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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
31

Materials evaluation using ultrasonic surface wave

Weston-Bartholemew, Winston January 1972 (has links)
The investigation reported in this thesis was prompted by the desire to develop an easy and reliable nondestructive method of testing to monitor changes in elastic properties of metals when they are subjected to creep , fatigue and case hardening. The possibilities of the useful application of such a test will be invaluable to the aircraft industry. It was 'therefore suggested that the specimens used should be made from the type of materials used in the aircraft industry. These were made of high strength alloys. The method of nondestructive testing was to excite ultrasonic surface waves on the surface of the specimens, and measuring the changes in surface wave velocity~when the material(specimens) are subjected to creep, fatigue, and case hardening. An ultrasonic goniometer was designed and constructed and,using the goniometer;critical angle reflectowetry was used to excite the surface waves on the surface of the specimens. For the measurements during fatigue,specimens were fatigued at high stress levels and low cycles. Constant amplitude alternating stresses with the specimen remaining in tension during the whole program type of loading was used. Most of the specimens were made of titanium (Ti230) but some work was also done on mild steel. The specimens used for creep were made from C263 Nickel alloy and , Titanium 230. Each specimen was subjected to creep to a certain degree. The case hardened specimens were made of steel with carburized case. It was found that using ultrasonic surface waves it was possible to determine the depth of case hardening. Using a calibration curve drawn from the results obtained of the change in surface wave velocity for specimens with known case depths, it Has possible to predict the case depths of specimens of unknown case depths; by measuring their surface wave velocities. Also , a frequency effect was observed whereby lower frequencies were found to be better for detecting larger. case depths When specimens which were subjected to creep were considered, it was found that ultrasonic surface waves can detect early stages of creep. This was the case for both materials studied. A difference was observed in' the response to the surface waves for the two materials studied. In both cases a frequency effect was observed in that higher frequencies were better for detecting early creep. For bo.th materials studied during fatigue, most of the change in surface wave velocity occurred during the very early stages of fatigue life. During the latter stages of fatigue life the rate of change was very much lower. No pattern was observed for the change in surface wave velocity with the number of fatigue cycles for either material. Lastly, measurements were done on brass, copper, aluminium, stainless steel and mild steel bar specimens, at the critical angle of incidence. For all the specimens it was found that the surface wave velocity changed with frequency.
32

Elastic follow-up and the interaction between applied and residual stresses

Horne, Graeme Christopher Milligan January 2013 (has links)
This dissertation considers the characterisation of the relaxation of residual stress in a component resulting from plasticity induced by externally applied loads with the elastic follow-up factor. Residual stresses are the result of an incompatibility of displacements or strains within a component, i.e. a misfit. However, elastic material surrounding the non-linearly deforming region results in the region experiencing loading conditions between the extremes of fixed-load and fixed-displacement conditions. This is known as elastic follow-up. Greater elastic follow-up, i.e. more compliance in the surrounding material, decreases the relaxation of residual stress with plasticity. An idealised residual stress model was developed to determine the residual component of load acting on a fracture specimen, as a function of the applied component of load, through the plasticity resulting from the total load on the specimen. The effect of elastic follow-up is shown on the rate of residual stress relaxation, additional plastic strain accumulation and the J-integral. The results from the model are compared with the existing and proposed R6 methodology. In general, the existing approach was found to be overly conservative with negligible elastic follow-up, reduced with the g-function approach, otherwise they were reasonably intolerant of elastic follow-up. The recently proposed relaxation equation showed a good agreement with the model for all levels of elastic follow-up. An experiment is presented to characterise the elastic follow-up associated with an idealised one-dimensional residual stress field. It is shown that the elastic follow-up factor is a function of the reduction in eigenstrain or misfit, rather than the increase in plasticity alone, for the associated reduction in residual stress. The rate of relaxation of residual stresses in a component is dependent on the eigenstrain, geometry and post-yield behaviour of the material. It cannot be uniquely described by the elastic follow-up factor as the rate of relaxation is also dependent on the position of all non-linear deformation in the body. Therefore the elastic follow-up factor is position-, loading- and defect-specific.
33

