Добірка наукової літератури з теми "Gorge annulaire"

The basal Cambrian marks the beginning of an important chapter in the history of life. However, most paleontological work on the basal Cambrian has been focused on skeletal animal fossils, and our knowledge about the primary producers—cyanobacteria and eukaryotic phytoplankton (e.g., acritarchs)—is limited. In this research, we have investigated basal Cambrian acritarchs, coccoidal microfossils, and cyanobacteria preserved in phosphorites and cherts of the Yanjiahe Formation in the Yangtze Gorges area (South China) and the Yurtus Formation in the Aksu area (Tarim Block, northwestern China). Our study confirms the occurrence in these two formations of small acanthomorphic acritarchs characteristic of the basal CambrianAsteridium–Comasphaeridium–Heliosphaeridium(ACH) assemblage. These acritarchs include abundantHeliosphaeridium ampliatimi(Wang, 1985) Yao et al., 2005, commonYurtusia uniformisn. gen. and n. sp., and rareComasphaeridium annulare(Wang, 1985) Yao et al., 2005. In addition, these basal Cambrian successions also contain the clustered coccoidal microfossilArchaeophycus yunnanensis(SonginLuo et al., 1982) n. comb., several filamentous cyanobacteria [Cyanonema majusn. sp.,Oscillatoriopsis longaTimofeev and Hermann, 1979, andSiphonophycus robustum(Schopf, 1968) Knoll et al., 1991], and the tabulate tubular microfossilMegathrix longusL. Yin, 1987a, n. emend. Some of these taxa (e.g.,H. ampliatum, C. annulare, andM. longus) have a wide geographic distribution but occur exclusively in basal Cambrian successions, supporting their biostratigraphic importance. Comparison between the stratigraphic occurrences of microfossils reported here and skeletal animal fossils published by others suggests that animals and phytoplankton radiated in tandem during the Cambrian explosion.

Objectives Although aortic annulus repair has a long history, there are still no ideal devices to control an aortic annulus. We have developed a new method involving the use of an expanded polytetrafluoroethylene graft with the support of a metallic ring holder during implantation from inside an aorta, with no dissection of the surrounding aortic annulus. Methods We used aortic annular rings of 18 to 24 mm made of Gore-Tex tubed grafts (W.L. Gore & Associates, Flagstaff, AZ, USA) and metallic ring holder. After cutting the graft circumferentially to a 5-mm thickness, it was compressed manually to decrease the thickness. Then, a metallic ring holder corresponding to the graft size was inserted into the graft. The metallic ring holder was fixed to the graft with one 4–0 monofilament suture using 6 holes and the side trench on the metallic ring holder. The proper size for the graft was determined, and the appropriate annuloplasty ring was selected. A row of 4–0 double-needle braided sutures with expanded polytetrafluoroethylene spaghettis was sewn from the aortic side into the left ventricular outflow tract with a horizontal mattress stitch. After all sutures around the annulus were placed, they were tied down and the metallic ring holder was removed. The leaflets were repaired if necessary. Results A total of 9 patients with tricuspid aortic valve have undergone this procedure since January 2015. The sizes of the aortic annular rings were 20 mm ( n = 3), 22 mm ( n = 5), and 24 mm ( n = 2). Aortic leaflets were repaired in 6 patients (6 central plications). Concomitant procedures were total aortic arch replacement ( n = 2), mitral valve repair ( n = 2), tricuspid valve repair ( n = 1), and coronary artery bypass grafting ( n = 2). There were no hospital deaths and no major morbidities. All patients were checked for mild or less than mild aortic regurgitation during the mean follow-up period of 13 months. Postoperative echocardiograms showed excellent peak pressure gradients compared with preoperative echocardiograms. Postoperative cardiac computed tomography scans were performed in 6 patients. The discrepancies between the ring size used and postoperative annular size were <1-mm diameter in all patients. Conclusions This new flexible ring is easy to use to fix an aortic annulus from the inside. Early results reveal excellent control of aortic regurgitation with the projected annular size. Further investigations are needed to ensure the effectiveness of the ring.

