Hormone replacement therapy

Martha Hickey, Jane Elliott, Sonia Louise Daviso

Statistical Modeling of Spatial Extremes

The areal modeling of the extremes of a natural process such as rainfall or temperature is important in environmental statistics; for example, understanding extreme areal rainfall is crucial in flood protection. This article reviews recent progress in the statistical modeling of spatial extremes, starting with sketches of the necessary elements of extreme value statistics and geostatistics. The main types of statistical models thus far proposed, based on latent variables, on copulas and on spatial max-stable processes, are described and then are compared by application to a data set on rainfall in Switzerland. Whereas latent variable modeling allows a better fit to marginal distributions, it fits the joint distributions of extremes poorly, so appropriately-chosen copula or max-stable models seem essential for successful spatial modeling of extremes.Comment: Published in at http://dx.doi.org/10.1214/11-STS376 the Statistical Science (http://www.imstat.org/sts/) by the Institute of Mathematical Statistics (http://www.imstat.org

Rejoinder to "Statistical Modeling of Spatial Extremes"

Rejoinder to "Statistical Modeling of Spatial Extremes" by A. C. Davison, S. A. Padoan and M. Ribatet [arXiv:1208.3378].Comment: Published in at http://dx.doi.org/10.1214/12-STS376REJ the Statistical Science (http://www.imstat.org/sts/) by the Institute of Mathematical Statistics (http://www.imstat.org

Post-Impact Thermal Evolution of Porous Planetesimals

Impacts between planetesimals have largely been ruled out as a heat source in the early Solar System, by calculations that show them to be an inefficient heat source and unlikely to cause global heating. However, the long-term, localized thermal effects of impacts on planetesimals have never been fully quantified. Here, we simulate a range of impact scenarios between planetesimals to determine the post-impact thermal histories of the parent bodies, and hence the importance of impact heating in the thermal evolution of planetesimals. We find on a local scale that heating material to petrologic type 6 is achievable for a range of impact velocities and initial porosities, and impact melting is possible in porous material at a velocity of > 4 km/s. Burial of heated impactor material beneath the impact crater is common, insulating that material and allowing the parent body to retain the heat for extended periods (~ millions of years). Cooling rates at 773 K are typically 1 - 1000 K/Ma, matching a wide range of measurements of metallographic cooling rates from chondritic materials. While the heating presented here is localized to the impact site, multiple impacts over the lifetime of a parent body are likely to have occurred. Moreover, as most meteorite samples are on the centimeter to meter scale, the localized effects of impact heating cannot be ignored.Comment: 38 pages, 9 figures, Revised for Geochimica et Cosmochimica Acta (Sorry, they do not accept LaTeX

Following microscopic motion in a two dimensional glass-forming binary fluid

The dynamics of a binary mixture of large and small discs are studied at temperatures approaching the glass transition using an analysis based on the topology of the Voronoi polygon surrounding each atom. At higher temperatures we find that dynamics is dominated by fluid-like motion that involves particles entering and exiting the nearest-neighbour shells of nearby particles. As the temperature is lowered, the rate of topological moves decreases and motion becomes localised to regions of mixed pentagons and heptagons. In addition we find that in the low temperature state particles may translate significant distances without undergoing changes in their nearest neig hbour shell. These results have implications for dynamical heterogeneities in glass forming liquids.Comment: 12 pages, 7 figure

The pancreatic-polypeptide family of peptides: their role in the brain-gut axis

In this volume of Biomedical Reviews, Rogers and Hermann present an interesting model for the regulation of gastrointestinal function by two peptides, neuropeptide Y (NPY), and peptide YY (PYY), both of which belong to a related group of peptides known as the pancreatic polypeptide (PP)-fold family of peptides. In this review, largely of their own innovative work, they develop the hypothesis that PYY, acting as a humoral (hormonal) agent, may be a major inhibitory factor in the regulation of the upper gastrointestinal tract. In contrast, NPY would appear to be a major excitatory transmitter. How can two structurally homologous peptides produce such divergent actions?Biomedical Reviews 1997; 8: 70-72

