A New Framework Based on a Discrete Element Method to Model the Fracture Behavior for Brittle Polycrystalline Materials

This work aims to develop and implement a linear elastic grain-level micromechanical model based on the discrete element method using bonded contacts and an improved fracture criteria to capture both intergranular and transgranular microcrack initiation and evolution in polycrystalline ceramics materials. Gaining a better understanding of the underlying mechanics and micromechanics of the fracture process of brittle polycrystalline materials will aid in high performance material design. Continuum mechanics approaches cannot accurately simulate the crack propagation during fracture due to the discontinuous nature of the problem. In this work we distinguish between predominately intergranular failure (along the grain boundaries) versus predominately transgranular failure (across the grains) based on grain orientation and microstructural parameters to describe the contact interfaces and present the first approach at fracturing discrete elements. Specifically, the influence of grain boundary strength and stiffness on the fracture behavior of an idealized ceramic material is studied under three different loading conditions: uniaxial compression, brazilian, and four-point bending. Digital representations of the sample microstructures for the test cases are composed of hexagonal, prismatic, honeycomb-packed grains represented by rigid, discrete elements. The principle of virtual work is used to develop a microscale fracture criteria for brittle polycrystalline materials for tensile, shear, torsional and rolling modes of intergranular motion. The interactions between discrete elements within each grain are governed by traction displacement relationships

Bimanual Interaction with Clothes. Topology, Geometry, and Policy Representations in Robots

Twardon L. Bimanual Interaction with Clothes. Topology, Geometry, and Policy Representations in Robots. Bielefeld: Universität Bielefeld; 2019.If anthropomorphic robots are to assist people with activities of daily living, they must be able to handle all kinds of everyday objects, including highly deformable ones such as garments. The present thesis begins with a detailed problem analysis of robotic interaction with and perception of clothes. We show that handling items of clothing is very challenging due to their complex dynamics and the vast number of degrees of freedom. As a result of our analysis, we obtain a topological, geometric, and functional description of garments that supports the development of reduced object and task representations. One of the key findings is that the boundary components, which typically correspond with the openings, characterize garments well, both in terms of their topology and their inherent purpose, namely dressing. We present a polygon-based and an interactive method for identifying boundary components using RGB-D vision with application to grasping. Moreover, we propose Active Boundary Component Models (ABCMs), a constraint-based framework for tracking garment openings with point clouds. It is often difficult to maintain an accurate representation of the objects involved in contact-rich interaction tasks such as dressing assistance. Therefore, our policy optimization approach to putting a knit cap on a styrofoam head avoids modeling the details of the garment and its deformations. The experimental results suggest that a heuristic performance measure that takes into account the amount of contact established between the two objects is suitable for the task

Developing Algorithms for Quantifying the Super Resolution Microscopic Data: Applications to the Quantification of Protein-Reorganization in Bacteria Responding to Treatment by Silver Ions

Histone-like nucleoid structuring proteins (HNS) play significant roles in shaping the chromosomal DNA, regulation of transcriptional networks in microbes, as well as bacterial responses to environmental changes such as temperature fluctuations. In this work, the intracellular organization of HNS proteins in E. coli bacteria was investigated utilizing super-resolution fluorescence microscopy, which surpasses conventional microscopy by 10–20 fold in spatial resolution. More importantly, the changes of the spatial distribution of HNS proteins in E. coli, by addition of silver ions into the growth medium were explored. To quantify the spatial distribution of HNS in bacteria and its changes, an automatic method based on Voronoi diagram was implemented. The HNS proteins localized in super-resolution fluorescence microscopy were segmented and clustered based on several quantitative parameters, such as molecular areas, molecular densities, and mean inter-molecular distances of the k-th rank, all of which were computed from the Voronoi diagrams. These parameters, as well as the associated clustering analysis, allowed us to quantify how the spatial organization of HNS proteins responds to silver, and provided insight into understanding how microbes adapt to new environments

Small scale tectonism on Venus: An experimental and image based study

Closely spaced parallel lineations in the plains of Venus are interpreted as extensional rubble-filled fractures because they show no structure and are radar-bright irrespective of look angle. Their formation was investigated using a new methodology which combines material science and fracture mechanics principles together with experimental measurement and the analysis of Magellan data. Mapping in the Guinevere and Sedna Planitia regions shows that the closely spaced parallel fractures (CSPF) follow a concentric pattern around the edge of the large topographic rise of Western Eistla Regio. 13 spacing profiles show that most of the CSPF have spacings of between 0.8 and 1.2km. Using a new fracture mechanics apparatus designed to simulate Venusian surface conditions (90bar of CO2, 450°C), the fracture toughness of basalt was measured from atmospheric to 200 bar confining pressure and from room temperature to 600°C. 1 /2 Fracture toughness was found to increase from -2.4 MPam1/2 at ambient pressure to about -3.0 MPam1/2 at 50 bar confining pressure. Higher confining pressures have no further effect. Fracture toughness shows no clear trend with temperature, rising from an ambient level of -2.4MPam1/2 to -3.0MPa1/2 at 150°C and returning to -2.4MPam1/2 at higher temperatures. A new, two-dimensional model based upon fracture mechanics is described. The depth of the CSPF is controlled by the stress intensity factor but their spacing is controlled by the initiation of new cracks. Application of a faulting criterion to limit the conditions under which the CSPF can form shows that the spacing is consistent with a regional tensile stress of 5.5-8.5MPa. This stress could have resulted from uplift of Western Eistla Regio by -2km

An advanced prototyping process for highly accurate models in biomedical applications

