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

Ceramic matrix composites (CMCs) are being developed for the exhaust nozzle of the Joint Strike Fighter (JSF) because they offer significant weight reduction and high temperature capability. The use of CMCs in JSF components, however, is hampered by the relative immaturity and unproven nature of design methods and life pre-diction theories. Materials Research & Design (MR&D) is proposing to develop Design and Life Prediction Methods for CMCs as a Phase I SBIR program. The program will initially focus on SiC/SiNC composites and environmental variables representative of flaps and seals on the exhaust nozzle, but the theory will be applicable to other CMC ma-terials and other propulsion components. The proposed program will be performed a team of MR&D, COI Ceram-ics, and Southern Research Institute and will include theoretical and experimental tasks. Phase I will include tasks on micromechanical model development and life prediction, material fabrication and characterization, and compari-sons of predicted and measured lifetimes. A literature review will assess the present state of the art in both modeling methods and properties of degraded CMCs. The micromechanical model will be based upon the Phase Average Stress theory in which the properties of the composite are directly related to the environmentally dependent proper-ties of the fibers, interface, and matrix. The Phase Average Stress theory computes average stresses in the fibers, interface, and matrix, which can be related to microcracking, diffusion, reaction, and degradation of constituent properties. This approach has proven successful in metal matrix composites, for example, where the composite properties are controlled by the history and temperature dependent, plastic behavior of the matrix. The experimental effort will address multiple environmental variables including temperature (e.g. 75F, 1500F, 2000F), stress level (e.g. above and below matrix cracking stress), load (e.g. mechanical, thermal, static, cyclic), atmosphere (e.g. air, humidity, salt fog), and time (e.g. 10, 100, 1000, 2500 hours). The Phase I test plan will assess a subset of these environmental variables to both address critical environments and remain within the funding guidelines of the Phase I SBIR program. Tests will be performed on both fiber-dominated and matrix-dominated properties. In addition to providing data to assess the micromechanical model, the test data will augment the existing database and assist on-going nozzle design projects. The comparison between model predictions and experimental results will assess the feasibility of the theoretical approach and provide a logical milestone for transition to a Phase II effort.