Kinds Of Imperfections Research Articles

Effect of rotation on flow instabilities during solidification of a binary alloy

Linear flow instabilities of the liquid and mushy regions during solidification of a binary alloy is investigated under a high gravity environment where the rotation axis is inclined to the high gravity vector. Results of stability analyses and numerical computations for a centrifugal mode of disturbance at several different rotation rates are determined which provide information for various flow features in the liquid and the mushy layers including neutral stability curves, streamlines and density plots for solid fraction perturbation in the mushy layer. The results indicate two distinct modes of convection whose dependences with respect to rotation rate suggest a procedure for possible elimination of chimneys formation tendency in the mushy layer. These chimneys are vertical channels of zero solid fraction and are kind of imperfections that reduce the quality of the solidified materials.

Coupled power equations in multimode waveguides: Several, random, correlated imperfections

Abstract In this paper we calculate the average power in a multimode fibre influenced by several kinds of imperfections occurring in parallel. The distribution of imperfections along the fibre is modelled as a stochastic process specified on the ensemble of similar fibres. We show how the power transmission is determined not only by the processes themselves but also by correlations occurring among them. The obtained results are a generalization of the wellknown Marcuse's equations (1974, Theory of Dielectric Optical Waveguides (New York: Academic Press)).

School choice and the search for an educational market

This article looks at attempts to create markets in schooling by increasing school choice, making particular reference to a study by the Organisation for Economic Cooperation and Development (OECD) of the impact of choice policies in six countries: Australia, England, the Netherlands, New Zealand, Sweden and the United States. The main policies examined are subsidies to private schools and enhancement of choice between schools within the public sector. The article identifies five kinds of imperfection in school markets in practice: the failure of clear alternatives to present themselves to consumers; the arbitrariness of criteria for choosing; limits to capacity at popular schools; the failure of supply to diversify in response to consumer pressures and the existence of negative externalities. It is argued that these imperfections make it necessary to intervene to maximise the benefits and minimise the risks of school choice.

Dephasing in neutron interferometry

Wigner functions are used to describe various dephasing effects in neutron interferometry which are caused by thickness variations and density fluctuations of the phase shifter and by variations of the beam parameters. It will be shown that separated coherent Schrodinger-cat-like states, which exist when large phase shifts are applied, are extremely fragile and sensitive to any kind of imperfections. The related dephasing factor depends quadratically on the spatial separation of the coherent states which permits the definition of an upper limit of feasible coherent packet separation. The results show that dephasing is an unavoidable effect caused by intrinsic fluctuations inherent to any physical system.

Optimum design with imperfection sensitivity coefficients for limit point loads

A computational method is presented for finding a sequence of optimum designs of a discrete system which exhibits limit point behaviour. Optimality conditions are derived in terms of the theory of imperfection sensitivity coefficients for the limit point load factor. Only those designs of the structures which exhibit limit point behaviour are considered as feasible designs, and the design change is conceived as generating a kind of imperfection. The efficiency of the proposed algorithm will be appreciated particularly for large structures, because incremental nonlinear analysis to find the limit point load factor needs to be carried out only once for the structure of trivial initial optimum design. The sequence of optimum designs is described by piecewise Taylor series expansions with respect to the specified limit point load factor. It is shown in the examples that the proposed method is efficient and of good accuracy for a large space truss.

(0001) quartz surface imperfections

Monocrystalline (0001) quartz surfaces may present two kinds of imperfections: point defects (surface oxygen vacancies) and tridimensional disorder/superficial amorphization). The point defects are studied thanks to Reflection Electron Energy Loss Spectroscopy experiments, which show that electronic transitions (5.1 and 7.2 eV) associated with surface oxygen vacancies appear in the gap of α-quartz under ion or electron bombardment and 900 o C vacuum healing. The defects responsible for these structures are located in the first nm under the surface. X-ray Absorption Near Edge experiments (XANES) at the O K-edge show that the 900 o C vacuum healing leads moreover to the destruction of the surface crystallinity (≃5 nm), as proved by the modification of the multiple scattering structures

