Design and Life Prediction Methodologies for Weight Efficient Ceramic Matrix Composite (CMC) Propulsion Components

The performance of the JSF can be improved by reducing the weight of the propulsion system. The JSF is powered by either Pratt & Whitney’s (P&W) F135 engine or General Electric’s F136 engine. Both engines use a common exhaust system produced by P&W. One attractive approach to reducing the weight of the JSF propulsion system is to replace high density metal nozzle compo-nents with CMCs. In fact, CMCs have shown promise as flaps and seals on F110 turbojets that are presently undergoing flight testing at Hill AFB. Very promising results have been exhibited by S200 CMC (Nicalon/SiNC) flaps and seals. Specifically several S200 5” x 21” x 0.070” 0/90 laminates have survived hundreds of hours of flight testing, including hundreds of afterburner lights. Not only do the CMCs weigh less than the existing metal components but they also eliminate the cracking and warping that the metal parts exhibit due to the high thermal gradients and high acous-tic loads. However, after several hundred hours of exposure the CMC flaps begin to delaminate and lose surface plies. Test experi-ence coupled with stress analysis indicates that if the material maintained its original properties the flaps would not suffer damage. However, the flight tests show that over time the CMC properties degrade, leading to delamination and ply loss. Thus in order to employ CMCs in JSF propulsion components it is nec-essary to understand the material degradation mechanisms and ap-ply that understanding to the design of CMC components. The degraded properties of the S200 CMCs are controlled by several environmental variables including loads, temperatures, atmosphere, time, and preconditioning. The loads include static and fatigue events, and mechanical and thermal loads. The ambi-ent temperature will affect the CMC behavior even if the CMC is undamaged and stress free. The chemistry of the surrounding at-mosphere may include air and moisture, which will affect the ma-terial behavior. Furthermore the CMC behavior will depend on exposure time. Finally the condition of the CMC prior to test-ing will theoretically affect the behavior so the program will address the properties of both virgin and precracked materials. It may be possible to develop design information on de-graded properties and CMC lifetimes through a purely empirical approach in which materials are exposed to various environments and tested to determine their properties. The number of mate-rial properties and environmental variables, however, will in-clude several hundred sets of data making a purely empirical ap-proach prohibitively expensive and time consuming. An alterna-tive approach, proposed here, is to develop a physics-based mi-cromechanical model, grounded by correlation with critical meas-ured properties that can compute degraded properties and life-times. This material model can be subsequently coupled with fi-nite element models of CMC components to design parts accounting for degraded properties and long term behavior.