Fracture toughness of a series of high chromium abrasion resistant alloys

The fracture properties of a series of alloys containing 15% chromium and 0.8 to 3.4% carbon are investigated using strain fracture toughness testing techniques. The object of the work is to apply a quantitative method of measuring toughness to abrasion resistant materials, which have previously been assessed on an empirical basis; and to examine the relationship between microstructure and K10 in an attempt to improve the toughness of inherently brittle materials. A review of the relevant literature includes discussion of the background to the alloy series under investigation, a survey of the development of fracture mechanics and the emergence of K10 as a toughness parameter. Metallurgical variables such as composition, heat treatment, grain size, and hot working are ???? to relate microstructure to toughness, and fractographic evidence is used to substantiate the findings. The results are applied to a model correlating ductile fracture with plastic strain instability, and the nucleation of voids. Strain induced martensite formation in austenitic structures is analysed in terms of the plastic energy dissipation mechanisms operating at the crack tip. Emphasis is placed on the lower carbon alloys in the series, and a composition put forward to optimise wear resistance and toughness. The properties of established competitive materials are compared to the proposed alloy on a toughness and cost basis.

Optical phase conjugation for dispersion and nonlinearity compensation in a 1600km, 42 Gb/s quasi-lossless system

We experimentally investigate a long-distance, high-bit-rate transmission system which combines optical-phase-conjugation with quasi-lossless amplification. Comparison with a conventional system configuration demonstrates the possibility of obtaining both dispersion compensation and improved nonlinear tolerance using proposed scheme.

Composite materials impact damage detection using neural networks

This thesis considers two basic aspects of impact damage in composite materials, namely damage severity discrimination and impact damage location by using Acoustic Emissions (AE) and Artificial Neural Networks (ANNs). The experimental work embodies a study of such factors as the application of AE as Non-destructive Damage Testing (NDT), and the evaluation of ANNs modelling. ANNs, however, played an important role in modelling implementation. In the first aspect of the study, different impact energies were used to produce different level of damage in two composite materials (T300/914 and T800/5245). The impacts were detected by their acoustic emissions (AE). The AE waveform signals were analysed and modelled using a Back Propagation (BP) neural network model. The Mean Square Error (MSE) from the output was then used as a damage indicator in the damage severity discrimination study. To evaluate the ANN model, a comparison was made of the correlation coefficients of different parameters, such as MSE, AE energy, AE counts, etc. MSE produced an outstanding result based on the best performance of correlation. In the second aspect, a new artificial neural network model was developed to provide impact damage location on a quasi-isotropic composite panel. It was successfully trained to locate impact sites by correlating the relationship between arriving time differences of AE signals at transducers located on the panel and the impact site coordinates. The performance of the ANN model, which was evaluated by calculating the distance deviation between model output and real location coordinates, supports the application of ANN as an impact damage location identifier. In the study, the accuracy of location prediction decreased when approaching the central area of the panel. Further investigation indicated that this is due to the small arrival time differences, which defect the performance of ANN prediction. This research suggested increasing the number of processing neurons in the ANNs as a practical solution.

Ionic and molecular diffusion in cementitious materials

The work described in this thesis is an attempt to provide improved understanding of the effects of several factors affecting diffusion in hydrated cement pastes and to aid the prediction of ionic diffusion processes in cement-based materials. Effect of pore structure on diffusion was examined by means of comparative diffusion studies of quaternary ammonium ions with different ionic radii. Diffusivities of these ions in hydrated pastes of ordinary portland cement with or without addition of fly ash were determined by a quasi-steady state technique. The restriction of the pore geometry on diffusion was evaluated from the change of diffusivity in response to the change of ionic radius. The pastes were prepared at three water-cement ratios, 0.35, 0.50 and 0.65. Attempts were made to study the effect of surface charge or the electrochemical double layer at the pore/solution interface on ionic diffusion. An approach was to evaluate the zeta potentials of hydrated cement pastes through streaming potential measurements. Another approach was the comparative studies of the diffusion kinetics of chloride and dissolved oxygen in hydrated pastes of ordinary portland cement with addition of 0 and 20% fly ash. An electrochemical technique for the determination of oxygen diffusivity was also developed. Non-steady state diffusion of sodium potassium, chloride and hydroxyl ions in hydrated ordinary portland cement paste of water-cement ratio 0.5 was studied with the aid of computer-modelling. The kinetics of both diffusion and ionic binding were considered for the characterization of the concentration profiles by Fick's first and second laws. The effect of the electrostatic interactions between ions on the overall diffusion rates was also considered. A general model concerning the prediction of ionic diffusion processes in cement-based materials has been proposed.

Optical phase conjugation for dispersion and nonlinearity compensation in a 1600km, 42 Gb/s quasi-lossless system

We experimentally investigate a long-distance, high-bit-rate transmission system which combines optical-phase-conjugation with quasi-lossless amplification. Comparison with a conventional system configuration demonstrates the possibility of obtaining both dispersion compensation and improved nonlinear tolerance using proposed scheme.

Isothermal fatigue of an aluminide-coated single-crystal superalloy:Part II. Effects of brittle precracking

The effect of brittle coating precracking on the fatigue behavior of a high-activity aluminide-coated single-crystal nickel-base superalloy has been studied using hollow cylindrical specimens at test temperatures of 600 °C, 800 °C, and 1000 °C. Three types of precrack were studied: narrow precracks formed at room temperature, wide precracks formed at room temperature, and narrow precracks formed at elevated temperature. The effect of precracking on fatigue life at 600 °C was found to depend strongly on the type of precrack. No failure was observed for specimens with narrow room-temperature precracks because of crack arrest via an oxidation-induced crack closure mechanism, while the behavior of wide precracks and precracks formed at elevated temperature mirrored the non-precracked behavior. Crack retardation also occurred for narrow room-temperature precracks tested at 800 °C - in this case, fatigue cracks leading to failure initiated in a layer of recrystallized grains on the inside surface of the specimen. A significant reduction in fatigue life at 800 °C relative to non-precracked specimens was observed for wide precracks and elevated temperature precracks. The presence of precracks bypassed the initiation and growth of coating fatigue cracks necessary for failure in non-precracked material. No effect of precracking was observed at 1000 °C.

Synergistic synthesis of quasi-monocrystal CdS nanoboxes with high-energy facets

Hollow nanostructures with a highly oriented lattice structure and active facets are promising for catalytic applications, while their preparation via traditional approaches contains multiple steps and is time and energy consuming. Here, we demonstrate a new one-step strategy involving two complementary reactions which promote each other; it is capable of producing unique hollow nanoparticles. Specifically, we apply synergic cooperation of cation exchange and chemical etching to attack PbS nanosized cubes (NCs) and produce CdS quasi-monocrystal nanoboxes (QMNBs) which possess the smallest dimensions reported so far, a metastable zinc-blende phase, a large specific surface area, and particularly high-energy {100} facets directly visualized by aberration-corrected scanning transmission electron microscopy. These properties in combination allow the nanoboxes to acquire exceptional photocatalytic activities. As an extension of the approach, we use the same strategy to prepare Co9S8 and Cu7.2S4 single-crystal hollow nanooctahedrons (SCHNOs) successfully. Hence, the synergic reaction synthesis strategy exhibits great potential in engineering unique nanostructures with superior properties.