Showing papers by "Mines ParisTech published in 1997"

Mechanical percolation in cellulose whisker nanocomposites

Abstract: Nanocomposites obtained by casting a mixture of a latex and an aqueous suspension of cellulose whiskers have been studied. Their mechanical properties (e.g. shear modulus) are found to increase by more than three orders of magnitude in the rubbery state of the polymer matrix, when the whisker content is 6 wt%. This large and unusual effect is discussed on the basis of different types of mechanical models, including semi-phenomenological and numerical finite element calculations. It is concluded that cellulose whiskers form a rigid network linked by hydrogen bonds. The formation of this network is assumed to be governed by a percolation mechanism.

Control of underactuated mechanical systems using switching and saturation

Abstract: In this paper we present some ideas on the control of underactuated mechanical systems using switching and saturation. We focus on the swingup control problem for a class of “gymnast” robots and also for the classical cart-pole system. The design methodology is based on partial feedback linearization in a first stage to linearize the actuated degrees of freedom followed by the control of the transfer of energy from the actuated to the unactuated degrees of freedom in a second stage. In a typical swingup control the desired equilibrium is unstable in the closed loop system as a consequence of the non-minimum phase behavior of the system. For this reason it is necessary to switch controllers at the appropriate time to a controller which renders the equilibrium stable. The successful implementation of the switching control has proved to be nontrivial, both in simulation and in experiment. We discuss both local and global design methods and present some simulation results.

Additive noise effect in digital phase detection

Abstract: The characteristic polynomials associated with the algorithms used in digital phase detection are used to investigate the effects of additive noise on phase measurements. First, it is shown that a loss factor η can be associated with any algorithm. This parameter describes the influence of the algorithm on the global signal-to-noise ratio (SNR). Second, the variance of the phase error is shown to depend mainly on the global SNR. The amplitude of a modulation of this variance at twice the signal frequency depends on a single parameter β. The material presented here extends previously published results, and as many as 19 algorithms are analyzed.

Mullite Based Oxidation Protection for SiC-C/C Composites in Air at Temperatures up to 1900 K

Abstract: For an industrial Si-SiC coated C/C material (reference material) the temperature dependence of the linear rate of mass loss is interpreted in the temperature range 773 < T < 1973 K. The Arrhenius plot of the thermogravimetrically determined oxidation rate shows four typical regimes. Only in the temperature range 1323 < T < 1823 K is the oxidation rate close to or lower than the limit for long-term application. Pulsed Laser Deposition (PLD) allows the ablation of nonconductive and high melting targets and the preparation of films with complex composition. High energy impact CO 2 laser pulses (j= 3.10 7 W cm -2 ) lead to melting and evaporation of the target material in a single step. Therefore the flux of the metal components is stoichiometric. Deposited green layers did not show IR peaks typical for mullite. After a short oxidation treatment (15 min at 1673K) the formation of mullite in the coating was completed as was confirmed by IR spectroscopy and XRD investigations. Thin PLD-mullite layers (900 nm) did not markedly improve the oxidation resistance of the reference material in the high temperature range 1473 < T < 1973 K. However, a preoxidation treatment of the substrate material and mullite coatings with a thickness of 2.5 μm improved the oxidation behaviour significantly. Because of SiO 2 formation at the mullite-SiC interface all samples exhibited a mass increase on oxidation. The inward diffusion of oxygen across the outer mullite-containing layer controlled the kinetics of the reaction as was deduced from 18 O diffusivity measurements in PLD mullite layers. The calculated oxidation rates resulting from the diffusion parameters in SiO 2 and mullite are close to the thermogravimetric data. For oxidation durations of three days only amorphous SiO 2 is formed at the mullite-SiC interface.

Size and geometry effects on ductile rupture of notched bars in a C-Mn steel: experiments and modelling

Abstract: The aim of this work was to investigate the effect of specimen size and geometry on ductile fracture of a C-Mn steel with high sulphur content. Uniaxial tensile tests were conducted at 300°C on axisymmetric notched specimens having different sizes and geometries. Geometry effects were studied using specimens with various notch radii, thus inducing different stress triaxiality levels. Size effects were evidenced using homothetic samples having the same geometry. Results show that ductility is reduced on specimens with sharp notches (which is a common observation). As specimen size increases, mean ductility as well as scatter are reduced (showing a clear size effect). In order to predict rupture, locally coupled (post-processing type) and fully coupled (continuum damage mechanics) Finite Element models were used. They are based on the plastic criteria introduced by Gurson and Rousselier. In order to model size effect (decrease of ductility and scatter), initial distribution of inclusion volume fractions, measured by quantitative metallography, was accounted for in the simulations. Comparison of experiments with simulations showed that both model types could predict mean values of ductility and scatter.

