Showing papers by "Polytechnic University of Milan published in 1979"
TL;DR: In this article, three objective methods for the propagation of a crack band of a blunt front are proposed based on the rate of energy release by the crack band and the angle of the crack front.
Abstract: A propagating smeared crack band of blunt front is much simpler to model by finite elements than a sharp interelement crack, especially when the propagation direction is unknown. For concrete or rock, a smeared crack band is also more realistic. A strength criterion is generally used for the propagation, but this is not objective because of a strong spurious dependence of results on the chosen element size, Three objective methods, which avoid the use of singularity elements, are proposed. Method A is based on the rate of energy release by the blunt crack band. In method B, the usual strength criterion is used but an adjustment of the strength value according to the element size is proposed. In method C, the propagation direction and crack advance are determined by fitting the Mode I asymptotic series to nodal displacements around the crack front, using an optimization subroutine. Special merits of each method are analyzed and solutions of example problems are compared with exact ones.
261 citations
TL;DR: In this article, the Cam Clay series of elastic-plastic models is used to define constitutive models for sand at a single initial density, but the way in which the constitutive parameters might be expected to vary with density is discussed.
Abstract: Although there is a large number of constitutive models for sand available in the literature it is believed that a fresh approach, striking a balance between complexity and theoretical rigour, is desirable.
The approach here has certain conceptual links with the Cam Clay series of elastic–plastic models, but includes the more general starting assumption that the yield function, plastic potential and failure locus should be given quite distinct mathematical expressions. Possible physical bases for the proposed forms are discussed.
Ways in which the parameters required to define the model may be determined are suggested and the use of the model is then demonstrated. Firstly, it is shown that, where a limited set of experimental data is available, the model is flexible enough to be able to match the test results. Secondly, it is shown that, where a wide range of test results has been produced, it is possible to determine the model constitutive parameters from a small number of tests and proceed to make satisfactory predictionsfor other, quite different, types of test.
The model is developed for sand at a single initial density, but the way in which the constitutive parameters might be expected to vary with density is discussed. The model is described for conditions of triaxial compression, and extension to more general stress states will be needed before it can be put to the test of incorporation in, for example, a finite element program.
242 citations
TL;DR: In this paper, the authors presented a technique for overcoming the problem of tissue absorption in emission tomography by solving a linear partial-differential equation that links the derivatives of the spectrum with respect to absorption and spatial frequencies.
Abstract: This paper presents a technique for overcoming the problem of tissue absorption in emission tomography. Given a set of equispaced projections in the interval (0, 2π), it is possible to derive an exact formula for recovering the spectrum of the image. The formula is obtained by solving a linear partial-differential equation that links the derivatives of the spectrum with respect to absorption and spatial frequencies. Examples of the application of the technique to synthetic and real data are given.
229 citations
TL;DR: In this paper, the orientation-dependent energy between a pair of segments is taken to be proportional to cos2 λij, where λj is the angle between the principal axes of their polarizability tensors, assumed to be cylindrically symmetric with respect to the molecular axis.
Abstract: A system of rigid, inpenetrable, rodlike molecules subject to orientation-dependent mutual attractions is treated by extension of the theory presented in the preceding paper. This energy is formulated, for a system at constant volume, by considering interactions between pairs of segments in contact, rather than in terms of interactions between entire molecules. The orientation-dependent energy between a pair of segments is taken to be proportional to cos2 Ψij, where Ψij is the angle between the principal axes of their polarizability tensors, assumed to be cylindrically symmetric with respect to the molecular axis. A characteristic temperature T∗ measures the intensity of these interactions. The orientational energy of the system as a whole is of the form derived by Maier and Saupe. The orientation distribution with respect to the domain axis and the partition function are formulated with T∗ and the axial ratio x as parameters, Steric effects of molecular shape asymmetry, embodied in x, are of for...
204 citations
TL;DR: In this article, the performances of diamond nuclear radiation detectors have been investigated using alpha and beta particles, and an energy for electron-hole pair creation of 13.19 eV has been determined for alpha particles in the 5-6 MeV energy range.
140 citations
TL;DR: In this article, an integral equation method for the numerical solution of the linear problems of elasticity of the homogeneous continuum was proposed. But this method is not suitable for the analysis of plate and axisymmetric cases.
Abstract: This paper concerns an integral equation method for the numerical solution of the linear problems of elasticity of the homogeneous continuum. The mathematical form adopted is sufficiently general to refer to both plane and three dimensional analysis as well as to analysis of plates and specialized axisymmetric cases. Having indirectly deduced the integral equations, the resolving procedure is based on variational principles and uses a convenient discretization of the boundary through the use of finite elements. Particular attention is given to the definition of the asymptotic conditions of the elementary displacement field to theoretically ensure a priori the conditioning of the resolving algebraic equations without any restriction as regards the nature of the boundary and the conditions thereby imposed. Efficiency, praticality and flexibility of use are characteristics particular of this method and have already been widely tested by the author in varying applications. Further information on the above can be found in the bibliography cited.