Library of geometric influences for stress intensity factor weight functions

Teh, Lay Seong January 2002 (has links)
This research thesis reports the development of a novel concept for Linear Elastic Fracture Mechanics (LEFM) analysis - Composition Theory of Stress Intensity Factor Weight Functions (CToWF). A generic closed form composition model has been derived to generate Mode I SIFs for an edge crack propagating in a symmetrically loaded two-dimensional body. The CToWF concept has demonstrated, by verification with published solutions and Finite Element Analysis (FEA), that the SIF weight function for a new cracked body can be evaluated by isolating and combining appropriate constituent geometries. Being a unique property of crack and component geometry, the newly determined weight function enables rapid generation of SIFs for the same cracked component under different stress systems. Over two thousand Finite Element (FE) models were analysed to provide constituent geometrical configurations and to validate the SIFs calculated from the CToWF model where published solutions were not available. These are Mode I SIFs for edge cracks emanating from two-dimensional notches i. e. semi-elliptical, U- and V-notches in semi-infinite bodies along with their associated stress distributions. Hence, a comprehensive database has been established. Using the versatile composition model with the database, a large number of new SIF solutions for edge cracks from equivalent notches in finite bodies have been obtained. This `Library' of geometric influences, which are presented as weight function coefficients in tabular form, can now be composed by the CToWF approach to generate SIFs for modelling crack propagation through residual stress fields and other complex stress systems. In general, this universal approach, which is easy-to-implement yet maintaining high accuracy, has tremendous potential in allowing rapid assessment of defects prone to linear elastic fracture behaviour via the evaluation of SIFs. Further work to enhance the understanding of this novel concept is proposed to develop a broader practical use in real engineering applications.
34

Study of effective methods of characterisation of magnetostriction and its fundamental effect on transformer core noise

Tabrizi, Shervin January 2013 (has links)
Magnetostriction of core laminations is one of the main sources of transformer acoustic noise. The magnetostriction of grain oriented silicon steel is extremely sensitive to applied compressive stress. A measurement system using piezoelectric accelerometers has been designed and built. This was optimized for magnetostriction measurements under stress within the range of 10 MPa to -10 MPa on large as-cut sheets. This system was used for characterization of wide range of grain-oriented grades. Laboratories around the world are using many different methods of measurement of the magnetostrictive properties of electrical steel. In response to this level of interest, an international round robin exercise on magnetostriction measurement has been carried out and eight different magnetostriction-measuring systems have been compared. Results show a reasonable correlation between the different methods. In this study the influence of factors such as the domain refinement process, curvature, and geometry on the magnetostriction of 3% grain oriented silicon steel were investigated. The study shows that both laser scribing and mechanical scribing have a similar effect on the sample’s domain structure and would cause an increase in magnetostriction. A proposed domain model was used successfully to estimate the effect of scribing on magnetostriction. Correlation between magnetostriction of 3% grain oriented silicon steel with transformer vibration was investigated. It was shown that increasing the clamping pressure to 4Nm can decrease the out of plane vibration in the joint regions due to the increase of friction and reduction of air gap which reduces the air gap flux and consequently the Maxwell forces. Also it has been shown that the primary source for the differences between the vibration of the cores under the same magnetic excitation and clamping pressure in the measured cores is due to the differences in the magnetostriction characteristics of the grades of electrical steels. Correlations between the magnetostriction harmonics and the vibration of the cores have been determined.
35

Hydrogen embrittlement of duplex stainless steel

Hutchings, D. January 1994 (has links)
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.
36

Nanoscale strain characterisation of modern microelectronic devices

Sanderson, Lisa January 2012 (has links)
Sources of stress and strain in modern microelectronics can be either beneficial to the electrical performance or detrimental to the mechanical integrity and ultimately lifetime of the device. Strain engineering is commonplace in state-of-the-art device fabrication as a means to boost performance in the face of device scaling limitation. The strain present in the device is directly related to the improvement factor and as such precise measurements and good understanding are of utmost importance due to the many thermal processing steps that can induce or cause relaxation of the strain. Front-end-of-line (FEOL) strain characterisation is becoming increasingly challenging due to the small volumes of material and nanoscale feature sizes being analysed. In this work, an extensive survey of strain characterisation techniques was undertaken. Narrow sSOI stripes were profiled using conventional Raman spectroscopy. Unlike with previous studies, it was shown that it is possible to achieve nanoscale measurements using current techniques. This study was supported by ANSYS FE simulation. The review of the literature briefly investigates the possibility of EBSD as a strain measurement tool. It is possible to calculate not just an absolute strain value as achievable with Raman spectroscopy, but the strain tensor. However, this is a difficult and complex process and not necessary for use in industry. This study proposes the possibility of a more simple method that would provide a good calibration technique to confirm Raman measurements. SERS and TERS are explored in detail as the most promising techniques when dealing with device scaling. Currently, SERS is a destructive technique not suitable for use in a highly cost driven industry such as semiconductor manufacturing. While it theoretically gives improved surface selectivity over conventional Raman spectroscopy, there is no improvement to the xy spatial resolution. With Si and SiGe samples, this study concludes there is also often no surface selectivity with either technique and the mechanisms behind the enhancement are not understood to the point of being able to implement the techniques in a process line. However, where a non-destructive technique is desired, outlined in this study is a method of achieving the SERS effect without sacrificing the sample. Aggressive scaling has forced the dimensions of the interconnecting wires that give the devices functionality to the deep submicron range. Copper, Cu has been introduced as a replacement to the traditionally used aluminium, Al because of its superior electrical and mechanical properties and scalability. However, as these wires begin to approach the dimensions of thin foils, the microtexture of the wires becomes significantly different from their bulk counterparts. This can affect the mechanical integrity of the interconnects and this has an impact on the reliability of the device. Failure mechanisms such as blistering, cracking and peeling caused by stress and strain are not uncommon and traditional methods of characterising residual stress in the thin films is no longer applicable to these narrow wires. The mechanical properties and microtexture of thin copper films annealed at temperatures comparative to those found in device manufacturing were characterised in some detail. EBSD was used to determine the grain size and structure of the films before nanoindentation confirmed properties such as hardness and elastic modulus. These results pave the way for investigation of strain applied along deep-submicron interconnects to lead to further understanding of what causes failure mechanisms from interconnecting wires.
37

