implied a high hydrogen concentration throughout the specimen. The austenite failed by ductile tearing and acted as a physical barrier to the propagation of cleavage cracks. As a result of SSR testing the best material was found to be the powder material; the fine equally dispersed austenite phase caused a lowering of the effective K value. The worst material was the high temperature heat treated type because it contained more ferrite (11:1 72%). The age-hardened material was also susceptible because of the hard and brittle ex' phase. However, regardless of the environment the UTS remained virtua]]y unchanged for each individual material, indicating that most cracking occurred in the post-UTS stage of the test. With the TCP test a lowering of the fracture load was found when an HE environment was used; daldt vs Kq curves were produced, however the DC potential drop equipment could not accurately measure crack growth because of the bridging effect of the austenite phase. The most susceptible microstructures were again the age-hardened and heat treated types. The hydrogen evolution reaction (HER) was also investigated by creating a fresh surface on the as-received DSS and studying the changes in the HER. This work showed that the effect of scratching is irreversible. Also the oxide film can not be totaHy reduced electrochemica]]y and only mechanical methods can remove the oxide films entirely. Fina]]y a means of detecting "475°C embrittlement" of DSS's was investigated using an electrochemical technique in 5M HCI. i-E curves were produced which showed the reactivation of the ferrite and austenite phases in the as-received material. By age-hardening at 475°C the two reactivation peaks merged.
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Parker, Daniel Edward. "Shear strength within reinforced concrete beam-column joints." Thesis, University of Bolton, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.492666.
Full textAbstract:
Reinforced concrete is an economical construction material and is widely used throughout the world in buildings and bridges. The shear strength within beam-column joints in reinforced concrete structures has been identified as an area where further research is still needed in order to form reliable design methods. The aim of this research programme has been to develop a rational analytical model which can be used conveniently in the design of beam-column joints. The work consists of a brief literature review, an extensive experimental programme and the development of a new analytical model for predicting the strength of beam-column joints. The new analytical model is a development of the strut-and-tie model and is believed to be original in two ways: (a) The influence of the shear span and the spacing of the links (if any) are considered directly. (b) The inclination of the compression field is determined by maximising the contribution of the concrete to the stiffness of the member in shear. The new analytical model is shown to predict the strength of the test specimens and of many specimens reported in the literature more reliably than current design codes and standards
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Ding, J. "Thermo-mechanical analysis of wire and arc additive manufacturing process." Thesis, Cranfield University, 2012. http://dspace.lib.cranfield.ac.uk/handle/1826/7897.
Full textAbstract:
Conventional manufacturing processes often require a large amount of machining and cannot satisfy the continuously increasing requirements of a sustainable, low cost, and environmentally friendly modern industry. Thus, Additive Manufacturing (AM) has become an important industrial process for the manufacture of custom-made metal workpieces. Among the different AM processes, Wire and Arc Additive Manufacture (WAAM) has the ability to manufacture large, low volume metal work-pieces due to its high deposition rate. In this process, 3D metallic components are built by depositing beads of weld metal in a layer by layer fashion. However, the non-uniform expansion and contraction of the material during the thermal cycle results in residual stresses and distortion. To obtain a better understanding of the thermo-mechanical performance of the WAAM process, a study based on FE simulation was untaken in this thesis. The mechanism of the stress generation during the deposition process was analysed via a 3D transient thermo-mechanical FE model which is verified with experimental results. To be capable of analysing the thermo-mechanical behaviour of large-scale WAAM components, an efficient FE approach was developed which can significantly reduce the computational time. The accuracy of this model was validated against the transient model as well as experimental measurements. With the help of the FE models studies on different deposition parameters, deposition sequences and deposition strategies were carried out. It has been proved that the residual stresses and the distortions are possible to be reduced by using optimised deposition parameters and sequences. In addition, a robot path generation prototype has been developed to help efficiently integrate these optimised process settings in the real-wold WAAM process.
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Lukaszewicz, Mikolaj. "Steam oxidation of advanced high temperature resistant alloys for ultra-supercritical applications." Thesis, Cranfield University, 2012. http://dspace.lib.cranfield.ac.uk/handle/1826/7917.
