Showing papers by "Mines ParisTech published in 1988"

Characterization and analysis of torrefied wood

Abstract: Physicochemical analysis are carried out on samples of torrefied pine obtained at 260°C in inert atmosphere. Gaseous effluents which evolve in the first moments of the pyrolysis as well as liquid phase are analyzed by gas-chromatography. It appears from the solid analysis, that the chemical structure of pine is strongly altered by the process. The roasted wood exhibits particular properties: pronounced hydrophobic character, improved energetic power, a noticeable increase in ash content, a decrease of the volatile components and a decrease of those extracted by neutral solvents. It is shown that the more substantial change occurs at the beginning of heating.

Combined Use of Hydraulic and Electrical Properties of an Aquifer in a Geostatistical Estimation of Transmissivity

Abstract: Many previous attempts have been made to establish an empirical relation between the electrical and hydraulic properties of aquifers. However, only regression models between transmissivity or permeability and a few electrical parameters have been used on the basis of the available pairs of data. Kriging, a geostatistical technique, estimates a regionalized variable at any point in space, and multivariate geostatistical techniques allow one to use several variables together to estimate any ‘spatial parameter. One such method, cokriging, is used to estimate the transmissivity based not only on measurements of transmissivity, but also on measurements of specific capacity and electrical transverse resistance. The studied aquifer is situated in the Medjerda Valley in Tunisia where very few data on transmissivity and specific capacity are available, but resistivity data are relatively abundant. It is shown that with the geostatistical technique, one can: (1) use several electrical or elastic properties, which are easily measured, in the estimation of the desired parameter without establishing any empirical relation; and (2) make the estimation at any point where none of these properties has been sampled and, at the same time obtain a variance of the estimation error. The method is also compared with the usual regression method.

Effect of random copolymerization on growth transition and morphology change in polypropylene

Abstract: The growth rate of an ethylene-propylene random copolymer with a 2.8% w/w ethylene content was studied in a similar manner to polypropylene. A growth regime transition associated with a birefringence change was observed at 130‡C, while the same phenomena appeared at 138‡C in isotactic polypropylene. In both polymers positive birefringence corresponds to Regime III, whereas negative birefringence of spherulites is associated with Regime II. The birefringence change is attributed to a change in the organization of crystalline lamellae: quadritic arrays of intercrossing lamellae at low temperature (Regime III) and preferentially radiating lamellae at higher temperature (Regime II). We confirm that such a morphological change can be interpreted using the concept of “non-adjacent re-entry” introduced in Hoffman's kinetic theory. Thus, quadritic morphology seems to have a partly kinetic origin. The shift of the transition temperature in the copolymer is due to the rejection of ethylene segments at the surface of crystalline lamellae of polypropylene.

High temperature, low cycle fatigue of IN-100 superalloy I: Influence of temperature on the low cycle fatigue behaviour

Abstract: Low cycle fatigue tests on cast nickel-based superalloy IN-100 were conducted at various temperatures from 20 to 1000°C in air under continuous-strain cycling at a constant total strain rate. The fatigue life was found to decrease with increasing temperature for a given total strain range. Fatigue cracks are partly crystallographic at low temperatures and strongly oxidized at high temperatures. The reduction in fatigue life is discussed using potential drop measurements and observations on interrupted test specimens. Testing at 1000°C is shown to reduce drastically the crack initiation period and this behaviour is attributed to oxidation.

Lawyers and Politics in France, 1814–1950: The State, the Market, and the Public

Abstract: Throughout the 19th century, lawyers in France were deeply involved in political action to pursue an overriding goal–to become recognized as spokesmen for the public. This strategy governed their history; it explains their brilliant social ascent and their subsequent slow decline. As long as the conflict between state and civil society raged, lawyers were able to we assets–political mobilization, the power of the word, the esteem enjoyed by law–which had allowed them faithfully to embody public opinion in its struggle to limit state powers. From this embodiment of public ideals they derived independence, prestige, and a dominant position in the state. But when the nature of the political regime ceased to be a bone of contention and when public life became organized around other cleavages, lawyers were gradually deprived of their representative function. This marked the beginning of a social decline that became visible between the two world wars and lasted until the 1950s.

