Showing papers by "Mines ParisTech published in 1986"

Influence of grain size on the mechanical behaviour of some high strength materials

Abstract: Combining in an additive or synergetic manner the most potent strengthening mechanisms available in an alloy is the art of the metallurgist. The various models proposed in the literature in order to interpret the Hall-Petch relation are critically reviewed by comparison with experimental data. The pile-up models and the work hardening theories must include the inner structure of the grain in the case of alloys hardened by a second phase. Similarly, the properties and structure of the grain boundaries are influenced by impurities or the presence of particles. Ultra-fine grain sizes can provide ductility to high strength materials when surface preparation eliminates microcracks. In steady-state creep equations, introducing the influence of grain size in complex alloys by incorporating the Hall-Petch stress as one component of the internal stress helps in rationalizing the existence of an optimal grain size where creep resistance is maximized. Slower crack growth rates can be obtained by controlling the grain boundary structure as well as grain size. Fatigue tests at room temperature clearly point out the interest of small grain sizes for reducing crack initiation, usually associated, however, with lower propagation threshold and somewhat faster growth rates.

Direct parallelization of call statements

Abstract: Asynchronous CALL statements are necessary in order to use more than one processor in current multiprocessors. Detecting CALL statements that may be executed in parallel is one way to fill this need. This approach requires accurate approximations of called procedure effects. This is achieved by using new objects called Region and Execution Context. An algorithm to find asynchronous CALL statements is given. It involves a new dependence test to compute data dependence graphs, which provides better results than previous ones even when no CALL statements are involved. This method has been implemented in Parafrase and preliminary results are encouraging.

Thermal dependence of nucleation and growth rate in polypropylene by non isothermal calorimetry

Abstract: Crystallization of polypropylene at a constant cooling rate is studied by differential scanning calorimetry. The results are interpreted using Ozawas theory, which allows the determination of the type of nucleation. In the studied polypropylene, a transition between a sporadic in time and an instantaneous nucleation is found at about 122 °C. Furthermore, an extension of Ozawa's theory is proposed to predict the thermal dependence of the spherulite growth rate. The results are consistent with those obtained by direct microscopic observation.

Experimental study and calculations of short glass fiber orientation in a center gated molded disc

Abstract: Short glass fiber orientation in a center gated molded disc of polyamide is studied using optical microscopy techniques. The very different orientation between the core and the surface of the molding is quantified with an orientation function. The influence of the molding conditions is investigated. A numerical scheme is used for modeling the mold filling with a viscous melted polymer. A computation method is introduced to describe the fiber movement during the flow. The theoretical results are in good agreement with the experimental ones. In particular, the very different orientation between the skin and the core of the disc is well predicted.

Plastic Deformation of Glassy Polymers: Constitutive Equations and Macromolecular Mechanisms

Abstract: Specific features of the deformation behaviour of amorphous polymers are reviewed, with particular reference to the current literature on glassy polycarbonate. A preliminary section is devoted to the structure and to the viscoelastic properties of disordered macromolecules at different temperatures. New experimental results on the plastic shear of polycarbonate below Tg are then presented, which show the great influence of thermo-mechanical history (including plastic cycling and annealing) on the deformation mode of polymeric glasses. The above effects are discussed on the basis of a model according to which plastic deformation would be controlled by the movement of linear defects similar to shear dislocations in the amorphous structure. The critical steps of this mechanism would be i) the multiplication of these defects in discrete shear bands during the transient deformation stage and ii) the elementary conformational changes of polymer chains at the core of a moving defect against the interaction of the surrounding molecules and the hardening effect of the gradual orientation of the chains.

The AMIGO Project: advanced group communication model for computer-based communications environment

Abstract: In this paper we discuss and elaborate on the conceptual requirements as well as the tools of the General AMIGO Model for group communication. The special features of the model are examined with particular reference to the social and ethical implications in the communication process. The applicability of the AMIGO Model is demonstrated by examples, and we give indications of further work to refine and advance the model.

Deformation of an inclusion in a viscous matrix and induced stress concentrations

Abstract: The strain rates and stresses developed within and around an elliptical (resp. spheroidal) inclusion embedded in a viscous matrix are determined for a prescribed plane (resp. axially symmetric) strain at infinity. An exact solution is derived for a linearly viscous matrix, providing analytical expressions for the strain rates and stresses within the whole material. In the more general case of a power-law viscous matrix, a variational approach based on the above velocity field is proposed. The inclusion strain rate ratios obtained in this way are compared with finite element results and experimental data from the literature. Finally, the induced stress concentrations at the inclusion-matrix interface are described in detail and discussed in connection with the problem of damage initiation.

Global Stability of a Direct Adaptive Control Scheme With Respect to a Graph Topology

Abstract: We study the stability given by a modified model reference adaptive controller. Modifications are projections of the adapted parameters into a convex compact set and normalization of the signals entering the adaptation law by a weighted l 2-norm of the I/O signals. The plant is assumed to satisfy properties which are proved to be robust with respect to a graph topology based on μ-exponential stability. Global stability is established and the mean square tracking error is shown to converge to zero linearly with the unmodeled effects.

Surface damage of CdTe produced during sample preparation, and determination of dislocation types near microhardness indentations

Abstract: The influence of cutting, grinding and polishing during sample preparation of CdTe Bridgman monocrystals has been investigated by an etch-pit technique. Dislocations, mainly produced by cutting the sample with a diamond wire saw, were found to move several hundreds of micrometres from the surface into the bulk of the samples. The structure of dislocation loops near microhardness indentations was analysed by the same etch-pit technique and by birefringence observations. The Burgers vectors of the dislocations were determined by X-ray topography. Long dislocation segments were found to be screw dislocations.