Full textAbstract:
Steam oxidation of heat exchanger tubing is of growing interest as increasing the efficiencies of conventional pulverised fuel fired power plants requires higher steam temperatures and pressures. These new, more severe steam conditions result in faster steam oxidation reactions, which can significantly reduce the lifetime of boiler components. This thesis reports results from an investigation of the steam oxidation of the high temperature resistant alloys. It covers an analysis of the impact of temperature, steam flow rate, specimen shape and specimen surface finish on oxidation of resistant materials. Additionally, the mechanism of steam oxidation was invastigated with the oxygen 18 water. The results show that an increased steam flow rate not only causes faster oxidation rates but also a change in oxide scale morphology. In case of T23, it triggers formation of micro-layered inner oxide, whereas for T92 it promotes the formation of an outer haematite layer. For austenitic steels, the faster steam flow increases the formation of initially protective oxide scales, but also accelerates the growth of oxide nodules with prolonged exposure times. The analysis of the different surface finishes show that clearly the change of the surface finish from ground to polish and pickled (as received) accelerates the oxidation process for austenitic steels, the ground specimens show the slowest oxidation, whereas the pickled specimens oxidise much faster and form thicker scales. Finally, the study of oxidation mechanism show that steam oxidation is not only controlled by the inner diffusion of the oxygen ions but the diffusion of the hydroxides have a significant impact on oxides formation. The results of the study suggest that the hydroxide ions influence formation of the inner oxides.
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Chirivi, Laura. "Effect of manufacturing parameters on TBC systems cyclic oxidation lifetime." Thesis, Cranfield University, 2011. http://dspace.lib.cranfield.ac.uk/handle/1826/7893.
Full textAbstract:
Aero-gas turbine engines have to meet reliability, durability and fuel e ciency requirements. High turbine inlet temperatures may contribute to minimise fuel consumption and, in turn, environmental impact of the engine. Over the past few years, new designs and engine optimisation have allowed increase of such temperatures at a rate of 15 C per year, with maximum operating temperatures currently exceeding 1650 C. Ceramic coatings (also known as Thermal Barrier Coatings or TBCs) in conjunction with advanced cooling technologies are adopted to protect stator vanes and high pressure turbine blades from excessive thermal loads. Nevertheless, even with these protections in place, such components may experience a continuous service temperature of 1050 C, and peak temperatures as high as 1200 C. Therefore, it is vital that engine rotating components are able to maintain their mechanical properties at high temperature, while being able to withstand thermal loads and having su cient oxidation resistance to preserve the integrity of the ceramic coating, and eventually reaching desired component lives. Such strict requirements can be met with the use of complex Thermal Barrier Coat- ing systems or TBC systems; these consist of a nickel-based superalloy component which is rst coated with an environmental resistant layer (identi ed as bond coat ) and then with a ceramic coating. As its name suggests, the bond coat must not only protect the metallic substrate against oxidation and/or corrosion but must also provide su - cient bonding of the ceramic top layer to the metallic substrate. This goal is achieved through the formation of a further layer between the bond coat and the ceramic. In gas turbine applications, such a layer (identi ed as Thermally Grown Oxide or TGO) is an alumina scale which is the result of the bond coat oxidation during the ceramic deposition. During engine service, several time and cycle related phenomena occur within the TBC system which eventually lead the system to failure by spallation of the top coat.Aero-gas turbine engines have to meet reliability, durability and fuel e ciency requirements. High turbine inlet temperatures may contribute to minimise fuel consumption and, in turn, environmental impact of the engine. Over the past few years, new designs and engine optimisation have allowed increase of such temperatures at a rate of 15 C per year, with maximum operating temperatures currently exceeding 1650 C. Ceramic coatings (also known as Thermal Barrier Coatings or TBCs) in conjunction with advanced cooling technologies are adopted to protect stator vanes and high pressure turbine blades from excessive thermal loads. Nevertheless, even with these protections in place, such components may experience a continuous service temperature of 1050 C, and peak