Finite element analyses of fatigue crack growth under small scale yielding conditions modelled with a cyclic cohesive zone approach

Mode I fatigue crack growth is modelled and investigated with a cohesive zone approach. A 2D finite element boundary layer model under plane strain and small scale yielding conditions is used to generate fatigue crack growth rate curves. This study focuses on the FE model generation with the aim to obtain reliable data of fatigue crack growth rates with computational costs kept as low as possible. In particular, this con- cerns the choice of geometric quantities of the boundary layer, appropriate mesh sizes and meshing strategies, and the time incrementation. In order to save resource consump- tion regarding computing time, licenses and memory, the ABAQUS URDFIL interface is used to observe the progress of fatigue crack growth, to control time incrementation and output generation, and to stop the simulation once a stable fatigue crack growth rate is reached. The latter is characterised by a constant amount of dissipated energy per load cycle and steady-state damage and stress profiles in the ligament. Different crack length definitions evaluating both profiles are compared. The resulting fatigue crack growth rate curves including threshold value, static failure load, and Paris region, qualitatively match experimental observations

Generating Visual Arguments: a Media-independent Approach

... In this paper, we take the position that certain types of arguments that can be presented visually in information graphics (e.g., bar charts and scatter plots) can be generated from an underlying media-independent representation of a presentation. In support of this claim, first we briefly describe the architecture we are developing for the generation of integrated text and information graphics presentations. In this architecture, mediaindependent communicative acts are transformed into user task specifications which are the basis for the automatic design of the presentation's graphics. Then we present an example showing correspondences between the media-independent representation of an argument and the tasks that would be used to design a graphic expressing the argument

Dynamic characterization of cellulose nanofibrils in sheared and extended semi-dilute dispersions

New materials made through controlled assembly of dispersed cellulose nanofibrils (CNF) has the potential to develop into biobased competitors to some of the highest performing materials today. The performance of these new cellulose materials depends on how easily CNF alignment can be controlled with hydrodynamic forces, which are always in competition with a different process driving the system towards isotropy, called rotary diffusion. In this work, we present a flow-stop experiment using polarized optical microscopy (POM) to study the rotary diffusion of CNF dispersions in process relevant flows and concentrations. This is combined with small angle X-ray scattering (SAXS) experiments to analyze the true orientation distribution function (ODF) of the flowing fibrils. It is found that the rotary diffusion process of CNF occurs at multiple time scales, where the fastest scale seems to be dependent on the deformation history of the dispersion before the stop. At the same time, the hypothesis that rotary diffusion is dependent on the initial ODF does not hold as the same distribution can result in different diffusion time scales. The rotary diffusion is found to be faster in flows dominated by shear compared to pure extensional flows. Furthermore, the experimental setup can be used to quickly characterize the dynamic properties of flowing CNF and thus aid in determining the quality of the dispersion and its usability in material processes.Comment: 45 pages, 13 figure

