Many modern materials are reliant on crack bridging to achieve adequate fracture resistance. As they are used in more cyclic loading applications, there is a need to make accurate fatigue reliability predictions. In bridging materials, 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 as 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 (ρ < 10 μm) 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. The accuracy of the bridging stress profile was verified by direct measurement of the bridging stresses using x-ray fluorescence spectroscopy. From the bridging stress profile, the fatigue threshold R-curve was calculated for more technically relevant crack geometries, such as a semi-elliptical surface crack. Finally, fatigue endurance strength predictions were made as a function of initial flaw size using the calculated fatigue threshold R-curve for a semi-elliptical surface crack.

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