temperatures as high as 1200 C. Therefore, it is vital that engine rotating components are able to maintain their mechanical properties at high temperature, while being able to withstand thermal loads and having su cient oxidation resistance to preserve the integrity of the ceramic coating, and eventually reaching desired component lives. Such strict requirements can be met with the use of complex Thermal Barrier Coat- ing systems or TBC systems; these consist of a nickel-based superalloy component which is rst coated with an environmental resistant layer (identi ed as bond coat ) and then with a ceramic coating. As its name suggests, the bond coat must not only protect the metallic substrate against oxidation and/or corrosion but must also provide su - cient bonding of the ceramic top layer to the metallic substrate. This goal is achieved through the formation of a further layer between the bond coat and the ceramic. In gas turbine applications, such a layer (identi ed as Thermally Grown Oxide or TGO) is an alumina scale which is the result of the bond coat oxidation during the ceramic deposition. During engine service, several time and cycle related phenomena occur within the TBC system which eventually lead the system to failure by spallation of the top coat.Aero-gas turbine engines have to meet reliability, durability and fuel e ciency requirements. High turbine inlet temperatures may contribute to minimise fuel consumption and, in turn, environmental impact of the engine. Over the past few years, new designs and engine optimisation have allowed increase of such temperatures at a rate of 15 C per year, with maximum operating temperatures currently exceeding 1650 C. Ceramic coatings (also known as Thermal Barrier Coatings or TBCs) in conjunction with advanced cooling technologies are adopted to protect stator vanes and high pressure turbine blades from excessive thermal loads. Nevertheless, even with these protections in place, such components may experience a continuous service temperature of 1050 C, and peak temperatures as high as 1200 C. Therefore, it is vital that engine rotating components are able to maintain their mechanical properties at high temperature, while being able to withstand thermal loads and having su cient oxidation resistance to preserve the integrity of the ceramic coating, and eventually reaching desired component lives. Such strict requirements can be met with the use of complex Thermal Barrier Coat- ing systems or TBC systems; these consist of a nickel-based superalloy component which is rst coated with an environmental resistant layer (identi ed as bond coat ) and then with a ceramic coating. As its name suggests, the bond coat must not only protect the metallic substrate against oxidation and/or corrosion but must also provide su - cient bonding of the ceramic top layer to the metallic substrate. This goal is achieved through the formation of a further layer between the bond coat and the ceramic. In gas turbine applications, such a layer (identi ed as Thermally Grown Oxide or TGO) is an alumina scale which is the result of the bond coat oxidation during the ceramic deposition. During engine service, several time and cycle related phenomena occur within the TBC system which eventually lead the system to failure by spallation of the top coat.This may have catastrophic consequences as the uncoated component would face temperatures higher than the melting point of the constituent metal. This is avoided by strict maintenance regimes based on the minimum expected life of the coating. While essential for safeguarding the aircraft, this approach prevents the TBC systems from being used to their full potential. This study investigates possible optimisation methods of the manufacturing process of TBC systems, with the aim of improving reproducibility in terms of time to failure, thereby extending their minimum life expectancy and reliability. Two di erent types of TBC systems are studied: a TBC system with a Platinum-di used bond coat and a TBC system with a Platinum-modi ed aluminide bond coat. The work focuses on the e ects due to modi cation of process parameters (varied within industrially accepted range) on the TBC systems lifetime in laboratory scale cyclic oxidation tests. Experimental results show that accurate monitoring of the metal substrate surface nish as well as of the Pt layer morphology and ceramic deposition temperature may result in a dramatic improvement in life expectancy of the system, up to sevenfold when compared to control samples, or threefold if compared to commercial coatings.
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Cini, Andrea. "Scribe marks at fuselage joints : initiation and propagation of fatigue cracks from mechanical defects in aluminium alloys." Thesis, Cranfield University, 2012. http://dspace.lib.cranfield.ac.uk/handle/1826/7895.
