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Transient reliability of ceramic structures



Present capabilities of the NASA Ceramic Analysis and Reliability Evaluation of Structures/Life (CARES/Life) code include probabilistic life prediction of ceramic components subjected to fast fracture, slow crack growth (SCG) (stress corrosion), and cyclic fatigue failure modes. Currently, this code has the capability to compute the time-dependent reliability of ceramic structures subjected to simple time-dependent loading. For example, in SCG type failure conditions CARES/Life can handle the cases of sustained and linearly increasing time-dependent loads, whereas for cyclic fatigue applications, it can account for various types of repetitive constant amplitude loads. In real applications applied loads are rarely that simple, but rather vary with time in more complex ways such as engine start up and shut down and dynamic and vibrational loads. In addition, when a given component is subjected to transient environmental and/or thermal conditions, the material properties also vary with time. The objective of this paper is to demonstrate a methodology capable of predicting the time-dependent reliability of components subjected to transient thermomechanical loads that take into account the change in material response with time. In this article, the dominant delayed failure mechanism is assumed to be SCG. This capability has been added to the NASA CARES/Life code, which has also been modified to have the ability of interfacing with commercially available finite element analysis codes executed for transient load histories. An example involving a ceramic exhaust valve subjected to combustion cycle loads is presented to demonstrate the viability of this methodology and the CARES/Life program.