Cyclic fatigue of alumina

Mechanical behavior of polycrystalline alumina under cyclic and static tensile loading was studied. Smooth cylindrical specimens of 99.8% alumina were cyclically loaded at both room temperature and 1200°C to produce stress-life curves. A decrease in strength was observed with an increase in temperature. Macroscopic fracture surfaces were found similar for both temperatures. A flat semicircular region that originated from the surface indicated a period of stable crack growth. The size of this zone agreed with the estimates from the fracture toughness of the material and maximum load during the fatigue cycle. Lifetimes of static fatigue specimens tested at 1200°C appeared to be shorter compared with cyclic fatigue tests. The viscous boundary phase may be the primary contributor to the improved fatigue resistance under cyclic loading. Specimens with two circumferential notches were loaded cyclically at 1200°C to simulate a component and study notch sensitivity effects. A further decrease in strength was observed as a result of the stress concentration. However, the alumina became increasingly less sensitive to the stress concentration factor, K_t, at the lower stresses, suggesting a fatigue notch factor, K_f, that is less than K_t.