Showing papers by "Polytechnic University of Turin published in 1991"
TL;DR: In this article, the main results of this theory are reviewed, with special attention to the most recent advances obtained in the case of componentwise bounds, where the uncertainty is described by an additive noise which is known only to have given integral (typically l 1 or l 2) or componentwise (l ∞) bounds.
708 citations
TL;DR: The interaction and the competition of a set of transverse cavity modes, which belong to a frequency-degenerate family, are analyzed and the predicted phase singularities in each pattern agree in detail with those found by theory.
Abstract: We analyze the interaction and the competition of a set of transverse cavity modes, which belong to a frequency-degenerate family. The laser turns out to be able to realize several different stationary spatial patterns, which differ in the transverse configuration of the intensity or of the field and are met by varying the values of the control parameters. A striking feature that emerges in almost all steady-state patterns is the presence of dark points, in which both the real and the imaginary part of the electric field vanish and such that, if one covers a closed loop around one of these points, the field phase changes by a multiple of 2\ensuremath{\pi}, which corresponds to the topological charge of the point. We show in detail the analogy of these phase singularities to the vortex structures well known in such fields as, for example, hydrodynamics, superconductivity, and superfluidity. In our case, at steady state, these singularities are arranged in the form of regular crystals, nd the equiphase lines of the field exhibit a notable similarity to the field lines of the electrostatic field generated by a corresponding set of point charges. We analyze in detail the patterns that emerge in the cases 2p+l=2 and 2p+l=3, where p and l are the radial and angular modal indices, respectively, and we compare the results with the experimental observations obtained from a ${\mathrm{Na}}_{2}$ laser. The observed patterns agree in detail with those found by theory; in particular, they exhibit the predicted phase singularities in each pattern. The transitions from one pattern to another, that one observes under variation of the control parameters, basically agree with those predicted by theory.
279 citations
TL;DR: In this paper, it was shown that all H-type groups which possess certain geometric properties, clearly possessed by Iwasawa N-groups, satisfy a Lie-algebraic condition (implicit in the work of B. Kostant [Kt2]) that we shall call the J'-condition.
226 citations
TL;DR: The area ratio index provides a quantitative approach to fatigue that is consistent with the intuitive definition of fatigue, and if applied to different myoelectric signal variables, the resulting area ratios may be interpreted as the components of a fatigue vector.
208 citations
TL;DR: In this article, the authors extended the cohesive crack model to mixed mode propagation and an experimental confirmation is provided by testing four-point shear specimens of concrete, where a constant crack mouth sliding displacement rate is imposed, so that is is possible to control and detect the snapback load vs deflection branches.
197 citations
TL;DR: A model density of states (DOS) for these materials is proposed, in accordance with their well-known microstructure, and explains the essential features of the optical-absorption spectra and ESR data.
Abstract: Amorphous carbon a-C and amorphous hydrogenated carbon a-C:H films were prepared by rf sputtering of a graphite target in argon and argon-plus-hydrogen atmospheres, respectively. The optical-absorption coefficients of these films were measured by a spectrophotometer in the high-absorption range and by photothermal deflection spectroscopy in the low-absorption range. They were also studied by electron-spin-resonance (ESR) measurements. The optical-absorption spectrum is found to have a rather broad peak, in contrast to the sharp rise in absorption near the band gap observed for normal semiconductors. To explain this broad peak, a model density of states (DOS) for these materials is proposed, in accordance with their well-known microstructure. This consists of a pair of broad Gaussian-like distributions lying above and below the Fermi level and separated by about 4 eV, arising out of the \ensuremath{\pi} states of the aromatic sixfold rings that comprise the bulk of the graphitic ${\mathit{sp}}^{2}$ regions, as well as pairs of discrete levels that are about 0.6 eV apart and are produced by fivefold and sevenfold rings present in these regions. The proposed DOS explains the essential features of the optical-absorption spectra and ESR data. The validity of the concept of the optical gap for these materials is also discussed.
182 citations
TL;DR: In this article, the constitutive equations of an elastic one-dimensional string are deduced from the stress-strain relations of nonlinear three-dimensional elasticity, by passing to the limit when the other dimensions go to zero.
Abstract: Using the variational point of view, the constitutive equations of an elastic one-dimensional string are deduced from the stress-strain relations of nonlinear three-dimensional elasticity, by passing to the limit when the other dimensions go to zero. The assumptions made on the three-dimensional model are not very restrictive.
