Silicon Nitride has traditionally been used as rolling contact bearing material owing to its superior performance compared to bearing steels. Its successful application as a bearing element has led to the development of Silicon Nitride in other rolling contact applications in the automotive industry and the power industry. However, a major limitation of its wider application is its high material and machining cost, especially the cost associated with the finishing process. In the present study, a low cost sintered and reaction-bonded Silicon Nitride is used to study the surface machining effects on its rolling contact fatigue performance. Studies have been carried out to link the surface strengths of Silicon Nitride derived from half-rod and C-Sphere flexure strength specimens to the rolling contact lives of Silicon Nitride rod and ball specimens. The rolling contact fatigue tests were carried out on ball-on-rod and modified four-ball machines. Three types of surface with coarse, fine and RCF-conventional finishing conditions were examined. Flexure strength tests on half-rod and C-Sphere showed an increasing surface strength from specimens with coarse, fine to RCF-conventionally machined conditions. During rolling contact fatigue tests of as-machined specimens, no failures were observed on either ball-on-rod or four-ball tests after 100 million stress cycles. However, a trend of decreasing wear volumes was measured on the contact path of rods and balls with coarse, fine and RCF-conventional conditions. In four-ball tests, spall failures were observed on pre-cracked specimens. There was a trend of increasing rolling contact fatigue lifetime from pre-cracked specimens with coarse, fine to RCF-conventional machining conditions. The study of Silicon Nitride machining was also carried out using an eccentric lapping machine to investigate the effect of eccentricity on the finishing rate of hot isostatically-pressed and sintered and reaction-bonded Silicon Nitride. The eccentricity had no significant impact on finishing rate as concluded in this study. The effect of lubricant viscosity and chemistry on the rolling contact fatigue performance of Silicon Nitride was also studied. The result is inconclusive.

In this thesis we examine three boundary-value problems combined with the presence of dead-load tractions in respect of transersely isotropic elastic materials. In particular, Chapter 1 mainly consists of existing preliminary remarks on the continuum (phenomenological) approach used here to study the mechanical response of elastic materials under large strains. More specifically, we discuss, always within the continuous framework, basic kinematical concepts, fundamental stress principles as well as balance laws; those also being appropriately specialized for material bodies under the state of equilibrium, i.e. for static problems. Description of the governing constitutive theory for Cauchy elastic isotropic and transversely isotropic solids follows, with reference to which, the notion of a hyperelastic solid is then prescribed. Further, the necessary connections with the classical linear theory of transversely isotropic solids are generated and finally some typical constitutive inequalities are summaraized. Then, in Chapter 2, we examine the classical problem of finite bending of a rectangular block of elastic material into a sector of a circular cylindrical tube in respect of compressible transversely isotropic elastic materials. More specifically, we consider the possible existence of isochoric solutions. In contrast to the corresponding problem for isotropic materials, for which such solutions do not exist for a compressible material [38], we determine conditions on the form of the strain-energy function for which isochoric solutions are possible. Based on those conditions, some general forms of strain-energy functions that admit isochoric bending are derived. We also, for the considered geometry and deformation, examine aspects of stability predicated on the notion of strong ellipticity. Expressly, for plane strain, we provide necessary and sufficient conditions for strong ellipticity to hold. The material incorporated in the chapter has been accepted for publication in [42]. In Chapter 3 we study the problem of (plane strain) azimuthal shear of a circular cylindrical tube of incompressible transversely isotropic elastic material subject to finite deformation. The preferred direction associated with the transverse isotropy lies in the planes normal to the tube axis and is disposed so as to preserve the cylindrical symmetry. For a general form of strain-energy function the considered deformation yields simple expressions for the azimuthal shear stress and the associated strong ellipticity condition in terms of the azimuthal shear strain. These apply for a sense of shear that is either 'with' or 'against' the preferred direction (anti-clockwise and clockwise, respectively), so that material line elements locally in the preferred direction either extend or (at least initially) contract, respectively. For some specific strain-energy functions we then examine local loss of uniqueness of the shear-strain relationship and failure of ellipticity for the case of contraction and the dependence on the geometry of the preferred direction. In particular, for a widely used reinforced neo-Hookean material (see e.g., [77, 63, 62, 47, 48]), we obtain closed-form solutions that determine the domain of strong ellipticity in terms of the relationship between the shear strain and the angle (in general, a function of the radius) between the tangent to the preferred direction and the undeformed radial direction. It is shown, in particular, that as the magnitude of the applied shear stress increases then, after loss of ellipticity, there are two admissible values for the shear strain at certain radial locations. Absoulutely stable deformations involve the lower magnitude value outside a certain radius and the higher magnitude value within this radius. The radius that separates the two values increases with with increasing magnitude of the shear stress. The results are illustrated graphically for two specific forms of energy function. The work of this chapter has been accepted for publication and will appear in [41]. Also, parts of this work have already been presented in SES-Penn State (2006) by the third author. In Chapter 4 we are concerned with circular cylindrical tubes composed of incompressible transversely isotropic elastic material subject to simultaneous finite axial extension, inflation and torsion. Here, a great deal of attention is given to the actual kinematics of the problem. Due to the incompressibility constraint, three independent deformaions quantities associated with each one of the processes comprising the combined deformation are identified. These serve, in essence, to measure stretch in the axial and azimuthal direction of the body as well as the amount of shear in the planes normal to its radial direction and hence they suffice to fully characterize the resulting strain. Analogously to the azimuthal shear problem examined in the previous chapter, the preferred direction associated with the transverse isotropy is distributed in the planes normal to the tube axis and is disposed so as, in any case, to preserve the cylindrical symmetry. For the considered geometry, the material line elements in the preferred direction always contract when axial extension of the tube is applied. Assuming that the body is held fixed in that extended state, inflation of the tube may be responsible for either further contraction (at least in early stages of the process) or relaxation of the preferred direction. In this situation, the sense of shear is of no importance since the torsional aspect of the deformation has no actual impact on the length of line aliments in that direction. The cylindrical polar components of the Cauchy stress tensor are written down by means of a general form of strain-energy function and then a new universal relation applying for the considered geometry and deformation is generated. In the special situation where the preferred direction lies along, in the undeformed configuration, the radial direction of the body, coaxiality between the Cauchy stress and the left stretch tensors is accomplished and the latter constitutive relation, under appropriate specialization, recovers a well known result holding in the corresponding isotropic theory (see, e.g., [32]). Finally, based on the governing equilibrium equations and in conjunction with the kinematics of the problem, we provide general formulas for the applied loads necessary to support the combined deformation. These are found to apply for a wide range of transversely isotropic materials as well as for isotropic materials. Analogous remarks are briefly made with respect to a specific class of cylindrically orthotropic tubes.

