K (lettre)

Résumé

K est la onzième lettre et la consonne de l'alphabet latin. Histoire Cette lettre de l'alphabet latin vient de l'étrusque ; c'est un descendant du kappa de l'alphabet grec (Κ en majuscule et κ en minuscule). centré|vignette|600x600px|Formes majuscules et minuscules manuscrites de la lettre K dans le Dictionnaire des abréviations latines et française d’Alphonse Chassant. Tracé On trouve couramment trois variantes du « K » (haut-de-casse), par ordre de fréquence :

la jambe se joint à l’autre diagonale avant d’atteindre le fût (la majorité des polices dont les plus courantes comme Arial) ;
les deux diagonales se rejoignent au même niveau sur le fût (un certain nombre de polices courantes comme Futura, Gill Sans, etc.) ;
les deux diagonales forment une jonction horizontale avant le fût (quelques rares polices comme Adam BP, Cordale, etc.).

On retrouve ces mêmes trois variantes pour le « k » (bas-de-casse) avec un quatrième tracé où la diagonale du haut f

Source officielle

https://fr.wikipedia.org/wiki/K_(lettre)

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New theory for. Mode I crack-tip dislocation emission

Predrag Andric, William Curtin

A material is intrinsically ductile under Mode I loading when the critical stress intensity K-Ie for dislocation emission is lower than the critical stress intensity K-Ic for cleavage. K-Ie is usually evaluated using the approximate Rice theory, which predicts a dependence on the elastic constants and the unstable stacking fault energy gamma(usf) for slip along the plane of dislocation emission. Here, atomistic simulations across a wide range of fcc metals show that K-Ie is systematically larger (10-30%) than predicted. However, the critical (crack tip) shear displacement is up to 40% smaller than predicted. The discrepancy arises because Mode I emission is accompanied by the formation of a surface step that is not considered in the Rice theory. A new theory for Mode I emission is presented based on the ideas that (i) the stress resisting step formation at the crack tip creates lattice trapping against dislocation emission such that (ii) emission is due to a mechanical instability at the crack tip. The new theory is formulated using a Peierls-type model, naturally includes the energy to form the step, and reduces to the Rice theory (no trapping) when the step energy is small. The new theory predicts a higher K-Ie at a smaller critical shear displacement, rationalizing deviations of simulations from the Rice theory. Specific predictions of K-Ie for the simulated materials, usually requiring use of the measured critical crack tip shear displacement due to complex material non-linearity, show very good agreement with simulations. An analytic model involving only gamma(usf), the surface energy gamma(s), and anisotropic elastic constants is shown to be quite accurate, serves as a replacement for the analytical Rice theory, and is used to understand differences between Rice theory and simulation in recent literature. The new theory highlights the role of surface steps created by dislocation emission in Mode I, which has implications not only for intrinsic ductility but also for crack tip twinning and fracture due to chemical interactions at the crack tip. (C) 2017 Elsevier Ltd. All rights reserved.

2017

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A current goal driving alloy development is the identification of alloy compositions for high temperature applications but with the additional requirement of sufficient ductility at ambient temperatures. Multicomponent, single-phase, polycrystalline High Entropy Alloys (HEAs) have recently emerged as a new class of metal alloys, and some refractory bcc HEAs composed mainly of Nb, V, Ta, Cr, Mo, and/or W show excellent strength retention up to very high temperatures but low ductility at room temperature (RT). Here, it is postulated that the macroscopic ductility in bcc elements and alloys is determined by the intrinsic competition between brittle cleavage and ductile dislocation emission mechanisms at an atomistically sharp crack. The stress intensities K-Ic for cleavage and K-Ie for emission are evaluated within Linear Elastic Fracture Mechanics and validated by atomistic simulations on model alloys. A RT ductility criterion based on K-Ie/K-Ic for critical crack orientations is proposed based on the elemental metals and is then applied to HEAs. Agreement with experimental trends in ductility vs. composition across a range of existing HEAs is demonstrated. The analysis is then extended across large composition spaces of the Mo-Nb-Ta-V-W and Mo-Nb-Ti alloy families, identifying new compositions with the potential for RT ductility.

PERGAMON-ELSEVIER SCIENCE LTD2021

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New synthetic seven-membered 1-azasugars displaying potent inhibition towards glycosidases and glucosylceramide transferase

Claudia Bello, Sylvain Favre, Tao Liu, Pierre Vogel

Several members of a new family of seven-membered azasugars, which can be seen as 1-azasugar ring homologues, have been I obtained by simple chemical transformations starting from a sugar-derived azidolactol. Unlike their piperidine counterparts, these molecules are chemically stable when they possess a hydroxy group at the pseudo-C-2 position. Biological assays with a range of carbohydrate-processing enzymes have revealed interesting potential for these compounds. A trihydroxymethyl-substituted azepane displayed strong competitive inhibition on almond beta-glucosidase (K-i = 2.5 mu M) while a trihydroxylated corboxylic acid derivative proved to be a potent and selective L-fucosidase inhibitor K-i = 41 nM). N-Butylation of these seven-membered 1-azasugars generated derivatives with some activity towards the Gaucher's disease-related glucosylceramide transferase (IC50) 75 mu M) that did not interact significantly with digestive glucosidases.

2008

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New theory for. Mode I crack-tip dislocation emission

Predrag Andric, William Curtin

2017