128 citations
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TL;DR: The incremental parser presented extends the conventional LR parsing algorithm and its performance is compared with that of a conventional parser.
Abstract: An incremental parser is a device which is able to perform syntax analysis in an incremental way, avoiding complete reparsing of a program after each modification. The incremental parser presented extends the conventional LR parsing algorithm and its performance is compared with that of a conventional parser. Suggestions for an implementation and possible extensions to other parsing methods are also discussed.
106 citations
TL;DR: A new approach to gain control of the discretization error by increasing the number of degrees of freedom in regions where the initial finite element model is not adequate is demonstrated by applications to two- and three-dimensional elasticity, including singularity problems.
76 citations
TL;DR: The structure of a real-time estimator for the estimation of the velocity of objects imaged by television cameras is presented and experimental results show that a very fine accuracy is obtained.
67 citations
TL;DR: A single procedure is presented for the determination of eigenvalue and eigenvector derivatives of general eigensystems, including those arising in flutter and divergence analysis, which unifies the determination a solution and of its derivatives in one single approach.
Abstract: A single procedure is presented for the determination of eigenvalue and eigenvector derivatives of general eigensystems, including those arising in flutter and divergence analysis. The method requires only the knowledge of the eigensolution under consideration and no information about the transposed problem. In the procedure, the eigenproblem is not presented as such, but as a nonlinear algebraic system of equations with the eigenvalue and the eigenvector as unknowns and the eigenderivatives coming from the solution of a linear system of equations that is determined trivially by differentiati ng the nonlinear equations with respect to a structural parameter. The technique can be extended easily to derivatives of any order and always requires solutions of the same linear system with different right-hand sides. The coefficient matrix of this linear system always maintains the structure of the matrix of the eigenproblem and three methods of solution are presented to take full advantage from its symmetry and sparseness, if present. Application to flutter and divergence problems is emphasized, as the basic idea of the method unifies the determination of a solution and of its derivatives in one single approach.
64 citations
TL;DR: In this article, a thorough investigation has been done about the behavior of natural diamond as a radiation detector material for a wide temperature range, and the average energy required to create an electron-hole pair and energy resolution have been measured in the 100 K - 400 K interval.
Abstract: A thorough investigation has been done about the behavior of natural diamond as a radiation detector material for a wide temperature range. Drift velocities and mean free drift time have been determined at temperatures ranging between 85 K and 700 K and with electric fields up to 60 KV/cm. Average energy required to create an electron-hole pair and energy resolution have been measured in the 100 K - 400 K interval. The spectroscopic features of diamond detectors have also been analyzed on a broad temperature interval. It was found that as T approaches 500 K polarization phenomena begin, thus establishing the upper temperature limit for the detector operation.
TL;DR: A digital lock- in detection technique which overcomes some practical limitations of the analog lock-in amplifiers is described, and an example of implementation is given, based on multichannel pulse analyzers.
Abstract: A digital lock‐in detection technique which overcomes some practical limitations of the analog lock‐in amplifiers is described. An example of implementation is given, based on multichannel pulse analyzers. Some representative results are reported for two typical application fields of lock‐in techniques, namely optical spectrometry and semiconductor profiling with C–V techniques. Advantages and limitation of the technique are discussed as well as adaptation to various modulation waveforms.
TL;DR: A formal framework for structuring and embedding the heuristic information is proposed in order to allow an algorithmic computation of the evaluation function ĥ (n) of the classical Hart-Nilsson-Raphael algorithm.
TL;DR: In this article, the electrochemical and corrosion behavior of 304L and 316L stainless steels, cold worked under various conditions, in 1M H 2 SO 4 and in 0.1M HCl de-aerated solutions.
TL;DR: In this paper, the reactions involved in the industrial production of propylene oxide from chlorohydrins have been kinetically investigated, both in the presence of slaked lime and of caustic soda, by potentiometric techniques.
TL;DR: A unified computational scheme is presented, and comparisons are made between the mathematical programming techniques and the so-called “direct” methods for nonlinear finite element analysis.
TL;DR: The fluorescence of several bacterial DNAs stained with quinacrine mustard have been investigated using a laser microfluorometer with a spatial resolution of ‐ 0.3 μm and a temporal resolution of ˜0.3 ns connected to a digital signal averager.
Abstract: —
The fluorescence of several bacterial DNAs stained with quinacrine mustard have been investigated using a laser microfluorometer with a spatial resolution of - 0.3 μm and a temporal resolution of ˜0.3 ns connected to a digital signal averager.
