Showing papers by "Mines ParisTech published in 1985"

Growth transition and morphology change in polypropylene

Abstract: The growth rate of isotactic polypropylene is deduced from microscopic observations during isothermal crystallizations. A change in the growth regime is observed at 138 ‡C and interpreted as a Regime III → Regime II transition, according to Hoffman's kinetic theory of polymer crystallization. A Regime II → Regime I transition is also theoretically predicted at 155 ‡C, i. e. at a temperature outside the investigated temperature range. The Regime III → Regime II transition is related to the positive to negative change in the spherulite birefringence, which is generally attributed to a change in the organization of crystalline lamellae: quadritic arrays of intercrossing lamellae atTc 138 ‡C (Regime II). It is suggested that such a morphological change could be interpreted using the concept of “non-adjacent re-entry” introduced in Hoffman's kinetic theory. This interpretation could also explain the interspherulitic ruptures observed in negative spherulites.

Plastic banding in glassy polycarbonate under plane simple shear

Abstract: Samples of amorphous polycarbonate were tested in plane simple shear at various temperatures (−100 to +150‡ C) and shear rates (3×10−5 to 3×10−2sec−1). In the glassy state, it was observed that the deformation concentrated at yield inside a single shear band whose elongation and widening phases correspond to well marked stages of the recorded stress-strain curve. Birefringence and X-ray diffraction in the growing band show that the molecular orientation follows a pseudoaffine evolution with the local plastic strain. Although the shear is inhomogeneous during the growth of the shear band, it is fairly uniform inside the band itself, down to the scale of 100 nm. After the band has completed its widening (for an overall shear of about 0.9) the overall shear in the whole specimen is homogeneous and then one can deduce the constitutive equation for steady state plasticity of the glassy material, up to shear strains as large as 2.0. It is characterized by a linear strain hardening whose value, such as the extrapolated yield stress, decreases gradually with temperature.

Representation of deviation from ideality in concentrated aqueous solutions of electrolytes using a mean spherical approximation molecular model

Abstract: A semiempirical expression for the Helmholtz energy of mixed electrolyte solutions in solvents mixtures is obtained from the solution of the mean spherical approximation given by Planche and Renon (1981) for a nonprimitive model of interactions. A consistent set of analytical equations for the related thermodynamic properties was further derived and used to represent osmotic coefficient data of strong aqueous electrolytes at 25°C. Data correlation was performed on more than 80 systems up to a 6 M molality using generally one adjustable parameter for each single electrolyte; from these parameters the properties of mixtures can be calculated in a predictive manner without additional parameters.

Water penetration through fractured rocks: Test of a tridimensional percolation description

Abstract: We show that the mathematical concept of percolation can provide a description of the permeability of fractured rocks in three dimensions. A numerical simulation of percolation of random two-dimensional plates contained in a cubic box displays a critical behavior, around percolation threshold, characterized by a correlation length exponent similar to usual site or bond percolation. Thus, general results obtained elsewhere for that class of problem could be applied to fractured rock hydrology.

Theoretical and experimental study of the molten polymer flow in the calender bank

Abstract: The flow of molten polymers in the calender bank has been computed using a finite-element method with stream function and vorticity. Two nonsymmetrical-recirculating regions have been obtained fully in agreement with the experimental observations on poly(vinyl chloride) melt banks. The pressure distribution along the flow axis is very close to the one obtained using the classical-lubrication approximation.

Brittle fracture of polycrystalline tungsten

Abstract: A comparative study of the brittle fracture of three varieties of polycrystalline tungsten has been carried out using mainly impact tests, scanning electron microscopy and Auger spectroscopy. It indicates the occurrence of two modes of brittle fracture, namely cleavage and intergranular fractures, whose proportions depend on temperature and on phosphorus segregation at the grain boundaries. Scanning Auger images indicate unambiguously that phosphorus is only located on the intergranular surfaces. No phosphorus is observed on cleavage planes. Fracture surfaces are the result of the propagation of a crack through the whole sample. As the propagation follows the path which requires the lowest expenditure of energy, it depends both on a geometrical factor and on the respective values of the cleavage (γcl) and intergranular (γl) works of fracture. Our results indicate an increase in γcl and γl with temperature (the effect being more marked in the case of γcl) and a large decrease in γl with phosphorus segregation at the grain boundaries. This impurity produces an intergranular embrittlement of the tungsten.

