Occurrence Of Kinks Research Articles

A new method to determine isothermal flow curves for integrated process and microstructural simulation in metal forming

Reliable isothermal flow curve data are often needed for the process simulations in metal forming technology. As a rule of thumb, at strain rates above 10−3 s −1 even isothermal compression tests are affected by deformation heating, such that the flow stress data should be corrected for stress decrease due to the temperature increase in the specimen. However, if the temperature increase during testing is larger than the interval of the nominal testing temperatures foreseen for individual tests, the compensated flow curves may show undesired kinks. When it comes to high strength materials, this phenomenon will be even more pronounced. The occurrence of kinks in compensated flow curves can be attributed to the interpolation methods used in the existing temperature compensation methods. As a remedy, an alternative strategy based on Thin Plate Spline interpolation is proposed on the example of two high strength materials. It is shown that this new strategy can be generally adopted to derive reliable isothermal flow curves over a wide range of temperatures and strain rates, especially for high strength materials.

Patološke elongacije karotidnih arterija - etiopatogeneza i kliničke manifestacije

Kinking and coiling of the internal carotid artery (ICA) is a special pathomorphological and pathophysiological entity within the cerebrovascular pathology. Coiling is attributed to a congenital origin and it is generally asymptomatic, in contrast to kinking, which is mainly associated with atherosclerosis and causes transitory ischemic attacks. Hemodynamic measurements show a significant reduction or even complete transient interruption of the flow due to kinking/coiling ICA in certain positions of the head. There is no statistically significant connection between kinking ICA and large neurological events, but there is a statistically significant correlation between the occurrence of kinking and transient vascular disorders. Extension of the head with ipsi - contralateral rotation emphasizes the importance of kinking and provokes the appearance of the symptoms.

Structure and electrical conductivity of Li2O-B2O3 glasses

The temperature-concentration dependence of the electrical conductivity of Li2O-B2O3 glasses in the temperature range of ∼180–310°C has been studied. For pure boron anhydride, the dependence logσ = f([1/T]) is linear, whereas for glasses with ∼2 mol % ≤ [Li2O] < ∼10 mol %, similar curves are kinked. At higher Li2O concentration the kinks disappear. Occurrence of kinks is attributed to variation of essence of current carriers from proton pattern for B2O3 to mixed proton-ion pattern for low-alkali glasses. Conductivity of glasses at [Li2O] ≥ 20 mol % is stipulated by the formation of a continuous sublattice of polar structuralchemical entities (entities) [BO4/2]−Li+ and the migration of lithium ions.

Aneurismi arteriosi intracranici associati ad altre anomalie vascolari

After subarachnoid hemorrage, the first aim in performing angiographic investigation, is to recognize the cause of bleeding. The arteriography can identify an aneurysm, the most frequent vascular malformation, but also associated vascular abnormalities. We report our experience on a study using angiographic investigation to explore intracranial aneurysms. Our data confirme high occurrence of kinking, coiling and others vascular abnormalities of the Willis circle and of the vertebral arteries, associated with aneurysms. This association suggests new concepts of aneurysm formation and must induce to detection of some aneurysms before rupture.

Structural flexibility of carbon nanotubes

We report high resolution electron microscope (HREM) observations and atomistic simulations of the bending of single and multi-walled carbon nanotubes under mechanical duress. Single and multiple kinks are observed at high bending angles. Their occurrence is quantitatively explained by the simulations, which use a realistic many-body potential for the carbon atoms. We show that the bending is fully reversible up to very large bending angles, despite the occurrence of kinks and highly strained tube regions. This is due to the remarkable flexibility of the hexagonal network, which resists bond breaking and bond switching up to very high strain values.

Lateral Organization of Liquid-Crystalline Cholesterol-Dimyristoylphosphatidylcholine Bilayers. Evidence for Domains with Hexagonal and Centered Rectangular Cholesterol Superlattices

The lateral organization of fluid cholesterol-dimyristoylphosphatidylcholine (DMPC) bilayers was studied by measuring the response of fluorescent membrane probes, dipyrenylphosphatidylcholines (diPyrxPCs) or merocyanine 540, to the variation of cholesterol concentration. Parallel absorbance and light-scattering measurements were also carried out. The excimer-to-monomer ratio of diPyrxPCs displayed abrupt deviations at particular cholesterol mole fractions (CMFs). The most notable of these occurred at CMFs of 0.15, 0.33, and 0.67. Deviations were also frequently observed at CMFs of 0.12, 0.20, 0.25, and 0.40. Merocyanine 540 reproducibly reported deviations at CMFs of 0.15 and 0.33 and frequently reported values close to 0.12, 0.20, and 0.25. In absorbance (turbidity) and light scattering versus CMF plots, well-defined kinks were observed at CMFs of 0.16, 0.33, 0.52, and 0.67. The occurrence of kinks or other deviations at those particular CMFs is most readily explained in terms of a superlattice model previously developed to explain the lateral distribution of pyrenylphospholipids in bilayers [Somerharju, et al. (1985) Biochemistry 24, 2773-2781; Virtanen, J. A., et al. (1988) J. Mol. Electron. 4, 233-236]. This model is based on the assumptions that (i) each cholesterol molecule replaces a single acyl chain in a hexagonal lattice, (ii) cholesterol molecules, because of their larger size, perturb the lattice, (iii) this perturbation is minimized when the cholesterol molecules are maximally separated from each other, and (iv) the maximal separation is achieved when the cholesterol molecules form a hexagonal or centered rectangular superlattice. All detected critical CMFs, except that at CMF 0.67, are predicted by the model, thus strongly supporting its validity. The critical CMF at 0.67 is a limiting case, which can be accounted for by assuming that cholesterol and phospholipid molecules form alternating rows, i.e., formation of a cholesterol superlattice with rectangular symmetry. As predicted by the superlattice model, composition-driven order-to-disorder transitions occur between the critical CMFs, as indicated by increased data scatter and sample fluctuations in those regions. Another important prediction of the superlattice model is that domains with different cholesterol superlattices should coexist at most cholesterol concentrations. Such domains do not have to be extensive to account for the critical events observed here; rather, they are expected to be dynamic entities of limited size. It is very likely that such microscopic domains with distinct cholesterol superlattices also coexist in biological membranes. This is expected to have remarkable effects on both the structure and functions of these membranes.