An integrated prototyping process for the derivation of complex medical models is introduced. The use of medical models can support today’s medicine by improving diagnosis and surgical planning, teaching and patient information. To withstand the challenges of time and accuracy, a process for generating accurate virtual and physical medical models is needed. The introduced process offers the possibility to derive virtual and physical models for biomedical engineering applications. Reviewing the current situation of medical virtual prototyping and rapid prototyping applications, limitations were found related to the influential variables of data acquisition, data processing, virtual reality use, and rapid prototyping manufacturing. An integrated prototyping concept (MPP) is introduced for embedding virtual prototyping and rapid prototyping in biomedical applications. Data processing and 3D modeling of complex anatomical structures from computerized image data were investigated and discussed in detail. Finally, parameter analyses were evaluated to derive optimal parameters needed for preparing 3D models for virtual prototyping and rapid prototyping processing in medicine. Summarizing from the accuracy analysis, the present investigation is the first to examine tomographic scanning as decisive factor for inaccuracy of medical prototyping models. The human nose is an example of a complex anatomical geometry, which has been an object of scientific research interest for several years. One of the applications introduced here uses the developed MPP concept as basis for a procedure that generates animated medical models in a virtual reality environment. Although, attempts are being made to reconstruct the human nose as an experimental rapid prototyping model, a process for accurate reconstruction as a transparent rapid prototyping model is still missing. The MPP concept allows fabricating individual models of the human nose with a high level of accuracy and transparency. Finally, temporal analysis revealed major time improvements in modeling complex anatomical models compared to approaches without optimized process sequences and approved parameters. The prototyping of the human hip was the second example used. The results of this particular example emphasized the strengths of the medial prototyping process in preparing hip models for presurgery planning. Here, accuracy was enhanced considerably. Rapid prototyping hip models can provide assistance as a surgical planning tool in complex cases, especially in improving surgical results and implant stability. Thus, the accuracy and time of model generation is improved, thereby establishing a defined process for medical model generation. Considering the novel findings of broad improvements in accuracy and time, a new field of research is emerging, serving both virtual surgery applications and physical implant generation. The MPP developed in this work can be viewed as an initial approach for launching international standards of prototyping technologies in medicine

Liquid crystalline phases in oligonucleotide solutions

In this thesis, we investigate the behavior of short complementary B-form DNA oligomers, 6 to 20 base pairs in length, exhibiting chiral nematic and columnar liquid crystal phases, even though such duplexes lack the shape anisotropy required for liquid crystal ordering. Structural characterization reveals that these phases are produced by the end-to-end stacking of the duplex oligomers into polydisperse anisotropic rod-shaped aggregates, which can order into liquid crystals. By use of polarized optical microscopy, X-ray micro-di raction and optical interferometry, we determine the phase diagram of DNA oligomers and we estimate the stacking energy to be 4-6 KBT. We also nd that upon cooling mixed solutions of short DNA oligomers, in which only a small fraction of the present DNA is complementary, the duplex-forming oligomers phase-separate into liquid crystal droplets, leaving the unpaired single strands in isotropic solution. This spontaneous partitioning is the combined result of the free energy gain from the end-to-end stacking and LC ordering of duplexes and of depletion-type interactions favoring the segregation of the more rigid duplexes from the flexible single strands. In a chemical environment where oligomer ligation is possible, such ordering and condensation would provide an autocatalytic link whereby complementarity promotes the extended polymerization of complementary oligomers. The possible relevance of these observations for prebiotic synthesis of nucleic acids is discussed

Supramolecular assembly, chirality, and electronic properties of rubrene studied by STM and STS

This thesis presents the first experimental results of a scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) investigation of rubrene at the supramolecular, molecular and submolecular level. Based on its semiconducting and fluorescent properties, this molecule is of particular interest in view of the emerging fields of molecular electronics and optoelectronics which could one day replace the conventional technology relying on semiconductors such as silicon and gallium arsenide. The goal is the substitution of these inorganic materials by cheap and flexible layers of semiconducting organic molecules for a new class of diodes and transistors, as well as the realization of electronic switches based on individual molecules. One fundamental approach is to take advantage of the molecular self-assembly behavior which results in the creation of well-ordered supramolecular structures. The investigations of the self-assembly of rubrene adsorbed on metal surfaces (Au(111), Au(100), Ag(111), and Ag(100)), which were carried out within the framework of this thesis, show a surprising diversity of supramolecular structures. Amongst other shapes, the molecules organize themselves into geometries of perfect hexagonal and pentagonal symmetry and create multifaceted patterns on the surface. A fascinating peculiarity consists in the spontaneous construction of nested structures which are built up by a hierarchical self-assembly of individual molecules into pentagonal supermolecules which form in a second step perfect supramolecular decagons. The geometric shape of rubrene is characterized by a structural asymmetry leading to the existence of two mirror imaged versions of the molecule which are not superimposable to each other, such as for instance our left and right hand or the helical DNA. The aspect of chirality is crucial for basic processes in living systems and calls for a fundamental understanding of the interaction mechanisms occurring between chiral molecules. The experiments on rubrene reveal that the intermolecular bonding differentiates between the two chiral types of the molecule (chiral recognition), yielding the self-organization into homochiral structures. These assemblies exhibit a geometry which is again chiral, demonstrating a propagation of chirality throughout the three stages of the supramolecular hierarchy. The semiconducting behavior of rubrene is furthermore probed by STS measurements detecting the energetic positions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The experimental data uncover that different adsorption conformations exhibit characteristic HOMO energies and reveal adsorption conformations of rubrene which preserve the intrinsic electronic structure of the free molecule. Furthermore, a switching of the molecular conformation and the electronic structure of one rubrene conformer is induced with the STM