Date: Thursday, Apr 22nd
Presenter: Sarah Gallops and Hailey Murdock, OSU Materials Science
Sarah Gallops Abstract:
Fatigue reliability predictions for grain bridging ceramics
The purpose of this research was to develop a new methodology to make fatigue reliability predictions for grain bridging ceramics. In bridging ceramics, the fatigue threshold, or stress intensity range below which fatigue cracks will not propagate, increases with crack extension in a manner similar to the fracture resistance. Thus, fatigue thresholds can be plotted versus crack extension in what is called a fatigue threshold R-curve. The fatigue threshold R-curve was measured for a 99.5% pure polycrystalline alumina. Crack growth was initiated from razor micro-notches (crack tip radius less than 10 microns) in compact tension specimens at a loading frequency of 25 Hz. and a load ratio of R = 0.1. The fatigue threshold was determined as a function of crack size by 1) decreasing the cyclic load until the crack growth rate slowed to less than 10^-10 m/cycle and 2) using varying initial crack length and load combinations to get varying final crack sizes. Using the measured fatigue threshold R-curve and fracture mechanics weight functions, the bridging stress profile, considered a true material property, was calculated. From the bridging stress profile, the fatigue threshold R-curve was calculated for more technically relevant crack geometries, such as internal penny and surface half-penny cracks. Finally, fatigue endurance strength predictions were made as a function of initial flaw size using the measured and calculated fatigue threshold R-curves.
Hailey Murdock Abstract:
Room Temperature Ductility of Chromium Vanadium Alloys
The next generation of high temperature structural alloys must exhibit exceptional resistance to fracture, creep, oxidation, and fatigue at high temperatures. Chromium is being considered due to its high melting point (>1800°C), relatively low density (~7.2 g/cc), high temperature strength, and oxidation resistance. Limiting chromium is its high ductile-to-brittle transition temperature. First principles calculations revealed elements, when alloyed with chromium, which have potential to reduce the ductile-to-brittle transition temperature. Experiments investigating room and elevated temperature deformation behavior of Cr-V alloys are presented. Cr-V samples were prepared from powders, hot isostatically pressed, and either: 1) heat treated at 1300°C or 2) heat treated at 1300°C and extruded at 1200°C. Extrusion achieved better homogeneity of the alloys as confirmed by electron microprobe data. Results of room and elevated temperature hardness tests, and three-point bend tests, and effect of Vanadium on the ductile-to-brittle transition temperature are described.