Stress rupture in ceramic-matrix composites: Theory and experiment
A micromechanically based model for the deformation, strength, and stress-rupture life of a ceramic-matrix composite is developed for materials that do not degrade by oxidative attack. The rupture model for a unidirectional composite incorporates fiber-strength statistics, fiber degradation with time at temperature and load, the state of matrix damage, and the effects of fiber pullout, within a global load sharing model. The constituent material parameters that are required to predict the deformation and lifetime can all be obtained independent of stress-rupture testing through quasi-static tension tests and tests on the individual composite constituents. The model predicts the tertiary creep, the remaining composite strength, and the rupture life, all of which are dependent critically, on the underlying fiber-strength degradation. Sensitivity, of the rupture life to various micromechanical parameters is studied parametrically,. To complement the model, an extensive experimental study of stress rupture in a Nextel 610/alumina-yttria composite at temperatures of 950degrees and 1050degreesC is reported. The Larson-Miller and Monkman-Grant life-prediction methods are inadequate to explain the current data. Constituent parameters for this material system are derived from quasi-static tests and literature data, and the micromechanical model predictions are compared with measured behavior. For a slow-crack-growth model of fiber-strength degradation, the lifetime predictions are shorter by two orders of magnitude. When the rupture life is fitted with one parameter, however, the model prediction of the tertiary creep and residual strength at 1050degreesC agrees well with the experimental results. For a more complex degradation model, the rupture life and tertiary creep at 1050degreesC can be predicted quite well; however, the spread in residual strength is not, and the lifetimes at 950degreesC are greatly overpredicted. Thus, the micromechanical model can be successful quantitatively but clearly shows that the rupture life of the composite is extremely sensitive to the detailed mechanisms of fiber degradation. The model has practical applications for extrapolating laboratory lifetime data and predicting life in components with evolving spatial stresses.