Mines Paris - PSL

Using X-ray computed tomography and focused ion beam-scanning electron microscopy (FIB-SEM) serial sectioning, this study investigated the three-dimensional propagation behavior of hydrogen-related intergranular cracks in martensitic steel with a tensile strength of 1.2 GPa, focusing on segments of prior austenite grain boundaries (PAGB segments). X-ray computed tomography revealed that the crack morphology was more continuous in the hydrogen-charged specimens. Through FIB-SEM serial sectioning, we found that crack-tip blunting and ductile rupture of un-cracked ligaments were associated with specific PAGB segments in the uncharged specimen. In the case of hydrogen-related intergranular crack propagation, even very fine, low-angle PAGB segments (sub-micrometer size) can act as obstacles for crack propagation. Based on these results, we propose that the misorientation of each PAGB segment has a large influence on the local arrestability of intergranular crack propagation.

The present study investigated the local arrestability of intergranular crack in high-strength martensitic steel through multi-scale three-dimensional (3D) analysis using X-ray computed tomography and focused ion beam machining (FIB)-scanning electron microscopy (SEM) serial sectioning combined with electron backscattering diffraction (3D EBSD). Macroscopic analysis using X-ray computed tomography demonstrated discontinuous propagation of the intergranular cracks, indicating local arrest of the crack propagation. An analysis of the opening displacement of each crack component revealed that the resolved normal stress was not the only factor determining the intergranular crack propagation path. The relationship between the microstructure and local crack-arrestability was microscopically analyzed using FIB-SEM serial sectioning. The 3D EBSD analysis clearly suggested that the crack propagations were arrested at the low-angle grain boundary plane segments and at the grain boundary triple junctions surrounded by relatively large martensite variants. Moreover, the larger martensite variants around grain boundaries contributed to the plastic accommodation of stress concentration and promoted crack-tip blunting. We propose that increasing the fraction of low-angle grain boundary plane segments as well as that of large martensite variants existing around grain boundaries can enhance local crack-arrestability and retard intergranular fracture of high-strength martensitic steels.

Using a vibrating tube density meter, the densities and speeds of sound of 1,2-dichloroethane, methyl tert-butyl ether, diisopropyl ether, and diethyl ether were measured at T = 278.15 to 333.15 K and at atmospheric pressure. The densities of the three binary mixtures (1,2-dichloroethane + ethers) were also experimentally determined. The Redlich–Kister data treatment and partial molar volume were applied to the excess molar volume of the mixtures. To conclude, the Prigogine–Flory–Patterson model was used to predict the excess molar volume of the three binary systems and to determine the most significant contribution to the excess molar volume.

SUMMARY Seismicity rate is sometimes observed to correlate with periodic stress oscillations, such as seasonal changes in hydrological loads, or solid-earth and oceanic tidal stresses. However, seismicity modulation is far from being systematic, often weak and the mechanical control is not yet fully understood. Here, a 3-D interacting fault population loaded by a periodic stress has been studied. For that a new earthquake simulator coupling rate-and-state friction and 3-D quasi-dynamic elasticity has been developed, and used to explore the mechanical conditions leading to seismicity modulation. It is first shown that seismicity modulation is maximized when the amplitude of periodic loading stress approaches aσ, a being the viscous parameter of the rate-and-state friction and σ the normal stress in the earthquake source region. The mode of modulation is frequency dependent: short loading periods lead to a stress control (seismicity rate follows the Coulomb stress) while longer periods lead to stressing rate control (seismicity rate tracks Coulomb stressing rate variations). An analytical expression for the critical period Tc separating the two regimes is derived from Dieterich’s theory, showing a dependence of the transition on Coulomb stress amplitude. Finally, the average seismicity rate either scales exponentially with Coulomb stress (stress control), or linearly with Coulomb stressing rate (stressing rate control), as suggested by Dieterich’s theory. These features indicate that for the range of parameters explored, stress redistribution within the fault population is a second order effect in the seismicity modulation, which is primarily controlled by the properties of remote loading. This paper therefore confirms with numerical simulations the Coulomb stress and stressing rate control of seismicity rate, and the limited importance of stress redistribution in controlling earthquake production anticipated by previous theoretical studies.

