Cohesive stress heterogeneities and the transition from intrinsic ductility to brittleness

International audienceThe influence of nanoscale cavities on the fracture of the Σ33{554}[110] symmetrical tilt grain boundary is studied by atomistic simulations. The crack crystallography is chosen such that dislo-cation emission is easy. A transition from a ductile behavior of the tip to a brittle one is obtained for a dense (coverage beyond 15% and inter-cavity spacing smaller than 4 nm) distribution of small cavities (sizes in between 1 and 2 nm). The results are in good agreement with recent experiments from the literature. Even at the highest coverage, the character of the crack is highly sensitive to the initial position of the tip and a mixture of ductile and brittle responses is found. This complexity is beyond the usual criterion based on the drop of the work of separation with the amount of damage in the structure. It is shown that a heterogeneous cohesive zone model, with parameters extracted from the simulations and enriched with a criterion for plasticity, can explain the simulations and reproduce the transition. Additional simulations show that outside this range of small sizes and dense packing, which gives essentially a two dimensional response (either crack opening or infinite straigt dislocation emission), dislocation half loops appear for inter-cavity spacing starting at about 4 nm. They constitute, together with regions of low covage/small cavities, efficient obstacles to brittle cracking. These results could be guidelines to designing interfaces more resistant to solute embrittlement, in general. The cohesive zone model is generic. Furthermore, the {554} single crystal was used to determine to which extent the results depend on the details of the core structure vs. the cavity distribution. These elements show that the conclusions reached have a generic character

Site stability and pipe diffusion of hydrogen under localised shear in aluminium

International audienceThis paper studies the effect of a plastic shear on the tetrahedral vs. octahedral site stability for hydrogen, in aluminium. Based on Density Functional Theory calculations, it is shown that the tetrahedral site remains the most stable site. It transforms into the octahedral site of the local hexagonal compact structure of the intrinsic stacking fault. The imperfect stacking is slightly attractive with respect to a regular lattice site. It is also shown that the shearing process involves a significant decrease of the energetic barrier for hydrogen jumps, at half the value of the Shockley partial Burgers vector, but not in the intrinsic stacking fault. These jumps involve a displacement component perpendicular to the shearing direction which favours an enhancement of hydrogen diffusion along edge dislocation cores (pipe diffusion). The magnitude of the boost in the jump rate in the direction of the dislocation line, according to Transition State Theory and taking into account the zero point energy correction, is of the order of a factor 50, at room temperature. First Passage Time Analysis is used to evaluate the effect on diffusion which is significant, by only at the nanoscale. Indeed, the common dislocation densities are too small for these effects (trapping, or pipe diffusion) to have a signature at the macroscopic level. The observed drop of the effective diffusion coefficient could therefore be attributed to the production of debris during plastic straining, as proposed in the literature

Étude comparative tensiomètre automatique versus doppler à ultrasons dans la mesure de l’index de pression systolique à la cheville (Ipsc)

ObjectifL’évaluation de l’index de pression systolique à la cheville (IPSC) est la méthode non invasive la plus simple de dépistage de l’artériopathie des membres inférieurs (AOMI) mais reste peu usitée en pratique courante, car elle est chronophage, implique un équipement et une formation appropriés. Le but de notre étude est d’évaluer la validité et la fiabilité de la mesure de l’IPSC à l’aide d’un tensiomètre automatique par comparaison avec les résultats obtenus par doppler continu, méthode de référence. Méthodes Les patients de 65 ans et plus hospitalisés au CHU d’Angers de juillet 2008 à mars 2009 en médecine interne et n’ayant pas d’AOMI documentée au moment de l’inclusion ont participé à l’étude. Les critères d’exclusion étaient la présence d’une fibrillation auriculaire et une incompressibilité des artères distales des membres inférieurs. Pour chaque patient, l’IPSC a été évalué par tensiomètre automatique et par la méthode Doppler de référence ; ces mesures étant faites par le même examinateur. La concordance entre les deux mesures était appréciée par la méthode de Bland et Altman et le coefficient de corrélation intraclasse (ICC). Résultats Sur 287 patients participant à l’étude, 221 ont étés inclus. Cinquante-six (25 %) de ces patients avaient un IPSC < 0,90. La concordance entre les deux méthodes était médiocre : ICC à 0,35 ± 0,08 et moyenne à 0,08 ± 0,26 selon la méthode de Bland et Altman. Conclusion La méthode de dépistage de l’AOMI par mesure de l’IPSC par tensiomètre automatique ne peut être retenue comme une alternative à la méthode Doppler

