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  • 1
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The protection of cells in the upper intestine against digestion by pancreatic trypsin depends on the prostanoid prostaglandin E2 (PGE2) and is mediated by protease-activated receptors in the epithelium,. As the airway epithelium is morphologically similar and also ...
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  • 2
    ISSN: 1573-5079
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Reduction of plastocyanin (PC) caused a change in the electric field at the surface of the molecule which resulted in a 0.3 pH unit increase in the pKa of a nitrated derivative of Tyr 83. This change in electrical potential could alter the affinity for cytochrome f which is known to bind at this site. Conversely, properties of the copper center, including the pH dependence of the reduction potential, are regulated by the charge on the surface of the molecule. Both the reduction potential and conformation (as measured by near-UV circular dichroic spectra) were pH dependent. Thus the conformation and electrostatic behavior of PC are dependent on oxidiation state, pH and surface charge, raising the possibility that its redox activity is controlled by the pH gradient.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    International journal of fracture 10 (1974), S. 565-583 
    ISSN: 1573-2673
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Description / Table of Contents: Résumé Des analyses numériques et théoriques ont été effectuées sur sent types d'éprouvettes d'essais, notamment des essais de décollement d'une membrane, de pelage à 90°, de torsion et de traction sur clines de différentes configurations. Ces éprouvettes sont en général le siège de champs de contraintes complexes comportant, en importances variables, des sollicitations de mode I, II ou III. Elles ont été façonnées à l'aide de polyméthylméthacrylate pour les supports, et d'un élástomère de polyuréthane transparent (Solithane® 113) pour l'adhésif. Cette combinaison a permis de faire des observations directes de la zone de liaison au cours de la mise en charge. En dépit de la présence, dans toutes les éprouvettes, de singularités telles que des manques d'adhérence, où les bords des liaisons, on a constaté que dans certains cas, les décollements n'ont pas débuté aux points singuliers d'amorce de rupture, ainsi qu'on devait normalement s'y attendre. On propose une explication à ce comportement. On compare également léffet du mode de mise en charge sur le comportement des éprouvettes où le décollement se propage au départ du point singulier que constitue le bord du joint. En outre, on discute l'effet de la compressibilité de l'adhésif, de son épaisseur, et de la courbure géométrique du plan de liaison, dans le cas de plusieurs des éprouvettes essayées.
    Abstract: Zusammenfassung Numerische oder analytische Untersuchungen wurden auf sieben verschiedenen Versuchsproben ausgeführt. bestehend aus Trennungsversuchen, 90° Abschälversuchen, Torsionsversuchen, und verschiedenen Kegelversuchen. Diese Proben zeigen im allgemeinen komplexe Spannungsfelder, die verschiedenliche Anteile von der Belastungsweise I, II and III enthalten. Die Proben wurden mit Polymethylmethacrylaten (PMMA) als Unterlagen und mit einem durchsichtigen Polyurethane-elastomer (Solithane® 113) als Klebstoff hergestellet. Diese Kombination erlaubte eine direkte Beobachtung der Verbindungslinie als die Last angebracht wurde. Obschon in allen Proben Anfangsentklebungen und Singularitäten an den Klebungsenden vorhanden waren, erfolgte in verschiedenen Fallen entgegen der Erwartung der Anfang der Entklebungen nicht an den Singularitätspunkten. Man gibt eine Erklärung für dieses Verhalten sowohl als ein Vergleich des Einflusses der Belastungsweise für die Proben wo das Entkleben von einem Singularitätspunkt an dem Klebungsende ausging. Zusätzlich wird der Einflu\ der PreBbarkeit und der Dicke des Klebstoffes Bowie die geometrische Krümmung der Verbindungslinie auf vielen Versuchsproben besprochen.
    Notes: Abstract Numerical or analytical analyses were performed on seven different test specimens including blister test, 90-degree peel test, torsion test, and various cone tests. These specimens are in general subjected to complex stress fields having various amounts of Mode I, Mode II, and Mode III loads. The specimens were then constructed using polymethylmethacrylate (PMMA) for the adherends and a transparent polyurethane elastomer (Solithane® 113) Solithane is a registered trademarked name for a polyurethane polymer manufactured by Thiokol Corporation. for the adhesive. This combination permitted direct observation of the bondline as load was applied. Although initial debonds as well as bond end termination singularities were present in all specimens, in some cases the debond did not initiate at the singularity points as would normally have been expected. An explanation for this behavior is presented, as well as a comparison of loading mode effect on those specimens for which the debond did propagate from a bond terminus singular point. In addition, the effect of adhesive compressibility, adhesive thickness, and bondline geometric curvature are discussed for many of the test specimens.
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    International journal of fracture 9 (1973), S. 421-436 
    ISSN: 1573-2673
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Description / Table of Contents: Résumé Les programmes de calcul par éléments finis ont été utilisés avec succès pour l'analyse des contraintes dans les pièces dont la géométrie ou la mise en charge est complexe, et l'emploi de ces programmes est à présent largement introduit dans l'analyse de la mécanique de la rupture par décohésion. Des procédures similaires ont été développées pour traiter de la mécanique de la rupture par décollement, mais on a constaté, tant sur le plan théorique que sur le plan expérimental, des oscillations prononcées de la contrainte au voisinage du point de décollement. Ce mémoire discute les effets que peuvent avoir ces variations sur le calcul de la contrainte et de l'énergie de déformation, et démontre que, dans ce problème difficile, les méthodes de calcul aux éléments finis peuvent néanmoins être utilisées avec une précision suffisante pour analyser les ruptures.