Abstract Evidence for coseismic temperature rise that induces dynamic weakening is challenging to directly observe and quantify in natural and experimental fault rocks. Hematite (U-Th)/He (hematite He) thermochronometry may serve as a fault-slip thermometer, sensitive to transient high temperatures associated with earthquakes. We test this hypothesis with hematite deformation experiments at seismic slip rates, using a rotary-shear geometry with an annular ring of silicon carbide (SiC) sliding against a specular hematite slab. Hematite is characterized before and after sliding via textural and hematite He analyses to quantify He loss over variable experimental conditions. Experiments yield slip surfaces localized in an ∼5–30-µm-thick layer of hematite gouge with &lt;300-µm-diameter fault mirror (FM) zones made of sintered nanoparticles. Hematite He analyses of undeformed starting material are compared with those of FM and gouge run products from high-slip-velocity experiments, showing &gt;71% ± 1% (1σ) and 18% ± 3% He loss, respectively. Documented He loss requires short-duration, high temperatures during slip. The spatial heterogeneity and enhanced He loss from FM zones are consistent with asperity flash heating (AFH). Asperities &gt;200–300 µm in diameter, producing temperatures &gt;900 °C for ∼1 ms, can explain observed He loss. Results provide new empirical evidence describing AFH and the role of coseismic temperature rise in FM formation. Hematite He thermochronometry can detect AFH and thus seismicity on natural FMs and other thin slip surfaces in the upper seismogenic zone of Earth’s crust.

Abstract Objective Recurrent mitral valve regurgitation after prior repair is usually treated with re–repair or replacement through a median sternotomy. We report our small initial experience with mitral valve re–repair through a repeated minimally invasive approach. Methods From July 2018 to November 2021, 10 patients (median age 52 years IQR 49–68; male 90%) underwent isolated mitral valve re–repair with a right mini–thoracotomy approach with peripheral arterial and venous cannulation. Median Euroscore II was 2.05 (IQR 2.03–2.54). Results The median time from the original operation to redo surgery was 4.7 (3.5–8.1) years. Median cardiopulmonary bypass and cross–clamp times were 128 (IQR 109–134) and 99.5 (IQR 81–112) minutes, respectively. Failure of previous mitral valve repair was mainly due to Gore–Tex chordal rupture (3 patients), annular detachment (3 patients), native chordal rupture (2 patients), and posterior leaflet prolapse (2 patients). Concerning the surgical terchnique, a complete mitral valve ring Corcym Memo 3D or 4D was used in 8 patients while additional repair techniques were needed 5 patients (PTFE neochords implantation, cleft or commissure closure). No conversion to full sternotomy and no reoperation for bleeding was necessary. Median ICU and hospital stay were 2.0 (IQR 2.0–2.0) and 7.5 (7.0–9.0) days respectively. Median follow up time was 3.5 (2.8–3.9) years. Neither deaths nor major postoperative complications occurred in the follow–up period. Conclusions In our experience, mitral valve re–repair can be an attractive option for patients with failure of previous mitral valve surgery and can be safely performed through a minimally invasive approach with good outcomes. Main advantages seem to be the avoidance of extensive surgical dissection, optimal valve exposure, and low risk of surgical complications.

In shearing sediment beneath glaciers, networks of grains may transiently support shear and normal stresses that are larger than spatial averages. Consistent with studies of fault-gouge genesis, we hypothesize that crushing of grains in such networks is responsible for surrounding larger grains with smaller grains. At sufficiently large strains, this should minimize stress heterogeneity, favor intergranular sliding and abrasion rather than crushing, and result in a self-similar grain-size distribution.This hypothesis is tested with a ring-shear device that slowly shears a large annular sediment sample to high strains. Shearing and comminution of weak equigranular (2.0–3.3 mm) sediment resulted in a self-similar grain-size distribution with a fractal dimension that increased with shear strain toward a steady value of 2.85. This value is significantly larger than that of gouges produced purely by crushing, 2.6, but it is comparable to values for tilts thought to be deforming beneath modern glaciers, 2.8 to nearly 3.0. At low strains, under a steady mean normal stress of 84 kPa, variations in normal stress measured locally ranged in amplitude from 50 to 300 kPa with wavelengths that were 100 times larger than the initial grain diameter. Crushing of grains, observed through the transparent walls of the device, apparently caused the failure of grain networks. At shearing displacements ranging from 0.7 to 1.0 m, the amplitude of local stress fluctuations decreased abruptly. This change is attributed to fine sediment that distributed stresses more uniformly and caused grain networks to fail primarily by intergranular sliding rather than by crushing of grains. Sliding between grains apparently produced silt by abrasion and resulted in a fractal dimension that was higher than if there had been only crushing.A size distribution with a fractal dimension greater than 2.6 is probably a necessary but not sufficient condition for determining whether a basal till has been highly deformed. Stress heterogeneity in subglacial sediment that is shearing through its full thickness should contribute to the erosion of underlying rock.