Geometrical effects in the impact response of composite structures

Yang, Fanjing January 2010 (has links)
The aim of this research is to investigate geometrical effects in the low velocity impact response of a wide range of composite structures. Attention focuses on understanding the impact response of circular CFRP and GFRP laminates, as well as the behaviour of aluminium honeycomb sandwich beams and determining the influence of target geometry on impact response. The impact contact force histories predicted by the mathematical models are presented and compared to the experimental results. The resulting damage in the structures was also examined by evaluating cross-sections of damaged structures. A series of low velocity impact tests have been carried out on a (0°,90°) glass fibre reinforced epoxy resin in order to investigate the influence of varying key parameters on the damage initiation threshold. Initial tests have confirmed observations made by previous researchers, that is that the impact force required to initiate damage, Peril, varies linearly with h3/1, where t is the target thickness. This relationship has been shown to apply for test temperatures between 23 and 90°C. The experimental evidence suggests that the influence of test temperature on damage initiation is complex, with the initiation force increasing with temperature for thinner laminates and decreasing in thicker panels. It has also been shown that this threshold does not depend on target size, for the range of plate geometries investigated here. A final series of tests to investigate the influence of impactor geometry have shown that Pcrit increases with indentor diameter. The damage initiation threshold was predicted using two previously-published models, one based on the Mode II interlaminar fracture toughness of the composite, GIIe, and the other on the interlaminar shear strength, ILSS. Here, it was shown that the latter can successfully predict the variation of Peril with both target size and impactor geometry. The second part of the study focuses on investigating scaling effects in the low velocity impact response of sandwich structures and plain composite materials using an instrumented drop-weight impact tower. Tests on carbon fibre reinforced epoxy laminates indicated that at energies above the first failure threshold, damage does not obey a simple scaling law, instead, becoming more severe as the scale size is increased. An examination of the damaged samples indicated that fibre damage was greater in the larger samples for a given scaled impact energy. Fibre damage took the form of large cracks extending across the panel in the 0 and 90 degree directions, suggesting that the energy absorbed in this mode of failure should scale with the square of the scale factor, i.e. n2 In contrast, the initial impact energy was scaled according to n3 indicating that there will be an excess of energy in the larger panels for a given scaled impact energy. Impact tests on the sandwich structures resulted in a more localised mode of fracture, involving perforation of the upper skin and localised crushing of the core, when the impact energy was sufficiently high. The resulting load-displacement traces indicated that the larger scale sizes absorbed more energy during impact than their smaller scale sizes. Cross-sections of perforated sandwich structures showed that, for a given scaled impact energy, the type and severity of damage were similar in all panels. This research also presents an experimental study for the normalised low-velocity impact response of composite plates. It is demonstrated that a characterisation diagram that shows the relationship of three non-dimensional parameters with the normalised maximum impact force can be used to fully characterise the response. With the governing non-dimensional parameters obtained experimentally, it is shown that impact tests having the same non-dimensional parameters, have dynamic similarity and the same non-dimensional response. Furthermore, the experiments can be placed in appropriate dynamic regions in which simplified dynamic models can be used to predict the response.
38

Study of Taylor instabilities and turbulence in lubricating films

Short, Michael Geoffrey January 1977 (has links)
The development of the wavy mode of Taylor vortices in the flow between a rotating inner cylinder and a stationary outer cylinder has been investigated by visual observations using a stroboscopic technique and by inner cylinder torque measurements. Radius ratios of 0.874, 0.911, 0.950 and 0.975 have been tested for the concentric and eccentric cylinder cases. The stability of wavy mode flow has been found to decrease with increasing radius ratio and/or increasing eccentricity. Observations of the flow suggested that the wavy mode can be classified into three regimes called, respectively, the primary, transitional and secondary modes. The three regimes were most easily identified using a radius ratio of 0.911 and concentric cylinders. In this case, the onset of the transitional state (so called because of the tendency for the circumferential wave number to change continually even at particular constant speeds) was found to be particularly violent, completely destroying the well defined nature of the primary wavy mode. Rather surprisingly, this quasi-turbulent regime of wavy mode flow was found to give way to the more regular secondary wavy mode as the speed of the inner cylinder was increased. Non-uniqueness is shown to be an important feature of wavy mode flow. Results are presented showing that circumferential wave number changes can often be detected by torque measurements. In particular, the onset of the transitional state results in a sudden reduction in the required driving torque, the magnitude of which is considerably influenced by the stability of the primary wavy mode flow. Friction coefficient characteristics derived from the torque measurement results are used to deduce a general relationship incorporating the effects of the eccentricity and radius ratios.
39

