Slow crack-growth behavior of alumina ceramics

The fracture behavior of high-purity alumina ceramics with grain sizes ranging from 2 to 13 mu m is studied by means of the double torsion method. Crack-propagation tests conducted in air, water, and silicon oil, for crack velocities from 10(-7) to 10(-2) m/s, show that slow crack growth is due to stress corrosion by water molecules. An increase of the grain size leads to enhanced crack resistance, which is indicated by a shift of the V-K-I (crack velocity versus applied stress intensity factor) plot toward high values of K-I. Moreover, the slope of the curve is apparently higher for coarse grain alumina. However, if the R-curve effect is substracted from the experimental results, a unique V-K-Itip (crack velocity versus stress intensity factor at the crack tip) law is obtained for all alumina ceramics, independently of the grain size. This means that the crack-growth mechanism (stress corrosion by water molecules) is the same and that the apparent change of the V-K-I law with grain size is a direct effect of crack bridging.