Recognising the difficulties encountered in measuring the adsorption of hydrogen at high pressure, a reliable volumetric differential pressure method of high accuracy and good repeatability has been developed for measurement up to ca 100 bar. The apparatus used has two identical limbs, a sample and a blank limb, between which a high accuracy differential pressure cell measures changes in pressure. By simultaneously expanding the two limbs and closely controlling the temperature of the entire system, many of the errors due to expansion of the gas can be avoided. In addition, helium blank measurements are used as a base line correction, which substantially reduces the effects caused by the rapid expansion of gas through a small port. Using this method, the hydrogen storage capacities of relatively small samples (1.0-2.5 g) of a selection of carbon materials have been accurately measured to a conservative limit of detection of 0.05 wt% and an accuracy of +/-0.02 wt%. The accuracy of the apparatus has been proven using lanthanide nickel (LaNi5), which has a known hydrogen storage capacity of 1.5 wt%, as a standard. The method has also been developed in order to analyse samples at elevated temperatures of up to 270 C. This has been demonstrated using lithium nitride (Li3N) compounds. The carbon materials studied include a series of activated carbons, carbon nanofibres (CNF) and carbon nanotubes (CNT). The activated carbons have displayed almost instantaneous hydrogen uptake independent of the degas method used, which indicates that sorption occurs via a physisorption mechanism. The series of powdered activated carbons have displayed direct correlation between the BET surface area and the hydrogen sorption capacity. The largest hydrogen sorption capacity observed for activated carbons was for a chemically activated carbon with a surface area of 3100 m2 g-1, achieving an uptake of 0.6 wt%. The preparation of CNF, grown from ethylene over mixed copper, iron and nickel alloy catalysts, has been extensively investigated. Control of the parameters of preparation has allowed the formation of CNF with surface areas of 10 - 500 m2 g-1, diameters of 100 - 1000 nm, lengths of 1-10s nm, gas conversions of 0-90 % and the formation of herringbone and platelet CNF structures. The CNF studied have been observed to be capable of adsorbing a maximum of 0.5 wt% hydrogen at 100 bar and ambient temperature. Only one of the materials studied was observed to break by a significant amount the trend of surface area vs hydrogen sorption capacity, observed for the activated carbons. This was a single-walled nanotube (SWNT) sample which achieved ca 1.6 wt% after slow carbon dioxide activation at low temperature. This larger sorption is hypothesised to result from the hydrogen slowly diffusing into the SWNT through defects in the structure and between the graphite planes in the CNF.