Full textAbstract:
Mechanical damages, like scratches, are commonly detected on the surfaces of aircraft components and structures. They can be accidentally introduced during machining or maintenance operations or be the result of wear and impacts during aircraft service. Under the action of service loads, such mechanical damage can generate fatigue cracks reducing the component fatigue life and compromising the aircraft structural integrity. The evaluation of the effect of scratches and other small mechanical defects on the structure and component fatigue lives is therefore necessary to define an inspections programme and ensure the structural safety. Conventional fatigue life prediction methods generally consider scratches tents of microns deep too shallow to appreciably affect the fatigue performances of structural components. However the discovery of the scribe marks on fuselage joints disproved that prediction. In fact several commercial airlines discovered during inspections that aircraft which have been repainted showed multiple scratches on the fuselage skin along longitudinal and circumferential joints. Those scratches, referred to scribe marks, appear to have been caused by use of sharp tools during sealant removal process prior repainting. Scratches less than 200 μm deep were capable of severely reducing the fatigue life performance under service load rendering some aircraft beyond economical repair. This thesis investigates the fatigue performances of 2024-T531 aluminium alloy sheets weakened by mechanically machined scratches. 2 mm thick clad and unclad samples were scribed at their gauge section using a diamond tipped tool. The scribing process produced very regular rounded V-shaped notches with an included angle of 60° across the sample width. Scratches from 25 μm to 185 μm deep, with 5 μm, 25 μm and 50 μm root radii were cut on the sample surfaces. Scribed sample were subsequently fatigue tested under constant amplitude tensile and bending load with a stress ratio of R=0.1 at a maximum stress of 200 MPa. Scribes were found to reduce the fatigue life of tension and bending samples up to 97% compared to that of smooth unscribed specimens. Both scribe shape and size affected the fatigue life of tensile and bending aluminium samples. The sharper and the larger the notch, the shorter the fatigue life. Post failure fractography investigations were performed on sample fracture surfaces by means of optical and scanning electron microscope. Crack nucleation sites, fracture morphology and peculiar features left during crack propagation were analysed. Finally crack propagation data under different loading conditions were obtained by striation counting performed on fracture surfaces. Multiple crack initiation occurred at scribe roots usually from inclusions, defects or weak points along the root. The number and density of crack nucleation sites appeared to be determined by the scribe; increasing for notches with larger stress concentrations. Scribe geometry did not affect the fatigue growth rate but the propagation life for cracks deeper than 50-100 μm was influenced. Cracks nucleated from scribe marks showed a typical short crack behaviour growing faster than long cracks with the same linear elastic stress intensity factor. Finite element calculations were performed on scribed samples evaluating how the presence of scribes altered the local stress and strain fields. Monotonic elastic and elastic-plastic and cyclic elastic-plastic analyses were carried out under tensile and bending loads. Local elastic-plastic stress and strain fields in the neighbourhood of different scribes were determined by the notch shape and size. According to the occurrence of mechanical similitude conditions, scribes with the same shape but different size showed similar plastic zone and stress and strain distributions. A stabilised cyclic plastic zone was developed just at the root of scribes with a ratio between the root radius and depth ρ/d≤0.2. No correlations were observed between the occurrence of a stabilised cyclic plastic zone and the sample nucleation lives defined as the number of cycles to obtain an initial crack 50 μm deep from the notch root. Traditional fatigue life prediction methods, based on the notch sensitivity factor, were not able to correctly characterise the effect of scratches few tens of microns deep on the fatigue life of 2024-T351 aluminium alloy components. An approach based on the critical distance theory was developed to characterise the total fatigue life reduction produced by the introduction of scribes relating the fatigue live to a critical stress range Δσlm. The critical stress range was capable of describing the effect of the elastic stress distribution produced by dissimilar notches on the nucleation and propagation of fatigue crack considering also the effect of the variation of the fatigue load nominal applied stress.
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Saifullah, Mohammad. "Effect of reinforced corrosion on bond strength in reinforced concrete." Thesis, University of Birmingham, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.496283.
Full textAbstract:
Premature deterioration of reinforced/prestressed concrete structures due to corrosion is of considerable current concern. One detrimental effect of corrosion of reinforcing/prestressing steel is the reduction of bond between steel and concrete due to the development of corrosion products at the interface. This thesis examines the influence of localized corrosion of reinforcing bars/ untensioned prestressing strands on their bond strength in concrete. In addition, an analytical study is conducted in order to investigate the complex behaviour due to cracking of cover concrete, non-uniform corrosion, presence of softened paste layer at the interface between steel and concrete, and stress relaxation due to creep in relation to the bond strength. The conditions of severe localized corrosion were simulated electrochemically. The main variables were cover-to-bar diameter ratio, reinforcement type, and corrosion rate. The influence of reinforcing/prestressing steel corrosion and cracking of the concrete cover on the behaviour of bond were studied at different stages of corrosion: non-corrosion, precracking, cracking, and postcracking levels. It was found that the bond strength increased with corrosion up to a certain amount. However, with the progressive increase