Experimental study of the draw resonance in fiber spinning

Abstract: An experimental fiber-spinning device has been built in order to study the draw-resonance phenomenon in nearly isothermal conditions The influence of the output rate, of the take-up speed and of the spinning length was studied for four different polyesters The experimental results are in accordance with non-isothermal and viscoelastic computations

Change of support for diffusion-type random functions

Abstract: Diffusion-type random functions are a first attempt at a non-Markovian and multidimensional generalization of the Ito stochastic integral theory. Within this framework, the variation δf of the pdf can be evaluated for a small change of support. These results are compared with the provisions of the usual approximate models. The conclusions are: the affine correction is false for the first order approximation, unless there is a linear factor. The isofactorial model is true for the first order, and in the multi-Gaussian case, almost correct for the second-order approximation. Counterexamples are given for the discontinuous case, and a more general model is suggested.

The effect of segregation on the formation of austempered ductile iron

Abstract: Alloy segregation at the solidification cell boundaries can have a very important influence upon the kinetics of the bainite reaction. The two stages usually observed in a homogeneous matrix after isothermal holding in the range of temperatures 350 to 450 °C can be described as follows: 1 -γ→ (α) + (γ); 2-(α) + (γ) → α + silicocarbides. These reactions are functions of the elements in solution in the matrix. For this reason, segregated elements affect very sensitively the evolution of the two basic reactions. The solidification cell boundaries are often not completely transformed to bainite, which explains the presence of martensite observed in these areas. The influence of the size of the solidification cells has also been studied. The austenitization conditions are also very important. At the opposite extreme, the bainitic reaction of the matrix located along the graphite interface evolves more quickly than that observed in the matrix or at the cell boundaries.

Ion induced surface compositional changes of α brass; a comparative study by AES and sputtered thermal ion mass spectrometry. Application to quantitative AES

Abstract: Surface concentrations and sputtered-off atom concentrations of α brass are measured during ion bombardment, respectively by Auger spectroscopy and by a new quantitative sputtered thermal ion mass spectrometry method A new formulation of the problem of surface composition change is proposed which leads to a definition of matrix effect coefficients, sputter elemental yields and a sputter correction factor μ, connecting surface concentrations to sputtered-off atom concentrations Implications in quantitative analysis by Auger spectroscopy or any mass spectrometry technique are discussed

Methodology and Applications of Local Criteria for the Prediction of Ductile Tearing

Abstract: Local criteria consist, within the field of continuum mechanics, in a simulation of the metallurgical damage occurring at a crack tip. The restrictions of more usual approaches such as mode I loading, plane strain state, isothermal situation, moderate yielding etc…, are no longer effective at the expense of non linear finite element calculations. Ductile damage is predicted through a simulation of cavities growth which either influences the constitutive relations or is a criterion for fracture assuming. a critical cavity size. Practically, two parameters, independent on temperature are necessary. They are measured using two different specimen geometries: one with a crack and the other one without. These two parameters allow the prediction of fracture of one mesh element in a finite element program. This procedure is used to predict crack initiation and stable crack growth in different structures.

On the Greedy Dimension of a Partial Order

Abstract: This paper introduces a new concept of dimension for partially ordered sets. Dushnik and Miller in 1941 introduced the concept of dimension of a partial order P, as the minimum cardinality of a realizer, (i.e., a set of linear extensions of P whose intersection is P). Every poset has a greedy realizer (i.e., a realizer consisting of greedy linear extensions). We begin the study of the notion of greedy dimension of a poset and its relationship with the usual dimension by proving that equality holds for a wide class of posets including N-free posets, two-dimensional posets and distributive lattices.

High strain rate ductility in uniaxial tension : a review

Abstract: The specific problems involved by high strain rate tension tests are first examined, as well as the various parameters describing ductility. Some experimental data relative to a high purity copper then illustrate the typical increase of dynamic ductility with strain rate. Finally, the main factors which are expected to determine ductility are reviewed. It is concluded that the dominant factor is the increase of stability with strain rate, which in turn may result from the occurrence of a linear flow rule and/or from inertia effects.