Die flow computations: A method to solve industrial problems in polymer processing†

Abstract: The flow of molten polymers through extrusion dies sets specific problems, usually connected to homogeneous distributions of flow rate and temperature. To overcome these problems, the computation of the flow by numerical techniques may be very useful. In this paper, approximation methods are developed, in order to treat industrial die geometries, without too complex and time-consuming computation programs. Different examples of results, for the main types of industrial dies, are presented.

Analysis of residual stresses in hot-rolled complex beams

Abstract: This paper deals with the analysis of residual stresses in hot-rolled complex beams After rolling, residual stresses appear during the cooling period when the temperature is not uniform in the cross-section These temperatures are calculated by a transient nonlinear program The thermal stresses are estimated by a two-dimensional thermoelastoplastic or thermoelasto-viscoplastic finite element idealization Bending effects are introduced in a generalized plane-strain formulation assuming circular curvature along the beam The method leads to a plane-strain calculation which is very interesting from the computational point of view; the temperature dependence of the physical properties can be taken into account The technique has important industrial applications It permits the optimization of the state of residual stress in hot-rolled complex beams by testing different cooling conditions

Two directional coextrusion flow of two molten polymers in flat dies

Abstract: A theoretical study has been carried out of the two directional isothermal flow of two molten polymers in a flat die. The viscous behavior of each fluid is described by a power-law. Using an iterative finite difference method, a numerical program predicts the influence of geometrical and Theological parameters on the variations of the interface position. Theoretical values are in good agreement with experimental results obtained on a fish-tail die. In the case of a coat hanger-die, the mean value of the interface position is correct, but the general shape may be quite different.

Automatic parallelization of FORTRAN programs in the presence of procedure-calls

Abstract: Scientific programs must be transformed and adapted to supercomputers to be executed efficiently. Automatic parallelization has been a very active research field for the past few years and effective restructuring compilers have been developed.

On the microstructure of rapidly solidified Al-15 wt pct mn ribbons: effect of annealing in the temperature range 300° to 600 °C

Abstract: A study has been made of the relationship between the microstructures formed in rapidly solidified Al-15 wt pct Mn ribbons and the precipitation processes following annealing in the regime 300° to 600 °C from 15 minutes to 100 hours. The as-melt spun ribbons exhibit the icosahedral structure (in an Al-matrix), which is found to be stable up to ∼320 °C as deduced from electrical resistivity measurements. X-ray and TEM analyses reveal that decomposition of this phase is detectable after annealing at 400 °C/1 h, giving rise to Al6Mn. At 450 °C, four phases, viz., the G, Al6Mn, G’, and G″ intermetallic phases, are observed (in addition to Al), which result from the direct decomposition of the icosahedral phase. At higher temperatures, only the Al6Mn phase + Al is observed. The effect of increase in annealing time is simply to coarsen the Al6Mn particles. Our X-ray and electron diffraction data favor the model proposed by Shechtman and Blech for the icosahedral structure. The investigations were carried out using density and electrical resistivity measurements, differential thermal analysis, as well as X-ray diffraction and optical and transmission electron microscopy.

Microstructural characterization of rapidly solidified Al-Li-Co powders

Abstract: A study of the combined effect of alloying elements and melt superheat has been carried out on the as-solidified structure of rapidly solidified Al-Li-Co powders. Three alloys,viz., Al-3 pct Li, Al-3 pct Li-0.4 pct Co, and Al-3 pct Li-0.8 pct Co were chosen, and the liquid melt in each alloy atomized from the temperatures 1173 and 1073 K, using the centrifugal atomization technique. The microstructural characterization was done using light, scanning, and transmission electron microscopy. Four types of microstructures,viz., dendritic, cellular, equiaxed-type, and featureless structures, were observed by light microscopy. The cooling rate, as determined from the same, lay in the range 104 to 106 Ks−1, but was seen to go beyond 107 Ks−1 when estimated from TEM micrographs. On the micro-level, the Al-Li powders were found to exhibit dendritic structures with differing morphologies, whereas low-angle cell walls with perturbed interfaces were the main structural features observed in the Al-Li-Co alloys. Increasing both cobalt content and powder particle diameter favored transition from dendritic into cellular structure. The featureless zone was comprised mainly of elongated columnar grains (0.2 μm width and 1.5 μm length). A mechanism describing the cellular structure formation has been proposed. Aging of the melt-quenched powders at 473 K for times up to 100 hours results in the dissolution of the cellular structure. A mechanism for the same has been postulated. The difference in the superheats chosen in the present work is found not sufficient to cause drastic microstructural changes.

Coextrusion flow of two molten polymers between parallel plates: non isothermal computation and experimental study

Abstract: A theoretical study of the one-directional coextrusion flow of two molten polymers between parallel plates has been carried out using a non-isothermal power-law model. A numerical method has been used to solve simultaneously the momentum and energy balance equations, in order to obtain the evolutions of the interface position, the pressure gradient, and the temperature profiles all along the flow. The theoretical results are in good agreement with experimental values measured on an industrial coextrusion line.

Mössbauer study of amorphous and crystallized Fe1-xCx alloys

Abstract: Fe1−xCx amorphous alloys have been studied with Mossbauer Spectroscopy for 0.19 -0.286, new carbides are formed by crystallization of such alloys. The chemical twinning model allows describing their structures.