Analysis of the diametral compression method for determining the tensile strength of transparent magnesium aluminate spinel

Attempts were made to determine the inherent tensile strength of a coarse-grained, hot-pressed magnesium aluminate spinel (MgAl₂O₄) using the diametral compression test. Thick (9.6 mm) disk specimens were machined from a large (356 mm square) plate of spinel. Two pairs of tungsten carbide (WC) platens, one with flat surfaces and the other with a 20° half-arc and radius matched to the disk diameter, were used to transfer the applied load. Specimens tested using the platens with the matched radius had strength values almost 50% higher than those tested using flat platens. Images of the fracture process captured using a high-speed camera showed that irrespective of the type of platens used, fracture consistently initiated at the loading interface, resulting in an invalid test. These results show that the diametral compression test method is not appropriate for determining the tensile strength of this spinel and it raises concerns about the applicability of the method for any advanced ceramic.     

Subsampling weakly dependent time series and application to extremes

This paper provides extensions of the work on subsampling by Bertail et al. in J. Econ. 120:295–326 (2004) for strongly mixing case to weakly dependent case by application of the results of Doukhan and Louhichi in Stoch. Proc. Appl. 84:313–342 (1999). We investigate properties of smooth and rough subsampling estimators for sampling distributions of converging and extreme statistics when the underlying time series is η- or λ-weakly dependent.     

Effect of Sn-doping on the sintering process of MgB<Subscript>2</Subscript> and its superconductive properties

Bulk (Mg_1.02B₂)_1−x Sn _x samples (x = 0.0, 0.01, 0.03 and 0.05) were synthesized by in situ sintering at 850 °C for one hour. Based on the phase identification and microstructure observation, the Mg₂Sn and Sn impurities are found as the main impurities in Sn-doped samples. According to the magnetization measurements, the low doping level of Sn was observed to have small influence on the grain connectivity, and thus a high critical current density was maintained at low field. However, the values of the critical current density at high field in the Sn-doped samples show a little decrease.     

Texture effect on microyielding of wrought magnesium alloy AZ31

Uniaxial tension tests and elastoplastic self-consistent (EPSC) simulations were conducted to study the effect of texture on the microyielding of highly textured AZ31 plate. The onset of microyielding is always associated with the activation of basal 〈a〉 slip irrespective of starting texture, while the value of the microyield stress does depend on the texture. The macroscopic yield stress is largely affected by the critical resolved shear stress of the deformation mode with the highest Schmid factor. An inverse relation is found between the microyield strength normalized by the macroscopic yield strength and the average Schmid factor of prism 〈a〉 slip, which is useful for a rough estimate of the microyield strength.     

Grain boundary microstructure and fatigue crack growth in Allvac 718Plus superalloy

The correlation between grain boundary microstructure and fatigue crack growth with hold-times was investigated for two conditions of the superalloy Allvac 718Plus; a Standard condition with the recommended distribution of grain boundary phases and a Clean condition with virtually no grain boundary phases. Fatigue testing was performed at 704 °C using 10 Hz cyclic load with intermittent hold-times of 100 s at maximum tensile load. Microstructural characterization and fractography were conducted using scanning- and transmission electron microscopy techniques. Auger electron- and X-ray photoelectron spectroscopy techniques were used for oxide analyses on fracture surfaces. It was found that in the Standard condition crack growth is mostly transgranular for 10 Hz loading and intergranular for hold-times, while for the Clean condition crack growth is intergranular in both load modes. The lower hold-time crack growth rates in the Standard condition are attributed to grain boundary δ-phase precipitates. No effect of δ-phase was observed for 10 Hz cyclic loading crack growth rates. Two different types of oxides and oxide colours were found on the fracture surfaces in the Standard condition and could be correlated to the different loading modes. For cyclic loading a bright thin Cr-enriched oxide was dominate and for hold-times a dark and slightly thicker Nb-enriched oxide was dominant These oxide types could be related to the oxidation of δ-phase and the matrix respectively. The influence of δ-phase precipitates on crack propagation is discussed.     

A perturbation method for guiding center orbit following calculations in the presence of Coulomb collisions

An orbit following guiding center Monte Carlo calculation is presented which employs a general flux coordinate system in an axisymmetric plasma equilibrium. The calculation incorporates collision effects. Furthermore, a perturbation expansion is applied for the guiding center calculations in the presence of a frictional source.     

Generalized derivative and π-derivative for set-valued functions

In this paper we study the generalized derivative and the π-derivative for interval-valued functions. We show the connections between these derivatives. Some illustrative examples and applications to interval differential equations and fuzzy functions are presented.     

Analytical Mode III electromagnetic permeable cracks in magnetoelectroelastic materials

Fracture mechanism is pertinent to minimize the catastrophic failures and optimize the structural design. An exact treatment on the electromagnetic permeable crack problems in a magnetoelectroelastic medium is presented by establishing a Hamiltonian system in terms of the symplectic eigenfunctions. The coefficients of the series are determined from the lateral boundary conditions at the crack surfaces and the outer boundary conditions along the geometric domain. Analytical formulations of intensity factors and distributions of dual variables are obtained by the symplectic expansion method. The various intensity factors are directly related to the first few terms of the non-zero eigenvalue solutions.     

On Homogeneous Nucleation of Dislocation Loops in Nanocrystalline Materials

In this article, we readdress the question of homogeneous nucleation of dislocation loops in the context of nanocrystalline materials. In this case, the commonly adopted assumption of an infinite medium is no longer valid, and image forces on dislocations must be accounted for in the analysis. An additional energy term associated with the presence of finite boundaries may act to promote homogeneous nucleation and growth of dislocation loops. Based on a simplified consideration of a circular dislocation loop in a spherical nanoparticle or nanosized grain in a polycrystal, energy calculations are carried out to estimate the activation energy for homogeneous nucleation of a dislocation loop in such a system. Two different cases are considered: (1) a single nanoparticle and (2) a grain in a polycrystalline nanomaterial. Based on simulations for aluminum, it is shown that this mechanism may be plausible in both cases, albeit only for small particles and grains in the nanometer range.