This thesis describes the advances made by the author on the direct modelling of the fatigue growth of planar cracks A versatile step-by-step 3D finite element technique has been developed, which calculates the stress intensity factors at a set of points on the crack front according to linear elastic fracture mechanics principles and then applies a fatigue crack growth la\v to this set of points to obtain a new crack profile The software DUCK developed has a good capability of automatic remeshing so that the fatigue propagation of arbitrary shaped cracks can be conveniently followed The stress intensity factor calculation using the 3D finite element method has been improved and widely verified to be of good accuracy generally, which provides an important guarantee for the fatigue growth prediction. Several major problems associated with the stress intensity factor calculation, such as crack front definition, free surface layer and mesh abutting the crack front, as well as J-integral path independence, have been discussed A variety of cracked geometries of either theoretical or practical significance have been modelled by the numerical technique, including internal defects in infinite solids, surface cracks in finite thickness plates, round bars and pressure vessels, and initially multiple surface small cracks. Many results useful for the fatigue assessment of each kind of crack, such as stress intensity factor variations, fatigue shape changes and fatigue lives, have been obtained. By comparing with existing numerical methods and some limited experimental data found in the literature, it is shown that the numerical simulation technique is reliable and can predict the fatigue propagation of complex crack configurations, which is seldom possible for methods with assuming crack front configurations Meanwhile, due to the highly automated procedure or the technique, it has become straightforward to model the fatigue growth of practical cracked components, as demonstrated by the computations in this thesis. It is believed that such a numerical simulation technique has a great potential and will play an useful role in the area of fatigue study and assessment.

No description available.

No description available.

The bearings in the air motors of modern jet aircraft engines must operate dry in hostile conditions at temperatures up to 500° C, where the thrust races in the actuators operate at temperatures up to 300° C. One of the few metallurgical combinations which can function efficiently under these conditions is martensitic stainless steel on tungsten carbide. The work described was initiated to isolate the wear mechanisms of two such steels in contact with tungsten carbide at temperatures up to 500° C. Experiments were carried out on angular contact bearings similar to these used in service, where both rolling and sliding is present and also for pure sliding conditions using a pin-on-disc apparatus. Wear measurements of the bearings were obtained with wear rates, friction and surface temperatures from the pin-on-disc machine for a series of loads and speeds. Extensive X-ray diffraction analysis was carried out on the wear debris, with also S.E.M. analysis and hardness tests on the worn surfaces along with profilometry measurements of the disc. The oxidational parameters of the steel were obtained from measurements of oxide growth rates by ellipsometry. Three distinct mechanisms of wear were established and the latter two were found to be present in both configurations. These involve an oxidational-abrasive mechanism at loads below 40 N with pin surface temperatures up to about 300 °C, with the mechanism changing to severe wear for higher loads. As the temperature increases a third wear mechanism appears due to transfer of relatively soft oxide films to the steel surface reducing the wear rate. Theoretical K factors were derived and compared with experimental values which were found to be in good agreement for the severe wear mechanism. The pin-on-disc experiments may be useful as a screening test for material selection, without the considerable cost of producing the angular contact bearings.

The cause of the respective rough and smooth fatigue failure surfaces of Neoprene GS : Neoprene W and Neoprene GS : natural rubber vulcanisates is investigated. The contrasting morphology of the vulcanisates is found to be the major factor determining the fatigue behaviour of the blends. Neoprene GS and Neoprene W appear to form homogeneous blends which exhibit physical properties and fatigue failure surfaces intermediate between those of the two horropolymers. Neoprene GS and natural rubber exhibit heterogeneity when blended together. The morphology of these blends is found to influence both the fatigue resistance and failure surface of the vulcanisates. Exceptional uncut and cut initiated fatigue lives are observed for blends having an interconnecting network morphology. The network structure and cross-link density of the elastomers in the blends and the addition of carbon black and antioxidant are all found to influence the fatigue resistance but not the failure mechanism of the vulcanisate.

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. This thesis reports on an experimental research programme into developing a new quantitative method, based on the Stiffness Damage Test (SDT), to be used in assessing fire-damaged concrete. The SDT subjects a concrete core to low-stress quasi-static load cycles. The energy dissipated during the load-cycle together with other characteristics of the stress-strain response are used as assessing tools of the fractured state of damaged concrete. In addition to the SDT, the extent of damage was also monitored by the following test methods: The Ultrasonic Pulse Velocity (UPV) technique The dynamic modulus measurements The crushing strength The full stress-strain response of fire-damaged concrete to failure The petrographic approach including: 0 Scanning Electron Microscopy (SEM) 0 X-Ray Diffraction (XRD) The investigated concrete was heated to various temperatures in the range (ambient - 5000C) to the point when the temperature became uniform and was then exposed to this temperature for various durations. Cooling of the heated concrete was either carried out in air or by means of spraying with tap water for a short duration. Both carbonate and siliceous aggregate concrete were investigated. Maximum aggregate sizes of 10mm and 20mm were used in the siliceous aggregate concrete. The SDT was shown to be a powerful method in quantifying the extent of fire-damage. The energy dissipated in a stress cycle was found to correlate extremely well with the % fired residual stiffness, R 2=0 . 95. Therefore, a new method of classification of fire-damage, based on such correlation is introduced. The currently accepted method of UPV was shown to be unsatisfactory in assessing fire-damaged concrete when moisture content is altered during the fire-fighting operations. The traditional method of assessing fire-damage using the phenomena of colour changes of heated concrete was also shown to underestimate the extent of internal disruption and damage. In this thesis a large body of evidence is presented to support the supposition that the extent of damage is not a sole function of the maximum temperature experienced. The extent of damage was found to be a complex function of: the maximum temperature, the method of cooling, the duration of exposure, and the type and size of aggregate used. Spraying hot concrete with water proved to be a major determinant in the post-firing residual stiffness of concrete.

