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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

Effect of Concurrent Partnerships and Sex-Act Rate on Gonorrhea Prevalence

The disease gonorrhea (GC) is a major public health problem in the United States, and the dynamics of the spread of GC through popula tions are complicated and not well understood. Studies have drawn attention to the effect of concurrent sexual partnerships as an influen tial factor for determining disease prevalence. However, little has been done to date to quantify the combined effects of concurrency and within-partnership sex-act rates on the prevalence of GC. This simulation study examines this issue with a simplified model of GC transmission in closed human populations that include concurrent partnerships. Two models of within-partnership sex-act rate are compared; one is a fixed sex-act rate per partnership, and the other is perhaps more realistic in that the rate depends on the number of concurrent partners. After controlling for total number of sex acts, pseudo-equilibrium prevalence is higher with the fixed sex-act rate than under the concurrency-adjusted rate in all the modeled partnership formation conditions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68414/2/10.1177_003754979807100404.pd

Ground-based observations of Saturn’s auroral ionosphere over three days:trends in H3+ temperature, density and emission with Saturn local time and planetary period oscillation

On 19–21 April 2013, the ground-based 10-m W.M. Keck II telescope was used to simultaneously measure View the MathML sourceH3+ emissions from four regions of Saturn’s auroral ionosphere: (1) the northern noon region of the main auroral oval; (2) the northern midnight main oval; (3) the northern polar cap and (4) the southern noon main oval. The View the MathML sourceH3+ emission from these regions was captured in the form of high resolution spectral images as the planet rotated. The results herein contain twenty-three View the MathML sourceH3+ temperatures, column densities and total emissions located in the aforementioned regions – ninety-two data points in total, spread over timescales of both hours and days. Thermospheric temperatures in the spring-time northern main oval are found to be cooler than their autumn-time southern counterparts by tens of K, consistent with the hypothesis that the total thermospheric heating rate is inversely proportional to magnetic field strength. The main oval View the MathML sourceH3+ density and emission is lower at northern midnight than it is at noon, in agreement with a nearby peak in the electron influx in the post-dawn sector and a minimum flux at midnight. Finally, when arranging the northern main oval View the MathML sourceH3+ parameters as a function of the oscillation period seen in Saturn’s magnetic field – the planetary period oscillation (PPO) phase – we see a large peak in View the MathML sourceH3+ density and emission at ∼115° northern phase, with a full-width at half-maximum (FWHM) of ∼44°. This seems to indicate that the influx of electrons associated with the PPO phase at 90° is responsible at least in part for the behavior of all View the MathML sourceH3+ parameters. A combination of the View the MathML sourceH3+ production and loss timescales and the ±10° uncertainty in the location of a given PPO phase are likely, at least in part, to be responsible for the observed peaks in View the MathML sourceH3+ density and emission occurring at a later time than the peak precipitation expected at 90° PPO phase

Physical tests for Random Numbers in Simulations

We propose three physical tests to measure correlations in random numbers used in Monte Carlo simulations. The first test uses autocorrelation times of certain physical quantities when the Ising model is simulated with the Wolff algorithm. The second test is based on random walks, and the third on blocks of n successive numbers. We apply the tests to show that recent errors in high precision simulations using generalized feedback shift register algorithms are due to short range correlations in random number sequences. We also determine the length of these correlations.Comment: 16 pages, Post Script file, HU-TFT-94-