In shearing sediment beneath glaciers, networks of grains may transiently support shear and normal stresses that are larger than spatial averages. Consistent with studies of fault-gouge genesis, we hypothesize that crushing of grains in such networks is responsible for surrounding larger grains with smaller grains. At sufficiently large strains, this should minimize stress heterogeneity, favor intergranular sliding and abrasion rather than crushing, and result in a self-similar grain-size distribution.This hypothesis is tested with a ring-shear device that slowly shears a large annular sediment sample to high strains. Shearing and comminution of weak equigranular (2.0–3.3 mm) sediment resulted in a self-similar grain-size distribution with a fractal dimension that increased with shear strain toward a steady value of 2.85. This value is significantly larger than that of gouges produced purely by crushing, 2.6, but it is comparable to values for tilts thought to be deforming beneath modern glaciers, 2.8 to nearly 3.0. At low strains, under a steady mean normal stress of 84 kPa, variations in normal stress measured locally ranged in amplitude from 50 to 300 kPa with wavelengths that were 100 times larger than the initial grain diameter. Crushing of grains, observed through the transparent walls of the device, apparently caused the failure of grain networks. At shearing displacements ranging from 0.7 to 1.0 m, the amplitude of local stress fluctuations decreased abruptly. This change is attributed to fine sediment that distributed stresses more uniformly and caused grain networks to fail primarily by intergranular sliding rather than by crushing of grains. Sliding between grains apparently produced silt by abrasion and resulted in a fractal dimension that was higher than if there had been only crushing.A size distribution with a fractal dimension greater than 2.6 is probably a necessary but not sufficient condition for determining whether a basal till has been highly deformed. Stress heterogeneity in subglacial sediment that is shearing through its full thickness should contribute to the erosion of underlying rock.

Le sertissage de tubes représente un enjeu important en ingénierie car c’est un besoin récurrent dans presque toutes les industries ayant besoin de solidariser des pièces tubulaires entre elles. On retrouve notamment ces assemblages dans l’industrie navale de défense pour la propulsion des bâtiments de surface ou des sous-marins. Au fil des années, de nombreux chercheurs se sont intéressés aux liaisons serties avec pour but de déterminer les paramètres procédé, géométriques et matériaux prépondérants en vue de les dimensionner de façon optimale. Afin d’augmenter leur résistance, ils ont essayé d’implanter une gorge au sein de la liaison en préconisant certaines formes géométriques mais en occultant les critères de conception indispensables à la tenue de la liaison dans le temps. Plus tard, nous avons assisté à l'émergence des procédés dynamiques qui grâce aux grandes vitesses de déformations qu'ils génèrent, permettent de réduire le retour élastique dans certains cas. Cela peut donc être un autre moyen de rendre des liaisons serties plus résistantes. Le premier objectif de ce travail de thèse consiste à construire et fiabiliser un modèle numérique de simulation du sertissage par expansion hydraulique (quasi-statique) en vue de proposer un profil de gorge annulaire optimisé respectant des critères de conception. Le second axe vise à analyser dans quelle mesure le sertissage électrohydraulique (dynamique) présente des bénéfices supplémentaires en termes de résistance à l'arrachement et d'étanchéité en comparaison d’un procédé quasi-statique
Tube crimping is a major challenge in engineering because it is a recurring requirement in nearly all industries that need toconnect tubular parts together. These assemblies are particularly found in the defense naval industry for the propulsion of surface vessels or submarines. Over the years, numerous researchers have been interested in crimped joints with the aim of determining the predominant process, geometric, and material parameters for optimal sizing. To increase their strength, they have attempted to introduce a groove within the joint by recommending certain geometric shapes but neglecting essentialdesign criteria necessary for the long-term integrity of the assembly. Later, we witnessed the emergence of dynamic processes that, thanks to the high strain rates they generate, can reduce springback in certain cases. This can, therefore, be another way to produce crimped joints more resilient. The primary objective of this thesis work is to build and validate a numerical simulation model of crimping by hydraulic expansion (quasistatic) in order to propose an optimized annulargroove profile respecting design criteria. The second one aims to analyze to what extent electrohydraulic crimping (dynamic) offers additional benefits in terms of pull-out strength and tightness compared to a quasi-static process

