Damage, contamination and surface treatment of electrical discharge machined materials

Electrical discharge machining (EDM) is a manufacturing process capable of machining electrically conductive materials regardless of their mechanical properties. It finds extensive usage across the aerospace, automotive, medical implant and mould/die industries, and is particularly useful for the micro-machining of precision components with complicated shapes. The surface integrity of materials machined by EDM is typically poor, and reduced service life is often expected as a result of surface properties. For example, reduced fatigue performance can result due to the presence of surface cracks as well as porosity, high surface roughness and tensile residual stress. Increased surface area due to surface cracks, porosity and surface asperities also inhibits corrosion performance. This thesis explores from a fundamental perspective the damage and contamination occurring in the surfaces of materials machined by EDM, and investigates the use of a novel surface modification technique, pulsed electron beam irradiation, to improve the most damaging surface property; surface cracking. A transmission electron microscopy (TEM) study was conducted on the surface of single-crystal silicon, which is a chemically and crystallographically homogenous material. For the first time, porosity, contamination and cracking were observed at a scale not visible to conventional imaging techniques such as SEM and optical imaging. The study suggested that conventional microscopic techniques such as SEM and optical microscopy are not sufficient to characterise recast layers created by EDM, and the properties of materials machined by the process are in fact determined by phenomena occurring at the nano-scale. The mechanism behind the movement of material between electrodes was investigated in this thesis. The flushing process in EDM is used to take machined material away from the machining region, and this material is not expected to reattach to electrode surfaces. Using the observation of single discharges and elemental analysis, the mechanism of attachment was determined to be a two-stage process, whereby material ejected at the end of discharge on-time is resolidified in the discharge gap by a successive discharge, which causes its fusion into the opposite electrode surface. This information is critical to the avoidance, or the deliberate deposition of foreign material on a workpiece. Pulsed electron beam irradiation was demonstrated as a rapid and simple method of repairing surface cracks induced by the EDM process. A 4 µm depth of surface cracks created by EDM of stainless steel could be completely eliminated in a pore-free layer. Only a small section of recast layer remained unaffected. The cathode voltage parameter was identified as key to increasing the depth of the remelted layer in future developments of the process. Roughness was at the same time reduced from 3.06 µm to 0.89 µm Sa value. A predominantly austenitic graded nanostructure with grain size down to 6 nm was characterised using TEM and XRD. Such structures have implications for improved mechanical properties via grain boundary strengthenin

Damage, contamination and surface treatment of electrical discharge machined materials

Electrical discharge machining (EDM) is a manufacturing process capable of machining electrically conductive materials regardless of their mechanical properties. It finds extensive usage across the aerospace, automotive, medical implant and mould/die industries, and is particularly useful for the micro-machining of precision components with complicated shapes. The surface integrity of materials machined by EDM is typically poor, and reduced service life is often expected as a result of surface properties. For example, reduced fatigue performance can result due to the presence of surface cracks as well as porosity, high surface roughness and tensile residual stress. Increased surface area due to surface cracks, porosity and surface asperities also inhibits corrosion performance. This thesis explores from a fundamental perspective the damage and contamination occurring in the surfaces of materials machined by EDM, and investigates the use of a novel surface modification technique, pulsed electron beam irradiation, to improve the most damaging surface property; surface cracking. A transmission electron microscopy (TEM) study was conducted on the surface of single-crystal silicon, which is a chemically and crystallographically homogenous material. For the first time, porosity, contamination and cracking were observed at a scale not visible to conventional imaging techniques such as SEM and optical imaging. The study suggested that conventional microscopic techniques such as SEM and optical microscopy are not sufficient to characterise recast layers created by EDM, and the properties of materials machined by the process are in fact determined by phenomena occurring at the nano-scale. The mechanism behind the movement of material between electrodes was investigated in this thesis. The flushing process in EDM is used to take machined material away from the machining region, and this material is not expected to reattach to electrode surfaces. Using the observation of single discharges and elemental analysis, the mechanism of attachment was determined to be a two-stage process, whereby material ejected at the end of discharge on-time is resolidified in the discharge gap by a successive discharge, which causes its fusion into the opposite electrode surface. This information is critical to the avoidance, or the deliberate deposition of foreign material on a workpiece. Pulsed electron beam irradiation was demonstrated as a rapid and simple method of repairing surface cracks induced by the EDM process. A 4 µm depth of surface cracks created by EDM of stainless steel could be completely eliminated in a pore-free layer. Only a small section of recast layer remained unaffected. The cathode voltage parameter was identified as key to increasing the depth of the remelted layer in future developments of the process. Roughness was at the same time reduced from 3.06 µm to 0.89 µm Sa value. A predominantly austenitic graded nanostructure with grain size down to 6 nm was characterised using TEM and XRD. Such structures have implications for improved mechanical properties via grain boundary strengthenin

