Fatigue crack growth in polycrystalline alumina at high temperatures
High temperature crack growth behavior in a commercial and a hot-pressed polycrystalline Al$\sb2$O$\sb3$ was examined under monotonic tensile loading (static fatigue) and under Mode-I tension-tension cyclic loading (cyclic fatigue). The investigation of fatigue crack growth in polycrystalline Al$\sb2$O$\sb3$ has confirmed the cyclic effect existing in ceramic materials, but this cyclic effect cannot be seen as the manifestation of static failure under cyclic loading. The temperature, load ratio, and microstructural factors, such as the grain boundary phase and grain size, play important roles in affecting high temperature crack growth behavior. Locally at the crack tip, the cyclic fatigue crack was found to advance in shear by the frictional sliding of grains on alternating sets of planes of the maximum shear. Evidence of shear-driven crack growth was supported by topological and morphological analyses of cyclic fatigue crack surface; grain sliding; frictional debris; and temperature-dependent of cyclic fatigue crack growth kinetics. Based on experimental observations, a new model of cyclic fatigue crack growth from alternating shears is proposed. The observed crack path morphology, the microstructure at the crack tip region, and the measured crack velocity under cyclic loading are distinctly different from those seen under static loading. At the crack tip, crack growth tends to follow the plane of the maximum tensile stress under static loading.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign