Deformation and Fracture of Alumina Joints in Bending Creep

Bending creep tests were conducted on alumina-alumina butt joints at temperatures of 788-888 K. The lifetimes in three-point bending tests under constant loads were measured and the specimens tested were investigated using scanning electron microscope. From the elastic stress-distribution at the joining layer calculated using the finite element method, it was confirmed that the thin metal film in the joining layer is under the conditions of tri-axial tension and the hydrostatic tensile stress. The results of SEM observation of creep fractured surfaces showed numerous small and large cavities in the tension side of the joining layer. Under the assumption that the mechanism of creep deformation and fracture for the thin metal film in the joining layer were the nucleation, growth and aggregation of cavity with the aid of the hydrostatic tensile stress component, the lifetimes in creep bending test were predicted. Time-dependent change of stress distribution related to initial outer fiber stress in bending has been calculated with formulas based on linear-elastic beam theory. Predicted lifetimes are in good agreement with the experimentally obtained lifetimes. The neutral-axis position measured from the photographs of the fracture surfaces can be explained by the predicted stress distribution in creep bending tests.