Particle displacements in the elastic deformation of amorphous materials: local fluctuations vs. non-affine field

We study the local disorder in the deformation of amorphous materials by decomposing the particle displacements into a continuous, inhomogeneous field and the corresponding fluctuations. We compare these fields to the commonly used non-affine displacements in an elastically deformed 2D Lennard-Jones glass. Unlike the non-affine field, the fluctuations are very localized, and exhibit a much smaller (and system size independent) correlation length, on the order of a particle diameter, supporting the applicability of the notion of local "defects" to such materials. We propose a scalar "noise" field to characterize the fluctuations, as an additional field for extended continuum models, e.g., to describe the localized irreversible events observed during plastic deformation.Comment: Minor corrections to match the published versio

What is the probability of connecting two points ?

The two-terminal reliability, known as the pair connectedness or connectivity function in percolation theory, may actually be expressed as a product of transfer matrices in which the probability of operation of each link and site is exactly taken into account. When link and site probabilities are $p$ and $\rho$, it obeys an asymptotic power-law behavior, for which the scaling factor is the transfer matrix's eigenvalue of largest modulus. The location of the complex zeros of the two-terminal reliability polynomial exhibits structural transitions as $0 \leq \rho \leq 1$.Comment: a few critical polynomials are at the end of the .tex source fil

Continuum limit of amorphous elastic bodies (III): Three dimensional systems

Extending recent numerical studies on two dimensional amorphous bodies, we characterize the approach of elastic continuum limit in three dimensional (weakly polydisperse) Lennard-Jones systems. While performing a systematic finite-size analysis (for two different quench protocols) we investigate the non-affine displacement field under external strain, the linear response to an external delta force and the low-frequency harmonic eigenmodes and their density distribution. Qualitatively similar behavior is found as in two dimensions. We demonstrate that the classical elasticity description breaks down below an intermediate length scale $\xi$, which in our system is approximately 23 molecular sizes. This length characterizes the correlations of the non-affine displacement field, the self-averaging of external noise with distance from the source and gives the lower wave length bound for the applicability of the classical eigenfrequency calculations. We trace back the "Boson-peak" of the density of eigenfrequencies (obtained from the velocity auto-correlation function) to the inhomogeneities on wave lengths smaller than $\xi$.Comment: 27 pages, 11 figures, submitted to Phys. Rev.

Bone Quality Changes In 3 Cases Of Hypophosphatasia: A Ftiri Study

International audienc

Cleavage fracture micromechanisms related to WPS effect in RPV steel

International audienceThe main objective of this paper is to characterise the changes caused by warm pre-stress (WPS) on the cleavage fracture micromechanisms of a 18MND5 (A533B) reactor pressure vessel (RPV) steel. In this purpose, different WPS fracture test results obtained on compact tensile (CT) and notched tensile (NT) geometries are presented and compared with isothermal test data. While confirming some well-established features of WPS, these experimental results, analysed through extensive fractographic investigations and finite element (FE) calculations, demonstrate a strong material aspect to WPS. One unveiled characteristic of the WPS effect is the deactivation of particles at high temperature through plastic straining under low stress, which enables breaking, or detaching from the matrix, the most critical particles without causing unstable cleavage propagation. In 18MND5 steel, these particles are TiN and carbide particles: according to the pre-stress level and fracture geometry, there is a progressive shift from TiN- to carbide-induced cleavage, up to the replacement of both by ductile type nucleation mechanisms