173 citations
TL;DR: Amorphous carbon thin films have been prepared by r.f. magnetron sputtering of graphite targets in an Ar atmosphere and their d.c. electrical dark conductivity has been measured as a function of temperature.
Abstract: Amorphous carbon (a-C) thin films have been prepared by r.f. magnetron sputtering of graphite targets in an Ar atmosphere and their d.c. electrical dark conductivity has been measured as a function of temperature. The results are compared with those obtained by earlier workers for a-C and hydrogenated a-C. A detailed analysis leads us to the conclusion that conductivity of amorphous C with or without H is determined mostly by the amount and distribution of sp2 sites in the material. Near room temperature the experimental results suggest hopping between neighbouring sp2 (graphitic) islands embedded in an sp3 (diamond-like) matrix if the sp2:sp3 ratio is below its percolation threshold value and hopping in the band tails if this ratio is above such a threshold. In the temperature region above 500 K hopping in the band tails is shown to be the only effective mechanism.
119 citations
TL;DR: This chapter studies the problem of estimating a given function of a vector of unknowns by using measurements depending non-linearly on the problem element and affected by unknown but bounded noise by using a method to compute the axis-aligned box of minimal volume containing the feasible solution set.
107 citations
TL;DR: In this paper, the energetic behavior of two samples of soft magnetic materials fed by inverters has been studied and the authors dealt with the increase of the iron losses due to a nonconventional voltage supply.
Abstract: The energetic behavior of two samples of soft magnetic materials fed by inverters has been studied. The authors dealt with the increase of the iron losses due to a nonconventional voltage supply. Six-step and PWM (pulse width modulated) inverter output voltages have been selected as the excitation waveforms for the samples under test. Extensive experimental results of high-quality grain-oriented silicon steel and amorphous-alloy-wound core samples are presented. A detailed description of the testing procedure is shown. The results are quite informative and quantify the iron-loss increase with nonsinusoidal supply in comparison with the results obtained by the standard tests (sinusoidal supply and Epstein method). The comparison can give useful indications to electromagnetic-device designers about the derating factors for the different magnetic materials that have to be used in designing devices fed by static power sources. >
100 citations
TL;DR: A general connection between laser physics and hydrodynamics is established, by reformulating the dynamical equations of the laser in the form of hydrodynamic equations for a compressible fluid, similar to the law of mass conservation and to the Bernoulli equation.
Abstract: We show that the stationary transverse patterns, formed by the interaction of the modes of a frequency-degenerate family, are selected by the laser according to a specific variational principle. We discuss the phenomenon of spatial multistability, which consists in the coexistence of two or more different configurations for the same values of the control parameters. We establish a general connection between laser physics and hydrodynamics, by reformulating the dynamical equations of the laser in the form of hydrodynamical equations for a compressible fluid, similar to the law of mass conservation and to the Bernoulli equation. This result provides a general framework for the discussion of the relations between optical and hydrodynamical turbulence.
30 Mar 1991-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the aging process of 6061 AA and SiC particle composite was studied by hardness measurements and differential scanning calorimetry (DSC) and the results showed that the two materials showed the same aging rate when solution treatment was complete.
Abstract: Kinetics of natural aging and precipitation hardening (T4 and T6 tempers) were compared in 6061 aluminium alloy (AA) and 6061 AA−14vol.%SiC particle composite. Aging processes have been studied by hardness measurements and differential scanning calorimetry (DSC). The latter technique is used as a screening tool to follow the aging sequence; in fact, the DSC thermogram depicts characteristic features of different heat treatments undergone by samples. DSC curves for both 6061 AA and 6061AA-SiCp, after solution treatment, display the same aging path; on the contrary, the two materials differ in solution temperature, which is about 30 °C higher in the case of the composite. The precipitation hardening (at 180°C) occurs faster in the composite than in 6061 alloy. The latter presents a broad hardness peak ranging between 4 and 10 h of aging while the composite shows a sharp peak after about 4 h. Probably this behaviour is due to the high dislocation density close to the metal matrix-SiC particle interface. In these strongly deformed zones nucleation processes are accelerated. The aging kinetics at room temperature for the composite is slower than for the 6061 AA. In fact the two materials show the same aging rate when solution treatment in the case of the composite is complete. On the contrary, the composite ages more slowly than 6061 AA. Hence the high dislocation density at the interface, between composite constituents, does not affect T4 treatment.
TL;DR: In this paper, the dispersion relation for an arbitrary general bianisotropic medium is derived in Cartesian coordinates, in a form well suited to imposing the boundary conditions when dealing with layered media with planar and parallel interfaces.
