Modeling static and cyclic fatigue in ceramics containing a viscous grain boundary phase

In ceramic materials at elevated temperatures failure is often the result of the propagation of a single dominant crack when the stress is sufficiently high. The surface behind the crack tip is extensively bridged by adhesive viscous ligaments and extent of crack tip shielding by these viscous ligaments depends on the microstructure, strain rate and temperature. A mechanistic model has been developed that takes into account the effects of grain size, width and viscosity of the viscous grain boundary layer and strain rate to predict the ratio of static to cyclic loading lifetime. A single dimensionless scalar parameter has been identified as a critical parameter to determine whether cyclic loading will be equally damaging or less damaging than the corresponding static loading at high homologous temperatures. Model predictions agree with experimental results of various authors.