Many items of medical, aeronautical, electronic and military equipment exposed to corrosive conditions, or required to have extreme performance characteristics, are sealed hermetically into micro packages. Laser beam welded (LBW) Ti6AI4V alloy has been adopted in anti-corrosion micro packages for the impeller of a left ventricular assistance device (LVAD). Thin and narrow welds were required for such medical equipment. A wide variety of laser types can be applied in sealing micro packages, which include traditional lasers (CO2, Diode and Nd: YAG) to the newest laser types (fibre). Compared with other LBW types, continuous wave fibre laser welds are well known for exhibiting narrow weld zones, low distortion, lower heat input and high efficiency. However, in this work significant porosity was found in the continuous wave fibre laser welds due to the high traverse speeds and high associated solidification rates. The largest distortion and melting area was found in the continuous wave diode laser welds due to the high heat generation. A pulsed Nd: YAG welding was suggested as the hermetic laser welding technique for sealing the micro packages, since it is a good-balance between low porosity, less distortion and a narrow weld zone. The microstructures of Ti6AI4V were complex and strongly affected the mechanical properties. These structures include: a' martensite, metastable ß, Widmanstätten, bimodal, lamellar and equiaxed microstructure. Bimodal and Widmanstätten structures exhibit a good-balance between strength and ductility. The microstructure of laser beam welded Ti6AI4V was primarily a' martensite, which showed the lowest ductility but not significantly high strength. A heat treatment at 950°C followed by furnace cooling can transform the microstructure in the weld from a' martensite structure into Widmanstätten structure. The fatigue fracture behaviour of laser beam welded thin sheet Ti6AI4V was examined in this project as the lifetime of the LVAD impeller has been seen to be limited by fatigue cracking. Grain size, phase content, stress ratio and frequency are all affected fatigue fracture behaviour. A transformed Widmanstätten structure in the weld gave the highest fatigue fracture life. A time of 8 hours heat treatment gave the highest fatigue fracture life due to a good-balance between the grain size and phase content; the existence of metastable (3 limits the crack propagation as well. The geometries, distortion and porosity of the welds were variable and depended on the types of the laser used for the welding process. These factors significantly affected the local stress levels during the tensile testing and fatigue testing. FEA was used to understand and evaluate these influences. By simulation in FEA, the maximum stress locations were observed to be strongly dependent on the specifics of the crosssectional geometries of the welds. For the pulsed Nd: YAG weld in the fatigue testing, the FEA-derived modified local stress amplitudes exhibited a increase of about 40% compared to the nominal applied stress.

