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

Approaches to fatigue life assessment applied in the very high cycle regime

Designers and calculation engineers are becoming increasingly interested in the latest results on very high cycle fatigue (VHCF). Often, the influence of loading with a very high number of cycles on component behaviour is estimated conservatively, but the exact safety margin is unknown. This paper gives an overview of failure mechanisms in the HCF- and VHCF-regions and of material and component related influences, which have to be considered in the fatigue life assessment. The state of the art of design codes, recommendations from the literature and initial investigations on variable amplitude loading in the VHCF-regime are presented. This review indicates that further research activities are necessary to improve fatigue life assessment in order to allow a reduction of safety margins

Additiv hergestellter Drehklemmhalter*/Additively manufactured turning tool holder - Fatigue strength of additive tool structures and open jet formation of cryogenic multi-component cooling

Additive Fertigungsverfahren gestatten die Herstellung innovativer Werkzeugsysteme mit erhöhter Funktionsintegration. Die vorliegende Arbeit liefert wichtige Erkenntnisse in Bezug auf additiv gefertigte Drehklemmhalter. Zum einen wird der Einfluss der Wärmebehandlung auf das Werkstoffgefüge und schlussendlich auf die Schwingfestigkeitseigenschaften untersucht. Zum anderen wird der Einfluss additiv hergestellter Zweistoffdüsen auf das resultierende Freistrahlverhalten experimentell ermittelt. &amp;nbsp; Additive manufacturing processes allow producing innovative tool systems associated with increased functional integration. This work provides important insights on additively manufactured turning tool holders. Firstly, it investigates how heat treatment affects both material structure and fatigue properties. Secondly, it determines experimentally how additively produced two-fluid nozzles influence the resulting open jet formation.</p

Failure prediction for advanced crashworthiness of transportation vehicles

During the past two decades explicit finite element crashworthiness codes have become an indispensable tool for the design of crash and passenger safety systems. These codes have proven remarkably reliable for the prediction of ductile metal structures that deform plastically; however, they are not reliable for joining systems and materials such as high strength steels, plastics and low ductility lightweight materials all of which are liable to fracture during the crash event. In order to improve crash failure prediction of materials and joining systems the CEC has recently funded a 3 year European research project dedicated to this topic. Specifically the project concerned aluminium, magnesium, high strength steels, plastics and two primary joining techniques; namely spotwelds and weldlines. Numerous new developments were undertaken including improved failure laws, adaptive meshing and element splitting to treat crack propagation. In the case of sheet stamping, investigations have also tried to account for process history effects and the metallurgical changes that occur during manufacture. This project has recently finished and this paper presents some of the key research results of the work concerning materials failure modelling