The generalized non-conservative model of a 1-planet system - revisited

We study the long-term dynamics of a planetary system composed of a star and a planet. Both bodies are considered as extended, non-spherical, rotating objects. There are no assumptions made on the relative angles between the orbital angular momentum and the spin vectors of the bodies. Thus, we analyze full, spatial model of the planetary system. Both objects are assumed to be deformed due to their own rotations, as well as due to the mutual tidal interactions. The general relativity corrections are considered in terms of the post-Newtonian approximation. Besides the conservative contributions to the perturbing forces, there are also taken into account non-conservative effects, i.e., the dissipation of the mechanical energy. This dissipation is a result of the tidal perturbation on the velocity field in the internal zones with non-zero turbulent viscosity (convective zones). Our main goal is to derive the equations of the orbital motion as well as the equations governing time-evolution of the spin vectors (angular velocities). We derive the Lagrangian equations of the second kind for systems which do not conserve the mechanical energy. Next, the equations of motion are averaged out over all fast angles with respect to time-scales characteristic for conservative perturbations. The final equations of motion are then used to study the dynamics of the non-conservative model over time scales of the order of the age of the star. We analyze the final state of the system as a function of the initial conditions. Equilibria states of the averaged system are finally discussed.Comment: 37 pages, 13 figures, accepted to Celestial Mechanics and Dynamical Astronom

Population Dynamics and Non-Hermitian Localization

We review localization with non-Hermitian time evolution as applied to simple models of population biology with spatially varying growth profiles and convection. Convection leads to a constant imaginary vector potential in the Schroedinger-like operator which appears in linearized growth models. We illustrate the basic ideas by reviewing how convection affects the evolution of a population influenced by a simple square well growth profile. Results from discrete lattice growth models in both one and two dimensions are presented. A set of similarity transformations which lead to exact results for the spectrum and winding numbers of eigenfunctions for random growth rates in one dimension is described in detail. We discuss the influence of boundary conditions, and argue that periodic boundary conditions lead to results which are in fact typical of a broad class of growth problems with convection.Comment: 19 pages, 11 figure

Geochemical analysis of bulk marine sediment by Inductively Coupled Plasma–Atomic Emission Spectroscopy on board the JOIDES Resolution

Geochemical analyses on board the JOIDES Resolution have been enhanced with the addition of a Jobin-Yvon Ultrace inductively coupled plasma-atomic emission spectrometer (ICP-AES) as an upgrade from the previous X-ray fluorescence facility. During Leg 199, we sought to both challenge and utilize the capabilities of the ICP-AES in order to provide an extensive bulk-sediment geochemical database during the cruise. These near real-time analyses were then used to help characterize the recovered sedimentary sequences, calculate mass accumulation rates of the different sedimentary components, and assist with cruise and postcruise sampling requests. The general procedures, sample preparation techniques, and basic protocol for ICP-AES analyses on board ship are outlined by Murray et al. (2000) in Ocean Drilling Program Tech Note, 29. We expand on those concepts and offer suggestions for ICP-AES methodology, calibration by standard reference materials, data reduction procedures, and challenges that are specific to the analysis of bulk-sediment samples. During Leg 199, we employed an extensive bulk-sediment analytical program of ~600 samples of varying lithologies, thereby providing several opportunities for refinement of techniques. We also discuss some difficulties and challenges that were faced and suggest how to alleviate such occurrences for sedimentary chemical analyses during future legs

Signature of small rings in the Raman spectra of normal and compressed amorphous silica: A combined classical and ab initio study

We calculate the parallel (VV) and perpendicular (VH) polarized Raman spectra of amorphous silica. Model SiO2 glasses, uncompressed and compressed, were generated by a combination of classical and ab initio molecular-dynamics simulations and their dynamical matrices were computed within the framework of the density functional theory. The Raman scattering intensities were determined using the bond-polarizability model and a good agreement with experimental spectra was found. We confirm that the modes associated to the fourfold and threefold rings produce most of the Raman intensity of the D1 and D2 peaks, respectively, in the VV Raman spectra. Modifications of the Raman spectra upon compression are found to be in agreement with experimental data. We show that the modes associated to the fourfold rings still exist upon compression but do not produce a strong Raman intensity, whereas the ones associated to the threefold rings do. This result strongly suggests that the area under the D1 and D2 peaks is not directly proportional to the concentration of small rings in amorphous SiO2.Comment: 21 pages, 8 figures. Phys. Rev. B, in pres

Self-optimization, community stability, and fluctuations in two individual-based models of biological coevolution

We compare and contrast the long-time dynamical properties of two individual-based models of biological coevolution. Selection occurs via multispecies, stochastic population dynamics with reproduction probabilities that depend nonlinearly on the population densities of all species resident in the community. New species are introduced through mutation. Both models are amenable to exact linear stability analysis, and we compare the analytic results with large-scale kinetic Monte Carlo simulations, obtaining the population size as a function of an average interspecies interaction strength. Over time, the models self-optimize through mutation and selection to approximately maximize a community fitness function, subject only to constraints internal to the particular model. If the interspecies interactions are randomly distributed on an interval including positive values, the system evolves toward self-sustaining, mutualistic communities. In contrast, for the predator-prey case the matrix of interactions is antisymmetric, and a nonzero population size must be sustained by an external resource. Time series of the diversity and population size for both models show approximate 1/f noise and power-law distributions for the lifetimes of communities and species. For the mutualistic model, these two lifetime distributions have the same exponent, while their exponents are different for the predator-prey model. The difference is probably due to greater resilience toward mass extinctions in the food-web like communities produced by the predator-prey model.Comment: 26 pages, 12 figures. Discussion of early-time dynamics added. J. Math. Biol., in pres

Origin and Evolution of Saturn's Ring System

The origin and long-term evolution of Saturn's rings is still an unsolved problem in modern planetary science. In this chapter we review the current state of our knowledge on this long-standing question for the main rings (A, Cassini Division, B, C), the F Ring, and the diffuse rings (E and G). During the Voyager era, models of evolutionary processes affecting the rings on long time scales (erosion, viscous spreading, accretion, ballistic transport, etc.) had suggested that Saturn's rings are not older than 100 My. In addition, Saturn's large system of diffuse rings has been thought to be the result of material loss from one or more of Saturn's satellites. In the Cassini era, high spatial and spectral resolution data have allowed progress to be made on some of these questions. Discoveries such as the ''propellers'' in the A ring, the shape of ring-embedded moonlets, the clumps in the F Ring, and Enceladus' plume provide new constraints on evolutionary processes in Saturn's rings. At the same time, advances in numerical simulations over the last 20 years have opened the way to realistic models of the rings's fine scale structure, and progress in our understanding of the formation of the Solar System provides a better-defined historical context in which to understand ring formation. All these elements have important implications for the origin and long-term evolution of Saturn's rings. They strengthen the idea that Saturn's rings are very dynamical and rapidly evolving, while new arguments suggest that the rings could be older than previously believed, provided that they are regularly renewed. Key evolutionary processes, timescales and possible scenarios for the rings's origin are reviewed in the light of tComment: Chapter 17 of the book ''Saturn After Cassini-Huygens'' Saturn from Cassini-Huygens, Dougherty, M.K.; Esposito, L.W.; Krimigis, S.M. (Ed.) (2009) 537-57

Skin microbiome prior to development of atopic dermatitis:early colonization with commensal staphylococci at 2 months is associated with a lower risk of atopic dermatitis at 1 year

Background: Disease flares of established atopic dermatitis (AD) are generally associated with a low-diversity skin microbiota and Staphylococcus aureus dominance. The temporal transition of the skin microbiome between early infancy and the dysbiosis of established AD is unknown. Methods: We randomly selected 50 children from the Cork Babies After SCOPE: Evaluating the Longitudinal Impact Using Neurological and Nutritional Endpoints (BASELINE) longitudinal birth cohort for microbiome sampling at 3 points in the first 6 months of life at 4 skin sites relevant to AD: the antecubital and popliteal fossae, nasal tip, and cheek. We identified 10 infants with AD and compared them with 10 randomly selected control infants with no AD. We performed bacterial 16S ribosomal RNA sequencing and analysis directly from clinical samples. Results: Bacterial community structures and diversity shifted over time, suggesting that age strongly affects the skin microbiome in infants. Unlike established AD, these patients with infantile AD did not have noticeably dysbiotic communities before or with disease and were not colonized by S aureus. In comparing patients and control subjects, infants who had affected skin at month 12 had statistically significant differences in bacterial communities on the antecubital fossa at month 2 compared with infants who were unaffected at month 12. In particular, commensal staphylococci were significantly less abundant in infants affected at month 12, suggesting that this genus might protect against the later development of AD. Conclusions: This study suggests that 12-month-old infants with AD were not colonized with S aureus before having AD. Additional studies are needed to confirm whether colonization with commensal staphylococci modulates skin immunity and attenuates development of AD