Cold gas dynamic spraying (CDGS) is a material deposition technique, in which powder particles are accelerated to speeds of between 300-1200 m/s and upon impact deform plastically and adhere. The overall aims of this research project were to understand the bonding behaviour in cold spraying of copper, aluminium and titanium, and to produce corrosion resistance barrier layer of titanium coatings using cold spraying. The mechanism of bonding in cold spraying is still a matter of some debate. In this thesis, copper has been cold sprayed onto aluminium alloy substrates, the surfaces of which had been prepared in a variety of ways. The coating - substrate bonding was assessed via a novel intermetallic growth method along with adhesive pull-off testing. The bond strength has been rationalised in terms of a modified composite strength model, with two operative bonding mechanisms, namely (i) metallurgical bonding and (ii) mechanical interlocking of substrate material into the coating. In most cases, mechanical interlocking is able to account for a large proportion of the total bond strength, with metallurgical bonding only contributing significantly when the substrate had been polished and annealed prior to spraying. In addition, grit-blasting has been shown to significantly reduce the bond strength compared to other substrate preparation methods. Aluminium has also been cold sprayed onto copper substrates, the mechanical interlocking of substrate material was not observed and the bond strength was relatively low. Titanium particles have been deposited onto three different steel substrates, namely low carbon steel, an Armco iron, and an austenitic stainless steel. Using the novel intermetallic growth method it was found that a barrier does exist at the interface of the titanium deposited onto the low carbon steel and Armco iron substrates which is not removed in either of the stages of impact or during the heat treatment process. On the other hand, in the case of titanium deposited onto the austenitic stainless steel, the barrier is removed. Cold spraying is believed to have the potential for the deposition of corrosion resistant barrier coatings. However, to be effective, a barrier coating must not have interconnected porosity. Titanium coatings were sprayed using nitrogen as an accelerant gas at two process gas temperatures of 600 and 800˚C to reduce porosity. A modified in-situ grit blasting was used to improve the coating-substrate adhesion. The mean bond strength of the titanium deposits was ~70 MPa and tensile strength was 250 MPa. Mercury intrusion porosimetry (MIP) was used to characterize the interconnected porosity over a size range of micrometers to nanometers. The MIP results showed that in cold sprayed deposits a significant proportion of the porosity was sub-micron and so could not be reliably measured by optical microscope based image analysis. A set of free standing deposits was also vacuum heat treated to further decrease porosity levels. The effect of porosity on the corrosion behaviour of titanium coatings onto carbon steels was investigated in 3.5 wt.% NaCl. The electrochemical measurements of the coatings showed significant substrate influence when the interconnected porosity of the coating was 11.3 vol.% but a decreased substrate influence with a porosity level of 5.9 vol.%. Salt spray (fog) tests confirmed these electrochemical findings and showed the formation of corrosion products following 24-h exposure. Laser surface melting (LSM) was used to seal the top ~140 μm of the coating to eliminate any interconnected porosity. The LSM titanium coatings showed no sign of corrosion after 100-h of salt spray tests, and the open circuit potential and passive current density values were similar to those of the bulk titanium.

The central theme of the work undertaken in this thesis involved modelling of diffusion – numerical and microstructural – occurring during high temperature exposure of selected materials and coatings. The materials and coatings and their high temperature treatments included carburization of steel (nonsteady-state diffusion of iron carburized at 950o C after 7.1 hours), a two component Cu-Ni diffusion couple subjected to diffusion anneal at 1054oC for 300 hours, a three component Pt-Ni-Al solid alloy subjected to oxidation and diffusion anneal, Multicomponent Ni -aluminide and Pt-aluminide coatings on MAR M002 subjected to 150 hours of diffusion treatment at temperature 1273K, a Jr and Jr/Pt Low-activity aluminide / MAR M002 system at 1100oC after 100 hours, aluminise coating on low alloy steels at 650oC, Jnnovatial coatings- Ti45Al8Nb coated with Al2Au subjected to air oxidation at 750oC for 1000 hours, and Ti45Al8Nb coated with TiAlCrY subjected to air oxidation at 750oC for 500 hours. Such coatings are being increasingly used to protect materials against high temperature (600-1000oC) degradation by oxidation. The demands for using such coatings have arisen because of the need to increase the efficiency by increasing the operation temperature in many areas of technological applications such as power operation, aero engines, and energy conversion systems and in processing industries. However the one of the major obstacles to use these coatings to prevent high temperature oxidation of materials is the degradation of the coatings due to the coating/substrate interdiffusion. Jnterdiffusion of critical elements from the coating to the substrate will deplete the protective scale forming elements in the coating.

The interaction of microorganisms with glass-reinforced polyester resins(GRP), both under laboratory and simulated operating conditions, has been examined following reports of severl! fungal biodeterioration. Although GRP was not previously associated with substantial microbial growth, small amounts of microbial activity would pose problems for products associated with comestible materials. The microbiology of the raw materials was investigated, two ingredients were supportive to microbial populations whilst five materials were biostatic or inhibitory in their action. Production laminate was not susceptible to microbial deterioration or inhibitory to microbes. Incorporation of zinc stearate, one of the supportive ingredients, at 300% manufacturing level or drastic undercuring produced laminate capable of supporting microbial growth but only after a non-biotic stage of degradation. Study of the long-term population dynamics of cisterns of GRP and competitive materials under conditions simulating in-service conditions, monitoring microbial numbers within the experimental vessels and comparing with the populations of the supply water, suggests that the performance of GRP cisterns is slightly superior to conventional competitive materials. An investigation of the biological performance of GRP cisterns in an isolated area of known microbiological hazard was conducted. Severe biodeterioration had been experienced with Preform GRP articles moulded using different production techniques, but substitution of current GRP articles resulted in no recurrence of the problem. All attempts to establish the fungal isolate responsible for the phenomena in cisterns under controlled conditions failed. Scanning Electron Microscopy of GRP surfaces showed that although differences exist between current and Preform laminates, these could not satisfactorily explain the differences in service behaviour. These results and the results of the British Plastics Federation Expert Working Group interlaboratory study are discussed in relation to the original report of gross fungal biodeterioration and, to the design of future testing programmes for the products of industrial concerns.