Abstract: The dispersion relation for an arbitrary general bianisotropic medium is derived in Cartesian coordinates, in a form well suited to imposing the boundary conditions when dealing with layered media with planar and parallel interfaces. Special cases of practical interest are also considered. Eleven fundamental coefficient families are identified by considering in detail all the symmetries present in the dispersion relation. An ad hoc expression of the determinant of the sum of two 3*3 matrices permits the use of a simple procedure to obtain the coefficients of the dispersion equation. The discussed symmetry properties have general validity, and this technique to evaluate the coefficients may be useful in other fields of application where dispersion relations are of importance. >
TL;DR: PERTOB, an object oriented language and methodology based on PROT nets, and the CASE environment that supports it are presented and a model of a Flexible Manufacturing System and its production control software are analysed as a case study.
TL;DR: In this article, exact solutions for buckling under axial compression and free vibrations of cross-ply laminated, simply supported, circular cylindrical and spherical composite shells are presented.
TL;DR: In this article, the spectrum of the beam reflected by a one-ended empty cavity was analyzed for the two cases in which the input corresponds to a squeezed vacuum, or to squeezed beam with a mean value much larger than the fluctuations.
TL;DR: The effects of silicon implementation on the backpropagation learning rule in artificial neural systems are examined and theoretical analysis is compared with ad-hoc simulations and results are discussed in detail.
Abstract: The effects of silicon implementation on the backpropagation learning rule in artificial neural systems are examined. The effects on learning performance of limited weight resolution, range limitations, and the steepness of the activation function are considered. A minimum resolution of about 20/22 bits is generally required, but this figure can be reduced to about 14/15 bits by properly choosing the learning parameter eta which attains good performance in presence of limited resolution. This performance can be further improved by using a modified batch backpropagation rule. Theoretical analysis is compared with ad-hoc simulations and results are discussed in detail. >
TL;DR: In this article, the analysis of a layered planar structure consisting of different bianisotropic materials is performed in the frequency domain, in which reflection and transmission coefficients are determined via a chain-matrix algorithm.
Abstract: The analysis of a layered planar structure consisting of different bianisotropic materials is performed, in the frequency domain. Reflection and transmission coefficients are determined via a chain-matrix algorithm. Numerical results are presented and discussed.
01 Dec 1991
TL;DR: A new algorithm which minimizes the number of calibration measurements for a full two-port error correction of an Automatic Network Analyzer is presented and proves that a slight better accuracy can be reached using the suggested procedure.
Abstract: A new algorithm which minimizes the number of calibration measurements for a full two-port error correction of an Automatic Network Analyzer is presented Since a reduced number of standard connections is required compared to that of other well known calibration techniques, the procedure allows saving time and standard elements Experimental comparisons carried out with other calibration techniques also prove that a slight better accuracy can be reached using the suggested procedure
TL;DR: A survey of the state of the art in the field can be found in this article, where the authors provide a detailed picture of the current state-of-the-art and some necessary information for future research.
Abstract: This paper provides a review of the various results available in the mathematical literature on the solution to the initial and initial-boundary value problem for the discrete Boltzmann equation, a nonlinear mathematical model of the kinetic theory of gases. References include papers published until 1990. The aim of this survey paper is to provide a detailed picture of the state of the art in the field and some necessary information for future research in the field.
TL;DR: In this paper, it was shown that the index bundle of the universal family of linear S.L. boundary value problems is non-orientable, and a one dimensional set of bifurcation points for nonlinear BVP of Sturm-Liouville type was found.
TL;DR: In this paper, a physical model for heat transfer between the heating plate and the tray containing the product, based on Knudsen conduction in a thin lay, was proposed.
TL;DR: In this paper, the authors proposed criteria for low-activation of fusion-reactor structural materials based on three fundamental characteristics: radioactive emissions in routine and off-normal conditions, maintenance operations after a shutdown, and finally waste management and disposal aspects.
TL;DR: The PROTOB object‐oriented methodology for the executable specification of large‐scale event‐driven systems is introduced and described, and a CASE environment with tools for specification, modelling, simulation and prototyping is presented.
Abstract: SUMMARY The PROTOB object-oriented methodology for the executable specification of large-scale event-driven systems is introduced and described. Two supporting features of PROTOB are also presented: the graphic and textual language that formally describes the behaviour of objects, which is based on highlevel Petri nets called “PROT nets” and which is demonstrated to be more powerful than SA/RT dataflows; and a CASE environment with tools for specification, modelling, simulation and prototyping. Use of PROTOB is illustrated by discussion of the automated generation of distributed systems running on a network of VMS and UNIX computers.
