Effect of viscous grain bridging on cyclic fatigue-crack growth in monolithic ceramics at elevated temperatures

The bridging tractions developed behind a crack tip are considered for a st ationary crack under cyclic loading conditions at elevated temperatures in high-toughness, monolithic ceramics. Assuming a temperature range where the grain-boundary phases are sufficiently soft such that bridging can occur d ue to a viscous layer in the boundary, a viscoelastic model is developed in which bridging forces associated with the shear resistance of the grain bo undary phase are transmitted across the surfaces of a crack. Throughout the work, cyclic and static damage mechanisms which may be operating ahead of the crack tip (e.g. creep cavitation) are ignored in order to focus exclusi vely on the role of viscous grain bridging. A primary goal is to incorporat e microstructural details like grain shape, grain-boundary thickness, and g lass viscosity, as well as the effects of external variables such as loadin g rate and temperature. A fully self-consistent numerical approach is adopt ed, which does not require any prescribed assumptions as to the shape of th e crack-opening profile. The self-consistent solution is compared to an ana lytical solution for a simplified parabolic approximation of the crack-flan k opening displacements. The model is applicable to a wide range of ceramic materials at elevated temperatures, and rationalizes the frequency and tem perature sensitivity not generally observed in ceramics at room temperature . Solutions identify a non-dimensional group associated with microstructure and external loading conditions, and solutions are presented over a range of this parameter. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Sc ience Ltd. All rights reserved.