No description available.

This study addresses a range of phenomena associated with dry frictional contacts. Particular emphasis is devoted to elastic dissimilarity. Problems are formulated such that coupling between normal and tangential tractions is permitted and the results, which must often be found by numerical means, are compared with classical solutions. The manipulation of objects in contact with frictional surfaces is investigated. The main focus is on objects resting upon a plane at three discrete points of support. When displaced by an arbitrary force such objects have a tendency to rotate about these points of support. The motion of both planar and three dimensional objects is predicted and the effect of friction at the pushing contact is considered. Rolling contact between two elastically dissimilar cylinders is addressed. By allowing full coupling between normal and tangential tractions it is found that an offset in the contact patch may result. This often leads to a significant resistive moment which balances the nett power supplied and the frictional losses. Three dimensional contacts form a major part of the study and a numerical method is developed to facilitate their solution. The evolution of an incomplete contact is evaluated to demonstrate how contact boundaries may be determined. The method is then applied to sliding contact between elastically dissimilar spheres. The traction distributions which result from a fully coupled formulation are compared with those predicted by a Hertzian solution. The parameters which cause the greatest differences are found. A Mindlin-type contact is re-evaluated and traction distributions are calculated which are compatible with local slip directions for both elastically similar and dissimilar bodies. A comprehensive set of results is presented revealing how material parameters influence the solution. Interactions between cracks and contacts are analyzed. It is shown that although the change in compliance of a flawed body may modify the traction distribution compared with a flawless one, the stress intensity factors at the crack tip are rarely affected. Finally, a method of determining the fracture toughness of brittle materials is explored based on the observation of surface cracks in the wake of a sliding indenter. Material characteristics are related to crack initiation, spacing and depth.

This thesis is concerned with investigating the surfactant aspects of corrosion inhibition. Corrosion inhibitors are of great industrial importance because they are a relatively inexpensive and easily applicable method of protecting oil pipelines against internal corrosion. The performance of corrosion inhibitors is known to depend upon the prevailing conditions within the pipeline. We have used a range of experimental techniques to study corrosion of steel surfaces, adsorption of corrosion inhibitors and the behaviour of inhibitors in mixtures of oil and water in order to understand more about the factors which affect the performance of corrosion inhibitors under oil field conditions. Firstly, electrochemical and weight-loss measurements were used to measure the corrosion rate of steel immersed in aqueous environments under atmospheric conditions and in the presence of dissolved carbon dioxide and hydrogen sulphide. The measurements were repeated in the presence of dodecylbenzyldimethylammonium chloride and I-aminoethyl-2-(8- heptadecenyl)-2-imidazoline acetate, which are thought of as model corrosion inhibitors. Both surfactants were found to effectively reduce the corrosion rate of steel when the dissolved gas was hydrogen sulphide, but were found to increase the corrosion rate in some aerated and carbonated environments. The optical technique ellipsometry has been used to study the adsorption of dodecylbenzyldimethylammonium chloride at the steel-water interface. We have determined the thickness of the surfactant film adsorbed from aqueous solution in the absence of electrolyte. The results are consistent with the formation of a monolayer at inhibitor concentrations close to the critical micelle concentration and possible multilayer formation at higher concentrations. Determination of inhibitor film thickness has not been possible in the presence of electrolyte and corrosive dissolved gases which cause rapid corrosion of the steel surface. The technique has been employed subsequently to study the initial stages of corrosion in the presence and absence of dodecylbenzyldimethylammonium chloride. Scanning electron microscopy and elemental analysis have also been used to examine the changes which occur to the steel surface during corrosion. These results show that the optical properties of the steel surface undergo changes as the steel is attacked by corrosive aqueous solutions and that dodecylbenzyldimethylammonium chloride can increase the rate of these changes or completely suppress them, depending on the nature of the dissolved gas present. Finally, we have investigated the phase behaviour of alkylbenzyldimethylammonium chloride and 1-aminoethyl-2-(8-heptadecenyl)-2- imidazoline acetate using equilibrium partitioning and emulsion phase inversion methods. The results show that variations in common oil field variables, such as temperature, electrolyte concentration and oil type, can lead to the consumption of the aggregated form of water-soluble corrosion inhibitors by partitioning to the oil phase. The results are discussed in terms of the effect of these variables upon the effective geometry of the inhibitor within the monolayer of the aggregates. We have also performed electrochemical corrosion rate measurements in systems containing oil and water, under conditions which cause virtually all of the inhibitor to exist in the oil phase. The results suggest that partitioning of the aggregated form of corrosion inhibitors from water to oil does not adversely affect corrosion inhibition in the aqueous phase.