A laser displacement sensor using the self-mixing effect for modal analysis and defect detection

Abstract: A low-cost, simple, noncontact displacement sensor using the self-mixing effect inside a cw single-mode laser diode has been developed for modal analysis and defect detection. The resonance frequencies of thin clamped metallic plates have been determined with a resolution of 0.2 Hz. With such targets, classic spectrum analyzers generate a measurement error up to 10% because of the weight of the accelerometer. The frequency-shift of the fundamental resonance frequency between a standard plate and the same one with a known defect can also be measured, the modal vector associated to this frequency being distorted at the place of the defect.

Systèmes linéaires sur les opérateurs de Mikusinski et commande d'une poutre flexible

Abstract: On propose une theorie algebrique des systemes lineaires sur les operateurs de Mikusinski, illustree par la commande d'une poutre flexible d'Euler-Bernoulli.

Bridging the gap between CSG and Brep via a triple ray representation

Abstract: Computing intersections between algebraic surfaces is an essential issue for Brep-based modellers, and a very difficult one. The more often, existing methods are not reliable, and reliable ones are hairy. We think there is another and simple-minded way which avoids this problem without loss of practicalities. The key idea is computing a triple ray representation by zbuffer, raytracing or whatever, and then using the popular marchiny cubes algorithm with some local improvements. 1 The gap between CSG and Brep Breps [Hof89] describe solid objects by their boundary: surface patches, edges and vertices with their connectivity relations. They typically use free-form patches, carefully sewn together lo form the consistent, boundary of a solid which is then called a free-form (or sculptured) object. The high geometric coverage of free-form surfaces and their design flexibility are very appealing. In the other hand, Boolean operations on solid objects are an essential practicality for end users. Unfortunately, performing Boolean operations on Breps involve computing the intersection between algebraic surfaces, which is a very difficult task. Existing methods are often not reliable, and when they are, they are anyway exceedingly complicated: see [Pat93, KM96, HPY96]. The CSG model [Hof89] represent solid objects by a tree whose nodes carry Boolean operators and leaves carry algcbraic half-spaces (algebraic inequalities: f(~, y, Z) 5 0). In contrast to Breps, the CSG representation does not suffer from reliahilit,y problems, and the surface to surface intersection problem ‘is not a crucial issue. The raytracing method permits to visualize CSG objects and to convert them to ray representations (rayrep for short). The recursive space subdivision method permits to evaluate (ie to voxelize, or tessel&e) them as in the SVLIS modeller [Bow951 or in Taubin’s method [Tau93]. As long as a CSG modeller does not rely on tesselation, the latter can even be locally inconsistent without afffscting thr modellrr. Note the divide and conquelapproach basically relies on the possibility of quickly and simply classifying a point with respect to an algebraic half-space (by evaluating and testing the sign of the corresponding formula f(~, y, 2)). It is then possible to compute, by an interval arithmetic (or some variant), ranges of the function f for boxes (a box is a point whose coordinates are intervals): a box B is classified inside if f(B) < 0 and outside if f(B) > 0. Otherwise the box is subdivided (into 2 or 8 smaller ones, according to implementations). Such a classification test is not available for free-form objects. Unfortunately CSG does not support the full range of free-form objects. Several attempts have been made to combine appeals of CSG and Brep: l Using soft objects is mainly restricted to the Animation field for the moment [IS’95]. l In t,he CAD/CAM field, J. Menon & B. Guo [MG96] use a restricted set of free-form surfaces, with a low degree implicit form (2 or 3): each free-form patch is assigned a companion tetrahedron which contains the patch, and whose vertices are, in some way, its control points. These tetrahedra permit to edit patches in an intuitive and interactive way. This modeller allows a bilateral CSG/Brep conversion. l A. Pasko & V. Adzhiev & A. Sourin & V. Savchenko [PASS931 describe the interior of all geometric objects: algebraic half-spaces, Boolean operations, sweeps, some kind of deformations and blends, free-form volumes (sometimes called cuboids) [MPS96]. . . , by a semi-algebraic inequality f(x, Y> 2) I 0. None of the previous solutions fully integrate in the CSG model all the free-form objects used in Brep-based modellers. l A last approach combines CSG and Brep in that frerform primitives are accepted at leaves of the CSG tree. However, the simplicity of the pure-CSG scheme is lost: these modellers face the surface to surface intersection and the robustness problems. Recent works illustrating this tendency are due to S. Krishnan & D. Manocha [KM96], and to C.Y. Hu & N. Patrikalakis & X. Ye [HPY96]. No doubt for us that these modellers are masterpieces, tours de force of geometric computing. But they are too much complicated. Moreover, they do not cover all the possible cases: sweeps (occurring for instance in NC-milling), blends or Minkowski sums. .so/rdhfocfe/rf7~ ’ 97 Atlnlltn GA 1 ISA Copyright 1997 ACM 0-X9791-946-7107102 .,$3.X)

Type checking higher-order polymorphic multi-methods

Abstract: We present a new predicative and decidable type system, called ML≤, suitable for languages that integrate functional programming and parametric polymorphism in the tradition of ML [21, 28], and class-based object-oriented programming and higher-order multimethods in the tradition of CLOS [12]. Instead of using extensible records as a foundation for object-oriented extensions of functional languages, we propose to reinterpret ML datatype declarations as abstract and concrete class declarations, and to replace pattern matching on run-time values by dynamic dispatch on run-time types. ML≤ is based on universally quantified polymorphic constrained types. Constraints are conjunctions of inequalities between monotypes built from type constructors organized into extensible and partially ordered classes. We give type checking rules for a small, explicitly typed functional language a la XML [20] with multi-methods, show that the resulting system has decidable minimal types, and discuss subject reduction. Finally, we propose a new object-oriented programming language based on the ML≤ type system.

