抄録
Although the relation between stress intensity factor (KI) and crack velocity (v) in polycrystalline materials is to be affected by their microstructures such as grain size, porosity, grain boundary and so on, the dependence of subcritical crack growth on microstructures has rarely been investigated. Therefore, in this study, the KI-v relations for single-and poly-crystalline α-alumina were examined in order to obtain the information about such dependence and to study the stress corrosion mechanism in α-alumina.
The specimens with a Chevron notch were fractured by the 4-points bending technique. The tests were carried out under a constant crosshead speed of 0.005mm/min. in air (20°C, 67%rh) and distilled water. The KI-v relations were obtained by the compliance analysis of load-deflection curves of stable fracture. Also the effective fracture energy (γeff) was measured by using the work-of-fracture technique.
The values of γeff for two polycrystalline bodies were 35.30J/m2 and 15.74J/m2 in air, which were larger than that of single crystal (3.50J/m2). The positions of the curves in KI-v diagram for three materials corresponded to the magnitude of γeff. The difference in γeff between two polycrystalline bodies was explained on the basis of grain size. The n-values in v=AKIn measured in air and distilled water were nearly equal, and they were 65, 50, and 27 for single-, large grain and small grain poly-crystalline bodies, respectively. This difference may be attributable to the purity of alumina in the bodies. Stress corrosion was hardly observed in single crystal. For the polycrystalline bodies, the stress corrosion resistance was better in a higher purity alumina body. Consequently, it seems that the stress corrosion resistance of polycrystalline bodies depends largely on that of glassy phases in grainboundaries.