Experiments performed on Micrococcus lysodeikticus samples show that both cytological preparations and the corresponding purified DNAs give coincident fluorescence curves, thus indicating that the fluorescence observed in the former case is to be attributed to the bacterial DNA only. Experiments thereafter performed on smears of several bacteria with known AT percentages show that each fluorescence decay curve, after a fast transient, can be fitted by an exponential decay law with a single time constant. This time constant has been found to depend linearly on the square of the AT percentage.
We explain this result on the basis of an energy transfer mechanism between dye molecules intercalating AT:AT sequences (donors) and dye molecules bound to either GC:GC or GC:AT sequences (acceptors). The agreement with the experimental data requires that all the bacteria considered present a common value for both the number of base pairs contained in a Forster sphere and for the maximum saturation of the strong binding process.
TL;DR: In this article, basic concepts of electrode geometry and signal processing for a time projection liquid argon ionization chamber are introduced, and the design criteria of a system of sensing electrodes are discussed.
Abstract: Basic concepts of electrode geometry and signal processing for a time projection liquid argon ionization chamber are introduced. The design criteria of a system of sensing electrodes are discussed. The signals induced by rectilinear particle tracks on the sensing electrodes are calculated by means of a detailed study of possible electric field configurations. Suitable signal processing filters are designed in order to achieve nearly equal position resolution in the three space coordinates. The electronic noise limits the position resolution of a single track to a few tenths of a millimeter and the capability to resolve and measure closely spaced multiple tracks to a few millimeters.
TL;DR: A computer program is described which, using as an input the polar coordinates of the real profile, gives as output the lift and accelaration laws of planar cams.
TL;DR: In this article, a mathematical model of a multistage unit for the synthesis of propylene oxide from propylene chlorohydrins is formulated, where both the reaction involved in the synthesis process and the distillation of polypropylene oxide are taken into account.
TL;DR: In this paper, the nonlinear Boltzmann equation with a discretized spatial variable is studied in a Banach space of absolutely integrable functions of the velocity variables Conservation laws and positivity are utilized to extend weak local solutions to a global solution.
Abstract: The nonlinear Boltzmann equation with a discretized spatial variable is studied in a Banach space of absolutely integrable functions of the velocity variables Conservation laws and positivity are utilized to extend weak local solutions to a global solution This is shown to be a strong solution by analytic semigroup techniques
TL;DR: In this paper, it is shown that the principal methods for reducing the indeterminacy of the sign of (δ M/δOi) (isotropic invariance of M, quantum mechanical calculations, determination of δM/ ΔOi via high resolution data or the Stark effect) lead, in the case of CH3X (X = F, Cl, Br, I) molecules, to contradictory conclusions.
TL;DR: In this paper, the atomic pseudopotentials of Ge, Sn, S and Se are investigated over the range of wavevectors that are sampled in band structure calculations of large unit cell crystals, such as the orthorhombic chalcogenides.
TL;DR: In this article, a method for the estimation of the parameters of generalized Streeter-Phelps river quality models is developed, which does not require any uniformity in the geometry of the measurement points and can be used in the case in which only DO data are available.
Abstract: A method for the estimation of the parameters of generalized Streeter-Phelps river quality models is developed in this paper. The method is quite flexible since it does not require any uniformity in the geometry of the measurement points and can be used in the case in which only DO data are available. The validity of the method is tested by means of real data collected on the Bormida river in northern Italy, and the corresponding results and difficulties are pointed out. Finally the sensitivity of the method with respect to missing data is analyzed.
TL;DR: The most relevant features of the computational algoritms developped to solve the Parameter Optimization Approach to the design of multivariable industrial regulators are outlined and illustrated by an application.
TL;DR: In this article, a slender beam with nonlinear, elastoplastic flexural characteristics is unilaterally supported in the plane by a frictionless, rigid profile, and the problem of determining the changes of the unilaterally supporting profile which reduce the curvatures of the beam below assigned values with the least total cost is addressed.
TL;DR: In this article, the transition flow of a gas between two reservoirs through a two-dimensional slot is studied, where the width of the slot is much smaller than the length but finite and the ratio of the equilibrium number densities in the reservoirs is near 1 and the equilibrium temperatures are the same.
Abstract: The transition flow of a gas between two reservoirs through a two-dimensional slot is studied. The width of the slot is much smaller than the length but finite. The ratio of the equilibrium number densities in the reservoirs is near 1 and the equilibrium temperatures are the same. Galerkin's method of solution is applied to the linearized BGK equation and the total mass flow is evaluated as a function of the inverse Knudsen number δ and the length-to-half width ratioL. Limits δ≫ 1, δ≪1 andL≫ 1 are investigated and good agreement with other authors is found. Simple analytical expressions are obtained for these cases.
TL;DR: In this paper, a synthetic kernel was used to study the effect of forward and backward scattering on the spherical albedo and the transmission factor for a finite atmosphere in the presence of a single particle.
Abstract: A synthetic kernel is used to study the effect of forward and backward scattering on the spherical albedo and the transmission factor for a finite atmosphere.