Doctor of Philosophy, University of Toledo, 2022, Engineering

Superelastic (SE) Nitinol (NiTi) could be a great candidate for a wide range of applications in the biomedical and aerospace industries. Despite its unique properties, fabrication still remains a challenge and is of high interest. To address the limitations, laser-based powder bed fusion (LPBF) additive manufacturing (AM) has been developed and used for the fabrication of superelastic NiTi components. However, in most SE applications NiTi components undergo cyclic loadings. There is, however, limited work on the fatigue life of NiTi components fabricated with LBPF. In general, different parameters starting from the powder preparation to process parameters, build conditions, and post-processing directly affect the microstructural and mechanical properties of the LBPF fabricated NiTi parts. After providing an introduction chapter, this work is organized into three papers, each forming one chapter of the dissertation, and there is a summary and conclusion as a final chapter. In the second chapter, the effect of build orientation on the fatigue behavior of the LBPF fabricated NiTi parts was evaluated. NiTi dog-bone samples with three different build orientations were fabricated and used to investigate the monotonic tensile and fatigue behavior of the material. In addition to the mechanical experiments, fracture surfaces of the monotonic and fatigue samples were evaluated, and different types of defects were assessed. It was shown that the samples fabricated on their edge has a low level of scattering in comparison to the other sample fabricated horizontally or in 45-degree. Since internal defects and unwanted porosities were recognized as a major cause of the fatigue failure, the remelting process was proposed as a potential solution to improve the parts' relative density and reduce internal pores. In the third chapter, to achieve a set of optimized process parameters for the remelting process, selective laser remelting with different process parameters was designe (open full item for complete abstract)

Committee: Mohammad Elahinia (Committee Chair); Mohammad J. Mahtabi (Committee Co-Chair); Mohamed Samir Hefzy (Committee Member); Ala Qattawi (Committee Member); Meysam Haghshenas (Committee Co-Chair) Subjects: Materials Science; Mechanical Engineering; Mechanics

Doctor of Philosophy, Case Western Reserve University, 2018, Materials Science and Engineering

A third generation Ag-free Al-Li alloy, 2070, is the focus of this dissertation. To determine its suitability for a range of applications, this alloy was first qualified through a series of room temperature mechanical tests. Samples from each orientation (e.g., L, T, or S) were excised from each section of an H-forging to determine the effects of prior work and degree of anisotropy. Samples were then tested at room temperature in tension, compression, and Charpy impact. Room temperature mechanical behavior of as-received H-forgings of 2070 was found to meet or exceed properties of second and third generation Al-Li alloys. The primary focus of this dissertation was to determine the effects of forging conditions (i.e., temperature and strain rate) on flow stress and microstructural evolution during/after hot deformation of this material. Subscale right circular cylinder samples were deformation processed under isothermal conditions at a range of temperatures (T = 300/425/450/475°C) and strain rates (0.01/s, 0.1/s, 5.0/s) to 100% true strain in order to determine these effects on the resulting flow stress and microstructure. Activation energy and power dissipation coefficients were determined for each temperature and strain rate combination followed by microstructure analyses via optical and scanning electron microscopy (SEM). Microstructure analyses included EBSD (electron backscatter diffraction) to determine the degree of dynamic recrystallization (DRX) or dynamic recovery (DRV) present after deformation processing at different temperatures and strain rates. Dynamic recovery was found to be the dominant deformation mechanism for the sample tested at 450°C and 0.01/s. Samples tested at 450°C and 5.0/s or 300°C and any strain rate (0.01, 0.1, or 5.0/s) were found to be dominated by dynamic recrystallization, with a large area fraction of unresolved highly deformed grains. When samples were solution heat treated (at 510°C for 1 hour) after deformation processing, (open full item for complete abstract)

Committee: John Lewandowski (Committee Chair); Gerhard Welsch (Committee Member); Jennifer Carter (Committee Member); Sunniva Collins (Committee Member) Subjects: Materials Science

Master of Science, The Ohio State University, 2017, Materials Science and Engineering

