Doctor of Philosophy, The Ohio State University, 2020, Materials Science and Engineering

Aluminum-magnesium (Al-Mg) alloys offer high strength-to-weight ratios, weldability, and excellent corrosion resistance in marine environments. However, the corrosion resistance of Al-Mg alloys with greater than 3 wt.% Mg degrades after long-term exposure to temperatures as low as 40 °C, a phenomenon called sensitization. Sensitization is caused by precipitation of β-phase (Al3Mg2) on α-Al grain boundaries. Grain boundary β precipitates are anodic to the α-Al matrix and increase susceptibility to intergranular corrosion, stress corrosion cracking (SCC), and corrosion fatigue. Until recently, a majority of corrosion fatigue tests were conducted at a high fatigue loading frequency (f) to estimate the effect of sensitization on corrosion fatigue crack growth rates (da/dN) during service. However, f in marine environments can be on the order of 0.01 Hz or lower. In this work, fracture mechanics-based experiments were utilized to understand the effects that f, sensitization level, and electrochemical potential have on corrosion fatigue da/dN of Al-Mg alloys used in marine environments. In Chapter 2, corrosion fatigue da/dN were quantified for AA5456-H116 as a function of f and sensitization level when loaded under freely corroding conditions in 3.5 wt.% NaCl at a constant stress intensity range (ΔK) and load ratio (R). A critical sensitization level of 24 mg/cm2 was established at and above which da/dN was inversely dependent on f. Sensitization to 70 mg/cm2 accelerated da/dN by approximately 2× at 10 Hz and 5× at 0.03 Hz compared to rates for the as-received microstructure (5 mg/cm2). Below 24 mg/cm2, da/dN were not higher than for the as-received microstructure and were f independent. Three possible explanations for the inverse f dependence were discussed. This research demonstrated the risk of overestimating fatigue life of sensitized AA5456-H116 in marine environments should the f dependence of da/dN not be considered. In Chapter 3, the interplay between SCC an (open full item for complete abstract)

Committee: Jenifer (Warner) Locke Ph.D. (Advisor); Gerald Frankel Ph.D. (Committee Member); Christopher Taylor Ph.D. (Committee Member); Eric Schindelholz Ph.D. (Committee Member) Subjects: Engineering; Materials Science; Metallurgy

Doctor of Philosophy, University of Akron, 2007, Civil Engineering

An accurate and practical methodology for stability analysis and design of drilled shafts reinforced slopes was developed utilizing limiting equilibrium method of slices. Complex soil stratifications and general failure slip surfaces can be handled in the developed method. The effect of soil arching due to the presence of the drilled shafts was accounted for by using a load transfer factor. The numerical values of the load transfer factor were developed based on 3-D FEM parametric study results. Many of the design variables controlling the slope/shaft systems, such: drilled shafts size, shafts location, shaft fixity (the necessary rock-socket length), and the required spacing between the drilled shafts to prevent soil from flowing around the shafts can be successfully determined from the developed method. The optimum location where the drilled shafts could be placed within the sliding soil mass so that the cost associated with the landslide repair using the drilled shafts is minimized can be searched for and determined from the developed methodology. From geotechnical point of view, the global factor of safety for slope/shaft systems can be determined. From structural point of view, the forces acting on the stabilizing drilled shafts due to the moving ground can be successfully estimated. In addition to the developed design methodology, Real-time instrumentation and monitoring were carried out for three landslide sites in the Southern part of Ohio. Various types of instruments were extensively installed inside the stabilizing shafts and the surrounding soils to monitor and better understand the behavior of slope/shaft systems. The UA Slope program developed by Dr. Robert Liang in corporation with ODOT and FHWA has been used in designing these landslides. The field instrumentation and monitoring processes have provided excellent and unique information on the lateral responses of shafts undergoing slope movements. Also, the results of the instrumented cases have provi (open full item for complete abstract)

Committee: Robert Liang (Advisor) Subjects: Engineering, Civil