Study of the resistance to crack propagation in alumina by acoustic emission

The resistance to crack propagation or R-curve effect is still extensively studied in polycrystalline ceramic materials. This effect is shown in the literature by the increase of the stress intensity factor KI versus crack extension, or by the decrease of crack velocity versus KI under flexure tests. We have studied this fracture resistance by using the acoustic emission and the Hertzian indentation technique. It was found that the acoustic emission count rate decreases, like crack velocity in conditions of sub-critical crack growth before the failure is reached. Two specimen of alumina, respectively with small and coarse grains, were tested. A high crack extension resistance is obtained for coarse-grained alumina. Mechanisms of crack bridging and crack shielding are responsible for the R-curve effect. These mechanisms are also operating during cyclic loading. The resistance to crack propagation increases more and more after each cycle. Also we obtain R-curve effect during static loading. On the other hand, toluene or silicone oil corrosive environment delays the crack propagation. But water medium has a reverse effect and the apparent toughness is reduced in this case.