Type Of Coarsening Research Articles

Estimation in discrete time coarsened multivariate longitudinal models.

We consider the analysis of longitudinal data of multiple types of events where some of the events are observed on a coarser level (e.g. grouped) at some time points during the follow-up, for example, when certain events, such as disease progression, are only observable during parts of follow-up for some subjects, causing gaps in the data, or when the time of death is observed but the cause of death is unknown. In this case, there is missing data in key characteristics of the event history such as onset, time in state, and number of events. We derive the likelihood function, score and observed information under independent and non-informative coarsening, and conduct a simulation study where we compare bias, empirical standard errors, and confidence interval coverage of estimators based on direct maximum likelihood, Monte Carlo Expectation Maximisation, ignoring the coarsening thus acting as if no event occurred, and artificial right censoring at the first time of coarsening. Longitudinal data on drug prescriptions and survival in men receiving palliative treatment for prostate cancer is used to estimate the parameters of one of the data-generating models. We demonstrate that the performance depends on several factors, including sample size and type of coarsening.

Design-based estimation in environmental surveys with positional errors

The recent increased availability of information about the micro-geographic positions of population units in environmental surveys has led to important developments in spatial sampling methodologies and, as a result, has improved the estimation accuracy. In real data, however, information about the location of units is often affected by inaccuracy about their exact spatial positions, and these non-sampling errors can affect the estimation procedure. This paper aims to investigate the effects of positional errors on total estimation through a Monte-Carlo simulation study based on real populations of trees. Starting from perfect positioning, we examine two typical types of coarsening that frequently impact two different species of trees. The simulation results show that the exploitation of spatial information to estimate population totals continues to be relevant in the context of environmental surveys, even in the presence of inaccuracies.

Effects of Grain Refinement by ECAP on the Deformation Resistance of Al Interpreted in Terms of Boundary-Mediated Processes

Results of a large set of tensile and compressive creep tests on pure Al were reanalyzed for the influence of low- and high-angle grain boundaries on the deformation resistance at the temperature T = 473 K = 0.51Tm where Tm is the melting point. Thermomechanical treatment by equal channel angular pressing followed by heating to T led to strong increase of areal fraction of high-angle boundaries in a structure of subgrains of ≈10−6 m in size, accompanied by significant reduction of subgrain strengthening and of the stress sensitivity of the deformation rate. (Sub)grain strengthening by low-angle boundaries is most effective; the strengthening effect virtually disappears during creep as the boundary spacings coarsen toward their stress-dependent, quasi-stationary size wqs. The same type of coarsening is found for (sub)grain structures with large fraction of high-angle boundaries; in the quasi-stationary state they lead to softening at low and strengthening at high stresses, and a significant increase in tensile fracture strain to values up to 0.8. The results are analogous to former results for Cu and are explained in the same way by the influence of boundaries on storage and recovery of crystal defects and the homogenization of glide.

Coarsening and thermalization properties of a quenched ferromagnetic spin-1 condensate

We examine the dynamics of a quasi-two-dimensional spin-1 condensate in which the quadratic Zeeman energy q is suddenly quenched to a value where the system has a ferromagnetic ground state. There are two distinct types of ferromagnetic phases, i.e. a range of q values where the magnetization prefers to be in the direction of the external field (easy-axis), and a range of q values where it prefers to be transverse to the field (easy-plane). We study the quench dynamics for a variety of q values and show that there is a single dynamical critical exponent to characterize the scale invariant domain growth for each ferromagnetic phase. For both quenches we give simple analytic models that capture the essential scale invariant dynamics, and correctly predict the exponents. Because the order parameter for each phase is different, the natures of the domains and the relevant topological defects in each type of coarsening is also different. To explore these differences we characterize the fractal dimension of the domain walls, and the relationship of polar-core spin vortices to the domains in the easy-plane phase. Finally, we consider how the energy liberated from the quench thermalizes in the easy-axis quench. We show that local equilibrium is established in the spin waves on moderate time scales, but continues to evolve as the domains anneal.