Whispering-gallery mirrors: fabrication tolerances and the effects of surface imperfections

Like all short-wavelength optics, whispering-gallery mirrors that are intended for use with soft x rays require a high-quality surface finish. A coupled-mode description of scatter in whispering-gallery mirrors provides a convenient way of understanding the effects of surface imperfections and of establishing fabrication tolerances. Different kinds of imperfection have different effects on mirror performance and can be tolerated to different degrees. Figure errors, for example, are not critical, but periodic toolmarks are potentially disastrous. The acceptable amounts of random waviness and microscopic roughness depend on the correlation lengths involved. Overall, the tolerances are strict but less severe than those for normal-incidence optics.

Microstructures at the interface between an a-axis oriented YBa 2Cu 3O 7− x film and a NdGaO 3 (110) substrate

The interface between an a-axis oriented YBa 2Cu 3O 7− x film and a NdGaO 3 (110) substrate has been investigated cross-section transmission electron microscopy (TEM). The orientational relationship between the a-axis oriented film and substrate is YBa 2Cu 3O 7− x [001]|NdGaO 3 [001]. This preferentially c-axis aligned direction of the Y YBa 2Cu 3O 7− x film would be caused by a very small lattice mismatch (0.1%) between the b(= a) lattice constant of YBa 2Cu 3O 7− x and a 2 + b 2) 1 2 /2 of the pseudo-cubic sub-lattices in NdGaO 3 at a substrate temperature of 750°C. Two kinds of imperfections have been observed in the crystal lattice of YBa 2Cu 3O 7− x near the interface; one is the deviation of YBa 2Cu 3O 7− x [301] from NdGaO 3 The other type is pair dislocations with a positive and negative Burgers vectors in the YBa 2Cu 3O 7− x (103) planes. These two kinds of defects at interface would act to reduce the tensile stress within a distance of about 1.1 nm from the substrate interface.

Microstructures at the Interface Between a-Axis Oriented YBa2Cu3O7−x and NdGaO3(110) Substrate

ABSTRACTThe interface between an a-axis oriented YBa2Cu3O7-x film and a NdGaO3(110) substrate has been investigated by cross-section transmission electron microscopy (TEM). The orientational relationship between the a-axis oriented film and substrate is YBa2Cu3O7-x[001] / NdGaO3[001]. This preferentially c-axis aligned direction of the YBa2Cu3O7-x film would be caused by a very small lattice mismatch (0.1%) between b(=a) lattice constant of YBa2Cu3O7-x and of the pseudo-cubic sub-lattices in NdGaO3 at a substrate temperature of 750°C. Two kinds of imperfections have been observed in the crystal lattice of YBa2Cu3O7-x near the interface; One is the deviation of YBa2Cu3O7-x [301] from NdGaO3 [111]. The other type is pair dislocations with a positive and negative Burgers vectors in the YBa2Cu3O7-x (103) planes. These two kinds of defects at the interface would be act to reduce the tensile stress within a distance of about l.lnm from the substrate interface.

Growth of emerald crystals from PbO · V2O5 flux

The flux growth of emerald crystals by slow cooling in the PbO · V2O5 flux is reported. The crystals exhibited the typical emerald-green colour, were up to 1.8 mm in size and transparent. Their form was a regular hexagonal rod bounded by the well-developed {0001} and {10¯10} faces. The solubility of emerald in PbO · V2O5 was also investigated; about 9.0 g emerald was dissolved in 100 g PbO · V2O5 at 1200 °C. The solubility decreased gradually with decreasing temperature. Taking the solubility data into account, the presence of undissolved particles in the solution was found to be a necessary condition for the growth of large emerald crystals. Unsaturated solution at a soak temperature produced good and relatively small emerald crystals in a subsequent cooling experiment. Advantages of the PbO · V2O5 flux are also mentioned. Emerald crystals showing various kinds of imperfections, and even good crystals, were also present.

Imperfection for realistic image synthesis

AbstractCreating objects with surface imperfections is accomplished through texture specification and generation techniques. Based on fractal subdivision techniques and relatively simple distribution models, a wide class of surface imperfections may be generated, combined and rendered. The surface effects include scratches, splotches, smudges, corrosion, mould, stains and rust. A rule‐based system is used to position the various surface imperfections on the texture map, and a simple natural language interface is used to specify the kinds of imperfections and their generative parameters through adverbs and prepositional phrases. Results along some of the imperfection dimensions are illustrated.

Effect of residual stresses on plastic buckling of cylindrical shell structures

Weld-induced residual stresses in shell structures tend to give earlier onset of plastic yielding and to reduce the critical buckling load. Geometrical imperfections may also have a detrimental effect on the load carrying capacity. The interaction between these two kinds of imperfections is studied here for various longitudinally welded cylindrical shell structures, with focus on short wave local buckling modes. It is found that structures with reduced bifurcation loads due to residual stresses show less sensitivity to geometrical imperfections, and after buckling mode deflections of the order of the shell thickness, the residual stresses play no role at all.

Radiation damage and deuterium trapping in deuterium ion irradiated austenitic stainless steel

The nature of radiation-induced defects and their roles in deuterium trapping in deuterium-ion-irradiated austenitic stainless steel, JPCA2, were studied by the complementary use of transmission electron microscopy (TEM) and thermal desorption spectroscopy (TDS). By irradiation with deuterium ions of keV energy at room temperature, very small dislocation loops and platelet-like deuterium clusters were formed. The comparison of the results of TEM and TDS showed that deuterium atoms trapped by the dislocation loops and by deuterium clusters dissociated at around 220°C and 300°C, respectively. Major trapping sites in JPCA2 are radiation-induced point defect clusters and some kind of imperfection in the matrix. Most of the deuterium atoms trapped by these sites are released at relatively low temperatures (< 180°C).

Analysis on lateral buckling load of beams by multi-component loadcell.

In experimenting with the lateral buckling of beams, there will be no lateral deflection, including torsion, up to the critical load in an ideal case. But actually, owing to various kinds of imperfections, the beam begins so deflect laterally, including torsion, with the beginning of loading. Thus the critical load is difficult to measure directly. In this paper, a modified Southwell's method is proposed to determine the critical load from the test data on the load and the reactive torsional moment at a clamped end of the beam. Experimenting with cantilever beams by using the multi-component loadcell developed by the authors, each critical load determined by the method agrees well with the theoretical value.

Defect characterization in natural diamonds by laser-light-scattering microtomography

Abstract Scanning microtomography using scattered laser light has been employed in a study of defects in small blocks of different types of diamond. Local changes in refractivity caused by various kinds of imperfections could be mapped. In relatively perfect yellow diamonds the scattered-light intensity distribution was related to growth zones, as was verified by stress-birefringence microscopy and cathodoluminescence. Scattered-light images of dislocation lines were also observed. In some diamonds it was found that {100} growth sectors showed a lower scattering intensity than the adjacent {111} sectors. Brown anisotropically scattering diamonds gave an inhomogeneous striated scattered-light distribution, not related to growth zones. A diamond with knots showed local differences in scattering intensity for the misoriented crystal volumes.

PROBABILISTIC STUDY ON CONSTRUCTION ACCURACY OF STEEL BRIDGES

Bacause of the presence of various kinds of imperfections (or errors) introduced during construction processes of bridges, the geometrical shape and stress distribution do not always conform to those intended by designers when completed. Such construction errors represent important quality characteristics of the bridge. The authors present a methodology to evaluate stochastically the effects of imperfections, such as the variation of dead load and stiffness parameters and dimensional imperfections of each member, upon construction accuracy of bridges by applying stochastic finite element method. The effects of cable length adjustment, which is usually taken at the erection of cable systems, on construction accuracy are also taken into account. Through numerical examples, some useful and important informations concerning the quality control of steel bridges are obtained.