The transition of newly developed materials from the laboratory to the manufacturing floor is often hindered by the task of quantifying the material's inherit variability which spans from the atomistic to macroscale. This impedance is coupled with the task of linking this variability observed at these length scales and ultimately correlating this multidimensional variance to the macroscale performance of the material. This issue has lead to the development of statistical and mathematical frameworks for evaluating material variability. In this work, the author employs one such methodology for the purpose of mesoscale variability quantification with the goal to further explore and enhance this framework while simultaneously presenting the pathway as a computational design tool. This stochastic representation of microstructure allows for the delineation of materials to be highly dependent upon the topology of the material's structure and allows for digital representation via statistical volume elements (SVEs). Quantification of the topology of these SVEs can be achieved through utilization of statistical microstructural descriptors (SMDs), which inevitably leads to an extremely high order data set for each microstructure realization. This high order data set can then be dimensionally reduced via kernel principal component analysis (KPCA), thus allowing for the variance of the microstructure to be observed through the generation of microstructure visualizations. Enhancement of these visualizations can then be achieved through the use of the 1-way multivariate analysis of variance (1-way MANOVA). The reduced order SMD data set can then be combined with property results determined via finite element analysis (FEA) producing microstructure-property maps, thus allowing for both the microstructure and property variance to be observed graphically. Lastly, predictive models can be trained on the reduced order SMD data sets and property results utilizing the machine learning te (open full item for complete abstract)

Committee: Stephen Niezgoda Dr. (Advisor); Dennis Dimiduk Dr. (Committee Member); Soheil Soghrati Dr. (Committee Member) Subjects: Materials Science; Mathematics; Statistics

Doctor of Philosophy, The Ohio State University, 2008, Mechanical Engineering

The overall goal of the research done in this dissertation is to develop next generation force feedback systems by combining novel Magnetorheological (MR) fluid based electromechanical systems with microstructural analysis and advanced control system design. Four MR fluid based systems are designed, prototyped and tested with medical applications: A two degree of freedom (2-DOF) force feedback joystick and a 5-DOF force feedback manipulator for telerobotic surgery application, a passive and a semiactive orthopedic knee brace for rehabilitation application. Furthermore, a force feedback steering wheel is modified using MR damper with application to steer-by-wire automobiles. The test results show the appropriate performance of MR fluid based systems used in haptic and force feedback applications.

Committee: Gregory Washington PhD (Advisor); Stephen Bechtel PhD (Committee Member); Vadim Utkin PhD (Committee Member); Giorgio Rizzoni PhD (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, Case Western Reserve University, 1995, Macromolecular Science

The morphology of polyethylenes, their blends and copolymers was examined as a function of thermal history and microstructural parameters. The blends were made with high and low-density polyethylene. The former is a linear homopolymer synthesized using low pressures. This leads to long chain branching but no short chain branching. Low-density polyethylene is synthesized under high pressures which results in both short and long branches along the main chain. Also, there is a distribution of short chain branches along the long chain branches. Blends of 75, 50, and 25 weight percent were melt-blended in a twin screw extruder. Compression-molding of the pellets was done and two different thermal histories were generated: slowly-cooled, approximately 18°C/hour, and rapidly-cooled, approximately 18°C/min. Both sets of blends showed two endothermic peaks which depended upon the blend concentration and thermal history. It is assumed that the homopolymers are phase-separated with an intermediate density phase composed of the long-chain branched HDPE and low branch content LDPE. The lamellae sizes decreased as the weight fraction of HDPE decreased. Furthermore, it appears as if a bimodal distribution of thicknesses is apparent, one due to the HDPE phase, the second to the LDPE phase. The te nsile properties of these blends showed that the systems do interact since the elongation at break exhibited negative deviations from the rule of mixtures. This is not due purely to the inhomogeneities in the blend but probably to the rougher interfaces between the crystalline regions. Both the HDPE and LDPE showed sharpening in the WAXS patterns over the undrawn blend indicative of crystallite perfection. A 50 weight percent HDPE blend had broader arcs than seen in the HDPE and no distinct higher order reflections. The fracture surface morphology showed a trend from fibrillar to ductile failure as the weight content of HDPE decreased. Linear low-density polyethylenes polymerized using sin (open full item for complete abstract)

Committee: Abdelsamie Moet (Advisor) Subjects: