Localized Preheating Approaches for Reducing Residual Stress in Additive Manufacturing

Uniform preheating can be used to limit residual stress in the solid freeform fabrication of relatively small parts. However, in additive manufacturing processes, where a feature is deposited onto a much larger part, uniform preheating of the entire assembly is typically not practical. This paper considers localized preheating to reduce residual stresses, building on previous work using a defined thermal gradient through the part depth as a metric for predicting maximum final residual stress. The building of thinwalled structures is considered. Two types of localized preheating approaches are compared, appropriate for use in laser- or electron beam-based additive manufacturing processes. In evaluating the effectiveness of each approach, a simplified thermomechanical model is used that can be related directly to analytical thermomechanical models for thermal stresses in unconstrained thin plates. Results are presented showing that one of the methods yields temperature profiles likely to yield reduced residual stresses at room temperature. Mechanical model results confirm this, showing a significant reduction in maximum stress values. A more complete thermomechanical simulation of thin wall fabrication is used to verify the trends seen in the simplified model results.Mechanical Engineerin

Process Maps for Laser Deposition of Thin-Walled Structures

In solid freeform fabrication (SFF) processes involving thermal deposition, thermal control of the process is critical for obtaining consistent deposition conditions and in limiting residual stress-induced warping of parts. In this research, nondimensionalized plots (termed process maps) are developedJrom numerical models of laser-based material deposition of thin-walled structures that.map out the effects of changes in laser power, deposition speed and part preheating on process parameters. The principal application of this work is to the Laser Engineered Net Shaping (LENS) process under development at Sandia Laboratories; however, the approach taken is applicable to any solid freeform fabrication process involving. a moving heat source. Similarly, although thinwalled structures treated in the current work, the same approach could be applied to other commonly fabricated geometries. A process map for predicting and controlling melt pool size is presented .and numerically determined results are compared against experimentally measured melt poollengthsfor stainless steel deposition in the LENS process.Mechanical Engineerin

Analysis of piezocone penetration under different drainage conditions with the two-phase Material Point Method

The piezocone penetration test (CPTU) is commonly used to identify the soil profile and to estimate material properties. Depending on the soil type, ranging from clay to sand, undrained, partially drained or drained conditions may occur during cone penetration. In silt and sand-clay mixtures the CPTU penetration is characterized by partially drained conditions, which are often neglected in data interpretation. The effect of drainage on CPTU measurements has been mainly studied experimentally. Numerical analyses are rare because taking into account large soil deformations, soil-water and soil-structure interactions, as well as non-linear soil behaviour is still a challenging task. This paper presents and discusses numerical simulations of CPTU in saturated soils with the two-phase Material Point Method. Soil behaviour is described with the Modified Cam Clay model. This study investigates the effects of pore pressure dissipation during penetration, cone roughness and horizontal stress state, comparing the results with experimental data. The paper discusses the effect of neglecting partial drainage in deriving the shear strength parameters for silty soils and suggests a procedure to estimate the consolidation coefficient performing CPTU at different penetration rates

Transient Changes in Melt Pool Size in Laser Additive Manufacturing Processes

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An analytical and experimental study of crack extension in center-notched composites

The normal stress ratio theory for crack extension in anisotropic materials is studied analytically and experimentally. The theory is applied within a microscopic-level analysis of a single center notch of arbitrary orientation in a unidirectional composite material. The bulk of the analytical work of this study applies an elasticity solution for an infinite plate with a center line to obtain critical stress and crack growth direction predictions. An elasticity solution for an infinite plate with a center elliptical flaw is also used to obtain qualitative predictions of the location of crack initiation on the border of a rounded notch tip. The analytical portion of the study includes the formulation of a new crack growth theory that includes local shear stress. Normal stress ratio theory predictions are obtained for notched unidirectional tensile coupons and unidirectional Iosipescu shear specimens. These predictions are subsequently compared to experimental results