Rheological characterization of suspensions

Bullivant, Susan Anne January 1977 (has links)
The aim of this research is to obtain a meaningful rheological characterization of deflocculated china clay suspensions. It is generally true that in the study of suspensions relatively little successful work has been carried out on the flow properties of highly concentrated suspensions as compared with dilute suspensions; it was decided therefore that the work, presented in this thesis, should be confined to the study of higher concentration suspensions. A survey is given of previous work on the rheological characterization of suspensions and the reasons for choosing the clay suspensions in particular, are discussed. Since a knowledge of the microscopic nature of the particles in suspension is important for the understanding of the macroscopic behaviour of the suspensions, a detailed account of the relevant aspects of clay and its rheological behaviour is presented. The investigation consists of a theoretical and experimental study of the suspensions. The experimental results are obtained by using a commercial rheometer, the Weissenberg Rheogoniometer. Experiments are performed which include steady shear studies, oscillatory shear studies and studies of a combined steady and low-amplitude oscillatory shear flow. A theory is developed for this latter flow situation and expressions for the percentage increase in couple are obtained based on different rheological equations of state. Concentration effects are examined and it is shown that, with increasing concentration, an initial shear thinning region is followed by a shear thickening one. It is also found that marked elastic properties are exhibited by these highly concentrated clay suspensions. Qualitative agreement is obtained between theory and experiment for all suspensions considered and at the highest concentrations it is shown (for the first time) that it is possible to characterize shear thinning and shear thickening properties of a fluid using one equation of state. The experimental results indicate that this programme of work may have important implications for certain industrial nearly viscometric flow situations as well as the whole approach being applicable to other concentrated suspension systems.
40

Characterization of shrinkage effects in micro-injection moulding (µ-IM)

Annicchiarico, Daniele January 2013 (has links)
This thesis characterizes the effects on shrinkage in microinjection moulding. The literature review considers four branches of investigation (material properties, processing parameters, mould design and specimen design). Two research gaps rise from the analysis of the literature review: the absence of a standardized methodology for measuring shrinkage of moulded parts at the micro-scale, and the absence of optimization stage that implements multiple quality criteria. Adequate research routes are set in order to address these gaps. The conventional standard for determining shrinkage at the macro scale is adapted to the micro-scale and this bridges the first gap. The micro-mould replicates the same design of the standard, and a preliminary stage solves some mouldability problems: the implemented mould extended the mouldability range of processing parameters for improving the reliability of results. After the micro-mould validation, the study of shrinkage at the micro-scale considers the influence of five processing parameters: the mould and melt temperature, the holding time and pressure, then the injection pressure. The design of experiment approach identifies the critical parameters that affect moulding, post-moulding and total shrinkage in parallel to and normal to the flow direction within an interval of confidence of 95% for POM and 90% for 316L feedstock. Statistical tools analyse the results, and the trends of critical factors found confirmation in the literature. This methodology at the micro-scale can fill the first gap because it is on purpose designed for the micro-scale. Moreover, the binder of feedstock is a mixture of POM based polymers, and the use of a common platform permits to compare directly the two materials and highlight the influence of powder loading. The optimization stage adopts desirability functions for achieving optimized values that simultaneously fulfil two requests: minimize shrinkage and maximize moulded part mass. The analysis of the literature review shows that few papers adopt multiple quality criteria approach as methodology for optimizing the results, and none consider jointly part mass and shrinkage. The optimized processing parameters allow moulding “optimized specimens”, and results demonstrate that their total shrinkage and part mass achieve the requests. Even if the use of desirability functions produce results thatrepresents a compromise between the requests, the results show that overall shrinkage decreases and part mass increases. This approach demonstrates its reliability and bridges the second gap. The last part of the thesis investigates the 316L feedstock behaviour for filling micro-features parallel to and normal to the flow oriented. The moulded features are investigated for studying the replication quality and the effect of the orientation of channels with dimension close to the feedstock lower mouldability value. These informations are available in the literature only for polymers, and the contribution of this part of thesis is to fill this gap by analysing a feedstock. The statistical approach permits to identify the critical factors that affect the feature replication quality. Optical investigations allow to identify the 316L feedstock lower mouldability value and to observe the influence of the orientation of features with dimensions near the lower limit.

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