Nichtlineare Schalentheorie in konvektiver Beschreibung mit anisotroper Plastizität

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Atomare Mechanismen der Legierungsbildung an Oberflächen : eine Untersuchung mit dem Rastertunnelmikroskop am Beispiel des Systems Mn/Cu (100)

Atomic processes important in surface alloy formation for MnlCu(100) have been studied using STM (Scanning Iunneling Microscopy). The investigations were performed for coverages between 0.005 and 1.0 monolayers (ML) deposited around and above 300 K. Due to a pronounced corrugation incorporated Mn atoms are imaged as small protrusions allowing the distinction between Mn and Cu atoms in the STM images. The height of the protrusions was determined to approximately 0.3 $\mathring{A}$. Mn is incorporated in the Cu(100) surface layer already at very low coverages ($\approx$ 0.005 ML Mn). No critical coverage for the incorporation of Mn was found. Below 0.25 ML Mn the incorporated Mn atoms are disordered. Around and above 0.25 ML Mn an ordering process into a c(2x2) structure starts which is completed at 0.5 ML Mn. The STM images of the c(2x2) surface alloy only show one type of atom. With regard to the strong corrugation of incorporated Mn at lower coverages presumably the Mn atoms are imaged. For small coverages a diffusion coefficient of D = 5,3 $\cdot$10$^{-18}$ cm$^{2}$s$^{-1}$ is determined from anevaluation of the Einstein relation. Different models for the diffusion of incorporated Mn are discussed. The experimental observations can be explained successfully by a diffusion mechanism that is based on diffusing surface vacancies. Lower and upper limits for the jump frequency of incorporated Mn atoms are derived from the STM images for several coverage regimes. This analysis shows that the mobility of incorporated Mn around 0.3 ML Mn is between one and two orders of magnitude higher than at low coverages. On the surface a ripening of alloyed islands is observed. Large islands grow at the expense of small islands. The mass transport between the islands is mediated by a particle transfer. Such a late-stage growth is called Ostwald-ripening. The ripening is particularly pronounced in the coverage regime where the mobility of the incorporated Mn is enhanced. This indicates that vacancies also playa decisive role for Ostwald-ripening in this system. The Mn atoms are preferentially incorporated in the vicinity of island edges and substrate steps. In the proximity of substrate steps, the rate of incorporation into the lower terrace increases with the kink density of the steps. This implies that the incorporation in the lower terrace is particularly effective near kinks. Different models responsible for the incorporation in the lower terrace are suggested. The experimental observations can be understood by kink-induced atomic exchange processes or by recombination processes between atoms and vacancies. The alloying of the upper terrace of substrate steps is driven by an attachment of Mn adatoms. This process is essentially independent of the step orientation. Therefore, the concentration of incorporated Mn in the vicinity of steps does not depend upon the step direction. Step fluctuations and vacancy mediated diffusion enable a transport of the attached Mn atoms into the upper terrace. The diffusion of the incorporated Mn atoms leads to a more homogeneous Mn distribution in the course of the time

Crash simulation of wound composite tubes based on multi-level modelling

Fibre reinforced plastics are widely used for energy absorbing parts. Due to their superior strength to density ratio they provide a high performance and are ideal for lightweight design for crashworthiness. For this, it is essential that the mechanical behaviour of fibre reinforced composites be predicted correctly by simulation. However, due to the complex inner structure, this is still a challenging task, in particular in case of highly nonlinear crash loading. In order to provide an alternative in this paper a virtual manufacturing simulation chain is proposed to gather detailed geometrical information about the roving structure of a filament wound tube on meso-scale. In addition effective material properties, based on calibrated models of the individual constituents, for the filament–matrix interaction are derived by micro-scale calculations. Both, combined with a USER MATERIAL model for the roving structure finally provide a complete finite element model which is used for the crash simulation of the filament wound tube. By comparing the numerical results to experimental data, the potential of the approach is shown and occurring differences are discussed as well as possible subsequent investigations are proposed

Atomare Mechanismen der Legierungsbildung an Oberflaechen Eine Untersuchung mit dem Rastertunnelmikroskop am Beispiel des Systems Mn/Cu(100)

Atomic processes important in surface alloy formation for Mn/Cu(100) have been studied using STM (scanning tunneling microscopy). The investigations were performed for coverages between 0.005 and 1.0 monolayers (ML) deposited around and above 300 K. Due to a pronounced corrugation incorporated Mn atoms are imaged as small protrusions allowing the distinction between Mn and Cu atoms in the STM images. The height of the protrusions was determined to approximately 0.3 A. Mn is incorporated in the Cu(100) surface layer already at very low coverages (#approx# 0.005 ML Mn). No critical coverage for the incorporation of Mn was found. Below 0.25 ML Mn the incorporated Mn atoms are disordered. Around and above 0.25 ML Mn an ordering process into a c(2 x 2) structure starts which is completed at 0.5 ML Mn. The STM images of the c(2 x 2) surface alloy only show one type of atom. With regard to the strong corrugation of incorporated Mn at lower coverages presumably the Mn atoms are imaged. For small coverages a diffusion coefficient of D=5,3.10"-"1"8 cm"2s"-"1 is determined from an evaluation of the Einstein relation. Different models for the diffusion of incorporated Mn are discussed. The experimental observations can be explained successfully by a diffusion mechanism that is based on diffusing surface vacancies. Lower and upper limits for the jump frequency of incorporated Mn atoms are derived from the STM images for several coverage regimes. This analysis shows that the mobility of incorporated Mn around 0.3 ML Mn is between one and two orders of magnitude higher than at low coverages. (orig.)SIGLEAvailable from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Optimierung der Schneidparameter beim Zerteilen von Aluminiumblechen zur Reduzierung der Gratbildung Schlussbericht

SIGLEAvailable from TIB Hannover: RO 146(135) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekArbeitsgemeinschaft Industrieller Forschungsvereinigungen 'Otto von Guericke' e.V. (AIF), Koeln (Germany); Bundesministerium fuer Wirtschaft, Bonn (Germany)DEGerman