Visual attention in primates and for machines - neuronal mechanisms

Visual attention is an important cognitive concept for the daily life of humans, but still not fully understood. Due to this, it is also rarely utilized in computer vision systems. However, understanding visual attention is challenging as it has many and seemingly-different aspects, both at neuronal and behavioral level. Thus, it is very hard to give a uniform explanation of visual attention that can account for all aspects. To tackle this problem, this thesis has the goal to identify a common set of neuronal mechanisms, which underlie both neuronal and behavioral aspects. The mechanisms are simulated by neuro-computational models, thus, resulting in a single modeling approach to explain a wide range of phenomena at once. In the thesis, the chosen aspects are multiple neurophysiological effects, real-world object localization, and a visual masking paradigm (OSM). In each of the considered fields, the work also advances the current state-of-the-art to better understand this aspect of attention itself. The three chosen aspects highlight that the approach can account for crucial neurophysiological, functional, and behavioral properties, thus the mechanisms might constitute the general neuronal substrate of visual attention in the cortex. As outlook, our work provides for computer vision a deeper understanding and a concrete prototype of attention to incorporate this crucial aspect of human perception in future systems.:1. General introduction 2. The state-of-the-art in modeling visual attention 3. Microcircuit model of attention 4. Object localization with a model of visual attention 5. Object substitution masking 6. General conclusionVisuelle Aufmerksamkeit ist ein wichtiges kognitives Konzept für das tägliche Leben des Menschen. Es ist aber immer noch nicht komplett verstanden, so dass es ein langjähriges Ziel der Neurowissenschaften ist, das Phänomen grundlegend zu durchdringen. Gleichzeitig wird es aufgrund des mangelnden Verständnisses nur selten in maschinellen Sehsystemen in der Informatik eingesetzt. Das Verständnis von visueller Aufmerksamkeit ist jedoch eine komplexe Herausforderung, da Aufmerksamkeit äußerst vielfältige und scheinbar unterschiedliche Aspekte besitzt. Sie verändert multipel sowohl die neuronalen Feuerraten als auch das menschliche Verhalten. Daher ist es sehr schwierig, eine einheitliche Erklärung von visueller Aufmerksamkeit zu finden, welche für alle Aspekte gleichermaßen gilt. Um dieses Problem anzugehen, hat diese Arbeit das Ziel, einen gemeinsamen Satz neuronaler Mechanismen zu identifizieren, welche sowohl den neuronalen als auch den verhaltenstechnischen Aspekten zugrunde liegen. Die Mechanismen werden in neuro-computationalen Modellen simuliert, wodurch ein einzelnes Modellierungsframework entsteht, welches zum ersten Mal viele und verschiedenste Phänomene von visueller Aufmerksamkeit auf einmal erklären kann. Als Aspekte wurden in dieser Dissertation multiple neurophysiologische Effekte, Realwelt Objektlokalisation und ein visuelles Maskierungsparadigma (OSM) gewählt. In jedem dieser betrachteten Felder wird gleichzeitig der State-of-the-Art verbessert, um auch diesen Teilbereich von Aufmerksamkeit selbst besser zu verstehen. Die drei gewählten Gebiete zeigen, dass der Ansatz grundlegende neurophysiologische, funktionale und verhaltensbezogene Eigenschaften von visueller Aufmerksamkeit erklären kann. Da die gefundenen Mechanismen somit ausreichend sind, das Phänomen so umfassend zu erklären, könnten die Mechanismen vielleicht sogar das essentielle neuronale Substrat von visueller Aufmerksamkeit im Cortex darstellen. Für die Informatik stellt die Arbeit damit ein tiefergehendes Verständnis von visueller Aufmerksamkeit dar. Darüber hinaus liefert das Framework mit seinen neuronalen Mechanismen sogar eine Referenzimplementierung um Aufmerksamkeit in zukünftige Systeme integrieren zu können. Aufmerksamkeit könnte laut der vorliegenden Forschung sehr nützlich für diese sein, da es im Gehirn eine Aufgabenspezifische Optimierung des visuellen Systems bereitstellt. Dieser Aspekt menschlicher Wahrnehmung fehlt meist in den aktuellen, starken Computervisionssystemen, so dass eine Integration in aktuelle Systeme deren Leistung sprunghaft erhöhen und eine neue Klasse definieren dürfte.:1. General introduction 2. The state-of-the-art in modeling visual attention 3. Microcircuit model of attention 4. Object localization with a model of visual attention 5. Object substitution masking 6. General conclusio

Scaling Effects in Laser-Based Additive Manufacturing Processes

Mechanical Engineerin

Process Mapping of Transient Melt Pool Response in Wire Feed E-Beam Additive Manufacturing of Ti-6Al-4V

Wire feed electron beam additive manufacturing processes are candidates for manufacturing and repair in the aerospace industry. In order to implement feedback or feedforward control approaches, the time needed for a change in process variables to translate into changes in melt pool dimensions is a critical concern. In this research, results from 3D finite element simulations of deposition of Ti-6Al-4V are presented quantifying the transient response of melt pool dimensions to rapid changes in beam power and travel velocity. Results are plotted in beam power vs. beam velocity space, following work by the authors developing P-V Process Maps for steady-state melt pool geometry. Transient responses are determined over a wide range of process variables. Simulation results are compared to initial results from experiments performed at NASA Langley Research Center.Mechanical Engineerin