in corrosion, the bond strength decreased very rapidly until the cracking of cover concrete, and then decreased at a very slow rate in the postcracking stage. The untensioned prestressing strands showed almost similar general behaviour but were found relatively more deteriorated due to corrosion in the postcracking stage under the similar corrosive conditions. The structural properties of steel such as yield strength and ultimate tensile strength were influenced significantly due to the enormous local reduction in cross-sectional area of the reinforcing/prestressing steel by corrosion. The test data showed that the percentage corrosion required to cause cracking of cover concrete varied linearly with cover-to-bar diameter ratio. Bond-slip studies at different stages of corrosion indicated that bond stiffness increases and then decreases with the increase of corrosion of reinforcing steel in concrete. Corroded prestressing strands exhibited a nonlinear bond-slip relationship. Corrosion rate was found to be a significant variable. Pullout bond specimens using deformed bars were exposed to the current densities of 4.0,2.0, 1.0,0.5,0.25,0.15,0.09, and 0.04 mA/cm'. Bond behaviour was studied at the cracking stage and after 20% corrosion. The results indicated a significant and non-linear effect of corrosion on bond strength. Both corrosion to cause cracking and bond strength as a ratio of the non-corroded bond strength increased with an increase of current density up to about 0.15-0.25 mA/CM2 , and then decreased with a further increase in current density. This explains the different results obtained by previous researchers at different corrosion rates. Finite element analysis of the effect of concrete cover at the stages of internal and surface cracking confirmed the behaviour found in the laboratory study. The presence of an interfacial softened paste layer showed no significant effect on the expansive pressure. The study of the relaxation of stresses due to creep showed that the high degradation in bond strength at high rate of corrosion was significantly contributed by the relaxation of stresses due to creep at the cracking stage. However, in the postcracking stage, creep showed little effect on the stresses in the concrete surrounding the reinforcing bar. It is concluded that it is extremely difficult to extrapolate laboratory data to field conditions. However, a simple assessment rule is proposed.
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Gouma, Pelagia-Irene. "Characterisation of gas-atomised powders of [gamma]-titanium aluminides." Thesis, University of Birmingham, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.537887.
Full textAbstract:
Gas-atomised powders of the Ti-48A1-2Nb-2Mn system have been produced from ingot material using the IRC facility and these powders have been examined before and after a range of HIPping treatments. The as-atomised powders and consolidated samples have been examined using optical microscopy, analytical scanning, transmission electron microscopy, and XPS spectroscopy. Some property measurements have been carried out on HIPped samples. The structure of the powders was, as expected, a function of particle diameter, with small particles (below 40μm diameter) showing the largest fraction of retained alpha. Detailed examination of the HIPped samples revealed a range of heterogeneous structures which were present in all size fractions. In particular, regions were found which are about 100μm in diameter where the Al content is very low, typically about 5% lower than the average alloy content, and which act as fracture-initiation sites during tensile testing. Examination of HIPped powders produced at different atomisation plants show similar regions with very low aluminium contents. The origin of these regions is argued to be the mushy zone which is present in the cold wall furnaces used to melt the alloy prior to atomisation. It is shown that heat-treatments aimed at removing these heterogeneities can be carried out as part of the HIPping process. The significance of these observations for powder-route products is briefly discussed.
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Al-Ani, Mahier Nawf Abbod. "The use of PFA in concrete." Thesis, University of Birmingham, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.496907.
Full textAbstract:
The study of previous classifications of pulverised fuel ash (pfa) has led the author to believe that a new approach is required to look Into the physical properties of pfa. In this study a large number of tests have been carried out In an attempt to establish a criterion which is related to the performance of, fresh and hardened pfa concrete. A classification for pfa Is proposed, based on Its particle size distribution, followed by , determination of the grading modulus (Gf). The results of the research show a close correlation between workability as measured by the VB time and. grading modulus with a marginally lower correlation between compressive strength and grading modulus. - The correlation values with Gf being higher than with the percentage mass retained on 45 m sieve. Taking Gf as the physical criterion which influences the behaviour of pfa concrete, the author proceeded to develop a mix design based upon Gf, whereby the required strength and workability can be achieved accurately by replacing a portion of the opc with pfa. Furthermore, the new design method is capable of achieving a wide range of workability and strength with different cement contents, and covers a variety of pfa finenesses (Gf). The cement and pfa content being adjusted in accordance with the properties and content of the aggregates. A newly developed technique, using the Scanning Electron Microscope, which investigated the hydration and porosity of hardened cementitious paste was implemented. The results showed that fog curing Is essential for the development of hydration products. The Interlock between the pfa particles and the matrix showed an influence on the strength of opc/pfa paste which was more pronounced than the porosity of the hydration products. Porosity was also measured, using an alcohol exchange method, which was found to give a higher porosity than the method using the Scanning Electron Microscope. The former measuring total porosity whilst the latter only measures pores in the matrix and does not take Into consideration cracks, capillaries or air voids.