Doctor of Philosophy, The Ohio State University, 2011, Materials Science and Engineering

Many metals, particularly ones with HCP crystal structures, undergo deformation by combinations of twinning and slip, the proportion of which depends on variables such as temperature and strain rate. Typical techniques to reveal such mechanisms rely on metallography, x-ray diffraction, or electron optics. Simpler, faster, less expensive mechanical tests were developed in the current work and applied to Mg AZ31B. An apparatus was designed, simulated, optimized, and constructed to enable the large-strain, continuous tension/compression testing of sheet materials at elevated temperature. Thermal and mechanical FE analyses were used to locate cartridge heaters, thus enabling the attainment of temperatures up to 350°C within 15 minutes of start-up, and ensuring temperature uniformity throughout the gage length within 8°C. The low-cost device also makes isothermal testing possible at strain rates higher than corresponding tests in air. Analysis was carried out to predict the attainable compressive strains using novel finite element (FE) modeling and a single parameter characteristic of the machine and fixtures. The limits of compressive strain vary primarily with the material thickness and the applied-side-force-to-material-strength ratio. Predictions for a range of sheet alloys with measured buckling strains from -0.04 to -0.17 agreed within a standard deviation of 0.025 (0.015 excluding one material that was not initially flat). In order to demonstrate the utility of the new method, several sheet materials were tested over a range of temperatures. Some of the data obtained is the first of its kind. Magnesium AZ31B sheets were tested at temperatures up to 250°C with strain rate of 0.001/s. The inflected stress-strain curve observed in compression at room temperature disappeared between 125°C and 150°C, corresponding to the suppression of twinning, and suggesting a simple method for identifying the deformation mechanism transition temperature. The temperature-depe (open full item for complete abstract)

Committee: Robert Wagoner PhD (Advisor); Suliman Dregia PhD (Committee Member); Katharine Flores PhD (Committee Member) Subjects: Materials Science; Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2010, Mechanical Engineering

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Committee: Robert. H. Wagoner (Advisor); June Key Lee (Committee Member); Sudarsanam Babu (Committee Member); Rebecca B. Dupaix (Committee Member) Subjects: Mechanical Engineering

Master of Science, The Ohio State University, 2009, Industrial and Systems Engineering

The increase in using Advanced High Strength Steel (AHSS) and aluminum sheet materials is accompanied by many challenges in forming these alloys due to their unique mechanical properties and/or low formability. Therefore, developing a fundamental understanding of the mechanical properties of AHSS, as compared to conventional Draw Quality Steel (DQS), is critical to successful process/ tools design. Also, alternative forming operations, such as warm forming or sheet hydroforming, are potential solutions for the low formability problem of aluminum alloys. In this study, room temperature uniaxial tensile and biaxial Viscous Pressure Bulge (VPB) tests were conducted for five AHSS sheet materials; DP 600, DP 780, DP 780-CR, DP 780-HY, and TRIP 780, and the resulting flow stress curves were compared. Strain ratios (R-values) were also determined in the tensile test and used to correct the biaxial flow stress curves for anisotropy. The pressure vs. dome height raw data in the VPB test was extrapolated to the burst pressure to obtain the flow stress curve up to fracture. Results of this work show that flow stress data can be obtained to higher strain values under biaxial state of stress. Moreover, it was observed that some materials behave differently if subjected to different state of stress. These two conclusions, and the fact that the state of stress in actual stamping processes is almost always biaxial, suggest that the bulge test is a more suitable test for obtaining the flow stress of AHSS sheet materials to be used as an input to FE models. An alternative methodology for obtaining the flow stress from the bulge test data, based on FE-optimization, was also applied and shown to work well for the AHSS sheet materials tested. Elevated temperature bulge tests were made for three aluminum alloys; AA5754-O, AA5182-O, and AA3003-O, using a special machine where the tools and specimen are submerged in a fluid heated to the required temperature. Several challenges were faced (open full item for complete abstract)

Committee: Taylan Altan (Advisor); Jerald Brevick (Committee Member) Subjects: Automotive Materials; Engineering; Industrial Engineering; Materials Science; Mechanical Engineering