Fatigue Oxidation Interaction in IN 100 Superalloy

Abstract: The interaction between fatigue and oxidation has been studied in IN 100,* a cast nickel based su-peralloy tested in laboratory air at 1000 °C. The effect of fatigue cycling on oxidation was studied by quantitative metallography on polished specimens which were oxidized in a furnace and on strain cycled low cycle fatigue specimens. Thickness measurements have shown that matrix oxidation obeys a parabolic kinetics and is strongly enhanced by fatigue cycling. Measurements of oxide spikes have shown that interdendritic oxidation obeys at1/4kinetics and is weakly affected by cyclic straining. A phenomenological equation was proposed which accounts for this interaction through fatigue induced fracture events in the oxide scale. Analysis of the distribution histograms shows that the actual life to crack initiation is identical to the number of cycles to break the matrix oxide scale in the most critical areas.

Thermal fatigue of mar‐m509 superalloy—i. the influence of specimen geometry

Abstract: — The thermal fatigue behaviour of a cast cobalt superalloy was investigated. Wedge type specimens were tested on a rig using flame heating. A standard cycle from 200 to 1100°C was used for most tests and the specimen geometry was changed to investigate a broad range of thermal fatigue lives. The maximum temperature was also varied for a single specimen geometry. The thermal fatigue life to create a 1 mm deep crack ranges from a few tens to one thousand cycles. Crack initiation occurs early in life at oxidised interdendritic carbides as in high-temperature, low-cycle fatigue. Precipitation of small chromium-rich carbides was found to occur in the dendrites during thermal fatigue cycling. An identical precipitate distribution could be induced by a two stage thermal treatment as shown by quantitative metallography. The isothermal stress-strain behaviour at high temperature was so shown to be almost insensitive to the microstructure changes induced by thermal fatigue.

Cyclic deformation mechanisms of a two-phase stainless steel in various environmental conditions

Abstract: The physical mechanisms which control fatigue damage in air and in a 3.5% NaCl solution of a two-phase (ferritic (α)-austentic (γ)) stainless steel with a 50% ferrite content are examined. The study focuses on the damage mechanisms leading to crack initiationat an imposed plastic strain amplitude in various environmental conditions: dry laboratory air, conditions of passivity, conditions of pitting and at cathodic potentials in the 3.5% NaCl solution. The fatigue properties of the two-phase alloy in air are related to a mechanical coupling effect between the two phases. At plastic strain amplitudes higher than 10−3, the fatigue behaviour of the α-γ alloy is related to the basic cyclic deformation mechanisms of the α phase (which exhibits twinning and pencil glide) and at lower plastic strain amplitudes to the cyclic properties of the γ phase (which exhibits planar slip). Finally the better resistance to corrosions fatigue of the α-γ alloy in comparison with those of its individual phases is emphasized and explained by both mechanical and electrochemical coupling effects between the α and γ phases of the two-phase alloy.

Improving the validity of Mössbauer hyperfine parameter distributions: The maximum entropy formalism and its applications

Abstract: The maximum entropy principle from information theory is applied to the problem of determining hyperfine distributions from Mossbauer spectra. This method is compared to the usual matrix method and it is shown that the smoothing parameter of the latter can be avoided. The method is applied to several simulated spectra calculated from model distributions, and to an experimental spectrum showing a spin density wave (or Overhauser) distribution. It is shown that the maximum entropy method can lead to a more model-independent determination of the distribution. It is especially useful in cases where the hyperfine distribution contains a sharp maximum.

High temperature, low cycle fatigue of IN-100 superalloy II: Influence of frequency and environment at high temperatures

Abstract: Low cycle fatigue tests on cast nickel-based superalloy IN-100 were conducted at 1000°C in air using sawtooth strain cycles over a wide range of frequencies (4 × 10−3 −2 Hz) and tensile strain hold time tests. The influence of environment was investigated using tests under vacuum at the same temperature. The low cycle fatigue life was found to be strongly frequency dependent between 5 × 10−2 and 1 Hz. Testing in vacuum results in a much longer fatigue life than in air except for the higher test frequency. Crack growth rate data were measured using the potential drop technique and were used to show that the fatigue life in air is dominated by crack growth and to assess the influence of oxidation effects.

Creep and Creep-Fatigue Intergranular Damage in Austenitic Stainless Steels: Discussion of the Creep-Dominated Regime

Abstract: Creep and strain controlled low-cycle fatigue tests with an imposed hold time at maximum tensile strain were carried out, mainly at 600°C, on two heats of 316L stainless steel. Very long times to failure (≃ 10 4 h) corresponding to low applied strain and long dwell period (24 h) were investigated. Both fatigue and creep damage were measured by quantitative metallography. Creep intergranular surface cracking shortens the initiation stage of fatigue cracks, while creep intergranular bulk cracking accelerates the average fatigue crack growth rate. These observations are the basis for a creep-fatigue interaction model previously proposed using intergranular damage as a life correlating parameter. The applicability of this model is tested with results of 10 4 h creep-fatigue tests showing a saturation effect. The saturation in creep-fatigue life observed for long dwell periods is explained by an improvement in creep ductility. This intergranular damage approach is discussed in the light of the creep dominated regime concept. It is shown that pure creep failure data cannot provide reliable prediction of creep fatigue life. The need to use a model that takes into account the effect of creep intergranular damage on fatigue crack propagation is emphasized.

Thermal fatigue of mar-m509 superalloy—ii. evaluation of life prediction models

Abstract: — Thermal fatigue data on MAR-M509 reported in a companion paper were used to evaluate four life prediction models The temperature-stress-strain history of the critical element at the thin edge of wedge specimens was computed for this purpose The analysis method uses a finite element computation of the temperature field and a uniaxial calculation of the stress-strain cycle using a cyclic viscoplastic constitutive equation The influence of specimen geometry and of maximum temperature on the thermal fatigue life to initiate a macroscopic crack was accounted for by variations in stress and mechanical strain ranges at the thin edge The accumulated and cyclic creep damage models were found to overestimate thermal fatigue life in all the cases Models which describe oxidation fatigue crack growth interactions, and which are borne out by metallographic observations on wedge specimens, were found to give reliable predictions of thermal fatigue life

Indirect adaptive nonlinear control

Abstract: The authors consider how to make a nonlinear control law adaptive. Assuming that the uncertainties interfere linearly, they answer this question for a certain class of systems. In particular, they propose an estimation-based adaptive controller with corrective terms for more general families than robot arms. Assuming some structural properties, they obtain global convergence under far less restrictive growth assumptions than previously. >

Une approche micromécanique du comportement des polycristaux

Abstract: Description theorique du comportement inelastique multiaxial des polycristaux par une approche situee entre les approches classiques (macro-macro) et les theories des lignes de glissement (micro-macro). Dans cette approche, on introduit les lois de comportement macroscopiques au niveau microscopique en utilisant un formalisme general de type autocoherent. Application a l'acier inoxydable austenitique AISI 316 soumis a des chargements multiaxiaux ou uniaxiaux, proportionnels ou non. Comparaison avec les resultats experimentaux

Topological instability of metallic lattices and glass formation

Abstract: The problem of glass formation is addressed in this paper in an attempt to describe how amorphization takes place rather than to explain why it occurs. Although sophisticated models that consider the detail of the atomic interactions are undoubtedly relevant, it is also well known that glass formation is very much related to the atomic size ratio of the alloy constituents. A general framework for the interpretation of such experimental evidence is proposed on the basis of a comparison with the formation of crystalline intermetallic compounds which result from the instability of metallic solid solutions because of inadequate matching of atomic sizes. The case of interstitial solid solutions is explored into some detail with reference to computer simulations of the topological instability and modelling of the glass structure. Icosahedral ordering in liquid alloys is briefly examined.