The needle-wire-dilator technique for the insertion of chest tubes in pediatric patients.

We evaluated the needle-wire-dilator (NWD) technique, using commercially available sets, for insertion of chest tubes in 24 pediatric patients who were admitted to our pediatric intensive care unit (PICU). Fourteen patients had pneumothoraces, three had hemothoraces, two had pneumonia with empyema, four had pleural transudate effusions, and one had chylothorax. The ages of the patients ranged from four months to 24 years, and the sizes of the inserted chest tubes ranged from 10F to 20F. All insertions were successful, and the time from invasion of the pleural space by the needle to completion of chest tube insertion and connection to the tubing drainage system ranged from four to seven minutes. In four patients the procedure had to be performed while a significant coagulopathy existed. However, none of the 24 patients developed hemorrhagic complications. The only complication observed was a kink in the chest tube in five patients, resulting in recurrence of pneumothorax in four and pleural effusion in one. These adverse occurrences were corrected by repositioning the chest tubes in three patients, and by replacing the chest tubes with the stiffer, trochar type, chest tubes in the other two. We conclude that the NWD technique for chest tube insertion is quick, safe, and easy to perform in all pediatric age groups. The commercially available chest tubes used in our study were somewhat softer than the trochar type chest tubes available, which explains the occurrence of kinks in some of them.

Molecular dynamics simulation of a phospholipid membrane.

We present the results of molecular dynamics (MD) simulations of a phospholipid membrane in water, including full atomic detail. The goal of the simulations was twofold: first we wanted to set up a simulation system which is able to reproduce experimental results and can serve as a model membrane in future simulations. This goal being reached it is then further possible to gain insight in to those properties that are experimentally more difficult to access. The system studied is dipalmitoylphosphatidylcholine/water, consisting of 5408 atoms. Using original force field parameters the membrane turned out to approach a gel-like state. With slight changes of the parameters, the system adopted a liquid-crystalline state. Separate 80 ps runs were performed on both the gel and liquid-crystalline systems. Comparison of MD results with reliable experimental data (bilayer repeat distance, surface area per lipid, tail order parameters, atom distributions) showed that our simulations, especially the one in the liquid-crystalline phase, can serve as a realistic model for a phospholipid membrane. Further analysis of the trajectories revealed valuable information on various properties. In the liquid-crystalline phase, the interface turns out to be quite diffuse, with water molecules penetrating into the bilayer to the position of the carbonyl groups. The 10-90% width of the interface turns out to be 1.3 nm and the width of the hydrocarbon interior 3.0 nm. The headgroup dipoles are oriented at a small angle with respect to the bilayer plane. The resulting charge distribution is almost completely cancelled by the water molecules. The electron density distribution shows a large dip in the middle of the membrane. In this part the tails are more flexible. The mean life time between dihedral transitions is 20 ps. The average number of gauche angles per tail is 3.5. The occurrence of kinks is not a significant feature.

Stripe-geometry d.h. lasers with linear output/current characteristics

The occurrence of kinks in the light-output/current characteristic of d.h.-laser diodes can be avoided by stabilising the lasing path. This is done by slightly misorienting the stripe contact with respect to the normal at the mirrors, or by changing the shape of the stripe contact. By this procedure the beam waist position with respect to the mirror is also stabilised.

Conformation of U.V.-irradiated DNA.

SummaryThe effects of u.v. on DNA have been found to be manifold. The initial effect characterized by a fall in absorbance, rise in sedimentation coefficient and appearance of kinks in many places of the molecule has been attributed to dimerization of the pyrimidines and formation of single-strand breaks. Together with the occurrence of kinks, the molecule becomes folded up and assumes a ring-like appearance at several places; this is partly due to intramolecular crosslinks. With an increase in dose a third effect, i.e. double-strand break which results in back-bone scission, comes into play and predominates. This effect is manifested in a further decrease in absorbance, a fall in sedimentation constant, and the appearance of shorter molecules in the electron microscope. At still higher doses, an equilibrium state is reached when both absorbance and sedimentation coefficient remain steady. The resulting molecules were so short that the effect of dimer formation with the increase in dose was negligible.