Coherent memory functions for finite molecular systems. A direct method of calculation

A new direct method for calculation of coherent memory functions (MFs) entering the Nakajima-Zwanzig (or GME) equation is presented for rigid molecular chains with natural boundary conditions. The method is illustrated on the chains with three and five molecules and various kinds of imperfections. Obtained results imply a necessity of a more careful interpretation of experimental data (e.g. diffusivity).

一般設計過程

Designing process is studied in an axiomatic approach. In the previous paper, the axiomatic approach was found capable of deriving a general design theory which clarifies fundamental principles of human designing activity. In the present paper, the similar method is applied to study dynamic aspects of design, that is designing process. Under the same axioms as the previous paper, the designing process by a real designer is analysed. The real designer is defined as one who deviates from the ideal one by several kinds of imperfections in the memory structure and the thinking process. In case a designer has no imperfections, say an ideal knowledge, it is understood that he reaches a design solution which satisfies the specification given in a mathematical way or algorithmically. A theorem, on the contrary, is deduced which insists that a real designer with some imperfections cannot design unless he is given some specific knowledges which directly link the function to the attribute compensating his imperfections. From the study, four models are proposed by which a real designer can design compensating his imperfections; the catalogue model, the calculation model, the production model and the paradigm model which have correspondences to actual design activities of designers and also have applicability to CAD system constructions.

X-ray diffraction characteristics of curved monochromators for synchrotron radiation

A theoretical study is presented concerning the diffraction characteristics of curved monochromators for X-ray synchrotron radiation used at the laboratories of Hamburg, Orsay and Stanford. The investigation was performed by extending to the X-ray case a simple model recently developed and fruitfully employed to describe the neutron diffraction properties of curved monochromators. Several diffraction patterns were obtained corresponding to different monochromator materials (Ge, Si) used by the different laboratories, for different reflecting planes (111), (220). asymmetry angles, X-ray wavelengths (Mo Kα, Cu Kα, Cr Kα) and curvature radii. The results are discussed in physical terms and their implications on the design of curved monochromators for synchrotron radiation are presented. In particular, the study shows that all the monochromators used in the different laboratories should behave practically as perfect crystals and therefore should have a very low integrated reflectivity corresponding to an optimized wavelength passband Δλ/λ ≃10−4. The gain that can be obtained by increasing the curvature, by introducing a gradient in the lattice spacing or by any other kind of imperfection is quite limited and much lower than the desirable value. The adopted model can help in obtaining a possible moderate gain in intensity by also taking into consideration other parameters, such as crystal material, reflecting plane, asymmetry of the reflection and X-ray wavelength.

Ultimate Strength of Biaxially Loaded H-Columns with initial deflections and eccentricities

For the purpose of finding the effect of biaxial imperfection on column strength, a series of tests on H-columns with initial deflections and eccentricities under compression combined with biaxial bending were carried out. The standard test conditions of total 28 columns are that the end sections are biaxially simply supported with warping constrained and that columns have no residual stresses before tests. Only two of them are specimens with residual stresses and the end sections of another two are simply supported with respect to weak axis bending but clamped about strong axis bending.The elastic plastic behaviors of columns tested were numerically analyzed with use of an incremental stiffeness method. The ultimate strength of columns with imperfections were also calculated from an approximate strength formula with consideration of inelastic stability. Reasonable agreement was found between the observed column strength and both calculated values.Two kinds of imperfection, eccentricity and initial deflection, have so similar effects on column strength that the difference between them seems to be negligible for designers.Such a middle-length column that its elastic buckling load equals to its fully plastic axial load is the most sensitive to imperfections. Therefore, such a column is considered as a critical one in order to determine the allowable values of imperfections.