Development of a new microscopy method: optical photon reassignment microscopy

In this thesis a promising new superresolution technique called Optical Photon Reassignment (OPRA) microscopy is introduced and applied to the field of fluorescence microscopy. The method is a optical realization of the computer-based reassignment principle in confocal microscopy introduced by Sheppard in 1988 [17]. There, the spatial information in the pinhole plane is used to increase the resolution. In the year 2010 this method received a lot attention as Müller and Enderlein published the same principle including experimental data from fluorescent samples [23]. As in OPRA the computational reassignment process is done optically, any necessary processing is avoided and only one camera readout is required. The microscopy concept, together with proof-of-principle experiments and a mathematical framework, was published and patented in 2013 [24, SR1]. The method combines several important properties of fluorescent imaging towards non invasive live-cell imaging in a unique way. OPRA achieves two-fold resolution enhancement in the focal plane if compared with the cut-off frequency of standard widefield microscopy. As OPRA does not require any processing, it is suitable for extremely fast imaging, especially if the method is parallelized. The imaging speed of the method is mostly limited by the scan speed. This is a system inherent advantage to other superresolution techniques as SIM or the pointillistic methods as PALM and dSTORM where several camera readouts are necessary. In [SR2] the special property of OPRA named "superconcentration" is introduced and discussed. This property describes, that the peak-intensity, compared to widefield microscopy, is increased and therefore the light is better concentrated than classical limits predicts. The theoretical background of this measurement concept was used for a general comparison of the microscopes OTF [SR4]. As the reassignment of the photons occurs in the pinhole plane, it is in general a two dimensional process. However, there is an effect along the optical axis as well. In [SR3] it could be shown, that it is possible to improve the axial resolution by more than 10% over the resolution of a confocal microscope for reasonable big pinholes if OPRA is combined with structured illumination. This combination has the advantage, that the relatively large pinhole maintains the high signal level of OPRA. In figure 2.2 the interaction of several important properties in modern superresolving fluorescent microscopy are illustrated in a simplified scheme. OPRA is able to link several characteristics in a new, unrivalled way. The described properties of OPRA have the potential to replace the confocal microscope as standard technique for biomedical imaging. OPRA in the parallelized realization is suitable for imaging of fast processes in living cells and is therefore a promising method to help answering important biomedical questions in the coming years of research. These properties of the optical reassignment methods lead to the development of several commercial products as the Re-Scan Confocal [25, 26], the spinning-disk variant (SD-OPR) by Yokogawa Electric Corporation [27, 28] or the multi-beam realisation called Vt-SIM by Visitech Int. [29].Die hier vorgelegte Arbeit beschäftigt sich mit einem neu entwickelten superauflösenden Verfahren der Fluoreszenz-Mikroskopie. Das Prinzip der optischen Photonen-Zuweisung (Optical Photon Reassignment, OPRA) macht es möglich auf der Basis von Laser-Scanning-Mikroskopen (LSM) in Kombination mit einem ortsaufgelösten Detektor superauflösende Bilder zu erhalten [SR1]. Die OPRA Methode basiert auf dem Prinzip der computergestützten Pixelzuweisung, welches erstmals 1988 von C. Sheppard beschrieben wurde [17]. Viel Aufmerksamkeit erlangte dieses Mikroskopie-Konzept 2010 unter dem Namen “Image Scanning Microscopy (ISM)“, als mit Hilfe moderner Kameras hoch auflösende Bilder fluoreszierender Polymerkugeln aufgenommen wurden. In OPRA wird dieses computerbasierte Reassignment-Verfahren optisch realisiert, so dass es möglich ist, vollständig auf die Verwendung aufwendiger Rechenverfahren zu verzichten. Anhand der Messung von fluoreszenten Proben konnte gezeigt werden, dass es mit diesem neu entwickelten Konzept möglich ist, in nur einer einzigen Kamerabelichtung Bilder mit einer, im Vergleich zum Abbe-Limit, deutlich verbesserten Auflösung aufzunehmen. Zudem konnte gezeigt werden, dass dabei kein Fluoreszenzsignal verloren geht und somit das Licht besser auf der Kamera konzentriert wird, als es das Gesetz der Étendue-Erhaltung vorherzusagen scheint. Diese Eigenschaft wurde in Anlehnung an der etablierten Begriff der “Superauflösung“ als “Superkonzentration“ bezeichnet [SR2]. In [SR3] wurde das Verfahren auf die dritte Dimension erweitert. Dabei wurden verschiedene Ansätze zur Verbesserung der axialen Auflösung mit Hilfe einer Detektionsapertur und strukturierter Beleuchtung unter Berücksichtigung der “Superkonzentration“ diskutiert und experimentell evaluiert. Zudem konnte dieses Konzept in [SR4] verwendet werden um ein allgemeines Konzept zur Beurteilung von Mikroskopietechniken zu entwickeln. Dieses Konzept der normierten optischen Transferfunktionen (OTF) wurde am Beispiel der rechnergestützten Pixelzuweisung eingeführt. Diese Ergebnisse unterstreichen das Potenzial der Methode für die biomedizinische Bildgebung, da sie in vorher nicht bekannter Weise wichtige Eigenschaften hochauflösender Mikroskopie wie Sensitivität (was zu einer Verringerung der erforderlichen Beleuchtungsintensität führt) und Auflösung miteinander verbindet. Dabei schaffen es die Methoden, welche auf dem Prinzip der optischen Photonen-Zuweisung beruhen, zweifache laterale Auflösungserhöhung (in Bezug auf die Grenzfrequenz der Weitfeld-Mikroskopie) mit erhöhter Sensitivität bei nur einer benötigten Kamerabelichtung zu verbinden ohne dabei spezielle Anforderungen an die Probenpräparation zu stellen. Aufgrund der, im Vergleich zu herkömmlichen Laser-Scanning-Verfahren, erhöhten Sensitivität ist es möglich, die Beleuchtungsintensitäten zu verringern. Damit eignen sich die Konzepte, welche auf der optischen Photonen-Zuweisung beruhen, besonders für die Untersuchung biologischer Prozesse in lebenden Zellen. Der Zusammenhang zwischen wichtigen Eigenschaften optischer Mikroskopiemethoden ist in Abbildung (1.1) dargestellt. Es ist leicht verständlich, dass erhöhte (zeitliche und räumliche) Auflösung immer mit erhöhter Beleuchtungsdichte einhergeht, weshalb dieEigenschaft der “Superkonzentration“ ein entscheidender Parameter in der praktischen Anwendung von OPRA ist. Der Probenpräparation kommt aufgrund der Vielfalt an Markierungstechniken und entsprechenden Fluorophoren eine besondere Bedeutung zu. Gerade die pointillistischen Methoden und STED stellen an die verwendeten Fluorophore hohe spezifische Anforderungen [18, 19]. Hier besitzt OPRA als generelles Konzept eine deutlich vergrößerte Vielseitigkeit. Zudem ist das Prinzip der Pixelzuweisung nicht auf den Prozess der Fluoreszenz beschränkt und lässt sich auch auf andere Scanning-Verfahren, wie z.B. konfokale Raman-Mikroskopie, erweitern [20]

Investigation of volume rendering performance through active learning and visual analysis

Volume visualization has many real world applications such as medical imaging and scientific research. Rendering volumes can be done directly by shooting rays from the camera through the volume data, or indirectly by extracting features such as iso-surfaces. Knowing the runtime performance of visualization techniques enables for optimized infrastructure planning, trained models could also be reused for interactive quality adaption. Prediction models can make use of information about renderer and datasets to determine execution times before rendering. In this thesis, we present a model based on neural networks to predict rendering times, by using volume properties and rendering configuration. Moreover, our model actively intervenes the sampling process to improve learning while decreasing the amount of necessary measurements. For this, it estimates how likely a drawn sample will improve future predictions. Our model consists of multiple submodels, using their disagreement about certain samples as criteria for possible improvement. We evaluate our model, using different sampling strategies, loss functions and volume rendering techniques. This includes predictions based on measurement data of a volume raycaster, as well as a continuous setup with interleaved execution and prediction of an indirect volume renderer. Our indirect renderer utilizes marching cubes to extract iso-surfaces as triangle mesh from a density field and organizes them in an octree. This way, highly parallel sorting on the graphics card is enabled that is necessary for rendering transparent surfaces in correct order