Flame retardant and mechanical properties of a copolymer PP/PE containing brominated compounds/antimony trioxide blends and magnesium hydroxide or talc

Abstract: The incorporation of two brominated compounds/antimony trioxide blends into a PP–PE copolymer were studied. Both brominated trimethylphenyl indane (FR 1808) and poly(pentabromobenzyl acrylate) (FR 1025) confer good flame retardancy at 40% loading. A comparison was made with the incorporation of magnesium hydroxide at higher loadings (up to 64%) in the same copolymer. The mineral filler improves both fire resistance and stiffness of the copolymer, nevertheless the high loading causes a dramatic decrease in impact resistance. Moreover, the necessary surface treatment of the filler significantly lowers the maximal tensile strength. In order to maximize both fire resistance and mechanical properties, we combined each brominated flame-retardant system (20% in weight) with magnesium hydroxide or talc (20% in weight). Talc is used as reference. These composites show interesting fire-retardant properties in comparison with the copolymers containing only 40% of the brominated flame-retardant systems. In addition, the mechanical properties are on the whole improved. © 1997 John Wiley & Sons, Ltd.

Robust segmentation of vessels from retinal angiography

Abstract: This paper presents an algorithm based on mathematical morphology and linear processing for vessels recognition in a noisy retinal angiography A geometrical model of undesirable patterns is defined in order to separate the vessels from their environment Bright round peaks are first extracted, allowing segmentation of microaneurisms from suitable angiograms of diabetic patients Next, linear structures are extracted using mathematical morphology, then appropriate differential properties are computed using a Laplacian filter Finally, vessels are extracted using curvature differentiation Results on various medical data from a normal eye and from a set of different abnormalities are presented and show that this algorithm can be used as a robust segmentation tool We intend to use it as a first step for image registration

Corrosion Behavior of Rapidly Solidified Magnesium-Aluminum-Zinc Alloys

Abstract: Rapidly solidified magnesium alloys with 8 at%, 15 at%, and 20 at% Al and 1 at% and 3 at% Zn were fabricated by centrifugal atomization followed by hot extrusion. Microstructure of the alloys was composed of a fine-grain magnesium matrix (0.5 μm) with β-Mg17Al12 precipitates. Electrochemical and weight-loss tests were performed in borate and ASTM D 1384 solution (chloride, carbonate, and sulfate). In both media, corrosion current of the alloys decreased with increases in aluminum or zinc content. In borate solution, a passivating plateau was observed from the corrosion potential (Ecorr) to Ecorr + 1,200 mV. Current density decreased with aluminum and zinc concentrations. Electrochemical behavior of the synthesized matrix and precipitates was characterized. Zinc increased Ecorr of the two phases, with a corresponding decrease of corrosion current. The same trend was noticed for aluminum but with a less dramatic effect. The corrosion mechanism was suggested result from galvanic coupling of the matr...

Design of phase-detection algorithms insensitive to bias modulation.

Abstract: For digital phase detection, the characteristic polynomial method permits algorithms that are insensitive to the harmonic content of the signal and insensitive to miscalibration to be designed easily. It is shown here that this method can also be used to design algorithms that are insensitive to a possible bias modulation of the intensity.

Using TOPMODEL towards identifying and modelling the hydrological patterns within a headwater, humid, tropical catchment

Abstract: Increasing pressure on the tropical environment requires a more thorough understanding of hydrological processes as part of reconciling the conflicting demands of economic development vis-a-vis sustainable land management. Using TOPMODEL, a physically based semi-distributed topohydrological model, we test its validity in modelling the stream flow dynamics (hydrograph) in a 1 ha tropical rainforest catchment in French Guiana. Another objective is through field validation of TOPMODEL to ascertain possible runoff generation mechanisms. The field validation of the temporal and spatial hydrodynamics across a rainfall-runoff event reveals that TOPMODEL may be suited for applications to this particular tropical rainforest environment; in fact, this is possibly the first successful application of such a model within the humid tropics. The main reasons why the model was successful are the presumed low hydraulic conductivities of the subsoil, coupled with the absence of an additional deep groundwater body, the contribution from which has caused difficulties in application of topographically, 'physically' based runoff models elsewhere in the humid tropics.