C-ring specimen geometries are used to ascertain Weibull parameters for spatially distributed flaw populations in ceramic gun barrels. A review of previously published results used to compute effective areas and effective volumes for this test specimen geometry is presented along with deficiencies associated with these computations. In addition, the results of the numerical analyses presented utilizing finite-element analysis and component level reliability algorithms clearly indicate that geometric constraints called out in the current ASTM standard for C-ring testing must be revised.

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The need for improved performance in high temperature environments is prompting industry to consider the use of structural ceramic materials in heat exchanger tubes and other high temperature components. In recognition of this need, the U. S. Department of Energy has supported work for the development of nondestructive methods for evaluating flaws in monolithic ceramic components, and the associated establishment of criteria for the acceptance of flawed components. Under this development of flaw assessment criteria, DOE supported the work being presented in this paper. The approach to developing the life prediction model combines finite element predictions, considering creep behavior, with continuum damage mechanics and Weibull reliability statistics. ABAQUS is used to predict time dependent creep response of the component based on experimental creep data. A continuity parameter is then calculated at each time step following continuum damage mechanics methods. Finally, Weibull statistics are used with the resulting continuity parameter to predict the reliability at each time step, through the use of the NASA-Lewis computer program CARES, interfaced to ABAQUS with ABACARES. There is very limited data available to characterize the creep, continuum damage and reliability behavior of the material. For the life prediction model reported, it is assumed that the material damages isotropically. Directional effects of the damage can be added as material databases improve.

A methodology is proposed to estimate creep rupture life for advanced ceramics such as continuous fiber reinforced ceramic matrix composites (CFCMC). Based on the premise that the damage pattern takes the form of a heterogeneous distribution of grain boundary cavities in the majority of creep life, a damage parameter is incorporated in various creep strain rate equations. The resulting constitutive equations for creep strain and accumulated damage are cast in terms of stress, and other affinities. It is pointed out that these affinities can be derived from a scalar creep potential in nonequilibrium thermodynamics. The evolutionary laws are formulated based on many micro-mechanical models. The time-dependent reliability or hazard rate for a Sic is then established by damage mechanics with Weibull analysis. A unit cell model is presented for predicting life of a uni-directional CFCMC subjected to a constant far-field stress. A system of coupled first order ordinary differential equations is derived from which the evolution of creep damage can be solved giving the rupture life. It is shown that the stress dependence on the lifetime is very sensitive to the type of damage mechanisms active at the microstructural level.