The Effect of Direct Laser Deposition Process Parameters on Microstructure and Mechanical Properties of Ti-6Al-2Sn-4Zr-6Mo

Blown powder Direct Laser Deposition (DLD) is a type of Additive Manufacturing (AM) that is of interest to the aerospace industry as a method of performing high-integrity repairs of critical components. The properties of the deposited material are largely influenced by process parameters such as beam power, velocity, hatch spacing, beam radius and powder feed rate. It is critical for a high-quality repair, that the effect of these process parameters on the solidification microstructure and hence the mechanical properties are fully understood. The work presented here focuses on quantifying the effect of process parameters on DLD of the α+β titanium alloy Ti-6Al-2Sn-4Zr-6Mo (Ti-6246). This alloy demonstrates high strength and good corrosion resistance and is a suitable replacement for Ti-6Al-4V in aerospace applications. This is due to its ability to perform at higher temperatures which is important as gas turbine engines push towards higher efficiencies and hence elevated operating temperatures. A Design of Experiment (DoE) was used to map a potential process window that would be suitable for Ti-6246 DLD repair of compressor bladed disks (Blisks). The aim was to identify combinations of process parameters that resulted in a fully-dense defect-free build that produced repeatable mechanical properties comparable to the parent Ti-6246 blisk material. Ten deposits were built with five different parameter sets using an RPM 557 laser deposition machine. Tensile specimens were machined from the build for uniaxial tensile testing. Small sections of each build were also retained for microstructural analysis, with the aim to correlate process parameters with the size of the resultant α+β lamellar microstructure. The α-lath width was found to generally increase with decreasing line energy density (beam power divided by velocity), although the effects of additional process parameters such as powder feed rate is also important and the influence of this is also explored. The results from this work were used to determine response surfaces relating process inputs such as energy density to process outputs such as 0.2% yield stress. These were then used to provide recommendations for future work with the aim of optimizing the DLD process window for Ti-6246 as a suitable repair method. The experimental work was supported by the development of a thermal model. This helped to inform how process parameters influenced the laser deposition conditions. The thermal model was calibrated against a thin-wall aerofoil-type build and reasonable agreement was found between predicted and measured melt depths for a range of process parameters. The thermal model also can help to provide predictions about the how further optimisation of the process window may affect mechanical properties. Some of the key findings and outcomes of this work are: • Development of an automated process to measure the size of Ti-6246 α+β lamellar microstructure produced by DLD. This automated process was validated using manual measurement techniques and was found to be a robust and trustworthy method that significantly decreases the time to gather microstructural data. • Size of the α-laths were generally found to be <1µm, apart from a dendritic zone at the top of each of the builds which has remained fine due to lack of coarsening from repeated thermal cycles. • Definition of a process window for the DLD of Ti-6246 which can produce dense builds with minimal defects (as revealed by both SEM and XCT analysis). • Testing of Ti-6246 DLD builds showed mechanical properties (tensile strength, 0.2% yield stress and elongation) comparable to parent forged material and within requirements set by Rolls-Royce for repair purposes. • Linear regression and response surface analysis showed that laser beam velocity (v) had the most effect on mechanical properties, particularly the 0.2% yield stress. Hatch spacing had little to no quantifiable effect on the mechanical properties. • Recommendations for process optimisation and productivity gains include increasing the hatch spacing and/or beam velocity to increase productivity. • Development of a Gaussian-based thermal model used to define a new parameter – melt pool saturation level (MPSL), this being the ratio between melting capacity of the laser and the actual amount of material being melted during the DLD process. • The MPSL was used to calculate an upper limit to the PFR and DLD process inputs were used to define a lower limit or “aspirational” PFR. Hence, the model developed in this work is useful in an industrial setting as it can reduce the number of test deposits needed to down-select the best process parameters and therefore define a suitable process window