Fatigue of alumina at room and high temperatures

Fatigue behavior of a polycrystalline alumina at room temperature and 1200$\sp\circ$C was investigated. Uniaxial tensile tests were conducted in both static and cyclic loading to produce stress-life curves. Cyclic loading at room temperature was found to be a deleterious effect on lifetime, as compared to static loading with a similar maximum stress. Cyclic lifetime was also found to be cycle dependent at room temperature.A variety of loading wave forms were applied during the cyclic tests at 1200$\sp\circ$C. High temperature results indicated that cyclic loading would not cause more detrimental effects on failure time in comparison to static loading. Cyclic lifetime at 1200$\sp\circ$C was found to be cycle shape dependent. Specimens cyclically loaded with a short duration of maximum stress in the loading cycle took a much longer time to fail than did the static loading specimens under the same maximum applied loads. Failure time for cyclic loading with a longer hold time at maximum stress was comparable to the static loading results.Failure analysis results suggest that the activities behind the crack tip may be the primary sources of the cyclic fatigue effects at both room and high temperatures. Detrimental cyclic effect at room temperature was likely related to the interlocking grains, frictional interlocking of asperities, and trapped grains behind the crack tip. High temperature beneficial cyclic effect (compared to static loading) might be attributed to the rate-sensitivity of the deformation of the viscous glassy phase bridging the crack surfaces behind the crack tip. A simplified model of crack bridging by the viscous glassy phase was applied to calculate the effective stress intensity factor at the crack tip under various loading conditions. Trends of the calculated results are consistent with the lifetime data obtained at high temperatures.U of I OnlyETDs are only available to UIUC Users without author permissio