Mit ePortfolios selbstgesteuert lernen. Ein Ansatz zur Reflexionsförderung im Rahmen eines hochschulweiten ePortfoliosystems

Der vorliegende Artikel beschreibt einen Ansatz zur medienbasierten Reflexionsunterstützung Studierender. Ausgehend vom lebenslangen Lernen und dessen individuellen Anforderungen wird zunächst näher auf selbstgesteuertes Lernen und den Stellenwert von Reflexionsfähigkeit eingegangen. Ein Modell lernbezogener Einflussvariablen dient anschliessend der Verdeutlichung verschiedener möglicher Reflexionsebenen, -gegenstände und -ziele. Am Beispiel von (e)Portfolios werden daraufhin Möglichkeiten mediengestützter Reflexionsförderung beschrieben, bevor schliesslich die reflexionsunterstützenden Massnahmen im ePortfoliosystem der Technischen Universität Hamburg-Harburg dargestellt werden

Comparing the predictions of a non-Schmid crystal plasticity model with phenomenological hardening and dislocation density hardening rules for superconducting cavities used in particle accelerators

High purity niobium is used to make superconducting radio frequency cavities. One potential fabrication method involves deep drawing cup-like shapes form slices of an ingot containing only a few grains. Thus, material models for large strain deformation of multicrystals are needed to predict conditions that lead to forming instabilities and to predict future responses to heat treatments to remove as many dislocations as possible. Deformation behavior of BCC materials does not follow the Schmid law. Therefore, the classical crystal plasticity cannot be used to predict their behavior. In BCC materials, core of screw dislocations expand on three different \u3c110\u3e planes, and influences from the stresses on each of these planes. This is known as non-Schmid behavior. In this study, a non-Schmid crystal plasticity model with a phenomenological hardening rule is developed and calibrated for single crystal niobium (Nb). The model accurately predicts the stress–strain response of Nb; however, the estimated crystal orientation evolution is not as accurate. A dislocation density hardening model is implemented to increase the accuracy of texture predictions. The results of these two hardening models are compared

Study of slip in high purity single crystal niobium for accelerator cavities

One of the ultimate goals in particle accelerator cavity design is to establish a model that integrates the grain orientations of an ingot slice, slip activity during deep drawing, and dislocation substructure evolution because of slip system interactions, which governs subsequent recrystallization. In order to understand the slip behavior during deep drawing of large grain Nb, it is first necessary to know how slip occurs in the uniaxial tension of single crystal Nb. In this study, the slip behavior of Nb samples taken from an ingot slice before and after an 800ºC/2h anneal is compared. The as-cut specimens were deformed monotonically to 40% engineering strain using a tensile tester (Instron 4302). The heat treated specimens were deformed in an SEM in situ using an Ernest Fullam stage, with strain increments of ~10%, and held in a loaded condition while microscopy measurements were made. After the heat treatment, the yield strengths decreased significantly for almost all specimens, and there was no longer a slight drop of flow stress between yield and 15% strain which is present in several of the as-received specimens. This implies that the dislocations initially present in the ingot play an important role in deformation, and that the heat treatment effectively removed many dislocations. In most cases, the deformation paths for corresponding specimens exhibit a different sense of rotation, even though the specimens have nearly identical starting orientations. This implies that pre-existing dislocations affected which slip systems were activated during deformation. Although the initial orientation differences were small, the pre-existing dislocations led to final orientations that differed by more than 10? after 40% strain. There is a noticeable prevalence of {110} slip in the heat treated specimens as compared to a dominance of {112} slip for the as-received specimens. This provides evidence that existing dislocations lead to conditions that favor operation of slip on {112} planes. Very linear slip traces were observed on the heat treated specimens, while there was a range of slip trace inclinations observed in the as-received group, which suggests that pre-existing dislocation entanglements may have led to bursts of slip on different planes in the same Burgers vector direction

Characterization of grain boundary deformation processes in commercial purity titanium using nanoindentation and crystal plasticity modeling

The nature of plastic deformation across grain boundaries in α-titanium has been studied using instrumented sphero-conical nanoindentation in complement with crystal plasticity modeling. Following crystal orientation mapping using electron backscattered diffraction, indention was preformed (i) in the center of grains to mimic single crystal indentation and (ii) near grain boundaries to carry out bicrystal experiments. The pile-up topography that forms as a function of nanoindentation axis was characterized using atomic force microscopy (AFM). Crystal plasticity finite element simulations were carried out iteratively by varying the slip parameters until a best fit was found between the experimentally measured single crystal pile-up topographies and the simulated single crystal pile-up topographies of several orientations. AFM characterization of bicrystal nanoindents revealed changes in the pile-up topographies compared to those for single crystal nanoindents carried out in the same grain. In general, the pile-up topographies were not strongly influenced by low angle grain boundaries, indicating significant strain transfer across the boundaries. In contrast, many high angle boundaries strongly influenced the pile-up morphology, with changes in the pile-ups noted in both the grain containing the pile-up and in the neighboring grain. The ability to accurately simulate the influence of grain boundaries pile-up development was examined by carrying out CPFE simulations of bicrystal nanoindentation. Direct comparison of the AFM-measured topographies with simulated topographies reveals that the general nature of bicrystal pile-up topography is well simulated, but that in general, the measured pile-up transfer is somewhat smaller than the simulated pile-up transfer. The results of the study will be discussed in terms of a number of generalized slip transfer criteria. This research was supported the National Science Foundation through a Materials World Network Grant DMR-1108211 and the complementary Deutsche Forchungsgemeinschaft grant ZA523/3-1

Nondestructive determination of subsurface grain morphology

Recent progress in experimental and numerical methods enables to scrutinize simulated polycrystal surface micromechanics at high spatial resolution. For the correct interpretation of similarities and deviations between experiment and simulation, the consideration of subsurface grain morphology is imperative because of its significant impact on the surface layer boundary condition. A novel method is presented that coarsely scans a relatively large area for subsurface crystallite orientation up to depths of ~0.2 mm by means of differential aperture X-ray microscopy. The resulting point set is categorized into grains according to proximity in physical and orientation space. Reconstruction of the subsurface grain structure starts with a Voronoi tessellation using the categorized set as seed points. Progressive smoothing of the resulting ragged grain boundary surfaces is achieved through mean curvature flow. As it turns out that the reconstruction quality of the bulk and on the surface are related, the latter can serve as guidance for optimum subsurface reconstruction

Offenheit von Wissen

Poster zur Offenheit von Wissen: Eine globale digitale Welt, die an Nachhaltigkeit orientiert ist, ist immer mehr auf Offenheit angewiesen. Diese umfasst u.a. den kostenfreien und öffentlichen Zugang zu Literatur und anderen Materialien im Netz -> Open Access, zu Lern- und Lehrmaterialien -> Open Educational Resources sowie zu Datenbeständen jedweder Art -> Open Data: Dieser Zugang wird durch eine offene technische Infrastruktur, z.B. -> Open Source Software unter Nutzung offener Standards, sichergestellt, die auch offene Entwicklungsprozesse ermöglicht -> Open Development, Open Peer Review, Open Research,…

20th Annual UD International Tea Scheduled

News release announces the 20th Annual International Tea at the University of Dayton with the theme Where In The World Are You