Fractographic Criteria for Subcritical Crack Growth Boundaries in 96% Al2O3

The percent intergranular fracture (PIF) was measured along radii extending from fracture origins in 96% A1₂O₃ specimens, fractured at various loading rates and temperatures, and plotted vs estimates of stress intensity factors ( K ₁) at the corresponding crack lengths. Two types of curves were observed. The first was similar to curves previously observed for hot-pressed alumina. In this case the subcritical crack-growth boundary was located approximately where the minimum in the PIF occurred near K ₁=4MPa·m^½, as was also the case for hotpressed alumina. Therefore, the location of this minimum or the projecting grams formed by intergranular fracture as the crack velocity increased can be used as criteria for locating the subcritical crack-growth boundary. The second type of curve lacks the minima in PIF characteristic of the first type and is characterized by a gradual trend toward higher PIF beginning at K ₁=3MPa·m^½. This type of curve may be caused by acceleration of the crack to high crack velocities at values of K ₁ approximately equal to or slightly greater than those necessary to cause critical crack growth on the lower fracture-energy planes in sapphire. Assuming that this is the case, the K ₁ at which the trend toward higher PIF begins can be used to calculate the radius to the critical flaw boundary for this type of fracture.