The Japanese MMX sample return mission to Phobos by JAXA will carry a rover developed by CNES and DLR that will be deployed on Phobos to perform in situ analysis of the Martian moon’s surface properties. Past images of the surface of Phobos show that it is covered by a layer of regolith. However, the mechanical and compositional properties of this regolith are poorly constrained. In particular, from current remote images, very little is known regarding the particle sizes, their chemical composition, the packing density of the regolith as well as other parameters such as friction and cohesion that influence surface dynamics. Understanding the properties and dynamics of the regolith in the low-gravity environment of Phobos is important to trace back its history and surface evolution. Moreover, this information is also important to support the interpretation of data obtained by instruments onboard the main MMX spacecraft, and to minimize the risks involved in the spacecraft sampling operations. The instruments onboard the Rover are a Raman spectrometer (RAX), an infrared radiometer (miniRad), two forward-looking cameras for navigation and science purposes (NavCams), and two cameras observing the interactions of regolith and the rover wheels (WheelCams). The Rover will be deployed before the MMX spacecraft samples Phobos’ surface and will be the first rover to drive on the surface of a Martian moon and in a very low gravity environment.Graphic

Iron and steels are allotropes, meaning they exhibit different crystal configurations. The martensitic transformation is crucial for a variety of processes, such as hardening. It is induced by a combination of undercooling and mechanical deformation. Due to the changing material properties within the phases, and due to topological changes that might occur during the transformation, a phase field approach was chosen that incorporates both the mechanical and the chemical aspect of this problem. A comparison of the Voigt/Taylor approach to the Khachaturyan approach within a multi-variant phase field modeling of the martensitic transformation including a chemical and a mechanical energy contribution is presented in this paper. The model was implemented in the finite element codes FEAP and Z-set independently. Numerical examples are given in order to highlight the features of this model.

A fuel microemulsion based on blend of diesel fuel:sunflower oil (4:1), water, 1-butanol (co-surfactant), and a blend of Tween 80, Span 80 and 1-buthyl-3-methylimidazolium nitrate ionic liquid as surfactant is formulated and optimized regarding the amounts of co-surfactant, water and oily phase. The fuel microemulsions were for upto one year stable on the bench and their density and viscosity were comparable to neat diesel fuel. The size of water droplets in fuel microemulsions are measured by dynamic light scattering (about 10 nm) with a very narrow size distribution. The optimum results are obtained by using a blend of Span 80:Tween 80:1-buthyl-3-methylimidazolium nitrate, 70:26.25:3.75, respectively, as surfactant. The engine combustion tests at various engine speeds and full load indicated that the brake-specific fuel consumption, torque, brake power and exhaust gas temperature are decreased in fuel microemulsion compared to neat diesel fuel. Carbon dioxide and nitrogen oxide emissions were reduced by 4–5%, and unburned hydrocarbons and carbon monoxide emissions were increased by 18.4% and 1.18%, respectively, for optimum fuel microemulsion compared to neat diesel fuel. The results are promising to introduce a way to improve the diesel fuel performance in terms of its environmental impacts.

Cet article est une contribution à la fois empirique et théorique à la problématique de la construction d’expertise, dans ses rapports à l’organisation et à l’innovation au sein d’une grande entreprise. Il est centré sur un cas d’étude célèbre : l’invention du transistor au sein des Bell Labs d’AT&T entre 1936 et 1950. Une réexploration de cette histoire, mobilisant la théorie C/K de manière rétrospective pour suivre au plus près la démarche de conception du transistor, permet d’analyser le processus d’élaboration d’une expertise nouvelle. Les Bell Labs lancent en effet, dès le milieu des années 1930, un programme de recherche fondamentale sur la physique des semiconducteurs comme préalable à la conception de dispositifs électroniques nouveaux. L’inconnu en jeu tient aux lacunes théoriques et technologiques considérables qu’il s’agit de combler, par la construction de connaissances, de savoirs et de savoir-faire totalement neufs. Le programme, accéléré par les ruptures techniques et organisationnelles du second conflit mondial, aboutira à l’invention de plusieurs prototypes de transistors en 1947-1948 et à une expansion sans précédent des technologies des semiconducteurs, reposant sur l’expertise construite. L’analyse éclaire ainsi l’interdépendance entre stratégie d’entreprise, innovation et construction d’expertise, en même temps qu’elle alimente les réflexions récentes sur les modes d’élaboration d’expertise sur un domaine inconnu.