3 research outputs found
Experimental investigation of microstructure and properties in structural alloys through image analyses and multiresolution indentation
This work addresses the challenges in the investigation of structural alloy microstructures and their mechanical properties at multiple length scales. The investigations are performed on small volume ferrite-pearlite steel samples that were excised from in-service gas turbine components after prolonged exposure (up to 99,000 hours) to elevated temperatures, which promotes microstructural changes (spheroidization of pearlite and graphitization) as well as their yield strengths. Recent advances in spherical indentation protocols are combined for the first time to investigate the mechanical response of microscale ferrite-pearlite constituents and estimates of bulk properties on macroscale. It is shown that indentation yield strength captured with large indenter tips on an ensemble of ferrite-pearlite grains correlate strongly to the bulk yield strength evaluated with tensile measurements. Measurements on the individual ferrite and pearlite constituents follow a similar trend of decreasing yield strength as the bulk measurements.
Second, to advance the reliability and accuracy of microstructure characterization, an image segmentation framework is developed that consists of five main steps designed to achieve systematic image segmentation on broad classes of microstructures utilizing widely available image processing tools. The flexibility and modularity of the framework was demonstrated on various types of microstructures images. The developed framework was used to segment the microstructures of ferrite-pearlite samples. The extracted microstructure statistics from the segmented images and multiresolution indentation yield strength measurements were used to evaluate established composite theory estimates and have demonstrated highly consistent estimates for these material systems.Ph.D
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Colour object search
The visual search process is required when locating an object in some region of space. To perform this search two capabilities must be available: the ability to recognise the object when it comes into view; and a way of selecting these views. Visual search is often complicated by object occlusion and low spatial resolutions of the object. Although the human visual system performs this task effortlessly, the mechanisms of it are not properly understood. Object colour and geometry, however do play an important role. This thesis develops an object search methodology which assumes that a computer vision system captures both wide-angle and zoomed images of the scene containing the object. Since most of the research has focused on object recognition using geometry, this system is purely colour-based. It is not expected that object colour will always give a definitive solution, however database pruning will often occur leading to reduced search times.
The thesis argues that because colour is salient and more resilient than geometry to decreases in spatial resolution, it is more appropriate for visual search when the object occupies a small spatial resolution in an image with a large field of view. It also demonstrates that colour can be used to recognise objects when they occupy most of the field of view; as well as discriminate between database models with similar colour proportions but different region topologies. These conclusions are supported by the results produced by three algorithms, two of which perform colour object search and one that performs colour object recognition.
The first object search algorithm uses image locations containing salient object colours as a method of selecting views. Each of these views are ranked indicating which view most likely contains the object. The second object search algorithm identifies image regions with similar colour and topology as the object. These results are produced in a best-first order. The object recognition algorithm uses an invariant based on region area to identify three corresponding model and image regions. A transformation is calculated to bring the model and object into the same viewpoint where region matches are based on position and colour.
Each of these methods produced good results in complex indoor scenes with fluorescence and/or tungsten filament lighting; also the search speeds were impressive
MULTISCALE CHARACTERIZATION OF ΑLPHA-BETA TITANIUM ALLOYS USING HIGH THROUGHPUT SPHERICAL INDENTATION TEST PROTOCOLS
Traditionally, new materials follow well stablished paths from their manufacturing beginnings to their final application. Development, evaluation, certification and deployment are some of the steps in these processes. However, some of these stages are characterized for very specialized protocols, resulting in prolonged timelines from beginning to end. The design process of materials can sometimes be defined as ambiguous due to the lack of fundamental knowledge at salient length-scales. This can be attributed to the absence of trustworthy testing methods that provide meaningful and reliable knowledge on the behavior of materials at length-scales over different orders of magnitude. In addition to this characteristic, cost and time efficiency are also crucial attributes in the materials design field, as large amounts of data covering wide ranges or parameters provide stronger bases for physics-based models that can reverse-engineer the whole process.
The work presented here, evaluates spherical nano-indentation protocols as a high-throughput approach for the mechanically characterization of several α- and α/β titanium alloys at the grain-scale level. We start by the exploring the mechanical response of primary-α grains, and their dependence on the HCP lattice orientation and the corresponding grain chemical composition. Next, we move into the mechanical behavior of single grains in fully basket-weave titanium microstructures. By looking into the grain responses, a reduction on the multiple microstructural features in this type of morphologies is accomplished, leading to better statements of the influence of lath-microstructure and α-lath orientations on the indentation properties. This work is accompanied by a thorough microstructural characterization/quantification of the basket-weave morphology that is later subjected to a dimensionality reduction for a simplified understanding. And finally, we aim to bridge multiresolution indentation measurements form a bimodal titanium microstructure for the subsequent evaluation of composite theories in the prediction of the effective indentation yield.Ph.D