TL;DR: In this article, three phases, all containing Bi(V), are identified: Sr(Sr.5 Bi.5 )O 2.75 (A); Sr(sr.2 Bi.8 )O 3 (B) and Sr 24 Bi 14 O 52 ( Sr Bi = 1.714 ) (C).
TL;DR: A coherent optical CDMA network employing Gold sequences for the spreading and despreading operations is analysed in terms of the signal to noise ratio penalty as a function of the number of simultaneous users incurred to maintain a given value of the bit error rate.
Abstract: A coherent optical CDMA network employing Gold sequences for the spreading and despreading operations is analysed in terms of the signal to noise ratio penalty as a function of the number of simultaneous users incurred to maintain a given value of the bit error rate. The binary PSK and OOK modulation schemes are considered. Exact results in the presence of noise and interfering signals (interferers) are obtained.
TL;DR: This investigation compared the corrosion resistance of the palladium-cobalt ferromagnetic alloy coupled with the samarium- cobalt magnets embedded in the removable part of the denture to simulate clinical conditions.
Abstract: In recent years magnetic retention has gained increasing popularity in dental practice. This investigation compared the corrosion resistance of the palladiumcobalt ferromagnetic alloy (constituent of the keeper cemented on the abutment teeth) coupled with the samarium-cobalt magnets embedded in the removable part of the denture. The behavior of three couples (cobalt-palladium, cobalt-palladium/titanium, and cobalt-palladium/palladium) has been studied. The magnets, because of their poor corrosion resistance, are encapsulated in various materials. To simulate clinical conditions, characterized by the continuous movement of the keeper with respect to the magnet, the experiments were conducted in artificial saliva under intermittent and continuous wear.
TL;DR: In this paper, the alignment of a nematic liquid crystal layer with negative dielectric anisotropy (Δe < 0) was found to be homeotropic below a critical temperature Tc, (low temperature range) and above that particular temperature the homeotropic alignment abruptly transforms into a planar one, which remains with increasing temperature up to the clearing point TNI (high temperature range).
Abstract: The inner surfaces of a cell of conventional type were covered with an SiO, aligning layer evaporated at an angle α = 60° and subsequently treated with lecithin in order to achieve symmetrical boundary conditions. The alignment of a nematic liquid crystal layer with negative dielectric anisotropy (Δe < 0) was found to be homeotropic below a critical temperature Tc, (low temperature range). Above that particular temperature the homeotropic alignment abruptly transforms into a planar one, which remains with increasing temperature up to the clearing point TNI (high temperature range). The alignment transition was found to be reversible, and is attributed to a packing change of the lecithin layer with the temperature. A simple model based on the different temperature dependence of the anchoring strengths W H, (T) and W P, (T), characterizing the homeotropic and planar alignment respectively, is proposed to explain the surface induced alignment transition.
TL;DR: The authors propose a novel algorithm which fully implements the multiway rendezvous of LOTOS within a distributed execution model based on a number of parallel processes.
Abstract: It is noted that the implementation of the multiway rendezvous mechanism of the International Standards Organization (ISO) LOTOS specification language for protocols is very important in the development of tools for the execution of LOTOS. It involves problems such as global knowledge in a distributed environment and distributed agreement. The authors propose a novel algorithm which fully implements the multiway rendezvous of LOTOS within a distributed execution model based on a number of parallel processes. The processes are organized in a hierarchical topology and communicate with each other only by message transfers. The performance of the proposed algorithm is evaluated and is shown to be better than that achieved by other algorithms proposed in the literature. A formal specification of the algorithm in LOTOS is provided. >
TL;DR: Simulation is used to estimate the performance of media access control protocols derived from carrier-sense multiple access with collision detection and operating in local area networks comprising several parallel broadcast channels, and results indicate that the multichannel option provides reductions in both the packet delay average and variance.
Abstract: Simulation is used to estimate the performance of media access control (MAC) protocols derived from carrier-sense multiple access with collision detection (CSMA/CD), and operating in local area networks comprising several parallel broadcast channels. The influence of possible protocol and system alternatives on the network performance is discussed, based on results of the packet delay average, variance, mean square, coefficient of variation, and histogram, as well as the packet rejection probability due to lack of buffer space. The delay incurred by multipacket messages is estimated, comparing the single channel to the multichannel option. Numerical results indicate that the multichannel option provides reductions in both the packet delay average and variance, even when stations are only able to simultaneously receive from a subset of channels. >