No abstract

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 950°and 1050°C 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 fiberstrength 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 1050°C agrees well with the experimental results. For a more complex degradation model, the rupture life and tertiary creep at 1050°C can be predicted quite well; however, the spread in residual strength is not, and the lifetimes at 950°C 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.

Abstract. Fresh mine spoil at a pH of 3.84 was collected and transferred to leachate columns in early August 1988. The spoil was leached weekly with simulated precipitation at pH values of 5.6, 5.0, 4.6, 4.2, and 3.8. Control samples of mine spoil were simultaneously leached with distilled water at a pH of 6.47. Leachate was collected and analyzed by standard methods for cations, anions, conductivity, and pH. The cations and anions of major interest were iron, aluminum, manganese, and sulfate. The leachate initiallY was pH 2

Precipitation and resultant runoff were sampled for a series of storm events over the period of one year. The test site was the parking lot of a large suburban shopping mall in the Syracuse, New York, area. Both precipitation and runoff were tested for lead, zinc, copper, cadmium, and petroleum hydrocarbons: substantial amounts were detected in each. No correlation was found between precipitation contaminant concentration and the length of the antecedent dry period. A weak, but apparently inverse relationship was noticed between concentration and amount of precipitation. Poor correlations were obtained between runoff contaminant concentration and the antecedent dry period. The variability attributable to different precipitation volumes was removed by converting to a unit‐area basis. The variability attributable to precipitation contaminant load was removed by subtraction. The resultant value, dryfall accumulation, then correlated well with the length of the antecedent dry period. Metal ions were found in both precipitation and runoff and were hypothesized to come from atmospheric fallout as a result of distant emissions and from very localized sources, primarily vehicle traffic on the parking facility. Petroleum residues were believed to be the sole result of automobile losses, since none could be detected in precipitation samples.