    Abstract: Zusammenfassung Finite Komputerverfahren wurden erfolgreich angewandt zur Untersuchung der Spannungen in Körpern mit komplizierter Geometrie und Belastung, und der Gebrauch dieser Verfahren ist jetzt weitgehend angenommen im Gebiet der kohesiven Bruchmechanik. Ähnliche numerische Verfahren wurden für die adhesive Bruchmechanik ausgearbeitet, aber ziemlich starke Spannungsschwankungen nahe am Trennungspunkt wurden sowohl analytisch wie numerisch gefunden. Dieser Bericht befaßt sich mit dem Einfluß dieser Schwankungen auf die Berechnung der Spannungen und der Verformungsenergie, und beweist daß in diesem sehr schwierigen Gebiet die finite Verfahren mit genügend Genauigkeit zur Untersuchung des Bruches angewandt werden können.
    Notes: Abstract Finite element computer programs have been used successfully for stress analysis of bodies with complex geometries and loads, and the use of these programs in cohesive fracture mechanics analysis is now widely accepted. Similar numerical procedures have been developed for use in adhesive fracture mechanics, but rather strong stress oscillations near the point of debonding have been shown to occur both analytically and numerically. This paper discusses the effect of these oscillations upon the calculation of the stress and strain energy, and demonstrates that in this potentially difficult problem, finite element codes can be used with sufficient precision for fracture analysis.
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    International journal of fracture 39 (1989), S. 191-200 
    ISSN: 1573-2673
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Description / Table of Contents: Résumé Lorsqu'on veut prédire la capacité de charge portante d'un joint collé, on peut être conduit à des erreurs d'un ordre de grandeur en utilisant le critère de contrainte moyenne. On montre qu'une approche par mécanique de rupture peut permettre de prédire la charge de rupture d'un joint collé réalisé avec un polyuréthane ou une époxy relativement fragile, lorsqu'on considère de manière appropriée la mode de mise en charge, la température et la vitesse de sollicitation. La principale contribution de cette étude réside dans une extension de la théorie de la mécanique de la rupture à des domaines où n'existent pas de point singuliers classiques. On joint à l'analyse des données d'essais en vue de déduire une tallle de défaut “inhérente”. On montre que la combinaison de cette dimension de défaut inhérent et de la vitesse critique de relaxation de l'énergie, peut être utilisée pour prédire la résistance d'une grande variété de joints collés.
    Notes: Abstract Using an avarage-stress oriterlon can lead to errors of an order of magnitude in predicted load carrying capability of a bonded joint. A fracture mechanics approach is shown to accurately predict failure load in a joint bonded with either a polyurethane or a relatively brittle epoxy when proper consideration is given to loading mode, temperature, and rate. The principal contribution of this paper is in extending fracture mechanics theory to regions where classical singular points fo not exist. Analyses are combined with test data to deduce an “inherent” flaw size. It is shown that the combination of this inherent flaw size with the critical energy release rate can be used to predict strength in a variety of bonded joints.
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Experimental mechanics 8 (1968), S. 411-417 
    ISSN: 1741-2765
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Experimental data have been obtained which duplicate the stress and strain field in a solid-propellant rocket-motor grain under pressurization. Development of a hollow ellipsoidal specimen has made possible the acquisition of these data. This test also allows a more thorough evaluation of the failure mechanism in multiaxial stress and strain fields. Failure data were obtained in the tensile-tensile-compressive (++−) stress and biaxial tensile-compressive (+−) strain octants by applying pressure to the internal surface of an ellipsoidal specimen. Data were obtained in the triaxial compressive (−−−) stress and biaxial tensile-compressive (+−) strain octants by simultaneous application of pressure to both internal and external surfaces of the ellipsoidal specimen and a gradual reduction of external pressure until failure. A stress analysis of the specimen is presented and data-reduction techniques are discussed. Data obtained with a hollow spherical specimen and with the newly developed hollow ellipsoidal specimen are compared. These data are obtained in the same stress and strain octants; however, the relative magnitudes of the parameters and, therefore, the positions within the stress and strain octants are different. This results in a change in the magnitudes of the failure parameter. Data from uniaxial specimens of the same propellant also are presented.
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    Experimental mechanics 16 (1976), S. 11-15 
    ISSN: 1741-2765
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Several test specimens have been proposed for obtaining adhesive-fracture energy, γa, in bond systems. These tests include blister, cone, lap-shear and peel tests. Peel tests have been used for many years to compare relative strengths of different adhesives, different surface-preparation techniques, etc. This paper demonstrates the potential use of peel tests in obtaining γa values. There are several reasons for devloping the peel test for fracture-mechanics work. First, most laboratories have facilities for preparing peel specimens. In addition, the adhesivefracture energy has recently been shown to be a function of loading mode. In peel tests, various combinations of Mode I and Mode II loadings can be applied by varying the peel angle. Peel-test-analysis methods discussed include closed-form solutions for particular peel-specimen geometries loaded with a given force and numerical techniques for general peel-specimen analysis. This paper also points out the difference between debond load and maximum peel load. The debond-load to maximum-load ratio is shown to depend upon adhesive type but independent of load rate over three decades of time for two different adhesive systems tested.
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 14 (1970), S. 735-745 
    ISSN: 0021-8995
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The time-temperature dependence of the cohesive fracture energy is deduced from experiments on a centrally cracked sheet of butadiene-acrylonitrile-acrylic acid viscoelastic terpolymer crosslinked with an epoxy curing agent. Analytic results based upon a cylindrical flaw model of the crack permit the segregation of the fracture energy time dependence from that of the relaxation modulus.
    Additional Material: 5 Ill.
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