BEHAVIOR AND TEST METHOD OF SUBCRITICAL CRACK-GROWTH AT ELEVATED-TEMPERATURES IN CERAMICS

Behavior and test method of subcritical crack growth (SCG) at elevated temperatures were investigated on an alumina with 90% purity. The proposed test method enabled to measure SCG at temperatures over 1000-degrees-C using the piston displacement of testing machine and fractographic information. In order to evaluate SCG behavior based on the linear elastic fracture mechanics (LEFM), larger specimens are recommended, because a small scale creep (SSC) condition is required in expressing the crack tip singular stress field by LEFM parameter, i.e. stress intensity factor K. Since creep is dependent on both thermal and mechanical factors, requirement of SSC condition, which implies that the creep damage zone is localized at the crack tip region, may be influenced by those factors. Detailed observations of SCG behavior at 900 and 1050-degrees-C revealed that a main crack grew by coalescences with microcracks initiated in the creep damage zone at the crack tip. Thus, tortuosity of SCG path increased with increasing size of the creep damage zone. Temperature dependence of SCG characteristics was discussed in terms of the crack growth mechanism and creep deformation scale.