Pour caractériser certains comportements des gouges de failles, des travaux à l'Appareil de Cisaillement Simple Annulaire ont été réalisés. Ils consistent à cisailler un modèle de gouge épaisse sur des distances plurimétriques. La diminution progressive, à grand déplacement, du frottement apparent est représentée par une nouvelle loi puissance qui prend en compte le niveau de confinement et dans laquelle l'effet de l'eau est négligeable. Les perturbations en déplacement, en dessous d'un certain seuil, en vue de relâcher le cisaillement, induisent une cicatrisation lors de la recharge où l'on a mis en évidence le rôle des déformations volumiques, et plus particulièrement la dilatation empêchée de la bande de cisaillement. Par ailleurs, la variation du confinement, sous forme sinusoïdale ou par paliers, entraîne une déformation volumique et une variation du cisaillement proportionnelles. Ces perturbations ont peu d'influence sur un échantillon ayant subi dans son passé un confinement supérieur à celui pratiqué par les variations. Les variations sinusoïdales se font sans déphasage et sans qu'il y ait résonance ; l'effet de la période est plus présent à faibles valeurs (< 20 s). La bande de cisaillement montre une réduction importante de sa perméabilité à l'eau attribuée à la production de fines particules résultant du broyage des grains de sable. Un nouveau dispositif permet d'avoir accès à des mesures de la pression normale au voisinage de la bande de cisaillement pour une interface rugueuse. La comparaison avec les travaux antérieurs sur une interface lisse conduit à une convergence satisfaisante ; une exception importante est le cas des phases d'adoucissement à long terme. La réponse globale des essais semble résulter de l'interaction entre la bande de cisaillement et le reste de l'échantillon.

Nous présentons une série de résultats expérimentaux visant à modéliser le comportement mécanique d'une zone de faille. Nous avons utilisé un appareil de cisaillement simple annulaire permettant d'appliquer des cisaillements plurimétriques à des échantillons épais de gouge granulaire (sable de quartz). Nos résultats mettent en évidence un nouveau processus spectaculaire d'adoucissement en glissement: le coefficient de frottement effectif de la gouge décroît de 70% sur des distances caractéristiques décimétriques. La diminution progressive du frottement avec le glissement est bien représentée par une loi puissance sans échelle caractéristique de longueur. On constate que ce nouveau processus d'adoucissement permet de rendre compte quantitativement des distances d'adoucissement apparentes et des énergies de fracture mises en jeu durant les grands séismes. Nous proposons de l'inclure dans une extension des lois de frottement classiques de type RSF (``rate- and state-dependent friction''). Outre ces résultats macroscopiques, nous avons aussi pu mesurer, par corrélations d'images, les déformations microscopiques à l'intérieur des échantillons. Dès le début du glissement, la déformation se localise dans une bande de cisaillement broyée d'épaisseur constante. Cette bande est séparée du reste du matériau par une fine zone de transition indurée. On remarque que le processus d'adoucissement macroscopique est systématiquement associé à un effet de relaxation lente des incréments de déformation hors de la bande de cisaillement. Nous interprétons ces deux processus concomitants comme résultant d'un découplage progressif entre la bande de cisaillement et le reste de l'échantillon. L'épaisseur mécanique effective de la gouge est donc significativement plus grande que l'épaisseur de la bande de cisaillement. Nous proposons d'extrapoler nos résultats aux zones de faille naturelles, dont la structure présente souvent de fortes analogies avec celle de nos échantillons cisaillés.