Ceramics Analysis and Reliability Evaluation of Structures/Creep (CARES/CREEP) is a computer program that predicts the creep lives of ceramic structural components. [CARES/CREEP should not be confused with the related program CARES/LIFE, which was described in "Program for Evaluation of Reliability of Ceramic Parts," NASA Tech Briefs, Vol. 20, No. 3 (March 1996), page 28.] CARES/CREEP is integrated with the commercially available ANSYS finite-element code and is intended to be used, in conjunction with ANSYS, to design ceramic structural components of advanced turbine engines.
The need to predict creep lives for hot engine structures arises from the expected service conditions and durability requirements. The high-temperature properties of ceramic structural components make them attractive for use in advanced turbine engines, the design operational lives of which can exceed 10,000 hours. In order to enable ceramic components to last such long times, one must subject them to relatively low stresses. The combination of high temperatures and low stresses typically causes failures of monolithic ceramic components to occur in the creep regime.
CARES/CREEP utilizes the finite-element heat-transfer and nonlinear-stress-analysis capabilities of ANSYS to obtain temperature and stress distributions in a ceramic component. CARES/CREEP takes account of time-varying creep-strain distributions (stress relaxation). The creep life of a component is discretized into short time steps, during each of which the stress and strain distributions are assumed constant. The increment of damage at each time step is calculated on the basis of a modified Monkman-Grant creep-rupture criterion. The cumulative damage is subsequently calculated as time elapses in a manner similar to that of Miner's rule for cyclic fatigue loading. Failure is assumed to occur when the normalized cumulative damage at any point in the component reaches unity. The elapsed time to occurrence of such a failure is considered to be the creep rupture life of the component.
CARES/CREEP is run as a post-processor of ANSYS output. The program (see figure) consists of two modules: The first module is a parameter-estimation subprogram that computes (1) the primary creep parameters based on the time hardening rule (a widely accepted constitutive equation that closely approximates empirical data on creep as a function of stress, temperature, and time), (2) the steady-state parameters based on the Norton equation, and (3) the creep-rupture parameters based on the modified Monkman-Grant criterion. The second module calculates the cumulative damage and thus the creep-rupture life of the component in question. Among the outputs of the program is a cumulative-damage plot for graphical rendering of critical regions of the component.
This work was done by John P. Gyekenyesi of Glenn Research Center, Lynn M. Powers of Cleveland State University, and Osama M. Jadaan of the University of Wisconsin. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.
Inquiries concerning rights for the commercial use of this invention should be addressed to
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Commercial Technology Office,
Attn: Steve Fedor,
Mail Stop 4 —8,
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Refer to LEW-16917.