Polymorphism of crystalline phases of calcium stearate

Abstract: The structures of the low temperature (T⩽400 K) phases of calcium stearate have been investigated by X-ray diffraction. Several forms are shown to exist at room temperature, depending on hydration and thermal treatment. New transitions at cryogenic temperatures are discussed. Transitions at higher temperatures have been investigated together with their reversibility. Thermogravimetry and two calorimetric techniques have been used as a complement. This data completes the already somewhat complex picture of the phase transitions of calcium stearate.

Finite element computation of bulging in continuously cast steel with a viscoplastic model

Abstract: The bulging of continuously cast steel between the support rolls has been computed using a viscoplastic finite element model. Both two-dimensional and three-dimensional models are based on the minimization of the functional associated with the Norton–Hoff law and use the streamlines integration to calculate the bulging. By using industrial data, the influence of both numerical parameters and process parameters on the extent of the calculated bulging values has been investigated. The comparison of our results with experimental measurements confirms the relevance of the hypothesis of viscoplastic behaviour.

A New Approach to Low Cycle Fatigue Behavior of a Duplex Stainless Steel Based on the Deformation Mechanisms of the Individual Phases

Abstract: This paper concerns the low-cycle fatigue (LCF) behavior of a duplex (austenitic-ferritic) stainless steel with a 50% ferrite content. The study focuses on the damage mechanisms leading to crack initiation in the duplex alloy cycled at imposed plastic strain amplitude. Furthermore, the behavior of two other alloys of compositions close to those of the α and γ phases is simultaneously examined and correlated to the duplex alloy cyclic properties. The LCF properties of the duplex alloy approach those of the ferrite phase at high strain amplitudes and those of the austenite phase at low strain amplitudes. Many observations confirm this result: the Coffin-Manson curves, the hardening/softening curves, the cyclic stress-strain curves, and the crack initiation sites indicate a change in the duplex behavior near a plastic strain amplitude of 10 - 3 . The proposed analysis of the LCF behavior of the duplex alloy is based on the cyclic plastic deformation mechanisms of the a phase (which exhibits twinning and pencil glide) and of the γ phase (with planar slip). Finally, the influence of a 3.5% NaCI solution on the LCF properties of the duplex alloy clearly underlines the occurrence in this alloy of two different types of behavior according to the applied plastic strain amplitude.

Sign determination of the57Fe electric field gradient in Al-based crystals and icosahedral quasicrystals

Abstract: The icosahedral quasicrystals i-AlMn isomorphically suostituted by 28 at.% Fe or by (CrFe) are studied for the first time by in-field Mossbauer spectroscopy (MS) in order to determine the sign and asymmetry parameter of the dominant EFG term. In addition, the orthorhombic o-Al(MnFe) and cubic α- and hexagonal β-Al(MnFe)Si phases are studied. We show that previous MS results are inadequate to determine if there are two sites in the i-AlMn structure in the ratio of the golden number. Our results for i-Al(MnFe) show that the dominant EFG is negative, with asymmetry parameter of about 0.6. For i-Al(CrFe), essentially no deviations are found from the model of Czjzek or the Gaussian isotropic model. The β phase is found to have a very similar quadrupole effect as that in i-Al(MnFe).

Structure and anisotropy of antiphase boundaries in rapidly solidified Ni3Al

Abstract: Antiphase boundaries (APBs) were studied by transmission electron microscopy in rapidly solidified Ni3Al compounds. It is shown that the antiphase vector can be decomposed into R = R o + r. The main term R o = ½ [110] gives rise to a strong fringe pattern, of the π type, when superlattice reflections are present. A small relaxation term r, giving a faint fringe contrast when only fundamental reflections (i.e. 111) are acting, was found to be maximum when lying in the boundary plane. For a given R o, the modulus of r varies with the boundary orientation. Comparison between experimental and simulated images indicated that r is at least one order of magnitude smaller than R o. Statistical determinations of the APB orientations evinced no anisotropy in their distribution. It is concluded that r restores the isotropy of the APB energy, which is unexpected if R = R o.