Fatigue Reliability Predictions for Crack Bridging Materials

Abstract

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.