It has been observed that post weld heat treatrents designed to reduce or remove residual welding stresses, may cause weld metal eTnbrittlen. ent. In this investigation, the effects of post weld heat treatment on three high strength submerged are weld metals were examined in terms of changes in'mechanical properties, principally fracture toughness. Metallographic work was carried out to determine the cause of ezbrittlement. Post weld heat treatment was performed in the 450 to 650 0C temperature range for times up to 50 hours and embrittlement was meatured in terms of Charpy, COD and J contour integral tests. It was found that each weld metal could be enbrittled and that two distinct forms of embrittlement occurred. The first was due to alloy carbide precipitation which promoted trans-granular cleavage. Vanadium at 0.12% could cause severe embrittlement while molybdenum, at up to 0.6% did not. The second forn of embrittle-ment was classical temper embrittlement which caused low energy decohesion along prior austenite grain boundaries. This was caused by the migration of phosphorus to these boundaries during heat treatment and bulk phosphorus concentraticrs of 0.011% could cause severe erbrittlement. Embrittlement occurred in the 450 to 550 0C temperature range and increased with time and decreasing cooling rate. Marten-sitic microstructures were more susceptible than acicular ferrite. Defect tolcrance calculations based on COD-and J determinations showed that defect tolerance could increase after heat treatment despite-embrittlement although the final defect tolerance was strongly dependent on residual stress levels remaining, after heat treatment. Heat treatment procedures for the three weld metals examined were suggested, along with general guidelines for the post weld heat treatment of other high strength weld metals.

No description available.

Le travail de thèse porte sur l’étude des phénomènes de l'absorption d'une onde de choc par un matériau granulaire, de l’impact balistique dans un matériau granulaire, et de l’impact balistique sur un tissu. Afin d'évaluer l'absorption d'une onde de choc par un matériau granulaire, un dispositif expérimental a été conçu, qui permet d'évaluer à la fois la pression incidente et réfléchie, et la déflexion et l'accélération d'une plaque de référence, en fonction du temps. Le comportement du système étudié est décrit par un modèle masse-ressort-amortisseur à un degré de liberté.La performance balistique d'un granulat, soumis à l'impact d'un projectile sphérique, est étudiée sur la base de l'évolution de la vitesse de la bille dans le matériau. L'impact dans le matériau granulaire peut être considéré comme un impact hydrodynamique. Au niveau de l'étude numérique, deux approches sont utilisées pour l'application de la sollicitation sur le matériau granulaire : application directe de la pression de l'onde de choc, et propagation de l'onde dans un domaine Eulérien. De même, pour le granulat, plusieurs modèles de comportement sont étudiés. Dans le modèle numérique, le matériau granulaire est modélisé par un matériau équivalent homogénéisé. Un protocole expérimental spécifique a été développé en vue de la mesure continue de la vitesse d'un projectile lors de l'impact sur un tissu. La confrontation entre les résultats expérimentaux et un modèle analytique sur la base du principe de la conservation de quantité de mouvement, démontre que le modèle analytique simple est capable de décrire l'évolution de la vitesse pendant la première phase après l'impact. / This thesis deals with the absorption of shock wave energy by a granular material, and with the study of ballistic impact in fabrics and granular materials. For the assessment of shock wave absorption by a granular material, a test set-up was developed. This set-up allows the evaluation of incident and reflected pressure, and deflection and acceleration of a reference plate as a function of time. An analytical mass-spring-dashpot model with one degree of freedom can predict the deflection correctly. The ballistic performance of a granular material, impacted by a spherical projectile, is evaluated in terms of velocity decay through the material. The experimental results indicate that the impact can be considered as a hydrodynamic impact.In the numerical study, two approaches are used to apply the pressure on the granular material: direct application of the pressure on the sample, and propagation of the shock wave through an Eulerian domain. For the granular material, considered as homogeneous and isotropic, different models have been analysed. A new test set-up was developed for the continuous measurement of the projectile velocity during the penetration in ballistic fabric. The experimental results are compared to an analytical model, based on the conservation of momentum. It appears that the very simple analytical model describes the velocity evolution very well during the first phase after impact.