Next articleSignificant improvements in propulsion and power generation for the next century will require revolutionary advances in high-temperature materials and structural design. Advanced ceramics are candidate materials for these elevated temperature applications. High-temperature and long-duration applications of monolithic ceramics can place their failure mode in the creep rupture regime.
An analytical methodology in the form of the integrated design program-Ceramics Analysis and Reliability Evaluation of Structures/ Creep (CARES/ Creep ) has been developed by the NASA Lewis Research Center to predict the life of ceramic structural components subjected to creep rupture conditions. This program utilizes commercially available finite element packages and takes into account the transient state of stress and creep strain distributions (stress relaxation as well as the asymmetric response to tension and compression). The creep life of a component is discretized into short time steps, during which the stress distribution is assumed constant. Then, the damage is calculated for each time step on the basis of a modified Monkman-Grant (MMG) creep rupture criterion. The cumulative damage is subsequently calculated as time elapses in a manner similar to 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 corresponding time is the creep rupture life for that component.
To account for the deteriorating state of the material due to creep damage (cavitation) as time elapses as well as the effects of tertiary creep, we implemented a creep life prediction methodology based on a modified form of the Kachanov-Rabotnov Continuum Damage Mechanics (CDM) theory. In this theory, the uniaxial creep rate is described in terms of stress, temperature, time, and the current state of material damage. This scalar damage state parameter is basically an abstract measure of the current state of creep in the material. The damage rate is assumed to vary with stress, temperature, time, and the current state of damage itself. Multiaxial creep and creep rupture formulations of the CDM approach have been characterized.
The CARES/ Creep code predicts the deterministic life of a ceramic component. Future work involves the role of probabilistic models in this design process. The complete package will predict the life of monolithic ceramic components, using simple, uniaxial creep laws to account for multiaxial creep loading. The combination of the CARES/ Creep and CARES/ Life codes gives design engineers the tools necessary to predict component life for the two dominant delayed failure mechanisms, creep and slow crack growth.
Find out more about this research.
Powers, L.M.; Jadaan, O.M.; and Gyekenyesi, J.P.: Creep Life of Ceramic Components Using a Finite Element Based Integrated Design Program (CARES/ CREEP ). ASME Paper 96-GT-369, 1996.
Jadaan, O.M.; Powers, L.M.; and Gyekenyesi, J.P.: Creep Life Prediction of Ceramic Components Subjected to Transient Tensile and Compressive Stress States. ASME Paper 97-GT-319, 1997.
Lewis contacts: Lynn M. Powers, (216) 433-8374,
Lynn.M.Powers@grc.nasa.gov,
and Lesley A. Janosik, (216) 433-5160,
Lesley.A.Janosik@grc.nasa.gov
Authors: Lynn M. Powers and Osama M. Jadaan
Headquarters program office: OASTT
Programs/Projects: CARES/
Creep
, Propulsion Systems
R&T, P&PM
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