Adsorbate-enhanced transport of metals on metal surfaces: Oxygen and sulfur on coinage metals

Coarsening (i.e., ripening) of single-atom-high, metal homoepitaxial islands provides a useful window on the mechanism and kinetics of mass transport at metal surfaces. This article focuses on this type of coarsening on the surfaces of coinage metals (Cu, Ag, Au), both clean and with an adsorbed chalcogen (O, S) present. For the clean surfaces, three aspects are summarized: (1) the balance between the two major mechanisms—Ostwald ripening (the most commonly anticipated mechanism) and Smoluchowski ripening—and how that balance depends on island size; (2) the nature of the mass transport agents, which are metal adatoms in almost all known cases; and (3) the dependence of the ripening kinetics on surface crystallography. Ripening rates are in the order (110)>(111)>(100), a feature that can be rationalized in terms of the energetics of key processes. This discussion of behavior on the clean surfaces establishes a background for understanding why coarsening can be accelerated by adsorbates. Evidence that O and S accelerate mass transport on Ag, Cu, and Au surfaces is then reviewed. The most detailed information is available for two specific systems, S/Ag (111) and S/Cu(111). Here, metal-chalcogen clusters are clearly responsible for accelerated coarsening. This conclusion rests partly on deductive reasoning, partly on calculations of key energetic quantities for the clusters (compared with quantities for the clean surfaces), and partly on direct experimental observations. In these two systems, it appears that the adsorbate, S, must first decorate—and, in fact, saturate—the edges of metal islands and steps, and then build up at least slightly in coverage on the terraces before acceleration begins. Acceleration can occur at coverages as low as a few thousandths to a few hundredths of a monolayer. Despite the significant recent advances in our understanding of these systems, many open questions remain. Among them is the identification of the agents of mass transport on crystallographically different surfaces e.g., 111, 110, and 100.

Low cycle fatigue of a nickel based superalloy at high temperature: deformation microstructures

The microstructural characteristics of a single crystalline nickel based superalloy (AM1) tested under high temperature fatigue at 950°C are reported. For repeated fatigue ( R ε =0) through a range of cycle numbers N with imposed total strain amplitude Δ ε t, two main types of behaviour are found depending on N and Δ ε t. This allows a map of microstructures versus the number of cycles N and Δ ε t to be constructed. A domain called A presents anisotropic microstructures due either to a partition of the plasticity throughout the γ channels, or to an oriented coarsening of the γ′ precipitates of the so-called type N (rafts perpendicular to the loading axis). The domain called H shows homogeneous deformation microstructures. The coarsening observed in repeated fatigue is accounted for on the basis of a model proposed earlier in the literature by Véron. Alternate fatigue ( R ε =−1) also leads to the same type of coarsening, even for a very low number of cycles ( N=113). The microstructures observed are ascribed to internal stress effects and are related to the possible shearing of the precipitates and to different diffusion processes.

Lithofacies and depositional processes from a Carboniferous lake, Sierra de Narváez, Northwest Argentina

Thick and widespread Late Carboniferous lacustrine successions occur in the Sierra de Narváez area, northwest Argentina. A detailed lithofacies analysis forms the basis for the definition of fourteen lithofacies clustered into three assemblages. The delta assemblage consists of channelized cobble to pebble conglomerates, quartz-rich sandstones, and carbonaceous mudstones. This assemblage is present at the top of the lake succession and records the progradation of a short-headed stream delta. Wave- and current-rippled sandstones, and a heterolithic facies make up the shallow-lake assemblage. The nearshore area was probably composed of small bars, shoreline sand flats and low-energy sand-mud flats. The deep-lake assemblage is represented by two types of coarsening- and thickening-upward lithofacies sequences. The first type consists, from bottom to top, of laminated mudstones, thin-bedded turbidites, classic turbidites and massive sandstones. Each sequence is thought to represent turbidite lobe progradation in a basinal setting. The second type of sequence consists, from base to top, of underflow current sandstones and mudstones, thin-bedded and classic turbidites, deformed sandstones and hummocky cross-stratified sandstones. Each sequence records progradation of turbidite lobes on the delta front. The analyzed lithofacies record deposition in a hydrologically open lake. Final lake infill was related to the progradation of a deltaic system. It is suggested that four distinct processes controlled sediment transport and deposition in the Carboniferous lake: (1) delta progradation, (2) episodic sediment density surges, (3) river plume dispersion, and (4) wave action. During periods of high discharge, fine to very fine sand and silt bypassed mouth-bar zones and were deposited farther into the lake by underflow currents. Sands supplied by the delta system accumulated in the upper delta front in unstable conditions and were later redeposited by episodic turbidity currents. Wave action was able to rework shallow-water sands supplied by the delta system or emplaced by turbidity currents.